Versatile CO2 Hydrogenation–Dehydrogenation Catalysis with a Ru–PNP/Ionic Liquid System

High catalytic activities of Ru-PNP [Ru = ruthenium; PNP = bis alkyl- or aryl ethylphosphinoamine complexes in ionic liquids (ILs) were obtained for the reversible hydrogenation of CO2 and dehydrogenation of formic acid (FA) under exceedingly mild conditions and without sacrificial additives. The novel catalytic system relies on the synergic combination of Ru-PNP and IL and proceeds with CO2 hydrogenation already at 25 °C under a continuous flow of 1 bar of CO2/H2 (1:5), leading to 14 mol % FA with respect to the IL. A pressure of 40 bar of CO2/H2 (1:1) provides 126 mol % of FA/IL corresponding to a space-time yield (STY) of FA of 0.15 mol L-1 h-1. The conversion of CO2 contained in imitated biogas was also achieved at 25 °C. Furthermore, the Ru-PNP/IL system catalyzes FA dehydrogenation with average turnover frequencies up to 11,000 h-1 under heat-integrated conditions for proton-exchange membrane fuel cell applications (<100 °C). Thus, 4 mL of a 0.005 M Ru-PNP/IL system converted 14.5 L FA over 4 months with a turnover number exceeding 18,000,000 and a STY of CO2 and H2 of 35.7 mol L-1 h-1. Finally, 13 hydrogenation/dehydrogenation cycles were achieved with no sign of deactivation. These results demonstrate the potential of the Ru-PNP/IL system to serve as a FA/CO2 battery, a H2 releaser, and a hydrogenative CO2 converter.