Halogen-free and solvent-free CO2 cycloaddition via adjusting the types of surface defect and doped metal on the metal-doped M/AlOOH catalysts

Abstract

The development halogen-free, solvent-free heterogeneous catalysts for CO₂ cycloaddition with epoxides remains critical, as conventional homogeneous systems face challenges in product separation and leave halogen residues. This work addresses these limitations by designing a series of halogen-free M/AlOOH catalysts with different types of doped metal M and surface defects, to optimize CO₂ cycloaddition with ethylene oxide (EO) by the regulation of acid-base properties. Theoretical and experimental analyses reveal that Lewis acid-base pairs composed of M∙∙∙V_H and M∙∙∙V_OH could stabilize the ring-opening intermediates (LO/LC) and promote EO ring-opening over M/AlOOH catalysts. Crucially, activated CO₂^δ− species mimic halogen’s nucleophilic role in attacking EO, with catalytic activity exhibiting an inverted volcano relationship to the Bader charge of CO₂^δ− species, Optimal performance occurs when the CO₂^δ− charge within 0.23 ∼ 0.55 e, where enhanced nucleophilicity drives cycloaddition. This study establishes a rational strategy for designing sustainable heterogeneous catalysts that eliminate halogens and solvents while maintaining high activity, offering a blueprint for green catalytic systems in CO₂ utilization.