Ionic liquids tailored and confined by one-step assembly with mesoporous silica for boosting the catalytic conversion of CO2 into cyclic carbonates

On the basis of economic and energy-saving criteria, the minimum effective dose of ionic liquids (ILs) for the catalytic conversion of CO2 into cyclic carbonates was first explored by confinement. With one-step assembly of mesoporous silica (mSiO(2)) by using a fixed amount of silicon source with varying amounts of ILs as templates, certain amounts of 1-ethyl-3-methyl imidazolium bromides (EmimBr) were tailored and confined in mSiO(2). The confined ILs (EmimBr@mSiO(2)) retained the advantages of homo- and heterogeneous catalysts, exhibiting higher performance than bulk EmimBr under identical reaction conditions. Amongst all the prepared materials, EmimBr@mSiO(2) with the lowest amount of EmimBr (6.9 wt%) exhibited the biggest improvement in catalytic activity, achieving a TOF of 112.6 h(-1) which is almost 1.7 times the bulk phase with good recyclability. The boosted catalytic activity could be attributed to the larger proportion of mesopores, the better dispersity of EmimBr (Si/Br = 25) and the synergistic effect from more exposed silanol groups (Si-OH/Br = 8) in the structure. The good recyclability was then explained by the XPS analysis and density functional theory (DFT) calculation, which confirmed that the compressing effect from Si-OH could enhance the cation and anion interaction to stabilize ILs in the space more firmly when less ILs were confined.