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

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.