Task-specific basic ionic liquid immobilized on mesoporous silicas: Efficient and reusable catalysts for Knoevenagel condensation in aqueous media

摘要

N-(3-aminopropyl), N(3)-(3-triethoxysilylpropyl)-4,5-dihydroimidazolium bromide hydrobromide was prepared and successively grafted onto mesoporous silicas of MCM-41 and SBA-15. Finally, the proton-ated amine groups in the resultant organic-inorganic hybrid mesoporous silicas were neutralized by treating with potassium bicarbonate, leading to the immobilized basic amino-containing task-specific ionic liquids (TSILs). Using Knoevenagel condensation as test reaction, the immobilized basic TSILs showed higher efficiency (particularly reusability) than that of similar pure basic TSIL of N-(3-aminopro-pyl), N(3)-(methyl)-imidazolium tetrafluoroborate or amino-functionalized mesoporous silicas in aqueous media. Much higher catalytic performance of the immobilized basic TSILs, as compared with the pure basic TSIL, could be due to the depressed reaction between amine groups and the hydrogen at C2 of imidazolium derivative cations in the immobilized basic TSILs and to the acid-base cooperativity between the silanols on the pore surface of the siliceous supports and amine groups. On the other hand, that amine groups nucleophilically attack the Si—O—Si bonds on the pore surface was suppressed due to steric hindrance resulted from the existence of dihydroimidazolium cations in the organic structure of the immobilized basic TSILs, leading to low leaching level of the active species in the immobilized TSILs during the reuse process. As such, the immobilized basic TSILs showed much better reusability in comparison with the amino-functionalized mesoporous silicas.