Hydrogenation of CO₂ to Formate over Ruthenium Immobilized on Solid Molecular Phosphines

Formic acid is a promising hydrogen storage medium and can be produced by catalytic hydrogenation of CO₂ . Molecular ruthenium complexes immobilized on phosphine polymers have been found to exhibit excellent productivity and selectivity in the catalytic hydrogenation of CO₂ under mild conditions. The polymeric analog of 1,2-bis(diphenylphosphino)ethane exhibited the highest activity and turnover numbers up to 13 170 were obtained in a single run. This catalyst was already active at 40 °C and with a catalyst loading of only 0.0006 mol %. Recycling experiments revealed a loss of activity after the first run, followed by a gradual decrease during the subsequent runs. This is attributed to a change in the catalytically active complex during the hydrogenation reaction. High selectivity towards formate and low leaching were maintained in the absence of CO formation. Based on the catalyst characterization, a mechanism for the CO₂ hydrogenation is proposed.