Catalytic synthesis of n-butyl carboxylate with immobilized ionic liquid based on response surface methodology optimization

Four kinds of functional ionic liquids (ILs) ([C(3)SO(3)Hnmp]HSO(4)), 1-(3-sulfopropyl)-1-methylpyrrolidone phosphate ([C(3)SO(3)Hnmp]H(2)PO(4)), 1-(3-sulfopropyl)-1-methylpyrrolidone p-toluene sulfonate ([C(3)SO(3)Hnmp]CH(3)SO(3)H) and 1-(3-sulfopropyl)-1-methylpyrrolidone methyl sulfonate ([C(3)SO(3)Hnmp]C(6)H(6)SO(3)H)) were prepared and the catalytic activity of these ILs during esterification of carboxylic acids (formic acid, acetic acid, propionic acid, butyric acid) with alcohols was investigated. The results indicated that the IL ([C(3)SO(3)Hnmp]HSO(4)) exhibited an optimal catalytic performance. And then the IL ([C(3)SO(3)Hnmp]HSO(4)) was immobilized to the silica gel. The immobilized IL performed more excellent catalytic activity than the unsupported [C(3)SO(3)Hnmp]HSO(4). The effects of reaction temperature, reaction time, molar ratio of acid to alcohol and catalyst dosage were investigated. The response surface methodology based on the Box–Behnken design (BBD) was used to explore the best reaction condition of different experimental variables. Accordingly, a high n-butyl butyrate yield of 97.10% under the deduced optimal reaction conditions was obtained, in good agreement with experimental results and that predicted by the BBD model. The immobilized IL [C(3)SO(3)Hnmp]HSO(4) maintained high catalytic activity after five cycles.