Synthesis of hydrophobic Pd-poly(ionic liquid)s with excellent CO2 affinity to efficiently catalyze CO2 hydrogenation to formic acid

Abstract

The catalytic hydrogenation of carbon dioxide (CO2) into formic acid is among the most important reactions to alleviate global warming and realize the carbon cycle. How to achieve rapid activation of CO2 remains a challenge. In this work, we designed two novel hydrophobic poly(ionic liquid)s (PILs) with a good affinity for CO2. Since the abundant N atoms in the PILs can anchor palladium (Pd) nanoparticles, the designed catalyst can rapidly activate CO2 and H2 simultaneously, thus efficiently catalyzing the hydrogenation of CO2 to formic acid under mild conditions. Due to the synergy from the bis(trifluoromethanesulfonyl)amide anion ([Tf2N−]) with strong hydrophobicity and CO2 affinity and trimethylamine (NEt3) as a low dissociation energy alkaline additive, the resultant Pd/PIL-bip-Tf2N exhibited the best catalytic performance on the hydrogenation of CO2 to formic acid. The TOF value reached 1,190 h−1 in the aqueous solution of NEt3 at 100 °C, 1 h, and a total pressure of 4 MPa. The activation energy and dissociation energy during the catalytic process were calculated by DFT, indicating that the synergistic effect of NEt3 is better than that of other alkaline additives. In addition, the catalytic activity of the catalyst remained stable after several cycling experiments. Based on the obtained results, we proposed a possible reaction pathway. Therefore, PILs have great potential as catalyst carrier material for CO2 hydrogenation.