PREPARATION AND IMMOBILIZATION OF
PALLADIUM(II) AND NICKEL(II) SALEN COMPLEXES
ONTO MCM-41 FROM RICE HUSK FOR
SUZUKI-MIYAURA AND OXIDATION REACTIONS
SALIH HAMZA ABBAS
UNIVERSITI SAINS MALAYSIA
2015
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i
PREPARATION AND IMMOBILIZATION OF
PALLADIUM(II) AND NICKEL(II) SALEN COMPLEXES
ONTO MCM-41 FROM RICE HUSK FOR
SUZUKI-MIYAURA AND OXIDATION REACTIONS
by
SALIH HAMZA ABBAS
Thesis submitted in fulfillment of the requirement for the degree of
Doctor of Philosophy
August 2015
iii
ACKNOWLEDGEMENT
In the Name of Allah, Most Gracious, Most Merciful
Praise is due to Allah Whose worth cannot be described by speakers.
I am pleased to place on record my special gratitude and thanks to the
government of Iraq, the government of Malaysia, University of Basrah and
University Science Malaysia for the financial and technical supports.
My sincere thanks go to my supervisor, Professor Farook Adam for his
guidance, insightful comments and constant support during the experimental part and
in the writing of the thesis.
I would like to thank the staff members of Institute of Postgraduate Studies
(IPS), School of Chemical Sciencess, School of Physics and School of Biological
Sciences, USM for their help in the use of the necessary equipments and analysis.
I would like to give special thanks to my past and present labmates and my
friends for their help, the knowledge sharing and instilling confidence.
Last but not least, my heartfelt gratitude towards my family: particularly my
sisters, my brother, my loving wife and my kids for their continuous love, prayer,
support and patience throughout my PhD study. Thank you all so much.
iv
TABLE OF CONTENTS
Page
DEDICATIONS ............................................................................................... ii
ACKNOWLEDGEMENT .............................................................................. iii
TABLE OF CONTENTS ................................................................................ iv
LIST OF TABLES ........................................................................................... x
LIST OF SCHEMES ...................................................................................... xx
LIST OF SYMBOLS .................................................................................... xxi
LIST OF ABBREVIATIONS ..................................................................... xxiv
ABSTRAK ................................................................................................. xxvii
ABSTRACT ................................................................................................ xxix
CHAPTER 1
INTRODUCTION .......................................................................................... 1
1.1 General entrance....................................................................................... 1
1.2 Heterogenization of homogeneous catalysts ............................................ 2
1.3 Silica as support material ......................................................................... 4
1.3.1 Silica abundance............................................................................... 4
1.3.2 Surface nature of silica ..................................................................... 4
1.3.3 Rice husk ash ................................................................................... 5
1.4 Mesoporous hybrid materials ................................................................... 7
1.4.1 Amorphous silica ............................................................................. 8
1.4.2 Ordered mesoporous silica ............................................................... 8
1.4.3 Synthesis of ordered mesoporous silica ......................................... 10
1.5 Organically functionalized mesoporous materials ................................. 12
1.6 Metal incorporation in mesoporous materials ........................................ 14
v
1.6.1 Incorporation of metals without organic ligands ........................... 15
1.6.1.1 Metal salts as metal precursors .................................................. 15
1.6.1.2 Metal complexes as metal precursors ........................................ 15
1.6.2 Incorporation of metal ions within organic ligands ....................... 16
1.6.2.1 Grafting incorporation (post-synthesis method)........................ 16
1.6.2.1.1 Indirect grafting incorporation ............................................ 17
1.6.2.1.2 Direct grafting method ........................................................ 19
1.6.2.2 One-pot incorporation ............................................................... 21
1.7 Salen Schiff base ligand and metal-salen complex ................................ 22
1.7.1 Incorporation of metal-salen complexes onto mesoporous
materials ......................................................................................... 23
1.7.2 Application of metal-salen in catalysis .......................................... 27
1.7.2.1 Suzuki-Miyaura reaction ........................................................... 31
1.7.2.2 Oxidation of benzyl alcohol ...................................................... 32
1.8 Objectives of the study ........................................................................... 36
1.9 Outline of thesis ..................................................................................... 37
CHAPTER 2
EXPERIMENTAL METHODS .................................................................. 40
2.1 Raw material and chemicals................................................................... 40
2.2 Extraction of silica ................................................................................. 40
2.2.1 Washing of rice husk ...................................................................... 40
2.2.2 Treatment of rice husk with acid .................................................... 41
2.2.3 Rice husk calcination ..................................................................... 41
2.3 Preparation of silica MCM-41 ............................................................... 41
2.4 Functionalization of silica MCM-41 with CPTES ................................. 42
2.5 Synthesis of MCMSalenPd catalyst ....................................................... 43
vi
2.5.1 Synthesis of N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine
palladium(II) complex ................................................................... 43
2.5.2 Immobilization N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine
palladium(II) complex on functionalized silica Cl-MCM-41 ........ 44
2.6 Synthesis of MCMSalenNi catalyst ....................................................... 45
2.6.1 Synthesis of N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine
Schiff base ..................................................................................... 45
2.6.2 Synthesis of N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine
nickel(II) complex ......................................................................... 45
2.6.3 Immobilization N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine
nickel(II) complex on functionalized silica Cl-MCM-41 .............. 46
2.7 Physco-chemical characterization .......................................................... 46
2.7.1 Fourier Transform Infrared Spectroscopy (FT-IR) ........................ 46
2.7.2 Powder X-ray Diffraction (XRD) .................................................. 47
2.7.3 1H,
13C Liquid State NMR Spectroscopy ....................................... 48
2.7.4 13
C, 29
Si CP/MAS NMR Spectroscopy .......................................... 49
2.7.5 Scanning Electron Microscopy-Energy Dispersive (SEM/EDX) .. 49
2.7.6 Transmission Electron Microscopy (TEM) ................................... 49
2.7.7 Nitrogen Sorption Analysis (BET/BJH) ........................................ 50
2.7.8 Atomic Absorption Spectrometry (AAS)....................................... 51
2.8 Catalysts reactions .................................................................................. 52
2.8.1 MCMSalenPd catalyst .................................................................... 52
2.8.1.1 Suzuki-Miyaura reaction ........................................................... 52
2.8.1.2 Gas chromatography and mass spectroscopy analysis (GC &
GC-MS) ..................................................................................... 53
2.8.1.3 Leaching and reusability of MCMSalenPd ............................... 54
2.8.1.4 Oxidation reaction ..................................................................... 55
2.8.1.5 Gas chromatography and mass spectroscopy analysis (GC &
GC-MS) ..................................................................................... 55
vii
2.8.1.6 Leaching and reusability of MCMSalenNi ............................... 56
CHAPTER 3
CHARACTERIZATION OF SALEN LIGAND & METAL-SALEN
COMPLEXES ............................................................................................... 58
3.1 Introduction ............................................................................................ 58
3.2 H2((OH)2-salen) Schiff base ligand ........................................................ 58
3.2.1 Fourier Transform Infrared Spectroscopy (FT-IR) ........................ 58
3.2.2 Liquid State 1H NMR spectroscopy ............................................... 59
3.3 N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine palladium(II)
complex ................................................................................................. 60
3.3.1 Fourier Transform Infrared Spectroscopy (FT-IR) ........................ 60
3.3.2 Liquid state 1H NMR Spectroscopy ............................................... 61
3.3.3 Liquid State 13
C NMR Spectroscopy ............................................. 62
3.4 N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine nickel(II) complex ... 63
3.4.1 Fourier Transform Infrared Spectroscopy (FT-IR) ........................ 63
3.4.2 Liquid State 1H NMR Spectroscopy .............................................. 64
3.4.3 Liquid state 13
C NMR spectroscopy .............................................. 65
CHAPTER 4
RESULT AND DISCUSSION ..................................................................... 67
4.1 Characterization of the ordered silica MCM-41 .................................... 67
4.1.1 Fourier Transform Infrared Spectroscopy (FT-IR) ........................ 67
4.1.2 X-ray Diffraction (XRD)................................................................ 68
4.1.3 Nitrogen sorption studies ............................................................... 69
4.1.4 Electron Microscope (SEM and TEM) .......................................... 70
4.2 Characterization of Cl-MCM-41 ............................................................ 74
4.2.1 Fourier Transform Infrared Spectroscopy (FT-IR) ........................ 74
4.2.2 X-ray Diffraction (XRD)................................................................ 74
viii
4.2.3 Nitrogen sorption studies ............................................................... 75
4.2.4 Electron Microscope (SEM and TEM) .......................................... 76
4.3 Characterization of MCMSalenPd catalyst ............................................ 80
4.3.1 Fourier Transform Infrared Spectroscopy (FT-IR) ........................ 80
4.3.2 X-ray Diffraction (XRD)................................................................ 81
4.3.3 Nitrogen sorption studies ............................................................... 82
4.3.4 Electron Microscope (SEM and TEM) .......................................... 85
4.3.5 13
C solid state NMR ....................................................................... 88
4.3.6 29
Si solid state NMR ...................................................................... 88
4.3.7 AAS and EDX ................................................................................ 89
4.4 Characterization of MCMSalenNi catalyst ............................................ 90
4.4.1 Fourier Transform Infrared Spectroscopy (FT-IR) ........................ 90
4.4.2 X-ray Diffraction (XRD)................................................................ 91
4.4.3 Nitrogen Sorption studies ............................................................... 93
4.4.4 Electron Microscope (SEM and TEM) .......................................... 96
4.4.5 13
C solid state NMR ....................................................................... 99
4.4.6 29
Si solid state NMR ...................................................................... 99
4.4.7 AAS and EDX .............................................................................. 100
CHAPTER 5
CATALYTIC ACTIVITY OF THE CATALYSTS ................................ 102
5.1 Catalytic activity of MCMSalenPd ...................................................... 102
5.1.1 Effect of time and temperature ..................................................... 102
5.1.2 Effect of mass of catalyst ............................................................. 105
5.1.3 Effect of mole ratio of reactants ................................................... 106
5.1.4 Effects of solvents and bases for the Suzuki-Miyaura reaction ... 107
5.1.5 Leaching test of MCMSalenPd catalyst ....................................... 109
ix
5.1.6 Reusability of MCMSalenPd catalyst .......................................... 110
5.1.7 Reaction kinetics .......................................................................... 111
5.1.8 Summary ...................................................................................... 114
5.2 Catalytic activity of MCMSalenNi ...................................................... 114
5.2.1 Effect of time and temperature ..................................................... 115
5.2.2 Effect of mass of catalyst ............................................................. 118
5.2.3 Effect of mole ratio of reactants ................................................... 119
5.2.4 Effects of oxidants for the oxidation of benzyl alcohol ............... 119
5.2.5 Leaching test of MCMSalenNi catalyst ....................................... 120
5.2.6 Reusability of MCMSalenNi catalyst .......................................... 121
5.2.7 Reaction kinetics .......................................................................... 122
5.2.8 The proposed mechanism of oxidation of benzyl alcohol ........... 125
5.2.9 Summary ...................................................................................... 126
CHAPTER 6
CONCLUSION AND RECOMMENDATIONS ..................................... 127
6.1 Conclusion ........................................................................................... 127
6.2 Probable future works .......................................................................... 128
REFERENCES ............................................................................................. 129
APPENDICES .............................................................................................. 143
x
LIST OF TABLES
Page
Table 1.1: Comparison of homogeneous and heterogeneous catalysts. ....................... 3
Table 1.2: Amorphous dispersed systems. ................................................................... 9
Table 1.3: Some Jacobsen catalysts and their applications with different metal ions
and bearing different substituents. .......................................................... 29
Table 1.4: Heterogeneous catalysts for the oxidation of benzyl alcohol. .................. 34
Table 2.1: GC and GC-MS program for identification and confirmation of the
product of Suzuki-Miyaura reaction. ...................................................... 54
Table 2.2: GC and GC-MS program for identification and confirmation of the
product of oxidation of benzyl alcohol. .................................................. 57
Table 4.1: Physicochemical textural parameters of MCM-41. .................................. 73
Table 4.2: Physicochemical textural parameters of Cl-MCM-41. ............................. 79
Table 4.3: Physicochemical textural parameters of MCM-41, Cl-MCM-41 and
MCMSalenPd. ......................................................................................... 87
Table 4.4: Physicochemical textural parameters of MCM-41, Cl-MCM-41 and
MCMSalenNi. ......................................................................................... 98
Table 5.1: The effect of catalyst’s mass, temperature and time for the Suzuki-
Miyaura reaction using MCMSalenPda. ................................................ 104
xi
Table 5.2: Effects of different solvents and bases for the Suzuki-Miyaura cross-
coupling reaction using MCMSalenPd as the catalysta. ........................ 108
Table 5.3: The kinetic parameters for the conversion of bromobenzene using
MCMSalenPd catalyst. .......................................................................... 113
Table 5.4: Effect of the temperature, weight of catalyst, and mmols of oxidant for
the oxidation reaction of benzyl alcohola. ............................................. 117
Table 5.5: The kinetic parameters for the conversion of benzyl alcohol using
MCMSalenNi catalyst. .......................................................................... 124
xii
LIST OF FIGURES
Page
Figure 1.1: The various types of siloxanes and silanols in silica matrix. .................... 5
Figure 1.2: The conversion of RH (a) to RHA (b, c) and their physical appearances.
RHA (b) is a result of incomplete burning resulting black coloured char
(Turmanova et al., 2011). .......................................................................... 7
Figure 1.3: Structures of M41S materials: a) MCM-41 (2D hexagonal, p6mm), b)
MCM-48 (cubic, Ia3d) and c) MCM-50 (lamellar, p2) (Kresge et al.,
1992)........................................................................................................ 10
Figure 1.4: Probable pathways for the templated synthesis of ordered MCM-41 by
structure-directing agents: a) true liquid-crystal template, b) cooperative
liquid-crystal template mechanism (Monnier et al., 1993). .................... 12
Figure 1.5: Three synthesis approaches for mesoporous hybrid materials (Hoffmann
et al., 2006). ............................................................................................. 14
Figure 1.6: Indirect grafting of palladium(II) complex onto silica gel (Sarmah et al.,
2012)........................................................................................................ 18
Figure 1.7: Indirect grafting of nickel(II) complex onto silica MCM-41 (a)
functionalization, b) condensation, c) metal complexation) (Bhunia and
Koner, 2011)............................................................................................ 19
Figure 1.8: Synthesis of copper(II) complex covalently anchored on silica (s = silica)
(Mishra et al., 2012). ............................................................................... 20
xiii
Figure 1.9: Immobilization of palladium complex onto silica gel through Schiff-
base condensation (Sarmah et al., 2013). ................................................ 20
Figure 1.10: Preparation of immobilized catalysts via sol-gel process. ..................... 21
Figure 1.11: Synthesis of salen Schiff base ligand and salen complex (Jacobsen
and Larrow, 1998). .................................................................................. 23
Figure 1.12: Sol-gel immobilization of Co-salen and Cu-salen complexes on silica
gel (Baiker et al.2001). ............................................................................ 24
Figure 1.13 Immobilization of Mn (salen) catalysts via the linkage groups axially
connected to Mn atoms (Zhang et al., 2008). .......................................... 25
Figure 1.14: Different tethered M-Salen-SBA catalysts (M = metal) (Yang et al.,
2011)........................................................................................................ 26
Figure 1.15: Salen Schiff base ligand of Jacobsen catalyst. ...................................... 28
Figure 2.1: Hexagonal pore structure of ordered silica MCM-41. ............................. 48
Figure 2.2: Types of gas sorption isotherms. ............................................................. 51
Figure 3.1: The FT-IR spectrum of H2((OH)2-salen) Schiff base ligand. .................. 59
Figure 3.2: 1H NMR spectrum of H2((OH)2-salen) ligand in DMSO-d6 (300 MHz). 60
Figure 3.3: The FT-IR spectrum of N,Nʹ-bis(4-hydroxysalicylidene)
ethylenediimine palladium(II) complex. ................................................. 61
xiv
Figure 3.4: 1H NMR spectrum N,Nʹ-bis(4-hydroxysalicylidene)ethylenediimine
palladium(II) complex in DMSO-d6 (500 MHz)..................................... 62
Figure 3.5: 13
C NMR spectrum of N,Nʹ-bis(4-hydroxysalicylidene)
ethylenediimine palladium(II) complex in DMSO-d6 (125 MHz). ......... 63
Figure 3.6: The FT-IR spectrum of N,Nʹ-bis(4-hydroxysalicylidene)
ethylenediimine nickel(II) complex complex. ........................................ 64
Figure 3.7: 1H NMR spectrum of N,Nʹ-bis(4-hydroxysalicylidene)
ethylenediimine nickel(II) complex in DMSO-d6 (300 MHz). ............... 65
Figure 3.8: 13
C NMR spectrum of N,Nʹ-bis(4-hydroxysalicylidene)
ethylenediimine nickel(II) complex in DMSO-d6 (125 MHz). ............... 66
Figure 4.1: FT-IR spectrum of silica MCM-41. ......................................................... 68
Figure 4.2: The X-ray diffraction pattern of the prepared MCM-41. ........................ 69
Figure 4.3: The nitrogen adsorption-desorption isotherm of MCM-41. .................... 70
Figure 4.4: The SEM image of MCM-41 at 30,000 magnification. .......................... 71
Figure 4.5: The TEM images of MCM-41 at 60,000 magnification. ......................... 72
Figure 4.6: The X-ray diffraction patterns of Cl-MCM-41. ...................................... 75
Figure 4.7: The nitrogen adsorption-desorption isotherm of Cl-MCM-41. ............... 76
Figure 4.8: The SEM image of Cl-MCM-41 at 30,000 magnification. ..................... 77
Figure 4.9: The TEM images of Cl-MCM-41 at 60,000 magnification..................... 78
xv
Figure 4.10: FT-IR spectrum of Cl-MCM-41, Pd(4-OH)2salen and MCMSalenPd. . 80
Figure 4.11: The X-ray diffraction patterns of MCMSalenPd catalyst. ..................... 81
Figure 4.12: Low angle XRD diffraction patterns of Cl-MCM-41 and
MCMSalenPd catalyst compared to MCM-41. ....................................... 82
Figure 4.13: The nitrogen adsorption-desorption isotherms of MCM-41,
Cl-MCM-41 and MCMSalenPd catalyst. ................................................ 83
Figure 4.14: BJH adsorption pore size distributions for the MCM-41, Cl-MCM-41
and MCMSalenPd catalyst. ..................................................................... 84
Figure 4.15: The SEM image of MCMSalenPd catalyst at 10,000 magnification. ... 85
Figure 4.16: The TEM images of MCMSalenPd catalyst at 60,000 magnification. .. 86
Figure 4.17: The 13
C solid state NMR spectrum of MCMSalenPd catalyst
(100 MHz). .............................................................................................. 88
Figure 4.18: The 29
Si solid state NMR spectrum of MCMSalenPd catalyst (79.4
MHz). ...................................................................................................... 89
Figure 4.19: Certain area, table of element continent and EDX pattern of
MCMSalenPd catalyst. ............................................................................ 90
Figure 4.20: FT-IR spectrum of Cl-MCM-41, Ni(4-OH)2salen and MCMSalenNi. . 91
Figure 4.21: The X-ray diffraction patterns of MCMSalenNi catalyst. ..................... 92
xvi
Figure 4.22: Low angle XRD diffraction patterns of Cl-MCM-41 and
MCMSalenNi catalyst compared to MCM-41. ....................................... 93
Figure 4.23: The nitrogen adsorption-desorption isotherm of MCM-41,
Cl-MCM-41 and MCMSalenNi catalyst. ................................................ 94
Figure 4.24: BJH adsorption pore size distributions for the MCM-41, Cl-MCM-41
and MCMSalenNi catalyst. ..................................................................... 95
Figure 4.25: The SEM image of MCMSalenNi catalyst at 10,000 magnification. .... 96
Figure 4.26: The TEM images of MCMSalenNi catalyst at 60,000 magnification. .. 97
Figure 4.27: The 13
C solid state NMR spectrum of MCMSalenNi catalyst (100
MHz). ...................................................................................................... 99
Figure 4.28: The 29
Si solid NMR spectrum of MCMSalenNi catalyst (79.4 MHz). 100
Figure 4.29: One spot, table of element continent and EDX pattern of
MCMSalenNi catalyst. .......................................................................... 101
Figure 5.1: The effect of the time and temperature on Suzuki-Miyaura reaction
using MCMSalenPd, Reaction conditions: bromobenzene (1 mmol),
phenylboronic acid (1 mmol), base K2CO3 (2 mmol), catalyst (0.05 g,
0.184 mol% pd), DMF:H2O (8:1 mL) solvent. ..................................... 103
Figure 5.2: The variation of conversion with catalyst weight loaded for the
Suzuki-Miyaura reaction with MCMSalenPd as the catalyst................ 105
xvii
Figure 5.3: Effect of mole ratio of reactants for the Suzuki-Miyaura reaction with
MCMSalenPd, Reaction conditions: base K2CO3 (2 mmol), catalyst
(0.05 g, 0.184 mol% pd), DMF:H2O (8:1 mL) solvent, 2 h at 130 oC. . 106
Figure 5.4: Leaching test of MCMSalenPd catalyst. The catalyst was removed
after 0.5 h, subsequent bar charts show the conversion without
catalyst in the reaction. .......................................................................... 110
Figure 5.5: Reusability of MCMSalenPd catalyst, Reaction conditions:
bromobenzene (1 mmol), phenylboronic acid (1 mmol), base K2CO3
(2 mmol), catalyst (0.05 g, 0.184 mol% pd), DMF:H2O (8:1 mL), 2 h
at 130 oC. ............................................................................................... 111
Figure 5.6: The first order rate plots for the conversion of bromobenzene using
MCMSalenPd catalyst at different temperature, R2 is the correlation
coefficient. ............................................................................................. 112
Figure 5.7: Arrhenius plot for the conversion of bromobenzene using
MCMSalenPd catalyst, R2 is the correlation coefficient. ...................... 113
Figure 5.8: The effect of the time and temperature on the oxidation reaction of
benzyl alcohol, Reaction conditions: benzyl alcohol (1.0 mmol),
periodic acid (1.1 mmol), catalyst (0.05 g, 0.132 mol% Ni), acetonitrile
(12 mL). ................................................................................................. 116
Figure 5.9: The variation of conversion with catalyst mass in oxidation of benzyl
alcohol, Reaction conditions: benzyl alcohol (1.0 mmol), periodic acid
(1.1 mmol), acetonitrile (12 mL), 90 min at 90 oC................................ 118
xviii
Figure 5.10: Effect of mole ratio of benzyl alcohol and oxidant, Reaction
conditions: catalyst (0.05 g, 0.132 mol% Ni), acetonitrile (12 mL),
90 min at 90
oC. ..................................................................................... 120
Figure 5.11: Leaching test of MCMSalenNi catalyst. The catalyst was removed
after 15 min, subsequent bar charts show the conversion without the
catalyst in the reaction. .......................................................................... 121
Figure 5.12: Reusability of MCMSalenNi catalyst, Reaction conditions: benzyl
alcohol (1.0 mmol), periodic acid (1.1 mmol), catalyst (0.05 g, 0.132
mol% Ni), acetonitrile (12 mL), 90 min at 90 oC. ................................. 122
Figure 5.13: The pseudo first order rate plots to determine the ka at different
temperature for the conversion of benzyl alcohol using MCMSalenNi
catalyst, R2 is the correlation coefficient. .............................................. 123
Figure 5.14: Arrhenius plot for the conversion of benzyl alcohol using
MCMSalenNi catalyst, R2 is the correlation coefficient. ...................... 124
Figure 5.15: The proposed mechanism of oxidation of benzyl alcohol. .................. 126
Figure A.1: FT-IR spectrum of the obtained biphenyl. ............................................ 143
Figure A.2: 1
H NMR spectrum of obtained biphenyl in CD2Cl2-d2 (500 MHz). ..... 144
Figure A.3: 13
C NMR of obtained biphenyl in CD2Cl2-d2 (125 MHz). ................... 144
Figure A.4: GC chromatograms of Suzuki-Miyaura reaction at 0 h and after 2 h
respectively............................................................................................ 145
xix
Figure A.5: GC-MS chromatogram of Suzuki-Miyaura reaction. ........................... 146
Figure A.6: GC-MS data for the produced biphenyl of Suzuki-Miyaura reaction. . 146
Figure A.7: GC chromatograms of oxidation of benzyl alcohol reaction at 0 min
and after 90 min respectively. ............................................................... 147
Figure A.8: GC-MS chromatogram of oxidation of benzyl alcohol reaction. ......... 148
Figure A.9: GC-MS data of oxidation of benzyl alcohol, (a g)............................. 155
xx
LIST OF SCHEMES
Page
Scheme 1.1: General mechanism for the Suzuki-Miyaura cross-coupling reaction. . 32
Scheme 2.1: The preparation steps of MCM-41. ....................................................... 42
Scheme 2.2: Functionalization of MCM-41 with CPTES linking agent. .................. 43
Scheme 2.3: Immobilization of the metal salen complex on functionalized silica
Cl-MCM-41. ............................................................................................ 44
Scheme 5.1: The general reaction scheme for the Suzuki-Miyaura using the
MCMSalenPd catalyst. .......................................................................... 102
Scheme 5.2: The general reaction scheme for the oxidation of benzyl alcohol
using MCMSalenNi catalyst. ................................................................ 115
xxi
LIST OF SYMBOLS
A Frequency factor
ao Hexagonal unit cell parameter
Co Initial concentration
Ct Concentration at time
cm-1
Wavenumber
d100 Diffraction plane at 100
Ea Activation energy
g Gram
h Hour
Hz Hertz
K Kelvin
k Kilo, (1000)
ka Apparent rate constant
kcal Kilocalorie
kHz Kilo Hertz
kv Kilo volt
L Litter
ln Natural logarithm
log Logarithm
M Molarity
mA Milli amper
min minute
mL Milliliter
xxii
mm Millimeter
mm Hg millimeter of mercury
mmol Millimole
mol Mole
nm Nanometer
o Degree
oC Degree Celsius (degree centigrade)
P/Pο Relative Pressure
Q2
Silicon atom bearing two hydroxyl groups and bonded to two
silicon atoms via oxygen bond
Q3
Silicon atom bearing one hydroxyl groups and bonded to three
silicon atoms via oxygen bond
Q4
Silicon atom not bonded to any hydroxyl group and bonded to
four other silicon atoms via oxygen bond
R Gas constant
T Absolute temperature
T Tesla (unit for measuring magnetic induction)
t Time
Vp Pore volume
Wd Pore diameter
X Halogen
x Unreacted concentration
μL Microliter
λ Wavelenght
δ Chemical shift
xxiv
LIST OF ABBREVIATIONS
a Primary concentration
APTES 3-aminopropyltriethoxysilane
BET Brunauer, Elmmet and Teller method
BJH Barrett, Joyner and Halenda method
ca. Calculated
cm3/g Cubic meter per gram
13C NMR Carbon-13 Nuclear Magnetic Resonance
CPTES 3-(chloropropyl)triethoxysilane
CTAB Cetyltrimethylammonium bromide
CuKα X-ray spectral line for copper bombarded with energy
sufficient to give intense X-ray emission
d Doublet
d6 Deuterated solvent
DMF Dimethylformamide
DCM Dichloromethane
DMSO Dimethyl sulfoxide
EDX Energy Dispersive X–ray
Et3N Triethylamine
etc. et cetera
FAO Food and Agriculture Organization of the United Nations
FID Flame ionization detector
FT-IR Fourier Transmitance Infrared Spectroscopy
xxv
GC Gas Chromatography
GC-MS Gas Chromatography Mass Spectroscopy
H4 Hysteresis loop type 4
1H NMR Hydrogen-1 Nuclear Magnetic Resonance
IUPAC The International Union of Pure and Applied Chemistry
M41S Mesoporous silicas
m2/g Square meter per gram
MAS NMR Magic angle–spinning Nuclear Magnetic Resonance
MCM-41 Mobil Crystalline Material Number 41
PAG Polyethylene glycol
pg page
pH The Hydrogen Function
PMOs Periodic measoporous organoslicas
ppm Part per Million
R Alkyl group
RH Rice husk
RHA Rice husk ash
RMM Rice Market Monitor
Rt Retention time
r.t. Room temperature
s singlet
salen Salicyladimine
SBA-15 Santa Barbara Amorphous Material Number 15
SDAs Structure-directing agents
SEM Scanning Electron Microscopy
xxvi
29Si NMR Sillicon-29 Nuclear Magnetic Resonance
t Triplet
TEM TEM Transmission Electron Microscopy
TEOS Tetraethyl orthosilicate
TMOS Tetramethyl orthosilicate
t-BuOOH tert-butyl hydroperoxide
v/v Volume/ volume
XRD Powder X–ray Diffraction
xxvii
PENYEDIAAN DAN SEKATGERAK KOMPLEKS SALEN PALADIUM(II)
DAN NIKEL(II) KE ATAS MCM-41 DARIPADA SEKAM PADI UNTUK
TINDAK BALAS SUZUKI-MIYAURA DAN TINDAK BALAS
PENGOKSIDAAN
ABSTRAK
MCM-41 daripada abu sekam padi (RHA) telah berjaya disintesis dangan
menggunakan cetiltrimetilammonium bromida sebagai agen pegasal melalui kaedah
sol-gel pada sahu 100 oC. MCM-41 telah difungsikan dengan 3-
kloropropiltrietoksisilana (CPTES) sebagai agen pencantuman dan pepejal yang
diperoleh dilabel sebagai Cl-MCM-41. Ini diikuti dengan dua kali immobilisasi
kompleks N,N'-bis (4-hidroksisalisilidina)etilenadiimina-paladium(II) dan N,N'-bis
(4-hidroksisalisilidina)etilenadiimina-nikel(II) melalui dua kumpulan hidroksil pada
Cl-MCM-41. Dua mangkin heterogen ini telah diperoleh dan dilabel sebagai
MCMSalenPd dan MCMSalenNi. Pencirian kedua-dua mangkin dan
immobilisasinya telah buktikan dengan menggunakan spektrometer infra merah
(FT-IR), Pembelauan Sinar-X (XRD), penyerapan N2, mikroskop transmisi elektron
(TEM), mikroskop elektron imbasan (SEM), resonan nuklear magnet C-13 (13
C
MAS NMR), resonan nuklear magnet Si-29 (29
Si MAS NMR), tenaga serakan sinar-
X (EDX) dan spektroskop serapan atom (AAS). Mangkin MCMSalenPd telah
digunapakai untuk tindak balas Suzuki-Miyaura antara bromobenzena dengan asid
fenilboronik. Mangkin ini sangat stabil dan memberikan aktiviti yang berkesan
dalam masa 2 jam pada 130 oC. Peratusan penukaran adalah 98.7% dengan 100%
kepilihan. Kajian kinetik menunjukkan tindak balas mematuhi hukun kadar pseudo-
pertama. Tenaga pengaktifan adalah 0.393 kcal mol-1
. Mangkin ini telah diguna
semula lebih daripada lima kali tanpa menjejaskan aktiviti pemangkin. Mangkin
MCMSalenNi telah digunakan untuk tindak balas pengoksidaan benzil alkohol
xxviii
dengan asid periodik sebagai agen pengoksidaan. Mangkin ini sangat stabil dan
menunjukkan aktiviti pemangkinan yang berkesan. Peratusan penukaran adalah
98.4% dengan 48.8% kepilihan apabila tindak balas dilakukan untuk 1.5 jam pada
suhu 90 oC. Kajian kinetik menunjukkan hukum kadar pseudo-pertama dan tenaga
pengaktifan adalah 0.0538 kcal mol-1
. Mangkin ini boleh digunakan lebih daripada
enam kali.
xxix
PREPARATION AND IMMOBILIZATION OF PALLADIUM(II) AND
NICKEL(II) SALEN COMPLEXES ONTO MCM-41 FROM RICE HUSK
FOR SUZUKI-MIYAURA AND OXIDATION REACTIONS
ABSTRACT
MCM-41
was successfully prepared from rice husk ash (RHA) using
cetyltrimethylammonium bromide as a directing agent via sol-gel method at 100 oC.
MCM-41 was functionalized with 3-chloropropyltriethoxysilane (CPTES) as a
functioning agent and the solid obtained was labeled as Cl-MCM-41. This was
followed by double immobilization of N,Nʹ-bis(4-
hydroxysalicylidene)ethylenediimine palladium(II) and N,Nʹ-bis(4-
hydroxysalicylidene)ethylenediimine nickel(II) complexes via their two hydroxyl
groups onto Cl-MCM-41. Two new heterogeneous catalysts were obtained and
designated as MCMSalenPd and MCMSalenNi. The two prepared catalysts were
fully characterized and the double immobilization of the two complexes was
confirmed using fourier transform infrared (FT-IR), X-ray diffraction (XRD), N2
sorption measurements, transmission electron microscopy (TEM), scanning electron
microscopy (SEM), solid state C-13 nuclear magnetic resonance (13
C MAS NMR),
solid state Si-29 nuclear magnetic resonance (29
Si MAS NMR), energy dispersive
X-ray elemental analysis (EDX) and atomic absorption spectroscopy (AAS). The
MCMSalenPd catalyst was examined for Suzuki-Miyaura C-C coupling reaction of
bromobenzene with phenylboronic acid. The catalyst had good stability and excellent
activity within 2 hour at 130 oC. The conversion percentage was 98.7% with 100%
selectivity. The kinetic studies showed the reaction followed a pseudo first order rate
law. The activation energy was determined to be about 0.393 kcal mol-1
. The catalyst
was reused for more than five catalytic cycles without significant loss of activity.
The MCMSalenNi catalyst was examined for the oxidation reaction of benzyl
xxx
alcohol using periodic acid as oxidant. The catalyst was found to have good stability
and excellent activity. The conversion percentage was 98.4% with 48.8% selectivity
when the reaction was carried out for 1.5 hour at 90 oC. The kinetic study showed a
pseudo first order rate law and the activation energy was determined to be about
0.0538 kcal mol-1
. The catalyst was reused for more than six catalytic cycles with
good reusability.
1
1 CHAPTER 1
INTRODUCTION
1.1 General entrance
Catalysis is the increase in the rate of a chemical reaction due to the
partnership of a substance called a catalyst. Catalysts have become an even more
important area of chemical research since metals started to be used in 1796 by Von
Marum for dehydrogenation of alcohols. The term ―catalyst‖ was introduced as early
as 1836 by John Jacob Berzelius when he supposed that catalysts had special
abilities that could cause the convergence of chemical substances. Catalysis can be
divided into two types: homogeneous catalysis, in which either a gas (or a liquid)
phase was engaged in both as the catalyst and the reactant and heterogeneous
catalysis, in which the catalysts and the reactant are in different phases (Satterfield,
1991); solid catalysts are used generally with liquids or gaseous reactants.
Broadly speaking, there are three main functions for a substance which acts
as a catalyst; activity, selectivity and stability. The preference between them depends
on the specific demands in a certain chemical process (Hagen, 2006).
The term ―catalyst activity‖ refers to the rate of one or more reactions
continuing to chemical equilibrium in the presence of the catalyst. It depends on
many variable parameters, like pressure, temperature, the mole ratio of reactants and
products and the catalyst quantity. The term ―catalyst selectivity‖ means the extent
to which the catalyst expedites the reaction to shape one or more of the coveted
products. The term ―catalyst stability‖ means its active time in the industrial process;
it depends on several factors, like decomposition, cocking and poisoning. The
2
catalyst damping can be tracked by detecting either its activity or selectivity as a
function of time.
Furthermore, the important considerations which are most concerned about
in the designing of catalysts are that they be economically useful and friendly
towards the environment in their industrial applications. Many of these qualities can
be found in heterogeneous catalyst systems mainly due to their inherent properties;
the heterogeneous catalyst can be recovered easily by a simple filtration process so
the coveted product will be un-contaminated by transition metals or complexes
though some leaching which may occur. In addition, the catalyst usually can be
reused in subsequent reactions. In industrial use, both types of catalysts have
advantages and disadvantages, which are briefly compared in Table 1.1 (Hagen,
2006).
1.2 Heterogenization of homogeneous catalysts
To implement the separation and reusability of the homogeneous catalysts,
lots of approaches have been evolved to immobilize the homogeneous catalysts to
achieve heterogenization. The immobilization of the homogeneous catalyst can be
accomplished by supporting the homogeneous catalyst molecules by different ways
involving suitable inorganic, organic and so called hybrid materials. For example,
this can take place on inorganic support like silica, clay, zeolites and heteropoly
acids (Kuźniarska-Biernacka et al., 2011; Uruş et al., 2013; Yu et al., 2013) and for
organic support like carbon, dendrimers, polymeric ligands and and polyelectrolytes
(Chang et al., 2008; Deraedt et al., 2014; Wang et al., 2012). The theme of
immobilization has been a subject of diverse publications and several prior reviews
3
(Selvam et al., 2012; Wight and Davis, 2002). In this way many homogeneous
catalysts have been immobilized on different supports in order to get a better
understanding of this strategy in recent years (Steiner et al., 2004).
Table 1.1: Comparison of homogeneous and heterogeneous catalysts (Hagen, 2006).
Homogeneous Heterogeneous
Effectivity
Active centers All metal atoms Only surface atoms
Concentration Low High
Selectivity High Lower
Diffusion problems Practically absent Present
Reaction conditions Mild (50–200 oC) Severe (often >250
oC)
Applicability Limited Wide
Activity loss Cluster formation;
poisoning
Crystallites; poisoning
Catalyst properties
Structure/stoichiometry Defined Undefined
Modification possibilities High Low
Thermal stability Low High
Catalyst separation Often laborious: chemical
decomp., distillation,
extraction
Fixed-bed: unnecessary
Suspension: filtration
Catalyst recycling Possible Fixed-bed: unnecessary
Suspension: easy
Cost of catalyst losses High Low
4
1.3 Silica as support material
1.3.1 Silica abundance
A lot of homogeneous catalytic systems have been successfully immobilized
in relation to diverse silica supports. The silica and oxygen are the most abundant
components in the earth’s crust, with about 55% w/w of the earth's surface and its
existence either as silicate SiO3= or as silica with anhydrous formula SiO2 or with
hydrated formula SiO2.xH2O (Scott, 1993). The silica also exists in mineral ores like
quartz and granite and in cereal plants like rice and barley (Unger, 1979).
1.3.2 Surface nature of silica
Studies on silica started in the 1930s and led to the understanding of the
nature of the silica surface (Zhuravlev, 2000). Due to the considerable spectral and
chemical data, it has become clear that silica has two different functional groups, i.e.
the siloxane (≡Si–O–Si≡) in the bulk and numerous forms of silanol groups (≡Si–
OH) on the surface.
According to Nuclear Magnetic Resonance (NMR) studies, there are three
types of siloxane groups (≡Si–O–Si≡) which can be represented by the following
representation Qn = Si(OSi)n(OH)4-n, where n = 2–4 which refers to the number of
bridging bonds (–O–Si) bonded to the central Si atom, i.e. Q2, two siloxane bonds to
the central silicon atom, Q3, three siloxane bonds to the central silicon atom and Q
4,
four siloxane bonds to the central silicon atom as shown in Figure 1.1. Moreover,
experimentally there are two types of silanol groups found on the surface of silica:
the isolated group (a single hydroxyl group bonds to the silicon atom; ≡SiOH),
which is called the vicinal silanol group when it is bonded by an intramolecular
hydrogen bond with other silanol group; and the geminal group (two hydroxyl
5
groups bonded to one silicon atom =Si(OH)2) (Yang et al., 2006; Zhuravlev, 2000).
The surface of silica can be activated using modification reactions with an organic
compound to get new bonds, i.e. (≡Si–O–C≡, ≡Si–C≡, ≡Si–N꞊) through the reaction
of the silanol functional group.
Si
Si
Si O
Si
OSi
O Si
Si
SiOO
OO
O
OO
OHOH
OH
O
OO
O
O
HH
O
O
HH
O
O
H
H
Hydrated surface
Vicinal silanolGeminal silanol
Isolated silanol
Q2 Q3Q4
O OO
Siloxane
Figure 1.1: The various types of siloxanes and silanols in silica matrix.
1.3.3 Rice husk ash
Rice husk (RH), also called rice hull, is the hard outermost layer of rice seeds
which protect the seed during the growing season. It forms on average 22% of the
rice which is separated out during the husking process in the rice mills.
The world’s paddy production in 2014 was 744.4 million metric tonnes of
rice according to the rice market monitor, a report made by the Food and Agriculture
Organization of the United Nations at October 2014 and, consequently, more than
6
150 million metric tonnes of rice husk which are mostly burnt outdoors or thrown on
fallow land. Such cause disposal systems environmental problems. In many cases
the rice husk are ignited resulting in poisons fumes. Since RH is highly resistant to
moisture and fungus, it works as a good insulator (Mohanta et al., 2012).
Rice husk consists of 20% ash, 38% cellulose, 22% lignin, 18% pentose and
2% other organic trace compounds (Adam and Chua, 2004; Adam et al., 2006).
Because of its large silica content (SiO2), it rots slowly. When it is brought back to
the fields, it causes poor food returns and, when burnt, the rice husk gives out 3,410
kcal/kg. Therefore, it represents a cheap source of renewable energy, but when used
for energy generation, it gives about 17-26% ash, higher than the common fuels like
wood 0.2-2% and coal 12.2% (Flörke, 2008).
The rice husk ash (RHA) contains 87-97% of amorphous silica (Della et al.,
2002) and it has a variety of applications in material science, like being used in the
production of Portland cement, supplying a very compact concrete, as an absorbent
for oils and chemicals, a source of silicon, in the ceramics industry and as an
insulation material (Flörke 2008).
A more specialized application is as a natural, perfect source of amorphous
reactive silica, which is employed as an inorganic support for heterogeneous
catalysts (Chumee et al., 2009). Figure 1.2 shows the conversion of yellow rice husk
into white amorphous silica RHA.
7
Figure 1.2: The conversion of RH (a) to RHA (b, c) and their physical appearances.
RHA (b) is a result of incomplete burning resulting black coloured char
(Turmanova et al., 2011).
1.4 Mesoporous hybrid materials
Mesoporous organic-inorganic hybrid materials, a novel type of substance
with large surface areas and a tunable pore size (Fujita et al., 2009), have been
synthesised through the pairing of organic and inorganic constituents by using the
template system. There are three ways to incorporate functionalities; sequent
grafting of organic components onto the matrix of silica, one-pot condensation
reaction of silica as an inorganic component with silylated compound as organic
component and using bissilylated organic components to give periodic mesoporous
organosilicas (PMOs) (Hoffmann et al., 2006).
8
1.4.1 Amorphous silica
In nature, silica is inert and it exists in amorphous and crystalline forms. The
crystalline form comprises of a major portion of soil, sand and other various
minerals like quartz, cristobalite and tridymite (Iler, 1979). Amorphous silica is an
irregular form which is more highly hydrated than its crystalline form and is found
as opal and infusible earth (Gregg and Sing, 1982). Silica exists in various
amorphous forms called the dispersed system as described in Table 1.2.
1.4.2 Ordered mesoporous silica
The improvement of porous materials with high specific surface areas is
actually an attractive research area, especially with respect to prospective applied
fields such as gas stockpiling, sensor technology, adsorption, chromatography and
catalysts. Since 1990, a big breakthrough in material synthesis took place when a
Japanese group of researchers synthesized the first mesoporous nanoparticals of
silica (Kato et al., 1990). Later, in 1992, the researchers of Mobil oil company
developed periodic mesoporous silicate materials called Mobil Crystalline Materials
(M41S) (Beck et al., 1992). These M41S materials subrogated zeolite molecular
sieves which were held on a pore size about of 15Å; however, the M41S materials
posses pore diameters of about 2 10 nm and manifest amorphous pore walls.
According to International Union of Pure and Applied Chemistry (IUPAC)
classification, based on their pore size, microporous materials are classified as
materials whose pore diameters are below 2 nm, while mesoporous materials are
classified as materials whose pore diameters are in the range 2-50 nm and
9
macroporous materials are classified as materials whose pore diameters are above 50
nm.
Table 1.2: Amorphous dispersed systems.
Silica sol (colloidal) It is an important type of amorphous silica, which can be
prepared by sol-gel route (Brinker and Scherer, 1990) by
polycondensation and polymerization reactions of a soluble
silicate by adjusting the pH using an acid or base.
Silica hydrogel It is rigid, coherent and includes three dimensional
network, which can be prepared by proceeding of
condensing reaction until the gelation point.
Silica xerogel It is a partial collapsed gel, which can be made during the
dehydration process of hydrogel and losing its pore filling
liquid (mostly water).
Silica aerogel It is a porous ultralight material with high pore volume,
which can be prepared through supercritical drying process
of a gel and replacing the liquid component with a gas.
Precipitated silica It is dry silica with short distance characteristics structure
(Parfitt and Sing, 1976). It occurs when the ultimate silica
particles are coagulated as loose aggregates in the aqueous
medium.
Pyrogenic silica
(fumed)
It is anhydrous silica which is extremely low in bulk
density and has a high surface area which can be
synthesized by heating the silica at elevated temperatures
(Otto et al., 2008).
10
The structures of M41S materials can be divided into: MCM-41 with a
hexagonal arrangement of the mesopores; MCM-48 with a cubic arrangement of the
mesopores; and MCM-50 with a lamellar structure (Figure 1.3) (Kresge et al., 1992).
Figure 1.3: Structures of M41S materials: a) MCM-41 (2D hexagonal, p6mm), b)
MCM-48 (cubic, Ia3d) and c) MCM-50 (lamellar, p2) (Kresge et al.,
1992).
Eight years after the discovery of M41S materials, researchers from the
University of California in Santa Barbara prepared silica nanoparticles which were
called Santa Barbara Amorphous materials (SBA-15). These also involve a
hexagonal mesoporous arrangement with large surface areas and pore volumes and
act as excellent supports for several applications in chemistry (Zhao et al., 1998a).
1.4.3 Synthesis of ordered mesoporous silica
The synthesis of ordered mesoporous materials has been mentioned vastly in
the literature. A suggestion of the steps for the synthesis of MCM-41 is shown in
Figure 1.4. There are two routes: route (a) (true liquid-crystal templating procedure),
which was proposed initialy by Mobil’s researchers (Beck et al., 1992). The
11
synthesis of hexagonal silicate materials depends on the hydrophilic and
hydrophobic properties of the molecules of the structure-directing agents, mostly
long-chain alkyltrimethylammonium halides like cetyltrimethylammonium bromide
(CTAB). When it is dissolved in water, its molecules will arrange themselves to
form a micelle; the hydrophilic heads form the outside connect with the polar
solvent (i.e. water) and the hydrophobic tails are directed inwards of the micelle
because they are insoluble in polar solvents. These micelles will arrange themselves
together to form a micellar rod. Sequentially over time, these new micellar rods will
arrange to form a hexagonal array of micellar rods in solution. At the time of the
preform process, when adding the silicate source like the tetramethylorthosilicate to
the bulk solution, it will form a coating on the outside of the formed micelle rods.
Route (b) follows another procedure suggested by Monnier et al. (1993), which is
called cooperative liquid-crystal template mechanism. It implies that the silica and
the structure-directing agents are first mixed, and then the surfaces of micelles in the
silica environment are developed from sphere to rod and cluster-driven by non-
covalent weak interactions like hydrogen bonding, van der Waals forces and
electrostatic attraction (Zhao et al., 2012). After the process is completed, the
obtained white product is filtered, washed, dried and the template molecules are
removed by either calcination at 500 to 600 oC (Beck et al., 1992) or solvent
extraction, i.e., by using ethanol, acid ethanolic solution or using the ion-exchange
procedure (usually using NaCl or NH4Cl) (Inagaki et al., 1999) to get the
mesoporous material MCM-41.
The mesoporous material SBA-15 are synthesized by the same pathway, but
the difference is that the SBA-15 is synthesized using non-polar surfactants like
12
polyethylene glycol as template agent in acidic conditions while MCM-41 is
synthesized using CTAB in basic conditions (Zhao et al., 1998a, 1998b).
Figure 1.4: Probable pathways for the templated synthesis of ordered MCM-41 by
structure-directing agents: a) true liquid-crystal template, b) cooperative
liquid-crystal template mechanism (Monnier et al., 1993).
1.5 Organically functionalized mesoporous materials
The amalgamation of the properties of organic and inorganic construction
units within a single material is a most interesting notion for scientists’s estimation
of materials. This would merge an enormous collection of organic functionality with
the advantages of the inorganic substrate giving it robustness and thermal stability.
The participation of organic and inorganic components can produce a united
material whose properties vary comparatively from those of the solo, isolated
components. This is particularly applicable to the heterogenization of the
homogeneous catalysts when the inorganic materials can be modified with organic
functionalities, like alcohols, thiols, amines, carboxylic and sulfonic acids, so that a
real reaction can occur on a stable, hard inorganic matrix (Hoffmann et al., 2006).
13
Recently, more extensive studies have been developed to use hybrid organic-
inorganic materials for different applications, such as optics, sensing,
microelectronics, separation and catalysis.
In terms of the large pore size and high abundance of silanol groups on the
surface of the mesoporous silica, this makes it a serious nominee for supporting
different organic functions which can be located in the channels or in the pore walls.
In this respect, there are three main approaches for the synthesis of porous hybrid
materials depending on the organoalkoxysilane units (Figure 1.5);
1) Grafting approach; which is the subsequent amendments of the inner
walls of the inorganic mesoporous silica.
2) Co-condensation approach (one-pot synthesis); which is a potential direct
synthesis of mesoporous silica phase by the co-condensation of tetraalkoxysilanes
[(RO)4Si; i.e. Tetraethyl orthosilicate (TEOS) or Tetramethyl orthosilicate (TMOS)]
with terminal trialkoxyorganosilanes of the type (RO)3SiR using a structure-
directing agent (SDA).
3) Periodic mesoporous organosilica production approach; this was first
used in 1999 (Inagaki et al., 1999; Melde et al., 1999) by hydrolysis and subsequent
condensation reactions of the bridged organosilica precursors of the type (RO)3Si–
R–Si(OR )3, which had long been known from sol-gel chemistry (Loy and Shea,
1995; Shea and Loy, 2001).
14
Figure 1.5: Three synthesis approaches for mesoporous hybrid materials (Hoffmann
et al., 2006).
1.6 Metal incorporation in mesoporous materials
Metals incorporated in mesoporous materials have been widely reported for
various numerous approaches inflation to catalytic applications. Many different
metals have been inserted in the mesoporous materials, such as B, Al, Ga, In, Ti
(Adam and Ahmed, 2008), and different transition metals, such as Mn, Fe, Co, Ni,
Cu (Adam et al., 2013b; Sayari and Liu, 1997). The following sections will discuss
two types of metal incorporation onto silica:
a) Incorporation of metal ions onto silica without organic ligands.
15
b) Incorporation of metal ions within organic ligands onto silica.
1.6.1 Incorporation of metals without organic ligands
1.6.1.1 Metal salts as metal precursors
The metals incorporated within mesoporous materials can be done by one-
pot method or by post-treatment. In the one-pot method, metal salt is added from the
beginning in the initial gel while the post-treatment can be carried out either by
impregration or by ion-exchange; in both procedures the interaction occurs between
the metal salt and the initially formed mesoporous silica. The resulting mesoporous
materials resulting from one route is not necessarily similar to that of the other. The
post-treatment also can be carried via grafting method. Tilley et al. (2002)
synthesized single-site grafted catalysts using a technique called as precursor route.
(iPrO)Ti[OSi(
tBu)3]3 and Ti[OSi(
tBu)3]4 were used as precursor grafted onto MCM-
41 and SBA-15, respectively (Fujdala, 2003; Nozaki et al., 2002). By utilizing this
approach various mesoporous materials have been obtained with different single-site
metal centers, such as Cr, Fe and V.
1.6.1.2 Metal complexes as metal precursors
In some cases, the right selection of the metal precursor in the post-treatment
can produce a mesoporous material with high metal dispersion. (Bonneviot et al.,
1990) have noticed that when Ni(NH3)x(H2O)(6-2x) complexes are adsorbed on silica,
they loose the aqua ligands through the replacement by ≡SiO- from the silica
surface. Moreover, the water molecules can easily replace the monodentate NH3
ligands through drying and calcination process, forming nickel phyllosilicates and
nickel hydroxide on the mesoporous material (Che et al., 1995). Using a bidentate
16
ligand such as ethylenediamine instead of NH3 can produce more isolated metal
species after calcination with high dispersion on the surface of silica (Sun et al.,
2006).
1.6.2 Incorporation of metal ions within organic ligands
There are four identified methods for anchoring metal complexes onto
mesoporous materials; i) encapsulation, ii) exchanging, iii) post-synthesis (grafting),
and iv) one-pot synthesis. The first two methods are insignificant, with weak
electrostatic interactions between the metal complex and the mesoporous support.
Subsequently, the anchored complexes of these two methods are normally easily
leached through the reaction process. The significant immobilization of complexes
on solid support are usually accomplished by post-synthesis and one-pot synthesis
methods by virtue of the formation of stable covalent bonds between a functional
group (R) and the solid support ≡Si-O-Si-R.
1.6.2.1 Grafting incorporation (post-synthesis method)
There are numerous strategies for grafting metal complexes into the channel
of mesoporous materials to create the active centers in the solid support. The
activation of the mesoporous materials can be achieved efficiently due to their
confinement effect and changeable of the hydrophilic-hydrophobic chemical
environment compared to that of homogeneous complexes, which suffer
deactivation due to their oligomerization probability. Herein, we are tallying the
various grafting strategies to two kinds: direct grafting and indirect grafting.
17
1.6.2.1.1 Indirect grafting incorporation
The grafting of metal complexes can be established indirectly by grafting
organic functional groups onto mesoporous materials such as numerous functioned
propyltrialkoxysilanes to tethered organic moieties. A further modification can be
done by a suitable nucleophilic substitution on the functionalized ligand leading to a
metal complex through complexation with acertain transition metal. However, it is
not possible to be certain whether all of the added metal salt reacts and is
coordinated with the ligand or whether it is just physically adsorbed on the surface
or pores of the support, so it can not be proven that the catalytic activity is facilitated
due to the metal complex or the adsorbed free metal salt.
Herein, are selected examples from literature to illustrate this approach. In
2012, Sarmah et al. reported the indirect grafting of palladium(II) Schiff base
complex onto silica (see Figure 1.6), where the Schiff base has been formed by the
condensation between 3-aminopropyltriethoxysilane (APTES) functionalized silica
gel and acetamide and then developed into a complex with palladium acetate
(Sarmah et al., 2012). The materials obtained have been utilized for the Suzuki-
Miyaura cross-coupling reactions of aryl halides. For the Suzuki-Miyaura reaction,
the general catalyst used is Pd(OAc)2. However this is a homogeneous catalyst
which has all the disadvantages discussed earlier.
18
Figure 1.6: Indirect grafting of palladium(II) complex onto silica gel (Sarmah et al.,
2012).
In 2011, Bhunia and Koner reported the preparation of nickel(II) Schiff base
complex via post-synthesis method. The Schiff base obtained by the condensation of
salicylaldehyde and 3-aminopropyltriethoxysilane (APTES), was anchored on
MCM-41. The activity of the prepared catalyst was examined for epoxidation of
olefins by using tert-butyl hydroperoxide as an oxidant (Bhunia and Koner, 2011)
(Figure 1.7).
19
Figure 1.7: Indirect grafting of nickel(II) complex onto silica MCM-41 (a)
functionalization, b) condensation, c) metal complexation) (Bhunia and
Koner, 2011).
1.6.2.1.2 Direct grafting method
In the direct grafting, the metal complex is first produced at the molecular
level, then it is immobilized as a whole onto the surface of mesoporous materials
through a suitable organic silanes, whether it is inside the molecular construction of
the metal complex or functioning on the surface of the mesoporous materials, as
shown in the examples in Figure 1.8 and Figure 1.9 respectively.
20
Figure 1.8: Synthesis of copper(II) complex covalently anchored on silica (s = silica)
(Mishra et al., 2012).
Figure 1.9: Immobilization of palladium complex onto silica gel through Schiff-base
condensation (Sarmah et al., 2013).
21
1.6.2.2 One-pot incorporation
Successful mesoporous materials containing metal complex in their pore wall
can be synthesized by co-condensation of a silica source such as common
tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) and terminal trialkoxy
organosilanes precursors with structure-directing agents (SDAs) by sol-gel
chemistry. This is shown in Figure 1.10.
Figure 1.10: Preparation of immobilized catalysts via sol-gel process (Lijuan et al.,
2009).
22
1.7 Salen Schiff base ligand and metal-salen complex
In 1864, the German scientist Hugo Schiff described the condensation of an
aldehyde with an amine leading to the production of imine with general chemical
formula R1R2C=N-R3, known as the so-called Schiff base. A wide range of
numerous types of Schiff base ligands can be coordinated with different metals
through the imine nitrogen and another heteroatom, which can be oxygen, nitrogen
or phosphine usually linked to the original aldehyde or ketone molecule to construct
metal complexes (Cozzi, 2004).
The preparation of new metal complexes of Schiff bases is still interesting in
catalyst chemistry. This is due to the ligands being easily prepared, supports
substrate chirality, tuning the located metal, enhancing the solubility and stability.
As a result, they were widely used in homogeneous and heterogeneous catalysts
(Yoon and Jacobsen, 2003).
Salicyladimines (salens) are an example of Schiff base complexes which
have been known since 1933 (Pfeiffer et al., 1933) and which can be formed by the
condensation of salicyladehyde bearing different substituents with a primary 1,2-
diamine (Figure 1.11). The salen complexes have been comprehensively studied,
especially when Jacobsen (Jacobsen et al., 1990) and Katsuki (Katsuki et al., 1990)
detected the enantioselective epoxidation of unfunctionalized alkenes by using
catalysts of chiral Mn-salen complexes and went on to investigate various catalytic
reactions by these types of complexes such as oxidation of hydrocarbons (Lee et al.,
1998), aziridination of alkenes (Omura et al., 2004), Diels-Alder reaction (McGilvra
and Rawal, 2004), hydrolytic kinetic resolution of epoxides (Shin et al., 2004),
alkylation of aldehydes (Maeda et al., 2004) and oxidation of sulfides to sulfoxides
23
(Kim and Rajagopalba, 2003). In this work, the salen ligand was synthesized by
condensation of 4-hdroxysalicyladehyde with 1,2-ethylenediamine and complexed
with palladium and nickel ions.
O
OH
salicylaldehyde
H2N NH2
ethylendiamine
O
N N
O
M
OH
N N
HO
Salen ligand
Salen complex
Alcohol solvent
reflux
Metal salt
reflux
Figure 1.11: Synthesis of salen Schiff base ligand and salen complex (Jacobsen and
Larrow, 1998).
1.7.1 Incorporation of metal-salen complexes onto mesoporous materials
The incorporation of metal-salen complexes onto mesoporous silica can be
accomplished by grafting or by one-pot synthesis methods as referred to the above.
In 2002, Baiker et al. was the first to immobilize the cobalt-salen and copper-salen
complexes on xerogel and aerogel silica by sol-gel approach via co-condensation of
N,N-ethylenebis(salicylidenaminato) (salen) metal complexes with tetraethoxysilane
under acid condition, with the complex possessing two imine bonds with metal
24
without any bridge connection for two nitrogen atoms. The material produced was
used as catalyst for aerobic allylic oxidation of isophorone (Baiker et al., 2001;
Murphy and Baiker, 2002) (Figure 1.12).
Figure 1.12: Sol-gel immobilization of Co-salen and Cu-salen complexes on silica
gel (Baiker et al.2001).
Below are some reported examples for the incorporation of salen complexes
formed by different linkages of the grafting or one-pot synthesis method.
In 2008, Zhang et al. synthesized the mesoporous silica-supported chiral Mn
(salen) catalysts via the linking groups which are axially connected to Mn atoms via
a linkage connected to the salen ligand. The prepared homogeneous and
heterogeneous Jacobsen catalysts were evaluated in the asymmetric epoxidation of
β-methylstyrene with NaClO as an oxidant (Zhang et al., 2008) (Figure 1.13).