One-step selective synthesis of 2-chlorobenzonitrile from 2-chlorotoluene: Via ammoxidation

Article abstract of DOI:10.1039/c7nj03728g

A series of V₂O₅/Al₂O₃ catalysts were prepared by a wet impregnation method and employed for a one-step synthesis of 2-chlorobenzonitrile from 2-chlorotoluene via ammoxidation in a fixed bed reactor at atmospheric pressure. The catalysts were characterized by BET-SA, XRD, FT-IR, TPR, UV-Vis DRS and NH₃-TPD to identify the molecular structural changes of the catalysts. The characterization results revealed that the addition of V₂O₅ to Al₂O₃ amplified the crystallinity of alumina and also resulted in the formation of an interactive species i.e. AlVO₄. Most of the V₂O₅ existed in the form of isolated surface and polymeric tetrahedral vanadia species in a highly dispersed state and/or having a strong interaction with the support. The formation of different kinds of vanadia species was entirely dependent on the content of vanadia present in the catalyst. A 10 wt% V₂O₅/Al₂O₃ catalyst was found to be the best and the high selectivity for 2-CBN was presumably due to the formation of the isolated surface and polymeric tetrahedral vanadia species along with aluminum vanadate species.

Full text of DOI:10.1039/c7nj03728g

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One-step selective synthesis of
2-chlorobenzonitrile from 2-chlorotoluene via
ammoxidation†
Cite this:DOI: 10.1039/c7nj03728g
abc
bc
a
Hari Babu B.,
*
Venkateswara Rao K. T.,a Suh Y. W.,
Sai Prasad P. S.
*
and Lingaiah N.*a
A series of V2O5/Al2O3 catalysts were prepared by a wet impregnation method and employed for a one-
step synthesis of 2-chlorobenzonitrile from 2-chlorotoluene via ammoxidation in a fixed bed reactor at
atmospheric pressure. The catalysts were characterized by BET-SA, XRD, FT-IR, TPR, UV-Vis DRS and
NH3-TPD to identify the molecular structural changes of the catalysts. The characterization results
revealed that the addition of V2O5 to Al2O3 amplified the crystallinity of alumina and also resulted in the
formation of an interactive species i.e. AlVO4. Most of the V2O5 existed in the form of isolated surface
and polymeric tetrahedral vanadia species in a highly dispersed state and/or having a strong interaction
with the support. The formation of different kinds of vanadia species was entirely dependent on the
content of vanadia present in the catalyst. A 10 wt% V2O5/Al2O3 catalyst was found to be the best and
the high selectivity for 2-CBN was presumably due to the formation of the isolated surface and
polymeric tetrahedral vanadia species along with aluminum vanadate species.
Received 29th September 2017,
Accepted 13th December 2017
DOI: 10.1039/c7nj03728g
rsc.li/njc
Supports play a vital role in influencing vanadia dispersions
and thereby the activity and selectivity of vanadia catalysts.
1. Introduction
Vanadium oxide has extensive applications in industrially Lemonidou et al.7 studied the oxidative dehydrogenation (ODH)
important reactions. However, it cannot be used in its bulk of propane over various supported vanadia catalysts and con-
form because of its poor thermal stability and mechanical cluded that V2O5/TiO2 is the most active catalyst whereas V2O5/
strength. In addition, it causes fast catalyst deactivation and Al2O3 is the most selective catalyst. It is well-known that g-Al2O3
favors deep oxidation. For this reason, V2O5 is usually supported allows the dispersion of vanadium cations by facilitating the
3À
on inorganic oxides such as TiO2, ZrO2, SiO2 and g-Al2O3. formation of inter-connected tetrahedral vanadate VO4 units
Generally, a support offers a sufficient surface area for a metal sharing corners and forming a two dimensional layer.8,9 Eon
or metal oxide dispersion and enhances the number of active et al.10 studied the ODH of propane with V2O5/Al2O3 catalysts and
sites and also improves the mechanical strength and thermal found that the bridging V–O–V groups from a two-dimensional
stability. The surface structure of supported vanadia species is VO4 arrangement are the active sites for this reaction. Supports
different from that of the bulk phase and consequently plays a are often found to modify the physico-chemical properties of
unique role in catalytic performance. Supported vanadia cata- vanadia catalysts. Traditionally, vanadia catalysts supported on
lysts are well-known and extensively employed in a variety of Al2O3 and ZrO2 are found to be active for the ammoxidation of
industrial processes such as oxidation of hydrocarbons,1 methanol toluene.11,12 Likewise, Reddy et al.13,14 studied the ammoxidation
oxidation,2 oxidative dehydrogenation of alkanes,3 selective catalytic of 3-picoline over V2O5/Al2O3 catalysts. Even though Al2O3 sup-
reduction of NOx with NH3,4 and ammoxidation of substituted ported V2O5 catalysts are studied for ammoxidation reactions,
aromatic and heteroaromatic compounds.5,6
there has been a lot of scope to apply these materials to other
methyl aromatics.
a Catalysis Laboratory, I&PC Division, CSIR-Indian Institute of Chemical
Technology, Hyderabad 500 007, India. E-mail: haribabu.bathula@gmail.com
b Department of Chemical Engineering, Hanyang University, Seoul 133-691,
Republic of Korea
Conversely, halogen substituted benzonitriles have a wide
range of applications in the fine chemical industry for the
synthesis of different dyestuffs, herbicides, pharmaceuticals,
and pesticides.15–19 Hence, the preparation of these com-
pounds by ammoxidation is of particular interest. In particular,
2-chlorobenzonitrile is an important chemical intermediate used
for the production of industrially important azo dyes namely,
c Research Institute of Industrial Science, Hanyang University, Seoul 133-691,
Republic of Korea
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7nj03728g
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2-amino-5-nitrobenzonitrile and 2-cyano-4-nitroaniline. Another was obtained by calcination at 450 1C for 4 h in an air flow. The
dynamic use of 2-chlorobenzonitrile is its utilization as a starting same procedure was used for the preparation of 10 and 15 wt%
material for the synthesis of 2-chlorobenzylamine, which can be V2O5/Al2O3 catalysts by varying the amounts of ammonium
used as an analgesic, antihypertensive, antitumor active, blood metavanadate and oxalic acid. This series of catalysts were
stream disorder disease remedy, anticoagulant, antifolate and denoted by their weight percentage of V2O5. For example
anticholesteremic agent.20 From a bibliographic standpoint, 5 wt% V2O5/Al2O3 indicates that the catalyst contains 5 wt%
only few have reported the synthesis of 2-chlorobenzonitrile V2O5 over Al2O3.
(2-CBN) from 2-chlorotoluene (2-CT) by means of an ammoxida-
tion process. Comparatively, inadequate literature is available on
2.2. Characterization of the catalysts
the ammoxidation of 2-chlorotoluene over other alkyl aromatics. The BET surface areas of the catalysts were determined on a
Ammonium containing vanadium phosphate,21–23 alumina sup- QUADRASORB SI (Quantachrome Instruments, USA). Prior to
ported V2O519 and DC-108 (composition not disclosed)24 catalysts nitrogen adsorption at À196 1C, about 100 mg of the sample was
were effective for this reaction. The formation of undesirable by- degassed at 200 1C for 2 h. A cross-sectional area of 0.164 nm2 of
products led to low selectivity towards the nitrile compound over the N2 molecule was assumed in the calculation of surface area
these catalysts. Therefore, it is highly desirable to develop a using the Brunauer, Emmett, and Teller (BET) method.
heterogeneous catalyst for ammoxidation of 2-chlorotoluene
which would yield high selectivity towards 2-CBN.
X-ray diffraction patterns of the catalysts were obtained on a
Rigaku Miniflex (Rigaku Corporation, Japan) using Ni filtered
Recently, we demonstrated the influence of various pro- Cu Ka radiation (1.5406 Å) with a scan speed of 21 minÀ1 and a
moters (i.e. MoO3, WO3 and La2O3) and their loading on the scan range of 101–801 at 30 kV and 15 mA. FT-IR spectra were
structural and catalytic functionalities of V2O5/Al2O3 catalysts recorded on a Biorad-Excalibur series (USA) spectrometer using
for ammoxidation of 2-chlorotoluene.18 All those catalysts the KBr disc method.
selectively yielded 2-chlorobenzonitrile from 2-chlorotoluene
Temperature programmed reduction (TPR) was performed
via ammoxidation. The present investigation depicts the effect in a flow of 5% H2/Ar mixture gas at a flow rate of 30 ml minÀ1
of V2O5 loading and reaction temperature on the catalytic with a temperature ramp of 10 K minÀ1. Prior to the TPR run
performance of V2O5/Al2O3 catalysts in the ammoxidation of the catalyst sample (20 mg) was pretreated with argon gas at
2-chlorotoluene and also finding the reason for selective for- 250 1C for 2 h. The hydrogen consumption was monitored
mation of 2-chlorobenzonitrile is one more objective of this using a thermal conductivity detector (TCD).
study. The catalysts prepared with different V2O5 loadings have
Diffuse reflectance spectra were recorded in the UV-Visible
been characterized by various physico-chemical techniques to region 200–800 nm at a split width of 1.5 nm and a scan speed
identify the active species responsible for the ammoxidation of 400 nm per minute using a GBC Cintra 10E UV-Visible
activity. Correlations have been drawn between the physico- spectrometer. Pellets were made by taking 15 mg of the catalyst
chemical properties of the catalysts and the activity and selectivity sample and dried KBr, and grinding them thoroughly for uni-
in the ammoxidation of 2-chlorotoluene to 2-chlorobenzonitrile.
form mixing before making the pellets; the spectra were recorded
at room temperature.
Temperature programmed desorption of ammonia (TPD-NH3)
was performed in laboratory built equipment containing a pro-
grammable temperature controller and a gas chromatograph
with a thermal conductivity detector (TCD). About 100 mg of
2. Experimental
2.1. Preparation of the catalysts
All the chemicals employed were of analytical grade and used as the catalyst was taken and pretreated at 300 1C for 1 h in He flow
received without further purification. Ammonium metavana- (30 ml minÀ1). Then the catalyst was exposed to 10% NH3
date (NH4VO3) and oxalic acid (C2H2O4) were purchased from balanced He gas for 1 h at 100 1C. The physisorbed NH3 was
S.D. Fine Chem. Pvt. Ltd, India, whereas the support, g-Al2O3, removed under He flow (30 ml minÀ1) for 50 min at the same
was supplied by Engelhard. A series of alumina supported V2O5 temperature. Then the temperature of the sample was increased
catalysts were prepared by varying the vanadium content from up to 800 1C at a heating rate of 10 K minÀ1. The desorbed NH3
5 to 15 wt%, by a conventional wet impregnation method. In gas was monitored with the gas chromatograph using the thermal
this method, impregnation of the Al2O3 support was carried out conductivity detector.
with a solution of a vanadium oxalate precursor. To prepare
5 wt% V2O5/Al2O3 catalyst, the vanadium oxalate precursor
2.3. Catalytic performance
solution was prepared by adding 0.68 g of ammonium meta- Ammoxidation of 2-chlorotoluene (2-CT) was carried out in a
vanadate to 1.05 g of oxalic acid in a molar ratio of 1 : 2. The continuous-down flow fixed bed reactor operated under atmo-
resulting mixture was thoroughly stirred at room temperature spheric pressure. In each catalytic run, about 3 g of the catalyst
until complete dissolution of the solid. The color of the (crushed to 18/25 BSS sieve to eliminate mass transfer limita-
solution turned deep blue. After that, this solution was added tions), diluted with double amount of quartz grains, was posi-
to 10 g of g-Al2O3 support under constant stirring. The excess tioned between two layers of quartz wool at the centre of the
water was removed using a water bath and the catalyst masses reactor. The upper portion of the reactor was filled with quartz
were dried in an air oven at 120 1C for 12 h. Finally, the catalyst beads that served both as a pre-heater and a mixer for
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the reactants. Prior to the reaction, the catalyst was reduced
with hydrogen (60 ml minÀ1) at 450 1C for 2 h. After bringing
the reactor temperature to a desired temperature, the reaction
feed, with a molar ratio of 2-CT : ammonia : air = 1 : 4 : 15, was
fed into the preheated portion of the reactor. 2-CT was fed into
the reactor through a microprocessor controlled metering
pump (B. Braun, Germany). The reaction temperature was
monitored using a thermocouple with its tip located at the
middle of the catalyst bed. The reactions were carried out in the
temperature range of 360–450 1C with temperature intervals of
30 1C. After allowing the catalyst to attain a steady-state for 1 h
at the desired reaction temperature, the liquid product was
collected for 30 min in a cold trap kept at À10 1C. The products
were analyzed using a gas chromatograph, separating on a ZB-5
capillary column using a flame ionization detector (FID). From
the analysis of the non-condensable exit gas mixture, it was
confirmed that the amount of any organic species or oxides of
carbon was negligible.
Fig. 1 XRD patterns of the V2O5/Al2O3 catalysts: (a) g-Al2O3, (b) 5 wt%
V2O5/Al2O3, (c) 10 wt% V2O5/Al2O3, (d) 15 wt% V2O5/Al2O3; (@) g-Al2O3,
(*) V2O5, (K) AlVO4.
characteristic peaks of V2O5 and AlVO4 with a very low intensity
along with those of Al2O3. The formation of metal vanadate
species (i.e. AlVO4) is very common when V2O5 is supported on
Al2O3 due to the solid state reaction between dispersed vanadia
and alumina. These findings correlate well with the previous
literature.28–31
3. Results and discussion
3.1. BET specific surface area analysis
Table 1 shows the BET surface area values of V2O5/Al2O3 cata-
lysts with varying V2O5 loadings along with that of the alumina
support. The alumina support exhibited a BET surface area
of 205 m2 gÀ1. A substantial loss in the BET surface area was
observed upon impregnation of Al2O3 with V2O5 from 5 to 15 wt%.
This is a common phenomenon usually noticed in supported
catalysts when the support is impregnated with an active
material.19,25,26 The decrease in surface area after impregnation
of vanadia can be ascribed to the penetration of dispersed
vanadium oxide species into the pores of alumina which can
taper the pore diameter. Alternatively, some of the pores in the
support might have been blocked by dispersed vanadia. Besides,
the formation of a new species might have been observed by the
solid state reaction between dispersed vanadium oxide and the
support with a low surface area, as observed from XRD data in
the following paragraphs. Similar interpretations have been
made in previous reports and the present results are in good
agreement with those reports.25–28
Interestingly, the addition of vanadia amplifies the intensity
of the characteristic peaks of alumina. From this observation,
one can conclude that the addition of V2O5 enhances the
crystallization of Al2O3. Klose et al.32 reported that the addition
of V2O5 induces the crystallization of amorphous alumina and
also leads to the formation of aluminium vanadate species. A
similar observation was also made by Kanervo et al.29 However,
the intensity of the characteristic peaks of alumina slightly
decreased at higher loadings of V2O5, which could be due to the
high surface coverage of the support by vanadia and/or also the
induced formation of metal vanadates. Meanwhile, only a slight
development in the crystalline growth of V2O5 and AlVO4 was
observed with the 10 wt% V2O5/Al2O3 catalyst, indicating that
the vanadia and aluminium vanadate species were in a highly
dispersed state. Further addition of V2O5 to 15 wt% dominated
the crystalline formation of aluminium vanadate and vanadia
species. Shiju et al.31 studied a V2O5/Al2O3 catalytic system by
varying the V2O5 content and found that the vanadia and
vanadate species were absent below 10 wt% V2O5 loading due
to the high dispersion of vanadia on alumina with particle sizes
lower than the detection limits of XRD and the presence of
these species was identified at 20 wt% V2O5 loading. The
present study corroborates the previous reports28–33 that the
formation of microcrystalline V2O5 and AlVO4 starts at 5 wt%
loading gradually increasing up to 10 wt% and dominating at
15 wt% V2O5 loading. These results suggest that the interaction
of vanadia with alumina increases from 5 to 10 wt% loading
and further addition of V2O5 to 15 wt% improves the crystalline
growth of vanadia and vanadate species. As a consequence of
this, the interaction of vanadia with Al2O3 would be reduced at
higher loadings. These results are in good agreement with the
3.2. Powder X-ray diffraction studies
XRD patterns of the catalysts with various loadings of V2O5 on
Al2O3 are shown in Fig. 1. Diffractograms of bare Al2O3 are
also provided for the sake of comparison. All the catalysts dis-
played features indicative of alumina at 2y values of 45.6 and
66.61 (JCPDS 50-0741). The 5 wt% V2O5/Al2O3 catalyst showed
Table 1 BET surface area and total acidity of the support and V2O5/Al2O3
catalysts
Catalyst
Surface area (m2 gÀ1
)
Total acidity (mmol gÀ1
)
g-Al2O3
5 wt% V2O5/Al2O3
205
177
321
1158
1125
1071
10 wt% V2O5/Al2O3 151
15 wt% V2O5/Al2O3 125
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BET surface area values where a continuous loss in the surface water and surface OH groups decreased considerably in this
area of the catalysts was observed.
catalyst due to the possible high surface coverage of alumina by
vanadia and/or the involvement of the surface OH groups in
polymerization. The appearance of the absorption bands at 845
and 654 cmÀ1 with the 15 wt% V2O5/Al2O3 catalyst evidently
confirms the crystalline nature of vanadia at this loading.19,32
On the other hand, the band noticed at 936 cmÀ1 ascribed to
the Al–O–V vibrations is a clear indication of the formation of
AlVO4 crystallites by the interaction of vanadia with alumina.32,39,40
According to Nakagawa et al.41 vanadium oxide on a support
surface exists in an amorphous V2O5 form at lower loadings,
whereas it exists as amorphous and crystalline V2O5 or crystal-
line AlVO4 at higher loadings. Analogous results were noticed
in the present study wherein we observed the absence of the
bands corresponding to crystalline vanadia up to 10 wt% V2O5
loading, and the apparent presence of the absorption bands of
V2O5 and AlVO4 at 15 wt% V2O5 loading indeed suggests that
the V2O5 and AlVO4 species are in a highly dispersed state up to
10 wt% loading. These findings support the XRD and BET
results, where crystalline V2O5 and AlVO4 species were found
at higher V2O5 loadings in XRD patterns and a substantial
decrease in the surface area was observed for catalysts with high
vanadia content.
3.3. Fourier transform-infrared spectra of the catalysts
Fig. 2 displays the FT-IR spectra of the V2O5/Al2O3 catalysts with
varying V2O5 contents along with the support, Al2O3. In the
present investigation, 5 and 10 wt% V2O5/Al2O3 catalysts mainly
showed three absorption bands at 1410, 1643, and 3453 cmÀ1
.
Additionally, these catalysts also displayed broad absorption
bands below 1000 cmÀ1. From the literature, it is known that
crystalline V2O5 exhibits bands at 1020, 826 and 609 cmÀ1. The
absorption bands at 1020 cmÀ1 and 826 cmÀ1 can be attributed
to the stretching vibrations of the isolated VQO vanadyl groups
in the V2O5 trigonal bipyramidal structure and the deformation
of the V–O–V bridges, respectively. The band at 609 cmÀ1 is
assignable to bending vibrations.34–36 The absorption bands of
crystalline vanadia below 1000 cmÀ1 were not detected in the
present catalysts up to 10 wt% V2O5 loading. As the Al2O3
support possesses strong IR absorptions below 1000 cmÀ1 due
to the ionic character of Al–O bonds, the bands of crystalline
vanadia are obscured.37
However, the absorption bands observed at 1643 cmÀ1 and
3453 cmÀ1 are due to the deformation vibrations of absorbed
water and the surface OH groups.28,38 The intensity of these two
bands decreased with the increase in vanadia content from 5 to
10 wt%. Therefore, the possibility of interaction between vanadia
and alumina and the formation of polymeric vanadia species
cannot be ruled out. The band detected at 1410 cmÀ1 can be
related to ammonia formed during the preparation, which
remained absorbed on the catalyst surface even after calcination.
To further confirm this absorption band, we treated Al2O3 with
NH3 at 450 1C and collected the FT-IR spectrum, which clearly
showed an absorption band at 1410 cmÀ1 (Fig. S1, ESI†). 15 wt%
V2O5/Al2O3 displayed three new absorption bands with a very
low intensity in the region below 1000 cmÀ1 at 936, 845 and
654 cmÀ1 along with the 1410, 1643, and 3453 cmÀ1 absorption
bands. The absorption bands of the ammonium ion, absorbed
3.4. H2-Temperature programmed reduction studies
TPR is a highly useful technique to characterize the redox pro-
perties of different metal oxides. TPR profiles of V2O5/Al2O3
catalysts with varying V2O5 contents are presented in Fig. 3.
Both 5 and 10 wt% V2O5/Al2O3 catalysts showed a single broad
reduction peak in the temperature region of 420–731 1C with
Tmax at 605 and 637 1C, respectively. The 15 wt% V2O5/Al2O3
catalyst mainly exhibited a broad reduction peak in the region
of 485–676 1C with Tmax at 608 1C. This catalyst also displayed a
small and broad reduction peak in the region of 374–480 1C with
Tmax at 436 1C. The single reduction peak observed in the present
study is in good accordance with previous reports.25,28,31,32,42–45
The single peak was found for reported catalysts46,47 with less
Fig. 2 FT-infrared spectra of the V2O5/Al2O3 catalysts: (a) g-Al2O3, (b) 5 wt%
Fig. 3 TPR profiles of the V2O5/Al2O3 catalysts: (a) 5 wt% V2O5/Al2O3,
V2O5/Al2O3, (c) 10 wt% V2O5/Al2O3, and (d) 15 wt% V2O5/Al2O3.
(b) 10 wt% V2O5/Al2O3, and (c) 15 wt% V2O5/Al2O3.
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than four layers of V2O5 deposited on the support. However, higher temperature in the 15 wt% V2O5/Al2O3 catalyst is due to
with the formation of a single peak, it is difficult to distinguish the presence of crystalline AlVO4 in conjunction with a decrease
the different kinds of vanadium oxide species formed on Al2O3. in the interaction of the polymeric vanadia species with alumina.
It is well-known that the textural properties of support The TPR results support the XRD and FT-IR data, in which
materials (e.g., surface area, porosity), impurities (foreign com- crystalline V2O5 species was found at higher loadings. From the
pounds), and preparation method, and calcination temperature TPR results, one can conclude that the isolated surface vanadia
play an important role in the reducibility of vanadia. In addi- and polymeric vanadia species are in a highly dispersed state
tion to these the reduction temperature is also sensitive to the and/or having a strong interaction with the Al2O3 support up to
reduction conditions, such as H2 partial pressure and heating 10 wt% V2O5 loading and their interaction decreases consider-
rate. Therefore, it is difficult to identify a common feature for the ably at 15 wt% V2O5 loading.
reduction of catalysts. For example, Xiang et al.44 studied various
supported vanadium catalysts and reported that 5 V/nano-Al2O3
3.5. UV-Vis diffuse reflectance spectra of the catalysts
was reduced at 569 1C, whereas 5 V/com-Al2O3 was reduced at
552 1C, suggesting that the interactions of vanadia and nano-
Al2O3 were stronger than those of vanadia and com-Al2O3. Chen
et al.,45 in their study on V2O5/Al2O3 catalysts, observed a single
reduction peak in the range of 500–520 1C below a vanadium
loading of 1.50 mmol gÀ1-Al2O3, while two reduction peaks were
found, one at around 530–550 1C and the other at 625 1C at
higher loadings of vanadia. The reduction peak noticed at lower
temperature was attributed to the isolated surface vanadia
species while the high temperature one was due to the reduction
of the polymeric vanadia species. da Silva et al.42 reported a
broad and intense reduction peak at 638 1C with a small
shoulder at 777 1C for a V2O5/Al2O3 catalyst. The vanadia species
reduced at 638 1C belonged to the polymeric vanadia species
while the vanadia species reduced at 777 1C was the mono-
vanadate (VQO) species. Ferreira and Volpe,48 in their theore-
tical and experimental study of supported vanadium catalysts,
revealed that the structure, coordination and VOx–support
interaction were strongly influenced by the nature of support.
From the calculations of stabilization and formation energies,
they found that the presence of dimeric vanadia species on SiO2
supports was ruled out, while V(QO)2 and VQO species were
the most plausible species. The dimeric vanadia species were
most conceivable on Al2O3 and TiO2 supports.
UV-Vis diffuse reflectance spectra of the catalysts with various
loadings of V2O5 on Al2O3 are depicted in Fig. 4. It is known that
the energy of the oxygen to vanadium charge transfer (LMCT)
absorption band is strongly influenced by the number of oxygen
ligands surrounding the central vanadium ion. This informa-
tion can be used to determine the nature of the coordination of
vanadium ions in different types of surface species.50–52 According
to previous reports,50–54 vanadium can be distributed on supports
with the formation of different kinds of species i.e. tetrahedrally
coordinated monomeric or isolated vanadia species and small
clusters or linear chains usually observed at absorption maxima
around 230–250 nm. The oligo or polymeric vanadia species
formed by the condensation of monomeric vanadia species are
found at around 270–300 nm, by retaining the tetrahedral coordi-
nation geometry. The band observed at 400 nm is related to the
vanadium ion surrounded by a square pyramid of five oxygen
ligands, with a distant sixth ligand in a bridging position to the
support. These species, generated by a two-dimensional conden-
sation reaction of immobilized vanadia, can also be present as
ribbons, patches or two-dimensional surface layers. The absorp-
tion bands of V5+ falling in the 430–570 nm region are character-
istic of distorted octahedral coordination or bulk-like vanadia
crystallites. The d–d transitions of V4+ and V3+ ions appear at
above 570 nm.
It was acknowledged from previous studies28,45,48,49 that the
isolated surface vanadia species easily underwent reduction
than polymeric or bulk like V2O5 species. Thus, from the TPR
results, one can conclude that the Tmax observed at 605 1C for
the 5 wt% V2O5/Al2O3 catalyst is due to the reduction of the
polymeric vanadia species (i.e. V5+ to V4+). At 10 wt% V2O5
loading, the Tmax shifts towards higher temperature (i.e. 637 1C)
with a greater intensity. The improved intensity with the high
temperature shift indicates the induced formation of the poly-
meric vanadia species. This also suggests a strong interaction
between polymeric vanadia and Al2O3. But one cannot rule out
the presence of isolated surface vanadia species in 5 and
10 wt% V2O5 catalysts. In the case of the 15 wt% catalyst, the
reduction profile had changed significantly. This is due to
the formation of isolated surface vanadia species (i.e. low
temperature peak) along with polymeric vanadia species (i.e.
high temperature peak). But one cannot ignore the reduction of
AlVO4 which can be seen between 480–650 1C29, whose forma-
tion is dominant in the 15 wt% V2O5/Al2O3 catalyst which is
All the catalysts in the present study exhibited broad absorp-
tion bands lying in the region of 200–475 nm. The appearance
Fig. 4 UV-Vis DR spectra of the V2O5/Al2O3 catalysts: (a) 5 wt% V2O5/Al2O3,
known from the XRD results. Hence, the Tmax shift found at a (b) 10 wt% V2O5/Al2O3, and (c) 15 wt% V2O5/Al2O3.
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of a broad band in all the catalysts indicates the presence of
different kinds of vanadium species. The 5 wt% V2O5/Al2O3
catalyst showed two absorption maxima at 238 and 278 nm,
respectively. In the case of the 10 wt% V2O5/Al2O3 catalyst, there
is a tiny red shift in both the absorption bands towards the
higher wavelength (i.e. 239 and 282 nm) with an improved
intensity. A noticeable change was observed in 15 wt% V2O5/Al2O3.
The red shift now occurred at 243 and 286 nm. Very often,
literature reports on this subject assign these absorption bands
based on the changes in the coordination and/or polymeriza-
tion degree of vanadium atoms. Following this criterion, the
former absorption band observed at around 238–243 nm can be
ascribed to the monomeric or isolated tetrahedral vanadia
species while the latter absorption band perceived in the region
of 278–286 nm can be assigned to the polymeric tetrahedral
vanadia species.50–54
Fig. 5 NH3-TPD profiles of the V2O5/Al2O3 catalysts: (a) g-Al2O3 (b) 5 wt%
V2O5/Al2O3, (c) 10 wt% V2O5/Al2O3, and (d) 15 wt% V2O5/Al2O3.
According to the investigations of Shiju et al.31 and Khodakov
et al.,33 vanadia dispersed on alumina shows a broad range of
All the V2O5/Al2O3 catalysts clearly exhibited two broad
VOx structures. They suggested that V2O5 predominantly exists desorption peaks except for the 15 wt% V2O5/Al2O3 catalyst, in
as monomeric or isolated vanadia species with tetrahedral which three broad desorption peaks were noticed. It is noteworthy
coordination at lower loadings while polymeric vanadia species that the first desorption peak appearing in the low temperature
are observed at higher loadings due to progressive condensation region i.e. below 350 1C in all the catalysts splits into two peaks
of the monomeric species. Besides, bulk-like V2O5 crystallites with Tmax lying in the temperature ranges of 140–160 1C and
were noticed at above 20 wt% V2O5 loadings. Xiang et al.44 and 200–215 1C, respectively. In the 5 wt% V2O5/Al2O3 catalyst, these
Liu et al.55 observed a broad absorption band at 300 nm which two peaks were identified at 150 1C and 210 1C. The Tmax values
was indicative of polymeric vanadia species. They found a new shifted to a lower temperature in the 10 wt% V2O5/Al2O3 catalyst
absorption band at 450 nm for the catalyst with 25 wt% V2O5 and were observed at 140 and 200 1C. These Tmax values moved
loading, suggesting the formation of bulk-like vanadia crystal- to a higher temperature and were detected at 160 and 215 1C in
lites due to further polymerization of the vanadia species. the 15 wt% V2O5/Al2O3 catalyst. The desorption peak seen at
Similar views were also expressed in the literature31,33,44,55,56 140–160 1C can be ascribed to the isolated surface vanadia
stating that the appearance of a broad band at higher wave- species and the latter visible at 200–215 1C can be attributed to
lengths for the catalysts with high loadings of V2O5 is due to the the Lewis acidity of uncovered alumina. The broad desorption
growth in size and dimensionality of vanadia species. In the peak appearing above 350 1C is due to the Bronsted acidity of
present investigation, the absorption profiles, showing an the highly dispersed and/or polymeric vanadia species.58–60 The
increase in the intensity of both absorption bands with the broadness of the second peak extending to a higher tempera-
addition of V2O5 and the red shift, are indicative of an increase ture is separated into two desorption peaks in the case of high
in the degree of vanadium condensation resulting in the for- V2O5 loadings.
mation of the polymeric vanadia species. These findings corre-
Khader59 studied the surface acidity of V2O5/Al2O3 catalysts
late with previous reports31,57 and are also in good agreement by IR and TPD. The V2O5/Al2O3 catalyst showed Bronsted acidity
with the FT-IR results, where we found a decrease in the inten- due to V–OH species while Lewis acidity corresponded to the
sity of the surface OH region due to the polymerization. The uncovered Al ions at lower vanadia loadings. He also suggested
broadness of the absorption bands also indicates the growth of that the Lewis acidity of the monolayer catalyst was caused by
the crystalline vanadia species. The absence of the absorption the isolated surface vanadia species. Nonetheless, at higher
band in the visible region suggests the absence of the d-d loadings of vanadia, the Bronsted acidity of the catalysts was
transitions of the V4+ and V3+ ions.
prominent due to two kinds of vanadia species i.e. the V–OH
species of adsorbed vanadium oxide on alumina and the V–OH
species of crystalline V2O5. However, the decrease in the Lewis
3.6. Temperature programmed desorption of NH3
The NH3-TPD profiles of the catalysts are presented in Fig. 5, acidity at higher loadings of V2O5 is due to the surface coverage
and the total acidity values of the catalysts are given in Table 1. of crystalline V2O5 and/or decrease in the number of OH groups
The Al2O3 support showed two desorption peaks centered at attached to the vanadium atoms due to the consumption of the
200 and 655 1C indicating that alumina had both weak and V–OH species during polymerization. Miyata et al.61 mentioned
strong acidic sites with a total acidity of 321 mmol gÀ1 58
In that the Bronsted acidic sites of monolayer or two-dimensional
.
general, the acid site distribution observed from the NH3-TPD or polymeric vanadia species are stronger than those obtained
studies can be differentiated into three temperature regions viz. in higher vanadia loadings due to crystalline V2O5.
100–350 1C (weak acidic sites), 350–600 1C (moderate acidic
sites) and above 600 1C (strong acidic sites).
The 5 wt% V2O5/Al2O3 catalyst exhibited approximately four
fold higher acidity (i.e. 1158 mmol gÀ1) than the pure support.
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Such kind of massive enrichment in the catalyst acidity can be
caused by the formation of polymeric vanadia species on the
support. There was no significant decrease in the acidity with
the addition of V2O5 up to 10 wt%. A substantial decrease in the
acidity was noticed for 15 wt% V2O5/Al2O3. The observed results
can be easily understood from the XRD, FT-IR, TPR and UV-Vis
DRS information. All these studies recommended the crystalline
growth of vanadia species as a result of increased polymerization
of the vanadia species from 5 to 15 wt% V2O5 loading. The
apparent enhancement of the low temperature peak intensity
and the broadness of the peak at a high temperature in the
10 wt% V2O5/Al2O3 catalyst was due to the induced formation of
isolated and polymeric vanadia species. The appearance of three
desorption peaks at low, moderate and high temperatures in the
15 wt% V2O5/Al2O3 catalyst can be described as follows: the low
temperature desorption peak is due to the isolated surface
vanadia species and uncovered alumina, the moderate tempera-
Fig. 6 Effect of V2O5 loading on the catalytic activity of the ammoxidation
of 2-CT at 420 1C.
ture desorption peak is ascribed to the V–OH species of crystal- among the series of catalysts. Such kind of huge improvement
line V2O5 and the high temperature desorption is due to the in the catalytic activity can be conceivable, due to the induced
V–OH species of polymeric vanadia. These results are in good formation of the isolated surface and polymeric vanadia species,
agreement with previous reports.59,61 The significant fall in the which in turn result in a stronger interaction with the support
acidity of the 15 wt% V2O5/Al2O3 catalyst is due to the formation Al2O3. The noticed decrease in the catalytic activity of the 15 wt%
of the crystalline vanadia and vanadate species.
V2O5/Al2O3 catalyst was due to the enormous formation of
crystalline V2O5 and AlVO4 and a subsequent loss in the highly
dispersed and/or polymeric vanadia species. These results are in
3.7. Ammoxidation of 2-chlorotoluene (2-CT)
All these catalytic systems were used for the evaluation of good agreement with the previous literature.19
ammoxidation of 2-chlorotoluene (2-CT) in a fixed bed reactor
It is well known from the literature62,63 that metal vanadates
at atmospheric pressure. The main product obtained in the are active and selective catalysts for ammoxidation reactions.
ammoxidation of 2-CT was 2-chlorobenzonitrile (2-CBN). Due Dhachapally et al.62 carried out ammoxidation of 2-methyl-
to the exothermic nature, the formation of by-products is very pyrazine over metal vanadate catalysts and observed that the
common in ammoxidation reactions. The usual by-products seen AlVO4 catalyst showed higher catalytic activity. They also
in the ammoxidation of 2-CT are as follows: chloro-benzene mentioned that a small proportion of crystalline V2O5 species
(due to loss of methyl group) and carbon oxides (i.e. CO and in the catalysts are also beneficial for ammoxidation reactions.
CO2, due to the partial, total oxidation of methyl group/2-CT). In addition, the acidity of the catalysts also plays a decisive role
However, in the present study, we did not find the formation of in the ammoxidation reaction by the way of enhancing the
any by-products and observed the exclusive formation of the absorption capacity of one of the reactants, ammonia. From
desired product, 2-CBN.
NH3-TPD studies, it was noted that only a small difference in
3.7a. Effect of V2O5 content on the catalytic activity. acidity was perceived between the 5 and 10 wt% V2O5/Al2O3
Ammoxidation of 2-CT was carried out over V2O5/Al2O3 catalysts catalysts. However, a large disparity in the acidity was noticed
in order to study the variation in catalytic activity as a function in the 15 wt% V2O5/Al2O3 catalyst due to the low availability of
of V2O5 content at 420 1C and the results are depicted in Fig. 6. the surface OH groups as they were involved in the polymeriza-
Irrespective of V2O5 loading, all the catalysts showed 100% tion, which ultimately led to the formation of crystalline V2O5.
selectivity towards a single product i.e. 2-CBN. No other by- These results suggest that 10 wt% V2O5/Al2O3 is the best
product was detected. A considerable variation in the 2-CT catalyst among the catalytic systems studied due to the induced
conversion was observed with respect to the V2O5 loading. formation of isolated surface and polymeric vanadia species
The 5 wt% V2O5/Al2O3 catalyst showed only 28% conversion. with reasonable acidity. Besides, the presence of small propor-
However, progress in the catalytic activity was noticed with the tions of crystalline V2O5 and AlVO4 species is favorable to obtain
addition of V2O5. The 10 wt% catalyst achieved higher catalytic high catalytic activity.
activity with 41% conversion. A further increase in the active
3.7b. Effect of temperature on the catalytic activity. The
component to 15 wt% decreased the conversion and it reached effect of the reaction temperature on the catalytic activity of the
a value of 35%. The difference noticed in the catalytic activity 10 wt% V2O5/Al2O3 catalyst is projected in Fig. 7. It is obvious
can be correlated with the observed characteristics of the that the conversion of 2-CT increases with an increase in the
catalysts.
reaction tempearatue from 360 to 450 1C. The conversion of
The lower catalytic activity acquired by the 5 wt% V2O5/Al2O3 2-CT was two fold when the temperature increased from 360 to
catalyst can be due to the low content of V2O5. But the 10 wt% 390 1C. A further increase in the temperature enhanced the
V2O5/Al2O3 catalyst achieved the highest catalytic activity conversion of 2-CT and the maximum conversion was attained
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large amount of crystalline AlVO4, which is known to have lower
combustion activity than V2O5.63–65 Hence, these results further
confirm that bulk and crystalline V2O5 are detrimental for high
product selectivity whereas isolated surface vanadia, polymeric
vanadia and aluminum vanadate species are accountable for
the high selectivity towards 2-CBN. Therefore, from these
results one can conclude that vanadia catalysts with a high
content of isolated surface and polymeric tetrahedral vanadia
species with resonable acidity can exhibit considerable activity
and high selectivity in ammoxidation reactions. More to the
point, the presence of small grains of V2O5 and AlVO4 species is
always crucial in these catalysts to attain higher catalytic activity
along with selectivity.
Herein, we discussed the mechanistic aspects reported in
the literature using Al2O3 supported V2O5 catalysts for the
ammoxidation of toluene and 2-chlorotoluene. Murakami
et al.12 and Niwa et al.68 investigated the reaction mechanism
Fig. 7 Effect of the reaction temperature on the catalytic activity of the
10 wt% V2O5/Al2O3 catalyst in the ammoxidation of 2-CT.
at 450 1C i.e. 51%. All the catalysts gave only one product, of ammoxidation of toluene using V2O5/Al2O3 catalysts by FTIR
2-CBN, at every temperature without the formation of any spectroscopy. They claimed that the reaction proceeds by the
byproduct. Dhachapally et al.63 and Kung et al.64,65 reported interaction of ammonium ions with surface benzoate ions.
that metal vanadates have lower combustion activity than V2O5, Otamiri and Andersson69 proposed the formation of vanadium
which is reflected in showing high selectivity towards the desired imido species (VQNH) and vanadium hydroxylamino species
product even at higher temperatures. Owen and Kung64 used (V–NH–OH) as nitrogen insertion sites in V2O5 catalysts. These
different metal vanadates for the ODH of butane and suggested species, in turn, reacted with adsorbed toluene to form an amine
that orthovanadate materials exhibit high selectivity towards the (R–CH2–NH2) and/or an imine (R–CHQNH) surface inter-
desired product, wherein the VO4 units are separated from each mediate, which then transformed into nitrile as the final
other by MOx units. In such materials, most of the lattice oxygen product. Very recently, Dwivedi et al.19 proposed a mechanism
ions are located in the solid bridge between the Vm+ and Mn+ ions for ammoxidation of 2-CT to 2-CBN over Al2O3 supported V2O5
resulting in low availability of readily removable lattice oxygen, catalysts using DFT computations. According to their study, the
which leads to low combustion or total oxidation.
V2O5/Al2O3 catalyst was first reduced by NH3 to form vanadium
Recently, Dhachapally et al.66 recounted that oligomeric/ imido (VQNH) species and then a 2-CT molecule would adsorb
polymeric vanadia species are considered to be selective for through a vanadyl oxygen (VQO) to produce C6H5QCH2 species.
the formation of 2-cyanopyrazine during ammoxidation of Subsequently, the formed C6H5QCH2 species was adsorbed on
2-methylpyrazine. Similarly, Bilde et al.67 recommended that the vanadium imido (VQNH) species to produce 2-CBN via three
the presence of isolated surface vanadia species or of low size successive dehydrogenation steps. For the DFT calculations, they
oligomeric vanadium oxide species in the catalysts was respon- used four-coordinated alumina supported polymeric vanadium
sible for the higher selectivity of acrylonitrile in the ammoxida- oxide as the active species, and the catalytic systems of the
tion of propane. In the present study, all the catalysts possess present study are composed of these species and thus follow
isolated surface and polymeric vanadia species along with the the same reaction pathways.
aluminum vanadate species. As a result, these catalysts selec-
It is well-known that ammoxidation reactions of methyl
tively yielded 2-CBN as the product in the ammoxidation of aromatics follow a redox mechanism as proposed by Mars and
2-CT even at higher temperatures. To further confirm that the van Krevelen.70 According to this mechanism, lattice oxygen is
isolated surface vanadia, polymeric vanadia and aluminum consumed in the reaction and the reduced site formed would be
vanadate species were responsible for the selectivity of 2-CBN, reoxidized by gaseous oxygen. Hence, the reduction of vanadium
ammoxidation was carried out on 20 and 25 wt% V2O5/Al2O3 based catalysts is always pursued prior to the ammoxidation
catalysts which possess more crystalline V2O5 and AlVO4 than the reaction to generate more coordinatively unsaturated sites (V4+),
studied catalysts (see Fig. S2 in ESI† for XRD). These catalysts which facilitate the redox mechanism (V4+/V5+) and in turn the
yielded around 40 and 36% of 2-CT conversion with 97% ammoxidation activity. Another important point to be con-
selectivity towards 2-CBN at 450 1C. The observed variation in sidered in almost all ammoxidation reactions is ammonia
the ammoxidation activity could be associated with the difference oxidation. There is always a competition between NH3 oxidation,
in the crystallinity of the catalysts. These results are well corro- and ammoxidation on the catalyst surface. Moreover, the contact
borated by the previous literature.19 In addition, these catalysts of ammonia with the catalyst surface, particularly at high tem-
exhibited lower selectivity than the studied catalysts due to peratures, causes a partial reduction of the oxide surface due to
transformation of most of the isolated and polymeric vanadia the occurrence of NH3 oxidation. Therefore, control of the rate of
species into more crystalline V2O5 and AlVO4. But they still unselective oxidation of ammonia to N2 (NOx)71,72 is an important
showed the highest selectivity to 2-CBN due to the presence of a factor in determining the selectivity of the nitrile product in the
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ammoxidation reaction, because this side reaction limits the was also financially supported by the Basic Science Research
availability of surface ammonia (N-insertion) species that are neces- Program through the National Research Foundation of Korea
sary for nitrile formation. It is known from the literature71,72 that the (NRF) funded by the Ministry of Education, Republic of Korea
oxidation of ammonia is faster than the ammoxidation reaction; (NRF-2016R1A6A1A03013422).
excess ammonia modifies the surface oxidation characteristics of a
catalyst and decreases the ammoxidation activity. Nevertheless, it is
also very difficult to completely avoid ammonia oxidation but it can
only be minimised to the maximum possible extent. Besides,
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Al2O3 supported V2O5 catalysts are active and selective for the one-
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