Bifunctional immobilized catalyst and N, N-dimethylformamide synergistically synthesize propylene carbonate from CO2 and propylene oxide

Abstract

In the present work, bifunctional immobilized catalysts, incorporating Lewis acid and nucleophilic reagent, were prepared by supporting Zn–Br with ZSM-5 for the synthesis of propylene carbonate from CO₂ and propylene oxide. When the total loading of Zn–Br, ω, is 7.5wt%, and the molar ratio, n(Br)/n(Zn), is 3.0, ZnO-KBr-7.5-3.0@ZSM-5 exhibits the highest activity with the solvent N, N-dimethylformamide (DMF). The conversion of propylene oxide and the selectivity of propylene carbonate are up to 97.73% and 98.83%, respectively. Gas chromatography results reveal the presence of by-products, specifically acetone and propylene glycol, in addition to propylene carbonate. Characterizations identify the active components in ZnO-KBr-7.5-3.0@ZSM-5 as ZnO and KBr. Meanwhile, the experimental results prove that DMF serves as a co-catalyst. The catalytic mechanism of DMF and ZnO-KBr-7.5-3.0@ZSM-5 on the cycloaddition is proposed, and the reaction process is simulated. DMF contains a tertiary nitrogen structure, able to function as a Lewis base, thereby activating CO₂. ZnO and KBr in ZnO-KBr-7.5–3.0@ZSM-5 work synergistically to boost epoxide ring opening. Subsequently, the activated CO₂ integrates into the ring-opened epoxide, resulting in the formation of propylene carbonate. Additionally, four cycles of replicate experiments show that the catalyst ZnO-KBr-7.5-3.0@ZSM-5 possessed good stability.