Triazole-Based Protic Ionic Liquid Grafted onto Mesoporous SiO ₂ as a Metal-Free Catalyst for CO ₂ and Epoxide Coupling Reaction

3-Amino-1,2,4-triazole-5-carboxylic acid was transformed into triazolium-based protic ionic liquid (TzPIL) via one-step acid–base neutralization. TzPIL was then immobilized onto a mesoporous SiO 2 (mSiO 2 ) support through quaternization, affording a mSiO 2 -supported protic ionic liquid catalyst (TzPIL-4-mSiO 2 ) with multiple hydrogen-bond donor sites. Benefiting from its nitrogen-rich units and mesoporous structure, TzPIL-4-mSiO 2 exhibited an excellent CO 2 adsorption capacity. Under metal- and solvent-free conditions, this catalyst efficiently promoted the coupling reaction of CO 2 with propylene oxide, affording propylene carbonate in 98% yield at 90 °C and 1.0 MPa within 6 h. Moreover, after six consecutive cycles, the propylene carbonate yield remained 94%, demonstrating excellent recyclability. Finally, a plausible mechanism for the catalytic reaction was proposed: the protic hydrogen atoms and −COOH groups in TzPIL-4-mSiO 2 cooperatively activated the epoxide; Br – sites served as nucleophile to initiate ring-opening, while the −NH 2 and −NH– groups in TzPIL facilitated CO 2 adsorption and activation.