Investigation of the catalytic activity of niobium phosphates for liquid phase alkylation of anisole with benzyl chloride

Article abstract of DOI:10.1016/j.apcata.2010.07.030

In this work crystalline niobium phosphate with different degrees of crystallinity was synthesized by two different methods as catalysts for benzylation of anisole with benzyl chloride. The catalysts were characterized by XRD, N₂ physisorption, solid state ³¹P and ⁹³Nb NMR, structural FTIR and FTIR after adsorption of pyridine. The catalytic activities were measured in the benzylation of anisole with benzyl chloride. The NMR technique seems to be more sensitive in evaluating the presence of amorphous phase, compared with DRX data. The crystalline niobium phosphate prepared by recrystallization of a commercial sample presented activity comparable to its parent compound, in contrast with the crystalline catalyst prepared from niobic acid. The activity was attributed to the Lewis sites present mostly on the amorphous phase of the catalysts.

Full text of DOI:10.1016/j.apcata.2010.07.030

Applied Catalysis A: General 386 (2010) 60–64
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Investigation of the catalytic activity of niobium phosphates for liquid phase
alkylation of anisole with benzyl chloride
Marcus Henrique C. de La Cruza, Ângela S. Rochab, Elizabeth R. Lachterc,∗, Aline M.S. Forresterc,
Michele Castro Reisc, Rosane A.S. San Gild, Stefano Caldarellie, Andrea D. Fariasf, Wilma A. Gonzalezg
a Instituto Nacional de Controle de Qualidade em Saúde, Fundac¸ ão Oswaldo Cruz, Rio de Janeiro, Brazil
b Universidade Federal do Rio de Janeiro, Nucat-COPPE, Rio de Janeiro, Brazil
c Universidade Federal do Rio de Janeiro, Lab. de Polimeros e Catalise, Instituto de Quimica, 21941-900, Rio de Janeiro, Brazil
d Universidade Federal do Rio de Janeiro, Lab. Multiusuario de RMN de Solidos, Instituto de Quimica, Rio de Janeiro, Brazil
e Université Paul Cézanne, Aix Marseille III, France
f
Instituto Nacional de Tecnologia, Rio de Janeiro, Brazil
g Instituto Militar de Engenharia, Rio de Janeiro, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work crystalline niobium phosphate with different degrees of crystallinity was synthesized by two
different methods as catalysts for benzylation of anisole with benzyl chloride. The catalysts were charac-
terized by XRD, N2 physisorption, solid state 31P and 93Nb NMR, structural FTIR and FTIR after adsorption
of pyridine. The catalytic activities were measured in the benzylation of anisole with benzyl chloride. The
NMR technique seems to be more sensitive in evaluating the presence of amorphous phase, compared
with DRX data. The crystalline niobium phosphate prepared by recrystallization of a commercial sample
presented activity comparable to its parent compound, in contrast with the crystalline catalyst prepared
from niobic acid. The activity was attributed to the Lewis sites present mostly on the amorphous phase
of the catalysts.
Received 11 April 2010
Received in revised form 7 July 2010
Accepted 18 July 2010
Available online 22 July 2010
Keywords:
Niobium phosphate
93Nb NMR
31P NMR
© 2010 Elsevier B.V. All rights reserved.
Friedel–Crafts reaction
Benzylation
Benzyl chloride
1. Introduction
uid phase benzylation of benzene and various aromatic substrates
including benzyl chloride and benzyl alcohol is a key process to the
The benzylation of benzene and toluene by benzyl chloride
and benzyl alcohol is important for producing dielectric fluids,
pharmaceutical intermediates and fine chemicals. Such reactions
are carried out industrially, using conventional strong homoge-
neous acid catalysts, such as AlCl3, FeCl3, BF3, ZnCl2 and H2SO4
[1,2]. In parallel, the use of solid heterogeneous catalysts has
become important in liquid phase applications, since these cata-
lysts can be easily separated from the reactant mixture. They are
also reusable and also have high activity for Friedel–Crafts reac-
tions [1,2]. Solid catalysts like zeolites [3,4], sulphated zirconia
[5], Fe-containing mesoporous materials [6], ion-exchanged clays
[7], zirconium-pillared clays [8], ion-exchange resins [9,10], iron-
containing aluminophosphate [11], Zr-SBA-15 [12] and Ga-, Al-,
AlGa-SBA-15 [13] have been reported for this reaction. The liq-
synthesis of diphenylmethane and substituted diphenylmethanes
that are building blocks in organic synthesis for the preparation
of intermediates in the pharmaceutical and fine chemical indus-
tries [13]. The alkylation of toluene with benzyl chloride has been
reported to be catalyzed by Lewis acid sites [14].
Efforts have also been made in developing new applications
of niobium phosphate for chemical processes. In previous papers
[15–17], the present authors have demonstrated that niobium
phosphate can be used as a catalyst in the benzylation of anisole
with benzyl alcohol, giving high yield and selectivity. The available
literature indicates that niobium phosphate shows potential appli-
cation in alkylation reaction of aromatic compounds with benzyl
alcohol [16–18]. The use of niobium catalysts in the benzylation of
aromatic compounds with benzyl chloride has not been reported so
far. Therefore, the objective of this work is to evaluate the benzyla-
tion of anisole with benzyl chloride catalyzed by different niobium
phosphates. We have also investigated the characteristics of the
niobium phosphates by N2 physisorption, XRD, FTIR in the struc-
tural region and in the region of adsorbed pyridine and by 31P and
93Nb solid state NMR.
∗
Corresponding author.
E-mail address: lachter@iq.ufrj.br (E.R. Lachter).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.07.030

M.H.C. de La Cruz et al. / Applied Catalysis A: General 386 (2010) 60–64
61
2. Experimental
Anisole (Merck) and benzyl chloride (Merck) were dried before
use. Three niobium phosphate samples were studied: a commer-
cial calcined material (NbP-C, NbOPO4·3H2O supplied by CBMM,
Araxá, Brazil); the same recrystallized (NbP-R); and a crystalline
sample prepared from a HF route (NbP-S). Commercial niobium
phosphate was dried and calcined at 773 K for 4 h. Recrystallized
niobium phosphate (NbP-R) was obtained by adding 35 mL of HF
40% (v/v) to 9.67 g of NbP-C at 353 K with agitation. After homog-
enization, the obtained solution was maintained under heating
and without stirring for 7 days, to allow slow precipitation. The
final solid was separated by centrifugation, washed with distilled
water until the washing water reached a constant pH, and dried
at 333 K for 24 h [18]. Crystalline niobium phosphate (NbP-S) was
synthesized by a procedure described in the literature [18,19]. A
solution was obtained by reacting 6 g of niobic acid (supplied by
CBMM) with 35 mL of HF 40% (v/v) under agitation, and 20.5 mL
of H3PO4 (85%) were added to this solution. After homogeniza-
tion, the solution obtained was maintained without stirring or
heating, until a precipitate was formed, after 21 days. When the
first crystals appeared, the final volume was adjusted to 100 mL
with distilled water. After the precipitation, the solid obtained was
washed with 140 mL of a 5 M HNO3 solution, separated by cen-
trifugation, washed with distilled water until pH 5 (in the washing
water) and dried in a vacuum oven at 333 K for 24 h. Specific surface
area and porous volume of NbP samples were determined through
a BET conventional method using an ASAP 2010 apparatus. The cat-
alysts were characterized by X-ray powder diffraction collected in
a Miniflex Rigaku instrument using Cu K␣ radiation. The samples
were scanned over the angular range 8–38◦ (2ꢀ), with a step size
of 0.04◦ and a scan rate of 1 s per step. Solid state 31P MAS NMR
spectra were obtained on a Bruker DRX300 spectrometer by using
Bloch decay pulse sequence (90◦ pulse length 10 ␮s), with a recy-
cle delay of 60 s. H3PO4 85% was used for chemical shifts reference
(ı = 0.0 ppm). Isotropic chemical shift and half-height width data
were obtained through spectral line simulation performed by using
the MestReNova® program. Solid state 93Nb NMR spectra were
obtained on a Bruker DRX300 (7.05T) and Bruker Avance III-400
spectrometers, operating at 73.46 and 97.98 MHz, respectively. The
4 mm CPMAS Bruker probes were used in both instruments, with
ZrO2 rotors and Kel-F caps. The spectra were acquired by using spin-
echo pulse sequence (90-ꢁ-180-ꢁ-aq) with recycle delay of 1 s. A
solution of 0.1 M NbCl5·CH3CN was used for both chemical shifts
reference (ı = 0.0 ppm) and 90◦ pulse width calibration (1 ␮s). The
spectral simulation was performed using WSOLIDS1 program [20].
FTIR measurements were carried out in a Perkin Elmer Spectrum
Fig. 1. X-ray diffraction patterns obtained for samples NbP-C, NbP-R and NbP-S.
drawn and analyzed through high-resolution gas chromatography.
Reaction products were identified by gas chromatography–mass
spectrometry analysis (GC–MS) in a HP 6890 instrument, using a
DB-5 (30m) fused silica column, and helium as carrier gas with the
same programming used in the VARIAN GC.
3. Results and discussion
The X-ray diffraction patterns of the three niobium phosphate
samples are presented in Fig. 1. No diffraction peak was observed
for NbP-C (commercial niobium phosphate) indicating that this
material is amorphous. Samples NbP-R and NbP-S present quite
similar XRD patterns [18].
The textural and acidic properties of niobium phosphate sam-
ples are presented in Table 1.
The recrystallized phosphate NbP-R and the prepared by the
HF route, NbP-S present very low surface areas and pore volumes
(23 m2 g−1, 0.06 cm3 g−1and 21 m2 g−1, 0.02 cm3 g−1 respectively),
while the commercial phosphate NbP-C presents higher surface
area (187 m2 g−1, 0.29 cm3 g−1).
The surface area of niobium phosphate can change with pre-
treatment temperatures. Martins et al. [21] found a surface area of
182m2 g−1 after calcinations at 773 K of commercial niobium phos-
phate. The preparation method can also change the surface area
of niobium phosphate. Values from 8 to 167 m2 g−1 were found
when the niobium phosphate was prepared from orthophospho-
ric acid and potassium niobate [22]. Armarolli et al. [23] found
for amorphous niobium phosphate specific areas in the range
150–180 m2 g−1. Our results are thus comparable to the literature.
The niobium phosphates acidities were previously studied by
FTIR of adsorbed pyridine after evacuation at different tempera-
tures [18]. The amount of Lewis acid sites (LAS) and Brönsted acid
sites (BAS) are presented in Table 1. The commercial phosphate pre-
sented the highest LAS concentration per gram of material. Martins
et al. [21] evaluated the relative acid strength of commercial nio-
bium phosphate pre-treated at different temperatures (373–773 K),
by infrared spectra of adsorbed pyridine and showed both Brön-
sted and Lewis acid sites are present. Both crystalline phosphates
present lower capacity to chemisorb pyridine compared with NbP-
C, and the recrystallized material presented more LAS than BAS. The
One spectrophotometer, between 4000 and 400 cm−1
.
The benzylation reactions of anisole with benzyl chloride were
carried out in a round-bottomed 50 mL 3-necked flask provided
with a reflux condenser, a nitrogen gas inlet and a septum for
sample removal. The reaction mixture was magnetically stirred at
atmospheric pressure and the temperature was kept at the reflux
temperature of the mixture by means of a constant-temperature
bath. Samples of the reaction mixture were periodically with-
Table 1
Properties of the niobium phosphate samples studied.
Niobium phosphate
Textural properties
Acidity
S.A.a
PVb
Brönsted
Lewis
Total
m2 g−1
cm3 g−1
␮mol g−1
␮mol g−1
␮mol g−1
NbP-C
NbP-R
NbP-S
187
23
21
0.29
0.06
0.02
163.3
32.3
24.4
160
46.3
17.4
323.3
78.6
41.8
a
Surface area determined by BET method.
Pore volume determined around saturation pressure.
b

62
M.H.C. de La Cruz et al. / Applied Catalysis A: General 386 (2010) 60–64
Fig. 2. 31P MAS NMR spectra (121 MHz) obtained for samples NbP-C, NbP-R and
NbP-S.
amount of LAS on NbP-R is higher than in NbP-S and much lower
than NbP-C.
Solid state 31P NMR spectra obtained for the three samples stud-
ied are depicted in Fig. 2. For sample NbP-C a broad signal centered
at −13 ppm could be seen. In contrast, for the samples NbP-R and
NbP-S a sharper signal centered at −23 ppm was obtained (Table 2),
with similar signal width. These results could be correlated with the
crystallinity of the samples obtained by XRD, although the differ-
ence between samples NbP-R and NbP-S detected in the 31P spectra
was not observed in DRX patterns. Amorphous NbP-C presented the
largest width, similar to those obtained previously for commercial
NbP samples [24], followed by sample NbP-R and NbP-S. On the
other hand, the difference in chemical shifts reflects the presence
of residual water in coordination to phosphorous sites [25]. In this
sense sample NbP-C present more water molecules coordinated,
compared with NbP-R and NbP-S ones.
The 93Nb NMR spectra obtained for NbP-S samples are shown
in Fig. 3. The 93Nb spin-echo signal of the three materials reflects
the degree of crystallinity of the samples. The spectrum of NbP-S
(Fig. 3b) has a lineshape resembling the ones of previously analyzed
crystalline phosphates [24]. However, a deviation from the perfect
crystalline lineshape is indicative, even for this sample, of a certain
degree of disorder. Thus, the spectra could be simulated by using
the data published recently [25], with the addition of secondary
sites in order to mimick to first order the charge disorder (Table 2).
Spectrum of NbP-C is a broad Gaussian, which corresponds to an
amorphous material, in accordance with DRX pattern. Reis et al.
[26] obtained the same result for mesostructured NbP. NbP-R, on
the other hand, has a spectrum (Fig. 3c) that can be decomposed
in the sum of a crystalline (39%) and an amorphous (61%) line. This
result is in accordance with the 31P results, in contrast with DRX and
evidences that solid state NMR is a more powerful tool in detecting
the presence of different phases in NbP samples.
Fig. 3. 93Nb static spin-echo NMR spectra of samples NbP-C (a); NbP-S experimental
(b) and simulation (b1); NbP-R experimental (c) and decomposition in amorphous
(c1) and crystalline (c2) contributions, with WSOLIDS program.
The structural IR spectra (Fig. 4) of the samples are typical of
phosphates, as described in the literature [17,22,27]. The samples
were manipulated under air, so bands around 3406 and 1629 cm
Table 2
Solid state 31P and 93Nb data obtained for niobium phosphate samples studied.
Niobium phosphate
31P MASa
93Nb Hahn–echob
Chemical Shift
Width
Isot. chem.
%
CSA
ꢄ
Quad.
ꢄQ
(ı, ppm)
(ꢂꢃ1/2, Hz)
shift (ıiso, ppm)
(ı, ppm)
(CQ, kHz)
NbP-C
NbP-R
NbP-S
−13.5
−22.6
−23.3
2517
2093d
1806
−1302c
−1290c,d
−1330
−1330
−1330
−1330
–
–
85
5
5
5
–
–
–
–
0.2
0.2
0.2
0.2
–
–
25
18
16
14
–
–
0
0
0
0
−880
−780
−900
−900
a
From optimization with MestReNova® processing program.
From WSOLIDS program [17].
b
c
Maximum intensity of amorphous signal, not the ıiso
.
d
Superposition of the signals of the type NbP-C and NbP-S.

M.H.C. de La Cruz et al. / Applied Catalysis A: General 386 (2010) 60–64
63
Fig. 4. Structural FTIR obtained for samples NbP-C, NbP-R and NbP-S.
Fig. 6. Effect of temperature on the conversion (%) of benzyl chloride in the presence
of NbP-C (m = 125 mg) in function of time of reaction (min).
Table 3
Benzylation of anisole with benzyl chloride catalyzed by different niobium
phosphates.
Time (min)
Conversion (%)
Catalyst
Fig. 5. Benzylation reaction of anisole with benzyl chloride.
NbP-C
NbP-R
NbP-S
−1
5
10
20
30
40
74
100
100
100
100
–
97
100
100
100
8
24
65
98
100
typical of water−1and hydroxyls are observed. Bands around
1220 and 1020 cm
P
may be related to stretching vibrations of
O groups, ꢃass (P O) and ꢃs (P O), respectively. Modes related
to vibrations of Nb-O and Nb O groups appear around 640–585
and 910 cm−1, respectively. The two crystalline phosphates present
well defined bands, as expected for highly organized structures, as
the vibration bands of the commercial niobium phosphate NbP-C
appear broadened, similar to those results obtained in a previous
work for microporous niobium phosphate [18].
The reaction of anisole with benzyl chloride in the presence of
NbP as catalysts to yield alkylated products is presented in Fig. 5.
The main products of the alkylation of anisole are isomers of the
benzyl anisole and, in some cases, dialkylated products can be
found.
The reaction of anisole with benzyl chloride catalyzed by NbP-C
occur at temperatures >343 K (Fig. 6). The temperature chosen for
the reaction was under anisole reflux, e.g. 427 K. FeCl3/Al2O3 was
evaluated in the benzylation of benzene with benzyl chloride in
temperature range of 323–393 K [28]. It was found that conversion
was very poor at lower temperature (323 K) and a temperature of
at least 353 K is required for appreciable conversion, similar to our
findings.
Table 3 presents the results of the catalytic activities observed
in the alkylation of anisole with benzyl chloride by varying the
reaction time. The conversion of benzyl chloride was 100% in all
cases, but the reaction occurred slowly in the presence of NbP-S
due probably to lower number of Lewis acid sites in this catalyst.
Conditions: anisole/substrate = 15/1; 427 K; 60 mg of catalyst pre-treated at 773 K.
The selectivity in monobenzylation product was superior to 90%
(Table 4). The isomer composition in benzylation products from
aromatic reaction gave predominantly ortho–para substitution in
accordance with a typical electrophilic aromatic substitution path-
way.
Although the specific area and number of acid sites of crystalline
NbP-S are lower when compared to NbP-C, the benzyl chloride
conversion in the presence of both catalysts was complete. In a
previous paper, by using benzyl alcohol as alkylating agent conver-
sions lower than 30% were achieved [18]. The higher performance
of NbP-R compared to NbP-S could be correlated with the presence
of amorphous phase in this material as confirmed by 93Nb solid
state NMR. Oliveira et al. [29] also attributed to amorphous phase
the activity of VPO-NbP catalysts for mild oxidation of n-butane
to maleic anhydride. Costa et al. [30] reported the activities of
amorphous and crystalline zirconium phosphates in Friedel–Crafts
alkylation of anisole. They suggested that the higher activity of the
amorphous catalyst compared to the crystalline catalyst could be
interpreted on the basis of lower accessibility of acid sites present
in the crystalline material.
Table 4
Results of benzylation of anisole with benzyl chloride in the presence of heterogeneous catalysts.
Catalyst
Time (min)
Conversion (%)
Products distribution (%)
Monoalkyl.
Ratio o/p
Ref.
Dialkyl.
NbP-C
NbP-R
NbP-S
HZSM-5
Ga/SiO2
Fe/SBA-15
10
20
40
1440
90
75
100
100
100
0
3.7
89.1
92.9
99.6
95.7
–
100
88.5
7.1
0.4
4.3
–
–
–
0.88
0.94
0.92
–
–
0.82
This work
This work
This work
[31]
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64
M.H.C. de La Cruz et al. / Applied Catalysis A: General 386 (2010) 60–64
Fig. 7. Lewis sites for alkylation reaction.
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In conclusion benzyl chloride can be used as reagent for alky-
lation of anisole, and reacts with aromatic compounds via a
mechanism involving Lewis sites. In this pathway, a carbenium ion
formed on acid sites attacks the aromatic compound forming the
monoalkylated product. o- and p-alkylated products are predomi-
nantly formed, in agreement with a typical eletrophilic aromatic
substitution pathway. 31P and 93Nb Solid state NMR proved to
be more sensitive in evaluating the presence of different Nb-P
phases compared with DRX. Both crystalline and amorphous nio-
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