Rationalizing the role of the anion in CO2 capture and conversion using imidazolium-based ionic liquid modified mesoporous silica

Abstract

Covalently supported ionic liquids in mesoporous materials were prepared by grafting 1-methyl-3-(3-trimethoxysylilpropyl)imidazolium chloride in MCM-41. Subsequently, the [Cl⁻] anion was changed to [BF₄⁻], [PF₆⁻] or [Tf₂N⁻]. These materials that present an advantageous combination of the properties of mesoporous solid materials and ionic liquids were evaluated for CO₂ sorption as well as catalysts for CO₂ conversion into cyclic carbonate using propylene oxide. The material with the [Cl⁻] anion had the best performance for both CO₂ sorption and conversion. A CO₂ sorption of 11 w/w% on the adsorbent was achieved and the cycloaddition reaction exhibited a conversion of 67% with 82% selectivity with the catalyst remaining active after 5 cycles, proving that the same sorbent/catalyst setup can be used for both CO₂ capture and conversion. Based on the experimental data and electronic-structure numerical simulations, we have hypothesized two major reasons why chloride out performs other anions when adsorbed on MCM-41 unlike unsupported ionic liquids.