The role of niobia location on the acidic and catalytic functionalities of heteropoly tungstate

Article abstract of DOI:10.1016/j.apcata.2015.06.023

12-Tungstophosphoric acid (TPA) was modified systematically by incorporating niobium into its primary and secondary structure. TPA supported on niobia catalyst was also prepared by impregnation method. These catalysts were characterized by FT-infra red, X-ray diffraction, Laser Raman, X-ray photo electron spectroscopy, X-ray absorption spectroscopy, UV–vis diffuse reflectance spectroscopy, solid state ³¹P nuclear magnetic resonance spectroscopy, temperature programmed desorption of ammonia and pyridine adsorbed FT-IR spectroscopy. Characterization results suggest the presence of intact Keggin structure after modification of parent TPA. Incorporation of Nb into the secondary structure results in generation of Lewis acidic sites. Nb containing TPA catalysts exhibited high acidity compared to the TPA supported on niobia catalyst. The activity of the catalysts was evaluated for benzylation of anisole with benzyl alcohol. Catalyst activity depended on the location of niobium ion in the heteropoly tungstate.

Full text of DOI:10.1016/j.apcata.2015.06.023

Applied Catalysis A: General 502 (2015) 297–304
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
The role of niobia location on the acidic and catalytic functionalities of
heteropoly tungstate
Ch. Ramesh Kumara, S. Gatlab, O. Mathonb, S. Pascarellib, N. Lingaiaha,∗
a Catalysis Laboratory, Inorganic & Physical Chemistry Division, CSIR–Indian Institute of Chemical Technology, Hyderabad, Telangana 500 007, India
b Electronic Structure and Magnetism (ESM) Group, European Synchrotron Radiation Facility (ESRF) 71, Avenue des Martyrs, 38000 Grenoble, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 March 2015
Received in revised form 17 June 2015
Accepted 19 June 2015
Available online 23 June 2015
12-Tungstophosphoric acid (TPA) was modified systematically by incorporating niobium into its primary
and secondary structure. TPA supported on niobia catalyst was also prepared by impregnation method.
These catalysts were characterized by FT-infra red, X-ray diffraction, Laser Raman, X-ray photo elec-
tron spectroscopy, X-ray absorption spectroscopy, UV–vis diffuse reflectance spectroscopy, solid state
31P nuclear magnetic resonance spectroscopy, temperature programmed desorption of ammonia and
pyridine adsorbed FT-IR spectroscopy. Characterization results suggest the presence of intact Keggin
structure after modification of parent TPA. Incorporation of Nb into the secondary structure results in
generation of Lewis acidic sites. Nb containing TPA catalysts exhibited high acidity compared to the TPA
supported on niobia catalyst. The activity of the catalysts was evaluated for benzylation of anisole with
benzyl alcohol. Catalyst activity depended on the location of niobium ion in the heteropoly tungstate.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Anisole
Benzyl alcohol
Dibenzylether
Keggin ion
Niobium
Heteropoly tungstate
1. Introduction
The acidic and redox properties of the modified HPA catalysts
depend on the nature of metal ion and its location in Keggin
Polyoxometalates (POMs) are composed of early transition-
metals of group V and VI in their highest oxidation states [1]. POMs
are classified into two categories, namely isopolyoxometalates
and heteropolyoxometalates. Among the heteropolyoxometalates,
Keggin type are the most frequently used in catalysis, represented
ion. HPAs with incorporation of V into its primary structure were
reported [7,10]. This modification resulted in the increase of the
redox nature of HPAs particularly heteropoly molybdate for dif-
ferent oxidation reactions [11–13]. The modification of HPAs by
incorporating different metal ions in their primary and secondary
structures will totally provide new catalytic functionalities. It is
anticipated that by modifying the heteropoly tungstate which is
the strongest acid among the HPAs by different metal ions such as
Nb will result in much stronger acidic catalysts. Niobia is known for
its acidic behavior and expected to modify the acidity and stability
of HPA to a great extent.
In the present work, the role of niobium in the heteropoly
tungstate is investigated by preparing (i) incorporation of niobium
in the primary structure of Keggin ion of heteropoly tungstate
(TPANb1), (ii) incorporation of niobium in the secondary struc-
ture i.e., substitution of counter cations (H+) of TPA with niobium
(Nb0.6TPA) and (iii) TPA supported on Nb2O5 (25% TPA/Nb2O5)
as illustrated in Scheme 1. These catalysts are characterized by
different techniques to derive their surface, structural and acidic
properties. The catalysts were evaluated for their catalytic activ-
ity by taking benzylation of anisole with benzyl alcohol as a
by the formula [XM12O40]
n−. The Keggin anion is made of an assem-
bly of twelve MO6 octahedrons sharing their corners or edges with
a central XO4 tetrahedron, where X is a hetero atom (P5+, Si4+),
n is the oxidation state and M is the addenda atom (Mo6+, W6+
,
n−
V5+). The metal oxide cluster molecule [XM12O40
]
is called as
primary structure. Secondary structure is a regular arrangement
of poly anions counter cations and also some polar molecule such
as water [2]. The catalytic properties of heteropoly acids (HPAs)
can be tuned in a systematic way by exchanging counter-cations
and hetero atoms [3–6]. Park et al. studied the acidic and oxidation
properties of HPA catalysts by exchanging the identity of addenda
atom with group-V metal ions such as V5+, Nb5+ and Ta5+ [7–9].
∗
Corresponding author. Fax: +91 40 27160921.
E-mail address: nakkalingaiah@iict.res.in (N. Lingaiah).
http://dx.doi.org/10.1016/j.apcata.2015.06.023
0926-860X/© 2015 Elsevier B.V. All rights reserved.

298
Ch. Ramesh Kumar et al. / Applied Catalysis A: General 502 (2015) 297–304
Scheme 1. Structure of (a) niobium incorporated Keggin ion (TPANb1 or [PW11Nb1O40
]
4−), (b) niobium salt of tungstophosphoric acid (Nb0.6TPA or Nb0.6PW12O40), and (c)
tungstophosphoric acid supported on niobia (25% TPA/Nb2O5 or 25% H3PW12O40/Nb2O5).
model reaction. The influence of different benzylating agents is also
studied.
and secondary graphite monochromatic. The measurements were
obtained in steps of 0.045◦ with count times of 0.5 s, in the 2ꢀ range
of 2–80◦.
Confocal micro-Raman spectra were recorded at room temper-
2. Experimental
ature in the range of 200–1200 cm−1 using a Horiba Jobin-Yvon
Lab Ram HR spectrometer with
a 17 mW internal He–Ne
2.1. Catalyst preparation
(Helium–Neon) laser source of excitation wavelength of 632.8 nm.
The catalyst samples in powder form (about 5–10 mg) were loosely
spread onto a glass slide below the confocal microscope for mea-
surements.
Nb0.6TPA catalyst was prepared as following. 5 g of TPA was dis-
solved in distilled water and 0.56 g of niobium oxalate dissolved
in 0.1 M oxalic acid solution was added to aqueous solution of TPA
with continuous stirring. The resultant mixture was stirred for 3 h.
Excess water was evaporated on a water bath and the dried cata-
lyst mass was kept for further drying in an air oven for overnight.
Finally the sample was calcined at 300 ◦C for 2 h.
TPANb1 catalyst was prepared according to the procedure
reported in the literature [7]. In a typical procedure, 10 g of niobium
oxalate was dissolved in 100 ml of 0.1 M oxalic acid solution and 6 g
of Na2HPO4 was dissolved in 100 ml of distilled water. These two
solutions were mixed together. 100 g of Na2WO4·2H2O was dis-
solved in 150 ml of distilled water, and subsequently added to the
solution containing niobium and phosphorous precursors with vig-
orous stirring. After heating the resulting solution to 80 ◦C, 60 ml
of H2SO4 was slowly added to it. The solution was further kept for
reflux at 80 ◦C for 8 h. After cooling the solution to room temper-
ature, the formed TPANb1 was extracted with diethyl ether. The
resulting etherate was maintained at 50 ◦C to obtain solid product,
dried in an oven and calcined at 300 ◦C for 2 h.
The acidity of the catalysts was measured by temperature pro-
grammed desorption (TPD) of ammonia. In a typical experiment,
0.1 g of catalyst was loaded and pre-treated in He gas 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 balanced He gas over the catalyst for 1 h. The catalyst surface
was flushed with helium gas at the same temperature for 2 h to flush
off the physisorbed NH3. TPD of NH3 was carried with a tempera-
ture ramp of 10 ◦C/min and the desorbed ammonia was monitored
using 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 (hꢁ = 1486.6 eV) was operated at 12.5 kV and 16 mA.
Before acquisition of the data each sample was out-gassed for about
3 h at 100 ◦C under vacuum of 1.0 × 10−7 T to minimize surface
contamination. The XPS instrument was calibrated using Au as
standard. For energy calibration, the carbon 1s photoelectron line
was used. The carbon 1s binding energy was taken as 285 eV. Charge
neutralization of 2 eV was used to balance the charge up of the
sample. The spectra were deconvoluted using Sun Solaris Vision-
2 curve resolver. The location and the full width at half maximum
(FWHM) value for the species were first determined using the spec-
trum 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.
TPA supported niobium oxide catalyst was prepared by impreg-
nation method. In a typical procedure, required amount of TPA
was dissolved in water and this solution was added to niobium
oxide with continuous stirring. The resultant mixture was allowed
to stand for 3 h and excess water was evaporated on a water bath.
The catalyst was kept overnight for drying in an air oven at 120 ◦C
and finally calcined at 300 ◦C for 2 h. The TPA content on niobia is
kept as 25 wt%. The catalyst is denoted as 25% TPA/Nb2O5.
The X-ray absorptions (XAS) experiments were performed at the
Nb K edge (18.986 KeV) at the BM23 beam line at the European Syn-
chrotron Radiation Facility (ESRF, Grenoble). Monochromatic X-ray
beam was obtained from the white beam by using Si(1 1 1) double
crystal; a harmonic rejection has been performed using Si coated
mirrors. Both the incident (I0) and transmitted (I1) monochromatic
beam intensities were measured by using ionic chambers filled
with 0.65 bar Ar and 2.1 bar Ar, respectively, and eventually I0 filled
up to 2 bar with He. The photon energy was calibrated with the edge
energy obtained from the maxima first derivative of the Nb K-edge
in the Nb foil (18.986 KeV). The reference samples (Nb foil, NbO2)
were measured in transmission mode. Nb2O5 is according to http://
ixs.iit.edu/database For the TPANb1, Nb0.6TPA and 25% TPA/Nb2O5
samples measurements were done in fluorescence mode with a
13-element Ge detector (Canberra Industries). The extraction of the
2.2. Characterization of catalysts
The FT-IR spectra were recorded on a Bio-Rad Excalibur series
spectrometer using the KBr disc method.
The nature of the acid sites (Bronsted and Lewis) of the catalyst
samples was determined by FT-IR spectroscopy 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 samples were degassed
under vacuum at 200 ◦C for 3 h and exposed to dry pyridine.
Then, the excess pyridine was removed by heating the sample at
120 ◦C. After cooling the sample to room temperature, FT-IR spec-
tra of the pyridine-adsorbed samples were recorded. X-ray powder
diffraction (XRD) patterns were recorded on a Rigaku Miniflex
diffractometer using Cu K␣ radiation (1.5406 ◦A) at 40 kV and 30 mA

Ch. Ramesh Kumar et al. / Applied Catalysis A: General 502 (2015) 297–304
299
dominated by the IR spectrum of niobia support to large extent.
This is because of strong absorption of niobia in the same infrared
region of Keggin ion [15]. Even so, the absorption bands at 1081 and
987 cm−1 still appeared, however, with low intensity suggesting
the presence of Keggin ion on niobia support.
(d)
(c)
3.2. X-ray diffraction patterns
XRD patterns of the samples are shown in Supplementary infor-
mation as Fig. S1. The patterns of parent TPA are also included for
the sake of comparison. TPANb1 and Nb0.6TPA catalysts showed the
most intense peaks at 2ꢀ of 10.2◦, 25.4◦ and 33.5◦ which are char-
acteristic patterns of Keggin heteropoly acid. No pattern related
to niobia was observed in the case of TPANb1 and Nb0.6TPA cat-
alysts. These results indicate the successful formation of TPANb1
and Nb0.6TPA Keggin ions [16]. In the supported catalyst weak
diffraction peaks of Keggin ion were observed at 2ꢀ of 10.2◦, 25.4◦,
33.5◦. Caliman et al. reported that the catalyst up to 40 wt% of
TPA on niobia and calcined below 450 ◦C shows only characteristic
peaks of niobia [15]. Similar result was observed in the case of 25%
TPA/Nb2O5 catalyst. These results substantiate that exchanging the
protons of TPA with Nb5+ ion and incorporation of Nb5+ in the pri-
mary Keggin structure of heteropoly anion do not alter the Keggin
structure of heteropoly tungstate.
(b)
(a)
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 1. FT-IR spectra of (a) TPA, (b) TPANb1, (c) Nb0.6TPA, and (d) 25% TPA/Nb2O5
catalysts.
EXAFS ␹(k) function has been done using Athena. Fourier transform
of EXAFS function ␹(k) into R space with k2 weighted factor and
3.3. Laser Raman spectra
Kaiser–Bessel window function has been performed in 2–10 Å−1
,
Laser Raman spectra of the catalysts along with spectra of par-
ent TPA are also presented in Supplementary information (Fig. S2).
Nb0.6TPA and TPANb1 catalysts showed characteristic bands at 991,
1010 cm−1 corresponding to asymmetric and symmetric stretch-
ing of W Ot of TPA [15,17]. In the case of 25% TPA/Nb2O5 sample,
less intense bands related to symmetric stretching frequency of
yielding a function ꢀ␹(R)ꢀ (Å−3).
The UV–vis diffuse reflectance spectra (UV–vis DRS) were
recorded on a GBC UV–visible Cintra 10e spectrometer in the range
of 200–800 nm.
31P nuclear magnetic resonance (NMR) spectra of solids were
recorded in a 400 MHz Bruker spectrometer. A 4.5 ␮s pulse (90◦)
was used with repetition time of 5 s between pulses in order to
avoid saturation effects. Spinning rate was 5 kHz. All the measure-
ments were carried out at room temperature using 85% H3PO4 as
standard reference.
W
Ot are noticed due to amorphous nature of niobia and lower
loading amount of TPA on niobia. Caliman et al. reported that with
increased amount of tungstophosphoric acid on niobia, the Keggin
peaks became more intense and well defined [15]. In case of TPANb1
and Nb0.6TPA, the peak in the range of 700–800 cm−1 related to
WO3 (decomposition product of TPA) is absent indicating the sta-
bility of TPA structure. These results point to the incorporation of
Nb5+ into the structure of heteropoly tungstate. Supporting of TPA
on niobia also does not cause any decomposition of Keggin ion but
reduced the intensity of the peaks. Besides, a strong band centered
at ca. 700 cm−1 generated, associated to the stretching modes of
different niobia polyhedra [15,18]. These results are in accordance
with those obtained by FT-IR and XRD.
2.3. General alkylation reaction procedure
The alkylation reaction was carried out in a 50 ml two-necked
round bottom flask provided with a reflux condenser with nitro-
gen inlet and a septum for sample removal. In a typical run, 10 g of
anisole and 3.37 g of benzyl alcohol (or 3.08 g dibenzylether) along
with 0.1 g catalyst were taken. The reaction was carried out at a
reaction temperature of 120 ◦C under nitrogen atmospheric condi-
tion. The reaction mixture was withdrawn at different intervals and
analyzed by gas chromatography (VARIAN GC-3800) equipped with
a SE-30 column and flame ionization detector. The identification of
products was made from GC–MS (SHIMADZU-2010) analysis.
3.3.1. In-situ laser Raman spectra
In order to know the structural changes during the calcina-
tion in niobium containing heteropoly tungstate, Nb0.6TPA and
TPANb1 catalysts were subjected to in-situ Raman studies during
calcination. The spectra were recorded in-situ at various temper-
atures ranging from room temperature to 600 ◦C. Fig. 2(a) and (b)
shows the in-situ Raman spectra of Nb0.6TPA and TPANb1 catalysts,
respectively. Generally Keggin bands of TPA were observed at 1010,
991, 237, 219, 158 and 102 cm−1 [15]. Raman spectra of the samples
recorded at room temperature shows bands at 991 and 1010 cm−1
corresponding to asymmetric and symmetric stretching frequen-
cies of W Ot bond of Keggin ion. Further increase in temperature
3. Results and discussion
3.1. FT-IR spectroscopy
FT-IR spectra of the catalysts are presented in Fig. 1. All the cat-
alysts showed the characteristic bands of Keggin ion in its finger
print region of 500–1200 cm−1 [8,14]. The bands observed at 1081
and 987 cm−1 are related to stretching vibrations of P O and W Ot
bonds of Keggin ion. Additionally, the bands present at 891and
790 cm−1 are associated to M Oc M and M Oe M bonds. All these
four characteristic bands of Keggin ion are noticed in the TPANb1
and Nb0.6TPA samples indicating the retention of Keggin ion struc-
ture. However, this is not the situation in the impregnated 25%
TPA/Nb2O5 sample. Instead, the bands related to Keggin ion are
to 200 ◦C these bands shifted to higher wave numbers to 1017 cm−1
.
The shift of W Ot vibration is due to the loss of crystal water and
removal of its hydrogen bonding [10]. On further increase in calci-
nation temperature up to 450 ◦C, not much variation in the spectra
was observed. But marginal changes to the spectra were noticed
above 500 ◦C only. At this temperature, the TPANb1 catalyst (Fig. 6)
showed partial degradation of Keggin ion, whereas destabilization

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Ch. Ramesh Kumar et al. / Applied Catalysis A: General 502 (2015) 297–304
Table 1
Acidity of TPANb1, Nb0.6TPA, and 25% TPA/Nb2O5 catalysts.
Catalyst
Weak
Acidity (mmol/g)
Moderate
Strong
Total
(g)
TPANb1
Nb0.6TPA
25% TPA/Nb2O5
0.19
0.18
0.29
–
0.03
0.03
0.84
0.29
0.04
1.03
0.50
0.36
(f)
(e)
(d)
(c)
(b)
(a)
200
400
600
800
1000
1200
Raman shift (cm-1)
(i)
(h)
(g)
(f)
(e)
Fig. 3. Pyridine adsorbed FT-IR spectroscopy (a) TPA, (b) TPANb1, (c) Nb0.6TPA, and
(d)
(c)
(d) 25% TPA/Nb2O5 catalysts.
the low temperature (120–350 ◦C). Conversely, the supported cata-
lyst showed less intense desorption peaks at high temperature but
slightly higher intense low temperature peak compare to TPANb1
and Nb0.6TPA samples. The high temperature desorption peaks are
correspond to the strong acidic sites of TPA, whereas low temper-
ature peaks are related to weak acidic sites mainly originated from
support niobia. Interestingly, Nb0.6TPA contains another small des-
orption peak at 410 ◦C which can be related to moderate Lewis
acidic sites. But the strong desorption peak in Nb0.6TPA catalyst was
observed at relatively lower temperature (≈550 ◦C) with low inten-
sity compared to its counterpart TPANb1 (≈610 ◦C). Shifting of this
desorption peak to higher temperature point out that TPANb1 has
strong acidic sites [9]. In the case of metal salts of TPA, shift in the
desorption peak towards lower temperature is an indication for the
exchange of H+ ion with the external metal cations [16,17]. Similar
situation is noticed in the present Nb0.6TPA catalyst, which specify
the substitution of H+ by niobium. These results suggests that the
catalyst in which Nb is incorporated in to the primary structure of
TPA exhibited highest acidity compared to other catalysts as shown
in Table 1. These results indicate that the location of Nb influences
the acidity of Nb containing TPA catalysts.
(b)
(a)
200
400
600
800
1000
1200
Raman shift (cm-1)
Fig. 2. In-situ Raman spectra of (A) Nb0.6TPA (B) TPANb1 catalysts during exposure
to different temperatures (a) RT (b) 200 ◦C (c) 300 ◦C (d) 400 ◦C (e) 450 ◦C (f) 500 ◦
(g) 600 ◦C.
C
of the Keggin structure is more prominent in Nb0.6TPA catalyst
(Fig. 2a). The Raman spectra of Nb0.6TPA sample recorded at 500 ◦C
showed two additional peaks at 946 cm−1 and a broad band in the
range of 600–800 cm−1. The band at 946 cm−1 is related to tungsten
oxide [19] and the broad band in the range of 600–800 cm−1 is due
to niobium oxide [15]. In case of TPANb1 sample even though the
restructuring initiated at 500 ◦C, the new bonds are apparent only
after 550 ◦C. As a whole, we can say that the Nb incorporated TPA
(TPANb1) catalyst is stable up to 500 ◦C and the Nb exchanged TPA
catalyst (Nb0.6TPA) is slightly less stable than that of TPANb1.
3.5. Pyridine adsorbed FT-IR spectroscopy
3.4. Temperature programmed desorption of ammonia
Pyridine adsorbed FT-IR spectra of niobium containing cat-
alysts were recorded and the results are shown in Fig. 3. The
pyridine adsorbed FT-IR spectra showed various features in the
region of 1400–1600 cm−1 due to the stretching vibrations of
M–N (metal–nitrogen) and N–H (pyridinium ion). The bands at
1536 cm−1 corresponds to Bronsted acidic sites and band related to
Lewis acidic sites appeared at 1440 cm−1. All the catalysts contain
bands at 1534 and 1485 cm−1 related to Bronsted acidic sites [20].
However, Nb0.6TPA catalyst exhibited one more band at 1440 cm−1
corresponding to Lewis acidic sites. This is due to presence of Nb
Acidity of the niobium containing heteropoly tungstate samples
were measured by TPD of NH3 experiments. The TPD of ammonia
profiles are shown in supplementary information as Fig. S3. The
acidity of TPANb1, Nb0.6TPA and 25% TPA/Nb2O5 catalysts were
determined from the peak area of desorbed ammonia and summa-
rized in Table 1. The total acidity of the samples is in the following
order: TPANb1 > Nb0.6TPA > 25% TPA/Nb2O5. Both the TPANb1 and
Nb0.6TPA catalysts showed strong NH3 desorption peaks at high
temperature (500–700 ◦C) and low intense desorption peaks in

Ch. Ramesh Kumar et al. / Applied Catalysis A: General 502 (2015) 297–304
301
Fig. 4. X-ray photo electron spectroscopy profiles of (A) O 1s, (B) W 4f, (C) Nb 3d, and (D) P 2p of (a) TPANb1, (b) Nb0.6TPA, and (c) 25% TPA/Nb2O5 catalysts.

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Ch. Ramesh Kumar et al. / Applied Catalysis A: General 502 (2015) 297–304
ions in the secondary structure of the Keggin ion. In the case of
TPANb1 catalyst band related to Lewis acidic sites was not observed
which indicates the successful incorporation of Nb into the primary
structure of Keggin ion. FT-IR of pyridine adsorption measurements
enabled us to distinguish the incorporated of Nb either into the
primary or secondary structures of TPA.
3.6. X-ray photo electron spectroscopy
XPS spectra of O 1s, P 2p, W 4f and Nb 3d of the TPANb1, Nb0.6TPA
and 25% TPA/Nb2O5 catalysts are shown in Fig. 4. The BE values of
the O 1s, P 2p, W 4f and Nb 3d for these catalysts are tabulated
and presented in supplementary information (Table S1). Fig. 4(A)
shows the O 1s XPS spectra of all the catalysts in the region of
530.9–533.1 eV. The B.E region of O 1s is deconvoluted into two
peaks. The first peak is attributed to oxygen present in M
O M,
and the second peak is ascribed to the oxygen of M P of the cat-
O
alyst. In the case of 25% TPA/Nb2O5 catalyst, first peak might be due
to the oxygen present in niobia, and latter might be related to oxy-
gen present in W
O W and/or W O P [17]. In general, the O 1s
spectra of oxygen attached to metal atom observed at 531 eV. In the
case of terminal oxygen of M O units, two bonds of the oxygen are
attached to metal atom. Therefore the binding energy of the O 1s
observed in the same region as that of W
O W bond. Similarly in
the case of supported heteropoly acid catalysts, the terminal oxygen
of W O might be interacted with the support. Fig. 4(B) shows the
W 4f doublet in the range of 36.2–38.9 eV. B.E values for W 4f7/2 and
W 4f5/2 were observed in the regions of 36.2–36.8 and 38.3–38.9
respectively, which are typical B.E values of W6+ oxidation state
[17,21].
Fig. 4(C) shows XPS of Nb 3d in the region of 207.4–210.9 eV. The
B.E in the range of 207.4–208.5 and 210.1–210.9 eV corresponds
to Nb 3d5/2 and Nb 3d3/2 of Nb5+ species, respectively [22]. Pres-
ence of Nb5+ confirms the exchange of Nb ions in the primary and
secondary structures of heteropoly tungstate. P 2p B.E region was
deconvoluted into three peaks in the range of 133.3–135.9 eV, as
shown in Fig. 4(D). The B.E values of P 2p confirms the presence
of phosphorus in the form of phosphate [17,21]. The XPS results
indicate that the exchanged and/or incorporated Nb is in its 5+
oxidation state.
Fig. 5. (a) Normalized Nb K-edge XANES and (b) k2 – weighted Fourier transform
(FT) of TPANb1, Nb0.6TPA, 25% TPA/Nb2O5, and Nb2O5.
surface of Nb sites from tetrahedral to octahedral geometry [24].
Such interaction of water molecule from the secondary structure
of Keggin ion [2] with the Nb sites in the Nb0.6TPA sample might
lowered the pre-edge peak compare to its counterpart (TPANb1)
catalyst. Since no water molecule exist in primary structure of TPA
the pre-edge intensity of TPANb1 is higher compare to all the sam-
ples, indicating the presence of lower oxygen atoms around Nb than
in Oh. Thus, FT of the EXAFS (Fig. 5b) also shows the low amplitude
for the Nb O signal for TPANb1 indicating low Nb O coordination.
In summary, XAS measurements revealed the nature, and locations
of the Nb ion present either in the primary or secondary structures
of TPA or exposed to moisture.
3.7. X-ray absorption spectroscopy
In order to get more information about the nature and location
of the Nb, samples were characterized by X-ray absorption spec-
troscopy (XAS). The normalized XANES (X-ray absorption near edge
structure) of TPANb1, Nb0.6TPA, 25% TPA/Nb2O5 along with Nb2O5
are compared in Fig. 5a. K2-weighted Fourier transforms (FT) of
EXAFS (Exntended X-ray absorption fine structure) spectra in R-
space are shown in Fig. 5b. Spectra of the samples in the XANES
region resemble to that of bulk Nb2O5 reference. To explain the
minute differences in the symmetry of the Nb species, we should
take the pre-edge features under consideration. Generally, the pres-
ence of a pre-edge peak associated with 1s-nd transitions can be
useful in determining the geometry of transition metal ions [23]. In
case of Nb, the intensity of the pre-edge peak due to the 1s-4d tran-
sition discriminates the tetrahedral (Td) and octahedral geometries
[24]. Intense and distinct pre-edge peak is associated to tetrahedral
geometry around the absorbing atom, while in octahedral geom-
etry this feature is much less prominent. The pre-edge features of
the samples are shown as inset diagram in Fig. 5a. The less intense
pre-edge (like a shoulder) in the 25% TPA/Nb2O5 could be associ-
ated to distorted octahedral (Oh) environment of Nb sites. Almost
similar geometry can be expected in the bulk Nb2O5 based on the
shape of pre-edge. The intensity of the pre-edge is also influenced
by the adsorption of water molecule, which can able to convert the
3.8. UV-diffused reflectance spectroscopy
Location of niobium ion was further confirmed by UV-diffused
reflectance spectroscopy (Fig. 6). Polyanions exhibits a strong
absorption band in the range of 200–500 nm due to oxygen-metal
charge transfer [25]. Pure TPA showed a broad band that extends
from 200 to 400 nm, ascribed to tungsten in octahedral coordina-
tion [26]. All the Nb containing catalysts also showed broad band
corresponding to Keggin ion. However, shift in band position to
higher wavelengths (Red shift) was observed in the case of TPANb1
and Nb0.6TPA catalysts. Whereas, in the case of 25% TPA/Nb2O5 the
absorption band was shifted to lower wavelengths (389 nm) com-
pared to TPA (Blue shift). This might be due to strong interaction
of Keggin ion of TPA with niobia. In the case of Nb0.6TPA cata-
lyst, absorption band was similar to that of parent TPA but slightly
extended to 415 nm (Red shift). This might be due to exchange of

Ch. Ramesh Kumar et al. / Applied Catalysis A: General 502 (2015) 297–304
303
Table 2
Activity results of niobium containing heteropoly tungstate catalysts for benzylation
of anisole with benzyl alcohol.
Catalyst
Conversion of benzyl alcohol (%) Yield (%)
Benzylanisole (o-,p-) Ether
25% TPA/Nb2O5 52.8
32.2
66.7
82.1
20.6
22.4
16.7
Nb0.6TPA
TPANb1
89.1
98.8
(d)
(c)
(b)
tungstate on niobia did not affect the P environment in Keggin
3−
unit and structure of the PW12O40
anion during the prepara-
tion of catalysts. These results are in consistent with the FT-IR and
XRD results. However, TPANb1 catalyst showed two signals in the
31P NMR spectra. Signal observed at −15.73 ppm can be assigned
to bulk TPA [22]. Another signal with high intensity observed
at −14.26 ppm is due to incorporation of Nb5+ into the primary
structure, which results a change in the P environment of Keggin
heteropoly tungstate. These two signals were observed due to pres-
ence of PW12 and PW11Nb1 units. The ratio of PW11Nb to PW12
is about 1.3. The percentages of PW11Nb and PW12 in the sam-
ple are 56 and 44 respectively. From the elemental analysis of the
TPANb1 sample the Nb/W ratio is 0.6/11.4. As TPANb1 sample con-
sists of both PW12 and PW11Nb1 units, therefore high content of
W was observed in the sample. This chemical shift is similar to
that reported by other authors [30,32]. From these results it can be
noted that majority of Nb is incorporated into Keggin heteropoly
acid and a small amount of TPA is also present. These results reveal
the successful preparation of Nb incorporated TPA catalyst.
(a)
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 6. UV-DRS spectra of (a) TPA, (b) TPANb1, (c) Nb0.6TPA, and (d) 25% TPA/Nb2O5
catalysts.
3.10. Evaluation of catalysts activity
The activities of the catalysts were evaluated for benzylation of
anisole with benzyl alcohol and the results are shown in Table 2.
In this reaction, o- and p- substituted benzylated products were
obtained as major products and a side product is also formed due
to dehydration reaction as shown in Scheme 2. Moderate to high
yields of benzylated products were obtained for the catalysts. Both
the Nb0.6TPA and TPANb1 catalysts showed high activity compared
to the TPA supported on Nb2O5 catalyst. The TPANb1 catalyst exhib-
ited better activity than Nb0.6TPA catalyst. The TPANb1 catalyst
showed near complete conversion with 82.1% yield towards ben-
zylated products.
The benzylation activity of the catalysts can be correlated to
their acidity. The acidity of the catalysts is largely influenced by
the location of Nb in TPA. Both the Nb containing catalysts exhib-
ited high acidity compared to supported catalyst. The benzylation
activity of the catalysts is also in the same order as that of acidity.
The TPANb1 catalyst showed maximum acidity and there by high
activity among all the catalysts. TPD of NH3 results are also in good
agreement with the catalytic activity of benzylation of anisole with
benzyl alcohol. The characterization results revealed that the Nb is
located as desired. The location of Nb is directing the properties of
the catalysts. These results suggest that heteropoly tungstae can be
modified to tune its catalytic properties.
Fig. 7. 31P solid state NMR spectra of niobium containing heteropoly tungstate
catalysts (a) 25% TPA/Nb2O5, (b) Nb0.6TPA, and (c) TPANb1.
protons of TPA with Nb5+ ions. Incorporation of metal ion into the
secondary structure does not change the primary Keggin ion struc-
ture. The red shift was more pronounced when Nb is incorporated
into the primary structure of TPA. The absorption band was broad
and extended to 457 nm for TPANb1 catalyst. These results corrob-
orate the presence of Nb in the primary and secondary structures
of the Keggin ion.
3.9. Solid state 31P NMR spectroscopy
Solid state 31P NMR spectroscopy is very sensitive method
to assess the structure and stability of heteropoly acids. By
using this technique one can determine purity of the het-
eropoly acid, degree of hydration, change in the environment
around the transition metal, central atom and formation of
The influence of benzylating agent was carried out by taking
dibenzyl ether as benzylating agent. In dehydration reaction, two
moles of benzyl alcohol yields one mole of dibenzyl ether as shown
in Scheme 2. In order to compare the influence of benzylating agent,
the amount of dibenzylether was taken as half of the moles of ben-
zyl alcohol. The activity results of niobium containing catalysts for
benzylation of anisole with dibenzyl ether at a reaction time of
30 min are shown in Table 3. In this case also the Nb containing cat-
alysts performance was high compared to 25% TPA/Nb2O5 catalyst.
Dibenzylether conversion up to 87.8% with 86.2% of yield towards
benzylated products was observed for TPANb1 catalyst.
n−3
species are due to interactions
[
M
OH2]n+[H3−nPW12O40
]
between the support and heteropoly acid [27–30]. Fig. 7 shows
the 31P NMR spectra of niobium containing heteropoly tungstate
catalysts. From the literature it can be noticed that 31P NMR
signal of crystalline bulk TPA is observed at −15.5 ppm corre-
sponding to the four co-ordinated phosphorous located in the
center of the Keggin anion [22,31]. 31P NMR spectra of Nb0.6TPA
and 25% TPA/Nb2O5 catalysts demonstrate single peaks at −15.79
and −15.81 ppm, respectively. This indicates that exchanging the
protons of heteropoly tungstate and/or supporting heteropoly

304
Ch. Ramesh Kumar et al. / Applied Catalysis A: General 502 (2015) 297–304
Scheme 2. Reaction scheme for the bezylation of anisole with benzyl alcohol and dibenzyl ether as benzylating agents.
Table 3
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Acknowledgements
ChRK thank Council of Scientific & Industrial Research, India for
financial assistance in the form of Junior & Senior Research Fellow-
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Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2015.06.
023