A Novel Br?nsted-Lewis acidic catalyst based on heteropoly phosphotungstates: Synthesis and catalysis in benzylation of p-xylene with benzyl alcohol

Article abstract of DOI:10.1016/j.mcat.2017.10.003

A novel Br?nsted-Lewis acidic catalyst, Hf_0.5[TEAPS]PW₁₂O₄₀, has been prepared by the replacement of protons in neat phosphotungstic acid with both organic and metal cations and characterized by FT-IR, Py-IR, XRD, TG-DTG, ICP-AES, BET, elemental analysis and n-butylamine potentiometeric titration techniques. This hybrid heterogeneous catalyst with both Br?nsted and Lewis acidity can efficiently promote the conversion of dibenzyl ether, the self-condensation product of benzyl alcohol, to the benzylation products, as well restrain the polybenzylation of aromatics. As a result, it exhibits excellent catalytic activity and selectivity in the benzylation of p-xylene with clean benzylation reagent benzyl alcohol. Thus, an environmentally friendly route for the benzylation reaction of p-xylene with high atomic economy has been developed by this reusable Hf_0.5[TEAPS]PW₁₂O₄₀ catalyst.

Full text of DOI:10.1016/j.mcat.2017.10.003

Molecular Catalysis 443 (2017) 110–116
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Research paper
A Novel Brønsted-Lewis acidic catalyst based on heteropoly
phosphotungstates: Synthesis and catalysis in benzylation of p-xylene
with benzyl alcohol
Bing Yuana, Yinglian Lia, Zhilei Wanga, Fengli Yua, Congxia Xiea,∗, Shitao Yub
a State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology,
Qingdao, 266042, China
b College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 March 2017
Received in revised form 2 September 2017
Accepted 3 October 2017
Available online 6 November 2017
A novel Brønsted-Lewis acidic catalyst, Hf0.5[TEAPS]PW12O40, has been prepared by the replacement of
protons in neat phosphotungstic acid with both organic and metal cations and characterized by FT-IR,
Py-IR, XRD, TG-DTG, ICP-AES, BET, elemental analysis and n-butylamine potentiometeric titration tech-
niques. This hybrid heterogeneous catalyst with both Brønsted and Lewis acidity can efficiently promote
the conversion of dibenzyl ether, the self-condensation product of benzyl alcohol, to the benzylation prod-
ucts, as well restrain the polybenzylation of aromatics. As a result, it exhibits excellent catalytic activity
and selectivity in the benzylation of p-xylene with clean benzylation reagent benzyl alcohol. Thus, an
environmentally friendly route for the benzylation reaction of p-xylene with high atomic economy has
been developed by this reusable Hf0.5[TEAPS]PW12O40 catalyst.
Keywords:
Benzylation
Benzyl alcohol
Brønsted-Lewis acid
Organic-inorganic heteropoly salts
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
acidity [10,13]. Some heteropoly compounds (HPCs) catalysts
with specific heteropoly anions have exhibited good catalytic
Friedel-Crafts benzylation of aromatic is
a
key direct
activities and selectivities for desired monobenzylated aromatics
in the reaction with BA being the benzylation reagent. However,
in most cases, these HPCs need to be impregnated onto or encap-
sulated into porous carriers to overcome the high solubility in
polar substance (especially for instance, water generating during
benzylation) or the low surface areas [14–17].
carbon–carbon bond forming reaction in organic synthesis to
produce diarylmethanes and their derivatives, commercial signifi-
cance pharmaceutical intermediates [1,2]. This industrial process
suffers from many problems like pollution, corrosion and difficulty
in the separation and recovery of catalysts due to the employment
of benzyl chloride as the reagent or homogeneous acid such as
AlCl3, FeCl3, and H2SO4 as catalysts [3,4]. Benzyl alcohol (BA) is a
friendly benzylation reagent to substitute benzyl chloride in view
of atom economy and environment factor. Many environmentally
friendly catalysts have been studied in benzylation reactions with
BA to overcome the problems of mineral acids [5–11]. However, it
has been found that the self-condensation of BA is more favorable
than benzylation of aromatics with less reactivity over the catalyst
without enough catalytic activities [12]. Moreover, the monoben-
zylated aromatics would further react with BA to produce bulky
polybenzylated aromatics at high reaction temperature or strong
Recently, a new modification for heteropolyacid (HPA) catalysts
has been developed via organic cations bonding ionically to het-
eropolyanion, just like ionic liquids (ILs) [18–27]. Leng [19] has
found that the acid strength of heteropoly ionic liquids could be
improved effectively by employing propane sulfonate (PS) func-
tionalized organic cations. However, the PS group in organic cations
could only provide enough acid strength for some reactions which
were easy to realize, such as esterification [23–25]. In our previous
work, heteropoly acidic salts with higher acid strength have been
developed via the partial replacement of protons by the organic
cation with acidic PS groups, and have exhibited good activity,
selectivity and stability in the benzylation of anisole with BA [28].
Unfortunately, as to the case of p-xylene (PX) with less activa-
tion degree, it was difficult to achieve an excellent selectivity for
monobenzylation via single Brønsted acid sites provided by the
heteropoly organic acidic salts, due to the forming of dibenzyl
ether (DBE) [29]. On the other hand, Hafnium-exchanged het-
∗
Corresponding author at: State Key Laboratory Base of Eco-chemical Engineer-
ing, College of Chemistry and Molecular Engineering, Qingdao University of Science
and Technology, Qingdao, 266042, China.
E-mail address: xiecongxia@126.com (C. Xie).
http://dx.doi.org/10.1016/j.mcat.2017.10.003
2468-8231/© 2017 Elsevier B.V. All rights reserved.

B. Yuan et al. / Molecular Catalysis 443 (2017) 110–116
111
eropoly acid has been found a favorable catalytic and recycling
performance due to the high Lewis acidity [30]. Herein, based on
the high charge numbers of heteropoly anions, sulfated organic
cations [TEAPS]+ (TEAPS: 3-(triethylammonio) propane sulfonate)
with Brønsted acidity and metal cations Hf4+ with Lewis acidity
were introduced together as counter ions for heteropoly anions
PW12O403−, establishing a novel heteropoly organic-inorganic salt
catalyst, Hf0.5[TEAPS]PW12O40, with both Brønsted and Lewis acid-
ity. As a solid catalyst, this heteropoly organic-inorganic salt which
has the similar structure of ionic liquids and enough water resis-
tance has exhibited excellent activity, selectivity and reusability in
the benzylation of PX with BA.
2. Experimental
2.1. Materials and methods
Fig. 1. FT-IR spectra of organic-inorganic heteropoly salts. (a) H3PW12O40, (b)
All chemicals were of analytical grade and phosphotungstic
acid hydrate was dried at 120 ◦C. The spectra of Fourier transform
infrared spectroscopy (FT-IR) for catalyst samples (for Py-IR test,
samples (0.50 g) were put in a vacuum drier together with pyridine
(3.0 mL) at 2 Kpa for 12 h prior to measurement) in KBr disks was
recorded on a Nicolet iS10 FT-IR instrument in the 400–4000 cm−1
range. The measurement of X-ray diffraction (XRD) was performed
by used a Rigaku DMAX 2500 PC diffractometer equipped with Cu
K␣ radiation in the 2ꢀ range of 5−90◦. The thermal analysis (TG-
DTG) was performed with Netzsch TG209F1 instrument in dry N2
at a heating rate of 20 ◦C/min from 30 to 800 ◦C. The content of the
elements in the catalyst was determined by Elementar Vario ELIII
system and Inductively Coupled Plasma Atomic Emission Spec-
troscopy (ICP-AES) with a Prodigy XP ICP spectroscope. The acidity
of prepared catalysts was determined by potentiometric titration
[31,32]. The Brunauer Emmett Teller (BET) specific surface areas
were determined with a Micromeritics ASAP 2010 apparatus.
[TEAPS]3PW12O40
, (c) Hf0.25[TEAPS]HPW12O40, (d) Hf0.25[TEAPS]2PW12O40, (e)
Hf0.5[TEAPS]PW12O40, (f) Reused Hf0.5[TEAPS]PW12O40.
The products were identified by GC–MS (HP6980/5973) analysis
after the separation of products on a DB-35 column with He as
carrier gas.
3. Results and discussion
3.1. Characterization of the organic-inorganic heteropoly salts
The FT-IR spectra (Fig. 1) of prepared organic-inorganic het-
eropoly salts, comparing with that of the neat H3PW12O40, show
that the catalysts Hfx[TEAPS]yHzPW12O40, [TEAPS]3PW12O40, and
reused Hf0.5[TEAPS]PW12O40 have four featured peaks similar to
those of H3PW12O40 [1080 (P-O), 984 (W = O), 891 (W-Ob1-W) and
802 cm−l (W-Ob2-W)], which are assigned to the Keggin structure.
On the other hand, feature peaks at 2989, 1483, 1394 and 1150 cm−1
are detected to verify the presence of [TEAPS]+ in these catalysts
[20,24]. Obviously, for the organic-inorganic heteropoly salts, the
still observed four peaks of Keggin structure together with the
simultaneous occurrence of characteristic peaks of organic groups
indicate that the structures of both organic cations and heteropoly
anions are well preserved. Moreover, the FT-IR spectrum of the used
Hf0.5[TEAPS]PW12O40 shows no obvious structural change in the
catalyst during benzylation (Fig. 1).
Besides, the Py-IR results of prepared organic-inorganic het-
eropoly salts (Fig. 2) show the existence of featured peaks
1536 cm−1 (referred to Brønsted acid) and 1447 cm−1 (referred
to Lewis acid), which proves the Brønsted-Lewis double acidity
of these hybrids and also reveals that both metal ions and sul-
fated organic cations have been introduced into the structure of
H3PW12O40 successfully.
2.2. Preparation of organic-inorganic heteropoly salts
Contrastive catalysts, [TEAPS]3PW12O40 and Hf0.5HPW12O40
,
were prepared according to the literatures [19,33]. Analogously,
equimolar triethylamine and 1,3-propanesultone were dissolved
in ethyl acetate and stirred at 50 ◦C for 24 h under nitrogen
atmosphere. The obtained white precipitate, 3-(triethylammonio)
propane sulfonate (TEAPS), was filtered, washed with ethyl acetate
and dried at 100 ◦C for 6 h. Next, a solution of intermediate TEAPS
and HfCl4 in water was dropped into another aqueous solution of
the calculated amount of H3PW12O40. The solution was stirred at
room temperature for 24 h, distilled to remove water, and washed
with ethyl acetate. Finally, the obtained solid was dried in a vac-
uum at 80 ◦C for 6 h to give organic-inorganic heteropoly salts
Hfx[TEAPS]yHzPW12O40(4x + y + z = 3).
N-butylamine
potentiometric
titration
results
of
2.3. General procedure for the benzylation of p-xylene with
benzyl alcohol
Hf0.5[TEAPS]PW12O40 and contrastive catalysts are shown in
Table 1. Generally, in terms of n-butylamine potentiometric, the
initial electrode potential indicates the acid strength of catalyst,
and the titration jump indicates the total acid amount. It can be
seen that introducing organic cations, even with highly acidic
sulfonic acid groups (TEAPS), into H3PW12O40 leads to a significant
reduction in acid strength. However, there is no distinct influence
on acid strength to introduce Hf4+ into phosphotungstic acid.
Moreover, the total acid amount of these catalysts prevailingly
depends on the volume of cations and anions, as well as their
bonding. Therefore, the prepared organic-inorganic heteropoly
salt, Hf0.5[TEAPS]PW12O40, demonstrates higher acid strength
and total acid amount than highly acidic heteropoly ionic liquid
[TEAPS]3PW12O40 because of the existing of Hf cations in struc-
The typical procedure for benzylation reactions was as fol-
lows: PX, BA, and catalyst Hfx[TEAPS]yHzPW12O40 were added
proportionally to a round-bottomed flask with a thermometer, a
magnetic stirrer and a reflux condenser. The resulting mixture was
stirred vigorously at 140 ◦C for 2 h then was cooled to room tem-
perature. The reaction solution, from which the generated water
and precipitated catalyst had been removed, was analyzed by a
gas chromatography (GC-9790) equipped with an FID detector
and a capillary column (OV-1701, 50 m × 0.25 mm × 0.25 ␮m) to
determine the conversion of BA and the selectivity of benzylation
products.

112
B. Yuan et al. / Molecular Catalysis 443 (2017) 110–116
Table 1
The acidity of organic-inorganic heteropoly salts.
No.
Catalyst
Solubility (25 ◦C)
In water
Acid strength (mV)
Total acid amount (mmol/g)
In p-xylene
1
2
3
4
5
6
H3PW12O40
Hf0.5HPW12O40
[TEAPS]3PW12O40
Hf0.25[TEAPS]HPW12O40
Hf0.5[TEAPS]PW12O40
Reused Hf0.5[TEAPS]PW12O40
Yes
Not
Almost not
Not
Not
Not
Not
Not
Not
Not
Not
814
811
657
673
685
683
0.89
0.94
0.82
0.80
0.84
0.83
Not
Fig. 2. Py-IR spectra of organic-inorganic heteropoly salts. (a) H3PW12O40, (b)
Hf0.25[TEAPS]HPW12O40, (c) Hf0.25[TEAPS]2PW12O40, d) Hf0.5[TEAPS]PW12O40, e)
Reused Hf0.5[TEAPS]PW12O40.
Fig. 3. XRD patterns of organic-inorganic heteropoly salts. (a) [TEAPS]3PW12O40
(b) Hf0.25[TEAPS]HPW12O40, (c) Hf0.25[TEAPS]2PW12O40, (d) Hf0.5[TEAPS]PW12O40
(e) Reused Hf0.5[TEAPS]PW12O40, (f) Raw materials mixture.
,
,
ture. In addition, it is found that the reused Hf0.5[TEAPS]PW12O40
has no obvious change in acid properties. In order to obtain
the specific surface areas of these catalysts, BET investigations
were also conducted. The results show that the introducing of
Hf4+ into H3PW12O40 structure (H3PW12O40: 1.199 m2/g) leads
to a slightly increased specific surface area (Hf0.5HPW12O40
:
1.627 m2/g). However, the introducing of TEAPS leads to a reduced
one ([TEAPS]3PW12O40
:
0.738 m2/g). Meanwhile, the catalyst
Hf0.5[TEAPS]PW12O40 with both Hf4+ and TEAPS has a specific
surface area which is lower than that of Hf0.5HPW12O40 but higher
than that of H3PW12O40 (Hf0.5[TEAPS]PW12O40
:
1.520 m2/g).
Furthermore, the specific surface area of Hf0.5[TEAPS]PW12O40 has
no obvious reduction after reused (1.457 m2/g).
Powder XRD patterns of prepared organic-inorganic heteropoly
salt, Hf0.5[TEAPS]PW12O40, and contrastive catalysts are shown in
Fig. 3, comparing with that of the raw materials mixture including
H3PW12O40, HfCl4 and the intermediate TEAPS. The broad diffrac-
tion peaks at 2ꢀ of 15−38◦ indicate smectic-state appearances of
the prepared heteropoly salt samples with organic group, differ-
ing from those of H3PW12O40 with high crystallinity [26,27]. The
phase change of the hybrid compounds has been due to replace-
ment of protons in H3PW12O40 with [TEAPS]+ [27]. On the other
hand, according to literature [33], lower angles shifts of the XRD
peaks would be observed when the protons of H3PW12O40 were
exchanged by metal ions such as Cs+, Sm3+and Hf4+ due to the
expansion of unit cell volume. Thus, shifts clearly observed in Fig. 3
also indicate exchanges of protons of H3PW12O40 by Hf4+. In addi-
tion, the XRD spectrum of the reused Hf0.5[TEAPS]PW12O40 shows
no obvious structural change (Fig. 3). These XRD results are in keep-
ing with the observations made from FT-IR (Figs. 1 and 3).
Fig. 4. TG-DTG profile of Hf0.5[TEAPS]PW12O40
.
organic framework in [TEAPS]+ (calculated weight loss: 6.63%) are
observed at 302.1 ◦C and 407.6 ◦C respectively. The decomposition
peak at 595.8 ◦C (calculated weight loss: 3.61%) is explained by the
decomposition of PW12O403−. The TG profile also indicates that the
prepared organic-inorganic heteropoly salt, Hf0.5[TEAPS]PW12O40
,
is quite stable below 302.1 ◦C.
The contents of Hf, P and W elements in Hf0.5[TEAPS]PW12O40
were characterized by ICP-AES to demonstrate the molecular orga-
nization of Hf0.5[TEAPS]PW12O40 (Table 2). The results show that all
of the three experimental values are in line with, but slightly lower
than the theoretical ones. It can be attributed to about 8 molecules
unbound or crystal water in the prepared catalyst which has been
proved by TG- DTG (Fig. 4).
The TG curve of the prepared organic-inorganic heteropoly salt,
Hf0.5[TEAPS]PW12O40 (Fig. 4) shows that about 8 unbound or crys-
tal water molecules were lost during 30–275 ◦C (calculated weight
loss: 4.85%). Moreover, decomposition peaks of sulfonic group and
The contents of C,
H
and
S
elements in fresh
Hf0.5[TEAPS]PW12O40 were characterized by elemental analy-

B. Yuan et al. / Molecular Catalysis 443 (2017) 110–116
113
Fig. 5. Influences of catalyst amount (a), reaction temperature (b), reaction time (c), and molar ratio of PX to BA (d) on the conversion and selectivity over Hf0.5[TEAPS]PW12O40
.
Reaction conditions: BA (0.05 mol), nPX:nBA = 2, 140 ◦C, 2 h (a); catalyst (0.1 g), BA (0.05 mol), nPX:nBA = 2, 2 h (b); catalyst (0.1 g), BA (0.05 mol), nPX:nBA = 2, 140 ◦C (c); catalyst
(0.1 g), BA (0.05 mol), 140 ◦C, 2 h (d).
Table 2
tion of PX with BA were investigated under the same conditions.
The conversion and product distribution was analyzed by GC and
The Hf, P and W elements contents of Hf0.5[TEAPS]PW12O40 by ICP-AES.
GC–MS (Table 4). It can be seen that PX cannot react with BA at
all in the conditions of no catalyst or the TEAPS being used, even
pure Lewis acid anhydrous HfCl4 was used, the catalytic activity is
still very low. As expected, due to the electron-donating effect of
methyl groups in PX, dibenzylation and even multiple benzylation
would occur over the catalysts with enough acid strength. Further-
more, another main by-product coming from the intermolecular
dehydration process of BA, DBE, would be generated easily over
the catalysts with only Brønsted acid sites, even the super acid
H3PW12O40. Meanwhile, the Lewis acidity from enough Hf4+ can
effectively avoid the intermolecular dehydration of BA to produce
DBE, but intensify the polybenzylation of PX. When both Hf4+ with
Lewis acidity and [TEAPS]+ with Brønsted acidity were introduced
together into the structure of PW12O403+ in a suitable ratio to form
Hf0.5[TEAPS]PW12O40 with Brønsted-Lewis double acidity, no DBE
and less polybenzylation products were found than Hf0.5HPW12O40
did. Kumar.et al. [30] reported a series of hafnium salts of het-
eropoly phosphotungstic acid(HfxTPA) with both Brønsted and
Lewis acidity and their catalytic performance of benzylation of
anisole with DBE. It was found that Hf0.5HPW12O40, the one with
appropriate ratio of Brønsted acid and Lewis acid was the most
efficient catalyst in the above reaction. This result can indicate the
synergistic effect of Brønsted and Lewis acid sites [34] and account
for the excellent performance of the prepared organic-inorganic
heteropoly salt, Hf0.5[TEAPS]PW12O40, compared to H3PW12O40
and HfCl4 in benzylation of PX with BA. (Table 4).
No.
Element
Expected content/%
Experimental content/%
1
2
3
Hf
P
W
2.80
0.97
69.12
2.74
0.92
66.14
Table 3
The C, H and S elements contents of Hf0.5[TEAPS]PW12O40 by elemental analysis.
No.
Element
Expected content/%
Experimental content/%
Fresh
Reused
1
2
3
C
H
S
3.39
0.66
1.01
3.23
1.25
0.97
3.20
1.26
0.95
sis and shown in Table 3. It can be seen that the experimental
values of C and S elements are also in line with, but slightly lower
than the theoretical ones. In contrast, the experimental value of H
is higher than the theoretical one. These results can be attributed to
the unbound or crystal water and can confirm the above results of
TG-DTG and ICP-AES. Moreover, the Hf0.5[TEAPS]PW12O40 catalyst
after used in the benzylation reaction shows similar elemental
content as the fresh one.
3.2. Catalytic performances of organic-inorganic heteropoly salts
in benzylation of PX with BA
Fig. 5 shows the effects of catalyst dosage, reaction temperature,
reaction time and the molar ratio of reactants on the catalytic per-
Catalytic
Hf0.5[TEAPS]PW12O40
performances
of
the
prepared
hybrid,
,
and contrastive catalysts in benzyla-

114
B. Yuan et al. / Molecular Catalysis 443 (2017) 110–116
Table 4
Catalytic benzylation performances of organic-inorganic heteropoly salts a
.
No.
Catalyst
Conv. of BA/%
Product distribution/%
Monobenzylationb
Polybenzylation
DBE
1
2
3
4
5
6
7
8
9
–
0
99.1
4.1
0
0
28.2
0.4
0
H3PW12O40
HfCl4
TEAPS
Hf0.5HPW12O40
[TEAPS]3PW12O40
Hf0.25[TEAPS]HPW12O40
Hf0.25[TEAPS]2PW12O40
Hf0.5[TEAPS]PW12O40
61.3
25.6
0
62.4
47.6
56.8
52.4
67.4
10.5
74.0
0
0
0
100
98.6
98.4
98.7
100
37.6
28.9
30.9
28.2
32.6
0
23.5
12.3
19.4
0
a
Reaction conditions: catalyst (0.1 g), BA (0.05 mol), nPX:nBA = 2, 140 ◦C, 2 h.
2,5-dimethylbenzylbenzene.
b
formance when Hf0.5[TEAPS]PW12O40 was utilized to catalyze the
benzylation of PX with BA. It can be seen that more DBE will gener-
ate in the case of low catalyst dosage being employed (Fig. 5a).
While, all the 0.05 mol of reactant BA could be transformed as
well as the formation of DBE could be inhibited under the con-
dition of using 0.1 g of Hf0.5[TEAPS]PW12O40 which has provided
enough catalytic active sites. However, the more catalyst dosage
was used, the more polybenzylated products would be observed.
Hence, it indicates 0.1 g of Hf0.5[TEAPS]PW12O40 is the suitable cat-
alyst dosage in view of 0.05 mol of reactant BA (Fig. 5a).
The catalytic activity of Hf0.5[TEAPS]PW12O40
, as well as
the selectivities for monobenzylation and polybenzylation, will
enhance with the increasing of reaction temperature (Fig. 5b).
A considerable amount of BA would undergo self-condensation
process with lower activation energy to produce DBE at lower
reaction temperature. It has been observed that all the BA would
turn into monobenzylated or polybenzylated products at an opti-
mal temperature of 140 ◦C, without DBE detected. However, higher
temperature than 140 ◦C would result in decreasing of selectivity
for monobenzylation due to further polybenzylation.
Fig. 6. Benzylation of PX with DBE being the benzylation reagent over
Hf0.5[TEAPS]PW12O40 Reaction conditions: catalyst (0.1 g), DBE (0.05 mol),
nPX:nDBE = 2, 140 ◦C, 2 h.
.
Under the above optimal catalyst dosage and reaction temper-
ature, the excellent catalytic activity of Hf0.5[TEAPS]PW12O40 has
been demonstrated (Fig. 5c). The conversion of BA can reach 100%
within 1 h. Furthermore, with the increasing of reaction time, the
selectivity for monobenzylated products enhances remarkably and
achieves a maximum within 2 h. At the same time, the content
of by-product DBE declines from 60% to zero Together with the
results of Fig. 5a and b, an opinion that the self-condensation of BA
with lower energy barrier competes with benzylation process of
which products have higher thermodynamic stability can be held
[12,29,35]. Moreover, DBE, the self-condensation product of BA, can
also attack the benzene ring of PX as a benzylated reagent to pro-
ceed monobenzylation, even polybenzylation process [36]. In order
to confirm this point, the reaction of PX with DBE being the benzy-
lated reagent was carried out under the same reaction conditions
(Fig. 6). As can be seen from Fig. 6, DBE could be consumed com-
pletely and converted into benzylated products within 2 h under
the above conditions. Meanwhile, the monobenzylated product
was liable to undergo consecutive reaction to produce polyben-
zylated products. Furthermore, either Brønsted or Lewis acid sites
can realize a catalytic effect in this reaction. To sum up the above
results and discussion, a plausible mechanism of PX benzylation
with BA over the Hf0.5[TEAPS]PW12O40 catalyst with Brønsted and
Lewis acidity is proposed (Scheme 1).
ability of polybenzylation had been found. However, once the molar
ratio of PX to BA was over 12, the content of DBE tended to
increase due to the excessive attenuation of catalytic active sites
by PX. From what has been discussed above, an excellent cat-
alytic benzylation result with 100% of BA conversion and 92.4%
of 2,5-dimethylbenzylbenzene selectivity has been achieved under
the optimal reaction conditions: Hf0.5[TEAPS]PW12O40 (0.1 g), BA
(0.05 mol), nPX:nBA = 12, 140 ◦C, 2 h.
3.3. Substrate extension
The benzylations of several benzene compounds which have
different groups with BA over Hf0.5[TEAPS]PW12O40 were also
investigated (Table 5). As can be seen, similar excellent conver-
sion and selectivity for monobenzylation with PX could be obtained
within 2 h in the case of activated benzene compounds. However,
as to the benzylation of chlorobenzene and benzene itself, the for-
mation of DBE was still hard to be avoided under the conditions of
this paper.
3.4. Reusability of Hf0.5[TEAPS]PW12O40 catalyst
The reusability of Hf0.5[TEAPS]PW12O40 in benzylation of PX
under the above optimal reaction conditions was investigated using
Besides, the influence of PX to BA molar ratio on the catalytic
performance of Hf0.5[TEAPS]PW12O40 in benzylation was inves-
tigated to restrain the polybenzylation process and improve the
selectivity for monobenzylation. The molar ratio has almost no
effect on the catalytic activity of Hf0.5[TEAPS]PW12O40 (Fig. 5d).
Consistent with the expected, the more PX served, the lower prob-
a seven-run recycling test. The solid catalyst, Hf0.5[TEAPS]PW12O40
,
precipitated to the bottom of the reactor after the reaction, allowing
separation from the solution via simple centrifugation. The sepa-
rated catalyst was washed with ethyl acetate, dried in a hot air oven
at 120 ◦C for 1 h and reused for the next run. Hf0.5[TEAPS]PW12O40

B. Yuan et al. / Molecular Catalysis 443 (2017) 110–116
115
Scheme1. Benzylation reaction mechanism of PX with BA over Hf0.5[TEAPS]PW12O40
.
Table 5
Benzylation of different aromatic substrates over Hf0.5[TEAPS]PW12O40
a
.
No.
Substrate
Benzylation product (major)
Conv.of BA/%
Product distribution/%
Monobenzylation (o-:p-)
polybenzylation
2.9
DBE
1
2
100
100
97.1 (48:52)
96.8
0
0
3.2
3
4
100
91.7 (47:53)
46.2
8.3
7.0
0
97.9
46.8
5
98.4
41.8 (6.3:93.7)
4.1
54.1
Reaction conditions: Hf0.5[TEAPS]PW12O40 (0.1 g), BA (0.05 mol), nsubstrate:nBA = 12, 140 ◦C, 2 h.
a
spectra, acidity, BET and elemental analysis of fresh and reused
Hf0.5[TEAPS]PW12O40 (Fig. 1) reveal that no obvious change of the
catalyst has occurred after benzylation. However, only 0.0803 g of
the recycled catalyst after 6 runs was found due to operational loss,
which can account for the lower catalytic performance of catalyst
after 6 runs. As respected, when 0.0200 g of fresh catalyst was added
into the recycled catalyst in the 7th run, the catalytic performance
recovered to the initial level. The above results exhibit the favor-
able reusability of Hf0.5[TEAPS]PW12O40 in benzylation of PX with
BA.
4. Conclusions
Introducing sulfated organic cation [TEAPS]+, and metal cation
Hf4+
,
together as counter ions for PW12O40
to form het-
3−
eropoly organic-inorganic salt can obtain a novel efficient catalyst,
Hf0.5[TEAPS]PW12O40, for benzylation of PX with BA. The suit-
able Brønsted-Lewis double acidity both from the positive ions
part of Hf0.5[TEAPS]PW12O40 catalyst can effectively restrain the
formation of DBE and polybenzylation products, and has led to
excellent catalytic activity as well as selectivity for monobenzy-
lation. Besides, the prepared solid Hf0.5[TEAPS]PW12O40 catalyst,
dissolving neither in organic phase nor in the water formed during
the reaction, has exhibited quite steady reusability demonstrated
Fig. 7. The performance of the reused catalyst. Reaction conditions:
Hf0.5[TEAPS]PW12O40 (0.1 g), BA (0.05 mol), nPX:nBA = 12, 140 ◦C, 2 h.
catalyst could maintain excellent activity and selectivity after
5 cycles (Fig. 7). ICP-AES was employed to test the dissolved
residue of catalyst in the reaction product phase. It has been
found that no P element was detected. In addition, the FT-IR, XRD

116
B. Yuan et al. / Molecular Catalysis 443 (2017) 110–116
by a seven-run recycling test. Under the optimum conditions, 100%
of BA conversion and as high as 92.4% of selectivity for monoben-
zylation have been achieved.
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The financial support provided for this research by the National
Natural Science Foundation of China (31470595, 31270615), the
Taishan Scholar Project of Shandong Province (ts201511033) and
Opening Foundation of Beijing National Laboratory for Molecular
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