Mesoporous silicas functionalized with aminopropyl via co-condensation: Effective supports for chiral Mn(III) salen complex

Abstract

The 3-aminopropyl-functionalized mesoporous materials with various amounts of 3-aminopropyl groups and nanopore sizes were synthesized through direct co-condensation of tetraethoxysilane with 3-aminopropyltriethoxysilane (APTES) using alkyltrimethylammonium bromide as template and applied to immobilize chiral Mn(III) salen complex. All the mesoporous hybrids and heterogeneous catalysts had ordered hexagonal mesostructure with uniform pore size distributions. The obtained heterogeneous catalysts exhibited comparable conversion and enantiomeric excess to those of the homogeneous counterpart for the asymmetric epoxidation of olefins even with a very low catalyst dosage (0.6 mol%), which were thrice more active than the catalysts supported on the 3-aminopropyl graft-modified mesoporous silicas. The improved catalytic efficiency was mainly attributed to the homogeneous distribution of chiral Mn(III) salen complexes on the surface of co-condensation modified supports. Both the APTES dosage and nanopore size of mesoporous hybrid material were found to have great influences on catalytic performance, and the catalyst with suitable APTES content and larger pore size showed higher catalytic activity and enantioselectivity.