Incorporation of Mg/Al metal oxide into ionic liquids for CO₂ capture and conversion into cyclic carbonate under solvent-free conditions: effect of coordination ability, recyclability, and catalytic study

The direct conversion of carbon dioxide (CO₂) and propylene oxide (PO) into propylene carbonate (PC) offers a green way to utilize anthropogenic CO₂. However, this reaction is limited by low conversion of PO and harsh reaction conditions. In this study, we solve this problem using ionic liquids (ILs)/metal oxide composites (ILs@MAO). The catalytic activity of MAO-500 (500 = annealing temperature) is poor evidenced by its low conversion of PO (24.94%). However, ILs@MAO-500 has a high conversion of PO (97.54%) under similar reaction conditions (2 h at 1.5 MPa CO₂ pressure, 90 ℃, and 0.85 g catalyst). The ILs consist of imidazolium cation with weak coordinated [NTf₂]^– anion leading to outward movement of anion resulting in the formation of “heterodinuclear complex”. This complex generates an amorphous-crystalline intermediate with balanced acid-base sites that activate PO and stabilize the catalytic intermediate. In large part, the high PO conversion is theorized to be primarily due to the abundant reactive sites in the ILs that are covalently immobilized on the MAO-500 carrier. Furthermore, even after multiple recycling, ILs@MAO-500 remains stable and exhibits high yield and selectivity. The proposed solvent-free catalytic system is mild, kinetically fast, and naturally safe for coupling CO₂ and PO into PC synthesis.