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 Akade ´miai 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 (VO 2? ) in the as-synthesized catalysts and as highly dispersed V 5? 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 V 5? 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: [email protected]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 (2012) 105:469–481 DOI 10.1007/s11144-011-0383-3
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V-Mn-MCM-41 catalyst for the vapor phase oxidation of o-xylene
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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 / Published online: 21 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
Fig. 5 EPR spectra of catalysts (a) as-synthesized V-MCM-41 (50) and (b) V-MCM-41 (50) spent
476 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 477
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
478 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 479
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
480 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.
Reference
1. Horvath IT (2003) Encyclopedia of catalysis, Vol 6, Wiley Interscience Publication, p 141