Highly Efficient Catalyzing and Separation System for CO₂ Hydrogenation to Formic Acid in Formate-Based Ionic Liquids

Though the hydrogenation of CO2 to formic acid (HCOOH) has been widely investigated, due to the strong HCOOH–base interaction and HCOOH decomposing easily upon heating, the design of a formic acid reaction and isolation system is a long-lasting challenge. Herein, three systems containing a Ru/KAP-Ph-PPh3 catalyst and formate-based ionic liquids were reported for the hydrogenation of CO2 to formate under base-free conditions. A very high HCOOH concentration of 3.38 mol/kg was obtained at 90 °C, 2 MPa of CO2, and 5 MPa of H2 in [bmmim][O2CH]. HCOOH can be successfully isolated with a separation yield of 87% and a molar percentage exceeding 99% by simple distillation. The catalyst can be recycled at least 6 times in [N4444][O2CH] and [bmmim][O2CH] with essentially constant reactivity.