Cooperative Catalysis by Bifunctionalized Mesoporous Silica

Abstract

The objective of my work was to prepare heterogeneous catalysts that can perform cooperative catalysis. Cooperative catalysis occurs when the presence of two or more functional groups provide an acceleration of a chemical reaction beyond what is possible when either of the two species is used independently. New catalytic materials were synthesized by functionalizing mesoporous silica (SBA-15) with two different functional groups with the groups distributed either randomly or arranged into heterodimeric pairs. The dependence of catalytic activity and selectivity on the surface arrangement (random vs. paired, distance between paired species) was investigated for several different condensation reactions. Catalysts featuring both sulfonic acid and thiol groups were investigated for the synthesis of various bisphenols. Paired alkylsulfonic acid/thiol catalysts outperformed randomly-distributed catalysts in the synthesis of bisphenol A and bisphenol Z. The distance between the two groups in the acid/thiol pair was varied and the activity and selectivity were found to diminish rapidly as the acid/thiol distance grows. Paired arylsulfonic acid/thiol catalysts outperformed randomly-distributed catalysts in the synthesis of bisphenol Z, whereas the synthesis of bisphenol A was insensitive to spatial arrangement. The second reaction investigated was the aldol reaction in order to investigate the possibility of acid/base cooperativity. A catalyst containing alkylsulfonic acid and primary amines grouped into pairs were generated by opening surface sultone rings with ammonia. This material was catalytically inactive in the aldol reaction due to acid/base neutralization, whereas randomly-distributed acid-base materials exhibit good catalytic activity. Primary amine/carboxylic acid cooperativity was also investigated, both with homogeneous amino acids and bifunctional heterogeneous silicas. While amine/acid cooperativity was conclusively demonstrated with the homogeneous catalysts, in the heterogeneous case the cooperativity due to surface silanol groups actually overshadowed the effect of the carboxylic acids. The results obtained provide evidence that the spatial arrangement of disparate functional groups on the surface of a heterogeneous catalyst can have profound effects on the activity and selectivity of the catalyst.