Investigating the role of SBA-15 silica on the activity of quaternary ammonium halides in the coupling of epoxides and CO2

Abstract

The conversion of CO2 into cyclic carbonates in the presence of epoxides is one of the most promising methods to valorize this important C1 building block. Tetrabutylammonium halide salts are known to catalyze this reaction with moderate activity, and often a co-catalyst is added to increase efficiency. Herein, the influence of the addition of ultra-large pore SBA-15 silica on the catalytic activity of tetrabutylammonium halide salts for this reaction has been investigated in detail. Whereas a negative effect of silica was observed for the NBu4Cl catalyst, surface silanols were shown to strongly improve the yield of the reaction when NBu4Br and NBu4I were used as catalysts. The catalytic activity of the NBu4I salt could be increased by a factor of ten when a relevant amount of mesoporous silica was added. This latter catalytic system leads to high yields and selectivity for a wide variety of substrates under mild condition (80°C). Measurement of the activation parameters demonstrated that the rate enhancement is entropically driven: the pre-organization of the reactants and catalyst on the surface leads to a marked decrease of the activation energy of the reaction. A direct comparison between yield improvement induced by the addition of either an amorphous or a mesostructured silica highlights the crucial role played by the shape and concavity of the support.