Microporosity in Mesoporous SBA-15 Supports: A Factor Influencing the Catalytic Performance of Immobilized Metalloporphyrin

Abstract

MnTMPyP cationic metalloporphyrin was immobilized by means of ion exchange on a series of aluminated SBA-15 mesoporous silica supports prepared by different methods. The solids were characterized with XRD, HRTEM, HRSEM, chemical analysis, and nitrogen sorption isotherms. The catalysts were tested in the reaction of cyclohexene oxidation with iodozobenzene. It was found that immobilization significantly enhances catalytic activity as compared to the homogeneous system. In contrast to previously investigated metalloporphyrin catalysts immobilized on aluminated HMS, MCM-41 or FSM-16 type supports, where too narrow pores limited the formation of epoxide and enhanced allylic oxidation, the use of aluminated large pore SBA-15 solids favoured the epoxidation pathway and resulted in yields significantly higher than in the case of homogeneous reaction. The catalysts showed important differences in the level of allylic oxidation. Analysis of various factors potentially influencing the product distribution demonstrates that the key role in determining the contribution of allylic oxidation is the microporosity nature of the support, in particular, the lack or presence of supermicropores capable of accommodating metalloporphyrin species. The MnTMPyP centres confined in supermicropores experience steric limitations, which do not allow for the formation of epoxide and favour allylic oxidation.