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) Ni 2 (OH) 2 (C 8 H 4 O 4 )(NH 2 ) (Ni-BDC-NH 2 ), this work designed and synthesized a novel core-shell structured composite catalyst, 1-C-3-M@Ni-BDC-NH 2 . CO 2 adsorption and contact angle experiments revealed that 1-C-3-M@Ni-BDC-NH 2 performed good adsorption capacity for reactant molecules CO 2 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 CO 2 for 10 h without adding any cocatalyst or solvent. The efficient catalytic performance from 1-C-3-M@Ni-BDC-NH 2 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 CO 2 ; and the carboxyl groups, functioning as hydrogen bond donors (HBDs), activated epoxides. Besides, 1-C-3-M@Ni-BDC-NH 2 demonstrated a favorable CO 2 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 CO 2 conversion. • A core-shell structured composite catalyst (1-C-3-M@Ni-BDC-NH 2 ) with multiple active sites comprising metal-organic framework and ionic liquid was designed and synthesized. • Ionic liquid shell of 1-C-3-M@Ni-BDC-NH 2 exhibited good affinity for both CO 2 and ST, which could enhance the adsorption of reactants and facilitate its catalytic activity during direct oxidative carboxylation • Due to synergistic effects of multiple active sites, 1-C-3-M@Ni-BDC-NH 2 demonstrated excellent catalytic activity for direct oxidative carboxylation of CO 2 and styrene under mild reaction conditions (90°C, 1.0 MPa, 10 h) without adding any cocatalyst or solvent. • As a heterogeneous catalyst, 1-C-3-M@Ni-BDC-NH 2 is easy to separate from the product and has good thermal stability and cycle stability. Therefore, it can be repeatedly utilized, representing a highly promising catalyst for direct oxidation carboxylation of CO 2 and styrene