Organo-Laponites as Novel Mesoporous Supports for Manganese(III) salen Catalysts

Abstract

A Mn(III) salen complex was immobilized onto the Laponite surface using three different methodologies: method A, direct immobilization of the complex on the parent Laponite; method B, covalent anchoring through cyanuric chloride (CC); and method C, covalent anchoring through CC into a 3-aminopropyl)triethoxysilane (APTES) modified Laponite. All of the materials were characterized by FTIR, XPS, thermogravimetry, XRD, and nitrogen isotherms at 77 K, to gather information on the modifications introduced by the organo spacers within the Laponite surface, as well as on the anchored complex integrity; the Mn based materials were screened in the heterogeneous epoxidation of styrene. The results have shown that the immobilization of the manganese(III) salen complex by methods B and C have occurred at the edges of the clay particles through the spacers (APTES and CC) that have been anchored onto the Si-OH groups, whereas in method A, the complex is distributed throughout the clay surface, including the interlayer region. Therefore, the manganese loadings on the Laponites were as follows: materials prepared by method A >> method B > method C. All of the heterogeneous catalysts showed high styrene epoxide selectivity, with that prepared by method A showing comparable styrene epoxide selectivity as the homogeneous phase reaction. The styrene epoxide yields decrease in the following order: materials prepared by method A > method B > method C (1st cycles), which parallel the respective support catalytic activity and decreasing of manganese content. The heterogeneous catalysts prepared using methods B and C could be reused at least for four times, with the former exhibiting the most stable catalytic activity, but that prepared by method A showed a significant decrease after two catalytic cycles.