Efficient fixation and conversion of CO2 into dimethyl carbonate catalyzed by an imidazolium containing tri-cationic ionic liquid/super base system

Abstract

The synthesis route used to prepare dimethyl carbonate from CO₂ and methanol is a most attractive route from a green chemistry point of view. Herein, we systematically studied binary catalyst systems for the synthesis of dimethyl carbonate from CO₂ and methanol. It was found that [GLY(mim)₃][NTf₂]₃IL/DBU, [GLY(mim)₃][NTf₂]₃/MTBD and [GLY(mim)₃][NTf₂]₃/TBD are effective binary catalysts for the direct synthesis of DMC without the need of a dehydration system. In particular, the [GLY(mim)₃][NTf₂]₃ IL/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) system was found to have 37% MeOH conversion and 93% DMC selectivity under mild reaction conditions. However, the tri-imidazolium salts were compatible with DBU for effective CO₂ adsorption, possibly because of the reactive C(2)–H in the imidazolium ring. Furthermore, the [GLY(mim)₃][NTf₂]₃ IL/DBU catalytic system could be easily recovered and reused three times without any obvious loss of catalytic activity. The catalytic reactivity of [GLY(mim)₃][NTf₂]₃ IL/DBU for the synthesis of DMC was proved by its predictable mechanism on the basis of ¹³C NMR spectroscopy.