Multiple-site activation induced by guanidine ionic liquid decorated chromium (III) terephthalate for coupling of carbon dioxide with epoxides

Cycloaddition, as a sound strategy for high-value utilization of carbon dioxide (CO₂), has been long pursued, wherein the challenging substrate activation process is a top priority for devising novel heterogeneous catalysts. In this study, a guanidine-based ionic liquid tethering –NH₂ groups was designed and integrated with chromium (III) terephthalate (MIL-101(Cr)) through the coordination with unsaturated Cr³⁺ centers. The developed [NH₂TMG]Br@MIL-101(Cr) (TMG represents tetramethylguanidine) decorated with plentiful basic functional groups created a fast channel for the capturing and binding of CO₂, while the highly-accessible Lewis acidic sites (Cr³⁺) and hydrogen bond donors (N⁺-H) embedded within the nanocomposite synergized to activate the epoxide, synchronously. Under the reaction conditions optimized by response surface methodology (RSM) (103.2 °C, 1.03 MPa, 1.85 h, and 2.53 wt% of catalyst), a satisfactory chloropropene carbonate (CPC) yield of 98.2 % with a selectivity of 99.2 % were achieved. We further demonstrated the heterogeneity and recyclability of [NH₂TMG]Br@MIL-101(Cr), and ascertained the substrate expansibility. Moreover, the in-situ diffuse reflectance infrared Fourier-transform spectra (DRIFTS) and density functional theory (DFT) computations afforded deep insights into the proposed multiple-site activation mechanism for CO₂ coupling. This study highlighted an innovative pathway for constructing durable IL@MOFs nanocomposites and presented a tangible route to effectively converting CO₂.