Zwitterionic Liquid Supported SBA-15: a Multifunctional Heterogeneous Catalyst Material

Abstract

Ionic liquids have attracted considerable attention in the fields of chemistry and chemical engineering due to their unique properties including extremely low volatility, nonflammable, wide liquid range and good solubility. Particularly, task-specific ionic liquids (TSILs) exhibit a great potential in replacement of conventional catalysts1 because of their designable property. Despite infinite utility, the application of TSILs are always limited by some disadvantages, such as high synthesis cost, high viscosity, difficulties of mass transfer and separation. Therefore, immobilization of TSILs on suitable solid supports is highly desirable, which is an alternative approach to solve above-mentioned problems.2 In this work, a zwitterionic liquid precursor N-(3-triethoxysilylpropyl)-N-(3-sulflopropyl)-amine (SNSA) was synthesized and characterized, which was further immobilized on SBA-15 mesoporous materials by using sol-gel and grafting methods respectively. The as-synthesized materials were characterized by X-ray diffraction (XRD), solid-state 29Si and 13C CP/MAS NMR, N2 adsorption, TEM, SEM and elemental analysis, which indicated that the precursor had been incorporated into the hexagonal silica matrix of SBA-15. Interestingly, the materials could be acidified or alkalized to form the functionalized ionic liquid supported phase (Scheme 1). Aldol condensation3 of 4-nitrobenzaldehyde and acetone (Scheme 2) was used as a model reaction to test the activity of the alkalized product. The conversion ratio was 77%, and the selectivity was 90% for hydroxide compound. The catalysts could be reused for three times without significant change of the catalytic activity