Catalytic esterification reactions over immobilized Brnsted ionic liquid

Three designed and synthesized Brnsted acidic ionic liquids(ILs)[(n-Bu-SO3H)MIm],[(n-Bu-SO3H)MIm][p-CH3C6H4SO3]and ,as well as another commercial ionic liquid [CF3SO3]were used to catalyze the esterification reaction of hexanoic acid and ethanol.Among them,[(n-Bu-SO3H)MIm]exhibited the highest activity,which was later selected to be immobilized on the silica gel by both impregnation and sol-gel methods.The immobilized ILs were then characterized by 1H NMR,elemental analysis,FTIR and activity test.The results showed that the loading amounts of ionic liquid by the impregnation method A,B and sol-gel methods were 15.5%,20.5 % and 40.5 %(mass),respectively.For the first time the three immobilized ILs were used to catalyze the esterification of hexanoic acid and ethanol,and the yields of ethyl hexanoate were 75.3%,92.6% and 92.6%,respectively.However,the activity of the ionic liquid immobilized impregnation decreased rapidly with recycle runs since the load amount of ionic liquid dropped to 3.0 %(mass)after 4 recycle runs for method A and dropped to 7.0 %(mass)after 8 cycle runs for method B.By contrast,the ionic liquid immobilized by the sol-gel method was much stable,and could be reused for 10 times with only little decrease in activity.Moreover,the sol-gel method immobilized ionic liquid also showed acceptable activity(ester yields90.0%)for the esterification of other carboxylic acid with ethanol.Therefore,the sol-gel method is an excellent method for the immobilization of ionic liquid to catalyze esterification reactions.