Benzylation with Benzyl Alcohol Catalyzed By [ChCl][TfOH]2, a Br?nsted Acidic DES with Reaction Control Self-Separation Performance

Article abstract of DOI:10.1007/s10562-018-2403-7

Abstract: A deep eutectic solvent with strong Br?nsted acidity was prepared easily by choline chloride and trifluoromethanesulfonic acid. The obtained [ChCl][TfOH]₂ catalyst which was characterized by FT-IR, ¹H NMR and TG, possessed greatly enhanced stability than trifluoromethanesulfonic acid, and exhibited excellent catalytic performance in benzylation with benzyl alcohol. Especially, the prepared [ChCl][TfOH]₂ catalyst could dissolve into the reactants to realize homogeneous catalysis, and then self-separate from the organic phase after reaction due to the entire consumption of benzyl alcohol. As a result, the catalyst could be recovered and reused simply for six-runs without noticeable loss of catalytic performance. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2403-7

Catalysis Letters
https://doi.org/10.1007/s10562-018-2403-7
Benzylation with Benzyl Alcohol Catalyzed By [ChCl][TfOH]2,
a Brønsted Acidic DES with Reaction Control Self-Separation
Performance
Bing Yuan1 · Yinglian Li1 · Fengli Yu1 · Xiaoqiang Li1 · Congxia Xie1 · Shitao Yu2
Received: 20 March 2018 / Accepted: 29 April 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
A deep eutectic solvent with strong Brønsted acidity was prepared easily by choline chloride and trifluoromethanesulfonic
acid. The obtained [ChCl][TfOH]2 catalyst which was characterized by FT-IR, 1H NMR and TG, possessed greatly enhanced
stability than trifluoromethanesulfonic acid, and exhibited excellent catalytic performance in benzylation with benzyl alcohol.
Especially, the prepared [ChCl][TfOH]2 catalyst could dissolve into the reactants to realize homogeneous catalysis, and then
self-separate from the organic phase after reaction due to the entire consumption of benzyl alcohol. As a result, the catalyst
could be recovered and reused simply for six-runs without noticeable loss of catalytic performance.
Graphical Abstract
Keywords Benzylation · Benzyl alcohol · Deep eutectic solvent · Reaction control self-separation
* Bing Yuan
1 Introduction
yuanbing@qust.edu.cn
* Congxia Xie
Diarylmethanes and their derivatives obtaining from Frie-
xiecongxia@126.com
del–Crafts benzylation of aromatic compounds are impor-
1
State Key Laboratory Base of Eco-chemical Engineering,
College of Chemistry and Molecular Engineering, Qingdao
University of Science and Technology, Qingdao 266042,
China
tant fine chemicals as pharmaceutical intermediates, dyes
and perfumes [1–4]. Benzyl halide is the frequently-used
benzylating agent in the industrial manufacture of diaryl-
methanes and their derivatives, releasing hydrogen halide
gas with strong corrosivity [5–7]. In addition, the involved
2
College of Chemical Engineering, Qingdao University
of Science and Technology, Qingdao 266042, China
Vol.:(0123456789)
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B. Yuan et al.
inorganic homogeneous acids catalysts in the above process,
such as AlCl3, FeCl3 and H2SO4, suffer from the issues like
pollution, difficulty in separation and recovery [8, 9]. As a
more environmentally friendly benzylating agent than benzyl
halide, benzyl alcohol (BA) tends to undergo intermolecu-
lar dehydration to produce dibenzyl ether (DBE) by-product
due to its lower activity when the reaction is catalyzed by
the catalyst without enough acidity [7, 10, 11]. Moreover,
water generated during the benzylation and etherification of
BA will lead to the deactivation of numerous catalysts [12].
Thus, the key issue of benzylation with BA is to develop
an efficient catalyst with strong acidity, water tolerance and
good separation performance.
discs were recorded on a Nicolet iS10 FT-IR instrument in
the 400–4000 cm−1 range. 1H NMR spectra were recorded
on a 500 MHz Bruker AV-400 spectrometer. TG analysis was
performed with a Netzsch STA 449C instrument in dry N2
at a heating rate of 20 °C/min from 30 to 800 °C. The analy-
sis of S element content was measured by a WK-2D micro-
coulomb analyzer (Taizhou Guochang Analytical Instruments
Company, Ltd.). The viscosity was measured by an Anton-
Paar rolling-ball viscometer Lovis 2000 M. Densities data
was obtained by an Anton Paar DMA 5000 M vibrating-tube
densimeter.
2.2 Preparation of Brønsted Acidic DES
Some acidic heteropoly ionic liquids have exhibited
excellent reaction control self-separation catalytic perfor-
mance in esterification of specific acid [13–15]. Both good
catalytic efficiency and self-separation characteristic have
been achieved due to the polar converting of reaction system
[16–21]. In the case of benzylation, a catalyst with property
of being soluble in benzylating agent, BA, but insoluble
in substrates and products will conduct a self-separation
catalytic process. Deep eutectic solvents (DESs) [22] has
emerged as a novel kind of green solvents in organic reac-
tions thanks to their low toxicity, nonvolatility, low flam-
mability, biodegradability, simple and low-cost preparation
[23, 24]. In addition, tunable characters of hydrogen-bond-
ing donor (HBD) and hydrogen-bonding acceptor (HBA)
enable functionalized DES to play catalyst roles in organic
reactions. Herein, a DES, [ChCl][TfOH]2, was found had
kept the strong acidity of its HBD, trifluoromethanesulfonic
acid (TfOH), but exhibited improved stability thanks to the
combination with its HBA, choline chloride (ChCl) [25].
As a result, efficient benzylation of p-xylene (PX) with BA
could be realized over [ChCl][TfOH]2 as a catalyst. Moreo-
ver, the [ChCl][TfOH]2 DES catalyst, being soluble in BA
and water, but insoluble in aromatic substrates and their
products, would self-separate out from the organic phase and
dissolve into the generated water when BA was consumed.
Hence, an environmentally friendly catalytic benzylation
process with reaction control self-separation property has
been achieved.
The Brønsted acidic DES, [ChCl][TfOH]2, was prepared by
blending TfOH and ChCl in a molar ratio of 1:2 in an ice-water
bath, and warming up to 80 °C for 2 h with stirring. Contras-
tive catalyst, [ChCl][p-TsOH]2 (or [ChCl][CH3SO3H]2, [ChCl]
[CF3COOH]2, [ChCl][CCl3COOH]2, [ChCl][CH3COOH]2)
was prepared by blending p-TsOH (or CH3SO3H, CF3COOH,
CCl3COOH, CH3COOH) and ChCl in a molar ratio of 1:2
warming up to 80 °C for 2 h with stirring.
2.3 Detection of HCl Gas
The reaction system of synthesizing [ChCl][TfOH]2 was
connected to a gas absorption bottle of silver nitrate solution
(0.1 mol/L), precipitate generation could be observed. Then
extra nitric acid was added, the precipitation did not disappear,
indicating the production of HCl.
2.4 General Procedure for the Benzylation of PX
with BA
The typical procedure for benzylation reaction was as fol-
lows: PX, BA, and catalyst were added proportionally to a
round-bottomed flask with a thermometer, a magnetic stir-
rer and a reflux condenser. The resulting mixture was stirred
vigorously at 140 °C for 1 h then cooled to room tempera-
ture. The reaction mixture, from which the generated water
and water-soluble catalyst had been removed by themselves,
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 µm) to determine the conversion of
BA and the selectivity of benzylation products.
2 Experimental
2.1 Materials and Methods
The products were identified by GC-MS (HP6980/5973)
analysis on a DB-35 column with He as carrier gas.
All chemicals were purchased from Macklin or Sinopharm
Chemical Reagent Co. Ltd of China, and used without fur-
ther purification. Fourier transform infrared spectroscopy
(FT-IR) spectra for ChCl samples on KBr pellet was recorded
on a Nicolet iS10 FT-IR instrument in the 400–4000 cm−1
range. FT-IR spectra for TfOH and [ChCl][TfOH]2 in KBr
1 3

Benzylation with Benzyl Alcohol Catalyzed By [ChCl][TfOH]2, a Brønsted Acidic DES with…
3 Results and Discussion
The viscosity and density of the prepared [ChCl][TfOH]2
at room temperature were 193.06 mPs/s and 1.5302 g/cm3,
respectively. Compared with ChCl in solid state, the viscos-
ity decreased significantly. HCl gas was detected during the
preparation process of [ChCl][TfOH]2 by blending TfOH
and ChCl with a molar ratio of 2, indicating that there was
partial electrostatic interaction between ChCl and TfOH
and a kind of ionic liquid formed. Meanwhile, the FT-IR
spectrum of [ChCl][TfOH]2 (Fig. 1) showed characteristic
peaks of both ChCl and TfOH. However, the red shift of
the stretching vibration of –OH (around 3476 cm−1) from
that in TfOH (around 3568 cm−1) indicated the formation
of hydrogen bonds in [ChCl][TfOH]2 instead of complete
1
formation of ionic liquid. The H NMR spectra of [ChCl]
[TfOH]2, TfOH and ChCl in both DMSO and D2O exhibited
in Fig. 2 also confirmed the above results. A low broad peak
appeared at δ 8–10 in 1H NMR spectra of [ChCl][TfOH]2
when DMSO was used as a solvent, but disappeared when
D2O was used as the solvent (Fig. 2a), which indicated an
active hydrogen. Also, the active hydrogen peak of –OH in
TfOH at δ 8.970 (Fig. 2b) disappeared in D2O. It was found
from Fig. 2c that ChCl was structurally correct and did not
have active hydrogen. Thus, the evidence above confirmed
the formation of hydrogen bonds based on the peak shape,
and illustrated that the involved TfOH retained the acidity
partly in the prepared [ChCl][TfOH]2 DES. The TG curves
Fig. 2 1H NMR spectra for various samples. a [ChCl][TfOH]2 (1) in
in Fig. 3 showed that the DES catalyst, [ChCl][TfOH]2, was
more thermally stable than TfOH, which could be attrib-
uted to the action of hydrogen bond. In summary, [ChCl]
[TfOH]2 was an excellent catalyst with both stability and
acidity due to the partial electrostatic interaction as well
DMSO, (2) in D2O; b TfOH (1) in DMSO, (2) in D2O; c ChCl (1) in
DMSO, (2) in D2O
Fig. 1 FT-IR spectra of various samples. (1) ChCl, (2) TfOH, (3)
[ChCl][TfOH]2, (4) Reused [ChCl][TfOH]2
Fig. 3 TG profiles of various samples. (1) ChCl, (2) TfOH, (3)
[ChCl][TfOH]2
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B. Yuan et al.
Fig. 4 The possible form of [ChCl][TfOH]2
partial hydrogen bond interaction between ChCl and TfOH
(Fig. 4).
Several Brønsted acidic DES catalysts with various HBDs
were also prepared and employed in the benzylation of PX
with BA. The catalytic results of these DES catalysts and
[ChCl][TfOH]2 were listed in Table 1. Excellent catalytic
performance of [ChCl][TfOH]2 could be observed and BA
achieved an entire conversion within 1 h, while no DBE, the
intermolecular dehydration product of BA, formed. How-
ever, other Brønsted acidic DES catalysts, [ChCl][p-TsOH]2,
[ChCl][CH3SO3H]2, etc., almost did not exhibit catalytic
activity in this reaction, due to their low acid strength.
Figure 5 exhibited the reaction control self-separation
performance of [ChCl][TfOH]2 in the benzylation of PX
with BA. As shown in Table 2, [ChCl][TfOH]2 could
dissolve in BA and water between 25 and 140 °C, while
could not dissolve in the substrates, PX, and its benzylated
products. However, due to the mutual solubility of PX and
BA, the DES catalyst, [ChCl][TfOH]2, could dissolve in
the organic phase to form a homogeneous catalytic sys-
tem before and during the reaction process. Consumption
of BA and generation of water during benzylation process
promoted the self-separation of [ChCl][TfOH]2 out of the
organic phase and into the generated water phase.
Fig. 5 Self-separation of [ChCl][TfOH]2 in benzylation with BA
Table 2 Solubilities of [ChCl][TfOH]2
Temperature In PX
(°C)
In BA
In benzylated
products
In water
25
140
Not
Not
Yes
Yes
Not
Not
Yes
Yes
benzylation of the substrates with electron-donating groups,
such as anisole, mesitylene, xylenes and methylbenzene,
catalyzed by [ChCl][TfOH]2 all achieved 100% conversion
of BA and over 90% monobenzylation selectivity. Moreover,
only a small amount multibenzylation by-products, but no
DBE by-product were detected owing to the strong acidity of
[ChCl][TfOH]2 catalyst. As far as benzene was concerned,
the conversion of BA also could reach as high as 97.9%
within 1 h. However, 22.9% DBE by-product was found due
to the low activity of benzene [7]. When the reaction time
Benzylations of various benzene compounds with dif-
ferent substituent groups catalyzed by [ChCl][TfOH]2
were also studied. It could be seen from Table 3 that the
Table 1 Benzylation of PX with
Entry
Catalysts
Reaction
time (h)
Conversion of Product selectivity (%)
BA catalyzed by various DES
catalysts
BA (%)
Monobenzyla- Multiben-
DBE
tion
zylation
1
2
3
4
5
6
7
8
None
ChCl
2
2
1
1
1
2
2
2
–
–
100
0.5
0.4
–
–
–
91.5
38.5
23.4
–
–
–
8.5
0
0
–
–
–
–
0
61.5
76.6
–
[ChCl][TfOH]2
[ChCl][p-TsOH]2
[ChCl][CH3SO3H]2
[ChCl][CF3COOH]2
[ChCl][CCl3COOH]2
[ChCl][CH3COOH]2
–
–
–
–
–
Reaction conditions: [ChCl][TfOH]2 (0.1 g), BA (0.01 mol), nPX:nBA =15, 140 °C.
1 3

Benzylation with Benzyl Alcohol Catalyzed By [ChCl][TfOH]2, a Brønsted Acidic DES with…
Table 3 Benzylations of
Entry Substrate
Reaction
Reaction Conversion Product selectivity (%)
benzene compounds with
different substituent groups
catalyzed by [ChCl][TfOH]2
temperature time (h) of BA (%)
(°C)
Monobenzylation (o:p) Multi-
DBE
benzyla-
tion
1
2
3
4
5
6
7
Anisole
160
160
140
140
1
1
1
1
1
1
6
100
100
100
100
100
97.9
100
91.2 (48:52)
90.3
90.6 (31.6:68.4)
90.8 (19.1:80.9)
92.3 (47:53)
74.7
8.8
9.7
9.4
9.2
7.7
2.4
17.4
0
0
0
0
0
22.9
0
Mesitylene
o-Xylene
m-Xylene
Methylbenzene 110
Benzene
Benzene
80
80
82.6
Reaction conditions: [ChCl][TfOH]2 (0.1 g), BA (0.01 mol), nSubstrate:nBA =15
alterations of selectivity could be attributed to the minimal
loss of the catalyst during the operation.
4 Conclusions
In conclusion, the prepared DES catalyst, [ChCl][TfOH]2,
exhibited excellent catalytic activity, selectivity and stability
in the benzylation using BA as a clean benzylating reagent.
Especially, the DES catalyst, [ChCl][TfOH]2, achieved a
reaction control self-separation property in the above reac-
tion process due to its solubility characteristic in reactants
and products.
Acknowledgements This work was supported by the National Natural
Science Foundation of China (31470595 and 31270615), the Shandong
key R & D plan (public welfare special project) (2017GGX40105), and
the Taishan Scholars Projects of Shandong (ts201511033).
Fig. 6 The performance of the reused catalyst
Compliance with ethical standards
was extended to 6 h, all BA could be consumed. The gener-
ated DBE could also act as benzylating reagent to attack the
benzene ring and then be consumed entirely [5–8, 10, 11,
26, 27]. However, the benzylated product, benzylbenzene,
tended to undergo further benzylation more easily than ben-
zene [10, 28, 29]. As a result, only 82.6% monobenzylated
product selectivity could be achieved under the above
conditions.
Conflict of interest The authors declare no conflict of interest.
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