Metal-organic frameworks MOF-808-X as highly efficient catalysts for direct synthesis of dimethyl carbonate from CO_2 and methanol

A series of metal-organic frameworks MOF-808-X(6-connected)were synthesized by regulating the ZrOCl2·8H2O/1,3,5-benzenetricarboxylic acid(BTC)molar ratio(X)and tested for the direct synthesis of dimethyl carbonate(DMC)from CO2 and CH3OH with 1,1,1-trimethoxymethane(TMM)as a dehydrating agent.The effect of the ZrOCl2·8H2O/BTC molar ratio on the physicochemical properties and catalytic performance of MOF-808-X was investigated.Results showed that a proper ZrOCl2·8H2O/BTC molar ratio during MOF-808-X synthesis was fairly important to reduce the redundant BTC or zirconium clusters trapped in the micropores of MOF-808-X.MOF-808-4,with almost no redundant BTC or zirconium clusters trapped in the micropores,exhibited the largest surface area,micropore size,and the number of acidic-basic sites,and consequently showed the best activity among all MOF-808-X,with the highest DMC yield of 21.5% under the optimal reaction conditions.Moreover,benefiting from the larger micropore size,MOF-808-4 outperformed our previously reported UiO-66-24(12-connected),which had even more acidic-basic sites and larger surface area than MOF-808-4,mainly because the larger micropore size of MOF-808-4 provided higher accessibility for the reactant to the active sites located in the micropores.Furthermore,a possible reaction mechanism over MOF-808-4 was proposed based on the in situ FT-IR results.The effects of different reaction parameters on DMC formation and the reusability of MOF-808-X were also studied.