A novel imidazolium ionic liquid-functionalized UiO-66-NH₂ catalyst for efficient conversion of CO₂ into cyclic carbonates under both solvent-/cocatalyst-free conditions

The cycloaddition of CO₂ with epoxides proceeds with 100% atom economy, making it a particularly attractive route for CO₂ chemical conversion. However, the chemical inertness of CO₂ demands the design of highly active catalysts. In this work, a task-specific imidazolium ionic liquid [BBDCMIM]Br, bearing two carboxyl groups on the imidazolium ring, was synthesized via quaternization. Subsequently, post-synthetic grafting of [BBDCMIM]Br with different loadings onto UiO-66-NH₂ support was achieved through nucleophilic substitution reaction between the alkyl bromide in [BBDCMIM]Br and –NH₂ in UiO-66-NH₂, affording [BBDCMIM]Br-x/UiO-66-NH₂ (x = 1–3) catalysts. Among them, [BBDCMIM]Br-2/UiO-66-NH₂ demonstrated outstanding catalytic activity under conditions of 100 °C, 1.0 MPa and 6 h, delivering 97% propylene carbonate yield with 99% selectivity. In addition, [BBDCMIM]Br-2/UiO-66-NH₂ exhibited excellent recyclability and broad substrate scope. Kinetic studies indicated that the activation energy of the [BBDCMIM]Br-2/UiO-66-NH₂-catalyzed CO₂ cycloaddition was 60.97 kJ·mol⁻¹. Finally, a reasonable mechanism was put forward for [BBDCMIM]Br-2/UiO-66-NH₂. In this mechanism, the coordinatively unsaturated Zr metal centers acted as Lewis acidic sites and –COOH groups served as hydrogen bond donors, collectively activating the epoxide. The Br⁻ then acted as a nucleophile to promote epoxide ring-opening. Meanwhile, the formed secondary amine (derived from partial –NH₂ conversion) on UiO-66-NH₂ and the tertiary amine in [BBDCMIM]Br served as basic sites and facilitated the activation of CO₂. Overall, [BBDCMIM]Br-2/UiO-66-NH₂ is a potential catalyst for sustainable conversion of CO₂ into cyclic carbonates.