V-Mn-MCM-41 catalyst for the vapor phase oxidationof o-xylene

C. Mahendiran • T. Maiyalagan • P. Vijayan •

C. Suresh • K. Shanthi

Received: 4 May 2011 / Accepted: 1 October 2011

� Akademiai Kiado, Budapest, Hungary 2011

Abstract The role of V and Mn incorporated mesoporous molecular sieves was

investigated for the vapor phase oxidation of o-xylene. Mesoporous monometallic

V-MCM-41 (Si/V = 25, 50, 75 and 100), Mn-MCM-41 (Si/Mn = 50) and bime-

tallic V-Mn-MCM-41 (Si/(V ? Mn) = 100) molecular sieves were synthesized by

a direct hydrothermal (DHT) process and characterized by various techniques such

as X-ray diffraction, DRUV-Vis spectroscopy, EPR, and transmission electron

microscopy (TEM). From the DRUV-Vis and EPR spectral study, it was found that

most of the V species are present as vanadyl ions (VO2?) in the as-synthesized

catalysts and as highly dispersed V5? ions in tetrahedral coordination in the calcined

catalysts. The activity of the catalysts was measured and compared with each other

for the gas phase oxidation of o-xylene in the presence of atmospheric air as an

oxidant at 573 K. Among the various catalysts, V-MCM-41 with Si/V = 50

exhibited high activity towards production of phthalic anhydride under the exper-

imental condition. The correlation between the phthalic anhydride selectivity and

the physico-chemical characteristics of the catalyst was found. It is concluded that

V5? species present in the MCM-41 silica matrix are the active sites responsible for

the selective formation of phthalic anhydride during the vapor phase oxidation of

o-xylene.

C. Mahendiran (&)

Department of Chemistry, Anna University of Technology Tirunelveli,

University College of Engineering, Nagercoil Campus, Nagercoil 629004, India

e-mail: cmmagi@gmail.com

T. Maiyalagan

School of Chemical and Biomedical Engineering, Nanyang Technological University,

Singapore 639798, Singapore

P. Vijayan � C. Suresh � K. Shanthi

Department of Chemistry, Anna University, Chennai 25, India

123

Reac Kinet Mech Cat

DOI 10.1007/s11144-011-0383-3

Keywords V and Mn-MCM-41 � Vapor phase � Oxidation � o-xylene �Phthalic anhydride

Introduction

Heterogeneous catalyzed gas phase oxidation plays a vital role in the chemical

industry. In fact, selective oxidation is the simplest functionalization method; in

particular, more than 60% of products synthesized by catalytic routes in the

chemical industry are obtained by oxidation reactions [1]. From the standpoint of

environmental friendliness, much attention has been paid to the development of

metal catalysts for the selective oxidation using molecular oxygen as an oxidant

[2–5]. The oxidative product pthalic anhydride is a commercially important and

versatile intermediate in organic chemistry. The primary use of phthalic anhydride

(PA) is as a chemical intermediate in the production of plastics from vinyl chloride.

Phthalate esters, which function as plasticizers, are derived from phthalic anhydride.

Phthalic anhydride has another major use in the production of polyester resins and

other minor uses in the production of alkyl resins used in paints and lacquers, certain

dyes (anthraquinone, phthalein, rhodamine, phthalocyanine, fluorescein, and

xanthene dyes), insect repellents, and urethane polyester polyols. It has also been

used as a rubber scorch inhibitor [6].

A method for converting naphthalene to phthalic anhydride using sulfuric acid as

the oxidizing agent in the presence of mercury salt as the catalysts was discovered

by E. Sapper, and was patented by the Badische Anilin and Soda Fabrik in 1896.

During the last decade of the nineteenth century, the growing demand for phthalic

anhydride for use in the preparation of xanthene and the indigoid dyes led to

research toward the discovery of cheaper processes for its manufacture [7]. In this

context, Dias et al. [8] found that V2O5 supported TiO2 as an efficient catalyst for

phthalic anhydride production. However, due to its poor mechanical strength of

V2O5/TiO2 and low surface area, attention has been focused on the development of

catalysts with high mechanical strength and high surface area. In this context,

mesoporous MCM-41 molecular sieves with high surface area and tunable pore size

came into existence [9]. The unique physical properties have made these materials

highly desirable for catalytic applications [10, 11]. Isomorphous substitution of

silicon with other elements is an excellent strategy in creating active sites and

anchoring sites for active molecules in the design of new heterogeneous catalyst.

Many metals, e.g., Al, Ti, Mn, Fe, B, Ni and V, have been incorporated into the

silica matrix of MCM-41 [12–15]. Molecular sieves containing redox active metals,

like Ti, V, Cr, Fe, or Co, are increasingly used as heterogeneous catalysts for the

selective oxidation of organic compounds. Among the metals, particularly

vanadium and manganese were found to have remarkable catalytic activity for

the selective oxidation of various organic molecules when incorporated into silicate

molecular sieve [16–21]. In this paper, the vapor phase oxidation of o-xylene to

phthalic anhydride over V-MCM-41, Mn-MCM-41 and V-Mn-MCM-41 catalysts

has been investigated and correlated with the structural, electronic and surface

results obtained.

C. Mahendiran et al.

123

Experimental

Synthesis of V-MCM-41

V-MCM-41 (Si/V = 50) was synthesized by hydrothermal method reported

elsewhere [21] using sodium metasilicate (CDH) as silica source, cetyl trimethyl

ammonium bromide (CTAB, OTTO Chemie) as the structure-directing agent with

the following molar gel composition SiO2:0.02 (VOS4�H2O):0.2 CTAB:0.89

H2SO4:160 H2O. In a typical synthesis, 21.32 g of sodium metasilicate and

0.63 g of vanadyl sulfate monohydrate were dissolved in 60 g of water. The

reaction mixture was stirred for 2 h. Meanwhile, CTAB (5.47 g) was dissolved in

20 g of water. Then, the resultant mixture of sodium metasilicate and vanadyl

sulfate monohydrate was added dropwise into the CTAB solution. The final mixture

was stirred for 1 h. The pH of the gel was adjusted to 10.5–11 using 2 M sulfuric

acid followed by stirring for 3 h. The obtained gel was placed into an autoclave and

heated to 413 K under static conditions for 12 h. The resultant precipitate was

filtered, washed with deionized water and dried in air at 375 K and then finally

calcined at 773 K for 1 h in N2 flow and for 12 h in CO2-free air flow. The catalysts

V-MCM-41 (Si/V = 25, 75,100), Mn-MCM-41 (Si/Mn = 50) and V-Mn-MCM-41

(Si/(V ? Mn) = 100) were also synthesized in a similar manner wherein only the

ratio of vanadyl sulfate monohydrate for vanadium source and manganese acetate

for manganese source was adjusted.

Characterization of the catalysts

Inductively coupled plasma (ICP) optical emission spectroscopy was used for the

determination of the metal content in each sample synthesized above. The

measurements were performed with a Perkin-Elmer OPTIMA 3000 and the sample

was dissolved in a mixture of HF and HNO3 before the measurements. XRD

analysis was performed on Rigaku Miniflex X-ray diffractometer. A germanium

solid state detector cooled in liquid nitrogen with Cu Ka radiation source was used.

The samples were scanned between 0.5� and 8.5� (2h) in steps of 0.02� with the

counting time of 5 s at each point. N2 adsorption studies were carried out to

examine the porous properties of each sample. The measurements were carried out

on a Belsorpmini II (BEL Japan. Inc) instrument. All the samples were pre-treated

in vacuum at 573 K for 12 h in flowing N2 at a flow rate of 60 mL/min. The surface

area and pore size were obtained from these isotherms using the conventional BET

and BJH equation. The coordination environment of vanadium and manganese

containing MCM-41 catalysts was examined by diffuse reflectance UV-vis

spectroscopy. The spectra were recorded between 200 and 800 nm on a Shimadzu

UV-vis spectrophotometer (Model 2450) using BaSO4 as the reference. Further-

more, the coordination environment of vanadium and manganese was confirmed by

EPR (Varian E112 spectrometer operating in the X-band 9.2 GHz frequency) at

room temperature. Transmission electron microscopy (TEM) images were obtained

by using a JEOL electron microscope with an acceleration voltage of 200 kV.

V-Mn-MCM-41 catalyst

123

Experimental procedure for the oxidation of o-xylene

The oxidation of o-xylene was carried out in a fixed bed down flow quartz reactor at

atmospheric pressure in the temperature range of 473–623 K with air flow of

0.02 mol h-1. Prior to the reaction, the reactor packed with 0.3 g of the catalyst was

preheated in a tubular furnace equipped with a thermocouple. The reactant (o-

xylene) was fed into the reactor through a syringe infusion pump at a predetermined

flow rate. The product mixture was collected at the time interval of 1 h and analyzed

by a gas chromatograph (GC-17A, Shimadzu) equipped with a flame ionization

detector. The gaseous products were analyzed by a TCD detector using an SE-30

column. After every run, the catalyst was regenerated to remove the coke deposit,

by passing a stream of pure dry air at a temperature of 773 K for 6 h. The effect of

various parameters, viz., temperature, weight hourly space velocity and time on

stream was studied on the regenerated catalyst.

Results and discussion

XRD

The XRD patterns of calcined V-MCM-41 materials with an atomic ratio of

(Si/V = 100, 75, 50, and 25), Mn-MCM-41 (50), and V-Mn-MCM-41

(Si/(V ? Mn) = 100) recorded at low diffraction angles are shown in Fig. 1 and

its inset. A strong intense peak observed in the 2h range between 2 and 38 for all the

samples is due to the reflection from (100) plane of MCM-41. Apart from this, low

intensity peaks in the 2h range 3–5�, corresponding to the higher order reflections

0 2 4 6 8 102 (Deg.)

Inte

nsit

y (a

.u.)

a

b

c

d

1 3 5 7 9 112 (Deg.)

Inte

nsit

y (a

.u.)

ab

Fig. 1 X-ray diffraction patterns of (a) V-MCM-41 (100), (b) V-MCM-41 (75), (c) V-MCM-41 (50) and(d) V-MCM-41 (25) catalysts. Inset (a) Mn-MCM-41 (50), (b) V-Mn-MCM-41 (Si/(V ? Mn) = 100)

C. Mahendiran et al.

123

such as (110) and (200) planes, were also observed, which confirms the mesoporous

nature of the samples. Higher angle XRD (not shown) does not show any peaks for

extra framework vanadium oxide. The unit cell parameter (a0) calculated using the

formula, a0 = 2d100/H3, and d spacing values obtained using the Bragg’s

equation 2dsinh = nk, where k = 1.54 A for Cu Ka radiation are presented in

Table 1. Upon introduction of V into the MCM-41, a slight decrease in the unit cell

parameter value was observed. However, when the metal content was increased,

the intensity of the diffraction peaks decreased, indicating that it may be due to

structural irregularity of the mesopores at high metal content as reported in

literature [22].

Nitrogen adsorption–desorption isotherms

The adsorption–desorption isotherms of the catalysts V, Mn and V-Mn-MCM-41

are illustrated in Fig. 2. A typical type IV isotherm as defined by IUPAC for

mesoporous material was obtained. The adsorption isotherm exhibits a sharp

increase in the P/Po range from 0.2 to 0.3 which is obviously characteristic of

capillary condensation within mesopores [23]. The P/Po position of the inflection

points is clearly related to the diameter in the mesopore range, and the step

indicates the mesopore size distribution. N2 adsorbed volumes at P/Po = 0.3, for

Si/V = 100, 75, 50, 25, Si/Mn = 50 and Si/Mn ? V = 100 are 350, 330, 315, 295,

275 and 250. The BET surface area, pore volume, and pore diameter, as a function

of V, Mn and V-Mn content are shown in Table 1. The increase in the vanadium

content slightly decreased the surface area, pore volume as well as pore diameter.

From the results, the N2 adsorption studies clearly indicate the successful incor-

poration of V and Mn. When V and Mn are used together, partial amorphization is

Table 1 Physico-chemical characteristics of the catalysts

Catalyst V

content

(wt%)a

Mn

content

(wt%)a

d-

spacing

(A)

Unit cell

parameter

a0 (A)

Surface

area

(m2/g)

Pore

diameter

(A)

Pore

volume

(cm3/g)

Wall

thickness

(A)

V-MCM-41 (25) 1.14 – 40.01 46.20 814 28.10 0.76 18.10

V-MCM-41 (50) 0.58 – 39.60 45.73 893 28.40 0.79 17.33

V-MCM-41 (75) 0.38 – 39.50 45.61 976 28.60 0.81 17.00

V-MCM-41

(100)

0.24 – 39.30 45.38 1,013 28.70 0.82 16.68

V-Mn-MCM-41

Si/(V ? Mn)

= 100

0.57 0.86 41.30 47.69 863 28.48 0.80 19.21

Mn-MCM-41

(50)

– 0.88 40.75 47.06 904 27.75 0.81 19.31

a Results obtained from ICP-AES analysis

V-Mn-MCM-41 catalyst

123

occurs. It may be due to metal oxides blocking the molecular sieves pore or partial

collapse of pore structure.

DR-UV-vis spectroscopy

The DRUV-Vis spectra of V-MCM-41 (Si/V ratio = 25, 50, 75 and 100) catalysts

showed the presence of two shoulder peaks at 260 and the other at 340 nm (Fig. 3a–

d). These correspond to the tetrahedral V5? ions inside the wall and the tetrahedral

V5? ions on the surface of the wall, respectively [24]. The intensity ratio of these

two peaks seems to be relatively high for a catalyst with high vanadium loading. It is

also evident from the spectra that as the ratio of Si/V increased, there is a

corresponding decrease in the intensity of the peaks due to decrease in the number

of vanadium ions. These bands were attributed to the low-energy charge transfer

transition between tetrahedral oxygen ligands and a central V5? ion [25, 26]. Such a

tetrahedral environment was typical for silica matrix V5? ions. A typical spectrum is

Fig. 2 N2 adsorption–desorption isotherms of catalysts (a) V-MCM-41 (100), (b) V-MCM-41 (75),(c) V-MCM-41 (50) and (d) V-MCM-41 (25) Inset (a) Mn-MCM-41 (50), (b) V-Mn-MCM-41 (Si/(V ? Mn) = 100)

C. Mahendiran et al.

123

recorded for the spent V-MCM-41 (50) catalyst (Fig. 3e). There are no absorption

bands and this indicates that V5? species are absent in spent catalyst.

EPR

The EPR spectra of the as-synthesized V-MCM-41 samples with varying values

of Si/V atomic ratio (25, 50, 75, and 100) were recorded at room temperature and

are shown in Fig. 4a–d. The source for the synthesis of vanadium containing

mesoporous materials was vanadyl sulfate (V4?, d1), and all the as-synthesized

V-MCM-41 exhibits its characteristic EPR signal. In comparison, the EPR spectra

of as-synthesized and spent V-MCM-41 (Si/V = 50) are also shown in Fig. 5a, b.

It is interesting to note the absence of the EPR signal in the spent catalyst (Fig. 5b)

which may be the indication for the complete utilization of the V5? species for

oxidation reaction.

TEM

TEM images of the calcined V-MCM-41 samples with Si/V atomic ratios of 100,

75, 50, and 25 are shown in Fig. 6a–d. A highly ordered mesoporous framework

with hexagonal arrays of cylindrical channels of the synthesized samples is

confirmed by TEM images [27]. These are virtually regular hexagonal arrays of fine

pore arrangement existing in these samples. This ordered arrangement, typical for

the MCM-41 materials, confirms the XRD data.

180 280 380 480 580 680

Wavelength (nm)

Abs

orba

nce

(a.u

)

a

b

c

d

e

Fig. 3 DR-UV-vis spectra of catalysts (a) V-MCM-41 (25), (b) V-MCM-41 (50), (c) V-MCM-41 (75),(d) V-MCM-41 (100) and (e) spent V-MCM-41 (50)

V-Mn-MCM-41 catalyst

123

Activity of V-MCM-41 catalyst

The activity of V-MCM-41 (50) catalyst was studied for the vapor phase oxidation

of o-xylene at 573 K with the flow rate of o-xylene 5.87 h-1 (WHSV) and CO2-free

air 0.02 mol h-1 over a period of 7 h. The results are illustrated in Fig. 7. The

percentage conversion and selectivity increased from 1 to 2 h and then decreased up

to 5 h. Beyond 5 h, the catalyst attained steady state activity. The initial increase in

conversion from 1 to 2 h is attributed to oxidation of V4? to V5?, which is necessary

for oxidation. The decrease in trend up to 5 h may be due to some carbon

deposition. Under this steady state reaction conditions, the activities of the catalysts

with varying Si/V ratios are compared.

0 2000 4000 6000 8000

Magnetic field strength (Gauss)

a

b

c

d

Fig. 4 EPR spectra of as-synthesized catalysts (a) V-MCM-41 (25), (b) V-MCM-41 (50), (c) V-MCM-41 (75), (d) V-MCM-41 (100)

Fig. 5 EPR spectra of catalysts (a) as-synthesized V-MCM-41 (50) and (b) V-MCM-41 (50) spent

C. Mahendiran et al.

123

In order to find out the optimum vanadium content, the vapor phase oxidation of

o-xylene was carried out at 573 K on V-MCM-41 catalyst with varying vanadium

content (Si/V ratio 25, 50, 75 and 100) and the results are given in Fig. 8. It is

observed that the conversion o-xylene increases with Si/V ratio till V-MCM-41

(50). Obviously, more vanadium loading can increase o-xylene conversion because

of the increased amount of available active sites. This is revealed from the low

intensity of DRUV-Vis spectral bands of V-MCM-41 (25) catalyst around 260

and 340 nm corresponding to V5? (Fig. 3a, b) compared to that of V-MCM-41 (50).

The optimum ratio is around 50. V-MCM-41 (50) exhibited the maximum catalytic

activity. However, beyond the Si/V ratio 50, there is a decrease in trend observed

with respect to its conversion. This may be because of the lack of dispersion of

vanadium even though available in large quantity. The decrease in conversion at

high Si/V value may be attributed to the decrease in the concentration of V5? active

sites as it is evident from the DRUV-Vis spectra where a decrease in the absorbance

intensity is noticed with increase in Si/V ratio from 50 to 100. Hence, the high

activity of V-MCM-41 (50) may be attributed to the availability of higher number

of V5? in V-MCM-41 (50) than in V-MCM-41 (75) and V-MCM-41 (100).

Fig. 6 TEM pictures of a V-MCM-41 (100), b V-MCM-41 (75), c V-MCM-41 (50), d V-MCM-41 (25)

V-Mn-MCM-41 catalyst

123

The dispersion and amount of V5? become important in order to account for high

conversion of o-xylene. The same trend was registered for the selectivity of phthalic

anhydride. The selectivity of o-toluic acid (OTA) remained reverse trend to that

of phthalic anhydride selectivity; hence it might be considered as the major

intermediate for phthalic anhydride formation as showed in the reaction scheme.

Based on the activity study and characteristics of catalysts, the vapor phase

oxidation of o-xylene is proposed to take place as suggested in reaction scheme

(Scheme 1). According to the scheme, molecular oxygen is activated by framework

vanadium. The activated O2 is inserted between carbon and hydrogen bond of the

methyl group in o-xylene. The resulting alcohol is rapidly oxidized to o-tolaldehyde

which is also subsequently oxidized to o-toluic acid. The same process is also

repeated on adjacent methyl group to yield phthalic acid. The product is

subsequently oxidized to phthalic anhydride.

Comparison of the catalyst supports

The activity of V-MCM-41 (50), Mn-MCM-41 (50) and V-Mn-MCM-41

(V:Mn = 50:50), was measured at 573 K with the WHSV of o-xylene 5.87 h-1

(WHSV). The results are compared under the optimized reaction conditions to

understand the influence of various metals on the oxidation reaction and presented

in Fig. 9. Among the three catalysts, it is the V-MCM-41 (50) catalyst that exhibited

maximum activity. The reason for the high activity of V-MCM-41 (50) may be due

to the availability of silica matrix V5? in MCM-41which is evident from DRUV-Vis

Fig. 7 Effect of reaction time on the oxidation of o-xylene. Reaction conditions: temperature = 573 K,weight of V-MCM-41 (Si/V = 50) = 0.3 g, WHSV = 5.87 h-1 and flow rate of air 0.02 mol h-1

C. Mahendiran et al.

123

V

O

SiSi

Si

O2O

OO

V

O

SiSi

Si

OO

O

CH2

CH3

H

CH2

CH3

OHO O

.

CH3

CH3

CHO

COOHCOOH

COOH

O

C

C

O

O

fast

fast

Repeated

Scheme 1 Vapor phase oxidation of o-xylene to phthalic anhydride

Fig. 8 Effect of Si/V ratio on the oxidation of o-xylene over V-MCM-41. Reaction conditions:temperature = 573 K, catalyst weight = 0.3 g, WHSV = 5.87 h-1 and flow rate of air 0.02 mol h-1;reaction time = 120 min

V-Mn-MCM-41 catalyst

123

spectra (Fig. 3). Further evidence of the elemental analysis results also (Table 1,

ICP-AES) reveals that decrease the Si/V ratios (from 100 to 25) there is increase the

incorporated metal content into the silica matrix. Hence, it is concluded that silica

matrix V5? was shown to be more active for the oxidation of o-xylene to phthalic

anhydride [28]. Manganese (Mn) incorporated into the MCM-41 is expected to

support oxidative dehydrogenation of hydrocarbons because of the presence of

successive acidic and redox sites. However, during the oxidative dehydrogenation

of o-xylene, the strong aromaticity will be lost significantly. Hence, Mn

incorporated MCM-41 does not support the oxidation reaction of o-xylene to

phthalic anhydride under these experimental conditions. Finally, based on the

literature, it can be understood that the poor activity of V-Mn-MCM-41 may be due

to the presence of lower number of silica matrix V5? in V-Mn-MCM-41 catalyst.

Conclusions

From the scrutiny of the above work, the following conclusions can be drawn:

1. Mesoporous V-MCM-41 molecular sieves with Si/V ratio 25, 50, 75 and 100

contains vanadyl ions (VO2?) in the as-synthesized form, whereas on

calcination, vanadyl ions (VO2?) is converted into highly dispersed V5?

species with tetrahedral coordination.

2. Enhancement of the activity of MCM-41 for the vapor phase oxidation of o-

xylene is achieved by incorporating vanadium. The high activity of V-MCM-41

(50) for phthalic anhydride formation could be accounted due to the presence of

large amount of well dispersed V5? on V-MCM-41. Both UV-Vis DRS and

0

20

40

60

80

100

% o

f C

onve

rsio

n &

Sel

ecti

vity

Conversion

Selectivity

V-MCM-41- Mn-MCM-41- V-Mn-MCM-41-(50+50)

Fig. 9 Comparison of activity of the catalysts for the oxidation of o-xylene. Reaction conditions:temperature = 573 K, weight of the catalyst = 0.3 g, WHSV = 5.87 h-1 and flow rate of air0.02 mol h-1; reaction time = 120 min

C. Mahendiran et al.

123

EPR spectroscopies provide valuable information about the surface structure of

V-MCM-41 catalysts.

3. When the activity of vanadium loaded MCM-41 is compared with Mn and

bimetal (V&Mn) loaded MCM-41, it is the vanadium that is the most preferred

metal for oxidation reaction.

Acknowledgments The authors would like to thank the Defence Research and Development

Organization (DRDO) of India for providing financial support.

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