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X. Li et al. / Ultrasonics Sonochemistry 17 (2010) 752–755
ture was stirred at room temperature until it became homoge-
neous phase then was introduced into the ultrasonic cleaning bath.
The esterification reaction was allowed to proceed at 40 2 °C for
4–6 h under ultrasonic irradiation. On completion, the reaction
mixture became two phase (exception of ethyl lactate). The upper
layer contained the expected ester and isolated by simple decanta-
tion. The lower layer was BAILs phase that contained the unreacted
materials, produced water and a small portion of the produced es-
ter. After washing BAILs phase with ethyl ether (3 Â 7.5 mL), BAILs
were regenerated by removing water under high vacuum at 105 °C
for 4–5 h.
aliphatic alcohols with lactic acid, and found that all reactions car-
ried out smoothly and good to excellent results were obtained,
these results were listed in Table 1 (entry 1 and entries 3–5). In or-
der to investigate the generality of this synthetic strategy for the
synthesis of lactate esters, the esterification of lactic acid with ste-
rically of hindered alcohols and aromatic alcohols were further
studied. It was exciting to observe that 3a, 3b also have demon-
strated excellent catalytic activity, these results are summarized
in Table 1 (entries 6 and 7). These good results could be explained
as following facts: because of the high degree of miscibility of 3a,
3b with water but hardly miscible with the produced esters, the
water formed during the reaction could easily enter into BAILs
phase from the esters, which induced a spontaneous phase separa-
tion, this behavior may drive the reaction toward to full conver-
sion, even though Fischer esterification is a reversible reaction. In
addition, 3a and 3b could be served as surfactant-type Brønsted
acid catalysts in this study. Under ultrasonic irradiation, the col-
lapse of the cavitation bubbles could caused the BAILs, organic sub-
strates and water from lactic acid and generated in reaction
forming emulsion droplets, which have a hydrophobic interior,
the BAILs molecules could concentrate protons on the surface of
the droplet and then shift the equilibrium to the product side.
Moreover, Forbes have reported that the existence of water in
BAILs was beneficial in carrying out Fischer esterification [15].
From Table 1, it was also found that with the increasing of carbon
chain of aliphatic alcohols, the conversion rate and yields of esters
were decreased. This is because the emulsification of ester was im-
proved with the increasing of its carbon chain, so the produced
water became more difficult to be separated from the formed es-
ters. Compared with aliphatic alcohols, esterifications of sterically
hindered alcohols and aromatic alcohols were more difficult,
needed longer reaction times and gave lower conversion rate and
yields still. Furthermore, different cations of BAILs also exhibited
different catalytic activity, 3a gave better results than 3b in this
study. Nowadays, SO3H-functionalized BAILs with acidic counter-
anion acted as catalyst and solvent have been applied successfully
in Fischer esterification [4a,4c,5a]. For the purpose of comparison,
the esterifications of lactic acid with n-butanol in 4a and 4b
(Scheme 1, they were synthesized according to the method re-
ported in the literature [4a]) under the same reaction conditions
were also carried out, although prolonging the reaction time to
5 h the conservation rate and yield of esters were 87%, 85% and
83%, 81%, respectively. As is evident, the catalytic performances
of 3a and 3b could be better than those of 4a and 4b, the reason
may be ascribed to the higher viscosity of 4a and 4b. Moreover,
the bank experiments of lactic acid reacted with n-butanol without
BAILs under the same ultrasonic irradiation conditions was also
performed but no esters produced. The silent experiments of lactic
3. Results and discussion
3.1. Synthesis of two novel BAILs 3a/3b
The synthesis of 3a and 3b involved a three-step process
(Scheme 1). N-Methylimidazole and pyridine were independently
alkylated by using benzyl chloride at the first step, the resulting
quaternary halides 1a and 1b then were transformed into 2a and
2b using the concentrated sulphuric acid as the anion exchange
agent in CH2Cl2 under reflux conditions. Then 2a and 2b were sul-
fonated by chlorosulfonic acid in CH2Cl2. The resulting 3a and 3b
were isolated as oil liquids with good mobility at room tempera-
ture. Their structures were confirmed by 1H NMR, 13C NMR, IR
spectra and elemental analyses. The viscosities of 3a and 3b were
211 and 198 cPa at 28 °C. The contents of water in 3a, 3b were
3.2% and 3.4% (Karl–Fischer titration). These BAILs were partially
immiscible with ethyl esters, butyl lactate ethers, n-hexane, tolu-
ene but readily soluble in water, methanol, ethanol, lactic acid
and dichloromethane. So compare with mineral acids, they can
be separated conveniently from produced esters by simple phase
separations. The halide impurities of 3a, 3b were evaluated by
ion chromatography according to the method reported in the liter-
ature [14]. In our case, the contents of halides in 3a, 3b range from
175 to 214 ppm. So we think these values may not be a problem for
catalytic applications in Fischer esterification.
3.2. Fischer esterification reaction in BAILs 3a/3b
Our initial efforts were directed toward investigating the syn-
thesis of butyl lactate from n-butanol and lactic acid using 3a, 3b
as the solvent and catalyst under ultrasonic irradiation. Under opti-
mum reaction conditions, the results of the experiments were
listed in Table 1 (entry 2). It was found that 3a, 3b showed high
catalytic activity for esterification, and excellent conversion rate
and satisfactory yield were obtained. Based on this success, we
continued to carry out the esterification of a series of straight chain
Table 1
Results of Fischer esterification lactic acid with different alcohols in 3a/3b under ultrasonic irradiation.a
Entry
Alcohol
Reaction time (h)
Conversion 3a/3b (%)b
Yield 3a/3b (%)c,d
1
2
3
4
5
6
7
Ethanol
n-Butanol
4
4
4
4
5
6
6
96/94
95/91
92/89
90/88
87/86
80/78
83/80
92/92
90/89
87/85
82/83
82/81
75/73e
80/77f
n-Hexanol
n-Octanol
Dodecanol
Cyclohexanol
Benzyl alcohol
a
Reaction conditions: reaction temperature 40 °C; ratio of acid/alcohol/BAILs = 1:1:0.2 (mol).
Conversion rate = (acid value of lactic acid À acid value of crude esters)/acid value of lactic acid  100%. Determination of acid value according to the procedures described
b
in GB/1668-95.
c
Yields of esters are based on isolated crude product.
d
e
f
Purity of crude esters were test by GC and the content of aliphatic esters no less than 90%.
Purity of crude esters were test by GC and the content of ester is 86%.
Purity of crude esters were test by GC and the content of ester is 87%.
Li, Xinzhong
