Y.Q. Cai et al. / Chinese Chemical Letters 23 (2012) 1–4
3
Table 2
The esterification reaction of benzoic acid with alcohols catalyzed by [C3SO3Hmim]HSO4.a
b
Entry
Alcohols
Time (h)
Yield (%)
1
2
3
4
5
Methanol
2.0
2.5
3.5
4.0
4.0
98
95
83
85
86
Butanol
Cyclohexanol
Benzenemethanol
Lauryl alcohol
a
The molar ratios of benzoic acid/alcohol/[C3SO3Hmim]HSO4/are 1:3:0.3, the temperature 95 8C.
Isolated yield based on benzoic acid.
b
and 5). Additionally, the optimal yield to the desired product was obtained when benzoic acid reacted with 3 equiv. of
ethanol at 95 8C for 2.5 h (Table 1, entry 6). Further inspection revealed that the anion influenced the esterification
dramatically, [C3SO3Hmim]HSO4 exhibited relatively better activity over [C3SO3Hmim]BF4, [C3SO3Hmim]PF6, and
[C3SO3Hmim]CF3SO3 under the same catalytic conditions (Table 1, entries 6, 11–13). The optimal condition was
achieved in the presence of [C3SO3Hmim]HSO4 (Table 1, entry 6), whereas lower yields of the products were
observed when [C3SO3Hmim]BF4, [C3SO3Hmim]PF6, and [C3SO3Hmim]CF3SO3 were used. The higher yields of
esters in [C3SO3Hmim]HSO4 are probably owing to higher Brønsted acidity of [HSO4]À as well as best immisciblity
of [C3SO3Hmim]HSO4 with the produced ester, which facilitates the shifting of the esterification reaction equilibrium
to the product side [1].
The recovery and reuse of catalysts are important from economic consideration and environmental concerns. Along
this issue, the stability and recyclability of [C3SO3Hmim]HSO4 were examined in the reaction of benzoic acid with
ethanol under the optimized reaction conditions. It was found that the activity and stability of [C3SO3Hmim]HSO4
maintained very well (yields: 95–98%) even after five recycles (Table 1, entries 6, 14–17).
To explore the generality of the esterification reaction catalyzed by [C3SO3Hmim]HSO4, a variety of alcohols were
selected to react with benzoic acid. As summarized in Table 2, [C3SO3Hmim]HSO4 showed good catalytic activities in
esterification of other alcohols like methanol, butanol, cyclohexanol, benzenemethanol, and lauryl alcohol with good
to excellent yields (83–98%). The results also showed that the steric bulk of alcohols affected the reactions.
Esterification of methanol and butanol (Table 2, entries 1 and 2) was easier compared with that of cyclohexanol,
benzenemethanol, as well as lauryl alcohol (Table 2, entries 3, 4, and 5) and higher yields were obtained.
In summary, we have developed an efficient, simple and recyclable method for esterification of benzoic acid with
alcohols catalyzed by imidazolium IL-supported sulfonic acids. Among the catalysis system with different anions,
[C3SO3Hmim]HSO4, [C3SO3Hmim]BF4, [C3SO3Hmim]PF6, and [C3SO3Hmim]CF3SO3, [C3SO3Hmim]HSO4
showed the highest activity and stability to catalyze the esterification reactions of benzoic acid. Compared to the
reported pyridinium IL-supported sulfonic acids, high yields of the esters were afforded with 0.3 equiv. of IL-
supported sulfonic acids. Moreover, the stability of [C3SO3Hmim]HSO4 facilitated high yields of the product without
the activity loss even after five recycling uses.
Acknowledgments
We acknowledge the National Natural Science Foundation of China (No. 20676033), China Postdoctoral Science
Foundation (No. 20070410169), and Shanghai Leading Academic Discipline Project (No. B507) for financial support.
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