Catalytic activity of vanadium-substituted molybdophosphoric acid supported on titania for the vapor-phase synthesis of isophthalonitrile

Article abstract of DOI:10.1080/24701556.2017.1357575

Different wt% of vanadium-substituted molybdophosphoric acid H4PMo11VO40 loaded on titania (TiO₂) are prepared by wet impregnation method. The catalytic activity of PMoV-supported titania catalyst for the vapor-phase ammoxidation of m-xylene at different temperatures is reported here. The synthesized catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, N₂ adsorption–desorption, thermogravimetric/differential thermal analysis, temperature programmed desorption (TPD), temperature programmed reduction (TPR) scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscopy?(TEM). FTIR studies confirmed that the Keggin structure of PMoV does not change even up to 460°C when loaded on TiO₂ surface. Among all the PMoV-supported TiO₂ catalysts, 1.7% PMoV/TiO₂ showed excellent activity for vapor-phase ammoxidation of m-xylene with 85.4% yield of isophthalonitrile.

Full text of DOI:10.1080/24701556.2017.1357575

Inorganic and Nano-Metal Chemistry
ISSN: 2470-1556 (Print) 2470-1564 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt21
Catalytic Activity of Vanadium Substituted
Molybdophosphoric Acid Supported on Titania for
the Vapor Phase Synthesis of Isophthalodinitrile
R.N. Senapati, P. Dutta, Surjyakanta Rana, K.M. Parida, S. Sahu & A. Sarkar
To cite this article: R.N. Senapati, P. Dutta, Surjyakanta Rana, K.M. Parida, S. Sahu & A. Sarkar
(2017): Catalytic Activity of Vanadium Substituted Molybdophosphoric Acid Supported on Titania
for the Vapor Phase Synthesis of Isophthalodinitrile, Inorganic and Nano-Metal Chemistry, DOI:
10.1080/24701556.2017.1357575
To link to this article: http://dx.doi.org/10.1080/24701556.2017.1357575
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Catalytic Activity of Vanadium Substituted Molybdophosphoric Acid Supported on
Titania for the Vapor Phase Synthesis of Isophthalodinitrile
R.N. Senapati1, P. Dutta*1, Surjyakanta Rana2, K.M. Parida3, S. Sahu4, A. Sarkar4
1 CSIR-Central Institute of Mining & Fuel Research, Dhanbad-828108, India
2University of KwaZulu-Natal, Durban 4000, South Africa
3Centre for Nano Science and nanotechnology, SOA University, Bhubaneswar-751030, India
4 Indian School of Mines University, Dhanbad-826004, Jharkhand, India
*Corresponding address: Dr. P. Dutta, CSIR-Central Institute of Mining & Fuel Research,
Dhanbad-828108, India. Email: pdutta2002in@yahoo.co.in
Abstract
Different wt % of vanadium substituted molybdophosphoric acid (PMoV) loaded on
titania(TiO2) are prepared by wet impregnation method. The catalytic activity of PMoV
supported titania catalyst for the vapour phase ammoxidation of m-xylene at different
temperatures is reported here. The synthesized catalysts were characterized by FT-IR, XRD, N2
adsorption-desorption, TG-DTA, TPD, TPR, SEM, EDX, and TEM. FT-IR studies confirmed
that the Keggin structure of PMoV does not change even up to 460oC when loaded on TiO2
surface. Among all the PMoV supported TiO2 catalysts, 1.7 % PMoV/TiO2 showed excellent
activity for vapour phase ammoxidation of m-xylene with 85.4% yield of isophthalodinitrile.
Keywords:
Heteropolyacid; TiO2; ammoxidation; m-xylene; isophthalodinitrile.
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Introduction
Isophthalodinitrile (IPN) and its derivatives are important chemical intermediates for the
production of various dyes, agro-chemicals, pharmaceuticals, synthetic fibre and pesticides.[1]
There are several methods for the synthesis of nitriles. Among these methods, vapour phase
ammoxidation process is one of the simple and economically viable method.[2,3] For vapour
phase ammoxidation reaction, vanadium and molybdenum are the key components for all the
catalysts.[4-12] The method of preparation and sources of vanadium/molybdenum influences the
[6]
typical catalytic properties of V2O5 and MoO3.
Ray et al reported that V2O5 prepared from
vanadyl oxalate source exhibited much better catalytic activity as compared to V2O5 prepared
from ammonium metavanadate.[6, 8] Hence, vanadia based catalysts of different composition
from various source materials have been prepared from time to time in our group and studied for
ammoxidation reaction.[6-8]
The V2O5-Cr2O3/Al2O3 catalyst system was investigated for ammoxidation of xylene by Ray et
al[6] which showed 58% yield of IPN using feed ratio of m-xylene: ammonia: air of 1:11:14. The
effect of high content of MoO3 in Vanadium Molybdenum Phosphorus Oxide (VMPO) catalyst
was studied for ammoxidation of 3-picoline exhibiting 96% yield by Roy et al.[8] Recently, Sai
Prasad et al reported vanadium incorporated molybdophosphoric acid (MPAV) to prepare Mo-V-
P oxide supported on Al2O3 for ammoxidation of m-xylene with high yield of m-tolunitrile
(82%) and 22% yield of IPN using feed ratio m-xylene: NH3: Air (1:10:40) at temperature 350-
430oC.[12] Scanty work is available on ammoxidation of m-xylene over heteropolyacids as
catalysts. Usually molybdophosphoric acid (PMo12) and vanado-molybdophosphoric acid
(PMoV) catalysts are used for oxidation and ammoxidation reactions.[13-17] The order of activity
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of the support is Nb2O5>ZrO2>Al2O3>SiO2>MgO for ammoxidation reaction.[18] Though PMoV
supported on TiO2 catalyst has been reported earlier but not studied for ammoxidation of m-
xylene. Here in, we report the vanadium incorporated molybdophosphoric acid (PMoV)
supported on TiO2 catalyst for the synthesis of IPN by ammoxidation of m-xylene using very low
ammonia feed ratio.
In the present work, vanadium incorporated molybdophosphoric acid (PMoV) was prepared and
supported on TiO2 by wet impregnation method in order to improve and modify the catalytic
properties, characterized by different techniques and evaluated for ammoxidation of m-xylene.
Experimental
Materials
Analytical reagent grade sodium molybdate, sodium metavanadate, disodium hydrogen
phosphate, diethyl ether and sulphuric acid were procured from CDH India. These reagents were
used without further purification in this investigation.
Preparation of catalyst
Vanadium incorporated molybdophosphoric acid; H4PMo11VO40 (abbreviated as PMoV) was
prepared according to the reported procedure.[19, 20] In brief, separately 3.05 g of sodium
metavanadate was dissolved in 100 ml boiling water and 3.55 g of disodium hydrogen phosphate
was dissolved in 100 ml distilled water to make a clear solution. Both the solutions were mixed
together and cooled. Then the mixture was acidified with 2.5 ml conc. H2SO4 when the colour of
the solution changed to red. To this mixture, 66.5 g sodium molybdate dissolved in 100 ml of
water was added. Finally 45 ml of concentrated sulphuric acid was added slowly with continuous
stirring revealing the change of coloration from dark red to light red. The PMoV was then
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extracted with 200 ml of diethyl ether. After separation of the oily (middle) layer as
heteropolyetherate, a stream of air was passed to make it ether free. The solid that settled down
was dissolved in water, concentrated to the first appearance of crystal in a desiccator over conc.
H2SO4 and then allowed to crystallize further and dried. Then a series of PMoV supported on
o
TiO2 (Aldrich make heated at 500 C) catalysts were prepared by wet impregnation method by
varying the wt% of PMoV from 0.7 to 2.2. The required quantity of PMoV was dissolved in
minimum amount of water and this solution was added to known amount of TiO2 with constant
stirring. Excess water was removed on a hot water bath, and the catalyst masses were dried in air
oven at 120 oC for 8 h. Then the catalysts were calcined at 460 oC for 2 h. Finally catalysts were
sized to 14-18 mesh.
Activity studies
Ammoxidation of m-xylene was carried out at atmospheric pressure in a continuous flow fixed
bed pyrex glass reactor. The reactor was packed with 10 g of catalyst between two layers of
porcelain beads. The upper layer of the porcelain beads served both as a preheater and a mixer
for the reactants. Prior to reaction, ammonia gas was fed to the catalyst bed with a flow rate of 50
o
ml/min at 450 C for 1 h. After bringing the reactor temperature to the required level, the molar
ratio of m-xylene: ammonia: air = 1: 3: 6, was fed into preheated portion of the reactor. m-xylene
was fed into the reactor through a syringe pump (Miclins). Ammonia gas and air were fed
through calibrated flow meters. The reaction temperature was monitored by a thermocouple with
its tip located at middle of the catalyst bed. The reactions were carried out at a temperature of
425 °C. Each experiment was of 2 h duration. The liquid product was collected in a trap kept at -
5 °C. The products were analyzed by gas chromatography, using carbowax 20 M column and
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using thermal conductivity detector (TCD). The analysis of the non-condensable exit gas mixture
was analysed using GC with TCD and it was confirmed that there were no organic species or
oxides of carbon in considerable range.
Physico-chemical characterization
The FTIR spectra of the samples were recorded using Varian FTIR-800 in KBr matrix in the
range of 4000-400 cm-1. Powder X-ray diffraction (PXRD) pattern of the samples were taken in
the range of 10 to 80o at a rate of 1.2o/min (Philips analytical 3710) using Cu Kα radiation. The
BET surface areas of the catalysts were determined by N2 adsorption-desorption at 77 K using
Tristar 3000 surface area analyser (Micromeritics, USA). Known amounts of the samples were
evacuated for 3 h at 150oC to remove physically adsorbed water prior to surface area
measurement. The acid character of the samples was studied by NH3-TPD AutoChem-II,
(Micromeritics) Chemisorptions analyser equipped with a thermal conductivity detector (TCD).
About 1 g of the powdered sample contained in a quartz ‘U’ tube was degassed at 250oC for 1 h
with ultra-pure nitrogen gas. After cooling the sample to room temperature, NH3 (20% NH3
balanced with helium) gas was passed over the sample while it was heated at a rate of 10oC min-1
and the profile was recorded. The H2-TPR of 1.7 % PMoV/TiO2 was carried out in an
AutoChem-II, (Micromeritics) instrument. Thermo gravimetric/differential thermal analysis
(TG/DTA) was performed under air with a Toledo-Metler TG/DTA/851e system at a heating rate
of 5oC min-1. To examine the composition, sizes and morphologies of 1.7 % PMoV supported
titania, SEM with EDX was performed at 15.0kV and at a magnification of 104. The images of
the sample were captured by using a Hitachi S3400N. The transmission electron micrograph of
PMoV supported titania was recorded by using TECNAI-G2 microscope with CCD camera.
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Results and discussion
FT-IR analysis
The FTIR spectra of PMoV, 0.7% PMoV/TiO2, 1.2%PMoV/TiO2, 1.7% PMoV/TiO2, 2.2%
PMoV/TiO2 are shown in Figure 1. The main characteristics bands of Keggin structure are found
in the range of 500-1550 cm-1 related to stretching vibrations of (P-Oa), (Mo-Od), (Mo-Ob-Mo),
(Mo-Oc-Mo) bands.[21-23] The bands at 1055-1100 cm-1 is due to P-O. The band at 1000-900 cm-1
is assigned to Mo-O stretching. The 980 cm-1 is for V-O and 900-700 cm-1 may be due to Mo-O-
Mo. Calcination of the catalyst does not lead to the disappearance of these bands. In fact TiO2
shows intense and broad vibration peaks originated from Ti-O-Ti located at lower wave number
900 cm-1. Vanadium substituted molybdophosphoric acid supported on titania shows two
shoulders corresponding to the P-O vibration at 1068 cm-1 and V-Ot at 959 cm-1. This confirms
the presence of V5+ inside the Keggin unit.[24]
After impregnation of different wt % of vanadium substituted molybdophosphoric acid on the
titania support, it is observed that the bands are shifted towards lower wave number region. For
supported vanadium containing catalysts, practically no change is observed in the IR bands of P-
O and Mo-O stretching modes. The higher wave number of the characteristic bands of the
heteropoly anions is found to be broadened after supporting on titania. The IR spectra revealed
that the vibration spectra of the heteropoly compounds with Keggin structure depend on the
nature of metal incorporated into it. Hence from the IR spectra, it can be concluded that V-Mo-P
are indeed incorporated into Keggin units. The absorption band at about 3200-3300 cm-1 is
stretching vibration of -OH and 1600-1700 cm-1 bending of -OH due to molecular and protonated
water respectively.
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XRD analysis
The X-ray diffraction profiles of different wt% of PMoV supported with titania are presented in
Figure 2. Strong peaks of anatase phase TiO2 is detected in all the XRD spectra. Only small
intense peak from the supported Keggin ions are observed in the spectra[25]. Whereas 2.2%
PMoV/TiO2 contains small intense peak indicating the presence of rutile phase that leads to loss
of surface area as confirmed by BET surface area analysis.[26]
N2 adsorption-desorption
Nitrogen adsorption-desorption isotherms of TiO2, 0.7% PMoV/TiO2, 1.2% PMoV/TiO2,
1.7%PMoV/TiO2, and 2.2% PMoV/TiO2 are summarized in Table 1. The specific surface area of
TiO2 is 44.05 m2/g and it decreases with an increase in loading of PMoV. The surface area is
observed to decrease from 41.46 m2/g to 31.06 m2/g with increase in PMoV loading from 0.7
wt% to 2.2 wt%. This may be due to the fact that PMoV molecules block the pores on the titania
surface[27]
.
TG-DTA analysis
The TGA-DTA profile of 0.7 % PMoV/TiO2, 1.2 % PMoV/TiO2, 1.7 % PMoV/TiO2, 2.2 %
PMoV/TiO2 catalysts are presented in Figure 3. There are two distinct stages of weight loss in the
catalyst. All the TGA profile showed weight loss between 50-160oC corresponding to desorption
of physically adsorbed water i.e., elimination of water of crystallization which binds the Keggin
unit in the primary structure forming water bridges [28, 29]. From the TGA curve, it is worthwhile
to mention that the catalyst is stable up to 590 oC may be due to the presence of titania. The peak
intensity appears to be proportional to the content of titania. The endothermic peak between 500-
590oC of DTA curve indicates the removal of residual sulphate and phosphate from titania.
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NH3 -TPD
The total acidity of PMoV supported catalysts were evaluated by NH3-TPD measurements. The
quantity of acidity present in different wt% of PMoV supported on TiO2 are given in Table 1.
The TPD profiles of 1.2% PMoV/TiO2 (a), 1.7% PMoV/TiO2 (b), 2.2% PMoV/TiO2 (c) are
shown in Figure 4. There are three temperature zones associated with the acid strength (weak,
medium and strong) which can be distinguished according to literature.[30, 31] Titania supported
o
heteropoly acids calcined at 450 C showed a broad desorption peak at 140-280oC. The low
temperature desorption peak at about 100oC corresponds to weak acid sites while the high
o
temperature broad desorption peak that appear at 400-500 C could be due to strong interaction
of ammonia with well dispersed 1.7%PMoV on titania. The strong acid site present in 1.7 wt%
of PMoV/TiO2 catalyst is shown in Figure 4(c).
TPR
The H2 -TPR spectrum of 1.7 % PMoV/TiO2 sample is shown in Figure 5. The sample revealed a
single broad reduction peak at 600oC indicating that the reduction of free metal oxides is formed
by the partial decomposition of Keggin ion during temperature programmed reduction. The
broad reduction peak ascribed to the reduction of segregated vanadium oxide species. The
substitution of molybdenum by V atom in the Keggin ion does not have a pronounced effect of
destabilization of the Keggin unit. This is an interesting observation that V can change its
oxidation state from V+5 to V+4 without destabilising the Keggin ion.[25]
SEM analysis
The SEM image of TiO2 and 1.7 % PMoV/TiO2 are shown in Figure 6(A) and (B). The Figure
6(A) suggests that the mesoporous titania heated at 600 oC have well-ordered tiny particle. After
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impregnation with 1.7% PMoV over titania support, the surface morphology of the catalyst 6(B)
has changed. It showed agglomerated irregular shaped crystalline structure indicating the
reduction of tetragonal symmetry due to the incorporation and dispersion of Keggin unit over the
surface of the support. No significant morphological changes in the SEM micrograph occurred
for the deactivated sample 6(C). Hence the material is highly stable for ammoxidation reaction.
EDX spectra
EDX investigation was performed on fresh and used 1.7 % PMoV/TiO2 sample to get
information regarding surface composition of the active components as shown in figure 7 (A)
and (B). The spectra of the catalyst suggest the presence of various elements like Ti, Mo, V, P
and O in appropriate proportion. Hence, EDX analysis confirms the incorporation of heteropoly
compounds on the surface of titania.
TEM analysis
The TEM images at magnification of (a) 50 nm of 1.7 % PMoV/TiO2 is shown in Figure 8. From
the image, it is clear that the material exhibits spherical morphology with average particle size
175-185 nm. The Keggin Units are well dispersed over the surface of the support material.
Catalytic activity
Very few studies were carried out on ammoxidation of m-xylene using vanadium substituted
heteropoly acid as catalyst. Lee et al reported vanadia/titania xerogel catalyst for ammoxidation
of m-xylene which gave 70% yield of IPN.[32] But the inherent disadvantages of the reaction
were high NH3 ratio, low yield and decreased catalytic activity. Cavani[33] reported that the
substitution of one or more molybdenum atoms by V in the primary structure of
molybdophosphoric acid leads to the enhancement of the oxidation potential of heteropoly acid
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(HPA) due to the vanadium reducibility. So, vanadium may act as a structural promoter in the
catalyst precursor facilitating the formation of the active Mo-O-V surface species.[34, 35] Due to
the structural flexibility and redox capability of vanadium, it may facilitate the direct
participation of V in the activation of gas phase oxygen, ammonia and m-xylene on the active
sites of the catalyst. Another important aspect of the titania support is that it is capable of
releasing oxygen from the surface due to mobility of lattice oxygen and to form oxygen
vacancies at relatively mild conditions.[31] We have carried out ammoxidation of m-xylene over a
series of PMoV supported TiO2 catalysts. Among them 1.7 % PMoV/TiO2 catalyst gave 88.6 %
conversion. As water is one of the by-products, its presence enhances the stability of the catalyst
because it can favour the surface reconstruction of heteropolyacids supported on titania during
the reaction.[33]
Preliminary experiments showed that no ammoxidation reaction takes place in vapour phase
in the absence of any catalyst. This reaction was then carried out using different wt% of
vanadium substituted molybdo phosphoric acid supported titania (Figure 9). From figure 9, it is
evident that from lower to higher wt% (0.7 -1.7) PMoV/TiO2, both the conversion and yield
increases. At higher wt% (2.2%) of PMoV supported on TiO2, the conversion and yield
decreases. This may be due to the first observation that the nitrile yield is found to be dependent
on PMoV loading. Secondly, it is strongly dependent on acid strength. High acid strength of 1.7
wt% PMoV supported materials resulting in high yield of IPN is due to enhancement of
adsorption capacity of ammonia. The high value of IPN yield obtained on 1.7% PMoV/TiO2
might be due to the manifestation of reduction of V+5 to V+4 when it is present in the Keggin
frame work. Apparently at temperature above 425oC the oxygen coordinated to the active site of
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the catalyst can be easily exchanged and can participate in partial oxidation reactions.
As shown in Scheme 1, a pathway is proposed for this reaction. The key step in this
proposed mechanism involves the formation of a carbanion intermediate from m-xylene by
abstraction of a proton by hetero polyanion(HVPOM) ion. The subsequent steps involve the
oxidation followed by rearrangement of the carbanion intermediate. Then after interaction with
NH3 with the intermediate, it is converted to IPN. In this Scheme 1, only one -CH3 group has
been shown to be converted to –CN group and same type of mechanism holds good for the other
-CH3 group.
Effect of temperature
The promotional effect of temperature of reaction has been investigated at six different
temperatures (350, 375, 400, 425 and 450 °C) keeping the other parameters fixed (Figure10). It
is observed that the conversion (42.9 % to 88.6%) and yield (41.6% to 85.4%) increases with
increasing temperature from 350 to 425 °C. After that, a decrease in the conversion of m-xylene
and yield of IPN is observed.
Reusability of the catalyst
Reusability is one of the important aspects of heterogeneous catalyst, which provides useful
information about the recycling capacity during the catalytic cycles. The catalyst with 1.7%
loading was used for recycling experiments. In order to regenerate the catalyst after completion
of reaction, air is passed over the catalyst at 450oC for 3 h. The regenerated catalyst was used for
further run in the ammoxidation of m-xylene. As shown in Figure 11, the yield decreased to 78
% in the regenerated catalyst after 20 cycles. The activity loss observed with the regenerated
catalyst may be due to reduced vanadium species as well as carbonaceous surface deposition on
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the catalyst during reaction.[11]
Conclusions
The vanadium substituted molybdophosphoric acid (PMoV) supported on TiO2 exhibited
excellent catalytic activity for ammoxidation of m-xylene using very low feed ratio of ammonia.
Among all the catalysts, the one with 1.7 wt% PMoV/TiO2 showed the highest conversion (88.6
%) and yield (85.4 %) of isophthalodinitrile. The high activity and selectivity of the catalyst
might be due to synergistic effect of high acid sites, presence of anatase phase of TiO2 and well
dispersion of the PMoV over the support. FTIR, XRD and EDX showed the incorporation of
vanadium in the Keggin unit of PMoV. The yield of IPN is found to be dependent on PMoV
loading. The reusability test exhibits a good recycling capacity of the PMoV supported on TiO2.
Acknowledgement
The authors thank the Director, CSIR-CIMFR for his permission to publish the paper. The
authors also thank CSIR-CIMFR for financial support.
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(34) Ressler, T.; Timpe, O.; Girgasdies, F.; Wienold, J.; Neisius, T.; J. Catal. 2005, 231, 279-
291.
(35) Satsuma, A.; Hattori, A.; Futura, A. ; Miyamoto, A.; Hattori, T.; Murakami, Y.; J. Phys.
Chem. 1989, 93, 1484-1490.
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Table 1: BET surface area and NH3-TPD of different wt% of PMoV/TiO2 catalysts
Sl
No
1
Catalyst
Surface area
(m2/g)
Acidity
(µmole/g)
-
TiO2
44.05
2
3
4
5
0.7 % PMoV/TiO2
1.2 % PMoV/TiO2
1.7 % PMoV/TiO2
2.2 % PMoV/TiO2
41.46
41.22
31.69
31.06
624.6
718.7
805.3
742.3
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Scheme 1: Proposed pathway of ammoxidation of m-xylene
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