Facile fabrication of aluminum-promoted vanadium phosphate: A highly active heterogeneous catalyst for isopropylation of toluene to cymene

Article abstract of DOI:10.1016/j.jcat.2012.02.004

Vanadium phosphate is well known as a heterogeneous catalyst in gas phase oxidation reactions. To date, there has been little interest in carrying out liquid-phase reactions with vanadium phosphate as catalyst. Herein, we report the catalytic activity of vanadium phosphate and aluminum-promoted vanadium phosphate toward liquid-phase isopropylation of toluene to cymene. The catalysts were unambiguously characterized by X-ray diffraction, N₂ adsorption-desorption, FT-IR technique, UV-vis DRS, and FE-SEM. The total acid sites were estimated by an NH₃ TPD analyzer. XPS was used as a powerful tool to know the electronic environment of the catalysts. The optimization of the reaction was carried out by varying temperature from 75 to 150 °C and molar ratio (toluene: isopropanol) from 1:1-1:3. Under optimum reaction conditions, 5 wt.% aluminum-promoted vanadium phosphate showed 90% conversion with 85% selectivity to p-cymene.

Full text of DOI:10.1016/j.jcat.2012.02.004

Journal of Catalysis 289 (2012) 190–198
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Facile fabrication of aluminum-promoted vanadium phosphate: A highly active
heterogeneous catalyst for isopropylation of toluene to cymene
⇑
Gobinda Chandra Behera, K.M. Parida , D.P. Das
Colloids and Materials Chemistry Department, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar 751 013, Odisha, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Vanadium phosphate is well known as a heterogeneous catalyst in gas phase oxidation reactions. To date,
there has been little interest in carrying out liquid-phase reactions with vanadium phosphate as catalyst.
Herein, we report the catalytic activity of vanadium phosphate and aluminum-promoted vanadium phos-
phate toward liquid-phase isopropylation of toluene to cymene. The catalysts were unambiguously char-
acterized by X-ray diffraction, N2 adsorption–desorption, FT-IR technique, UV–vis DRS, and FE-SEM. The
total acid sites were estimated by an NH3 TPD analyzer. XPS was used as a powerful tool to know the elec-
tronic environment of the catalysts. The optimization of the reaction was carried out by varying temper-
ature from 75 to 150 °C and molar ratio (toluene: isopropanol) from 1:1–1:3. Under optimum reaction
conditions, 5 wt.% aluminum-promoted vanadium phosphate showed 90% conversion with 85% selectiv-
ity to p-cymene.
Received 5 December 2011
Revised 1 February 2012
Accepted 13 February 2012
Available online 27 March 2012
Keywords:
Isopropylation
p-Cymene
Toluene
Vanadium phosphate
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
[22] suggests that combinations of V4+ and V5+ phases are required
for the catalyst to exhibit high activity and selectivity.
Due to emerging applications, metal phosphates and related
solids having low-dimensional structures (i.e., 1D or 2D) are
currently the subject of increasing interest [1,2]. Particularly,
oxovanadium phosphate chemistry is enjoying interest in view of
its industrial uses [3]. This has resulted in a rich crystal chemistry
that is associated, in part, with the ability of vanadium to adopt
different coordination polyhedra and to accede easily to various
oxidation states [4,5].
In view of the above context, vanadium phosphorus oxide (VPO)
catalysts are of great interest due to their potential applications
toward various organic transformation reactions. Although the
VPO catalyst system has been studied extensively in gas phase
oxidation, including catalyst structural stability, morphology,
crystalline structure, and surface composition after hundreds of
hours of testing, these types of studies are very limited in liquid-
phase reactions where VPO catalysts have been used [23–27]. It
is well known that the VPO catalysts are structurally sensitive to
its catalytic activation and reaction conditions [28–30].
A literature survey reveals that the best industrial catalyst from
the VPO family is generally not used in the form of pure VPO
phases, but incorporates a broad range of promoter compounds
[31,32]. Promoter compounds are added to induce enhancement
in activity or selectivity or to ensure the structural integrity of
the catalyst surface. In an earlier comprehensive review of the lit-
erature on this topic, promoters were assumed to act in two ways;
namely, structural effects due to enhancement of the surface area
of the activated catalysts and electronic effects [33].
Here, we explore the promotion of vanadium phosphate cata-
lysts by aluminum. Some research groups consider the acidity of
the catalysts to be an important aspect in controlling the catalyst
performance. An IR study of the acid sites using NH3, pyridine,
and acetonitrile as probe molecules showed the existence of Lewis
and Brønsted acid sites [34–39]. The presence of acid sites in VPO
materials encourages us to carry out acid-catalyzed reactions. But
the presence of acid sites in VPO is not sufficient for acid-catalyzed
Many well-characterized crystalline vanadium phosphate
phases have been identified, whose structures and catalytic
properties have been well documented. Some of the most widely
studied are the V5+ vanadyl orthophosphates (
a-, b-, c-, d-, e-,
and
x
-VOPO4 and VOPO4Á2H2O) and the V4+ vanadyl hydrogen
phosphates (VOHPO4Á4H2O, VOHPO4Á½H2O, VO(H2PO4)2), vanadyl
pyrophosphate ((VO)2P2O7), and vanadyl metaphosphate
(VO(PO3)2) [6–13]. Of these compounds, VOHPO4.½H2O (vanadyl
hydrogen phosphate hemihydrate) is of particular interest as a cat-
alyst precursor [14–17], which on activation gives a catalyst
mainly comprising (VO)2P2O7 (vanadyl pyrophosphate) [18]. A
literature survey reveals that the resulting catalyst comprises a
complex mixture of (VO)2P2O7 with primarily
a-II and d-VOPO4
phases [19]. Some researchers favor V4+ phases, e.g., (VO)2P2O7,
as the active phase [20,21]. However, in situ Raman spectroscopy
⇑
Corresponding author. Fax: +91 674 2581637.
E-mail address: paridakulamani@yahoo.com (K.M. Parida).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.02.004

G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
191
reactions. Promoting the catalyst with aluminum enhances the
Lewis acid sites on the surface of the catalyst, which play an impor-
tant role in the alkylation of alkyl aromatics.
These pellets were further used for recording FTIR spectra. UV–vis
investigations in diffuse reflectance mode were recorded in a
UV–vis spectrophotometer (Varian, CARY 100). The spectra were
recorded in the range 200–800 nm using boric acid as the reflec-
tance standard. The acid character of the catalysts was studied
with an NH3 TPD CHEMBET-3000 (Quantachrome, USA) analyzer
equipped with a thermal conductivity detector (TCD). About 0.1 g
of powdered sample was contained in a quartz U tube and
degassed at 250 °C for 1 h with ultrapure nitrogen gas. After the
sample cooled to room temperature, NH3 gas (20% NH3 balanced
with helium) was passed over the sample while it was heated at
a rate of 10 °C minÀ1 and the profile was recorded. The electronic
states of V, Al, and P were examined by X-ray photoelectron spec-
troscopy (XPS, Kratos Axis 165 with a dual-anode (Mg and Al)
The alkylation of the aromatic nucleus has been traditionally
carried out with well-known Lewis acids or organometallic
reagents. Isopropylation of benzene and toluene with isopropyl
bromide as alkylating agent and AlCl3 as catalyst has been studied
in detail by Olah et al. [40]. Alkylation with various alkylating
agents and Friedel–Crafts catalysts has provided insight into the
trends, in which activity and selectivity are mostly considered
[41–43]. Alcohols and alkenes can also serve as sources of electro-
philes in Friedel–Crafts reactions in the presence of strong acids. In
many cases, more than a stoichiometric amount of AlCl3 is used for
the reaction, giving poor selectivity because of degradation,
polymerization, and isomerization. Al-promoted VPO catalysts
play an important role in this area. The products of isopropylation
of toluene with isopropanol are o-cymene, p-cymene, and m-
cymene. p-Cymene is an important intermediate used in the
pharmaceutical industries and for the production of fungicides
and pesticides. It is also used as a flavoring agent and as a heat
transfer medium.
apparatus) using an Mg K
a source. All the binding energy values
were calibrated by using the contaminant carbon (C1s = 284.9 eV)
as a reference. Charge neutralization of 2 eV was used to balance
the charge of the sample. Binding energy values of the samples
were reproducible within 0.1 eV.
Field emission scanning electron microscopy (FE-SEM) was
performed with a ZEISS 55 microscope. Magnification in the range
15.83–44.90 KÂ was used to get a better micrograph.
ICP-OES (Optima 2100 DV, PerkinElmer) using microwave
pressure digestion (MDS 200; CEM) with hydrofluoric acid and
aqua regia at 9 bar was used to analyze the chemical composition
of fresh and used catalysts. All the samples were analyzed three
times, and the results presented here are the average values.
2. Materials and methods
2.1. Materials synthesis
2.1.1. Preparation of the bulk VPO precursor (VOHPO4Á0.5H2O)
The VPO precursor was prepared according to the following
procedure: V2O5 (5.0 g, SBMC, 98.5%) and o-H3PO4 (30 ml, 85%
Aldrich) were refluxed in deionized water (120 ml) for 24 h. A
yellow solid was recovered by vacuum filtration, washed with cold
water (100 ml) and acetone (100 ml), and dried in air (110 °C,
24 h). Powder X-ray diffraction studies confirmed that the solid
was the dihydrate VOPO4Á2H2O [44].
2.3. Catalytic isopropylation reaction
Isopropylation of toluene was conducted in the liquid phase in a
250-ml round-bottomed flask equipped with a reflux condenser
and a magnetic stirrer. In a typical reaction procedure, 0.05 g cata-
lyst was mixed with 25 mmol toluene and 25 ml nitrobenzene. To
this solution, 25 mmol isopropanol was added, and the whole solu-
tion was heated to the reaction temperature (150 °C) with constant
stirring for 7 h. The products were separated by column chroma-
tography and then analyzed by offline GC (Shimadzu, GC-17 A)
equipped with a capillary column (ZB-1, 30-m length, 0.5-nm ID,
The dihydrate (4 g) was refluxed with isobutanol (80 ml, 99%,
Spectrochem) for 21 h, and the resulting hemihydrate was recov-
ered by filtration, dried in air (110 °C, 16 h), refluxed in deionized
water (9 ml H2O/solid(g)) for 2 h, filtered hot, and dried in air
(110 °C, 16 h).
and 3.0-lm film thickness) using a flame ionization detector (FID).
2.1.2. Preparation of promoted VPO precursor
Different wt.% Al-promoted VPO catalysts were prepared by the
wetness impregnation method using isopropanol as solvent. The
requisite amount of promoter source (Al as isopropoxide) was
dissolved in isopropanol (30 ml, 99.8%, Spectrochem). The solution
was warmed to 70 °C in a water bath for some minutes, and the
desired amount of the precursor compound VOHPO4Á0.5 H2O was
added in powder form. The resulting slurry was evaporated to
dryness on a water bath followed by oven-drying at 120 °C for
16 h. All the materials were calcined at 400 °C for 5 h in air. The
final promoted VPO catalysts consisted of 5, 10, 15, and 20 wt.%
of Al. Most of the investigations were carried out using bulk VPO
and Al-promoted VPO catalysts.
3. Results and discussion
3.1. Characterization
3.1.1. X-ray diffraction
Fig. 1i and ii displays the XRD patterns of VPO catalysts. For the
precursor, all major diffraction peaks can be attributed to VOH-
PO4Á0.5H2O. XRD patterns corresponding to the planes at (001),
(101), (021), (121), (201), (220), (031), and (102) were observed
at 2h values of 15.6°, 19.7°, 24.5°, 27.4°, 28.9°, 30.4°, 32.1°, and
33.7°, respectively, matching well with the VOHPO4Á0.5H2O. This
is in accordance with JCPDS File 4–880. The intense peak at
2h = 30.4° of the (220) plane of the VPO catalysts (having d spac-
ing = 2.93 Å, cell parameter a = 8.31 Å) is in agreement with cubic
mesostructured VPO [45]. In Fig. 1ii, a small peak at 2h = 12.8°
was found which corresponds to the reflectance pattern of VOPO4.
This is due to the transformation of VOHPO4Á0.5H2O to VOPO4 after
calcination. This shows that the vanadium in the sole VPO catalyst
sample is predominantly in the V4+ state, with a small amount of
V5+ species. However, this peak was only found in the case of
5 wt.% Al-VPO and thereafter was reduced with the increase of
the aluminum loadings. This may presumably be due to the reduc-
tion of V5+ to V4+ with the increase of aluminum content in the pro-
moted catalysts. There are three peaks found at 2h values of 37.8°,
2.2. Characterization
The BET surface areas and pore volume distributions of the cat-
alysts were determined by N2 adsorption at 77 K (ASAP2020,
Micromeritics). A known amount of catalyst sample was evacuated
for 2 h at 110 °C to remove physically adsorbed water prior to
surface area measurements. Phase analysis of all materials was
performed by XRD (PANalytical, X’pert PRO) using CuKa radiation
at 1.543 Å. IR spectra of bulk and promoted VPO catalysts were
recorded on a Varian 800 FTIR spectrophotometer. Self-supporting
pellets were prepared with KBr and catalysts by applying pressure.

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G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
Fig. 1. (i) X-ray diffraction patterns of (a)
c
-Al2O3, (b) 5 wt.% Al-VPO, (c) 10 wt.% Al-VPO, (d) 15 wt.% Al-VPO, and (e) 20 wt.% Al-VPO; (ii) X-ray diffraction patterns of (a)
VOHPO4Á0.5H2O and (b) VPO (calcined VOHPO4Á0.5H2O).
46.1°, and 67.1° for pure Al2O3. But there are no such peaks found
in the case of Al-VPO catalysts. The lack of reflections correspond-
ing to crystalline alumina phases indicates that the aluminum spe-
cies in the VPO were amorphous or highly dispersed on the VPO
surface. Crystalline alumina is generally formed at temperatures
>500 °C (calcined temperatures). Since the present material was
calcined at 400 °C, amorphous alumina might be formed in VPO.
The XRD patterns reveal that there is a reduction of the peak in
the Al-VPO relative to the parent precursor. In all Al-VPO materials,
the intensity of the peak at 2h = 15.6° slightly increased while the
peak at 2h = 30.4° decreased in comparison with the bulk VPO.
3.1.2. BET surface areas and pore volume distributions
To understand the textural properties, the VPO and promoted
VPO catalysts were subjected to N2 adsorption–desorption mea-
surements. The results are depicted in Fig. 2a and b and also sum-
marized in Table 1. Among the promoted samples, 15 and 20 wt.%
Al-VPO showed typical type-IV isotherms, indicating the presence
of mesoporous structures. But as for the classification of porosity,
the materials having pore sizes between 2 and 50 nm are mesopor-
ous. In this context, all VPO materials fall into the mesoporous
range. Again, VPO has a surface area below 15 m2 gÀ1 and very
small pore volume. The Al-VPO samples exhibit significantly higher
Fig. 2. (a) Adsorption–desorption isotherm of VPO and different wt.% of Al-VPO samples; (b) pore size distribution curve of VPO and different wt.% of Al-VPO samples.

G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
193
Table 1
Textural properties and surface acidity of the catalysts.
Catalyst
Surface area
(m2/g)
Pore size
(nm)
Pore volume
(cm2/g)
Total acidity
(mmol/g)
Total acidity
(mmol/m2)
Lewis acid sites
Brønsted acid sites
VPO
14
27
76
111
127
10.0
5.8
3.5
2.9
2
0.05
0.16
0.24
0.31
0.34
2.37
6.274
7.02
7.77
9.17
0.169
0.232
0.092
0.07
1.28
5.9
6.0
6.21
7.03
0.09
0.374
1.02
1.56
2.14
5wt% Al-VPO
10 wt% Al-VPO
15 wt% Al-VPO
20 wt%Al-VPO
0.072
values of these parameters, relative to VPO. It can be seen that
20 wt.% Al-VPO presents the highest surface area and pore volume,
with all pores being in the mesopore range. The surface area of the
sample was observed to increase drastically with increasing alumi-
num loading. For this study, the deposit of aluminum did not sta-
bilize the VPO phase because the impregnation method appeared
to permit aluminum to be present on the VPO crystallite surface.
The surface area increases with the increase of aluminum loading
but is not linearly increased, that is why there are no separate
phases in the materials. The decrease in pore size indicates the
mesoporosity of the materials. The increase in surface area may
presumably be due to increased amorphous alumina in the VPO
materials.
VPO. However, it has been found that there is only a slight change
of the peak at 1639 cmÀ1 with the increase in aluminum loading.
3.1.4. UV–vis DRS studies
Optical absorption spectra of bulk and Al-VPO are depicted in
Fig. 4. The investigations were carried out to obtain information
on the vanadium oxidation state. The presence of a broad band
at 550–650 nm in the bulk and Al-promoted catalysts indicates
the presence of V4+ species in these catalysts [46]. However, the
broad band at 450 nm in the DR UV–vis spectra of Al-containing
samples can be related to the presence of V5+ species (VOPO4).
According to this, different VAPAO phases with V ions in different
oxidation states have been observed. However, this broad band at
450 nm was only prominently found in 5 wt.% Al-VPO, which indi-
cates the presence of V5+ in the Al-VPO materials. This is also con-
firmed from the XRD peak. Hutchings et al. [22] reported that
combinations of V4+ and V5+ phases are required for the catalyst
to exhibit high activity and selectivity. However, the peak at
450 nm gets reduced with increased aluminum content. This may
presumably be due to the reduction of the V5+ ion to V4+ with
the increased aluminum content, as it contained isopropanol units
during materials preparation.
3.1.3. FTIR studies
The FTIR spectra of the bulk and promoted VPO catalysts are
depicted in Fig. 3. All the catalysts showed sharp bands in the re-
gion of 400–3500 cmÀ1. The slightly broad spectra at 3370 cmÀ1
are due to the symmetric stretching mode of OAH groups. The
infrared spectra of the catalysts in the region 900–1200 cmÀ1 cor-
respond to the stretching modes of PAO and V@O groups. The band
at 642–415 cmÀ1 can be attributed to the deformation vibrations of
OAPAO groups of phosphate tetrahedral. Almost no shift is
observed in the catalysts (bulk and promoted catalysts), especially
in an important band at 970 cmÀ1 that corresponds to symmetric
stretching vibrations of V4+@O groups. The peak at 1045 cmÀ1
can be assigned to symmetric stretching vibrations of PO3 groups,
and the rest of the peaks at 1101 and 1200 cmÀ1 can be ascribed to
asymmetric stretching vibrations of PO3 groups. The peak at
2376 cmÀ1 may be due to the adsorption of atmospheric CO2. It
is strange to see there is no change in spectra obtained in Fig. 3.
It only confirms different stretching and bending vibrations of
3.1.5. Temperature-programmed desorption studies
To comprehend the acidic properties of the catalysts, VPO and
Al-VPO catalysts were subjected to NH3 TPD analysis. The typical
NH3-TPD profiles are depicted in Fig. 5i and also summarized in
Table 1. The total acidity of 2.37 mmol/g found for bulk VPO was
lower than that of promoted catalysts. The total acidity of the pure
Al2O3 was found to be 2.5 mmol/g. Again, the increase in the Al
content in the parent VPO enhances the total acidity of the catalyst.
That means that Al could contribute to the total acidity of the
catalyst. Typically, NH3 TPD curves showed two clear peaks, which
Fig. 3. FT-IR spectra of (a)
c
-Al2O3, (b) VPO, (c) 5 wt.% Al-VPO, (d) 10 wt.% Al-VPO,
Fig. 4. UV–vis DRS of (a)
c-Al2O3, (b) VPO, (c) 5 wt.% Al-VPO, (d) 10 wt.% Al-VPO, (e)
(e) 15 wt.% Al-VPO, and (f) 20 wt.%Al-VPO.
15 wt.% Al-VPO, and (f) 20 wt.% Al-VPO.

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G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
Fig. 5. (i) NH3 TPD plot of (a) VPO, (b) 5 wt.% Al-VPO, (c) 10 wt.% Al-VPO, (d) 15 wt.% Al-VPO, and (e) 20 wt.% Al-VPO. (ii) Pyridine TPD plot of (a) c-Al2O3, (b) VPO, (c) 5 wt.%
Al-VPO, (d) 10 wt.% Al-VPO, (e) 15 wt.% Al-VPO, and (f) 20 wt.% Al-VPO.
determine the existence of at least two types of acid sites. These
peaks are called the l-peak (lower temperature) and h-peak (high
temperature), respectively. The l-peak is attributed to desorption
of weakly bound ammonia. It is relevant to gas sensing applica-
tions, due to the influence of the weakly bound ammonia on the
proton mobility in VPO. The h-peak reflects desorption of ammonia
from the strong acid sites (here its Lewis acid sites), which deter-
mines the acidic properties of VPO. Brønsted and Lewis acid sites
were also quantitatively characterized by pyridine TPD (Fig. 5ii).
Adsorption of pyridine on Al-VPO produced two types of adsorbed
species: a pyridinium ion on Brønsted acid sites and a covalently
bound species on Lewis acid sites. It has been found that the Lewis
acid site increases with increased aluminum content. The peak
maximum for 15 wt.% at lower temperature was due to the
increase of Lewis acid sites along with Brønsted acid sites. Since
the materials contain two different phases, multiple peaks are ob-
served in the profile.
3.1.6. X-ray photoelectron spectroscopy studies
The XPS investigations reported here were aimed at the deter-
mination of the vanadium oxidation state present in the catalyst
samples. The representative photoelectron peaks of Al2p, V2p3/2,
O1s, and P2p are depicted in Fig. 6a–c, respectively. Since contro-
versial interpretations of the broad right V2p3/2 photoelectron
peak as well as somewhat contradictory values for the binding
energies for the Vn+ contributions can be found in the literature,
VOPO4 with phase structure found by UV–vis spectra was used
to calibrate the binding energy of the V2p3/2 (V5+) peak. The

G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
195
Fig. 6. XPS spectra for the Al-VPO catalysts, with Al loadings (wt.%) as indicated by each spectrum; (a) Al2p, (b) V2p, and O1s, and (c) P2p.
measured value of 517.5 eV for V5+ and the independently
obtained value of 516.3 eV for V4+ [47] are in good agreement with
the literature [48–50] and were used for the V2p3/2 peak deconvo-
lution for all samples studied. The binding energies, 74.6 and
132.9 eV, correspond to Al2p and P2p in +3 and +5 oxidation states,
respectively (Fig. 6a and c). The shift to higher binding energy of
the Al2p peak with increasing Al loading (Fig. 6a) may reflect a
change from Al2O3 clusters to Al2O3 monolayer deposition or a
change in overlayer–support interaction. The surface compositions
of the catalysts are shown in Table 2. The binding energy value
531.8 eV of the O1s peak indicated that the oxygen species was
O2À in oxides, and therefore, the increase of O/V ratio supports
the conclusion that a measurable fraction of V4+ at the equilibrated
surface is oxidized to V5+
.
3.1.7. Scanning electron microscopy studies
The FE-SEM micrograph of the catalysts revealed that the sam-
ples possess a slatelike morphology (Fig. 7). Further, aggregates
without regular shapes are observed in VPO. This is the reason
for the low surface area of the VPO catalyst than of Al-VPO. How-
ever, the morphology of VPO was not affected significantly by Al
doping.
3.1.8. Estimation of catalyst components by ICP-OES measurement
To estimate the actual contents of all catalyst components, all
the prepared catalysts undergo characterization using the
ICP-OES technique. ICP analysis results of various Al-VPO catalysts
are shown in Table 3. The contents of aluminum, vanadium, and
phosphorus estimated from ICP are in good agreement with the
nominal values. In addition, the contents of all these components
were also checked again in used catalysts and were quite compara-
ble to those of fresh samples. This fact suggests that there is no loss
of catalyst components during the course of reaction. The P/V ratio
detected here indicates the presence of V4+ species in the materials.
From the literature, it has been found that a P/V ratio >1 is
Table 2
Surface composition of the Al-VPO catalysts determined by analysis of the X-ray
photoelectron spectra.
Al2O3 (wt.%)
at.%
O
V
P
Al
N
P/V
Al/V
5
74.0
74.0
73.7
73.2
9.2
9.5
8.7
8.7
14.8
13.6
13
1.1
1.6
3.2
3.7
0.8
1.4
1.5
1.7
1.61
1.42
1.50
1.46
0.119
0.168
0.367
0.425
10
15
20
12.7

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G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
Fig. 7. Scanning electron micrographs of VPO and Al-VPO.
Table 3
parative understanding of the catalytic activity for the reaction.
Electrophilic substitution reactions such as isopropylation are cat-
alyzed by strong Lewis acid sites. It was also reported that surface
hydroxyl groups are responsible for the acidic nature of the cata-
lyst [54]. Thus, the surface oxygen and surface hydroxyl group
[55] play a crucial role in the catalytic activity of vanadium
phosphate.
The results of isopropylation of toluene with isopropanol using
VPO and different wt.% of Al-promoted VPO in nitrobenzene as sol-
vent are summarized in Table 4. It is clearly evident from Table 4
that VPO gives 80% p-cymene selectivity with 86% conversion in
a period of 7 h. Also, 5 wt.% Al-VPO showed higher catalytic activ-
ity, giving 85% p-cymene selectivity with 90% conversion.
However, the conversion decreases with increased aluminum
content of VPO. Presumably the high wt.% of aluminum blocks
the active site of the catalyst due to protonation of the hydroxyl
groups present in the catalyst. The high selectivity of p-cymene
may be due to the smaller steric hindrance of the methyl group
at the para position, which is favored from the shape selectivity
point of view [56].
Estimation of Al, V, P contents in wt.% from ICP-OES in Al-VPO catalysts with different
Al loadings.
Catalyst
Al
V
P
P/V
Fresh
Used
Fresh
Used
Fresh
Used
VPO
00
00
18.7
18.7
28.91
19.83
19.58
18.85
18.25
28.90
19.83
19.58
18.84
18.25
1.54
1.52
1.52
1.52
1.52
5 wt.% Al-VPO
10 wt.% Al-VPO
15 wt.% Al-VPO
20 wt.%Al-VPO
5.04
10.09
15.13
20.18
5.04
10.09
15.13
20.18
13.01
12.84
12.40
12.01
13.01
12.83
12.40
12.01
necessary for high catalytic performance [51]. It has also been
found that at P/V ratios of less than 1.0, a number of vanadium
sites remain inactive, but at the higher P/V ratios, all surface sites
are active [52]. This is in good agreement with our results.
3.2. Catalytic reaction
The isopropylation of toluene with isopropanol is an
electrophilic substitution reaction. Isopropylation reactions cata-
lyzed by acids are commonly considered as proceeding via a
carbonium-ion-type mechanism [53]. The reaction of toluene with
isopropanol is shown in Scheme 1. The effects of various parame-
ters on the isopropylation reaction are discussed later. The VPO
and Al-promoted VPO catalysts are tested in order to have a com-
3.2.1. Effect of reaction time
The effect of the reaction period on the isopropylation of tolu-
ene with isopropanol using Al-VPO catalyst was studied at 150 °C
with mole ratio 1:2 (toluene: isopropanol). The results are illus-
trated in Fig. 8. From the figure, it is found that the conversion of
Al-VPO
+
HO
+
+
150 o
C
,7h
isopropanol
toluene
m-cymene
o-cymene
Scheme 1. Isopropylation of toluene to cymene.
p-cymene

G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
197
Table 4
Comparison of the activity of the bulk and promoted catalysts.
Catalyst
Conversion (%)
Cymene selectivity (%)
Para
Ortho
Meta
Others
VPO
Al2O3
5 wt.% Al-VPO
10 wt.% Al-VPO
15 wt.% Al-VPO
20 wt.% Al-VPO
86
81
90
88
88
88
80
37
85
83
82
82
12
29
14
14
13
13
5
6
–
2
3
3
2
11
–
–
1
1
Note: Conditions: temperature 150 °C, time 7 h, molar ratio 1:1 (toluene:
isopropanol.
Fig. 9. Effect of reaction temperature on toluene conversion and p-cymene
selectivity over 5 wt.% Al-VPO. Conditions: catalyst 0.05 g, time 6 h, molar ratio
1:2 (toluene: isopropanol).
150 °C over 5 wt.% Al-VPO catalyst with varying toluene-to-
isopropanol molar feed ratios from 2:1 to 1:3. The results are
summarized in Fig. 10. In all cases, p-cymene was obtained as
the major product, along with small amounts of o-cymene and
m-cymene. It is clearly evident from the figure that toluene conver-
sion increases with decreasing toluene content in the feed. More-
over, there exists competitive adsorption between toluene and
isopropanol at the feed ratio 1:1. Hence with the increase in the
molar ratio, the conversion of toluene increases.
The p-cymene selectivity increases with the increase in the feed
ratio from 2:1 to 1:2. The decrease in the selectivity to p-cymene at
a feed ratio of 2:1 compared to 1:2 is due to the presence of more
toluene content in the former, which leads to more chemisorption
of toluene on the catalyst surface, thereby suppressing the chemi-
sorption of isopropanol, which in turn facilitates the dealkylation
rather than isopropylation of toluene. It is observed that isopropy-
lation is favored only if toluene reacts from the vapor phase and
isopropanol is chemisorbed on the catalyst surface, which could
also be due to the trans alkylation of diisopropyltoluene formed
due to the easy availability of isopropyl cations.
Fig. 8. Effect of time period on toluene conversion and p-cymene selectivity over
5 wt.% Al-VPO. Conditions: catalyst 0.05 g, temperature 150 °C, molar ratio 1:2
(toluene: isopropanol).
toluene increases from 78% at 1 h to a maximum of 90% at 7 h.
After 7 h, the conversion of toluene remains constant. This may
presumably be due to the blocking of active sites of the catalyst
with coke [57]. The selectivity to p-cymene exhibits a small but a
significant increase with time. The presence of more Brønsted sites
causes faster deactivation, which may be due to higher cracking
activity and coke formation at strong acid sites of the catalysts.
The coke deposits at the pore mouth and/or within the catalyst
may reduce the effective channel dimensions, resulting in a tighter
fit of the molecules and finer discrimination between the isomers
[58].
Further there is a marginal decrease in p-cymene selectivity
with the increase in the isopropyl content at a feed ratio of 1:3.
This may presumably be due to the dilution of toluene with isopro-
panol. As said above, at a feed ratio of 2:1, toluene is more prone to
3.2.2. Effect of reaction temperature
To accomplish better toluene conversion and maximum p-cym-
ene selectivity, liquid-phase isopropylation of toluene was carried
out over 5 wt.% Al-VPO catalyst at various reaction temperature,
and the results are depicted in Fig. 9. It has been found that the tol-
uene conversion increases up to 150 °C and remains constant
thereafter. This could be attributed to the coke deposition caused
by dealkylation, which in turn leads to the oligomerization of ole-
fins. On the other hand, the increase in toluene conversion at lower
temperatures may be due to the clustering of alcohols around the
Brønsted acid sites of the catalyst through H-bonding. As a result,
its dissociation to isopropyl carbocations will be suppressed at
higher temperatures. But at higher temperatures, the alcohols will
have a decreased tendency to form molecular clusters, and so they
can freely form carbonium ions via protonation.
3.2.3. Effect of mole ratio of the reactants
To study the effect of feed ratios on toluene conversion and
product selectivity, an isopropylation reaction was carried out at
Fig. 10. Effect of molar ratio on toluene conversion and p-cymene selectivity over
5 wt.% Al-VPO. Conditions: catalyst 0.05 g, temperature 150 °C, time 6 h.

198
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
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selectivity.
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