This document describes a study investigating V and Mn incorporated mesoporous molecular sieves for the vapor phase oxidation of o-xylene. Mesoporous monometallic V-MCM-41, Mn-MCM-41, and bimetallic V-Mn-MCM-41 molecular sieves were synthesized and characterized. Their activity was measured for the gas phase oxidation of o-xylene to phthalic anhydride. Among the catalysts, V-MCM-41 with Si/V = 50 exhibited the highest activity and selectivity towards producing phthalic anhydride under the experimental conditions. The physico-chemical properties of the catalysts, including metal content, surface area,

1. Reac Kinet Mech Cat (2012) 105:469–481
DOI 10.1007/s11144-011-0383-3
V-Mn-MCM-41 catalyst for the vapor phase oxidation
of o-xylene
C. Mahendiran • T. Maiyalagan • P. Vijayan •
C. Suresh • K. Shanthi
Received: 4 May 2011 / Accepted: 1 October 2011 / Published online: 21 October 2011
Ó 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 Á C. Suresh Á K. Shanthi
Department of Chemistry, Anna University, Chennai 25, India
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2. 470 C. Mahendiran et al.
Keywords V and Mn-MCM-41 Á Vapor phase Á Oxidation Á o-xylene Á
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.
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3. V-Mn-MCM-41 catalyst 471
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 (VOS4ÁH2O):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.
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4. 472 C. Mahendiran et al.
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
Intensity (a.u.)
Intensity (a.u.)
a
b
1 3 5 7 9 11
2 (Deg.)
a
b
c
d
0 2 4 6 8 10
2 (Deg.)
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)
123

5. V-Mn-MCM-41 catalyst 473
Table 1 Physico-chemical characteristics of the catalysts
Catalyst V Mn d- Unit cell Surface Pore Pore Wall
content content spacing parameter area diameter volume thickness
(wt%)a (wt%)a ˚
(A) ˚
a0 (A) (m2/g) ˚
(A) (cm3/g) ˚
(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 0.24 – 39.30 45.38 1,013 28.70 0.82 16.68
(100)
V-Mn-MCM-41 0.57 0.86 41.30 47.69 863 28.48 0.80 19.21
Si/(V ? Mn)
= 100
Mn-MCM-41 – 0.88 40.75 47.06 904 27.75 0.81 19.31
(50)
a
Results obtained from ICP-AES analysis
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
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6. 474 C. Mahendiran et al.
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)
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
123

7. V-Mn-MCM-41 catalyst 475
Absorbance (a.u)
a
b
c
d
e
180 280 380 480 580 680
Wavelength (nm)
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)
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.
123

8. 476 C. Mahendiran et al.
a
b
c
d
0 2000 4000 6000 8000
Magnetic field strength (Gauss)
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
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.
123

9. V-Mn-MCM-41 catalyst 477
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)
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).
123

10. 478 C. Mahendiran et al.
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
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
123

11. V-Mn-MCM-41 catalyst 479
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
O O . CH2 H
O O
V O2 V CH2 OH
O O O O CH3
O O
Si Si Si Si CH3
Si Si
fast
CHO
CH3
O
C COOH COOH
Repeated
O
fast
C COOH CH3
O
Scheme 1 Vapor phase oxidation of o-xylene to phthalic anhydride
123

12. 480 C. Mahendiran et al.
100
% of Conversion & Selectivity 80
60
Conversion
40
Selectivity
20
0
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 air
0.02 mol h-1; reaction time = 120 min
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
123

13. V-Mn-MCM-41 catalyst 481
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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