Hafnium salts of dodeca-tungstophosphoric acid catalysts for liquid phase benzylation of anisole with dibenzylether

Article abstract of DOI:10.1016/j.apcata.2013.11.001

A series of hafnium salts of heteropoly tungstate (Hf_xTPA) catalysts were prepared by exchanging the counter cations (hydrogen) of heteropoly tungstate with hafnium ions. The prepared catalysts were characterized by FT-IR, Laser Raman, temperat

Full text of DOI:10.1016/j.apcata.2013.11.001

Applied Catalysis A: General 471 (2014) 1–11
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Hafnium salts of dodeca-tungstophosphoric acid catalysts for liquid
phase benzylation of anisole with dibenzylether
Chowdari Ramesh Kumara,b, N. Rambabu , N. Lingaiah , P.S. Sai Prasad , A.K. Dalai
a
b,∗
b
a,∗
a
Department of Chemical and Biological Engineering, University of Saskatchewan, Saskatoon, SK, S7N 5A9, Canada
Catalysis Laboratory, I&PC Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, AP, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 June 2013
Received in revised form 29 October 2013
Accepted 1 November 2013
Available online 1 December 2013
A series of hafnium salts of heteropoly tungstate (Hf TPA) catalysts were prepared by exchanging the
counter cations (hydrogen) of heteropoly tungstate with hafnium ions. The prepared catalysts were char-
x
acterized by FT-IR, Laser Raman, temperature programmed desorption of ammonia, scanning electron
microscopy, and X-ray photo electron spectroscopy. FT-IR and Raman characterization results suggested
retention of the Keggin ion structure of heteropoly acid. Pyridine adsorbed FT-IR spectra of the catalysts
showed that generation of Lewis acidic sites was due to the presence of hafnium ions in the catalyst.
Activity of these catalysts was investigated for liquid phase benzylation of anisole with dibenzylether. In
the partial exchanged catalyst (Hf0.5TPA), higher conversion was due to high mobility of residual protons.
The catalytic activity of HfxTPA depended on the exchangeable hafnium ions in the secondary structure
of heteropoly tungstate that, in turn, relates to variation in acidity of the catalyst. Thermal stability and
structural properties of the Hf0.5TPA catalyst were studied by treating it at different temperatures. Diben-
zylether conversion and selectivity towards benzylated products were found to depend on the anisole to
dibenzylether ratio, reaction temperature, and catalyst loading.
Keywords:
Anisole
Benzylation
Dibenzylether
Hafnium
Heteropoly tungstate
Keggin ion
©
2013 Published by Elsevier B.V.
1
. Introduction
octahedrons sharing their corners or edges with a central XO4
tetrahedron. Hydrogen forms of these heteropolyoxometalates
Over the past two decades polyoxometalates (POMs) have
are called heteropoly acids (HPAs), for example, H [PW12O40],
3
received increasing interest in the area of catalysis [1,2] because
of their participation in important industrial processes related for
the materials [3], nanotechnology [4], biology [5,6], supramolecu-
lar materials [7,8], and colloidal science [9]. POMs are made up of
oxygen and early transition metals of groups V and VI in their high-
est oxidation states, such as V, Nb, Ta, Mo, W [10]. Depending on
the structural type, POMs are broadly classified into two types; (i)
isopolyoxometalates and (ii) heteropolyoxometalates. Isopolyox-
H [SiW12O40], and H [PMo12O40]. Among the Keggin type het-
4
3
eropoly acids, W-based heteropoly acids are relatively more
acidic compared to other heteropoly acids [12]. Misono et al.
[13,14] reported the primary, secondary, and tertiary hierarchical
structures for HPAs to understand their heterogeneous catalysis.
Accordingly, the heteropoly anion is a primary structure; the regu-
lar arrangement of the polyanions, counter cations, and additional
molecules in three-dimension is called a secondary structure; and
the assembly of this structure into solid particles is known as a ter-
tiary structure, which relates to their properties such as particle
size, surface area, and pore structure [15].
HPAs are widely used as catalysts in a number of homogeneous
and heterogeneous acid–base and redox type catalytic reactions,
due to their specific physicochemical properties such as high acidity
and oxidizing properties [16–18]. The redox properties of het-
eropoly acids can be tuned in a systematic way by substituting the
metal into primary and secondary structures [14,19–21]. Gener-
ally, heteropoly acids are highly soluble in polar solvents. HPAs can
0
ometalate is a metal oxide framework with only d metal cations,
6
+
5+
such as W , Mo [11]. Heteropolyoxometalates are composed of
a metal-oxide framework along with a hetero atom in the form of
3−
2−
anion (e.g., PO4 , SO4 ). There are many structures proposed for
heteropolyoxometalates in which Keggin type and Wells–Dawson
structures are well studied.
The Keggin type of heteropoly anion is generally represented
x−
as [XM12O40
]
, where X is the hetero atom, such as P, Si, Al, or
6+
Ge in tetrahedral fashion, and M is the addenda atom, usually W
or Mo . The Keggin ion is made of an assembly of twelve MO6
6
+
be made heterogeneous by supporting them on suitable supports
+
[
[
22–24] and/or substitution of counter ions (H ) with metal ions
25–29]. Metal exchanged HPAs can be classified into two groups:
∗ Corresponding authors. Tel.: +1 306 966 4771; fax: +1 306 966 4777.
E-mail addresses: nakkalingaiah@iict.res.in (N. Lingaiah), ajay.dalai@usask.ca
A.K. Dalai).
group A are the HPAs in which protons are exchanged with small
metal ions like Na , Al , or Cu ; and group B are those HPAs
+
3+
2+
(
0
926-860X/$ – see front matter © 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.apcata.2013.11.001

2
C.R. Kumar et al. / Applied Catalysis A: General 471 (2014) 1–11
containing large metal ions like Cs , Sn2+, or Sn4+ [25,30]. Substitu-
tion of certain metal ions generates Lewis acidic sites in heteropoly
acids, in which metal ions act as electron pair acceptors [25].
Friedel–Crafts alkylation (FCA) of aromatic compounds is an
important process in petroleum, chemical, and pharmaceutical
industries [31,32]. Conventionally, diarylmethane and substituted
diarylmethane derivates produced by the reaction of aromat-
ics with benzyl chloride produce HCl that causes corrosion and
is environmentally hazardous. Use of benzyl alcohol or diben-
zylether as a benzylating agent instead of benzyl halides, produces
water as a by-product, which is non-hazardous [33,34]. Shimizu
et al. [25,26] reported that the polyvalent-metal salts of dodeca-
+
was kept in a hot air oven for further drying and finally calcined
at 300 C for 2 h. The catalyst was denoted as HfxTPA, where x rep-
resents the number of hafnium ions, which varied from 0.25 to
0.75.
◦
2.3. Characterization of catalysts
The FT-IR spectra were recorded on a PerkinElmer spectrum 100
series (USA) infrared spectrometer using the KBr pellet method in
−
1
the wave number range of 4000–400 cm , with a resolution of
−
1
4 cm at room temperature. The nature of the acid sites (Bron-
sted and Lewis) of the catalysts was determined with chemisorbed
pyridine. The pyridine adsorption studies were carried out in the
diffuse reflectance infrared Fourier transform (DRIFT) mode. Prior
to the pyridine adsorption catalysts were degassed under vacuum
+
+
3+
3+
3+
3+
3+
4+
4+
tungstophosphate (Cs , Ag , Y , Fe , Bi , Ru , Al , Ti , Zr
,
4+
4+
4+
Ti , Hf , and Sn ) act as effective heterogeneous catalysts for
Friedel–Crafts acylation and alkylation of aromatics with carboxylic
acids and alcohols, respectively. Metal salts of heteropoly acid for
hydrolysis of cellobiose and cellulose are also reported in the liter-
ature [35]. However, the literature shows few reports on detailed
characterization of metal salts of dodeca-tungstophosphoric acid.
The application of simple hafnium salts is an emerging area in
organic synthesis [36–42]. Shiina et al. studied FCA of aromatic
nucleophiles with intermediary benzyl silyl ethers using HfCl4 as
Lewis acid [36]. Zhang et al. have developed HfCl /HfO catalyst for
◦
at 200 C for 3 h followed by suspending dry pyridine. Then, the
excess pyridine was removed by heating the sample at 120 C for
◦
1 h. After cooling the sample to room temperature, FT-IR spectra of
the pyridine-adsorbed samples were recorded. The Bronsted and
Lewis acid sites intensity ratios were calculated from the intensities
of the corresponding absorbance values.
The ratio of Bronsted and Lewis acidities was estimated from
the IR peak intensity corresponding to these acid sites.
Confocal micro-Raman spectra were recorded at room temper-
4
2
arylation of benzyl alcohols [42]. This system catalyzes the reaction
of dichloromethane with benzene to yield diphenylmethane with
high selectivity. However, most of these catalysts have limitation
to reuse. In our previous reports, modification of tungstophospho-
−
1
ature in the range of 200–1200 cm using a Renishaw Invia Reflex
Raman microscope equipped with an Ar+ laser (Spectra-Physics
model 127) operating at an excitation wavelength of 514.5 nm and
an 1800 line/mm grating. The laser was focused onto the powdered
sample using a 5× objective, giving a spot size of approximately
1 ␮m. The laser power was 1.8 mW measured at the sample. The
instrument’s calibration was verified with a Si(1 1 0) sample that
ric acid with metal ions such as Sn2+, Sm and Al was reported
3+
3+
[
27,28,33]. Since HfCl4 is Lewis acid, it can be used to modify the
4
+
heteropoly acids with Hf to generate Lewis acidic sites. To the best
of our knowledge, no detailed study has been reported on hafnium
salts of tungstophosphoric acid for organic synthesis.
−
1
was measured at 520 cm
.
The objective of the present study was to prepare hafnium salts
of tungstophosphoric acid catalysts with varying hafnium content
The acidity of the catalysts was measured by temperature
programmed desorption of ammonia (TPD–NH3). In a typical
experiment, 0.1 g of catalyst was loaded and pre-treated in a helium
and to characterize them by FT-IR, Laser Raman, NH -TPD, X-ray
3
◦
photoelectron spectroscopy, pyridine FT-IR, and SEM. The catalytic
activity of the prepared catalysts was investigated by benzylation
of anisole with dibenzylether, and changes in the surface and struc-
tural properties of the catalysts with calcination temperature were
studied using different characterization techniques. The effects of
reaction temperature, catalyst weight, and anisole to dibenzylether
molar ratio were also studied to optimize the reaction conditions.
atmosphere at 300 C for 2 h. After pre-treatment, the temperature
◦
was brought to 100 C and the adsorption of NH3 was carried out
by passing a mixture of 10% NH3 on He over the catalyst for 1 h.
◦
The catalyst surface was then flushed with He at 100 C for 2 h
to flush off the physisorbed NH3. TPD of NH3 was carried with a
◦
temperature ramp of 10 C/min and the desorbed ammonia was
monitored using a thermal conductivity detector (TCD) of a gas
chromatograph.
X-ray photo electron spectroscopy (XPS) measurements were
conducted on a KRATOS AXIS 165 with a DUAL anode (Mg and Al)
apparatus using Mg K␣ anode. The non-monochromatized Al K␣
X-ray source (hꢀ = 1486.6 eV) was operated at 12.5 kV and 16 mA.
2
. Experimental
2.1. Materials and chemicals
◦
Before analysis, each sample was out-gassed for about 3 h at 100 C
The following analytical grade chemicals were used for
−7
under vacuum of 1.0 × 10 T to minimize surface contamination.
catalyst preparation and benzylation of anisole reaction. 12-
tungstophosphoric acid (Sigma-Aldrich), hafnium chloride (Sigma-
Aldrich), benzyl alcohol (EM Science), dibenzylether (Alfa-Aesar),
The XPS instrument was calibrated using Au as standard. For energy
calibration, the carbon 1s photoelectron line was used. The carbon
1
2
s binding energy was taken as 285 eV. Charge neutralization of
eV was used to balance the charge up of the sample. The spectra
4
-methoxydiphenylmethane (Matrix Scientific, Columbia), anisole
Alfa-Aesar). Ammonia balanced with helium, hydrogen, air, and
nitrogen gases were procured from Praxair Canada.
(
were deconvoluted using Sun Solaris Vision-2 curve resolver. The
location and full width at half maximum (FWHM) value for the
species were first determined using the spectrum of a pure sample.
Symmetric Gaussian shapes were used in all cases. Binding energies
for identical samples were, in general, reproducible within ± 0.1 eV.
Scanning electron microscope images (SEM) were obtained
using a JEOL 6010LV instrument to observe the morphology of the
prepared samples.
2.2. Preparation of hafnium salt of tungstophosphoric acid
The hafnium-exchanged 12-tungstophosphoric acid (HfxTPA)
catalyst series was prepared according to the procedure previ-
ously reported in the literature [26]. The catalysts were prepared as
follows: required amount of TPA was dissolved in distilled water,
and to this solution, the calculated amount of HfCl dissolved in dis-
2.4. Alkylation reaction procedure
4
tilled water was added with continuous stirring, forming a white
precipitate. The resultant mixture was stirred for 2 h and the excess
water was evaporated on a rotary evaporator. The dried catalyst
The alkylation reactions were carried out in a 50 ml two-necked
round bottom flask with a reflux condenser with nitrogen inlet for

C.R. Kumar et al. / Applied Catalysis A: General 471 (2014) 1–11
3
(d)
(c)
(b)
(a)
2000
1800
1600
1400
1200
1000
800
600
400
-
1
Wavenumber (cm )
Fig. 2. X-ray diffraction patterns of (a) TPA, (b) Hf0.25TPA, (c) Hf0.5TPA, and (d)
Hf0.75TPA.
Fig. 1. FT-IR of (a) TPA, (b) Hf0.25TPA, (c) Hf0.5TPA, and (d) Hf0.75TPA.
nitrogen purging into the reaction mixture and a septum for sample
removal. In a typical run, 10 g of anisole and 3.08 g of dibenzyl ether
along with 0.1 g catalyst were taken into a flask. The reaction was
◦
carried out at temperature of 120 C.
(d)
The reaction mixture was withdrawn at different intervals and
analyzed by gas chromatography equipped with a stibil wax col-
umn and flame ionization detector. The reaction products were
identifiedusing GC–MSand also confirmedby using retentiontimes
of standard samples.
(
c)
3
. Results and discussion
(b)
3.1. FT-IR spectroscopy
Fig.
1 shows the FT-IR spectra of hafnium salts of 12-
(a)
tungstophosphoric acid along with the parent acid. Pure TPA
(
1
Fig. 1(a)) shows characteristic bands in the fingerprint region, at
081, 988, 896, and 796 cm that indicate P–O, W=Ot (Ot—refers to
2
00
400
600
800
1000
1200
−
1
-1
Raman shift (cm )
the terminal oxygen), W–Oc–W (Oc—refers to the corner oxygen),
and W–Oe–W (Oe—refers to edge sharing oxygen), respectively
Fig. 3. Raman spectra of (a) TPA, (b) Hf0.25TPA, (c) Hf0.5TPA, and (d) Hf0.75TPA.
[
5
28]. Apart from these four bands, two more bands observed at
−1
96 and 524 cm
are related to bending vibrations of P–O and
W–O, respectively. All these characteristic bands of TPA are present
in hafnium salts of TPA, indicating the retention of Keggin ions
after exchange of the counter ions with hafnium ions. The band
3.3. Raman spectra
The existence of the Keggin ion was further confirmed by Laser
Raman spectroscopy. This is an effective technique to determine
the presence of different metal oxides and molecular structure
of the immobilized samples because it is more sensitive to the
Keggin unit. Fig. 3 shows the Laser Raman spectra of the par-
ent 12-tungstophosphoric acid and hafnium salts of TPA. Pure
crystalline TPA (Fig. 3 (a)) showed peaks at 1006, 990, 900, 535,
−1
at 1630 cm is assigned to the H–O–H bending vibration mode
43].
[
3.2. X-ray diffraction patterns
Fig. 2 shows the X-ray diffraction patterns of HfxTPA catalysts.
◦
−1
TPA shows the characteristic peaks of the Keggin ion at 2ꢁ of 10.5 ,
231, and 215 cm [45–47]. Hafnium salt of heteropoly tungstate
◦
◦
◦
◦
◦
◦
1
8.3 , 23.7 , 26.1 , 30.2 , 35.6 , and 38.8 . The characteristic peaks
(Fig. 3(b–d)) also showed the characteristic peaks of the parent
acid. The bands at 1006 and 990 cm are characteristic peaks of
asymmetric and symmetric vibrations of W=Ot (t-terminal). These
−
1
of TPA were observed in all the catalysts. As reported in the liter-
ature, when TPA protons were exchanged with metal ions such as
+
3+
Cs and Sm the XRD peaks corresponding to TPA were shifted to
lower angles, indicating the expansion of unit cell volume [28,44].
Similarly, it was noticed that with Hf content increased from 0.25
to 0.75, the relative intensity of characteristic peaks related to the
Keggin ion decreased and shifted to lower angles, which indicates
that expansion of the unit cell volume as the size of the Hf4+ is
much greater than that of proton. The shift in XRD peaks is clearly
observed in the inset of the Fig. 2, which indicates an exchange
bands are unresolved in the Hf0 TPA sample (Fig. 3 (d)) that
.75
−
1
showed only one band at 1006 cm . The Raman bands at 900 and
535 are ascribed to asymmetric stretching vibrations of W–Oc–W
(Oc-corner sharing bridging oxygen atom) and symmetric stretch-
ing vibrations of W–Oe–W (Oe-edge sharing bridging oxygen
−1
atom), respectively. The peak observed at 231 cm is related to
W-O-W bending mode of vibration, and the peak observed at
−
1
215 cm
might be due to the coupling between two bending
4
+
of Hf ions for the protons of TPA. This was further supported by
FT–IR results.
modes of vibrations [45]. Hafnium salts of tungstophosphoric
acid show all the characteristic bands related to the Keggin ion,

4
C.R. Kumar et al. / Applied Catalysis A: General 471 (2014) 1–11
Fig. 4. X-ray photo electron spectroscopy of Hf0.5TPA (a) O 1s, (b) W 4f, (c) P 2p, and (d) Hf 4f.

C.R. Kumar et al. / Applied Catalysis A: General 471 (2014) 1–11
5
Table 1
(c)
Binding energy values of Hf0.5TPA catalyst.
Element
Hf (4f)
Binding energy (eV)
16.681
(
b)
(
a)
(
46.2%)17.905 (53.8%)
P (2p)
W (4f)
O (1s)
134.695
33.9%)135.763 (66.1%)
(
35.940
47.1%)37.910 (52.9%)
(
531.779
44.2%)532.776 (55.8%)
(
confirming the existence of the primary structure of heteropoly
tungstate.
1600 1580 1560 1540 1520 1500 1480 1460 1440 1420 1400
-1
Wavenumber (cm )
3.4. X-ray photo electron spectroscopy
Fig. 5. Pyridine adsorbed FT-IR spectra of (a) Hf0.25TPA, (b) Hf0.5TPA, and (c)
Hf0.75TPA.
X-ray photoelectron spectroscopy (XPS) is a versatile surface
analysis technique that can be used for compositional, chemical,
and electronic states of elements. XPS spectra of O 1s, W 4f, P 2p, and
Hf 4f of the Hf0.5TPA catalyst are shown in Fig. 4 and binding energy
values are summarized in Table 1. Fig. 4(a) shows the XPS spectra of
O 1s in the binding energy region of 531.8 and 532.8 eV as a doublet.
The peak at 531.8 eV is attributed to oxygen present in the W–O–W,
and the second peak at 532.8 eV is ascribed to W–O–P [28]. XPS
of W 4f deconvoluted into two peaks in the range of 35.9–37.9 eV
also decreased with an increase in the Hf content, these results
confirm that Lewis acidity was generated by the exchange of TPA
protons with Hf4+
.
3.6. Temperature programmed desorption of ammonia
Acidity of the hafnium salts of heteropoly tungstate cata-
lysts was measured by temperature programmed desorption of
ammonia. The desorbed ammonia from the catalyst surface was
calculated and acid strength distribution values are presented in
the Table 3. Fig. 6 shows the TPD of ammonia profiles, in which
two desorption peaks are observed for all samples. The low tem-
(Fig. 4(b)). Binding energies of 35.9 and 37.9 eV correspond to W
4
f7/2 and W 4f5/2, respectively that are typical binding energy values
6
+
of W oxidation state [28,33].
XPS spectra of P 2p, two peaks are observed with 1.1 eV separa-
tion owing to spin–orbit coupling (see Fig. 4(c)). The binding energy
values of P 2p (Table 1) confirm the presence of phosphorus in the
form of phosphate [46]. Binding energies of Hf 4f were observed at
◦
perature desorption peak at 150 C might be due to the ammonia
molecules adsorbed on Lewis acidic sites and the high temperature
◦
desorption peak at 570 C might be due to the ammonia molecules
1
6.7 (4f7/2) and 17.9 eV (4f5/2), which correspond to the Hf–O bond.
adsorbed on Bronsted acidic sites. When ammonia is adsorbed on
heteropoly acid, generally ammonium salt of heteropoly acid is
formed and desorption of ammonia occurs at higher temperatures
This suggests that Hf is combined onto the oxygen atom of the het-
eropoly anion to form the Hf–O bond and confirms the presence of
Hf state.
4+
[
48]. Fig. 6(a–c) shows that with an increase in the Hf content,
the desorption peak at higher temperatures was shifted to lower
temperatures. The intensity of the peak at higher temperatures
decreased with an increase in Hf content, whereas, at lower tem-
peratures the intensity of the desorption peak increased. These
results suggest that with an increase in hafnium content from 0.25
3
.5. Pyridine adsorbed FT-IR spectra
Pyridine FT-IR analysis was used to investigate the pres-
ence of Bronsted and Lewis acidic sites in the catalyst. The
pyridine-adsorbed FT-IR spectra show various features in the
region of 1400–1600 cm due to the stretching vibrations of M–N
−
1
(
metal–nitrogen) and N–H (pyridinium ion). The band assigned
Table 2
−
1
to the pyridinium ion is recorded at 1536 cm
1
sites. Pyridine FT–IR of HfxTPA samples were recorded at room
temperature and the results are shown in Fig. 5. Hafnium salt of
heteropoly tungstate catalysts contains both Bronsted and Lewis
and the one at
Bronsted to Lewis acid sites intensity ratio of Hf TPA catalysts.
x
−
1
449 cm is associated to pyridine adsorbed onto Lewis-type acid
S. no.
Catalyst
B/L ratio (I1533/I1440)
1.
2.
3.
Hf0.25TPA
Hf0.5TPA
Hf0.75TPA
1.56
1.17
1.16
−
1
acidic sites, as evidenced by bands at ꢂ = 1533 and 1440 cm
,
respectively [27,28]. The Bronsted and Lewis acid sites intensity
ratios were calculated from the intensities of absorbance at 1533
Table 3
−1
Acid strength distribution of HfxTPA catalysts.
and 1440 cm .The presented in Table 2. From this Table it can be
noticed that the Bronsted to Lewis acidity intensity ratio decreased
with an increase in the Hf content. The band at ꢂ = 1485 cm
Catalyst
Acidity (mmol/g)
−
1
Weak/moderate
Strong
Total acidity
corresponds to the combined band that originated from pyridine
adsorbed on Bronsted and Lewis acidic sites. Lewis acidity arises
due to the exchange of protons of TPA with hafnium ions. With
an increase in the hafnium content from 0.25 to 0.75 (Fig. 5(a–c),
respectively), intensity of the band associated with Lewis acidic
sites increased and those corresponding to Bronsted acidic sites
were decreased. Since the Bronsted to Lewis acidity intensity ratio
Hf0.25TPA
Hf0.5TPA
Hf0.75TPA
0.036
0.138
0.298
0.128
0.022
0.025
0.031
0.621
0.516
0.257
0.369
0.032
0.005
0.012
0.657
0.654
0.555
0.497
0.054
0.030
0.043
400-Hf0.5TPA
500-Hf0.5TPA
6
7
00-Hf0.5TPA
50-Hf0.5TPA

6
C.R. Kumar et al. / Applied Catalysis A: General 471 (2014) 1–11
activity of dibenzylether conversion increased from 68% to 97%,
when the x value increased from 0.25 to 0.5, and when hafnium
content was further increased to 0.75, the conversion of diben-
zylether decreased to 84%. In the case of the Hf0.25TPA catalyst,
6
8% of dibenzylether conversion was observed, selectivity towards
p- and o- were 53% and 45%, respectively. A by-product (benzyl
alcohol) was also found with a selectivity of 2%. The Hf0.75TPA cata-
lyst showed 84% of dibenzylether with 51:48 (p-:o-) selectivity. The
Hf0.5TPA catalyst showed much higher conversion of dibenzylether
(97%) and achieved a p-:o- selectivity of 52:48, without formation
of benzyl alcohol, even though the total acidity of the Hf0.25TPA
catalyst was same as the Hf0.5TPA catalyst (Table 3). The higher
conversion of dibenzylether in the case of Hf0.5TPA may be due to
high mobility of residual protons. Hf0.5TPA possesses both Lewis
and Bronsted acidic sites, as evidenced by pyridine FT-IR spectra
(Fig. 5). Acidity results of TPD of ammonia (Fig. 6) are also in good
agreement with the observed catalytic activity. A plausible mecha-
nism is proposed and presented in Scheme 1, in which Bronsted and
Lewis acid sites are involved in the reaction. Path I and II shows the
adsorption of dibenzylether molecule on the Bronsted and Lewis
acidic sites respectively, available on the catalyst surface. In the
second step attack of anisole molecule on adsorbed dibenzylether
molecule leads to cleavage of dibenzylether molecule and produces
benzylated products (o- and p-). In the next step reaction between
another anisole molecule and adsorbed benzyl alcohol (produced
due to cleavage of dibenzylether) produces benzylated products
or adsorbed benzyl alcohol which is produced due to cleavage of
dibenzylether may desorbed form the catalyst surface produces
benzyl alcohol as a by-product in the reaction mixture.
Fig. 6. Ammonia TPD of (a) Hf0.25TPA, (b) Hf0.5TPA, and (c) Hf0.75TPA.
to 0.75, Lewis acidity is increased. For the Hf0.25TPA sample, a weak
◦
desorption peak is observed in the region of 120–280 C due to low
hafnium content, whereas, the Hf0.75TPA sample showed a strong
◦
desorption peak in the same region (120–280 C). The intensity of
the desorption peak at higher temperatures is decreased, indicating
the presence of more Lewis acidic sites in the Hf0.75TPA catalyst. The
desorbed ammonia values related to weak/moderate acidic sites
also increased with an increase in Hf content (Table 3). The partial
exchanged catalysts Hf0.25TPA and Hf0.5TPA exhibited high acidity
compared to Hf0.75TPA catalyst.
FT-IR of pyridine adsorbed spectra results are also in good agree-
ment with these results. The partial exchanged catalyst (Hf0.5TPA)
showed moderate acidic sites related to Lewis acid, which resulted
in high mobility of the residual protons [27].
3
.8.1. Effect of catalyst calcination temperature
Hf0.5TPA was the most active catalyst and subjected to calcina-
tion at different temperatures to investigate the surface-structural
properties, thermal stability, and its influence on benzylation of
anisole with dibenzylether. Fig. 9 shows the effect of catalyst calci-
nation temperature on benzylation of anisole with dibenzylether.
Conversion of dibenzylether was decreased with an increase in cal-
cination temperature. The catalyst was calcined at a temperature
3.7. SEM analysis
Structural morphology of HfxTPA catalysts were studied by
scanning electron microscopy (Fig. 7). Hafnium salts of heteropoly
tungstate particles are irregular, not uniform, and agglomerated.
The Hf0.5TPA sample appears to have small particles compared to
the other two samples.
◦
of 300 C and showed maximum conversion of dibenzylether with
9
7% yield of benzylated products. When the catalyst was calcined
◦
at 400 and 500 C, conversion of dibenzylether was 84% and 1%,
respectively. To better understand the variations in catalytic activ-
ity of the catalysts with calcination temperature, they were further
characterized by FT-IR, Laser Raman, and TPD of ammonia.
Fig. 10 shows the FT-IR spectra of Hf0.5TPA catalyst calcined
at different temperatures. The IR bands related to the Keggin ion
are clearly observed in the catalysts calcined at 300 and 400 C,
indicating that the Keggin ion structure can be retained up to a
calcination temperature of 400 C (Fig. 10(a and b)). It was noticed
that a slight decrease in the intensity of the peaks related to Keggin
3.8. Evaluation of catalytic activity for benzylation of anisole
with dibenzylether
◦
Fig. 8 shows the conversion of dibenzylether and product distri-
bution for benzylation of anisole with dibenzylether over HfxTPA
catalysts along with the parent TPA. The main products in ben-
zylation of anisole are p-benzyl anisole and o-benzyl anisole. As
shown in Fig. 8, TPA showed only 35% of dibenzylether conver-
sion with selectivity toward desired products i.e., p- and o- are
◦
◦
ion, for the catalyst calcined at 400 C compared to catalyst cal-
cined at 300 C. This might be due to partial degradation of the
◦
4
7 and 41 (mol%), respectively. In the case of the HfxTPA catalyst,
Keggin ion of Hf0.5TPA catalyst. Decomposition of the Keggin ion
Fig. 7. SEM images of (a) Hf0.25TPA, (b) Hf0.5TPA, and (c) Hf0.75TPA.

C.R. Kumar et al. / Applied Catalysis A: General 471 (2014) 1–11
7
dibenzylether conversion
p-benzyl anisole
o-benzyl anisole
benzyl alcohol
100
80
60
40
20
0
Hf0.25TPA
Hf0.5TPA
Hf0.75TPA
◦
Fig. 8. Catalytic activity of the HfxTPA (where x = 0.25, 0.5, and 0.75) catalysts for benzylation of anisole with dibenzylether. Reaction conditions: Reaction temperature 120 C,
◦
catalyst calcination temperature 300 C, catalyst loading 100 mg, Anisole to dibenzylether molar ratio 12:2 and reaction time 1 h.
100
Dibenzylether conversion
p-benzyl anisole
o-benzyl anisole
(e)
80
60
40
20
0
(
d)
(
(
C)
b)
(a)
2
000
1500
1000
500
-1
Wavenumber (cm )
300
400
500
o
◦
◦
◦
Fig. 10. FT-IR spectra of Hf0.5TPA catalyst calcined at (a) 300 C, (b) 400 C, (c) 500 C,
(
Calcination temperature ( C)
◦ ◦
d) 600 C, and (e) 750 C.
Fig. 9. Effect of Hf0.5TPA catalyst calcination temperature on benzylation of anisole
◦
with dibenzylether. Reaction conditions: Reaction temperature 120 C, catalyst load-
ing 100 mg, anisole to dibenzylether molar ratio 12:2 and reaction time 1 h.
intensity, which indicates the partial degradation of Keggin ion.
These results are in good agreement with the results obtained from
FT-IR spectra. The catalyst calcined at 500 C (Fig. 11(c)) also shows
◦
◦
−1
started at 500 C, as it shows sharp IR bands related to the Keggin
a weak band at 1006 cm
related to asymmetric vibrations of
−1
ion (Fig. 10(c)). A further increase in calcination temperature (600
W=Ot, as well as strong bands at 790, 690, and 272 cm . The Raman
◦
−1
and 750 C) resulted in complete decomposition of the Keggin ion
bands at 790 and 690 cm are ascribed to stretching vibrations
−
1
of heteropoly tungstate into constituent metal oxide WO3 [23].
Fig. 11 shows the Laser Raman spectra of the Hf0.5TPA catalyst
of W–O–W and the band at 272 cm relates to the characteristic
bending mode vibration of WO3 [49]. With an increase in calci-
nation temperature of the catalyst from 600 to 750 C (Fig. 11(d
◦
◦
calcined at different temperatures in the range of 300–750 C. The
◦
catalysts calcined at 300 C (Fig. 11(a)) clearly shows that bands
at 1006 and 990 cm are associated with asymmetric and sym-
metric vibrations of W=Ot (Ot—terminal oxygen). Catalyst calcined
and e)), the band related only to WO3 phase was observed. These
−1
◦
results suggest that the catalyst is thermally stable upto 400 C and
that a further increase in temperature causes decomposition of the
Keggin ion of heteropoly tungstate.
◦
at 400 C also showed these peaks related to Keggin ion with low

8
C.R. Kumar et al. / Applied Catalysis A: General 471 (2014) 1–11
OCH3
OCH3
O
HfH[PW12O40
]
OCH3
O
Path-I (Bronsted acid)
H
HfH[PW12O40
]
O
HfH[PW12O40
]
O
OCH3
HfH[PW12O40
]
HO
HfH[PW12O40
]
O
HfH[PW12O40
]
OCH3
Path-II (Lewis acid)
H
OCH3
O
HfH[PW12O40
]
O
OCH3
HfH[PW12O40
]
Scheme 1. Plausible mechanism for benzylation of anisole with dibenzylether.
◦
Acidity of the catalysts calcined at different temperatures was
intensity compared to catalyst calcined at 300 C. However, in TPD
◦
◦
analyzed by NH -TPD (Fig. 12). The catalyst calcined at 300 C
of ammonia pattern of catalyst calcined at 400 C a shift in high
temperature desorption peak to higher temperature (700 C) was
3
◦
◦
(
Fig. 12(a)) showed a strong desorption peak centered at 620 C.
The catalyst calcined at 400 and 500 C (Fig. 12(b and c)) showed
a small desorption peak at higher temperatures (580 C) that
◦
observed. This might be due to partial degradation of the Keggin
units of Hf0.5TPA this may be accountable for the benzylation
activity.
The experimental result of catalytic activity for benzylation of
anisole indicates that catalytic activity depends on the acidity of
catalysts, which in turn is related to the existence of the heteropoly
anion. To optimize the dibenzylether conversion, the effect of
reaction parameters such as reaction temperature, catalyst weight,
and anisole to dibenzylether molar ratio were also studied.
◦
were absent for the catalysts calcined at higher temperatures
◦
of 600 and 750 C (Fig. 12(d and e)), indicating no considerable
acidity for these catalysts compared to the catalyst calcined at
◦
lower temperatures of 300 and 400 C. This might be due to the
decomposition of heteropoly anion at higher calcination tem-
◦
perature. The catalyst calcined at 400 C showed characteristic
peaks of Keggin ion in the FT–IR and Raman spectra with low

C.R. Kumar et al. / Applied Catalysis A: General 471 (2014) 1–11
9
Table 4
Effect of catalyst weight.
Catalyst weight (mg)
Conversion of dibenzyl ether (%)
Selectivity (%)
o-
p-
Benzyl alcohol
5
1
1
2
0
47.0
64.0
89.60
100.0
40.5
46.0
48.0
47.0
47.0
50.7
52.0
53.0
12.5
3.3
–
00
50
00
–
◦
◦
Reaction conditions: Reaction temperature 120 C, catalyst calcination temperature 300 C, anisole to dibenzylether molar ratio 12:2 and reaction time 30 min.
o-benzylanisole
p-benzylanisole
Benzyl alcohol
(e)
100
1
8
6
00
80
60
40
20
0
0
0
(d)
40
20
0
(
c)
b)
a)
(
(
200
400
600
800
1000
1200
-
1
Raman shift (cm )
◦
◦
Fig. 11. Raman spectra of Hf0.5TPA catalyst calcined at (a) 300 C, (b) 400 C, (c)
100
120
140
◦
◦
◦
5
00 C, (d) 600 C, and (e) 750 C.
o
Reaction temperature ( C)
Fig. 13. Effect of reaction temperature on benzylation of anisole with dibenzylether.
Reaction conditions: Anisole to dibenzylether molar ratio 12:2, catalyst (Hf0.5TPA)
loading 100 mg and reaction time 15 min.
amount from 50 to 200 mg. Experimental results are shown in
Table 4 at a reaction time of 30 min. An increase in catalyst weight
from 50 to 200 mg increased the conversion of dibenzylether. A
by-product was observed when a lower amount (50 and 100 mg)
of catalyst was used, but when 200 mg of catalyst was used, a com-
plete conversion of dibenzylether with selectivity of p- to o- of 53
and 47%, respectively, was observed without any by-product.
3.8.2.3. Effect of molar ratio. To assess its effect on catalytic activ-
ity and product selectivity over the Hf0.5TPA catalyst, benzylation
of anisole with dibenzylether was carried out by varying the molar
ratio of anisole to dibenzylether from 12:1 to 12:4 (Table 5). With an
increase in reactant molar ratio from 12:1 to 12:4, the conversion
of dibenzylether decreased. The higher concentration of diben-
zylether leads to a higher adsorption of dibenzylether molecules
onto the catalyst surface, and less availability of anisole molecules
retards the benzylation reaction. In all the cases, there was no
marginal change in product selectivity and maximum conversion
◦
◦
◦
Fig. 12. Ammonia TPD of Hf0.5TPA calcined at (a) 300 C, (b) 400 C, (c) 500 C, (d)
6
◦ ◦
00 C, and (e) 750 C.
3.8.2. Optimization of reaction parameters
3
.8.2.1. Effect reaction temperature. Benzylation of anisole with
dibenzylether was carried out at various temperatures ranging
from 80 to 140 C at a reaction time of 15 min (Fig. 13). There was
no significant conversion of dibenzylether at 80 C (not shown). An
increase in reaction temperature from 80 to 140 C increased the
dibenzylether conversion and complete conversion without any
by-products at 140 C. A by-product benzyl alcohol was observed
(100%) of dibenzylether was observed in the 12:1 molar ratio.
◦
◦
Table 5
◦
Influence of anisole to dibenzylether molar ratio.
◦
Anisole to DBE molar ratio
Conversion of DBE (%)
Selectivity (%)
when the reaction was carried out at lower temperatures. Selec-
tivity towards benzylated products increased with an increase in
reaction temperature.
o-
p-
12:1
12:2
12:4
100
85.16
75.81
47.0
47.7
48.2
53.0
52.3
51.8
◦
3
.8.2.2. Effect of catalyst weight. The effect of catalyst weight on
Reaction conditions: Reaction temperature 120 C, Catalyst calcination temperature
◦
dibenzylether conversion was studied by varying the catalyst
300 C, catalyst loading 100 mg and reaction time 30 min.

1
0
C.R. Kumar et al. / Applied Catalysis A: General 471 (2014) 1–11
Table 6
Comparison of the catalytic activity of Hf0.5TPA catalyst with other reported acid catalysts for benzylation of aromatics with dibenzylether.
◦
Catalyst
Substrate
Reaction temperature ( C)
Reaction time (h)
Conversion of dibenzylether (%)
Product yield (%)
100
Ref.
Hf0.5TPA
Anisole
Benzene
Benzene
Anisole
Benzene
140
90
180
90
0.25
1.67
24
0.5
12
100
50
100
Present study
[50]
[51]
[34]
[52]
HPW(50)/MCM-41
Sulfated zirconia/MCM-41
Ir/Sn bimetallic catalyst
FeCl3
95*
99
68
90
*
Diphenylmethane selectivity.
3.9. Comparison of the catalytic activity of Hf0.5TPA with
3.10. Catalyst reusability
reported catalysts
To determine the efficiency of the Hf0.5TPA catalyst, reusabil-
ity experiments were carried out. After completion of the reaction,
the catalyst was separated from the reaction mixture by centrifu-
gation, followed by ethyl acetate washes. The catalyst was then
The benzylation of aromatics with benzyl alcohol reaction was
studied well, but few reports are available on the benzylation of
aromatics with dibenzylether. In the present study, the catalytic
activity of optimized catalyst Hf0.5TPA was compared with those
of reported catalysts for the benzylation of aromatics with diben-
zylether (DBE) and the results are shown in Table 6. The Hf0.5TPA
catalyst exhibited a complete conversion of DBE within 0.25 h with
◦
dried in a hot air oven at 120 C for 1 h and reused for benzyla-
tion of anisole with dibenzylether. Catalytic activity of the reused
catalyst is presented in Fig. 14. The recycling results showed that
there is marginal change in the selectivity of the benzylated prod-
ucts. However, formation of the by-product (benzyl alcohol) was
observed when the catalyst was reused. The selectivity of benzyl
alcohol was only 5% after the third recycle run of the catalyst. It
was found that, the catalytic activity of the catalyst decreased by
∼6% after the fourth run when compared to the first run with a fresh
1
00% selectivity towards benzylated products when compared to
other catalysts. Kamalakar et al. studied the role of dibenzylether
◦
(
DBE) in benzylation of benzene at a reaction temperature of 90 C.
The time required for 50% conversion of DBE was about 1.67 h.
Al-Hazmi et al. examined the benzylation of benzene with DBE.
The conversion of DBE was 100% and selectivity of the product
catalyst. It is concluded that the Hf0 TPA catalyst can be reused for
.5
(
diphenylmethane) was 95%. However, 100% conversion of DBE was
benzylation reaction without any considerable loss in the activity.
◦
achieved only at high reaction time (24 h) and temperature (180 C).
Ir/Sn bimetallic catalyst was reported by Podder et al. for benzyla-
tion of arenes with ethers. The benzylation of anisole reaction was
4. Conclusions
◦
studied at 90 C and the selectivity of benzylated product was 99%
Hafnium salt of heteropoly tungstate catalysts was prepared
within 0.5 h. But the main disadvantage of this catalyst is limitation
to reuse. Wang et al. investigated the benzylation of benzene with
DBE over FeCl3 catalyst. The obtained desired product diphenyl-
methane yield was only 68% after 12 h of reaction time. However,
this catalyst is well known traditional homogeneous Lewis acid
catalyst and it cannot be recycled. This study indicates that the
Hf0.5TPA catalyst is highly active and selective for benzylation reac-
tion with DBE compared to above reported catalysts.
with retention of the Keggin ion structure by varying hafnium
content. Catalytic activity depends on exchangeable hafnium ions
with protons of TPA. Partial exchange of TPA protons with Hf
4+
ions results in high mobility of residual protons and exhibits high
activity and selectivity towards benzylated products compared to
parent TPA, due to the presence of both Lewis and Bronsted acidic
sites. A plausible mechanism, involving Bronsted and Lewis acidic
sites in the reaction is also proposed for benzylation of anisole with
dibenzylether. The optimized reaction conditions for benzylation of
anisole with dibenzylether are: Catalyst calcination temperature
◦
◦
3
1
00 C, 100 mg catalyst loading, 140 C reaction temperature and
2:2 molar ratio of anisole to dibenzylether.
Acknowledgments
Ramesh Kumar acknowledges the University of Saskatchewan
and the Canadian Bureau for International Education (CBIE) for
financial assistance through the Canadian Commonwealth schol-
arship program. Support from the Canada Research Chair (CRC)
program is also acknowledged.
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