Keywords: CO 2 valorization cyclic carbonates tetrabutylammonium halide salts organocatalysis ordered mesoporous silica A B S T R A C T The conversion of CO 2 into cyclic carbonates in the presence of epoxides is one of the most promising methods to valorize this important C 1 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 NBu 4 Cl catalyst, surface silanols were shown to strongly improve the yield of the reaction when NBu 4 Br and NBu 4 I were used as catalysts. The catalytic activity of the NBu 4 I 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.