Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Clean benzylation of anisole with benzyl alcohol over recyclable partially
sulfonated imidazole-exchanged heteropoly phosphotungstate
⁎
Bing Yuan, Wensheng Zhao, Fengli Yu, Congxia Xie
Key Laboratory of Eco-Chemical Engineering, Ministry of Education, College of Chemistry and Molecular 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 5 June 2014
Received in revised form 27 July 2014
Accepted 28 July 2014
Available online 6 August 2014
A series of organic heteropoly acidic salts with high acid strength have been prepared via the partial replacement
of protons in a neat phosphotungstic acid. These hybrid catalysts have exhibited good activity and selectivity in
the benzylation of anisole with benzyl alcohol. The “liquid-phase reaction at high temperature, solid-state sepa-
ration at room temperature” performance of obtained hybrid catalysts enables an easy recovery and steady reus-
ability demonstrated by a three-run recycling test.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Benzylation
Benzyl alcohol
Ionic liquids
Organic heteropolyacid salts
1. Introduction
employed [BMIM]3PW12O40([1-butyl-3-methylimidazolium]3PW12O40)
as the catalyst for esterification of ethanol with acetic acid. Ivanova
[15] studied the catalytic performances of [BMIM]3PW12O40 and
[BMIM]3PMo12O40 in the dehydration of methanol. Dai [16] found that
[HMIM]3PW12O40([1-methylimidazolium]3PW12O40) could form a
“temperature-controlled phase-separation” system in the acetalation
reaction of carbonyl compounds. Leng [13,17] synthesized a series of
heteropoly ionic liquids containing propane sulfonate (PS) functionalized
organic cations [MIMPS]3PW12O40([3-(1-methylimidazolium-3-yl)pro-
pane-1-sulfonate]3PW12O40), [PyPS]3PW12O40 ([pyridinium propane
sulfonate]3PW12O40) and [TEAPS]3PW12O40([3-(triethylammonio)pro-
pane sulfonate]3PW12O40). It was found that the use of the PS group
The benzylation of aromatic compounds to produce diarylmethanes
and their derivatives, which are key industrial compounds as pharma-
ceutical intermediates or fine chemicals, is one of the important reac-
tions in organic chemistry [1,2]. The general benzylation methods,
using benzyl chloride as reagent or employing homogeneous acid cata-
lysts such as AlCl3, FeCl3, and H2SO4, suffer from many problems like
pollution, corrosion and difficulty in the separation and recovery of
catalysts [3–5]. Considering that benzyl alcohol (BA) is a relatively
friendly benzylation reagent due to water as the byproduct, a cleaner
benzylation route can be carried out to reduce environmental pollution
[6,7]. Solid catalysts have been studied in benzylation reactions with
benzyl alcohol to overcome the problems of mineral acids [8,9]. Howev-
er, these heterogeneous catalysts have disadvantages as well, for
example, water instability, the tendency to deactivate, and high mass
transfer resistance.
Acidic functionalized ionic liquids have been revealed as promis-
ing green acid catalysts in view of their high activity, easiness in sep-
aration and recovery, just like the advantages of both homogeneous
and heterogenous catalysts [10,11]. Especially, owing to the variety
of ionic liquid structure available, “temperature-controlled liquid–
solid separation” or “reaction-controlled phase-transfer” catalysis can
be actualized by ionic liquid catalysts [10,12]. The organic salts of
heteropolyacids, which possess the structural characteristics of both
ionic liquids and heteropoly compounds, have raised much concern as
novel “task-specific” catalysts for acid catalyzed reaction [13]. Shi [14]
was an effective approach to improve the acid strength of heteropoly
3−
ionic liquids. More importantly, PW12
O
existing in the catalysts
40
would lead to higher melting points than conventional ionic liquids.
The above heteropoly ionic liquids with solid nature could act as “reaction
induced self-separation catalysts” in some esterification reactions due to
their different solubility in reactants and products. However, the PS group
in organic cations could only provide enough acid strength for some
specific reactions which were easy to realize, such as esterification
[18–20]. In our benzylation study of aromatic compounds with benzyl al-
cohol using [MIMPS]3PW12O40, considerable dibenzyl ether (DBE), the
dehydrated product of benzyl alcohol, was produced due to insufficient
acid strength of the catalyst.
In this paper, heteropoly acidic salts with acid strength even higher
than a neat phosphotungstic acid have been prepared via the partial re-
placement of protons by the organic cation with highly acidic PS groups.
These heteropoly acidic salts, which have enough water resistance and
the similar structure of ionic liquids, have exhibited good activity, selec-
tivity and stability in the benzylation of anisole with benzyl alcohol.
⁎
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