Highly Efficient Hydrogenation of CO2 to Formic Acid over Palladium Supported on Dication Poly(ionic liquid)s

Carbon dioxide (CO₂) emissions from the burning of fossil fuels have seriously aggravated the global greenhouse effect, leading to extreme environmental problems. Thus, the sufficient and effective use of CO₂ resources has always been an urgent issue. Herein, dication poly(ionic liquid)s (PILs)-supported palladium nanoparticles (Pd NPs) were designed to catalyze the hydrogenation of CO₂ to formic acid (FA) under mild conditions. The prepared catalysts were characterized by FT-IR, ICP, SEM, TEM, XPS, and other techniques. Results indicate that due to the strong metal-support interactions in the resultant catalysts, PILs are constructed by the IL units comprising dications and hydrophobic anions (bis(trifluoromethanesulfonyl)amide, Tf₂N⁻) that can uniformly disperse Pd NPs and enhance the catalytic hydrogenation of CO₂ to FA. The effect of various factors on CO₂ conversion is also investigated. Turnover frequency (TOF) can reach as high as 13.32 s ⁻¹ when using 1%Pd/PIL-2-Tf₂N as a catalyst under the conditions of 80 °C, 1 h, 2 MPa pressure of CO₂, and 0.88 M sodium borohydride aqueous solution, and the maximum amount of the obtained FA can reach up to 82% of the theoretical value. A possible mechanism of catalytic hydrogenation of CO₂ to FA is proposed according to the product analysis. This study highlights that PIL, as support, can promote the catalytic performance, and the resulting catalyst has potential application value for the CO₂ industry.