Organotin compounds immobilized on mesoporous silicas as heterogeneous catalysts for direct synthesis of dimethyl carbonate from methanol and carbon dioxide

Abstract

Organotin compounds of (MeO) 2ClSi(CH 2) 3SnCl 3 have been grafted on a series of mesosporous silicas, as proved by XRD, N 2 adsorption/desorption experiments and FTIR, diffuse reflectance UV–vis and 13C CP/MAS NMR spectroscopies. A further substitution of Cl − bonded to Sn 4+ by CH 3O − led to the formation of a series of active heterogeneous catalysts for the direct synthesis of dimethyl carbonate from methanol and CO 2. The catalytic properties of these catalysts depended on the structure, crystal size and surface properties of mesoporous silicas. SBA-15 as a host was superior to SBA-16 and large-pore Ia3d. In addition, the methods used for removing surfactants occluded in mesoporous silicas had strong influences on the catalytic performance as a result of significantly affecting the surface areas and the concentrations of surface hydroxyl groups of the mesoporous silicas. The extraction of surfactants by an ethanol–HCl solution should be available. An increase in the reaction time, reaction temperature, methanol amount and CO 2 pressure significantly increased the dimethyl carbonate yield before reaching thermodynamic equilibrium. The dimethyl carbonate yield could also be increased by adding dehydration agents to the reaction system. 2,2-Dimethoxypropane gave a much higher dimethyl carbonate yield than 3A molecular sieve and tetramethyl orthorsilicate. The prepared organotin/mesoporous silica catalysts were highly stable, as corroborated by the findings that the dimethyl carbonate yield obtained over the SBA-15-immoilized organotin compound did not decrease within six repeated runs with regeneration under the same reaction conditions and that the tetrahedral coordination state of Sn species was maintained after reaction.