Study on core-shell ionic liquid@metal-organic framework composites for direct oxidative carboxylation of CO₂ and olefins under cocatalyst and solvent-free conditions

The direct oxidative carboxylation reaction involves two sequential steps: epoxidation and cycloaddition. The specific catalyst for each step often leads to challenges in catalyst separation and recovery. Therefore, by depositing ionic liquids (ILs), 1-carboxypropyl-3-methylimidazolium bromide (1-C-3-M) on the external surface of the metal-organic framework (MOF) Ni₂(OH)₂(C₈H₄O₄)(NH₂) (Ni-BDC-NH₂), this work designed and synthesized a novel core-shell structured composite catalyst, 1-C-3-M@Ni-BDC-NH₂. CO₂ adsorption and contact angle experiments revealed that 1-C-3-M@Ni-BDC-NH₂ performed good adsorption capacity for reactant molecules CO₂ and styrene (ST), which could promote its catalytic activity. Using this catalyst, the yield of target product styrene carbonates (SC) reached 64.5% at 90 °C and 1.0 MPa CO₂ for 10 h without adding any cocatalyst or solvent. The efficient catalytic performance from 1-C-3-M@Ni-BDC-NH₂ was ascribed to the synergistic interaction among multiple active sites: Ni functioned as both oxidation center and Lewis acid center, facilitating the epoxidation and cycloaddition steps, respectively; Br⁻ served as both epoxidation and nucleophile agent; the amino and imidazolium groups acted as basic centers to activate CO₂; and the carboxyl groups, functioning as hydrogen bond donors (HBDs), could activate epoxides. Besides, 1-C-3-M@Ni-BDC-NH₂ demonstrated a favorable CO₂ adsorption capacity, medium specific surface, and abundant mesoporous structures, which were also conducive to the catalytic reactions. In addition, due to the stable core-shell structure formed by hydrogen bonding, the catalyst maintained its catalytic activity after five consecutive recycling runs, which made it a potential catalyst for CO₂ conversion.