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PRASEODYMIUM AND IRON INCORPORATED
AlPO-5 MOLECULAR SIEVES FOR ORGANIC
TRANSFORMATIONS
A THESIS
Submitted by
SUNDARAVEL B
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
FACULTY OF SCIENCE AND HUMANITIES
ANNA UNIVERSITY
CHENNAI 600 025
MAY 2014
ii
ANNA UNIVERSITY
CHENNAI 600 025
BONA FIDE CERTIFICATE
Certified that this Thesis titled “PRASEODYMIUM AND IRON
INCORPORATED AlPO-5 MOLECULAR SIEVES FOR ORGANIC
TRANSFORMATIONS” is the bona fide work of Mr. SUNDARAVEL, B.
who carried out the research work under my supervision. Certified further that
to the best of my knowledge the work reported herein does not form part of
any other thesis or dissertation on the basis of which a degree or award was
conferred on an earlier occasion on this or any other scholar.
Place : Chennai Dr. V. MURUGESAN
Date : SUPERVISOR
Professor of Chemistry (Retd.)
Anna University
Chennai 600 025
iii
ABSTRACT
Homogeneous catalysts such as sulfuric acid, hydrochloric acid,
hydrofluoric acid, phosphoric acid, etc., are frequently used in the chemical
industry. However, these catalysts are environmentally hazardous, toxic,
corrosive and non-recyclable. Porous materials such as zeoloites, zeotypes
and mesoporous solids have emerged as an alternative option for the chemical
industry due to their non toxicity, non corrosive nature, possibility of reusing
and more importantly their ecofriendly nature. The use of heterogeneous
catalysts is highly desirable for compliance with the principles of green
chemistry, offering low energy routes to products, eliminating the
requirement of auxiliary species and facilitating catalyst recovery to minimise
waste generation. The use of porous solid acids as heterogeneous catalyst is
gained significant importance in organic synthesis due to their environmental
compatibility combined with good yield and selectivity.
The isomorphously substituted AlPOs found applications in aerial
oxidation of linear and cyclic hydrocarbons using molecular oxygen.
Compared to zeolites, MeAlPO molecular sieves possess large variety of acid
sites and broader acid sites distribution. Because of these valuable properties,
metaloaluminophosphates (MeAPOs) become one of the important materials
as heterogeneous catalyst for important organic transformations. The present
work focused on the synthesis, characterization and catalytic evaluation of
PrAlPO-5 and FeAlPO-5.
PrAlPO-5 with different (Al+P)/Pr ratios (25, 50, 75, 100, 150 and
200) were successfully synthesized by hydrothermal method in fluoride
iv
medium. These molecular sieves were characterized using XRD, DRS UV-
vis, BET, SEM, TEM, 27Al and 31P MAS-NMR ESR, XPS, TPD-NH3, ex-situ
pyridine adsorbed IR, TPR, TGA and FTIR studies. The incorporation of
praseodymium in the framework of AlPO-5 was confirmed by XRD, DRS
UV-vis and 27Al and 31P MAS-NMR spectra. The increase of lattice
parameters also supported the incorporation of Pr in AlPO-5 framework. ESR
spectrum revealed the presence of adsorbed oxygen. The ammonia-TPD study
confirmed the presence of weak and moderately strong acid sites whereas
ex-situ pyridine adsorbed IR spectrum confirmed the presence of Lewis acid
sites. The BET surface area of PrAlPO-5 was found to be in the range of
239 – 272 m2g-1. The textural parameters varied linearly with increase in
praseodymium content. The hysteresis loop observed just below the relative
pressure (p/p0) of one is due to inter-particle voids. The SEM and HRTEM
images also revealed such large number of voids due to aggregation of
particles.
Iron containing AlPO-5 with AFI topology was synthesized
hydrothermally in fluoride medium. The framework incorporation of Fe3+,
and the absence of separate iron(III) oxide phase in AlPO-5 were confirmed
by XRD and DRS UV-vis studies. The tetrahedral geometry of iron in [FeO4]-
was identified by ESR and DRS UV-vis studies. The ammonia-TPD study
revealed the presence of weak acid sites in FeAlPO-5. The ex-situ pyridine
adsorbed IR spectrum confirmed the presence of Lewis acid sites. The
textural parameters especially average pore size of fresh and used FeAlPO-5
catalysts remained almost the same and in good agreement with the parent
AlPO-5, suggesting that the zeotype kept its original structure.
v
The catalytic activity of PrAlPO-5 (25, 50, 75 and 100) was tested
in the liquid phase aerobic oxidation of ethylbenzene. Acetophenone was
found to be the major product with more than 90% ethylbenzene conversion.
PrAlPO-5 (25) showed better selectivity than other catalysts at 120 °C in 6 h
reaction time. The decrease of (Al+P)/Pr ratio increased the conversion and
selectivity. This correlated the dependence of Pr content in AlPO-5 and
reactivity. It was also observed that weak and moderately strong acid sites
created by the framework incorporation of praseodymium in AlPO-5 favored
side chain oxidation rather than ring hydroxylation. Further, the change in
electron density around the benzylic hydrogen did not influence the selectivity
to acetophenone. This study concluded that ethylbenzene and different
substituted ethylbenzenes could be effectively oxidized using molecular
oxygen as oxidant over PrAlPO-5 at 120 oC. The ICP-OES analysis
confirmed the presence of praseodymium intact in the framework of AlPO-5
up to five cycles.
The catalytic activity of PrAlPO-5 (75, 100, 150 and 200) was
evaluated in the synthesis of campholenic aldehyde from -pinene.
Campholenic aldehyde was found to be the major product whereas -pinene
oxide, verbenol and verbenone were identified as minor products in the liquid
phase oxidation. The influence of reaction parameters such as temperature,
reaction time, solvent and Pr content in AlPO-5 was optimized with a view to
obtain campholenic aldehyde selectively. The epoxidation of -pinene by
chemisorbed oxygen followed by isomerisation of -pinene oxide over Lewis
acid sites on PrAlPO-5 yielded campholenic aldehyde selectively. Thus
PrAlPO-5 was proved to be active, selective and reusable catalyst.
vi
Synthesis of 5-arylidene-2,4-thiazolidinediones was carried out in
water-ethanol (4:1) solvent system at 100 °C over FeAlPO-5 (75, 100 and
150). Though the reaction proceeded faster in polar solvents like dimethyl
sulphoxide, acetonitrile and ethanol, the reaction parameters such as
temperature, iron content and reaction time were optimized in water-ethanol
solvent system with a view to design a greener route for the synthesis of 2,4-
thiazolidinedione (TZD) derivatives. The Knoevenagel condensation of
substituted benzaldehydes with TZD also showed 90% product selectivity
with high aldehyde conversion. Further, the structure of isolated products was
confirmed by 1H-NMR spectra.
The bi-functional nature of PrAlPO-5 catalyst will open new
prospect for potential application in the synthesis of fine chemicals. Further, it
is also concluded that FeAlPO-5 is an efficient catalyst for Knoevenagel
condensation.
vii
ACKNOWLEDGEMENT
I take this opportunity to express my profound thanks and
wholehearted gratitude to my supervisor Dr. V. Murugesan, Professor of
Chemistry (Retd.), Anna University, for his valuable guidance, stimulating
discussions, constant encouragement and moral support during the entire
course of my research work.
I am grateful to Dr. M. Palanichamy, Professor of Chemistry
(Retd.), Anna University, Chennai, for his unstinted support and valuable
suggestions. I thank Dr. P. Kannan, Professor and Head, Department of
Chemistry, Anna University, Chennai, for providing facilities to carry out my
research work. I record my sincere thanks to Doctoral Committee member,
Dr. M. Kandasamy for his valuable inputs.
I am thankful to the Department of Science and Technology, New
Delhi, for the award of Junior Research Fellow and financial support to carry
out this research. I wish to express my sincere thanks to Dr. S. Devika,
Dr. B. Palanisamy and Mr. C. M. Babu for their support for successful
completion of my work. I also like to thank my seniors and friends for their
support.
Finally, I extend my sincere gratitude to my beloved parents, brother
and sisters for their constant encouragement, forbearance and patience during
the entire course of my research work.
(SUNDARAVEL B)
viii
TABLE OF CONTENTS
CHAPTER NO. TITLE PAGE NO.
ABSTRACT iii
LIST OF TABLES xv
LIST OF FIGURES xvii
LIST OF SCHEMES xxi
LIST OF SYMBOLS AND ABBREVIATIONS xxii
1 INTRODUCTION 1
1.1 CATALYSIS 1
1.2 CATALYSIS AND GREEN CHEMISTRY 2
1.2.1 E-Factor 3
1.2.2 Atom Efficiency or Atom Economy 4
1.3 HETEROGENEOUS CATALYSIS 5
1.4 POROUS MATERIALS 6
1.5 ZEOLITES 6
1.5.1 Structure of Zeolites 7
1.5.2 Nomenclature of Zeolites 9
1.5.3 Classification of Zeolites Based on
Pore Size 10
1.5.4 Unique Properties of Zeolites 10
1.5.5 Applications of Zeolites 11
1.5.6 Molecular Sieves Versus Zeolites 11
1.5.7 Non-aluminosilicate Molecular Sieves 12
1.6 GENERAL ASPECTS OF
ALUMINOPHOSPHATES 13
1.6.1 Natural Aluminophosphates 15
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CHAPTER NO. TITLE PAGE NO.
1.6.2 Synthetic Aluminophosphate Molecular
Sieves 16
1.6.3 Nomenclature 17
1.7 STRUCTURAL ASPECTS OF
ALUMINOPHOSPHATES 20
1.7.1 Classification 20
1.7.2 Neutral Framework 20
1.7.3 Anionic Framework AlPOs 23
1.7.4 Bonding Patterns 23
1.8 TOPOLOGICAL CHEMISTRY OF
ALUMINOPHOSPHATES 24
1.8.1 Building Units 24
1.8.2 Al and P Coordinations and
Stoichiometries 27
1.9 TEMPLATING IN THE CONSTRUCTION OF
ALUMINOPHOSPHATES 28
1.9.1 Types of Templates 28
1.9.2 Role of Templating 31
1.10 ISOMORPHIC SUBSTITUTION 33
1.10.1 Metal Aluminophosphates 35
1.11 AlPO-5 MOLECULAR SIEVE 36
1.12 CATALYTIC APPLICATIONS OF MeAPO-5
MOLECULAR SIEVES 37
1.13 SCOPE AND OBJECTIVES OF THE PRESENT
INVESTIGATION 44
x
CHAPTER NO. TITLE PAGE NO.
2 EXPERIMENTAL METHODS 47
2.1 MATERIALS 47
2.1.1 Chemicals 47
2.2 PREPARATION OF CATALYSTS 48
2.2.1 Synthesis of PrAlPO-5 48
2.2.2 Synthesis of Praseodymium Oxide 49
2.2.3 Synthesis of AlPO-5 Supported Pr6O11 49
2.2.4 Synthesis of FeAlPO-5 49
2.2.5 Synthesis of Fe2O3 50
2.3 CHARACTERIZATION OF
SYNTHESIZED MATERIALS 50
2.3.1 X-ray Diffraction (XRD) 51
2.3.2 Diffuse Reflectance Ultraviolet -
Visible Spectroscopy 53
2.3.3 Nitrogen Sorption Studies 53
2.3.4 Fourier Transform - Infrared
(FT-IR) Spectroscopy 56
2.3.5 Thermogravimetric Analysis
(TGA) 57
2.3.6 Temperature Programmed
Desorption (TPD) 58
2.3.7 Temperature Programmed
Reduction (TPR) 59
2.3.8 Scanning Electron Microscopy
(SEM) 60
2.3.9 Transmission Electron
Microscopy (TEM) 61
xi
CHAPTER NO. TITLE PAGE NO.
2.3.10 X-ray Photoelectron Spectroscopy
(XPS) 62
2.3.11 Electron Spin Resonance (ESR)
Spectroscopy 63
2.3.12 Magic Angle Spinning - Nuclear
Magnetic Resonance (MAS-NMR)
Spectroscopy 64
2.3.13 Inductively Coupled Plasma–
Optical Emission Spectroscopy
(ICP-OES) 65
2.4 CATALYTIC STUDIES 66
2.4.1 Liquid Phase Reactions 66
2.4.1.1 Oxidation of ethylbenzene 68
2.4.1.2 Synthesis of campholenic
aldehyde from -pinene 68
2.4.1.3 Synthesis of 5-arylidene-
2,4-thiazolidenedione 68
2.5 PRODUCT ANALYSIS 69
2.5.1 Gas Chromatograph 69
2.5.2 Gas Chromatograph Coupled with Mass
Spectrometer 69
2.5.3 NMR Spectroscopic Analysis 70
3. PHYSICO-CHEMICAL CHARACTERIZATION
OF PrAlPO-5 AND FeAlPO-5 MOLECULAR
SIEVES 71
3.1 INTRODUCTION 71
xii
CHAPTER NO. TITLE PAGE NO.
3.2 PHYSICOCHEMICAL CHARACTERIZATION
OF PrAlPO-5 72
3.2.1 X-ray Diffraction (XRD) 72
3.2.2 Diffuse Reflectance Ultraviolet Visible
(DRS-UV-Vis) Spectroscopy 74
3.2.3 Surface Microstructure
(SEM and HR TEM) 76
3.2.4 Nitrogen Sorption Studies 78
3.2.5 Electron Spin Resonance
Spectroscopy (ESR) 80
3.2.6 X-ray Photoelectron
Spectroscopy (XPS) 83
3.2.7 27Al and 31P Magic Angle
Spinning – NMR 84
3.2.8 Temperature Programmed
Reduction (TPR) 88
3.2.9 Characterization of Acid Sites
(TPD-NH3 and ex-situ pyridine
adsorbed IR) 89
3.2.10 FT-IR Spectroscopy 92
3.2.11 Thermogravimetric Analysis (TGA) 93
3.3 PHYSICOCHEMICAL CHARACTERIZATION
OF FeAlPO-5 95
3.3.1 X-ray Diffraction (XRD) 95
3.3.2 Diffuse Reflectance Ultraviolet –
Visible Spectroscopy 97
3.3.3 Scanning Electron Microscopic
(SEM) Analysis 99
xiii
CHAPTER NO. TITLE PAGE NO.
3.3.4 Nitrogen Sorption Studies 100
3.3.5 Electron Spin Resonance
Spectroscopy (ESR) 101
3.3.6 X-ray Photoelectron
Spectroscopy (XPS) 102
3.3.7 Characterization of Acid Sites
(TPD-NH3 and ex-situ pyridine
adsorbed IR) 103
4 LIQUID PHASE AEROBIC OXIDATION OF
ETHYLBENZENE OVER PrAlPO-5
4.1 INTRODUCTION 106
4.2 CATALYTIC STUDIES 108
4.2.1 Effect of Temperature 110
4.2.2 Effect of (Al+P)/Pr Ratios 112
4.2.3 Effect of Reaction Time 112
4.2.4 Effect of Substituents 113
4.2.5 Catalyst Recycling 115
4.2.6 Conclusion 115
5 SYNTHESIS OF CAMPHOLENIC ALDEHYDE
FROM -PINENE OVER PrAlPO-5 116
5.1 INTRODUCTION 116
5.2 CATALYTIC STUDIES 118
5.2.1 Effect of Temperature 121
5.2.2 Effect of (Al+P)/Pr Ratios 121
5.2.3 Effect of Reaction Time 122
5.2.4 Effect of Solvents 124
xiv
CHAPTER NO. TITLE PAGE NO.
5.2.5 Structure Identification of Products 125
5.2.6 Catalyst Recycling 130
5.3 CONCLUSION 130
6 SYNTHESIS OF 5-ARYLIDENE-2,4-
THIAZOLIDINEDIONES OVER FeAlPO-5 131
6.1 INTRODUCTION 131
6.2 CATALYTIC STUDIES 133
6.2.1 Effect of Temperature and Al/Fe Ratios 136
6.2.2 Effect of Reaction Time 137
6.2.3 Effect of Solvents 138
6.2.4 Effect of Substituents 139
6.2.5 Structure Identification of Products 142
6.3 CONCLUSION 153
7 SUMMARY AND CONCLUSION 154
7.1 SUMMARY AND CONCLUSION OF
THE PRESENT WORK 154
7.2 SCOPE FOR FUTURE WORK 158
REFERENCES 159
LIST OF PUBLICATIONS 178
xv
LIST OF TABLES
TABLE NO. TITLE PAGE NO.
1.1 E factors in various segment of chemical industry 4
1.2 Structure type index as per IUPAC nomenclature 9
1.3 Classification of zeolites based on pore openings 101.4 Structure of aluminophosphate based molecular sieves 18
1.5 Acronyms for framework composition 19
1.6 Structures of AlPO4-n molecular sieves 221.7 Templates used for the preparation of specific
structure type 29
3.1 Lattice parameter values for AlPO-5 and PrAlPO-5 743.2 Nitrogen sorption results of calcined AlPO-5 and
PrAlPO-5 (25, 50, 75, 100, 150 and 200) 80
3.3 TPD-NH3 sorption results of calcined samples 91
3.4 Lattice parameters for calcined AlPO-5 and FeAlPO-5 97
3.5 Nitrogen sorption results for fresh and used
FeAlPO-5 catalyst 101
3.6 TPD-NH3 sorption results of calcined FeAlPO-5 catalysts 105
4.1 Effect of reaction temperature and (Al+P)/Pr ratios on the
oxidation of ethylbenzene 111
4.2 Effect of substituents on benzylic oxidation 114
5.1 Effect of reaction temperature and (Al+P)/Pr ratios on the oxidation of -pinene 122
5.2 Effect of solvents on the oxidation of -pinene 124
6.1 Effect of reaction temperature and (Al+P)/Pr ratios in the synthesis of 5-benzylidene - 2, 4 – thiazolidinedione 137
xvi
TABLE NO. TITLE PAGE NO.
6.2 Effect of solvents in the synthesis of
5-benzylidene - 2, 4 – thiazolidinedione 139
6.3 Effect of substituents in the synthesis of
5-benzylidene - 2, 4 – thiazolidinedione 141
xvii
LIST OF FIGURES
FIGURE NO. TITLE PAGE NO.
1.1 Secondary building units identified in zeolite frameworks 8
1.2 Representative AlPO4-n molecular sieves with different
pore openings 21
1.3 Secondary building units (SBUs) found in AlPO4-n based
framework 25
1.4 Eight distinct 2D sheet structures
(The SBUs constructing these sheets are also shown) 26
1.5 Cylindrical channel in AlPO-5 and the stacking of
encapsulated tetrapropylammonium hydroxide species 30
1.6 Partial periodic table with transition elements introduced
into aluminophosphates and silicoaluminophosphates 36
1.7 Framework topology of AlPO-5 (a) framework structure
of AlPO-5, (b) 12-ring channel view along (001) plane
and (c) 12-membered ring of AlPO-5 37
2.1 Schematic diagram of multiple reflection ATR system 56
2.2 The interaction between primary electron beam and
the sample in an electron microscope 60
2.3 Catalytic reaction set up for liquid phase reactions 67
3.1 XRD patterns of calcined (a) AlPO-5, (b) PrAlPO-5 (25),
(c) PrAlPO-5 (50), (d) PrAlPO-5 (75), (e) PrAlPO-5 (100),
(f) PrAlPO-5 (150), (g) PrAlPO-5 (200) and (h) Pr6O11 73
3.2 DRS UV-Vis spectra of (a) AlPO-5 and (b) AlPO-5
supported with 3 wt%Pr6O11 75
xviii
FIGURE NO. TITLE PAGE NO.
3.3 DRS UV-Vis spectra of (a) PrAlPO-5 (200), (b) PrAlPO-5 (150), (c) PrAlPO-5 (100), (d) PrAlPO-5 (75), (e) PrAlPO-5 (50) and (f) PrAlPO-5 (25) 75
3.4 SEM images of (a) AlPO-5, (b) PrAlPO-5 (200), (c) PrAlPO-5 (150), (d) PrAlPO-5 (100), (e & h) PrAlPO-5 (75), (f) PrAlPO-5 (50) and
(g) PrAlPO-5 (25) 773.5 TEM images of (a) AlPO-5, (b) PrAlPO-5 (200),
(c) PrAlPO-5 (150), (d) PrAlPO-5 (100), (e) PrAlPO-5 (75), (f) PrAlPO-5 (50) and (g) PrAlPO-5 (25) 78
3.6 N2 sorption isotherms of (a) AlPO-5, (b) PrAlPO-5 (200), (c) PrAlPO-5 (150),
(d) PrAlPO-5 (100), (e) PrAlPO-5 (75),
(f) PaAlPO-5 (50) and (g) PrAlPO-5 (25) 79
3.7 Small angle XRD pattern of PrAlPO-5 (75) 79
3.8 Room temperature ESR spectra of PrAlPO-5
samples 81
3.9 Low temperature ESR spectra of (a) PrAlPO-5 (25)
and (b & c) PrAlPO-5 (75) 83
3.10 XPS spectrum of calcined PrAlPO-5 (75) 84
3.11 27Al MAS-NMR spectra of AlPO-5 and PrAlPO-5
(25 and 75) 85
3.12 31P MAS-NMR spectra of AlPO-5 and PrAlPO-5
(25 and 75) 87
3.13 TPR profile of (a) AlPO-5 supported 3 wt% Pr6O11,
(b) PrAlPO-5 (100), (c) PrAlPO-5 (75),
(d) PrAlPO-5 (50) and (e) PrAlPO-5 (25) 88
xix
FIGURE NO. TITLE PAGE NO.
3.14 TPD-NH3 profile of (a) AlPO-5 and (b) AlPO-5
supported 3 wt% Pr6O11 89
3.15 TPD-NH3 profile of (a) PrAlPO-5 (200), (b) PrAlPO-5
(150), (c) PrAlPO-5 (100), (d) PrAlPO-5 (75),
(e) PrAlPO-5 (50) and (f) PrAlPO-5 (25) 90
3.16 Ex-situ pyridine adsorbed IR spectra of (a) PrAlPO-5
(150), (b) PrAlPO-5 (100), (c) PrAlPO-5 (75) and
(d) PrAlPO-5 (25) 91
3.17 FT-IR spectra of (a) AlPO-5, (b) PrAlPO-5 (200),
(c) PrAlPO-5 (150), (d) PrAlPO-5 (100),
(e) PrAlPO-5 (75), (f) PrAlPO-5 (50) and
(g) PrAlPO-5 (25) 93
3.18 TGA of as-synthesized (a) AlPO-5,
(b) PrAlPO-5 (100), (c) PrAlPO-5 (75),
(d) PrAlPO-5 (50) and (e) PrAlPO-5 (25) 94
3.19 TGA of calcined (a) AlPO-5, (b) PrAlPO-5 (100),
(c) PrAlPO-5 (75), (d) PrAlPO-5 (50) and
(e) PrAlPO-5 (25) 95
3.20 XRD patterns of (a) FeAlPO-5 (150),
(b) FeAlPO-5 (100), (c) FeAlPO-5 (75)
and (d) Fe2O3 96
3.21 DRS UV-Vis spectra of (a) FeAlPO-5 (150),
(b) FeAlPO-5 (100) and (c) FeAlPO-5 (75) 98
3.22 SEM images of (a) AlPO-5, (b) FeAlPO-5 (75),
(c) FeAlPO-5 (100) and (d) FeAlPO-5 (150) 99
3.23 Nitrogen sorption isotherms of (a) AlPO-5,
(b) fresh FeAlPO-5 (75) and (c) used FeAlPO-5 (75) 100
xx
FIGURE NO. TITLE PAGE NO.
3.24 ESR spectra of (a) FeAlPO-5 (75), (b) FeAlPO-5
(100) and (c) FeAlPO-5 (150) 102
3.25 XPS spectrum of calcined FeAlPO-5 (75) 103
3.26 TPD-NH3 profile of (a) FeAlPO-5 (150),(b) FeAlPO-5 (100) and (c) FeAlPO-5 (75) 104
3.27 Ex-situ pyridine adsorbed IR spectra of calcined
FeAlPO-5 (75) 1054.1 Effect of reaction time on ethyl benzene oxidation 113
5.1 Effect of reaction time 123
5.2 1H NMR spectrum of campholenic aldehyde 127
5.3 1H NMR spectrum of -pinene oxide 128
5.4 1H NMR spectrum of verbenone 129
6.1 Effect of reaction time 138
6.2 1H NMR spectrum of 5-(4-nitrobenzylidene)-1-
3-thiazolidine-2,4-dione 145
6.3 1H NMR spectrum of 5-(3-nitrobenzylidene)-1-3-
thiazolidine-2,4-dione 1466.4 1H NMR spectrum of 5-(3-methoxybenzylidene)-1-
3-thiazolidine-2,4-dione 147
6.5 1H NMR spectrum of 5-(2-hydroxy-3-methoxy benzylidene)-1-3-thiazolidine-2,4-dione 148
6.6 1H NMR spectrum of 5-(4-methoxybenzylidene)-1-
3-thiazolidine-2,4-dione 1496.7 1H NMR spectrum of 5-((benzo[d][1,3]dioxol-6-yl)
methylene)1,3-thiazolidine-2,4-dione 150
6.8 1H NMR spectrum of 5-((thiophen-2-yl)methylene)
thiazolidine-2,4-dione 151
6.9 1H NMR spectrum of 5-((pyridin-3-yl)methylene)
thiazolidine-2,4-dione 152
xxi
LIST OF SCHEMES
SCHEME NO. TITLE PAGE NO.
1.1 Generation of Brönsted acid sites in
aluminophosphate molecular sieves 34
4.1 Aerobic oxidation of ethylbenzene 109
4.2 Possible pathway for the oxidation of
ethylbenzene to acetophenone 110
5.1 Synthesis of campholenic aldehyde from -
pinene 119
5.2 Plausible pathway for the oxidation of -
pinene and isomerisation of -pinene oxide 120
6.1 Synthesis of 5-arylidene-2,4-
thiazolidinedione 133
6.2 Plausible mechanism for the synthesis of
water mediated TZD derivative 134
6.3 Plausible mechanism – The role of Lewis
acid sites in the synthesis of TZD derivative 135
xxii
LIST OF SYMBOLS AND ABBREVIATIONS
Å
N
°C
m
%
-
-
-
-
-
-
-
-
-
-
Alpha
Angstrom unit
Avagadro number
Beta
Degree Celcius
Gamma
Magic angle
Percentage
Theta
Wavelength
AlPO - Aluminophosphate
AFI - Aluminophosphate-five
a.u. - Arbitrary unit
BJH - Barrett - Joyner - Halenda
BE - Binding energy
BET - Brauner - Emmet - Teller
CRT - Cathode ray tube
cm - Centimeter
CCD - Charge coupled device
cm3/g - Cubic centimeter per gram
DTG - Derivative thermogram
W/ T - Difference in weight/difference in temperature
DTA - Differential thermal analysis
DRS-UV-vis
D
-
-
Diffuse reflectance ultraviolet visible spectroscopy
Dimensional
xxiii
ESR - Electron spin resonance
eV - Electron volt
EDAX - Energy dispersive x-ray analysis
FID - Flame ionisation detector
FT-IR - Fourier transform infrared spectroscopy
FWHM - Full width at half maximum
GC - Gas chromatograph
GC-MS - Gas chromatograph coupled with mass spectrometer
Hz - Hertz
HMS - Hexagonal mesoporous silica
h - Hour
H2O2 - Hydrogen peroxide
IUPAC - International union of pure and applied chemistry
keV - Kiloelectron volt
kHz - Kilohertz
kJ - Kilojoules
KIT - Korean advanced institute of science and technology
MAS - Magic angle spinning
MRP - Membered ring pore
m - Meta
MeAPO - Metal aluminophosphate
MeAPSO - Metal silicoaluminophosphate
m2g–1 - Metre square per gram
m - Micrometer
s - Microsecond
mg - Milligram
ml - Millilitre
min - Minute
MCM - Mobil composition matter
xxiv
M - Molar
mol - Mole
MOR - Mordenite
nm - Nanometer
NMR - Nuclear magnetic resonance
o - Ortho
p - Para
ppm - Parts per million
PILCS - Pillared interlayered clays
PBU - Primary building unit
RSF - Relative sensitivity factor
rpm - Rotation per minute
SBA - Santa Barbara
SEM - Scanning electron microscopy
SKM - Schuster - Kubelka - Munk
SBU - Secondary building unit
s - Second
SAPO - Silicoaluminophosphate
SSHC - Single-site heterogeneous catalyst
STP - Standard temperature and pressure
SDA - Structure directing agent
TPD
TPR
-
-
Temperature programmed desorption
Temperature programmed reduction
TBHP - tert-Butylhydroperoxide
tert - Tertiary
TCD - Thermal conductivity detector
TGA - Thermogravimetric analysis
TZD - 2,4-Thiazolidinedione
UHV - Ultra high vacuum
xxv
Wt % - Weight percentage
XRD - X-ray diffraction
XPS - X-ray photoelectron spectroscopy
ZSM - Zeolite sacony mobil
1
CHAPTER 1
INTRODUCTION
1.1 CATALYSIS
Catalysis is a process in which acceleration of chemical reaction is
induced in the presence of a material (catalyst) that is chemically unchanged
at the end of the reactionThe phenomenon of reducing the energy requirement
of a chemical process by changing the rate of attainment of equilibrium
through lowering of activation energy is termed as catalysis and the material
as catalyst. However, catalysts do not alter the equilibrium position of a
reaction which is controlled thermodynamically and require high pressures.
Recent estimations revealed that approximately 90% of chemicals ranging
from bulk chemicals to consumer products come into contact with a catalyst
at one stage or another of their manufacturing process. Depending upon their
relative reaction medium catalysts are classified into two basic types,
heterogeneous and homogeneous.
The world wide effort to replace homogeneous acid catalysts by
heterogeneous catalysts in all industries is to control pollution and waste. In
homogeneous type, the catalysts are in the same phase as the substrate and are
uniformly distributed. As the catalyst gets dissolved in the reaction medium
almost all the reactions under homogenous type takes place within the liquid
phase whereas in most cases of heterogeneous system the catalyst used is a
porous solid and the reaction takes place either on its external surface or
surface within the pores of the solid. Heterogeneous catalytic systems, in
2
which fluid reactants are reacted over solid acid catalysts, are the most widely
used catalytic processes in the manufacturing industries at present. The
following are the advantages of heterogeneous systemswhen compared to
their homogeneous counterparts:
Minimal pollution, less corrosion and wastes.
High activity, selectivity and suppression of side products.
Shapeselectivity.
Easy removal of product from the reaction mixture and
efficient recycling of the catalyst.
Use of renewable starting materials.
Easy separation of end products.
1.2 CATALYSIS AND GREEN CHEMISTRY
The concept of green chemistry has gained momentum among
researchers both in academic and industries as a tool for achieving
sustainability by promoting innovative chemical technologies that reduce or
eliminate the use or generation of hazardous substances in the design,
manufacture and application of chemical products. Strong legislative
enactments towards controlling discharge of waste products from industries
into the environment and their restrictions in the manufacture, transport,
storage and use of certain hazardous chemicals has sparked the introduction
of cleaner technologies. Realizing the unsustainable consequences of
exceeding the earth’s natural capacity in dealing with the waste and pollution
which society generates, Anastas&Warner (1998) coined a set of twelve
principles as green chemistry. Heterogeneous catalysis is an omnipotent tool
to realize all the twelve principles of green chemistry.
3
i) Waste prevention instead of remediation
ii) Atom efficiency
iii) Less hazardous/toxic chemicals
iv) Safer products by design
v) Innocuous solvents and auxiliaries
vi) Energy efficient by design
vii) Preferably renewable raw materials
viii) Shorter synthesis (avoid derivatization)
ix) Catalytic rather than stoichiometric reagents
x) Design products for degradation
xi) Analytical methodologies for pollution prevention
xii) Inherently safer processes
Heterogeneous system is more convincing in controlling
environmental pollution. The two significant factors of heterogeneous
catalysts that influence the environmental impact of cleaner chemical
processes are (1) E-factor, and (2) atom efficiency.
1.2.1 E factor
E factor is an important metric to understand the potential
environmental acceptability of chemical processes. It is defined as the mass
ratio of waste to desired product. The magnitude of the waste problems in
chemical manufacture is readily apparent from the consideration of typical E
factor in various segments of chemical industry (Table 1.1). A high E factor
means more waste and consequently more negative environmental impact.
The ideal E factor is zero. E factor can be calculated using the following
Equation (1.1)
4
Kgof secondary productsEfactorKgof desired product
(1.1)
Table 1.1 E factors in various segment of chemical industry
Industry segment Product tonnagea E(kg wasteb/kg product)
Oil refining 106 - 108 < 0.1
Bulk chemicals 104 - 106 < 1to 5
Fine chemicals 102 - 103 5to50Pharmaceuticals 10 - 103 25 to100a Represents annual production volume of a product at one site (lower end
of range) or world-wide (upper end of range) b Defines as everything produced except the desired product (including all
inorganic salts, solvent loss, etc)
For example, the original process of Friedel-Crafts acylation using
AlCl3 had an E factor of about 5 and required a chlorinated hydrocarbon or
nitroaromatic solvent. The new process with zeolite (H-beta)
catalysedFriedel-Crafts acylation, in contrast, has an E factor of < 0.01 and no
solvent is required. The substantially higher E factors in fine chemicals and
pharmaceuticals compared with bulk chemicals is a reflection of more
widespread use of stoichiometric reagent and multi-step synthesis in the
former steps. Thus, replacement of stoichiometric protocols in the fine and
pharmaceutical industries by catalytic methods will help to reduce E factors in
these sectors and thus will help to achieve the goals of green chemistry.
1.2.2 Atom Efficiency or Atom Economy
The concept of atom economy was developed by Trost (1991 and
1995). This is a method of expressing how efficiently a particular reaction
5
makes use of the reactant atoms. Thus if all the reactants are completely
incorporated in to the product, the synthetic pathway is said to 100% atom
efficiency because it will not generate any waste. Atom economy of a
particular reaction can be calculated by the following Equation (1.2).
Atom ef iciency =Molecular weight of the product
Sum of the molecular weight of the reactants(1.2)
1.3. HETEROGENEOUS CATALYSIS
Unlike a homogeneous catalytic system, both the catalyst and the
reactants of a heterogeneous system are in different phases such as solid,
liquid or vapour. Solid acids and their salts find important application as
heterogeneous catalysts. The most common are silica, alumina,
aluminosilicates and aluminophosphates. Industrial processes involving
dehydrogenation, oxidation, ammoxidation and polymerization are catalyzed
using metals, metal oxides, clays and zeolites. A landmark in the history of
heterogeneous catalysis was achieved by Fritz Haber in 1970 (Smil
1999)when he prepared large quantities of ammonia from nitrogen and
hydrogen in the presence of Fe2O3 catalyst using a high pressure reactor.
Similarly oxidation with metallic platinum, dehydrogenation with metallic
nickel and Fischer-Tropsch process over cobalt and iron catalysts are other
examples. Lewis and Brönsted acidities in the catalysts are two fundamental
active centres in most of the solid acid catalysts. Chromia is a well known
example of a Lewis solid acid catalyst (Auroux&Gervasini 1990). On the
other hand bulk oxides with loosely bound protons associated with oxide ions
are examples of Brönsted acid catalyst. V2O5 and ZrO2 contain both Lewis
and Brönsted acid sites (Auroux&Gervasini 1990 and Kawai et al 1981).
Yadav et al(1993) and Yadav&Thorat (1996) have reported alkylation of
6
toluene with benzyl chloride, benzyl alcohol and benzyl ether over sulfated
zirconia.
1.4 POROUS MATERIALS
Porous materials have widespread applications such as catalysts,
catalyst supports, adsorbents and sensors due to their high thermal,
hydrothermal, mechanical and chemical stabilities as well as high specific
surface area, large specific pore volume and pore diameter. The IUPAC has
recommended specific nomenclature for classification of porous materials
into three groups based on their predominant pore size: microporous (pore
diameter < 2nm), mesoporous (2nm < pore diameter <50 nm) and
macroporous (50 nm < pore diameter). Zeolites, zeotype materials and
activated carbons are examples of microporous materials. M41S family,
mesoporousAlPOs, aero-gels and most recent SBA-1, SBA-15 and KIT-5 are
few examples of mesoporous materials. Examples of macroporous materials
include silica-gel, activated charcoal and CPG (controlled porous glass).
1.5 ZEOLITES
Zeolite is a unique class of oxides, consisting of microporous, crystalline aluminosilicates found in nature or synthesized artificially
(Thomas et al 1999). These materials were discovered in 1756 by the Swedish
mineralogist Axel Frederick Cronstedt, who found that the mineral stilbite
lost significant amount of water when heated. The word zeolite stems from
Greek and means boiling stone. It took almost two centuries before zeolite
received the attention of chemists. Nowadays, new zeolites and associated
materials are still being discovered in laboratories worldwide. Zeolites are
used in various potential applications such as household detergents, desiccants
and toothpaste, whereas their acidity makes them attractive catalysts. In the
middle of 1960s, Raboet al(1966) at Union Carbide demonstrated that
7
faujasitic zeolites are very interesting solid acid catalysts. Since then, a wealth
of zeolite-catalyzed reactions of hydrocarbons has been discovered. For
fundamental catalysis they offer the advantage that the crystal structure is
known and that the catalytically active sites are thus well defined. The fact that zeolite possess well-defined pore systems in which the catalytically active
sites are embedded in a defined way gives them some similarity to enzymes.
1.5.1 Structure of Zeolites
Zeolites are crystalline aluminosilicates having three dimensional
framework made up of primary buildings units (PBU) of SiO4 and
AlO4tetrahedra (known as TO4) by sharing a common oxygen atom in their
corners. The PBUs are joined together to form a secondary building unit
(SBU) and twelve such SBUs were identified by Meier & Olson (1987) as
shown in Figure 1.1.These SBUs are arranged in a specific geometrical
pattern to form a definite crystal structure and uniform pore size.
Theoretically thousands of structures can be arrived but only around 160
have been synthesized till today. Out of these, only 40 of them are naturally
occurring zeolites. Zeolites are represented by the following empirical
formula (Breck 1964).
Mx/n [(AlO2)x (SiO2)1-x] . zH2O (1.1)
where M is a cation with valency n, x represents the number of
AlO2tetrahedra and z represents the number of water molecules.
8
Figure 1.1 Secondary building units identified in zeolite frameworks
9
The presence of extra negative charge in the framework,
compensated by cations especially protons, is the main cause for Brönsted
acidity. Lewis acidity is generated by the formation of trigonally co-ordinated
Al and Si sites by the removal of two hydroxyl groups from the framework
(Uytterhoeven et al 1965). While zeolites are synthesized commercially for
specific uses, many natural zeolites are readily available as minerals from the
earth crust. Their unique properties made tremendous applications in
petrochemical cracking, ion-exchange and in separation and removal of gases
and solvents (Piera et al 1998 and Tomita et al 2004). The other applications
are in agriculture, animal husbandry, construction, etc.
1.5.2 Nomenclature of Zeolites
International Zeolite Association Structure Commission and
IUPAC have assigned structural codes to known natural and synthetic zeolites
(Barrer 1983). Designations consist of three letter abbreviation derived from
the names of species which do not include numbers and characters other than
Roman letters. Some examples are shown in Table 1.2.
Table 1.2 Structure type index as per IUPAC nomenclature
Structure type code
Species Structure type code Species
MFI ZSM-5 LAU Laumonite MOR Mordenite LTA Linde Type A MTN ZSM-39 LTL Linde Type L MTT ZSM-23 MEL ZSM-11 BEA Beta AST ALPO-16 MTW ZSM-22 ATS ALPO-36 AEL ALPO-11 ATT ALPO-33 AFI ALPO-5 CHA Chabazite
10
1.5.3 Classification of Zeolites Based on Pore Size
Zeolites exhibit ion-exchange property, extreme thermal stability
and possess channels like pore systems approaching molecular size which
made them attractive for a variety of industrial applications. Barrer (1983)
classified zeolites based on the pore openings(Table 1.3).
Table 1.3 Classification of zeolites based on pore openings
S. No. ClassPore
opening
Pore
diameter (Å) Example
1. Small pore 8 3-4 A, ZK-5
2. Medium pore 10 5-6 ZSM-5, ZSM-11
3. Large pore 12 6-8 X, Y, BEA
4. Ultra large pore 18*, 20# 8-12 VPI-5*, Cloverite#
1.5.4 Unique Properties of Zeolites
The following are the unique and salient properties of zeolites
which made them useful in many areas.
Crystallinity Uniform pore systems
High internal surface area Ion-exchange capabilities
Non-toxic Microporosity
Pore channels or cages High thermal stability
Acidity Environmentally safe
11
1.5.5 Applications of Zeolites
Adsorption:In drying, purification and separation,zeolites can absorb up to 25% of their weight in water.
Ion-exchange: Zeolites are builders in washing powder, where
they gradually replaced phosphates to bind calcium. Calcium
and to a lesser extent magnesium in water are exchanged for
sodium in zeolite A. This is the largest application of zeolites
today as they are essentially non-toxic and pose no
environmental risk. Zeolites are also applied in toothpaste, again
to bind calcium and prevent plaque.
Catalysis: Zeolites possess acid sites that are catalytically
active in many hydrocarbon reactions. The pore system allows
molecules that are small enough to enter and hence it affects the
selectivity of reactions by excluding both the participation and
formation of molecules that are too large for the pores.
1.5.6 Molecular Sieves VersusZeolites
McBain (1932) proposed the term molecular sieve. According to
him molecular sieves are materials with the capability of separating
components in a mixture on the basis of molecular size and shape differences.
The two classes of molecular sieves, namely, zeolites and microporous
charcoals were known when McBain formulated his definition. The list now
includes silicates, metalloaluminates, aluminophosphates, silico and
metalloaluminophosphates, mesoporous and macroporous materials in
addition to zeolites. These materials are structurally analogous but differ only
in their elemental composition. Although all the materials stated above are
molecular sieves, only aluminosilicates carry the classical name of zeolites.
12
1.5.7 Non-aluminosilicate Molecular Sieves
The preparation of zeolites and zeo-type structures containing
framework components other than aluminium and silicon has become need of
the hour to press forward in the area of new molecular sieves. Zeolites offer
ion-exchange property, high thermal stability, high acidity and shape selective
structural features. However, modification and subsequent improvement of
these properties have served as a driving force for changing the composition
of these microporous materials. Structures with pores larger than the known
12 ring type of zeolite Y (ultra large pore molecular sieves) have not yet been
produced in zeolite types. However, structures containing other composition
offer the possibility that such ultra pores may be realized. Aluminophosphate
(VPI-5) containing 18-membered ring (Davis et al 1988) and Cloverite
containing 22-membered ring pore systems (Merrouche et al 1992) have been
synthesized in non-aluminosilicate system. The change of elemental
composition not only produced ultra large pore materials but also added a new
dimension to find tailor-made molecular sieves. Gallium can easily substitute
aluminium, and germanium for silicon in aluminosilicate system. In addition
to these materials, zeo-type structures crystallized in the presence of organic
cations have been claimed to contain boron, iron, chromium, cobalt, titanium,
zirconium, zinc, beryllium, hafnium, manganese, magnesium, vanadium
and tin.
Since tetravalent germanium crystallizes to form molecular sieve
structures, it is also possible for other tetravalent ions that can occupy
tetrahedral oxide sites to crystallize to form such structures. Based on the
theory formulated by Barrer (1984), titanium could substitute into molecular
sieve structures. Perego et al (1986) synthesized titanium contining ZSM-5
structures. CeAlPO-11 was synthesized by Araujo et al (1997) without
13
affecting the structure of AlPO-11. Zahedi-Niaki et al (2000) reported a
comparative study of VAPO-5, -11, -17 and -31 aluminophosphate molecular
sieves.
1.6 GENERAL ASPECTS OF ALUMINOPHOSPHATES
At the onset of 1980s, a novel class of crystalline,
microporousaluminophosphates (AlPOs) was reported by Wilson et al (1982
and 1982a) at the Union Carbide, representing the first family of framework
oxide molecular sieves synthesized without silica. This discovery opened the
door to a new era in open-framework inorganic materials (Cheetham et al
1999). The aluminophosphate molecular sieves known as AlPO4-n(nrefers to
a distinct structure type) were prepared with a wide range of pore sizes by
hydrothermal synthetic technique in the presence of organic amines or
quaternary ammonium cations as templates or structure directing agents
(SDA) (Pastore et al 2005 and Wilson 1991 and 2001). These molecular
sieves are built from strict alternation of AlO4 and PO4tetrahedra. The primary
building units are formed by Al-O-P linkages instead of Si-O-Al or Si-O-Si
bridges of zeolite (Chen et al 1994). The lack of P-O-P and Al-O-Al in these
materials, constraining the structure to be alternate Al and P tetrahedra, limit
the structural building units to only even-numbered rings (Szostak 1989).The
AlPO contains Al3+ and P5+ in tetrahedral position and the resultant
framework is neutral and therefore there are no charge compensating ions as
in zeolites. Thus Brönsted acidity is intrinsic to AlPOs and they are not
suitable for acid catalysis (Pujado et al 1992).
The exciting property of AlPOmaterials is that Al or P can be
replaced by silicon to form SAPO (silicoaluminophosphate) materials resulted
in Brönsted acidity and they can be used as acid catalyst (Gielgens et al
1995). The isomorphous substitution of divalent or trivalent metal ions in
AlPO and SAPO forms MeAPO and MeSAPO respectively (Levi et al 1991
14
andMontes et al 1990). Such substitution introduced charge imbalance in the
framework which was balanced by protons, thus generating Brönsted acidity
and offering catalytic activity and ion-exchange capability in these molecular
sieves (Flanigen et al 1988). The overall composition of aluminophosphate
molecular sieves is represented as xR: Al2O3: 1.0 ± 0.2P2O5: yH2Owhere R is
an organic amine or quaternary ammonium ion. The quantities x and y
represent the amount of organics and water respectively that fills the pores of
the crystal as AlPO requires no counter ions (Flanigen et al 1986). The
aluminium to phosphorous ratio of these molecular sieves is always unity.
Aluminophosphate molecular sieves include more than 40
structures. Among these 25 are three dimensional framework structures, of which at least six are two dimensional layered materials and the others are
microporous. Most of the three dimensional structures are novel. AlPO and
SAPO molecular sieves cover a wide range of structure types, some are analogous to certain zeolites such as SAPO-42 (zeolite A structure), SAPO-34
(chabazite structure) and SAPO-37 (faujasite structure). But there is also a
large number of aluminophosphates such as AlPO-5, AlPO-11 or VPI-5, which possess unique structures with no zeolitic analogue (Davis et al 1988).
Aluminophosphate based molecular sieves exhibit excellent thermal and
hydrothermal stability compared to those observed in stable zeolites. Many are thermally stable and resist loss of structure even at 1000 ºC (Wilson et al
1982). Their surface selectivity is mildly hydrophobic. Their general formula
can be expressed as [(AlO2)x(PO2)x]·yH2O indicating that, unlike most
zeolites, aluminophosphate molecular sieves are ordered with Al/P ratio is
always unity. However, in spite of this, aluminophosphate molecular sieves exhibit enhanced structural diversity.
The discovery of open-framework AlPOs has brought some
conceptual breakthrough for traditional microporous compounds, e.g. the
framework elements are not only limited to Al and Si atoms; the upper limit
15
of pore size is not only delimited to 12-ring; the primary building units are not
only defined to tetrahedral. The on-going search for new structures
particularly provides some mechanistic clues on the formation of open-
framework materials. Ultimately, the crystallisation mechanism of
microporous materials must be understood in order to rationalise the
synthesized materials with desired structures, compositions and properties.
The discovery of AlPOs also improved the current application areas of
microporous materials(Thomas et al 1999 and Thomas 1999). One of the
important and promising areas of application of AlPOs is in catalysis where
aerial oxidations are possible using linear and cyclic hydrocarbons (Thomas
et al 2001). Selective oxidation reactions are also carried out using AlPOs
(Li et al 2010).The nanosized channels of AlPO-n also present suitable host
systems for the fabrication of advanced functional materials such as nanosized
single walled carbon nanotubes (Wang et al 2000).
1.6.1 NaturalAluminophosphates
The interactions between aluminium and phosphorus oxides to form
stable structures occur to a considerable extent in nature. The nine naturally
occurring neutral aluminophosphate minerals are berlinite (AlPO4), variscite
and metavariscite (AlPO4.2H2O), augelite (Al2PO(OH)3), senegalite
(Al2(OH)3(H2O)(PO4)), wavellite (Al3(OH)3(PO4)2H2O), trolleite
(Al4(PO4)3(OH)3), bolivarite (Al2(PO4)(OH)):4-5H2O) andevansite
(Al3PO4(OH)6 6H2O). To date, at least 200 structure-types of open framework
AlPOs have been identified. These include neutral open framework AlPO4-n
molecular sieves, their isomorphous substitute analogues and anionic AlPOs
framework.
16
1.6.2 Synthetic AluminophosphateMolecular Sieves
The researchers at Union Carbide Corporation, USA discovered aluminophosphate materials, a novel class of crystalline microporous solids
that represents the first family of framework oxide molecular sieves
synthesized apart from the well knownaluminosilicates (zeolites) and silica molecular sieves (Flanigen 1976). The periodic table was viewed as potential
scope for new framework compositions and structures. This resulted in the
discovery of aluminophosphate molecular sieves as reported by Wilson et al (1982).
The family of aluminophosphate molecular sieves is microporous
crystalline oxides, many of which contain pores within their framework
structure like zeolites. In aluminophosphate molecular sieves (AlPO4) the
framework sites are occupied by Al3+ or P5+ and the average ionic radius of Al3+ (0.39 Å) and P5+ (0.17 Å) is 0.28 Å, which is very close to the ionic
radius of Si4+ (0.26 Å). The notable feature of AlPO4 composition is the
invariant Al2O3/P2O5 ratio which is in direct contrast to the variable compositions of SiO2/Al2O3 found in zeolite structures. Unlike zeolite
molecular sieves, which contain Al3+ and Si4+ in tetrahedral positions and
exhibit a net negative framework charge, aluminophosphate molecular sieves contain Al3+ and P5+ in tetrahedral position and the resultant framework is
neutral. Structural diversity is observed in AlPO4 materials even though there is only a limited variation in chemical composition.
The linkage of SiO4, AlO4, PO4 and other cationtetrahedra will decide the three dimensional framework shape and final structure type of the material. The structure of aluminophosphate molecular sieves contains 4-, 6-, 8- and 12- rings of alternating AlO4 and PO4tetrahedra. The avoidance of Al-O-Al and P-O-P bonds in aluminophosphate frameworks (Löwenstein’s rule) made their structures contain only even-numbered rings. Therefore zeolitic structures of pentasil family such as ZSM-5 and ZSM-11 were not
17
found in AlPOs. Table 1.4 presents five categories of these structures viz., very large pore, large pore, medium pore, small pore and very small pore groups.
1.6.3 Nomenclature
The aluminophosphate materials are classified into (i) binary, (ii) ternary, (iii) quaternary, (iv) quinary and (v) senary. The composition of aluminophosphate molecular sieves depends on the number of elements contained in the cationic framework sites of any given structure. The normalised TO2 formula represents the relative concentration offramework elements in the composition (ElxAlyPz)O2 where El is the incorporated element and x, y and z are the mole fractions of the respective elements in the composition. Acronyms describing the framework composition are shown in Table 1.5 (Flanigen et al 1986).The structure type is indicated by an integer following the compositional acronym, e.g. SAPO-5 is a (Si,Al,P)O2 composition with type 5 structure. The numbering of structure type is arbitrary and bears no relationship to structural numbers used previously in the literature, e.g. ZSM-5. It only identifies structures found in the aluminophosphate based molecular sieves. The same structure number is used for a common structure type with varying framework composition.
18
Table 1.4 Structure of aluminophosphate based molecular sieves
Species Structure type Ring size Pore size (Å) Very large pore VPI-5 novel 18 12AlPO-54 novel 18 12
Large pore
AlPO-5 novel 12 8 -36 novel 12 8 -37 faujasite 12 8 -40 novel 12 7 -46 novel 12 7Intermediate pore AlPO-11 novel 10 6.0 -31 novel 10 6.5 -41 novel 10 6.0Small pore AlPO-12 novel 8 4.0 -14 novel 8 4.0 -17 erionite 8 4.3 -18 novel 8 4.3 -34 chabazite 8 4.3 -35 levynite 8 4.3 -44 chabazite-like 8 4.3Very small pore AlPO-16 novel 6 3 -20 sodalite 6 3 -25 novel 6 3 -28 novel 6 3
19
Table 1.5 Acronyms for framework composition
TO2, T = Acronym
Al, P AlPO
Si, Al, P SAPO
Me, Al, P MeAPO
Mg, Al, P Zn, Al, P Co, Al, P Mn, Al, P
MAPO ZAPO
CoAPO MnAPO
Me, Al, P, Si Mn, Al, P, Si Mg, Al, P, Si Zn, Al, P, Si Co, Al, P, Si
MeAPSO MnAPSO
MASO ZnAPSO CoAPSO
Other elements El, Al, P El, Al, P, Si
ElAPO ElAPSO
20
1.7 STRUCTURAL ASPECTS OF ALUMINOPHOSPHATES
1.7.1 Classification
The open framework AlPOs reported to date comprise a wide range
of structures and compositions. In terms of electrostatic properties and
Al/P ratios of the frameworks, they can be classified into two major
categories viz.,(i) neutral framework AlPO4-n with Al/P = 1 and (ii) anionic
framework AlPOs with Al/P 1.
1.7.2 Neutral Framework
The characters of AlPO4-n include a neutral framework and a
univariant framework composition with Al/P = 1 (Bennett et al 1986).
Subsequent efforts to incorporate other elements led to the formation of
AlPO4-based molecular sieves such as SAPO (S: Si), ElAPO (El: Li, Be, B,
Ga, Ge, As, Ti, etc.), ElAPSO, MeAPO (Me: metal) and MeAPSO. Even
though some of them have not been found pure AlPO4-n counterpart yet, these
structures can be ideally described using a hypothetical AlPO4-n lattice with
alternate Al and P sites as the basis. The AlPO4-based molecular sieves
include 51 unique structure types withextra-large pores (>12-ring), large pores
(12-ring), intermediate pores (10-ring), small pores (8-ring) and very small
pores (6-ring). These structures include 16 zeolite analogues such as chabazite
(AlPO4-n = 34, 44 and 47), erionite (AlPO4-17), faujasite (AlPO4-37),
gismondine (AlPO4-43), levynite (AlPO4-35), linde type A (AlPO4-42),
sodalite (AlPO4-20) and 35 novel structures such as VFI (VPI-5), AEL
(AlPO4-11) and AFI (AlPO4-5). Figure 1.2 illustrates several representative
AlPO4-n molecular sieves with different pore openings and dimensions
including VPI-5 (VFI): 18-ring (1.27 × 1.27 nm), AlPO4-8 (AET): 14-ring
(0.79 x 0.87 nm), AlPO4-5 (AFI): 12-ring (0.73 x 0.73 nm), AlPO4-11 (AEL):
10-ring (0.40 × 0.65 nm), AlPO4-41 (AFO):10-ring (0.43 × 0.70 nm) and
21
AlPO4-25 (ATV): 8-ring (0.30 × 0.49 nm). Apart from aluminosilicate or
silica zeolites, AlPO4-based molecular sieves constitute a major class of
zeolitic materials. These AlPO4-based materials are normally stable upon
removal of the occluded template molecules and exhibit excellent thermal
stability up to 1000 ºC. These materials are mildly hydrophilic. The major
structures in the AlPO4-nmolecular sieves are listed in Table 1.6.
Figure 1.2 Representative AlPO4-n molecular sieves with different pore openings
22
Table 1.6 Structures of AlPO4-n molecular sieves
n IZAa structure code Pore diameter (nm)
(ring) Very Large Pore
8VPI-5
AET VFI
0.79 × 0.87 (14) 1.21 (18)
Large Pore 536374046
AFI ATS FAU AFR AFS
0.73 (12) 0.75 × 0.65 (12) 0.74 (12) 0.43 × 0.70 (10) 0.64 × 0.62 (12) 0.4 (8)
Intermediate Pore 113141
AEL ATO AFO
0.63 × 0.39 (10) 0.54 (12) 0.43 × 0.70 (10)
Small Pore 17183334353942434447
ERIAEI ATT CHALEV ATN LTA GIS
CHACHA
0.36 × 0.51 (8) 0.38 (8) 0.42 × 0.46 (10) 0.38 (8) 0.36 × 0.48 (8) 0.4 (8) 0.41 (8) 0.31 × 0.45 (8) 0.31 (8) 0.38 (8)
Very Small Pore 162025
AST SOD ATV
(6)(6)0.30 × 0.49 (8)
23
1.7.3 Anionic Framework AlPOs
In contrast to neutral framework AlPO4-n with Al/P = 1, most of
the anionic framework AlPOs have Al/P ratio less than unity (Yu &Xu 2003).
The structures of anionic AlPOs comprise three dimensional and low
dimensional frameworks made up of alternate Al-centered polyhedra (AlO4,
AlO5 and AlO6) and P-centered tetrahedra P(Ob)n(Ot)4-n (b = bridging,
t = terminal and n = 1,2,3 and 4) forming diverse stoichiometries. The
existence of terminal P–OH and/or P = O groups or Al(OP)n (n = 5 and 6)
polyhedra results in the deviation of Al/P ratio from unity in the framework.
Their Al/P ratios are found to 1/1, 1/2, 2/3, 3/4, 3/5, 4/5, 5/6, 11/12, 12/13,
13/18 and so on. Their frameworks exhibit fascinating structural architectures.
A notable example is JDF-20 with Al/P = 5/6 (Huo et al 1992), which has the
largest channel ring size of 20 among open framework AlPOs. Anionic
framework AlPOs have also been prepared with diverse low
dimensionalframework structures such as 2D layers with various porous
sheets and sheet stacking sequences and 1D chains which may act as
fundamental building blocks for complex structures. It is significant to note
that within each compositional family a wide variety of structure types have
been observed. For instance, the 2D frameworks with Al/P = 3/4 show diverse
layered structures. Most of the anionic framework AlPOs possess interrupted
open frameworks with terminal P–OH and/or P=O groups. They are unstable
upon removal of the occluded protonated template molecules by calcination.
1.7.4 Bonding Patterns
As that of zeolites, open-framework AlPOs made up of Al–O–P
bonds obey Löwenstein’s (1954) rule with avoidance of Al–O–Al bonds (only
one exceptional case was reported by Huang &Hwu (1999) in a layered AlPO
containing Al–O–Al linkages. The P–O–P bonds do not appear to be stable in
24
these structures. Thus the avoidance of Al–O–Al and P–O–P bonds endows
open-framework AlPOs featured by even-numbered rings.
In interrupted anionic frameworks, a part of Al–O–P linkages are missed and
terminal P–OH and/or P=O bonds are commonly observed that interact with
protonated templating molecules through H-bonds(Yu &Xu 2003). Using the
first-principle quantum chemical techniques, Cora&Catlow (2001)
characterized the bonding properties of crystalline AlPOs and compared them
with isostructural silica-based zeolites. Their calculation results revealed that
silica polymorphs and AlPOs differ in the nature of bonding. The silica
polymorphs consist of covalently bonded SiO4 units while AlPOs are shown
to be of molecular ionic character and comprised of discrete Al3+ and PO43-
ions. The ionicity of AlPO frameworks might be responsible for the major
contrast between the chemistry of AlPOs and that of aluminosilicates relative
to the nature and concentration of dopants that can be introduced into the
frameworks. In AlPOs, ionic substitutional dopants introduce minor
perturbations to the host electric structure and therefore more readily replace
Al in AlPOs than Si in zeolites.
1.8 TOPOLOGICAL CHEMISTRY OF ALUMINOPHOSPHATES
1.8.1 Building Units
The complex structures of open-framework AlPOs can be
understood on the basis of their construction from fundamental building units.
Topologically, the neutral framework AlPO4-n molecular sieves can be
described as four-connected 3D frameworks since Al and P atoms occupy the
4-connected vertices of 3D net. Most of the anionic framework AlPOs can be
described as interrupted frameworks because part of the Al-O-P linkages is
missed. The four connected 3D frameworks, typically for zeolite frameworks,
can be thought to be constructed of finite secondary building units (SBUs).
18 SBUs have been listed for zeolites in the fifth edition of the ATLAS
25
among which 8 occur in four-connected 3D AlPO4-n frameworks
(Figure 1.3). These SBUs are formed by primary building units (PBUs) of
AlO4 and PO4tetrahedra (known as TO4) by sharing a common oxygen atom
in their corners. Different linkages of these tetrahedral units lead to various
sheet topologies. Figure 1.4 shows eight distinct 2D sheet structures. These
SBUs are arranged in a specific geometrical pattern to form a definite crystal
structure and uniform pore size.
Figure 1.3 Secondary building units (SBUs) found in AlPO4-n based
framework
26
Figure 1.4 Eight distinct 2D sheet structures (The SBUs constructing
these sheets are also shown)
27
1.8.2 Al and P Coordinations and Stoichiometries
The structural and compositional richness of AlPOs are attributed to
the diverse coordination of Al and P atoms. The majority of AlPO4-n
molecular sieves are based on a four-connected network of corner sharing
tetrahedra, i.e., AlO4b and PO4b (b = bridging oxygen between Al and P).
There are a number of AlPO4-n with mixed-bonded frameworks containing
five orsix coordinated Al atoms with one or two extraframework oxygen
species such as OH and H2O (Chen et al 1999). For instance, both VPI-5 and
AlPO4-8 contain AlO4b (H2O)2 units; AlPO4-17, -18, -20, -21 and -31 contain
AlO4b(OH) units. By omitting the OH and H2O species, these frameworks can
be idealized as a four-connected framework. Combinations of alternate Al and
P atoms give rise to various framework structures and Al and P
stoichiometries. According to Löwensteinsrule, the number of Al–Ob bonds
must be equal to the number of P–Ob bonds in open framework AlPOs.
Consequently, the correlation of coordination environment of Al and P can be
described in the following equation (1.3) (Yu &Xu 2003).
AlOib AlOib POjb POjbi j
m i n j (1.3)
where i(j) is the number of bridging oxygen coordinated to Al(P), m(n) is the
number of AlOib (POjb) coordination, mAlOib nPOjb = Al/P, i = 3, 4, 5 and 6
corresponding to AlO3b, AlO4b, AlO5b and AlO6b units respectively, j = 1, 2, 3
and 4 corresponding to PO4 units with one, two, three and four bridging
oxygen respectively. Based on this equation, the detailed Al and P
coordination for a given stoichiometry can be enumerated.
28
1.9 TEMPLATING IN THE CONSTRUCTION OF
ALUMINOPHOSPHATES
Open framework AlPOs are synthesized by hydrothermal or
solvothermal crystallization of reactive aluminophosphate gels in the presence
of an organic base as the templating agent (or structure-directing agent) as in
the synthesis of high-silica zeolites. These template species occupy the pores
and cages of the structures and play an important role in directing the
formation of a specific structure.
1.9.1 Types of Templates
A large variety of organic templates can facilitate the synthesis of
open-framework AlPOs. So far, over 100 species have been used successfully
as templates, typically involving quaternary ammonium cations and various
organic amines including primary, secondary, tertiary and cyclic amines, and
alkanolamines. Some stable metal ligand complexes such as Cp2Co2+ and
Co(en)33+ have also been used in the synthesis of AlPO materials. Very
recently, ionic liquids have been used as both solvent and template for the
preparation of SIZ-n type AlPO materials (Cooper et al 2004 and Parnham
et al 2006). The one template multiple structure and multiple template one
structure phenomena are remarkable in open framework AlPOs. For example,
di-n-propylamine (Pr2NH) has been used in the synthesis of at least ten
different AlPO structure types such as AlPO4-11, -31, -39, -41, -43, -46, -47,
-50, H3/MCM-1 and H1/VPI-5/MCM-9, exhibiting low structure specificity.
On the other hand, some structures readily form from many different
templates, e.g., AlPO4-5 is much less template specific and can be synthesized
with more than 25 different templates. Few examples are given in Table 1.7.
Tetrapropylammonium hydroxide (TPAOH) is a typical template for the
synthesis of AlPO4-5, which is stacked in a tripod arrangement with the head
of one TPA ion suspended between three feet of the next TPA ion with a
29
hydroxyl group neatly suspended between them(Bennett et al 1986). As seen
in Figure 1.5, although this tripod arrangement is such a good geometrical fit
with the cylindrical wall, the TPAOH is not a template in the true sense
because of the inconsistency of the three fold molecular symmetry and six
fold channel symmetry.
Table 1.7Templates used for the preparation of specific structure type
Structure type Typical template(s)
AlPO4-5 tetrapropylammonium hydroxide, tripropylamine,triethylamine, etc.
AlPO4-11 dipropylamine, diisopropylamine
AlPO4-14 isopropylamine
AlPO4-17 quinuclidine, piperidine
AlPO4-18 tetraethylammonium hydroxide
AlPO4-20 tetramethylammonium hydroxide
AlPO4-31 dipropylamine
AlPO4-34 tetraethylammonium hydroxide
AlPO4-35 quinuclidine
AlPO4-36 tripropylamine
AlPO4-46 dipropylamine
AlPO4-47 dipropylamine and diethylethanolamine
30
Figure 1.5 Cylindrical channel in AlPO-5 and the stacking of
encapsulated tetrapropylammonium hydroxide species
A few AlPO structures exhibit high template specificity. For
example, AlPO4-20 can be crystallized only with tetramethylammonium
hydroxide (TMAOH). The spherical TMAOH molecule with 0.62 nm
diameter fits neatly into the sodalite cage. In some AlPOs, a mixture of
templates appears to cooperatively direct the formation of structures.For
instance, SAPO-37 is prepared by a mixture of TPAOH and TMAOH.
Structural characterization shows the presence of TMA in the sodalite cages
and TPA in the supercages. In the synthesis of AlPO4-52, both Pr3N and
TEAOH appear to be necessary but only TEAOH is occluded in the structure.
As with organic amine, water can also play an important structure directing
role. A notable example has been seen in VPI-5. Even though VPI-5 is
31
preferentially prepared in the presence of organic amines, the organic species
are not occluded into the extra large 18 ring pores. Instead, water molecules
form an intriguing H-bonded triple helix inside the channel(McCusker et al
1991).
1.9.2 Role of Templating
Templating has been a frequently discussed phenomenon in the
synthesis of zeolites and related open-framework materials (Davis &Lobo
1992 and Zones et al 1996). So far, the relationship between the templating
agents and the structures, usually known as templating effect, is still not fully
understood. The term templating has been frequently used in the context of
synthesizing high silica zeolites. One definition about templating was
described by Lok et al (1983) as the phenomenon occurring during either the
gelation or the nucleation process whereby the organic species organizes
oxide tetrahedra into a particular geometric topology around itself and thus
provides the initial building block for a particular structure type. The gel
chemistry is also essential for the formation of microporous
aluminophosphates. With the addition of organic base, the gel chemistry of
aluminophosphate is altered, and the templating becomes operative only in
the gel with right gel chemistry. Therefore the dual role of organic templates
in the synthesis of open framework AlPOs is evident. It serves the important
role of modifying the gel chemistry, and it also has a structure directing
effect.
The organic template plays at least two additional roles in the
product, i.e., stabilizing voids and balancing the framework charge. By
packing the cages and channels the organic template can increase the overall
thermodynamic stability of the template/lattice composite, so that the
metastablility of the lattice alone is less critical (Wilson 1991 and 2001). The
stabilizing and charge balancing role of organic templates is quite evident in
32
anionic framework AlPOs. After removing the occluded template molecules
by calcination, the anionic frameworks normally collapse. Furthermore, the
templates also determine the stacking sequences of 2D layers. The template
molecules interact with host inorganic network with certain regularity, and
their interaction can be well described based on the interaction between SBUs
and protonated amino groups. This allows the template molecules to be
located with a reasonable success.
Even though a true templating effect, i.e., hand in glove fit between
organic and inorganic lattice, is less pronounced in the synthesis of open
framework AlPOs, it seems that an encapsulated organic species in the void
space of the inorganic host can adopt configuration that conforms best with
the surrounding aluminophosphate framework. For example, AlPO4-12, -21
and -EN3 were synthesized with encapsulated ethylenediamine. It is stabilized
into optical isomers of the gauche form by intramolecular bonding in
AlPO4-12 and -21, while it occurs in AlPO4-EN3 as trans configuration with
N–C–C–N extended along a straight eight-ring channel. The empirical
evidence is that for a template to be successful there must be a good fit
between the guest molecule and the host framework formed. The importance
of template molecules appears not only in its role of structure directing but
also orientating the distribution of Si in the frameworks. Vomscheid et al
(1994) demonstrated the role of template in directing Si distribution in the
lattice of SAPO materials.
In metal-substituted AlPOs, the template molecules also influence the degree of metal ion substitution in the frameworks. Lewis et al (1996) studied the influence of organic templates on the structure and the concentration of framework metal ions in microporousAlPOs. Their calculations demonstrate that the degree of metal ion substitution in the framework is controlled not only by the relative stability of the framework but also by the need to accommodate the structure directing and charge
33
compensating template molecules. Templates with higher charge/size ratios will allow a greater control over the ratio of metal substitution or heteroatom incorporation in the framework.
1.10 ISOMORPHIC SUBSTITUTION
Apart from the structural similarity with zeolites, AlPO molecular sieves exhibit structural diversity due to their neutral framework in contrast to the negatively charged aluminosilicate. Secondly the aluminium atoms in the aluminosilicate framework are always tetrahedrally coordinated as compared to four, five or six coordinated aluminium atoms present in the AlPO framework as mentioned earlier in this chapter. Moreover, they also offer compositional diversity. The Al and/or P ions in the AlPO frameworks can be replaced by another element with similar cation radius and coordination environment. However, elements incorporated in the AlPO framework should possess radius ratios with oxygen, and T-O distances consistent with the applied crystal chemical concept for tetrahedral coordination. Their successful incorporation may be due to flexibility of microporousaluminophosphate framework and to specific interactions with organic template, coupled with mildly acidic gel chemistry used in the synthesis (Flanigen et al 1986).
Thus, the incorporation of silicon in aluminophosphate molecular sieves results silicoaluminophosphate, SAPO-n (Lok et al 1984). The addition of metal cations yield porous metal aluminophosphate, MeAPO-n or metalsilicoalumino phosphate, MeAPSO-n. In SAPO-n materials, silicon substitutes for phosphorous or for aluminium-phosphorous pair whereas metal cations substitute almost exclusively for aluminium. The MeAPO-n and MeAPSO-n materials encompass the characteristics of both zeolites and aluminophosphates which results in their unique catalytic, ion-exchange and adsorbent properties.
Flanigen et al (1988) reported framework incorporation of at least fifteen elements in the framework of aluminophosphate materials. The most
34
important of these cations are Si4+, Co2+, Zn2+, Mn2+, Mg2+, Cr6+, Ti4+ and V5+.It was suggested that successful incorporation of these elements is attributed to flexibility of the microporous framework and specific interactions with organic templates. The charge of the framework is balanced by template molecules in the as-synthesized materials and the charges in the calcined samples are compensated by H+ ions derived from the template. Thus Brönsted acidity generated in the neutral framework is as shown in Scheme 1.1. The strength of Brönsted acidity depends on the electronegativity of the element which is used for the isomorphous substitution.
Scheme 1.1 Generation of Brönsted acid sites in aluminophosphatemolecular sieves
When M2+ ions are substituted in the place of Al3+ ions, two units
of negative charge is generated in which one negative charge is balanced by
2+
Isomorphous substitution of M2+
Neutral AlPO
Isomorphous substitution of M4+
35
the positive charge on the P atom and other negative charge is balanced by H+
ion as shown in Scheme 1.1 (Lohse et al 1995). When M4+ ion substitute P5+
ion results a net negative charge on the Al atom and this is compensated by
protons derived from the template during calcination as shown in Scheme 1.1
(Prakash et al 1994).
1.10.1 Metal Aluminophosphates
Initially, Flanigen et al (1986) reported the incorporation of 13
elements into AlPO-5 including transition metal ions like titanium,
manganese, iron, cobalt and zinc. The incorporation of transition metal ions
into framework sites of aluminophosphate and silicoaluminophosphate
molecular sieves is also of particular interest for the design of novel catalysts.
Paramagnetic metal species are often introduced into the molecular sieves to
generate catalytically reactive species or site. Various pretreatment or
activation procedures are typically used to generate reactive metal ion valence
states which are often paramagnetic. Transition metal ions are incorporated by
three different methods viz., impregnation, ion-exchange and isomorphous
substitution. In the latter method the transition metal ion salt is incorporated
directly into the synthesis mixture. Since the comprehensive papers of
Flanigen et al (1986 and 1988) on aluminophosphates and the periodic table,
many studies have been published, claiming the isomorphous substitution of
transition metal ions into the framework of different structure types
(Hartmann &Kevan 1999).A variety of metals and transition metals can be
incorporated into aluminophosphate structure (Figure 1.6) but actual
incorporation into the tetrahedral framework is difficult to prove.
36
Figure 1.6 Partial periodic table with transition elements introduced intoaluminophosphates and silicoaluminophosphates
1.11 AlPO-5 MOLECULAR SIEVE
AlPO-5 is a microporousaluminophosphate. It consists of alternate
Al and P tetrahedra with cylindrical pores of diameter 0.73 nm. It possesses
hexagonal crystal symmetry with 24 tetrahedral oxide (TO2) units per unit
cell. The novel three dimensional structure of AlPO-5 was determined by
single crystal X-ray method (Bennett et al 1983). It has hexagonal symmetry
with a = 13.72 Å and c = 8.47 Å. It contains one dimensional channels
oriented parallel to the c axis bounded by 12-membered rings. The framework
structure of AlPO-5 is shown in Figure 1.7(Bennett et al 1983).It can be
synthesized with at least 24 different amines and quaternary ammonium
compounds as template. The drawback of AlPO-5 molecular sieve is neutral
and there is no acidity. Adsorption properties of AlPO-5 have been studied by
Stach et al (1986) and Lohse et al (1987) using hydrocarbon and water as
adsorbates.The incorporation of divalent metal ions in the framework of
AlPO-5 creates Brönsted acidity.
Ti V Cr Mn Fe Co Ni Cu Zn Zr Nb Mo Tc Ru Rh Pd Ag CdHf Ta W Re Os Ir Pt Au HgRf Ha
Framework incorporation
claimed
Ion-exchange and impregnation Not Studied
37
(a) (b) (c)
Figure 1.7 Framework topology of AlPO-5 (a) framework structure of AlPO-5, (b) 12-ring channel view along (001) plane and (c) 12-membered ring of AlPO-5
1.12 CATALYTIC APPLICATIONS OF MeAPO-5 MOLECULAR
SIEVES
The use of microporous solid catalysts such as zeolites and related
molecular sieves has an additional benefit in organic synthesis. The highly
precise organisation and discrimination between molecules by molecular
sieves endow them with shape selective properties, reminiscent of enzymatic
catalysts. The incorporation of transition metal ions and complexes into
molecular sieves extends their catalytic scope to redox reactions and a variety
of other transition metal catalysed processes (Sheldon & van Bekkum 2001).
The metal ion substituted aluminophosphate molecular sieves are
interesting as they possess more density of acid sites for catalytic applications.
Lin et al (1993) carried out one-step liquid phase oxidation of cyclohexane over
CoAPO-5 catalyst in the presence of glacial acetic acid as the solvent. Since
the conversion and selectivity reported in this study were moderately good
and hence CoAPO-5 was proved to be a useful catalyst for this reaction.
38
Concepcion et al (1997)carried out oxidative dehydrogenation of ethane over
MgVAPO-5 catalyst. This catalyst is very active and selective in this reaction.
The activity is related to the presence of Mg2+ in the framework of AlPO-5.
The low catalytic activity of VAPO-5 can be related to its lower reducibility.
Although catalysts with high vanadium or magnesium content could be
prepared, their low crystallinity could decrease the number of effective active
sites.
Suresh et al (2004) reported isopropylation of benzene with
2-propanol over alkaline earth metal substituted MeAPO-5 (Me = Mg, Ca, Sr
and Ba) molecular sieves. The selectivity of cumene and benzene conversion
are in the order: Mg >Ca>Sr>BaAPO-5. Among these catalysts,
MgAPO-5 is more active than other catalysts due to the presence of more acid
sites. Dumitriu et al (2002) reported trans-alkylation of toluene with
trimethylbenzenes over various MeAPO-5 catalysts. The activity of
MeAPO-5 catalysts follows the order:
SiAPO>MgAPO>MnAPO>ZnAPO>CoAPO which can be correlated with
acidic properties of the catalysts. The strength of acid sites of the catalyst
influences the competition among various reactions that occur during the
trans-alkylation process. Generally,
trans-alkylation or disproportionation reactions occur on strong acid sites
while isomerisation of xylenes predominates on weak acid sites.
Hentit et al (2007) investigated the alkylation of benzene and other
aromatics over AlPO-5, AlPO-11, FeAPO-5 and FeAPO-11 catalysts using
benzyl chloride as the alkylating agent. Among the catalysts FeAPO-5 and
FeAPO-11 showed both high activity and selectivity due to their pore size and
acidity. The activity of these catalysts for benzylation of different aromatic
compounds is in the following order: benzene > toluene >p-xylene > anisole.
The interesting observation is that this catalyst could be reused in the
39
benzylation of benzene for several times. Hsu and Cheng (1998) reported
pinacol rearrangement over V, Cr, Co, Cu, Ti and Zn substituted AlPO-5,
Fe substituted VPI-5, AlPO-11 and silicalite-1. Among the transition metal
ions substituted in the AFI crystal structure, Fe3+, Cu2+ and Ni2+ showed the
highest pinacol conversion and pinacolone selectivity. The catalytic activity
was found to exhibit no direct correlation with acid strength or amount of acid
sites in the catalysts. Besides, comparison of the catalytic activities of Fe-
substituted molecular sieves of different crystalline structures, the activity
decreased in the order: AlPO-5 > AlPO-l1 > AlPO-8 > VPI-5 > silicalite-1.
Since the catalytic activity is independent of pore diameter, the liquid phase
reaction is considered to proceed mainly on the outer surface of the catalysts.
The hydrophilicity of aluminophosphate surface is in favour of catalysing the
pinacol rearrangement.
Vijayaraghavan& Raj (2004) carried out vapour phase ethylation of
benzene with ethanol over AlPO-5, MgAPO-5, ZnAPO-5 and MnAPO-5.
MnAPO-5 was found to be more active than other catalysts. Although
isomorphic substitution in MnAPO-5 is nearly the same as in MgAPO-5 and
ZnAPO-5, the increased conversion over MnAPO-5 is attributed to the
presence of unpaired electrons in the d-subshell of manganese.
Elangovan&Murugesan (1997) studied the catalytic transformation
of cyclohexanol over AlPO-5, AlPO-11, SAPO-5, SAPO-11, VAPO-5,
VAPO-11, CoAPO-5, CoAPO-11, NiAPO-5, NiAPO-11, ZnAPO-5 and
ZnAPO-11. SAPO-5 and VAPO-5 were found to be more active than other
catalysts because of the presence of more number of acid sites. The product
distribution is influenced by acidity, weight hourly space velocity (WHSV)
and temperature. Kannan et al (1998) reported ethylation of toluene with
ethanol over NiAPO-5, NiAPO-11, ZnAPO-5 and ZnAPO-11 in the vapour
phase. The products formed in this reaction were ethyltoluene, diethylether,
40
benzene and styrene. The conversion was found to be maximumover ZnAPO-
5 at 350 ºC but further increase in temperature decreased the conversion due
to coke formation.
Oxidation of alkylbenzenes is a promising subject in industrial
chemistry. Many bulk chemicals such as terephthalic acid, phenol, benzoic
acid, etc., are manufactured by homogeneous liquid phase oxidation with
oxygen. The large scale liquid phase oxidation is the conversion of
p-xylene into terephthalic acid which is chiefly used as polyethylene
terephthalate polymer material. m-Xylene is also commercially oxidised to
isophthalic acid. Benzoic acid derived from the oxidation of toluene is an
important raw material in the production of various pharmaceuticals and
herbicides. Commercially cumenehydroperoxide and
ethylbenzenehydroperoxide are also manufactured by aerobic oxidation of
isopropyl benzene and ethylbenzene respectively (Ishii &Sakaguchi 2006).
Singh et al (1999) studied the oxidation of ethylbenzene over
MeAPO-11 (Me = Co, Mn or V). The excellent incorporation of metal into
the framework has been achieved by synthesizing MeAPO-11 in the presence
of fluoride ions. In spite of their large crystallite size, MeAPO-11s obtained
from fluoride route are more active in the oxidation of ethylbenzene. The
complete change in the oxidation state of vanadium from lower valence state
(IV) to higher valence state (V) during calcination is observed in VAPO-11.
The redox behaviour of MeAPO-11 has a potential influence on the catalytic
activity during the oxidation of ethylbenzene. VAPO-11, which has
significant redox behaviour, is most active.
Subrahmanyam et al (2002) studied the vapour phase oxidation of
toluene with molecular oxygen over CrAlPO. They reported that CrAlPO
functions both as acid and redox catalyst and observed that in CrAlPO both
acidity (due to Al3+) and redox properties (due to Cr5+/6+) are competing,
41
thus leading to benzene and benzaldehyde respectively. Subrahmanyam et al
(2004) also reported the aerial oxidation of cyclohexane over FeAlPO under
highpressure conditions and reported that the reaction is probably taking place
through radical initiated mechanism. Subrahmanyam et al (2005) studied the
oxidation of toluene over mesoporousVAlPO, and the productsbenzaldehyde
and benzoic acid were obtained when the oxidising agent was 70% TBHP
while with 30% H2O2 cresols were formed. The activity of VAlPO has been
compared with those obtained with other similar porous materials likeV-
MCM-48, V-MCM-41, V-Al-Beta and VS-1.
Potter et al (2012) reported simultaneous framework incorporation
ofheavy metal ions such as Ru(III) and Sn(IV) intoaluminophosphate
architectures and evaluated the catalytic activity in cyclohexene oxidation.
The bimetallic catalyst facilitated synergistic interactions, affordinghigh
degree of selectivity and activity in the catalytic oxidation reactions as
compared with their corresponding transition metal analogues (Co and Ti).
They also reported that heavy metal dopants suchas Ru and Sn in the
framework architecture displayed enhancedcatalytic turnovers compared to
their correspondingtransition metal analogues (such as Co and Ti) in
selectiveoxidation reactions. In particular, the bimetallic analogues ofthe
former exhibit a concomitant enhancement in catalyticactivity when
compared with the corresponding bimetallictransition metal counterparts,
suggesting a synergistic enhancementin catalytic properties.
Raboin et al (2012) reported the grafting of titanium alkoxide over
mesoporousaluminophosphate and evaluated the catalytic activity in the
liquid-phase epoxidation of cyclohexene in the presenceof TBHP. The
catalyst showed 67% selectivity to epoxide formation. Further, they compared
the catalytic properties with Ti grafted SBA-15 catalysts and reported that
both SBA-15 and mesoporousAlPO showed comparable activities and
42
selectivities in cyclohexene oxidation. In addition, Ti-AlPO catalysts
exhibited higher tendency towards allylic oxidationin comparison with similar
Ti-SBA15 catalysts.
Wang et al (2013) synthesized Pt-Co/AlPO-5 catalysts for the
preferential oxidation of CO in H2rich gases. The optimized catalysts were
highly active and selective.CO could be puri ed below 10 ppm in the reaction
temperature range of 110 –125°C under 1% CO, 1%O2, 12.5% CO2, 15%
H2O, 50% H2 in volume and N2 balance at the space velocity of 24,000 ml
gcat1h 1. Pt–Co/CoAPO-5 exhibited the best catalytic performance and Pt–
Co/AlPO-5 was the most active catalystat low reaction temperature, in which
particles of Pt–Co alloy were formed and the particles were highlydispersed
on the surface of the support.
Devika et al (2011) reported the single sited CeAlPO-5 catalyst for
the oxidation of ethylbenzene to acetophenone in air atmosphere. The
selectivity to acetophenone was above 90% at all reaction temperatures.
Devika et al (2012) reported the vapour phase oxidation of diphenylmethane
to benzophenone. They reported that not only active ceriumsite isolation is a
requirement for selective oxidation but also themagnetic field of cerium sites
and free radicals produced duringoxidation were also suggested to play a
major role in theselective oxidation. In addition, free rotation across the
phenyland ethyl carbon bond also key factor forselective oxidation of
diphenylmethane.
Smet et al (1998) reported Pr6O11-MoO3 catalysts for the selective
oxidation of isobutene to methacrolein. The synergism of Pr with Mo played
an important role in the oxidation. Rovira et al (2012) reported the catalytic
activity of ceria-praseodymia nanotubes in the CO oxidation. The
43
incorporation of Pr in CeO2lattice improved the redox properties, and their
nanostructure also aided the catalytic oxidation of CO
Praseodymium incorporated AlPO-5 (PrAlPO-5) with different
(Al+P)/Pr ratios were synthesized under hydrothermal condition in the
presence of fluoride ions. Similarly, iron incorporated AlPO-5 (FeAlPO-5)
with different Al/Fe ratios were also synthesized. It was found that fluoride
ions exhibited several rolessuch as (1) they solubilisealuminium in the
reaction mixture leading to slower nucleation, thus rendering the formation of
dense aluminophosphate phases less favourable, (2) they lead to slow crystal
growth rates yielding crystals of larger size with fewer defects and (3) the
fluoride ions impart a structure-directing and templating effect by interacting
with the framework. In this last role, fluoride ions behave as bidentate ligands
linking two aluminium ions. Consequently, the aluminophosphate framework
requires a cation to balance the charge. Generally, a protonated organic amine
is the counter ion.
General characterization was performed to check the purity and
crystalline nature of the desired phase, its surface area, morphology and
chemical composition. In depth spectroscopic techniques were employed to
understand the accessibility, redox ability, coordination, acidity and oxidation
state of the metal ions in PrAlPO-5 and FeAlPO-5 for specific catalytic
application. The characterization of the samples was performed using XRD,
BET, TEM, SEM and ICP-OES analysis. DRS-UV-Vis,ESR, XPS, 27Al, 31P
MAS NMR and ex-situ pyridine adsorbed IR were used to understand the
nature and surface chemistry of materials.
44
1.13 SCOPE AND OBJECTIVES OF THE PRESENT
INVESTIGATION
Synthesis of fine chemicals using homogeneous catalysts possess
several problems such as difficulty in separation and recovery, disposal of
spent catalyst, formation of undesirable and/or toxic wastes. Efforts have been
made to replace homogeneous catalysts by reusable and easily separable
heterogeneous solid acid catalysts for the synthesis of fine chemicals.
The incorporation of one or more transition metal ions into AlPO
framework has gained considerable importancebecause of their redox
behavior and potential bi-functionality (Lewis and Bronsted acid sites). A
wide array of amines was used as structure directing agents for the
preparation of microporousAlPOs. Among the various AlPOs, AlPO-5 has
been extensively studied structure.AlPO-5 is not template specific,it can be
synthesized using more than one template.
The main objectives of the present investigation are
Hydrothermal synthesis of AlPO-5 in fluoride medium using
aluminiumisopropoxide, orthophosphoric acid and hydrofluoric
acid as the sources for aluminium, phosphorous and fluoride
respectively.
Hydrothermal synthesis of PrAlPO-5 with (Al+P)/Prratios of
25, 50, 75, 100,150 and 200 in fluoride medium using
praseodymium nitrate hexahydrate as the source for
praseodymium. The synthesis procedure was similar to that of
AlPO-5.
45
Calcination of PrAlPO-5((Al+P)/Pr = 25, 50, 75, 100,150 and
200)in air atmosphere at 550 ºC to remove occluded
template.Physico-chemical characterization of the materials
using XRD, SAXS, DRS-UV-vis, BET, SEM, TEM, TPD-
NH3, TPR, FT-IR, ICP-OES, TGA, 27Al and 31P MAS-NMR,
ex-situ pyridine adsorbed IR, ESR and XPS.
Study of liquid phase aerobic oxidation of ethylbenzene over
calcined PrAlPO-5((Al+P)/Pr = 25, 50, 75 and 100) between
60 and 140 °C. Analysis of the product using gas
chromatograph (GC) and gas chromatograph coupled with
mass spectrometer (GC-MS).Study of the influence of
temperature, reaction time, (Al+P)/Pr ratios and substituents on
ethylbenzene conversion and product selectivity.Optimisation
of the reaction parameters for maximum conversion with high
product selectivity. Study of the stability and reusability of the
catalyst.
Study of campholenic aldehyde synthesis from -pinene over
bi-fuctional PrAlPO-5with (Al+P)/Prratios of 75, 100, 150 and
200 catalysts. Analysis of the product using gas chromatograph
(GC) and gas chromatograph coupled with mass spectrometer
(GC-MS). Study of the influence of temperature, reaction time,
solvent and (Al+P)/Pr ratios on -pinene conversion and
campholenic aldehyde selectivity, and optimisation of the
parameters for maximum conversion with high product
selectivity. Study of the stability and reusability of the
catalyst.Separation of the products of the reaction by column
chromatography. Identification of the structure of the isolated
product by 1H-NMR
46
Hydrothermal synthesis of FeAlPO-5 with Al/Fe ratios of 75,
100 and 150 in fluoride medium using iron (III) nitrate
nanohydrate as the source for iron (III).
Calcination of FeAlPO-5(Al/Fe = 75, 100 and 150) in air
atmosphere at 550 ºC to remove occluded template. Physico-
chemical characterization of the materials using XRD, DRS-
UV-vis, BET, SEM, TPD-NH3, ex-situ pyridine adsorbed IR,
ESR, XPS and ICP-OES.
Synthesis of 5-arylidene-2,4-thiazolidinedione over FeAlPO-5.
Study of the influence of temperature, reaction time, solvent
and Al/Fe ratios on 5-benzylidene-2,4-thiazolidinedione
synthesis, and optimization of reaction parameters. The
influence of various substituted benzaldehydes and
heterocyclic aldehydes as substrate in Knovenegal
condensation. The structural identification of isolated product
by 1H-NMR.
47
CHAPTER 2
EXPERIMENTAL METHODS
2.1 MATERIALS
2.1.1 Chemicals
Hydrothermal synthesis of MeAPO-5 (Me = Pr and Fe) was carried
out using triethylamine (Merck) as the organic template.
Aluminiumisopropoxide (Merck), orthophosphoric acid (Merck),
praseodymium nitrate hexahydrate (Indian rare earths Ltd.), iron (III) nitrate
nonahydrate (Merck) and hydrogen fluoride (Merck) were used as the sources
for aluminium, phosphorus, praseodymium, iron and fluoride respectively.
The other chemicals such asethylbenzene, toluene,diphenylmethane, 4-chloro-
1-ethylbenzene, 4-bromo-1-ethylbenzene,4-fluoro-1-ethylbenzene, 4-iodo-1-
ethylbenzene, 4-nitro-1-ethylbenzene, 4-nitro-1-methylbenzene, 4-methoxy-1-
ethylbenzene,4-methoxy-1-methylbenzene and -pineneof analytical grade
(Sigma Aldrich) were used to study the catalytic activity of calcined PrAlPO-
5.The chemicals such as 2,4-thiazolidinedione, benzaldehyde, 4-
methoxybenzaldehyde, 3-methoxybenzaldehyde, 4-nitrobenzaldehyde, 3-
nitrobenzaldehyde, 2-hydroxy-3-methoxybenzaldehyde, 2-nitrobenzaldehyde,
piperonal, thiophene-2-carbaldehyde and pyridine-3-carbaldehyde were of
analytical grade (Sigma Aldrich) used in the study of the catalytic activity of
calcined FeAlPO-5.Deionised water was used in the catalyst synthesis and
product extraction. Solvents such as chloroform, dichloroethane, acetonitrile,
acetone, dichloromethane, N,N-dimethylformamide, dimethylsulfoxide, water
and ethanol were of analytical grade (Merck). Ether and ethylacetate wereof
48
analytical grade (Merck) used in product extraction. All the chemicals were
used as such without any further purification. All the glassware used in all the
experiments were either Borosil or Vensil.
2.2 PREPARATION OF CATALYSTS
2.2.1 Synthesis of PrAlPO-5
Zhao et al (2006) reported the synthesis of CeAlPO-5 molecular
sieves. The same procedure was adopted tosynthesize PrAlPO-5 except the
crystallization time was extended to 12 h. PrAlPO-5 molecular sieves were
synthesized by hydrothermal crystallization of gel with the following molar
composition: xPr(NO3)3.6H2O: 1.0Al2O3: 1.3P2O5: 1.6TEA: 1.3HF:
425H2O.The typical synthesis procedure adopted is as follows:
Aluminiumisopropoxide (7.15 g, Merck) was soaked in 50 ml double distilled
water and aged for 24 h. In a polypropylene bottle praseodymium nitrate
hexahydrate (0.493 g, Indian rare earths Ltd.) was dissolved in ortho-
phosphoric acid (4.9 g, Merck), double distilled water (15 ml) and
triethylamine (2.8 g, Merck), and stirred for 2 h. This was labelled as solution
A. The aged aluminium precursor stirred for 2 h was added to solution A and
the resultant mixture was stirred for 2 h. This was labelled as solution B. Then,
hydrofluoric acid (0.98 g, Merck) diluted with 5 ml double distilled water was
slowly added to solution B and stirred for another 2 h. Finally, the gel was
transferred to a teflon lined stainless steel autoclave and heated at 180 °C for
12 h. The hot autoclave quenched in ice-cold water gave crystallized product
which was filtered and washed several times with water. The crystalline
product was dried in an air oven at 120 °C and calcined at 550 °C in air for 6
h at a heating rate of 1 °C min-1. The same procedure was adopted for the
synthesis of AlPO-5 without the precursor for praseodymium.
49
2.2.2 Synthesis of Praseodymium Oxide
Praseodymium nitrate hexahydrate (2.18 g) was dissolved in double
distilled water (20 ml) and then 5 ml aqueous ammonia solution was added to
it slowly with constant stirring. A pale greenish precipitate was formed, and
the stirring continued for another 2 h. The precipitate was filtered,
washed thoroughly with double distilled water and dried at 120 °C. The dried
sample was crushed well and calcined at 550 °C for 12 h at a heating rate of
1 °C min-1.
2.2.3 Synthesis of AlPO-5 Supported Pr6O11
AlPO-5 supported 3 wt% of Pr6O11 was prepared as follows:
AlPO-5 (1 g) was added to 50 ml aqueous solution of Pr(NO3)3·6H2O (0.3 g).
The reaction mixture was stirred at ambient temperature for about 24 h to
obtain dry powder. The dry powder was calcined at 550 °C for 6 h in air
atmosphere. This sample was used in the TPR and DRS-UV-Vis studies to
establish the absence of extra framework praseodymium species in PrAlPO-5.
2.2.4 Synthesis of FeAlPO-5
FeAlPO-5 was synthesized by hydrothermal crystallization of gel
with the following molar composition: xFe(NO3)3.9H2O: 1.0Al2O3: 1.3P2O5:
1.6TEA: 1.3HF: 425H2O.The typical synthesis procedure is as follows:
Aluminiumisopropoxide (14.3 g, Merck) was hydrolysed in 100 ml double
distilled water for 24 h. In a polypropylene bottle iron(III) nitrate nonahydrate
(0.23 g, Merck) was dissolved in ortho-phosphoric acid (9.8 g, Merck),
double distilled water (30 ml) and triethylamine (5.6 g, Merck), and stirred for
2 h. This was labelled as solution A. The hydrolysedaluminium precursor was
added to solution A and the resultant mixture was stirred for 3 h. This was
labelled as solution B. Then, hydrofluoric acid (1.96 g, Merck) diluted with
50
10 ml double distilled water was slowly added to solution B and stirred for
another 4 h. Finally, the gel was transferred to a teflon lined stainless steel
autoclave and heated at 180 °C for 12 h. The hot autoclave quenched in ice-
cold water gave crystallized product which was filtered and washed several
times with water. The crystalline product was dried in an air oven at 120 °C
and calcined at 550 °C at a heating rate of 1 °C min-1 in air for 6 h.
2.2.5 Synthesis of Fe2O3
Iron (III) nitrate nonahydrate (2.42 g) was dissolved in double
distilled water (20 ml) and then 2ml aqueous ammonia solution was added to
it slowly with constant stirring. A dark brown precipitate was formed, and
thiswas stirred continuously for another 2 h. The precipitate was filtered,
washed thoroughly with double distilled water and dried at 120 °C. The dried
sample was crushed well and calcined at 550 °C for 12 h at a heating rate of
1 °C min-1. This sample was used in the XRD studies to establish the absence
of extra framework iron species in FeAlPO-5.
2.3 CHARACTERIZATION OF SYNTHESIZED MATERIALS
The structural, physical and chemical characteristics of catalysts are
essential for deriving correlation between physico-chemical properties and
catalytic activities of the materials. Different methods were employed to
characterize the as-synthesized and calcinedmaterials. The following physico-
chemical characterization techniques have been adopted in thepresent study.
51
2.3.1 X-ray Diffraction (XRD)
The X-ray diffraction (XRD) patterns were mainly used to identify
the structure, crystallographic phase present in the catalyst, degree of
crystallinity, unitcell parameter and crystallite size of the catalysts.
The conventional X-ray source consists of a target which is
bombarded with high energy electrons. Commonly CuK , CoK , FeK and
MoK are used as the sources for X-ray. Usually CuK radiation with an energy
of 8.04 keV and a wavelength of 0.154 nm was used as the source for X-ray.
The X-ray diffraction method involves the interaction between incident
monochromatised X-ray and atoms of periodic lattice. X-rays scattered by
atoms in an ordered lattice interfere constructively in the directions given by
Bragg’s law in Equation (2.1) (Bragg & Bragg 1949)
dSin2n (2.1)
where n = 1,2,3,… is the order of reflection, is the wavelength of X-rays,
d is the distance between two lattice planes and is the Bragg’s angle. This
law relates the wavelength of electromagnetic radiation to the diffraction
angle and the lattice spacing in a crystalline sample. Crystal structures
produce several thousand unique reflections, whose special arrangement is
referred as diffraction pattern. Indices (hkl) may be assigned to each reflection,
indicating its position within the diffraction pattern. The pattern has a
reciprocal Fourier transform relationship to the crystalline lattice and the unit
cell in the real space.
X-ray diffractograms revealedimportant and useful information
about a solid sample.
52
Whether the solid scanned is crystalline or amorphous in
nature.
Kinetics of the crystallization of a material during synthesis.
Presence of impurity phase can be identified by finger print
matching of a known sample with synthesized sample.
Changes in shape and size of unit cell with respect to the
position ofthe peak in a XRD profile.
Crystallite size (D) can be determined from the corrected line
broadening ( ) in a solid sample using Scherrer Equation (2.2)
D =K / Cos (2.2)
where D is the crystal size of the catalyst, is the X-ray wavelength (1.54 Å),
is the full width at half maximum (FWHM) of the sample, K is a
constant(equal to 0.89) and is the Bragg’s angle.
The XRD patternsof PrAlPO-5 and FeAlPO-5 wererecordedon a X-
ray diffractometer (PANalytical X’ Pert Pro) using CuK =1.54 Å) radiation
and liquid nitrogen-cooled germanium solid-state detector. The
diffractograms were recorded in the 2 range 5-80° in steps of 0.02° with a
count time of 5 s at each point. The small angle X-ray diffraction pattern of
PrAlPO-5 was recorded on a Bruker D8 advanced powder X-ray
diffractometer using CuK ( =1.5418 Å) as the radiation source. The
diffractograms were recorded in the 2 range 0.5-6o with a step size of 0.01o
and a step time of 1s at each point.
53
2.3.2 Diffuse Reflectance Ultraviolet - Visible Spectroscopy
Diffuse reflectance ultraviolet - visible spectroscopy
(DRS-UV-Vis) is known to be a very sensitive and useful technique for the
identification and characterization of metal ion coordination and its existence
in the framework or extra-framework position of a metal containing molecular
sieves. It deals with the study of electronic transitions between orbitals and
bands of atoms, ions or molecules in gaseous, liquid and solid state. This
technique is based on the reflection of light in the ultraviolet (UV) and visible
(vis) region by a powdered sample. The DRS-UV-Vis absorption spectra were
recorded using UV-Vis. spectrophotometer (Shimadzu model 2450) with
BaSO4 as the standard for measurements in the scan range of 190-900 nm.
The thickness of the quartz optical cell was 5 mm. The absorption intensity
was expressed as the Schuster-Kubelka-Munk function, F(R ) = (1- R )2/
2R , where R is the diffuse reflectance (DR) of a semi-infinitive layer and
F(R ) is proportional to the absorption coefficient.
2.3.3 Nitrogen Sorption Studies
Surface area measurement of solid materials is an important
parameter in catalyst characterization. Measurement of the quantity of
nitrogen adsorbed on a solid surface at a constant temperature with varying
adsorbate pressure has been studied extensively by many researchers
(Brunauer et al 1938, Barret et al 1951, Sing et al 1985). The Brunauer-
Emmett-Teller (BET) volumetric gas adsorption technique using N2 or Ar is
the standard method for the determination of surface area of a finely divided
porous material. The relationship between the amount of N2 adsorbed and
equilibrium pressure of the gas at a constant temperature is defined as the
adsorption isotherm. The specific surface area of a sample was calculated
using Brunauer-Emmett-Teller (BET) equation (2.3).
54
o m m o
p 1 C 1 pV(p p) V C V C p
(2.3)
where Vm is the volume of the gas forming monolayer on the adsorbent at
standard temperature and pressure (STP), p is the pressure at which volume
(V) of the gas is adsorbed, po is the saturated vapour pressure of the gas and C
is a constant.
The specific surface area, specific pore volume and average pore
diameter (BJH method) of the samples were determined by nitrogen
adsorption-desorption isotherms at 77 K using Belsorb mini II sorption
analyser. All the samples (50 mg) were degassed for 3 h at 250 °C under
vacuum (10-5 mbar) in the degas port of the adsorption analyser. Helium was
used as the carrier gas and thermal conductivity detector (TCD) as the
detector. Pore size was mapped directly from nitrogen adsorption isotherms
along with surface area of the catalyst. The same measurements were also
determined using Micromeritics ASAP 2020volumetric adsorption analyser.
The plot of )pV(p
p
o versus
opp gave a straight line with the
slope =m
C 1V C
. From the slope and intercept, Vm was derived. Then the number
of molecules of nitrogen adsorbed was calculated using equation (2.4).
Number of molecules of nitrogen adsorbed =V X 6.023 X 10
0.0224 (2.4)
The specific surface area (S) was obtained by multiplying the same with
cross-sectional area of nitrogen molecule, which is taken as 16.2 10-20 m2 as
shown in equation (2.5).
55
0224.0102.16V10023.6(S)areasurfaceSpecific
20m
23
(2.5)
The total surface area (St) of the sample was obtained using the following
equations (2.6 - 2.8).
t m csS n A N (2.6)
MWn m
m (2.7)
MNAWS csm
t (2.8)
where N is the Avogadro number (6.023 × 1023 molecules mol-1), M is the
molecular weight of the adsorbate, Wm is the weight of the adsorbate
constituting a monolayer surface coverage, nm is the amount adsorbed
constituting a monolayer surface coverage and Acs is the molecular cross-
sectional area of the adsorbate molecule. The specific surface area (S) of the
solid was calculated from the total surface area (St) and the degassed sample
weight (m) using the equation (2.9).
mSS t (2.9)
The total pore volume was calculated by converting the volume of
nitrogen adsorbed (Vads) into volume of liquid nitrogen (Vliq) using the
equation (2.10).
ads mliq
PV VVRT
(2.10)
56
where P and T are the ambient pressure and temperature respectively. Specific
pore volume (Vp) was calculated from equation (2.11).
mV
V liqp (2.11)
where m is the weight of adsorbent after degassing.
2.3.4 Fourier Transform - Infrared (FT-IR)Spectroscopy
Fourier transform infrared spectroscopy (FT-IR) is a
multidisciplinary analytical tool, which gives information pertaining to the
structural details of a material. In addition, it can be used to confirm surface
characteristics such as acidity and isomorphous substitution by other elements
in the material. FT-IR involves the absorption of electromagnetic radiation in
the infrared region of the spectrum which results changes in the vibrational
energy of a molecule. It is a valuable and formidable tool in identifying
organic compounds which have polar chemical bonds such as OH, NH, CH,
etc., with good charge separation. Since every functional group has unique
vibrational energy, the IR spectra can be seen as their fingerprints. IR spectra
of calcined samples were recorded on a FT-IR spectrometer (Perkin
Elmer,Spectrum Two) by ATR sampling technique (Figure 2.1).
Figure 2.1 Schematic diagram of multiple reflection ATR system
57
ATR uses a property of total internal reflection resulting in an
evanescent wave. A beam of infrared light is passed through the ATR crystal
in such a way that it reflects at least once off the internal surface in contact
with the sample. This reflection forms the evanescent wave which extends
into the sample. The penetration depth into the sample is typically between
0.5 and 2 micrometres, with the exact value being determined by the
wavelength of light, the angle of incidence, the indices of refraction for the
ATR crystal and the medium being probed. The number of reflections may be
varied by varying the angle of incidence. The beam is then collected by a
detector as it exits the crystal. In horizontal ATR (HATR) units, the crystal is
parallel sided plate, typically about 5 cm by 1cm, with the upper surface
exposed. The number of reflections at each surface of the crystal is usually
between five and ten, depending on the length and thickness of the crystal and
the angle of incidence.
2.3.5 Thermogravimetric Analysis (TGA)
In thermoanalytical technique, the change in sample weight is
measured while the sample is heated at a constant rate under air or nitrogen
atmosphere. This technique is effective for quantitative analysis of thermal
reactions that are accompanied by mass changes due to evaporation,
decomposition, gas adsorption, desorption and dehydration.
Thermogravimetric analysis (TGA) is widely used to study the structural
stability of molecular sieves. It provides information about the temperature
range required for expulsion of adsorbed water, decomposition of occluded
organic cations, structural modification and phase changes in the pores of
molecular sieves. The sample and reference material are simultaneously
heated or cooled at a constant rate. Reaction or transition temperatures are
then measured as a function of temperature difference between the sample and
the reference. It provides vital information about the materials with respect
58
totheir endothermic and exothermic behaviour at high temperatures. TGA of
the materials was performed (Perkin Elmer SII Diamond series) with 10 mg
of the sample under N2 atmosphere at a heating rate of 10 °Cmin-1 in the
temperature range 30-900 ºC.
2.3.6 Temperature Programmed Desorption (TPD)
The very common method applied to detect the presence of acid -
OH groups (Si-OH-Al groups or bridging -OH groups) is the temperature
programmed desorption (TPD) using basic gases such as ammonia and
pyridine with molecular size smaller than the pore size as probe molecule.
Due to their small molecular dimensions, pyridine and ammonia are suitable
to probe all -OH groups accessible in the pores, channels or windows of size
4 Å. Depending upon the number and strength of acid sites distributed on
the surface of the catalysts both its activity and selectivity vary. With proper
understanding of the acidic property requirement for various reactions,
catalysts with high activity and selectivity can be designed. In the TPD
method, desorption rate of pre-adsorbed base is continuously measured by
heating the catalyst in an inert gas stream of helium or nitrogen. The desorbed
amount of ammonia gave information about the number of -OH groups.
The quantitative analysis of the characteristic desorption curve
provides information about the strength and distribution of acid sites. The
strength and number of acid sites of PrAlPO-5 and FeAlPO-5were determined
by TPD of ammonia using Micrometrics Chemisorb 2750 pulse
chemisorption system. Approximately 50 mg of the sample was placed in a
U-shaped, flow-through, quartz tube. The surface of the sample was cleaned
by heating the sample at 250 C for 30 min with helium flow of 20 ml min-1.
The sample was then cooled to 100 C and saturated with 10% NH3-He
mixture. Then, isothermal desorption of ammonia from dead volume and
59
physically adsorbed ammonia were carried out in a flow of helium for 20
min.The temperature programmed desorption (10 oC min-1) of the sample was
recorded in the temperature range of 100 – 600 oC at a heating rate of 5 C
min-1under helium atmosphere. Ammonia molecules desorbed at low
temperature are weakly bound to the active sites and those desorbed at high
temperature are due to strong chemisorption on the active sites.
2.3.7 Temperature Programmed Reduction (TPR)
Temperature-programmed reduction (TPR) is yet another
technique used for the characterization of solid materials. It is often used in
the field of heterogeneous catalysis to find the most efficient reduction
conditions. The oxidized catalyst precursor is subjected to a programmed
temperature rise while a reducing gas mixture is flowed over it. Temperature-
programmed reduction (TPR) determines the number of reducible species
present on the catalyst surface and reveals the temperature at which the
reduction of each species occurs. The important aspect of TPR analysis is that
the sample need not possess any special characteristics other than the
presence of reducible metals.
Temperature programmed reduction (TPR) experiments were
carried out using a Micromeritics Chemisorb 2750. The reactor was a
U-shaped quartz tube and the sample was held in position by quartz wool
plugs. Prior to the TPR experiment, the reactor and its contents were flushed
with argon gas at a flow rate of 30 ml min-1 under controlled heating upto
150 °C, and then held isothermal for 30 minutes. The inert gas was then
switched over to a 10% H2/Ar mixture and the reduction was performed at a
controlled heating rate of 10 °C min-1 from 150to 900 °C. The hydrogen
consumption was monitored by a thermal conductivity detector (TCD).
60
2.3.8 Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) is one of the most widely
used techniques for the characterization of size and morphology of the
materials. SEM provides not only topographical information like optical
microscopes but also information pertaining to chemical composition near the
surface. SEM creates magnified images by using electrons instead of light.
It gives detailed 3D images at higher magnification than an optical
microscope. The various processes of interaction of primary electron beam
with catalysts in an electron microscope are shown in Figure 2.2. In the
scanning electron microscope, back scattered electrons and secondary
electrons are captured by a detector to form the image.
Figure 2.2 Theinteraction between primary electron beam and the sample in an electron microscope
Secondary electrons arise due to inelastic collision between primary
electrons (the beam) and loosely bound electrons of the conduction band or
tightly bound valence electrons. The energy transferred is sufficient to
overcome the work function which binds them to the solid and they are
ejected. The ejected electrons possess 5to10 eV energy and they are detected
by scintillator/photomultiplier tube. Back scattered electrons arise due to
e-
Sample
Unscattered electrons
Elastically scattered electrons
Interaction volume
Back scattered electrons
Auger electronsSecondary electrons
Inelastically scattered electrons
Incident electron beam
61
elastic collisions between the incoming electron and the nucleus of the target
atom (i.e. Rutherford scattering). Higher the atomic number, higher is the
number of back scattered electrons. They are detected by semiconductor
detectors. Most of the catalysts used in the present study are low conducting
specimens and hence the catalysts were coated with gold by sputtering
method. The morphology of the materials were recorded using a scanning
electron microscope (Hitachi-S-3400N) operated at an accelerating voltage of
16 kV. The samples were suspended in methanol and the specimen stub was
dipped into the liquid and removed.The sample powder deposited onto the
surface of the stub evenly when methanol was evaporated. Thisspecimen was
coated with gold for two minutes. The beam is scanned over the specimen
surface in synchronism with the beam of a cathode ray tube (CRT) display
screen. Materials can be studied properly only when they are electrically
conducting otherwise they give rise to charging phenomenon resulting blurred
images.
2.3.9 Transmission Electron Microscopy (TEM)
Transmission electron microscopy (TEM) has been extensively
used for determination of the size of nanoparticles. The electron gun consists
of a tungsten anode, an accelerating anode and a beam aperture control. An
electron lens system focuses the electron beam with very narrow cross section
at the point where it strikes the specimen. The transmitted electrons pass
through another set of electronic optics to finally fall on a fluorescent screen
where the image is produced, and recorded on a photographic film. TEM uses
transmitted electrons such as unscattered, elastically scattered and
inelastically scattered electrons. The use of high energy electrons (100 keV or
more) gives high resolution and reduces chromatic aberration. The resolution
of commercial instruments is 1 nm and the magnification is about 106 times
higher than ordinary optical microscope.
62
Transmission electron microscopic (TEM) images were recorded
using a JEOL TEM-3010 electron microscope operated at an accelerating
voltage of 300 keV. The samples for TEM analysis were prepared by
dispersion of the catalysts in ethanol under sonication and deposited on a
copper grid.
2.3.10 X-ray Photoelectron Spectroscopy (XPS)
The typical X-ray photoelectron spectrum (XPS) represents the plot of number of electrons detected versus binding energy (BE) of the electrons
detected. Each element produces a characteristic set of XPS peaks at
characteristic binding energy values that directly identify each element that exist in or on the surface of the material being analyzed. These characteristic
peaks correspond to the electronic configuration of the electrons within the
atoms. The number of detected electrons in each of the characteristic peak is
directly related to the amount of element within the area (volume) irradiated.
To generate atomic percentage values, each raw XPS signal must be corrected by dividing its signal intensity by a relative sensitivity factor (RSF) and
normalised over all the elements detected. To count the number of electrons at
each BE value, with the minimum error, XPS must be performed under
ultra-high vacuum (UHV) condition because electron counting detector in
XPS instrument is typically one meter away from the material irradiated with
X-rays. The main components of a XPS system include a source for X-rays,
an ultra-high vacuum stainless steel chamber with UHV pumps, an electron
collection lens, an electron energy analyzer, a magnetic field shielding, an
electron detector system, sample mounts and stage.
XPS is used to determine
Elements present in a sample and quantity of elements present
within approximately 10 nm of the sample surface.
63
Existence of very small contamination on the surface or in the bulk of the sample.
Empirical formula of a material which is free of excessive
surface contamination.
Identification of the chemical state of one or more elements in
the sample.
Binding energy of one or more electronic states.
Thickness of one or more thin layers (1 - 8 nm) of different
materials within the top 10 nm of the surface.
Density of electronic states.
The X-ray photoelectron spectrum was recorded on a Thermo
Multilab 2000 using monochrome AlK radiation as the excitation source.
Prior to analysis, the sample was evacuated under high vacuum and then
introduced into the analysis chamber. The spectra were recorded for Pr3d, O1s,
Al2p and P2p photoelectron peaks in PrAlPO-5 and Fe2p, O1s, Al2p and P2p in
FeAlPO-5. Each spectral region of photoelectron was scanned several times
to obtain good signal-to-noise ratios.The powder samples were fixed on a
steel holder with double-face adhesive tape and analysed as received. An
electron flood gun was used to reduce the charge effects.
2.3.11 Electron Spin Resonance (ESR) Spectroscopy
ESR technique is very sensitive to environmental symmetry of
transition metal cations as long as they are paramagnetic. The coordination
environment of praseodymium in PrAPO-5 and iron in FeAlPO-5 samples
were confirmed by this technique. The ESR spectrum was recorded on a
Bruker EMX Plus spectrometer at room temperature with microwave power
64
of 0.632 mW and modulation frequency of 100 kHz. The sample (40 mg) was
taken in a quartz tube with 4 mm outer diameter and then evacuated to
approximately 10-3 Torr. The tube was sealed under vacuum and then set in a
quartz Dewar vessel fitted in the EPR cavity.
2.3.12 Magic Angle Spinning - Nuclear Magnetic Resonance
(MAS-NMR) Spectroscopy
High resolution NMR will be of comparatively little value in the
catalyst field because broad lines observed in solids tend to obscure the shifts
of the resonance line. In powdered solids, chemical shifts and scalar
J couplings are largely hidden because of the presence of large anisotropic
interactions like dipolar couplings and chemical shift anisotropies, which lead
to significant broadening of the signals. Both the chemical shifts Hamiltonian
and the dipolar coupling Hamiltonian depend on the orientation of the
molecule with respect to the direction of the external field. Powdered solids
contain many crystallites with random orientation. The anisotropic
interactions thus lead to broad patterns since different molecular orientations
present in the sample give rise to different spectral frequencies. The resulting
lack of resolution obscures information contained in the spectrum. It is thus
necessary to apply special techniques to obtain high resolution spectra. This is
a major contrast with liquid state NMR, for which fast tumbling of molecules
causes anisotropic interactions to average themselves to isotropic values
resulting sharp lines.
Solid state MAS-NMR is a powerful tool in the structural analysis
of zeolites and zeo-types especially for obtaining information of local
structure, geometry and coordination of the building atoms such as Al and P
or the heteroatoms substituted. The anisotropic broadening was averaged to
zero when the sample physically rotates around = 54.74°. This angle is
65
referred to as magic angle ( m) since the resulting spectrum has similar
narrow lines characteristics to a liquid state spectrum. The associated
technique known as magic angle spinning (MAS) has been extensively used
in solid state NMR experiments with spinning rates routinely set in the range
of 10 - 30 kHz and up to 70 kHz. When the sample is spun around the magic
angle at a rate faster than the anisotropy of interaction, all the crystallites
appear to exhibit the same orientation. At the magic angle the chemical shift
anisotropy and the dipolar interactions are averaged to zero and thus reduction
of line broadening is observed.
Detailed information may thus be derived on the structure of a
catalyst, its thermal or chemical transformation, specific sorbent-sorbate
interactions, nature of chemically deposited species, catalytically active sites
and chemical reactions at the catalyst. 27Al MAS-NMR spectrum
quantitatively distinguishes between four and six coordinated aluminium
sites. The nature of interaction between aluminium and phosphorus is
obtained from 31P MAS-NMR spectroscopy. Solid state 27Al MAS-NMR and 31P MAS-NMR spectra of PrAlPO-5 sample were recorded on a Bruker NMR
spectrometer at 5 kHz. Solid state 27Al and 31P chemical shifts were externally
referenced to [Al(H2O)6]3+ in aqueous Al(NO3)3 and H3PO4(85 wt% in water)
respectively.
2.3.13 Inductively Coupled Plasma - Optical Emission Spectroscopy
(ICP-OES)
Chemical analysis was performed with ICP-AES Perkin Elmer
OPTIMA 5300 DV ICP-OES instrument. About 0.25 g of dried sample was
accurately weighed in a platinum crucible. The crucible was heated in an
electrical Bunsen burner to red-hot for 3 h and from the weight loss of the
material, the percentage of the volatile matter was calculated. To this sample,
66
5 ml of concentrated hydrochloric acid was added. It was stirred and the clean
solution was made up to 100 ml in a standard flask with double distilled water.
The aluminium,iron and praseodymium contents in the solution were then
determined using ICP-OES instrument.
2.4 CATALYTIC STUDIES
2.4.1 Liquid Phase Reactions
The reaction was carried out in liquid phase in a batch reactor
consisting of a double- necked round bottom flask fitted with a condenser.
The reactants and the respective catalyst were taken in a round bottom flask
of desired volume fitted with a reflux condenser. The flask with its content
kept inside an oil bath was heated at a constant temperature while being
stirred magnetically. The temperature of the oil bath was controlled using a
thermocouple. The progress of the reaction was monitored using the clear
centrifugate obtained by centrifuging the aliquots withdrawn from the hot
reaction mixture at regular intervals. The schematic diagram of the reactor
set-up is depicted in Figure 2.3.
67
Figure 2.3 Catalytic reaction set up for liquid phase reactions
68
2.4.1.1 Oxidation of ethylbenzene
PrAlPO-5 (200 mg) and ethylbenzene (50 mmol) were taken in a
two necked round bottom flask fitted with a condenser and a magnetic pellet
for stirring. To this heterogeneous reaction mixture, air was purged through a
balloon for 6 h. After completion of the reaction, the reaction mixture was
filtered and washed the catalyst with ether for 5 to 6 times. The filtrate was
poured into ice cold water and extracted with ether. The ether layer and
washings were combined and dried over anhydrous sodium sulphate. The
ether layer was evaporated to obtain the product.The products were analyzed
as given in section 2.5.
2.4.1.2 Synthesis of campholenic aldehyde from -pinene
PrAlPO-5 (100 mg), -pinene (5 mmol) and chloroform (10 ml)
were taken in a three necked round bottom flask (50 ml) fitted with a
condenser and a magnetic pellet was placed for stirring. To this heterogeneous
reaction mixture, air was purged through an aerator maintained at a flow rate
of 5 ml min-1 for 12 h. After completion of reaction, the reaction mixture was
filtered and washed the catalyst with ether. The filtrate was poured into ice
cold water and extracted with ether. The ether layer and washings were
combined and dried over anhydrous sodium sulphate. The ether layer was
evaporated to obtain the product. The products were analysed as given in
section 2.5. The structure of the major product was confirmed by 1H-NMR
spectrum.
2.4.1.3 Synthesis of 5-arylidene-2,4-thiazolidenedione
FeAlPO-5 (100 mg),2, 4-thiazolidinedione (5 mmol), aldehyde (5mmol),
water (4 ml) and ethanol (1 ml) were taken in a two necked round bottom flask (50
69
ml) fitted with a condenser. A magnetic pellet was placedinside the flask for stirring
(500 rpm) content in the flask. The reaction mixture was refluxed for an appropriate
time and then filtered. The progress of the reaction was monitored with TLC under
UV light. When the reaction was completed, product was filtered along with the
catalyst. The precipitate was washed with water and dissolved in ethyl acetate to
separate the catalyst. The organic layer was evaporated and crude product was
column purified using silica gel of 60-120 mesh with hexane-ethyl acetate as eluent.
The structure of the product was confirmed by 1H-NMR.
2.5 PRODUCT ANALYSIS
Quantitative analysis of the liquid products of the reactions was
carried out using a gas chromatograph (GC). The products were also
confirmed using a gas chromatograph coupled with a mass spectrometer.
The details of the product analysis using GC and GC-MS are given in the
following sections.
2.5.1 Gas Chromatograph
Quantitative analysis of the reaction products were carried out
using a gas chromatograph (GC; Shimadzu GC-17A) with DB-5 capillary
column (30 m x0.25 mm x 0.25 m) equipped with a flame ionisation detector
(FID). Nitrogen was used as the carrier gas. The oven temperature was fixed
at 70 °C and the column temperature was in the range of 100 - 250 °C at a
heating rate of 10 °Cmin-1. The sample (5 L) was injected in the injection
port and the gaseous sample travelled and separated in the column through an
oven and finally reached the flame ionisation detector (FID).
The conversion of reactant and selectivity of the products were calculated
from GC analysis.
70
2.5.2 Gas Chromatograph Coupled with Mass Spectrometer
The reaction products were also confirmed using a gas
chromatograph (Perkin-Elmer Clarus 500 Auto System XL with elite series
PE-5 capillary column, 30 m x 0.25 mm x1 m) coupled with a mass
spectrometer (GC-MS) (Turbo; EI, 70 eV). DB-1 (100%
dimethylpolysiloxane) column and helium was used as the carrier gas at a
flow rate of 1 mlmin-1. Each component of the product mixture was identified
from its characteristic m/e value and fragmentation pattern.
2.5.3 NMR Spectroscopic Analysis
The NMR spectrum proves to be of great utility in structure
elucidation because the properties it displays can be related to the chemist's
perception of molecular structure. The chemical shift of a particular nucleus
can be correlated with its chemical environment, the scalar coupling (or J-
coupling) indicates an indirect interaction between individual nuclei,
mediated by electrons in a chemical bond, and under suitable conditions, the
area of a resonance is related to the number of nuclei giving rise to it.
The 1H nucleus is the most commonly observed nucleus in NMR
spectroscopy. Hydrogen is found throughout most organic molecules and,
fortunately for chemists, the proton has high intrinsic sensitivity as well as
being almost 100% abundant in nature, all of which make it a favourable
nucleus to observe. The proton NMR spectrum contains a wealth of chemical
shift and coupling information and is the starting point for most structure
determinations. The structure of the products isolated in the synthesis of
campholenic aldehyde and 2,4-thiazolidinedione analogues were elucidated
by 1H-NMR. The sample was dissolved in appropriate deutrated solvent
(CDCl3 or DMSO-d6) and 1H-NMR spectrum was recorded on a Bruker AV
III 500 MHz FT NMR spectrometer.
71
CHAPTER 3
PHYSICO-CHEMICAL CHARACTERIZATION OF
PrAlPO-5 and FeAlPO-5 MOLECULAR SIEVES
3.1 INTRODUCTION
The substitution of Al by metal ions could generate Brønsted acid
sites (bridging OH groups) as well as Lewis acid sites (corresponding to
anionic vacancies deriving from missing lattice oxygens) in the
aluminophosphate lattice. So the incorporation of a transition metal or
lanthanide cation can easily change its oxidation number and also creates redox
active site. The coupling of acidic with redox properties opens up routes
towards shape selective bi-functional catalysts and design of novel catalysts.
The location and local structure (metal ion environment) of the incorporated
metal ions are necessary for optimization and control of the catalytic activity in
these systems. The catalytic properties of microporous materials are largely
determined by the composition and structure on the atomic scale. Therefore,
catalyst characterization is a highly relevant discipline in catalysis.
Different characterization techniques are used to gain an insight into
the location of the transition metal ions in aluminophosphate framework.
Generally, data on the location of the cations are collected with difficulty since
the metal concentration is usually low. It is necessary to use more than one
method if a reliable conclusion is to be reached. Characterization techniques
such as diffuse reflectance UV-Vis spectroscopy (DRS), electron spin
resonance (ESR), electron spin echo modulation (ESEM), Fourier transform
72
infrared (FT-IR) and diffuse reflectance infrared Fourier transform (DRIFT)
spectroscopies are commonly applied. Nuclear magnetic resonance
spectroscopy (NMR), Mössbauer spectroscopy, X-ray absorption near-edge
structure (XANES) and extended X-ray absorption spectroscopy for fine
structure (EXAFS) are also employed occasionally. Therefore, suitable
complementary characterization techniques are necessary to understand the
surface chemistry of microporous AlPO4 materials.
In the present study XRD, DRS-UV-Vis, BET, SEM, TEM, ESR,
XPS, 27Al and 31P MAS-NMR, FT-IR, TGA, TPD, TPR, ex-situ pyridine
adsorbed IR and ICP-OES were employed to correlate the physico-chemical
properties of the synthesized catalysts and their catalytic activities.
3.2 PHYSICOCHEMICAL CHARACTERIZATION OF PrAlPO-5
3.2.1 X-Ray Diffraction (XRD)
The XRD patterns of calcined AlPO-5, PrAlPO-5 (25, 50, 75, 100,
150 and 200) and Pr6O11 are shown in Figure 3.1. Pure Pr6O11 showed
reflections close to 28, 33, 47 and 56° (2 ) corresponding to (111), (200), (220)
and (311) reflections respectively (Ma ecka & Kepinski 2007, Abu–Zied &
Soliman 2008). All PrAlPO-5 catalysts exhibited characteristic reflections of
AlPO-5 (Fang et al 1997) and not that of Pr6O11, thus confirmed the absence of
extra framework praseodymium species in any of the PrAlPO-5.
Praseodymium incorporated AlPO-5 molecular sieves showed a change in the
order of intensity of reflections from that of parent AlPO-5. Since the intensity
of different reflections of PrAlPO-5 (25 and 50) exhibited the same order as
that of AlPO-5, it is presumed that all of them possessed the same crystal
morphology. Although the reflections of PrAlPO-5 (75, 100, 150 and 200)
exhibited similar to that of AlPO-5 and PrAlPO-5 (25 and 50), the intensity of
reflections changed. For example, the reflection at 2 = 18° became intense
peak, and based on the increase in intensity it could be inferred that
73
praseodymium was better located in (210) plane compared to other planes.
When praseodymium content was low as in the case of PrAlPO-5 (75, 100, 150
and 200), it chose specific plane and the intensity of reflections changed from
the parent AlPO-5. The lattice parameters and d spacing values for AlPO-5 and
PrAlPO-5 samples were calculated, and the results are presented in Table 3.1.
Praseodymium incorporated AlPO-5 showed higher lattice parameter values
than that of parent AlPO-5, and the inter-planar distance (d100) increased
linearly with increase of praseodymium content.
Figure 3.1 XRD patterns of calcined (a) AlPO-5, (b) PrAlPO-5 (25), (c) PrAlPO-5 (50), (d) PrAlPO-5 (75), (e) PrAlPO-5 (100), (f) PrAlPO-5 (150), (g) PrAlPO-5 (200) and (h) Pr6O11
74
The angular positions of reflections (2 ) of PrAlPO-5 samples showed a slight
shift from the parent AlPO-5. The shift in 2 values and change in the lattice
parameter values of PrAlPO-5 compared to AlPO-5 supported framework
incorporation of praseodymium.
Table 3.1 Lattice parameter values for AlPO-5 and PrAlPO-5
Catalyst
(calcined) a=b C
d(100)
(Å)
AlPO-5 13.4233±0.1645 8.4061±0.2453 11.40
PrAlPO-5(200) 13.7075±0.0153 8.6029±0.1889 11.95
PrAlPO-5(150) 13.7361±0.009 8.4824±0.0483 11.98
PrAlPO-5(100) 13.7886±0.0221 8.5110±0.0420 12.00
PrAlPO-5(75) 13.8549±0.0315 8.3912±0.0587 12.07
PrAlPO-5 (50) 13.8676±0.0135 8.4591±0.0147 12.12
PrAlPO-5 (25) 13.8709±0.0213 8.4672±0.0321 12.13
3.2.2 Diffuse Reflectance Ultraviolet-Visible(DRS-UV-Vis)
Spectroscopy
The DRS-UV-Vis spectra of AlPO-5 supported 3 wt% Pr6O11, AlPO-
5 are shown in Figure 3.2, and praseodymium incorporated AlPO-5 (25, 50, 75,
100, 150 and 200) catalysts are shown in Figure 3.3. AlPO-5 did not show any
absorbance in the UV-Vis region. The trivalent praseodymium (Pr3+) contained
two electrons in its 4f orbital which gave rise to a number of distinct
microstates whose term symbols are 3H4, 3H5, 3H6, 3F2, 3F3, 3F4, 1G4, 1D2, 3P0,3P1, 3P2, 1I6 and 1S0. Hence, a series of f-f transition were possible (Carnall et al
1989). The 3 wt% Pr6O11 loaded AlPO-5 showed a broad band between 415
and 590 nm with a fine structure which is attributed to f-f transition of Pr3+ ions
(Ma ecka & Kepinski 2007, Barrera et al 2007).
75
Figure 3.2 DRS UV-Vis spectra of (a) AlPO-5 and (b) AlPO-5 supported 3 wt%Pr6O11
Figure 3.3 DRS UV-Vis spectra of (a) PrAlPO-5 (200), (b) PrAlPO-5 (150), (c) PrAlPO-5 (100), (d) PrAlPO-5 (75), (e) PrAlPO-5 (50) and (f) PrAlPO-5 (25)
76
PrAlPO-5 molecular sieves showed three bands around 210, 262 and
380 nm. Donega et al (1995) reported the band around 210 nm is assigned to
O2- to Pr3+ charge transfer transition. The band at 262 nm is quite broad and it is
due to the appearance of induced electron dipole f-f transition (Dorenbos 2000)
(Jude-Ofelt theory) and O2- to Pr4+ charge transfer transition (Tankov et al
2011) in that region. The promotion of 4f electron into 5d sub-shell is Laporte
allowed and they are broad (around 250 nm) which overlap with O2- to Pr4+
charge transfer transition at 270 nm, and observed as a broad peak centered
around 262 nm. Further, the band at 210 and 270 nm are the characteristics of
Pr (III and IV) species in the tetrahedral position. These observations supported
the framework incorporation of Pr in AlPO-5 framework.
3.2.3 Surface Microstructure (SEM and HR TEM)
The SEM images of calcined AlPO-5 and PrAlPO-5 are shown in
Figure 3.4. The image exhibited hexagonal rods of smooth surface. Zhao et al
(2006) and Fang et al (1997) also reported similar morphology. The SEM
image of PrAlPO-5 (100) shown in Figure 3.4d, illustrates similar features as
shown in Figure 3.4a. There were hexagonal aggregated rods and most of the
rods were cleaved along the longitudinal direction. The SEM image of PrAlPO-
5 (75) (Figure 3.4e) was nearly the same as that of Figures 3.4a and 3.4g. Both
PrAlPO-5 (25) and PrAlPO-5 (50) also exhibited similar morphology (Figures
3.4g and 3.4f).
77
Figure 3.4 SEM images of (a) AlPO-5, (b) PrAlPO-5 (200), (c) PrAlPO-5 (150), (d) PrAlPO-5 (100), (e & h) PrAlPO-5 (75), (f) PrAlPO-5 (50) and (g) PrAlPO-5 (25)
Although the XRD pattern of PrAlPO-5 ((Al+P)/Pr = 75 and 100)
exhibited change in the order of reflections from that of parent AlPO-5, the
morphology of the material did not change. This revealed that the concentration
of praseodymium in the synthesis gel didn’t affect the nucleation process. The
appearance of tiny crystallites on the surface of hexagonal rods was due to slow
condensation of hydroxides of praseodymium with phosphates. The
aggregation of small particles created large number of voids, which are clearly
evidenced in the TEM images (Figure 3.5)
78
Figure 3.5 TEM images of (a) AlPO-5, (b) PrAlPO-5 (200), (c) PrAlPO-5 (150), (d) PrAlPO-5 (100), (e) PrAlPO-5 (75), (f) PrAlPO-5 (50) and (g) PrAlPO-5 (25)
3.2.4 Nitrogen Sorption Studies
The nitrogen sorption isotherms of calcined AlPO-5 and PrAlPO-5 with (Al+P)/Pr ratios of 25, 50, 75, 100, 150 and 200 molecular sieves are shown in Figure 3.6. The hysteresis loop observed just below the relative pressure (p/p0) of one is due to inter particle voids. The TEM images (Figure 3.5) also revealed such large number of voids due to aggregation of particles. The small angle XRD (Figure 3.7) pattern also supported the absence of mesopores as it did not exhibit any characteristic reflection at low 2 as that of ordered mesoporous materials. The BET surface area and pore volume of the materials are presented in Table 3.2. Li et al (2010) reported increase in surface area and pore volume due to incorporation of lanthanides in AlPO-5 framework. The increase in surface area and pore volume with Pr incorporation in AlPO-5 concurred with Li et al (2010) report. The larger ionic radii of Pr (III and IV) than that of Al (III) and P (V) ions altered the lattice parameters of AlPO-5 crystals. Such increase in lattice parameters increased the crystal size which in turn increased the surface area and pore volume of the materials. Thisincrease in surface area and pore volume also supported framework incorporation of praseodymium in AlPO-5.
79
Figure 3.6 N2 sorption isotherms of (a) AlPO-5, (b) PrAlPO-5 (200), (c) PrAlPO-5 (150), (d) PrAlPO-5 (100), (e) PrAlPO-5 (75), (f) PrAlPO-5 (50) and (g) PrAlPO-5 (25)
Figure 3.7 Small angle XRD pattern of PrAlPO-5 (75)
80
Table 3.2 Nitrogen sorption results of calcined AlPO-5 and PrAlPO-5 (25, 50, 75, 100, 150 and 200)
CatalystSurface
area, ABET
(m2/g)a
Pore volume Vp
(cm3/g)b
Pore diameter,
Dp
(Å)c
AlPO-5 228 0.20 4.36PrAlPO-5 (25) 272 0.28 5.18PrAlPO-5 (50) 265 0.27 5.03PrAlPO-5 (75) 258 0.25 4.89PrAlPO-5 (100) 245 0.24 4.69PrAlPO-5 (150) 242 0.23 4.56PrAlPO-5 (200) 239 0.22 4.50acalculated from N2 adsorption-desorption isotherm, b & cDetermined by BJH method
3.2.5 Electron Spin Resonance Spectroscopy (ESR)
The ESR spectra of PrAlPO-5 (25, 50, 75, 100, 150 and 200)
recorded at room temperature are shown in Figure 3.8. All the six spectra
showed signals around g values of 1.9 and 3.4 due to paramagnetic
praseodymium species. PrAlPO-5 (200, 150 and 100) showed sharp ESR
signals due to oxygen chemisorbed on Pr3+ sites whereas PrAlPO-5 (25, 50 and
75) showed broad and resolved signals for free Pr3+ and Pr4+ sites. Since the
metal content in PrAlPO-5 (75) was relatively higher than all other PrAlPO-5,
incorporation of high amount of praseodymium in the framework led to
collapse of structure. In order to avoid such a ring strain, part of Pr3+ species
was oxidized to Pr4+ by chemisorbed oxygen. Hence well resolved signals for
Pr3+ and Pr4+ appeared in the ESR spectrum of PrAlPO-5 (25, 50 and 75). The
ESR signal close to g value of 2.0 is attributed to oxygen
chemisorbed on praseodymium species. Maurelli et al (2012) and
81
82
Cheralathan et al (2000) already reported similar observations for metal
incorporated aluminophosphates. Lu et al (2005) reported that the signal with
similar g value of 2.0 was assigned to framework defects in AlPO-5. Trojan et
al (1992) reported the magnetic behavior of phthalocyanine ligands that
exhibited a strong magnetic interaction with lanthanide f-electrons and trivalent
Pr in bis(phthalocyaninato) complexes which showed a spin resonance signal at
g = 2.0. The low temperature ESR spectra of PrAlPO-5 (75) recorded at 200
and 110 K and that of PrAlPO-5 (25) recorded at 110 K (Figure 3.9) showed
two distinct peaks at g values of 2.0 and 2.045. The former g value suggested
magnetic interaction of Pr species with paramagnetic oxygen molecule and the
later suggested the presence of defective AlPO-5 sites. Hence the magnetic
interaction of oxygen with Pr species was confirmed and also supported the
framework incorporation of Pr in AlPO-5.
83
Figure 3.9 Low temperature ESR spectra of (a) PrAlPO-5 (25) and (b & c) PrAlPO-5 (75)
3.2.6 X-ray Photoelectron Spectroscopy (XPS)
The XPS spectrum (Figure 3.10) of Pr 3d core electron levels for
calcined PrAlPO-5(75) revealed the oxidation state of praseodymium in
AlPO-5. The two broad peaks around 938 and 959 eV are assigned to Pr 3d5/2
and Pr 3d3/2 states respectively (Barrera et al 2007). De-convolution of the peak
(3d5/2) established the presence of Pr3+ and Pr4+ species. The peak around
933 eV is assigned to Pr3+, and the peak around 935 eV is assigned to Pr4+
oxidation state. Similar de-convolution of Pr 3d3/2 peak results two peaks
around 953 and 955 eV. The comparison of intensity of the peaks revealed that
Pr3+ content is slightly lower than that of Pr4+.
84
Figure 3.10 XPS spectrum of calcined PrAlPO-5 (75)
3.2.7 27Al and 31P Magic Angle Spinning – NMR
27Al MAS-NMR of AlPO-5 and PrAlPO-5 (25 and 75) are shown in
Figure 3.11. The 27Al MAS-NMR of AlPO-5 exhibited a broad peak between
30 and 50 ppm (Akholekar & Ryoo 1996). It showed seven overlapped peaks
due to quadrupolar interactions. The 27Al MAS-NMR of PrAlPO-5 (25)
showed an intense peak at 37.93 ppm due to tetrahedral aluminium, and a weak
peak at 4.96 ppm due to aluminium in distorted trigonal bipyramidal co-
ordination. Similarly PrAlPO-5 also showed an intense peak at 37.63 ppm, and
a weak peak around 6.04 ppm. Both PrAlPO-5 (25) and PrAlPO-5 (75) showed
a peak around -15 ppm. The paramagnetic praseodymium species present in the
framework delocalized the unpaired electron through oxygen linkage which led
to the appearance of negative signals in the spectra (Zhao et al 2006)
85
Figure 3.11 27Al MAS-NMR spectra of AlPO-5 and PrAlPO-5 (25 and 75)
27Al MAS-NMR of AlPO-5
27Al MAS-NMR of PrAlPO-5 (25)
27Al MAS-NMR of PrAlPO-5 (75)
86
The 31P MAS-NMR of AlPO-5 and PrAlPO-5 (25 and 75) are shown in
Figure 3.12. AlPO-5 showed an intense sharp peak at -24.62 ppm (Li & Davis
1993). This confirmed only one type of crystallographic phosphorous in
AlPO-5. This peak is shifted to -27.86 ppm in PrAlPO-5 (25). The shoulder to
the main peak observed for PrAlPO-5 (25) suggested different environment for
framework phosphorous. The 31P MAS-NMR of PrAlPO-5 (75) also exhibited
a strong peak at -27.52 ppm along with a shoulder peak at -31.21 ppm. The
appearance of two resonance signals confirmed the framework substitution of
Pr for Al. Further, 31P MAS-NMR showed spinning side bands due to dipolar
interaction with paramagnetic praseodymium species. The MAS-NMR study
thus confirmed the framework incorporation of praseodymium by replacing
both Al and P ions in the framework of AlPO-5.
87
31P MAS-NMR of PrAlPO-5 (75)
* *
Figure 3.12 31P MAS-NMR spectra of AlPO-5 and PrAlPO-5 (25 and 75)
31P MAS-NMR of AlPO-5
31P MAS-NMR of PrAlPO-5 (25)
31P MAS-NMR of PrAlPO-5 (75)
88
3.2.8 Temperature Programmed Reduction (TPR)
The TPR profiles of AlPO-5 supported 3 wt% Pr6O11 and PrAlPO-5
with (Al+P)/Pr ratios 25, 50, 75 and 100 molecular sieves were recorded
between 100 and 900 oC and are shown in Figure 3.13. The AlPO-5 supported
3 wt% Pr6O11 showed two reduction peaks around 610 and 720 oC. Such
reduction peaks were not observed in all PrAlPO-5 molecular sieves. This
further confirmed the absence of non-framework praseodymium species in
AlPO-5.
Figure 3.13 TPR profile of (a) AlPO-5 supported 3 wt% Pr6O11,
(b) PrAlPO-5 (100), (c) PrAlPO-5 (75), (d) PrAlPO-5 (50) and
(e) PrAlPO-5 (25)
89
3.2.9 Characterization of Acid Sites (TPD-NH3 and ex-situ pyridine
adsorbed IR)
The surface acidity in PrAlPO-5 was investigated by temperature
programmed desorption of ammonia (TPD-NH3) and ex-situ pyridine adsorbed
IR. The TPD-NH3 results of AlPO-5, AlPO-5 supported 3 wt% Pr6O11 and
PrAlPO-5 with (Al+P)/Pr ratios 25, 50, 75, 100, 150 and 200 are depicted in
Figure 3.14 and Figure 3.15 respectively. The presence of weak and moderately
strong acid sites was evidenced by the appearance of two distinct peaks around
150 and 350 °C (Pastore et al 2005). The MAS-NMR study also confirmed that
Pr replaced both Al and P in the framework. The isomorphic substitution of
phosphorous by praseodymium created mild Bronsted acid sites.
Figure 3.14 TPD-NH3 profile of (a) AlPO-5 and (b) AlPO-5 supported 3
wt% Pr6O11
90
Figure 3.15 TPD-NH3 profile of (a) PrAlPO-5 (200), (b) PrAlPO-5 (150),
(c) PrAlPO-5 (100), (d) PrAlPO-5 (75), (e) PrAlPO-5 (50) and
(f) PrAlPO-5 (25)
The desorption peak between 300 and 410 °C attributed to moderately strong
acid sites also supported this view. The total acidity of the material increased
with increase in praseodymium content (Table. 3.3). The acidity created in
PrAlPO-5 molecular sieve was mainly due to incorporation of praseodymium
in the framework. Dumitriu et al (2002) illustrated the TPD profile for metal
incorporated AFI catalysts with intermediate acid strength. The results
concurred with Dumitriu et al (2002) report. The pyridine adsorbed IR spectra
of PrAlPO-5 (75, 100 and 150) are shown in Figure 3.16. The adsorption of
pyridine led to the formation of coordinated bands around 1445, 1490 and 1598
cm-1 (Huang et al 2003, Liu et al 2008). These are the characteristic bands of
Lewis acid sites present in a solid acid catalyst. The very weak band around
1547 cm-1 suggested the presence of a few number of Bronsted acid sites in the
PrAlPO-5 catalysts.
91
Table 3.3 TPD-NH3 sorption results of calcined samples
Catalyst Total aciditiya
(mmol NH3/g)
AlPO-5 -
PrAlPO-5 (25) 0.174
PrAlPO-5 (50) 0.170
PrAlPO-5 (75) 0.165
PrAlPO-5 (100) 0.158
PrAlPO-5 (150) 0.137
PrAlPO-5 (200) 0.106aNH3-TPD
Figure 3.16 Ex-situ pyridine adsorbed IR spectra of (a) PrAlPO-5 (150),
(b) PrAlPO-5 (100), (c) PrAlPO-5 (75) and (d) PrAlPO-5 (25)
92
3.2.10 FT-IR Spectroscopy
The FT-IR spectra of calcined PrAlPO-5 ((Al+P)/Pr = 25, 50, 75,
100, 150 and 200) and AlPO-5 samples were recorded between 4000 and
400 cm-1 (Figure 3.17). The asymmetric stretching vibration of TO4 groups (Al-
O-P and Al-O-Pr) appeared between 1300 and 850 cm-1, and the corresponding
bending vibration appeared around 750 cm-1 (Zecchina & Arean 1996). The
bending vibration of PrAlPO-5 samples showed decrease in intensity and
broadened with fine structure (peak around 512 and 580 cm-1) thus indicated
Pr-O linkage, which was absent in AlPO-5. This also supported the
incorporation of praseodymium species in AlPO-5 framework. The band
appeared below 500 cm-1 is due to vibrations in double ring region. The band
just below 3000 cm-1 due to C-H stretching vibration was absent in all the
calcined samples. This confirmed the complete removal of template during
calcination.
93
Figure 3.17 FT-IR spectra of (a) AlPO-5, (b) PrAlPO-5 (200), (c) PrAlPO-
5 (150), (d) PrAlPO-5 (100), (e) PrAlPO-5 (75), (f) PrAlPO-5
(50) and (g) PrAlPO-5 (25)
3.2.11 Thermogravimetric Analysis (TGA)
The thermograms of assynthesized AlPO-5 and PrAlPO-5 with
(Al+P)/Pr ratios of 25, 50, 75 and 100 are shown in Figure 3.18. All the
samples showed a weight loss between 70 and 100 oC due to removal of
physisorbed water. The second weight loss between 200 and 560 oC was due to
removal and decomposition of the template. The removal of template occurred
in two stages in the case of PrAlPO-5 molecular sieves. First the template
adsorbed at weak acid sites lost below 400 oC, and then the decomposition of
94
template occurred between 400 and 560 oC due to the presence of strong acid
sites. Similar behavior was already reported by Umamaheswari et al (2000).
Figure 3.18 TGA of as-synthesized (a) AlPO-5, (b) PrAlPO-5 (100),
(c) PrAlPO-5 (75), (d) PrAlPO-5 (50) and (e) PrAlPO-5 (25)
The thermograms of calcined AlPO-5 and PrAlPO-5 molecular
sieves are shown in Figure 3.19. The weight loss below 100 oC increased with
increase of metal content thus suggested the hydrophilic nature of the catalysts.
This confirmed not only the complete removal of template but also the
hydrophilic nature of metal incorporated AlPO-5 molecular sieves.
95
Figure 3.19 TGA of calcined (a) AlPO-5, (b) PrAlPO-5 (100),
(c) PrAlPO-5 (75), (d) PrAlPO-5 (50) and (e) PrAlPO-5 (25)
3.3 PHYSICO-CHEMICAL CHARACTERIZATION OF FeAlPO-5
3.3.1 X-ray Diffraction (XRD)
The XRD patterns of calcined FeAlPO-5 (75, 100 and 150) and
Fe2O3 are shown in Figure 3.20. The XRD patterns showed intense peaks at 2
values of 7.56, 13.04, 15.08, 20, 21.06 and 22.62° corresponding to the
diffractions of (100), (110), (200), (210), (002) and (211) planes respectively
[JCPDS: 41-0044]. These are the characteristic reflections of AlPO-5
molecular sieves (Shiju et al 2006). Further, the XRD patterns of FeAlPO-5
molecular sieves did not show any reflections corresponding to Fe2O3 (Soria
et al 2014) thus confirmed the absence of separate Fe2O3 phase. The lattice
parameters increased with increase of iron content, and the inter-planar
96
distance also linearly increased with increase of iron content. The angular
positions of reflections were shifted to lower 2 values with the increase of iron
content. The lattice parameters for AlPO-5 and FeAlPO-5 were calculated and
presented in Table 3.4.The change in lattice parameters, d-spacing values and
shift in 2 values are supportive for the framework incorporation of iron in
AlPO-5.
Figure 3.20 XRD patterns of (a) FeAlPO-5 (150), (b) FeAlPO-5 (100), (c) FeAlPO-5 (75) and (d) Fe2O3
97
Table 3.4 Lattice parameters for calcined AlPO-5 and FeAlPO-5
Catalyst a=b cd(100)
(Å)
V
(Å)3
AlPO-5 13.5495±0.1451 8.3805±0.2239 11.51 1332.43
FeAlPO-5(150) 13.6523±0.891 8.4513±0.0660 11.61 1364.17
FeAlPO-5(100) 13.6858±0.0100 8.4961±0.0158 11.75 1378.14
FeAlPO-5(75) 13.6975±0.0085 8.4857±0.0184 11.82 1378.80
3.3.2 Diffuse Reflectance Ultraviolet-Visible (DRS-UV-Vis)
Spectroscopy
DRS-UV-Vis spectra of calcined FeAlPO-5 (75, 100 and 150)
molecular sieves are presented in Figure 3.21. The calcined FeAlPO-5 samples
showed a single intense broad band around 253 nm in the UV region. This
band is attributed to the Laporte allowed ligand to metal (O2- to Fe3+) charge
transfer transition (Selvam & Mohapatra 2006, Wang et al 2004). The peak
around 253 nm also confirmed the tetrahedral geometry of Fe3+ in [FeO4]-.
Further, the absence of peak between 400 and 700 nm confirmed the absence of
clustering of iron species and extra-framework iron species (Wei et al 2008).
98
Figure 3.21 DRS UV-Vis spectra of (a) FeAlPO-5 (150), (b) FeAlPO-5 (100) and (c) FeAlPO-5 (75)
99
3.3.3 Scanning Electron Microscopic (SEM) Analysis
The SEM images of calcined AlPO-5 and FeAlPO-5 are shown in
Figure 3.22. The AlPO-5 and FeAlPO-5 (75, 100 and 150) showed hexagonal
morphology. Guo et al (2005) already reported hexagonal morphology for
AlPO-5 crystals. This suggested that the incorporation of iron in AlPO-5
framework did not affect the morphology of AlPO-5 crystals. The XRD
patterns also revealed that the crystals belonged to hexagonal space group of
P6cc.
Figure 3.22 SEM images of (a) AlPO-5, (b) FeAlPO-5 (75), (c) FeAlPO-5
(100) and (d) FeAlPO-5 (150)
100
3.3.4 Nitrogen Sorption Studies
The nitrogen sorption isotherms of calcined AlPO-5, fresh and used
FeAlPO-5 (75) are shown in Figure 3.23. The N2 sorption isotherm of
FeAlPO-5 at high relative pressure showed a hysteresis loop of H1 type, which
concurred with typical microporous material (Sing et al 1985).
Figure 3.23 Nitrogen sorption isotherms of (a) AlPO-5, (b) fresh FeAlPO-
5 (75) and (c) used FeAlPO-5 (75)
101
The BET surface area, pore volume and average pore diameter of the
materials are presented in Table 3.5. The textural parameters especially average
pore size of both fresh and used FeAlPO-5 catalysts remained almost the same
and in good agreement with the parent AlPO-5 suggesting that the zeotype kept
its original structure. Further, the incorporation of iron in the framework
slightly increased the BET surface area of FeAlPO-5. These observations also
supported the framework incorporation of Fe in AlPO-5
Table 3.5. Nitrogen sorption results for fresh and used FeAlPO-5 catalyst
Catalyst BET Surface area
(m2/g)
Micropore
volumea (cm3/g)
Mesopore
volumeb (cm3/g)
AlPO-5 225 0.101 0.284
FeAlPO-5(75) (fresh) 245 0.108 0.287
FeAlPO-5(75) (used) 241 0.107 0.288a Calculated by t-plot method, b Calculated by BJH method
3.3.5 Electron Spin Resonance Spectroscopy (ESR)
The ESR spectra of calcined FeAlPO-5 (75, 100 and 150) recorded
at room temperature are shown in Figure 3.24. All the three spectra showed
signals around g values of 4.3 and 2.0. Goldfarb et al (1994) reported that the g
value of 2.0 was due to Fe3+ in tetrahedral site while the g value of 4.3
attributed to distorted tetrahedral. However, the signal around g value of 2.0
could also be assigned to Fe3+ in oxidic clusters (Catana et al 1995). In order to
study the chemical location of Fe3+ ion, the ESR spectra were compared with
DRS-UV-Vis spectra. The absence of peak between 400 and 700 nm in DRS-
UV-Vis spectra excluded the possibility of extra framework Fe3+ and clustering
of iron species. Hence it is concluded that iron incorporated in AlPO-5
framework is in tetrahedral sites.
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Figure 3.24 ESR spectra of (a) FeAlPO-5 (75), (b) FeAlPO-5 (100) and (c)
FeAlPO-5 (150)
3.3.6 X-ray Photoelectron Spectroscopy (XPS)
The XPS spectrum was recorded in order to establish the oxidation
state of iron in AlPO-5. The core level XPS spectrum of iron in FeAlPO-5 (75)
is depicted in Figure 3.25. The peaks due to Fe 2p3/2 occurred at 710.1 eV and
that of Fe 2p1/2 at 723.9 eV. Similar values were reported in literature for Fe3+
ions by Yamashita and Hayes (2008) and Bhargaba et al (2007). Hence the
oxidation state of iron in FeAlPO-5 was established to be +3. The ESR and
DRS-UV-Vis studies supported the presence of iron in trivalent oxidation state.
103
Figure 3.25 XPS spectrum of calcined FeAlPO-5 (75)
3.3.7 Characterization of Acid Sites (TPD-NH3 and ex-situ pyridine
adsorbed IR)
The surface acidity in FeAlPO-5 was determined by temperature
programmed desorption of ammonia (TPD-NH3) and ex-situ pyridine adsorbed
IR. The TPD-NH3 results of FeAlPO-5 (75, 100 and 150) are depicted in Figure
3.26. A strong peak around 120 °C was observed, and this peak position shifted
towards higher temperature with slight increase of Fe content. This suggested
that the strength of acid sites increased with increase of metal content. The total
acidity of the material was calculated from the area under the peak and
presented in Table. 3.6. This confirmed the incorporation of iron in AlPO-5
created acidic nature to FeAlPO-5 catalysts (Dumitriu et al 2002, Pastore et al
104
2005). The ex-situ pyridine adsorbed IR spectra of FeAlPO-5 (75, 100 and 150)
are shown in Figure 3.27. The peaks observed at 1445, 1490 and 1598 cm-1 are
assigned to pyridine coordinated to Lewis acid sites (Huang et al 2003,
Liu et al 2008). The weak peak around 1545 cm-1 is due to interaction of
pyridine with Bronsted acid sites. This peak is very weak in comparison with
peak at 1445 cm-1, thus established that FeAlPO-5 contained more number of
Lewis acid sites than Bronsted acid sites.
Figure 3.26 TPD-NH3 profile of (a) FeAlPO-5 (150), (b) FeAlPO-5 (100) and (c) FeAlPO-5 (75)
105
Table 3.6 TPD-NH3 sorption results of calcined FeAlPO-5 catalysts
Catalyst Total aciditya
(mmol NH3/g)
AlPO-5 -
FeAlPO-5 (150) 0.107
FeAlPO-5 (100) 0.128
FeAlPO-5 (75) 0.145aNH3-TPD
Figure 3.27 Ex-situ pyridine adsorbed IR spectra of calcined FeAlPO-5 (75)
106
CHAPTER 4
LIQUID PHASE AEROBIC OXIDATION OF
ETHYLBENZENE OVER PrAlPO-5
4.1 INTRODUCTION
Selective catalytic oxidation of alkyl aromatics is a viable
technology to functionalize saturated and unsaturated hydrocarbons (Wentzel
et al 2000).The benzylic oxidation of alkyl aromatics is considered to be one
of the important catalytic reactions for the preparation of corresponding
carbonyl compound of the reactant as they are used in the synthesis of fine
chemicals and pharmaceuticals (Lu et al 2010). Parida& Dash (2009) studied
the liquid phase oxidation of ethylbenzene using TBHP as oxidant under mild
reaction conditions at a temperature of 80 °C. They reported 57.7%
ethylbenzene conversion and selectivity to acetophenone (82.2%) and
benzaldehyde (18%).The catalytic oxidation of ethylbenzene over Co-
substituted heteropolytungstate catalyst using H2O2 oxidant with acetonitrile
as solventgave acetophenone (93%) and 1-phenylethanol
(Kanjina&Trakarnpruk 2010).Kanjina&Trakarnpruk (2011) reported the
selective oxidation of ethylbenzene to acetophenone using tert-
butylhydroperoxide (TBHP) as oxidant at 130 °C in the presence of mixed
metal oxide catalysts. The reaction showed 87% ethylbenzene conversion and
92% selectivity to acetophenone.
Vanadia supported on ceria catalysts were used in the liquid phase
oxidation of ethylbenzene using H2O2 oxidant (Radhika et al 2007).
107
Acetophenone was found to be the major product along with
2-hydroxyacetophenone as minor product.The sintering at high temperature
and leaching of metal ions are serious drawbacks of transition metal oxide
based catalysts (Yu et al 2006). Organic peroxides, hydrogen peroxide and
molecular oxygen are cost effective oxidants for the oxidation of alkyl
aromatics in the presence of a suitable catalytic system. However, organic
peroxides are not environmentally benign as they leave large volume of
organic waste. Hydrogen peroxide decomposedrapidly above 70 °C, as a
consequence the formation of water in the reaction mixture decreased the
activity of the catalyst by decreasing the interaction between the substrate and
the catalyst surface. Molecular oxygen (air) is the cheapest and clean
oxidizing agent. Perkas et al (2001) reported the aerobic oxidation of
cyclohexane over mesoporous iron-titania catalyst. The reaction showed 90%
selectivity to cyclohexanol. Rajuet al (2008) reported supported Ni catalysts
for the aerobic oxidation of ethylbenzene. They concluded that Ni based
systems were active for the sidechain oxidation of ethylbenzene and the
formation of products was anchored by acidity of the catalysts.
Selective oxidation of ethylbenzene catalyzed by fluorinated
metalloporphyrins with molecular oxygen (Li et al 2007) gave 94%
acetophenone with a turnover number of 2719 under mild conditions. Solvent
plays an important role in the liquid phase reactions (Mal &Ramasamy 1996,
Jha et al 2006). However, the use of solvent also led to environmental
problems. Guo et al (2003) reported solvent free aerobic oxidation of
cyclohexene. Tusar et al (2011) reported solvent free oxidation of alkyl
aromatics to aromatic ketones using molecular oxygen. Zhan et al (2007),
Tian et al (2004), Araujo et al (2003) and Devika et al (2012) reported a
variety of metal incorporated AlPO-5 molecular sieves for the selective
oxidation of organic molecules. Devika et al (2011) interpreted the
paramagnetic behavior of Ce3+ ions in CeAlPO-5 molecular sieves and
108
oxygen chemisorption behavior in the selective oxidation of ethylbenzene. In
the lanthanide family, praseodymium the successor of cerium has two lone
pair electrons in the 4f shell which couldexhibit better interaction with
molecular oxygen than cerium. Since Pr3+ and Pr4+ are paramagnetic in nature,
they can activate molecular oxygen and thus facilitate the oxidationof
ethylbenzene.
The framework substitution of praseodymium in the molecular
sieves can combine the high activity and selectivity of homogeneous catalysts
with ease of recovery and recycling, which are characteristics of
heterogeneous catalysts. The high surface area is an additional advantage
acquired by framework incorporation of praseodymium into AlPO-5.
Further,the weak and moderate acid sitescreated in PrAlPO-5 framework
aided side chain oxidation rather than ring hydroxylation (Reddy et al 1993,
Mal et al 1995, Chen et al 1996, Selvam&Singh 1995, Chen &Sheldon 1995).
Keeping in mind the advantages of framework incorporation of
praseodymium into AlPO molecular sieves, in the present study PrAlPO-5
with different (Al+P)/Pr ratios in fluoride medium were synthesized and
evaluated their catalytic activity in the liquid phase aerobic oxidation of
ethylbenzene.
4.2 CATALYTIC STUDIES
Solvent free liquid phase aerobic oxidation of ethylbenzene was
carried over PrAlPO-5 molecular sieves in the temperature range 60-140 oC.Acetophenone was found to be the major product (>90%) and 1-
phenylethanol, 2-phenylethanol, phenyl acetaldehyde and phenyl acetic acid
as minor products (Scheme 4.1).
109
Scheme 4.1Aerobic oxidation of ethylbenzene
The influence of reaction parameters such as temperature, reaction
time, substituents and Pr content was also studied. The plausible mechanism
for the reaction is depicted in Scheme 4.2. In this mechanism, Pr3+ is oxidized
to Pr4+ by the chemisorbed oxygen. This then abstracts a hydrogen from the
methylene group of ethylbenzene, thus forming metal hydroperoxide and
phenyl ethyl radical. The formation of 1-phenylethanol is attributed to the
reaction between metal hydroperoxide and phenyl ethyl radical. Further, the
oxygen radical present in Pr4+ abstracts a hydrogen from 1-phenylethanol to
form a tertiary radical which eliminates a molecule of water thus resulting
acetophenone. The reaction was carried out between 60 and 140 oC. There
was practically no reaction in this temperature range in the absence of
catalyst. Further, the reaction did not proceed in nitrogen atmosphere instead
of air atmosphere. This supported the proposed reaction mechanism.
110
Scheme 4.2 Possible pathway for the oxidation of ethylbenzene to acetophenone
4.2.1 Effect of Temperature
Liquid phase oxidation of ethylbenzene in the presence of air was
carried over PrAlPO-5 (25, 50, 75 and 100) catalysts in the temperature range
60-140 oC. The experimental results are presented in Table 4.1. The ethyl
benzene conversion and acetophenone selectivity were found to be maximum
at 120 oC. When the reaction temperature was increased above 120 oC, there
was no significant improvement in the selectivity of 1-phenylethanol whereas
selectivity to others increased appreciably. The acetophenone selectivity also
decreased considerably above this reaction temperature. 2-Phenylethanol,
phenyl acetaldehyde and phenyl acetic acid were formed at 140 oC due to
activation of methyl group. Since the formation of acetophenone from
1-phenylethanol is a rapid reaction, the formation of 1-phenyl ethane-1,2-diol
is ruled out. The competition of additional reaction forming phenyl acetic acid
111
112
suppressed 1-phenylethanol formation, and this decreased the selectivity to
acetophenone. Thus, ethylbenzene conversion remained the same but the
selectivity to acetophenone decreased over PrAlPO-5 catalysts above 120 oC.
4.2.2 Effect of (Al+P)/Pr Ratios
PrAlPO-5 with different (Al+P)/Pr ratios viz., 25, 50, 75 and 100
were used for the aerobic oxidation of ethylbenzene and the results are
presented in Table 4.1. PrAlPO-5 with (Al+P)/Pr ratio 25 showed slightly
higher selectivity to acetophenonethan others at 120 °C. Since Pr content in
this catalyst was high, it could rapidly converted 1-phenylethanol to
acetophenone. The decrease of 1-phenylethanol selectivity also supported this
view. As 1-phenylethanol is a neutral compound, and there is also an
appreciable steric hindrance for adsorption on Pr sites through its –OH group,
it can rapidly diffuse out. Hence,increase of framework Pr content in AlPO-5
could rapidly converted 1-phenylethanol to acetophenone. This study revealed
the dependence of acetophenone selectivity on the framework Pr content in
AlPO-5.
4.2.3 Effect of Reaction Time
The influence of reaction time on conversion and selectivity was
studied between 1 and 10 h over PrAlPO-5 (25) at 120 oC and the results are
depicted in Figure 4.1. The percentage conversion of ethyl benzene increased
from 1 to 6 h reaction time. Though ethylbenzene conversion remained the
same upto 10 h, the selectivity to acetophenone decreased. The concentration
of acetophenone was found to be maximum after 6 h reaction time. It was
presumed that a small amount of acetophenone adsorbed on the acid sites
further oxidized to benzoic acid (Chumbhale et al 2005). Since the selectivity
to acetophenone was found to be maximum at 6 h reaction time, this was
chosen as the optimum condition.
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Figure 4.1 Effect of reaction time on ethyl benzene oxidation
4.2.4 Effect of Substituents
Aerobic oxidation of ethylbenzene over PrAlPO-5 (25) at 120 oC
yielded acetophenone as the major product along with 1-phenylethanol as
minor product. The various substituents in the aromatic ring
ofethylbenzenechanged the electron density around the benzylic hydrogen
atom. As stated already the reaction proceeded via hydrogen abstraction
mechanism. The free radical formed in the proposed mechanism could be
stabilized either by electron releasing or electron withdrawing substituent.
Hence, the electron density around the benzylic hydrogen may not alter the
selectivity to form the respective carbonyl compound. The benzylic oxidation
of various substituted ethylbenzenes was attempted and the results are
presented in Table 4.2. All the substituted compounds exhibited almost
similar selectivity (> 90 %) under the given reaction conditions. Since the free
radical formed at benzylic carbon is resonance stabilized by both electron
114
releasing and electron withdrawing substituents in the para position, this
catalyst is found to be suitable for the oxidation of substituted ethylbenzene
compounds.
Table 4.2 Effect of substituents on benzylic oxidation
S.No. R1 R2 Conversion
(%)a
Selectivity
(%)
Major product
1. CH3 H 95 95 Acetophenone
2. CH3 Cl 94 94 4-Chloroacetophenone
3. CH3 Br 94 95 4-Bromoacetophenone
4. CH3 F 93 95 4-Fluoroacetophenone
5. CH3 I 91 93 4-Iodoacetophenone
6. CH3 NO2 90 94 4-Nitroacetophenone
7. H NO2 91 93 4-Nitrobenzaldehyde
8. CH3 OCH3 94 93 4-Methoxyacetophenone
9. H H 94 93 Benzaldehyde
10. H OCH3 94 94 4-Methoxybenzaldehyde
11. C6H5 H 96 95 Benzophenone
aDetermined by GC-MS
115
4.2.5 Catalyst Recycling
In order to address the problem of leaching of Pr from AlPO-5, 5 mg of fresh catalyst was dissolved in aqua regia and elemental composition was analyzed using ICP-OES. The catalyst recovered from the reaction mixture after 2 h reaction time was washed with 5% dilute nitric acid, dried and dissolved completely in aqua regia. The elemental composition of this solution was also performed using ICP-OES. The results clearly showed that praseodymium content in the catalyst before and after the reaction remained almost the same. Hence it was concluded that praseodymium remained intact in AlPO-5 and well incorporated in the framework. The recovered catalyst after the reaction was washed well with ether and dried at 200 oC. The recycled catalyst was used in the reaction for five times under the same reaction conditions. It was found that the catalyst showed similar activity up to five reaction cycles.
4.3 CONCLUSION
The hydrothermal synthesis of PrAlPO-5 with different (Al+P)/Pr ratios was successfully accomplished in the fluoride medium. The ESR study confirmed the presence of chemisorbed oxygen on the catalyst, which concluded that this catalyst is found suitable for oxidation reactions. The TPR study revealed the absence of free Pr2O3 and PrO2 which confirmed that the oxidation reaction proceeded via chemisorbed oxygen in the aerobic oxidation of ethylbenzene. The decrease of (Al+P)/Pr ratio increased the conversion and selectivity. This correlated the dependence of Pr content in AlPO-5 and reactivity. It is also concluded that the weak and moderately strong acid sites created by the framework incorporation of praseodymium in AlPO-5 favored the side chain oxidation rather than ring hydroxylation. The electron density in the aromatic ring did not influence the selectivity to acetophenone. This study also concluded that ethylbenzene and different substituted ethylbenzenes can be effectively oxidized using molecular oxygen as oxidant over PrAlPO-5 at 120 oC.
116
CHAPTER 5
SYNTHESIS OF CAMPHOLENIC ALDEHYDE
FROM -PINENE OVER PrAlPO-5
5.1 INTRODUCTION
-Pinene is a cheap and renewable source for the manufacture of
many fine chemicals (Monteiro &Velos 2004).The oxidation of -pinene
yielded products such as -pinene oxide, verbenol and verbenone, which are
key precursors for the synthesis of artificial flavors, fragrances and drugs
(Chapuis &Jacoby 2001, Calogirou et al 1999, Wender &Mucciaro, 1992).
Coelho et al (2012) used Fe-MCM-41 catalysts for the transformation of -
pinene oxide into various value-added fragrance compounds such as trans-
sobrerol, campholenic aldehyde andtrans-carveol.The product distribution
remarkably depended on the nature of solvent. Ravasio et al (2008) also
reported the isomerisation of -pinene oxide to campholenic aldehyde using
Fe (III) acetylacetonate grafted silica. In this reaction verbenone, trans-
sobrerol and trans-carveol were also formed as minor products. Since
majority of the fragrance compounds are formed by the isomerisation of -
pinene oxide, it is important to selectively oxidize -pinene to -pinene oxide.
Skrobot et al (2003) reported the oxidation of -pinene with
H2O2/ammonium acetate at room temperature and atmospheric pressure,thus
produced -pinene oxide as the main product. -Pinene co-oxidation with
isobutyraldehyde selectively afforded -pinene epoxide, with the selectivity
depended on the amount of NH2 groups on the support and attained 94% at
117
96% alkene conversion. Woitiski et al (2004) reported epoxidation of natural
terpenes using hydrogen peroxide–dinuclear manganese(IV)complex as
oxidant. Eimer et al (2008) reported -pinene oxidation over TS-1 using H2O2
as the oxidizing agent.
Ajaikumar et al (2011) reported the oxidation of -pinene over gold
containing bimetallic nanoparticles supported on reducible TiO2 using
t- butylhydroperoxide (TBHP). The copper–gold containing bimetallic
catalysts were found to be active and selective towards the formation of more
amount of verbenone than other products in the oxidation of -pinene under
liquid phase condition at mild temperatures. Allal et al (2003) explained the
allylic oxidation of -pinene to verbenol and verbenone using H2O2 and
t- butylhydroperoxide respectively. Lu et al (2009) obtained -pinene oxide as
the major product over nanosized metal oxides in the aerobic oxidation of
-pinene. TBHP was used as an initiator in the reaction. Kuznetsova et al
(2007) studied the liquid-phase oxidation of -pinene with oxygen in the
temperature range of 70–90 °C in the presence of Pd, Pt, Ru, Rh, and Ir
supported on carbon. They reported that the conversion of -pinene and the
selectivity of the main reaction products, namely, verbenol, verbenone and
-pinene oxidedepended on the nature of the metal, on its oxidation state, the
extent of metal dispersion and on the admixtures introduced into the system.
Maksimchuk et al (2005) carried out aerobic oxidation of -pinene
over cobalt-substituted polyoxometalate supported on amino-modified
mesoporous silicates. Timofeeva et al (2012) studied the -pinene oxidation
using molecular oxygen over vanadium containing nickel phosphate catalyst.
They evaluated the effect of vanadium content on the activity and selectivity
of the catalyst. Patil et al (2007) studied cobalt cation-exchanged zeolite
Ybased catalysts in the epoxidation of -pinene with molecular oxygen in the
pressure range of 20-100 psi usingN,N-dimethylformamide (DMF) as a
118
solvent at 373 K. The best results were obtained using NaCsCoY20 with 47%
-pinene conversion and 61 % epoxide selectivity at 80 psi pressure. The
solvent was observed to play an important role in the epoxidation of
-pinene, and best results were obtained in N,N-dimethylformamide as the
solvent. Lajunen (2001) explained the beneficial effect of
t- butylhydroperoxide in the catalytic epoxidation of -pinene.
It is clear from the literature that the selective oxidation of -pinene
to -pinene oxide and isomerisation of -pinene oxide to campholenic
aldehyde depends on the nature of oxidant, medium of reaction and initiators
used in the reaction. Hence, it is necessary to choose a catalyst which carries
both redox and acid property for the one pot synthesis of campholenic
aldehyde from -pinene. van der Waal et al (1996) reported bi-functional
nature of Ti4+ ions in the liquid phase Meerwein–Ponndorf–Verley (MPV)
reduction of 4-tert-butylcyclohexanone to 4-tert-butylcyclohexanol. The
isolated Ti4+ ions performed the role of both redox and acid sites.
In this chapter the catalytic activity of PrAlPO-5 (75, 100, 150 and
200) was evaluated in the synthesis of campholenic aldehyde from -pinene.
This study illustrated that PrAlPO-5 is not only a redox catalyst but also a
mild Lewis acid catalyst for the formation of isomerised products. The active
sites aided one pot synthesis of campholenic aldehyde from -pinene.
5.2 CATALYTIC STUDIES
Liquid phase aerobic oxidation of -pinene was carried over
PrAlPO-5 molecular sieves in the temperature range of 30-70 oC using
chloroform as the solvent with a view to obtain selective oxidation products.
Campholenic aldehyde was found to be the major product after 12 h reaction
time whereas -pinene oxide, verbenol and verbenone were also identified as
the minor products in the liquid phase oxidation (Scheme5.1).
119
Scheme 5.1 Synthesis of campholenic aldehyde from -pinene
The influence of reaction parameters such as temperature, reaction
time, solvent and Pr content in AlPO-5 was also studied. The formation of
campholenic aldehyde exemplified the dual nature of the catalyst. -Pinene
was first converted to epoxide by the chemisorbed oxygen present on the
praseodymium sites of PrAlPO-5. The praseodymium sites also contained
Lewis acidity which in turn isomerised -pinene oxide to campholenic
aldehyde. Based on the product selectivity and the sequence of reaction,
plausible mechanism is proposed as depicted in Scheme 5.2. In this
mechanism molecular oxygen chemisorbs on the praseodymium sites and the
distant oxygen site of chemisorbed oxygen homolytically cleaves the carbon-
carbon double bond to form the radical intermediate (1). The radical
intermediate (1) rearranges to form -pinene oxide. Pr-O (2) formed during
the catalytic epoxidation of -pinene abstracts the allylic hydrogen to form
-pinenyl radical (3). This -pinenyl radical (3) acts upon by the distant
oxygen site of chemisorbed oxygen to form metal -pinene peroxide species
(5). The peroxide species (5) decompose to form -pinene oxy radical (6).
120
Scheme 5.2 Plausible pathway for the oxidation of -pinene and isomerisation of -pinene oxide
121
-Pinene oxy radical (6) abstracts allylic hydrogen of -pinene to form trans-
verbenol and -pinenyl radical (3). The trans-verbenol formed in the reaction
further oxidises by Pr-OH (4) species to form verbenone. This reaction is
essential to generate Lewis acidic Pr sites for the isomerisation of -pinene
oxide to campholenic aldehyde. PrAlPO-5 co-ordinates with epoxide oxygen
to form a carbocation (7). This six membered ring carbocation rearranges to
form less stable five membered ring carbocation (8). The campholenic
aldehyde is formed by the rearrangement of carbocation (8). The reaction
pathway is supported by the generation of campholenic aldehyde during the
latter stage of the reaction.
5.2.1 Effect of Temperature
-Pinene undergoes rapid and non-selective auto-oxidation without
any catalyst above 100 oC (Lajunen 2001). Liquid phase oxidation of
-pinene in the presence of air was carried over PrAlPO-5 (75, 100, 150 and
200) catalysts in the temperature range of 30-70 oC using chloroform as the
solvent. The experimental results are presented in Table 5.1. The -pinene
conversion and campholenic aldehyde selectivity increased with increase of
temperature. The selectivity to -pinene oxide decreased with increase of
temperature whereas selectivity to verbenol increased slightly. The reaction
temperature was maintained at 70 oC for 12 h to obtain the highest -pinene
conversion with high selectivity to campholenic aldehyde. It is concluded that
70 oC is the optimum temperature for the synthesis of campholenic aldehyde
from -pinene.
5.2.2 Effect of (Al+P)/Pr Ratios
PrAlPO-5 with different (Al+P)/Pr ratios viz. 75, 100, 150 and 200
were attempted for campholenic aldehyde synthesis and the results are
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presented in Table 5.1. PrAlPO-5 with (Al+P)/Pr ratio 75 showed higher
selectivity to campholenic aldehyde than others at 70°C. Since Pr content in
PrAlPO-5 (75) was high, it possessed large number of Lewis acid sites. The
high concentration of Lewis acid sites increased the isomerisation of -pinene
oxide to campholenic aldehyde. -Pinene conversion also increased with
increase in praseodymium content. Hence, it is concluded that PrAlPO-5 (75)
is better than others for the synthesis of campholenic aldehyde from -pinene.
Table 5.1 Effect of reaction temperature and (Al+P)/Pr ratios on the oxidation of -pinene
Catalyst Temperature(°C)
Conversion(Wt%)
Selectivity (%)
-Pinene oxide Campholenic aldehyde Verbenol Others
20030 16 8 84 5 350 28 4 85 7 470 37 5 85 7 3
15030 27 6 86 5 350 43 3 86 6 470 51 3 87 6 4
10030 41 6 86 5 350 68 4 87 5 470 79 3 89 6 3
7530 59 6 88 4 250 78 4 89 5 270 92 3 90 5 2
Reaction condition: chloroform = 10 ml, 12 h, air (5ml/min)
5.2.3 Effect of Reaction Time
The influence of reaction time on -pinene conversion and
selectivity to -pinene oxide and campholenic aldehyde was studied between
1 and 14 h over PrAlPO-5 (75) at 70 oC using chloroform as the solvent.The
results are depicted in Figure 5.1. The percentage conversion of
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-pinene increased steadily from 1 to 12 h reaction time and thereafter
stabilized. The selectivity to -pinene oxide was found to be maximum upto
5 h reaction time which then underwent slow decomposition over Lewis acid
sites present on PrAlPO-5 (75), thus yielded campholenic aldehyde. After 5 h
reaction time the selectivity to campholenic aldehyde increased steadily at the
expense of -pinene oxide. This observation also supported the proposed
mechanism. It is evidently clear that -pinene oxide is the precursor for the
formation of campholenic aldehyde in this reaction. The selectivity to
campholenic aldehyde below 5 h reaction time is low ( 12 %) compared to
epoxide selectivity. Since the concentration of -pinene is maximum during
the initial stage of the reaction, it is oxidized by chemisorbed oxygen. When
the concentration of -pinene oxide reached considerable amount it is acted
upon by Lewis acid sites on PrAlPO-5 to form campholenic aldehyde.
Figure 5.1 Effect of reaction time
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5.2.4 Effect of Solvents
Aerobic oxidation of -pinene was carried out at 70 °C over
PrAlPO-5 (75) using different solvents and the results are presented in Table
5.2. Among the solvents, chloroform and dichloroethane showed good
selectivity to campholenic aldehyde. Although polar solvents like acetonitrile,
acetone, dimethylacetamide enhanced the formation of -pinene oxide, these
solvents favored the isomerisation of -pinene oxide to give products like
trans-sobrerol, trans-pinocarveol and trans-carveol. Though acetonitrile
enhanced the conversion of -pinene to -pinene oxide, it decreased the
selectivity of campholenic aldehyde. trans-Pinocarveol, trans-sobrerol and
trans-carveol were also formed along with the desired product. trans-Carveol
was formed along with campholenic aldehyde when dimethylacetamide was
used as the solvent. From this study, it is concluded that polarity and basicity
of the solvents played a key role in the isomerisation of -pinene oxide
(Rocha et al 2008).
Table 5.2 Effect of solvents on the oxidation of -pinenea
Solvent Conversion
(%)b
Selectivity (%)
Campholenic
aldehyde
-Pinene
oxide
Others
Chloroform 90 91.4 7.3 1.3
Dichloroethane 81 76.7 16.7 6.6
Acetonitrile 88 37.8 12.4 49.8
Acetone 80 34.9 11.5 53.6
Dimethylacetamide 92 18.4 10.3 71.3aReaction condition: -Pinene (5 mmol), solvent:10 ml, catalyst: PrAlPO-5 (75)
(100 mg), for 12 h at 70oC. b Determined by gas chromatography.
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5.2.5 Structure Identification of Products
The separation of the products was carried out with column
chromatography using neutral alumina (Merck) with ethyl acetate: hexane in
the ratio 1.2:10 as eluent. Nuclear magnetic resonance (NMR) spectra were
recorded on a Bruker (500 MHz) instrument using TMS as internal standard
and CDCl3 as solvent. The 1H-NMR spectra of campholenic aldehyde
(Figure 5.2), -pinene oxide (Figure 5.3) and verbenone (Figure 5.4)
confirmed the structure of the products.
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NMR spectral data:
Campholenic aldehyde
1H NMR (500 MHz, CDCl3): 1.26 (6H, s), 1.76-1.78 (3H, s), 2.18 (1H, m),
2.43-2.47 (2H, dd), 2.56-2.57 (2H, t), 5.60-5.61(1H, t) and 9.73-9.82 (1H, t).
- Pinene oxide
1H NMR (500 MHz, CDCl3): 0.94 (3H, s), 1.29-1.33 (6H, s), 1.72-1.91
(1H, t), 1.87-1.97 (1H, m), 1.97-2.02 (4H, dd) and 2.89-3.05 (1H, t).
Verbenone
1H NMR (500 MHz, CDCl3): 1.02 (3H, s), 1.50 (3H, s), 2.02 (3H, s), 2.08-
2.45 (2H, dd), 2.62-2.65 (1H, t), 2.65-2.78 (1H, t) and 5.71 (1H, s).
127
128
129
130
5.2.6 Catalyst Recycling
In order to check the reusability of the catalyst, 5 mg each of fresh
and used catalyst was dissolved in aqua regia and analysed using ICP-OES.
The results showed almost same praseodymium content in the fresh and used
catalyst. The recovered catalyst after the reaction was washed with
acetonitrile and dried at 200 °C. The recycled catalyst used in the reaction
showed almost similar activity upto 5 reaction cycles. This confirmed that
leaching of Pr from AlPO-5 did not happen during the catalytic reaction. It is
confirmed that the catalyst can be used for five reaction cycles without loss of
activity.
5.3 CONCLUSION
PrAlPO-5 catalysts with different (Al+P)/Pr ratios were
successfully synthesized under hydrothermal condition in fluoride medium.
Spectroscopic and other characterization studies evidenced the incorporation
of praseodymium into AlPO-5 framework. The decrease of (Al+P)/Pr ratio
increased the conversion and selectivity to campholenic aldehyde. The
oxidation of -pinene occurred via chemisorbed oxygen on the
praseodymium sites. It is also concluded that Lewis acid sites created by the
framework incorporation of praseodymium in AlPO-5 favored the
isomerisation of -pinene oxide to campholenic aldehyde. These results
confirmed the bi-functional nature of PrAlPO-5. The weakly basic and non-
polar solvents favored the isomerisation of -pinene oxide to campholenic
aldehyde. This study concluded that campholenic aldehyde can be selectively
synthesized from -pinene over bifunctional PrAlPO-5 catalysts at 70 oC
using non-polar solvents.
131
CHAPTER 6
SYNTHESIS OF 5-ARYLIDENE-2,4-THIAZOLIDINEDIONES
OVER FeAlPO-5
6.1 INTRODUCTION
The thiazolidinediones, also known as glitazones, are a class of
drugs used in the treatment of type 2 diabetes mellitus (Bhat et al 2004).
2, 4-Thiazolidinedione (TZD) derivatives gained significant importance as
they possess various biological activities such as antimicrobial (Bonde &
Gaikwad 2004), cytotoxic (Patil et al 2010), antimalarial (Sunduru et al 2009),
antihyperglycemic (Cantello et al 1994a) and neuroprotective (Youssef et al
2010). The typical synthesis of these 2,4-thiazolidinedione analogues was
achieved by Knoevenagel condensation of aromatic aldehydes in the presence
of organic bases like triethylamine, pyridine, piperidine and more (Bruno et al
2002, Zask et al 1990, Cantello et al 1994b). These low molecular weight
organic amines are known to exhibit toxic effect to the human beings. Since
the reaction is catalyzed by both acid and base, it could be possible to
synthesis TZD derivatives using solid acid catalysts.
Several solid acid catalysts were reported as environmental friendly
catalysts in the synthesis of many organic compounds. Reddy et al (2001)
reported the synthesis of substituted coumarins by the reactions of resorcinol
and substituted resorcinol with ethyl acetoacetate and ethyl
-methylacetoacetate (Pechmann reaction). The production of
environmentally harmful waste streams is minimized by the use of novel solid
132
acid catalyst in this reaction. Huang & Fu (2013) summarized recent advances
in the hydrolysis of cellulose by different types of solid acids such as
sulfonated carbonaceous based acids, polymer based acids and magnetic solid
acids. The acid strength, acid site density, adsorption of the substrate and
micropores of the solid material are the key factors for effective hydrolysis
processes. Shimizu and Satsuma (2011) reviewed the role of solid acid
catalysts in the biomass conversion and Friedel-Crafts acylation reaction.
Saravanamurugan et al (2005) studied the liquid phase Claisen–
Schmidt condensation between 2’-hydroxyacetophenone and benzaldehyde to
form 2’-hydroxychalcone, followed by intramolecular cyclisation to form
flavanone over zinc oxide supported metal oxide catalysts under solvent free
condition. Saravanamurugan et al (2004) also studied the reaction of
2’-hydroxyacetophenone with benzaldehyde over H-ZSM-5, Mg-ZSM-5 and
Ba-ZSM-5 catalysts at 140 °C. The products were 2-hydroxychalcone and
flavanone. They also studied the role of solvents to enhance the conversion.
Recently, Pachamuthu et al (2013) reported Mannich type reactions
over SnTUD-1 materials. SnTUD-1 with interesting Lewis acidity provided
excellent activities in the one-pot three-component Mannich-type reactions of
ketones with aldehydes and amines at room temperature. They reported the
synthesis of a range of -aminocarbonyl compounds in moderate to very good
yields under very mild reaction conditions. However, green synthesis of TZD
derivatives using solid acid catalysts is not reported so far.
Iron incorporated AlPO-5 molecular sieves have unique catalytic
properties and are widely used as promising catalysts in many reactions (Shiju
et al 2006, Hentit et al 2007 and Cheng et al 2012). In this study, the
hydrothermal synthesis of FeAlPO-5 with Fe/Al ratios of 75, 100 and 150
molecular sieves in the presence of fluoride ions were found to be
catalytically active in the synthesis of 5-arylidene-2, 4-thiazolidinedione
133
analogues. Various substituted benzaldehydes were allowed to react with
2, 4-thiazolidinedione in order to understand the effect of electron density in
the synthesis of TZD analogues. It was found that the reusable solid acid
catalyst could replace the role of toxic mineral acids, organic and inorganic
bases in the TZD analogue synthesis.
6.2 CATALYTIC STUDIES
The nature and type of acid sites and chemical location of iron in
FeAlPO-5 were well established by various characterization techniques. The
catalytic activity of FeAlPO-5 was evaluated in the Knoevenagel
condensation of substituted aldehydes with 2,4-thiazolidinedione using water-
ethanol (4:1) mixture as solvent (Scheme 6.1).
Scheme 6.1 Synthesis of 5-arylidene-2,4-thiazolidinedione
Knoevenagel condensation was catalysed by both Bronsted and
Lewis acid sites (Rao & Venkataratnam 1991, Bao et al 1996, Wang et al
1997). The catalytic activity study was focused to design a greener route for
the synthesis of 5-arylidene-2,4-thiazolidinedione. Though the reaction
proceeded faster in polar solvents like dimethyl sulphoxide, acetonitrile and
ethanol, the reaction parameters such as temperature, iron content and
reaction time were optimized in water-ethanol solvent system with a view to
design a greener route for the synthesis of TZD derivatives. The general
mechanism for the condensation of aldehyde with activated methylene group
is depicted in Scheme 6.2.
134
Scheme 6.2 Plausible mechanism for the synthesis of water mediated TZD derivative
The reaction initiates with protonation of carbonyl oxygen of
aldehyde by Bronsted acid sites generated by the interaction of water with
135
Lewis acid sites, thus facilitates the carbonyl carbon of the aldehyde
vulnerable to nucleophilic attack. Similar protonation of 2,4-thiazolidinedione
yields enolic form which in turn attacked the carbonyl carbon. The desired
product is formed by subsequent elimination of water molecule. The reaction
exhibits significant product yield in aprotic solvents as presented in
Scheme 6.3. The carbonyl group can also be activated by Lewis acid sites
itself.
Scheme 6.3 Plausible mechanism – The role of Lewis acid sites in the synthesis of TZD derivative
136
The Lewis acid sites co-ordinate with carbonyl group and facilitate
the nucleophilic attack on the carbonyl carbon. This was confirmed by using
polar aprotic solvent as medium. The aromatic aldehydes are least soluble in
water and hence a small quantity of ethanol was used in the reaction mixture
when solid aromatic aldehydes were used in the reaction. The influence of
electron density in the synthesis of 5-arylidene-2,4-thiazolidinedione was also
studied.
6.2.1 Effect of Temperature and Al/Fe Ratios
Synthesis of 5-benzylidene-2,4-thiazolidinedione was carried in the
liquid phase over FeAlPO-5 (75, 100 and 150) catalysts in the temperature
range 30-100 oC using water-ethanol as solvent for 8 h reaction time. The
conversion of aldehyde increased with increase of temperature while the yield
of 5-benzylidene-2,4-thiazolidinedione was above 95 % at all temperatures.
The reaction was monitored with TLC. After 8 h reaction time the catalyst
was separated from the reaction mixture by filtration, and the organic layer
was extracted with ethyl acetate and dried over anhydrous sodium sulphate.
The organic layer was vaccum distilled and column purified over silica gel of
60-120 mesh using 20 % ethyl acetate in hexane. The isolated product yield
and the amount of unreacted starting material were calculated and presented
in Table 6.1. The reaction reached a maximum conversion of 98.5 % at 100 oC
over FeAlPO-5 (75) catalyst. The FeAlPO-5 (75) possessed larger number of
acid sites than the other two catalysts, which clearly demonstrated that
increase in acidity increased the aldehyde conversion. Hence, it is concluded
that FeAlPO-5 (75) is better than other catalysts for the synthesis of 5-
benzylidene-2,4-thiazolidinedione by Knoevenagel condensation of
benzaldehyde and 2,4-thiazolidinedione. The structural identification of the
compound was confirmed by 1H-NMR.
137
Table 6.1 Effect of reaction temperature and Al/Fe ratios in the synthesis of 5-benzylidene - 2, 4 – thiazolidinedione
Catalyst Temperature
(°C)
Conversion
(Wt %)
Isolated product yield
(Wt %)
P1 Others
150
30 20.8 96.5 3.5
70 41.5 97.3 2.7
100 53.4 97.5 2.5
100
30 41.9 97.7 2.3
70 69.7 98.2 1.8
100 80.7 98.5 1.5
75
30 59.6 98.6 1.4
70 87.5 98.8 1.2
100 98.5 98.8 1.2
Reaction condition: Solvent: Water-ethanol (4:1) = 5 ml & Reaction time: 8 h
P1: 2,4-Thiazolidinedione
6.2.2 Effect of Reaction Time
The influence of reaction time on benzaldehyde conversion and
selectivity to 5-benzylidene-2,4-thiazolidinedione was studied between 1 and
10 h over FeAlPO-5 (75) at 100 oC using water-ethanol as the solvent. The
results are depicted in Figure 6.1. The percentage conversion of benzaldehyde
increased steadily from 1 to 8 h reaction time and then stabilized. The isolated
product yield of 5-benzylidene-2,4-thiazolidinedione was above 95 % till the
end of the reaction time. Beyond 8 h reaction time, there was no further
increase in the percentage conversion of benzaldehyde.
138
Figure 6.1 Effect of reaction time
6.2.3 Effect of Solvents
The nature of solvent played significant role in the synthesis of
5-benzylidene-2,4-thiazolidinedione. The effect of various solvents in the
synthesis was studied and the results are presented in Table 6.2. Polar solvents
like DMSO and DMF chemisorbed on the acid sites of FeAlPO-5, and the
protonated form of DMSO or DMF (Drexler & Amiridis 2003) favored the
polarization of carbonyl group and facilitated the condensation reaction.
Water and ethanol also favored the condensation but not as fast as in the case
of DMSO. Non-polar solvents like chloroform and dichloroethane suppressed
the polarization of carbonyl group and thus retarded the condensation
reaction. Water and ethanol in the ratio 4:1 showed relatively high conversion
than other solvents except DMSO. Though DMSO exhibited better
139
conversion than water-ethanol mixture, it was not chosen as the solvent
system for the synthesis of 5-benzylidene-2,4-thiazolidinedione as this solvent
is not ecofriendly. Water-ethanol (4:1) mixture was opted as solvent since this
could be a greener method for the synthesis of TZD analogues.
Table 6.2 Effect of solvents in the synthesis of 5-benzylidene-2,4- thiazolidinedione
Solvent Conversion
(Wt %)
Product yield (Wt %)
P1 Others
Chloroform# 59.5 97.4 2.6
Dichloroethane# 63.8 95.7 4.3
DMSO *97.8 98.7 1.3
DMF *97.5 98.1 1.9
Water 96.5 98.5 1.5
Ethanol# *97.2 97.1 2.9
Water-Ethanol(4:1) 98.5 98.8 1.2
Reaction condition: Benzaldehyde (5 mmol), 2,4 – thiazolidinedione (5 mmol),
solvent: 5 ml, catalyst: 100 mg of FeAlPO-5 (75), Reaction time: 8 h at 70 oC. #Reaction mixture refluxed, *Maximum conversion reached before 8 h,
P1: 2,4-Thiazolidinedione
6.2.4 Effect of Substituents
The reaction was carried out with various substituted
benzaldehydes using FeAlPO-5 (75) in water-ethanol solvent system at
100 °C to understand the influence of various substituents on the benzene ring
which is also considered as an important parameter in this reaction. The
effect of different substituted benzaldehydes, piperonal and heterocyclic
aldehydes on the conversion and yield of the product are presented in Table
6.3. All p-substituted benzaldehydes showed moderate to high yield of
140
5-arylidene-2,4-thiazolidinedione irrespective of their electron density
whereas o-substituted benzaldehyde showed slightly low product yield
(70%). This is attributed to steric hindrance in the ortho position. All the
p-substituted benzaldehydes showed almost maximum conversion and
product yield upto 98%. The optimized reaction conditions were applied for
the synthesis of 5-((benzo[d][1,3]dioxol-6-yl)methylene)thiazolidine-2,4-
dione by condensing piperonal and TZD over FeAlPO-5 (75). This
condensation yielded 93% product. This compound is used as a precursor for
many biologically active compounds (Chinthala et al 2013). The reaction of
thiophene-2-carbaldehyde and pyridine-3-carbaldehyde with
2,4-thiazolidinedione resulted 95% aldehyde conversion and selectivity
above 90%. It is concluded that FeAlPO-5 catalyst can be
utilized for the synthesis of 5-substituted TZD analogues.
141
142
6.2.5 Structure Identification of Products
The separation of the products was carried out with column chromatography using silica gel of 60-120 mesh (Merck) and ethyl acetate: hexane (2:10) as eluent. Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker (500 MHz) instrument using TMS as an internal standard and DMSO-d6/CDCl3 as the solvent. The chemical shifts are reported in ppm. The 1H-NMR spectra of various 5-arylidene-1,3-thiazolidine-2,4-dione analogues are shown in figures 6.2 to 6.9
NMR spectral data: 5-(4-Nitrobenzylidene)-1-3-thiazolidine-2,4-dione
1H NMR (500 MHz, DMSO - d6): 7.29 (1H, s), 7.63-7.64 (2H, d), 8.36-8.37 (2H, d) and 10.19 (1H, s) 5-(3-Nitrobenzylidene)-1-3-thiazolidine-2,4-dione
1H NMR (500 MHz, CDCl3): 7.27-7.29 (1H, s), 7.70-7.73 (1H, m), 7.82-7.84 (1H, m), 8.30 (1H, m), 8.31-8.39 (1H, m) and 8.89 (1H, s). 5-(3-Methoxybenzylidene)-1,3-thiazolidine-2,4-dione
1H NMR (500 MHz, DMSO - d6): 3.80-3.81 (3H, s), 7.05-7.08 (1H, d), 7.14-7.17 (1H, s), 7.42-7.47 (1H, m), 7.75-7.78 (1H, s) and 12.63 (1H, s)
143
5-(2-Hydroxy-3-methoxybenzylidene)-1-3-thiazolidine-2,4-dione
1H NMR (500 MHz, DMSO - d6): 3.84 (3H, s), 6.92-6.95 (1H, m), 7.08-7.10
(2H, m), 8.05 (1H, s), 9.72 (1H, s) and 12.63 (1H, s)
5-(4-Methoxybenzylidene)-1,3-thiazolidine-2,4-dione
1H NMR (500 MHz, DMSO - d6): 3.34-3.35 (3H, s), 6.91-6.93 (2H, d), 7.45-
7.47 (2H, d), 10.31 (1H, s) and 12.63 (1H, s)
5-((Benzo[d][1,3]dioxol-6-yl)methylene)1,3-thiazolidine-2,4-dione
1H NMR (500 MHz, DMSO - d6): 6.08 (2H, s), 6.90-6.93(1H, d), 6.97 (1H,
s), 7.03-7.06 (1H, d) and 7.67 (1H, s)
144
5-((Thiophen-2-yl)methylene)thiazolidine-2,4-dione
1H NMR (500 MHz, DMSO - d6): 7.13-7.19 (1H, m), 7.61(1H, s), 7.71-7.77
(1H, d) and 7.87-7.93(1H, d)
5-((Pyridin-3-yl)methylene)thiazolidine-2,4-dione
1H NMR (500 MHz, DMSO - d6): (7.33 1H, t), 7.75 (1H, s), 7.88-7.91 (1H,
d), 8.07-8.08 (1H, d), 8.25 (1H, s) and 9.97 (1H, s)
145
146
147
148
149
150
151
152
153
6.3 CONCLUSION
FeAlPO-5 with different Al/Fe ratios were successfully synthesized
under hydrothermal condition in fluoride medium. The isomorphic
substitution of iron in MO4 tetrahedra of AlPO-5 was confirmed by ESR and
DRS-UV-Vis studies. The framework incorporation of Fe created mild acid
sites which were found to be useful in the synthesis of TZD derivatives.
Further, the reaction was carried out in water-ethanol solvent system as a
greener route for the synthesis of TZD analogues although DMSO and DMF
were found to facilitate the condensation more easily. This Lewis acidic
material is also under investigation for the synthesis of ciproflaxacin
analogues.
154
CHAPTER 7
SUMMARY AND CONCLUSION
7.1 SUMMARY AND CONCLUSION OF THE PRESENT WORK
Catalysis is one of the most important tools of green chemistry as it
minimises waste production in chemical reactions. Conventional synthesis of
fine chemicals and pharmaceuticals utilizes stoichiometric equivalents of
homogeneous catalyst such as mineral acids or Lewis acid catalysts. Selective
oxidation reactions are pivotal transformation in organic synthesis which
generates huge amount waste and the disposal is an environmental issue.
Many of these transformations are currently performed both in the laboratory
and industrial scale by the use of hazardous stoichiometric inorganic oxidants
like Cr(VI) and Mn(VI) reagents. These acid and redox catalysts generate
large amount of inorganic wastes. Further, the catalyst separation and reuse,
disposable of the spent catalyst, corrosion and toxicity are series issues which
lead to unfavourable ecological conditions. Solid acids find important
application as heterogeneous catalyst. The surface area of the catalyst is an
important factor because it increases the catalytic activity by exposing more
active centers. Bronsted and Lewis acid sites present act as active sites in
zeolites and zeotype molecular sieves. The linkage of SiO4, AlO4, PO4 and
other cations tetrahedra decides the framework shape and final structure type
of zeolites and zeo-type molecular sieves.
The discovery of aluminophosphate (zeotypes) molecular sieves
(AlPOs) widened the scope to tailormade neutral framework.However, the
155
drawback of AlPO isits neutral zeotype. The incorporation of desirable
heteroatom created active sites for several organic transformations. The
framework incorporated PrAlPO-5 with different (Al+P)/Pr ratios were
synthesized using appropriate precursors for aluminium, phosphorous and
praseodymium in fluoride medium. The catalytic activity was evaluated in the
oxidation of ethylbenzene and synthesis of campholenic aldehyde. Similarly,
iron incorporated AlPO-5 molecular sieves with different Al/Fe ratios were
synthesized and the catalytic activity was evaluated in the synthesis of
biologically active 2,4-thiazolidinedione analogues. The summary and
conclusions drawn from the study are delineated below.
The hydrothermal synthesis of PrAlPO-5 with different (Al+P)/Pr
ratios viz., 25, 50, 75, 100, 150 and 200 was successfully accomplished in
fluoride medium.The characterization using XRD, DRS-UV-vis,BET, and 27Al and 31P MAS-NMR techniques confirmed the incorporation of Pr in
AlPO-5 framework.
The phase purity of PrAlPO-5 molecular sieves was confirmed by
correlating the XRD patterns of PrAlPO-5 andXRD patterns of Pr6O11. The
lattice parameters calculated for PrAlPO-5 were different from the parent
AlPO-5. PrAlPO-5 showed higher surface area and pore volume than AlPO-5.
All these observations confirmed the incorporation of praseodymium into the
framework.The DRS-UV-Vis, ESR and XPS studiesexplained the electronic
environment of praseodymium in PrAlPO-5. The appearance of two bands
around 210 and 262 nm in DRS-UV-Vis spectra of PrAlPO-5 confirmed the
presence of Pr3+ and Pr4+species in tetrahedral environment. Further, the
absence of peak between 415 and 590 nm in PrAlPO-5 confirmed the absence
of extra-framework praseodymium species. The TPR-H2trace of PrAlPO-5
also confirmed the absence of praseodymium oxide species.The low
temperature ESR studies confirmed the chemisorption of oxygen on
156
paramagnetic praseodymium speciesand unsymmetrical environment of
praseodymium. The XPS spectrum of PrAlPO-5 confirmed the existence of
praseodymium in +3 and +4 oxidation states. The incorporation of
praseodymiumsubstitution both Al and P in TO4tetrahedra was evidently
proved from 27Al and 31P MAS-NMR spectra of PrAlPO-5. The SEM and
TEM images established the presence of inter-particle voids in PrAlPO-5. The
TPD-NH3studyand the ex-situ pyridine adsorbed IR spectra revealed the
nature and strength of acidty. Thus, the presence of both redox and acid sites
in PrAlPO-5 was established. TGA and FT-IR spectra of PrAlPO-5 confirmed
the complete removal of template.
The liquid phase aerobic oxidation of ethylbenzene over PrAlPO-5
with different (Al+P)/Pr ratios (25, 50, 75 and 100) demostrated thatthe
conversion of ethylbenzene and selectivity to acetophenone increased in the
order: PrAlPO-5 (25) > PrAlPO-5 (50) > PrAlPO-5 (75) > PrAlPO-5 (100).
Thisstudy also concluded that weak and moderately strong acid sites favoured
side chain oxidation rather than ring hydroxylation. PrAlPO-5 (25) showed
slightly higher selectivity to acetophenone (95%) and ethylbenzene
conversion (95%) at 120 °C. The substituted ethylbenzenes alteredthe electron
density around the benzylic hydrogen atom.Since benzylic oxidation was
ascribed byhydrogen abstraction of the C-H bond, the electron density around
benzylic hydrogen did not alter the selectivity to form the respective carbonyl
compound.
PrAlPO-5 with different (Al+P)/Pr ratios viz., 75, 100, 150 and 200
were attempted in the synthesis of campholenic aldehyde from -pinene. The
catalytic activity of the catalysts increased in the order:PrAlPO-5 (75) >
PrAlPO-5 (100) > PrAlPO-5 (150) > PrAlPO-5 (200). The effect of reaction
parameters such as reaction temperature, (Al+P)/Pr ratio, reaction time and
solvent were optimised over PrAlPO-5 (75) for the synthesis of campholenic
157
aldehyde. The catalyst was found to play dual role both as redox and acid
catalyst. The oxidation of -pinene occurred via chemisorbed oxygen on the
praseodymium sites.The campholenic aldehyde formation was attributed to
the isomerisation of -pinene oxide over Lewis acid sites of PrAlPO-5. This
study concluded that weakly basic and non-polar solvents favoured the
formation of campholenic aldehyde. This study also established the bi-
functional nature of PrAlPO-5 catalysts.
The hydrothermal synthesis of FeAlPO-5 with different Al/Fe
ratios (75, 100 and 150)was successfully accomplished in the fluoride
medium.The characterizationof the materials using XRD, DRS-UV-vis,BET,
SEM, ESR, XPS, TPD-NH3 and ex-situ pyridine adsorbed IR revealed the
physico-chemical characteristics of the materials.
The powder XRD patterns of FeAlPO-5catalysts displayed
characteristic reflections of AlPO-5, which were indexed to P6cc space group.
The increase of unit cell parameter from 11.51 to 11.82Å with increase of Fe
contentwas explained on the basis of atomic radius of Al and Fe. The atomic
radius of Fe3+ (0.49 Å) is larger than Al3+ (0.39 Å), assuming the coordination
number of both the atoms as four.This led to large Fe-O distance which
ultimately changed the unit cell constant of the materials. Moreover, these
observations suggested that Fe atoms were successfully incorporated into
AlPO-5 framework. FeAlPO-5 samples exhibited type I nitrogen adsorption-
desorption isotherms characteristics of microporous material. TheUV-Vis
DRS spectra of FeAlPO-5 samples revealed that most of the Fe atoms in
AlPO-5 framework occupied tetrahedral position, which was further
confirmed from the results obtained from ESR spectroscopy. The TPD-
NH3study and ex-situ pyridine adsorbed IR spectra revealed the presence of
mild Lewis acidic nature in FeAlPO-5.
158
The catalytic activity of FeAlPO-5 catalysts with different Al/Fe
ratios was tested in the Knoevenagel condensation of aromatic aldehydes with
2,4-thiazolidinediones. The catalytic activity of the catalysts increased in the
order: FeAlPO-5 (75) > FeAlPO-5 (100) > FeAlPO-5 (150). The reaction
parameters such as reaction temperature, reaction time and solvent were
optimised with FeAlPO-5 (75) for high conversion and selectivity. The
Knoevenagel condensation of several substituted benzaldehydes and
heterocyclic aldehydes with 2,4-thiazolidinedione under optimised reaction
conditions using water-ethanol (4:1) mixture as solvent was successful. This
study revealed that FeAlPO-5 is a good catalyst for green synthesis of a range
of TZD analogues.
7.2 SCOPE FOR FUTURE WORK
The present investigation results concluded that Pr incorporated
AlPO-5is a convenient eco-friendly redox catalyst for aerobic oxidation of
alkyl aromatics. PrAlPO-5 can be used not only as a redox catalyst but also
solid acid catalyst. The bi-functional nature of PrAlPO-5 catalyst will open
new prospect for potential application in the synthesis of fine chemicals.
Further, it is also confirmed that FeAlPO-5 is an efficient catalyst for the
Knoevenagel condensation. With suitable modifications and reaction
conditions, it is expected these materials can be exploited for other organic
transformations. Synthesis of ciprofloxacin analogues using FeAlPO-5 is in
the pipeline.
159
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LIST OF PUBLICATIONS
1. Sundaravel, B, Babu, CM, Palanisamy, B, Palanichamy, M, Shanthi, K & Murugesan, V 2013, ‘Praseodymium incorporated AlPO-5 molecular sieves for aerobic oxidation of ethylbenzene’, Journal of Nanoscience and Nanotechnology, Vol. 13, pp. 2507-2516.
2. Sundaravel, B, Babu, CM, Palanisamy, B, Palanichamy, M & Murugesan, V 2014, ‘Green synthesis of 5-arylidene-2,4-thiazolidinediones over FeAlPO-5 catalysts’, Advanced Porous Materials (Accepted)
3. Visuvamithiran, P, Sundaravel, B, Palanichamy, M & Murugesan, V 2013, ‘Oxidation of alkyl aromatics over SBA-15 supported cobalt oxide’, Journal of Nanoscience and Nanotechnology, Vol. 13, pp. 2528-2537.
4. Babu, CM, Palanisamy, B, Sundaravel, B, Palanichamy, M & Murugesan, V 2013, ‘A novel magnetic Fe3O4/SiO2 core-shell nanorods for the removal of arsenic’, Journal of Nanoscience and Nanotechnology, Vol. 13, pp. 2517-2527.
5. Venkatachalam, K, Visuvamithiran, P, Sundaravel, B, Palanichamy, M & Murugesan, V 2012, ‘Catalytic performance of Al-MCM-48 molecular sieves for isopropylation of phenol with isopropyl acetate’, Chinese Journal of Catalysis, Vol. 33, pp. 478-486.
6. Devika, S, Sundaravel, B, Palanichamy, M & Murugesan, V 2013, ‘Vapour phase oxidation of toluene over cerium substituted AlPO-5 molecular sieves’, Journal of Nanoscience and Nanotechnology (Accepted).
179
CONFERENCES
1. Sundaravel, B., Visuvamithiran, P., Palanichamy M. and Murugesan, V. “Allylic oxidation of cycloalkene over CeFeAlPO-5”, International Conference on Frontiers in Materials Science for Energy and Environment, LIFE, Loyola College, Chennai, January 11-13, 2012.
2. Sundaravel, B., Palanichamy M. and Murugesan, V. “Liquid phase aerobic oxidation of ethylbenzene over PrAlPO-5 catalysts”, 21st National Symposium on Catalysis for Sustainable Development, CSIR-Indian Institute of Chemical Technology, Hyderabad, February 11-13, 2013.
3. Sundaravel, B., Visuvamithiran, P., Palanichamy M. and Murugesan, V. “Aerobic oxidation of cycloalkene over CeFeAlPO-5”, National Conference on Interface between Chemical Sciences and Technologies, National Institute of Technology, Warangal, December 29-30, 2011.
4. Sundaravel, B., Devika, S., Visuvamithiran, P., Palanichamy M. and Murugesan, V. “Chemoselective oxidation of dialkyl aromatics over Ce-AlPO-5 ”, 15th National Workshop on The Role of New materials in Catalysis, Indian Institute of Technology, Chennai, December 11-13, 2011.
5. Visuvamithiran, P., Sundaravel, B., Palanichamy, M. and Murugesan, V. “Aerobic chemoselective oxidation of alkyl aromatics over Fe3O4@Ag core- shell nanoparticle”, International Conference on Frontiers in Nanoscience and Nanotechnology and their Applications, Panjab University, Chandigarh, February 16-18, 2012.