Synergistic Defect and Halide Catalysis for CO<sub>2</sub> Cycloaddition on ZIF-8: Mechanistic and Energetic Insights from Density Functional Theory.

Abstract

Understanding the mechanism of CO₂ cycloaddition with epoxides under mild conditions is vital for advancing green carbon capture and utilization strategies. This work investigates the catalytic role of a defective ZIF-8 model with Zn-OH-Zn moieties and bromide (Br^-) assistance in promoting the conversion of CO₂ and propylene oxide into propylene carbonate. Density-functional theory calculations reveal that, in the absence of a catalyst, the reaction proceeds through high activation barriers (52.02 kcal mol^-1 and 59.31 kcal mol^-1 for the α and β pathways, respectively). Upon introducing the Zn-OH-Zn site and Br^-, the energy barrier for the rate-limiting ring-opening step is drastically lowered to 14.45 kcal mol^-1, confirming the synergistic effect between Lewis acid/base sites and halide assistance. The calculated reaction energy of -13.89 kcal mol^-1 aligns well with the experimental enthalpy change (-12.64 kcal mol^-1). This study provides molecular-level insights into the cooperative catalytic mechanism and supports defect-engineering strategies for metal-organic frameworks in CO₂ fixation applications.