Mesoporous silica-supported palladium catalysts functionalized with ionic liquids for aqueous CO₂ hydrogenation under mild conditions

A series of palladium-based catalysts were synthesized by grafting a silylated imidazolium ionic liquid onto two types of silica supports: mesoporous SBA-15 and commercial silica. The ionic liquid served as both a stabilizing matrix and anchoring platform for palladium ions, introduced via aqueous impregnation. The influence of support activation temperature, reduction treatment, and textural properties on catalytic performance was systematically studied. Catalysts were tested in the aqueous-phase hydrogenation of CO₂ to formic acid under mild conditions (40 bar H₂/CO₂, room temperature). SBA-15-based catalysts showed superior performance, achieving up to 50 molHCOOH·mol⁻¹Pd·h−1 — over three times higher than the flash silica counterpart. High support surface area and pore volume enabled greater ionic liquid loading and palladium dispersion, as confirmed by TEM, SEM-EDX and ICP analyses. Pre-reduction with NaBH₄ eliminated induction periods in the kinetic profile and further enhanced activity, whereas no such improvement was observed for flash silica, likely due to palladium aggregation. These findings demonstrate that combining mesoporous supports with covalently anchored ionic liquids enables the design of efficient, water-compatible CO₂ hydrogenation catalysts under mild conditions.