A study on the reaction pathway and mechanism of urea alcoholysis by the disassociation and conjugation of groups†

The chemical conversion of alcohols to carbonates, which are high value-added products, is considered a green transformation route; however, the underlying reaction mechanism remains unclear and needs further study. Considering the synthesis of ethylene carbonate (EC) from urea and ethylene glycol (EG) as a model reaction using tetrabutylphosphonium bromide ([P₄₄₄₄][Br]) and zinc bromide (ZnBr₂) as the binary catalyst, the yield and selectivity of EC could reach 83.07% and 95.38%, respectively. Subsequently, the qualitative and quantitative analyses of major components in the reaction were performed via GC-MS, ¹H NMR, ¹³C NMR, and kinetics studies. Additionally, three catalysis processes catalyzed by [P₄₄₄₄][Br], ZnBr₂, and [P₄₄₄₄][Br]/ZnBr₂ catalysts were detected by in situ FT-IR spectra. Meanwhile, the possible reaction pathway and mechanism of the formation of EC from urea and EG was systematically studied according to the variation tendencies of a range of functional groups. The introduction of a catalyst as a judging criterion for determining the rates of group disassociation and conjugation on substrates and products provided a new reference basis for the evaluation of catalytic reaction processes and the performance of catalysts.