Carbon Dioxide Hydrogenation To Formic Acid Over Supported Ruthenium-based Catalysts

The efficient fixation and transformation of carbon dioxide?CO₂?into practical chemicals could reduce the overall anthropogenic carbon footprint and then attract more and more attention.Hydrogenation of CO₂ to formic acid has contributed to significant contemporary interest in formic acid production since formic acid has been considered as a candidate for methanol-alternative fuels in methanol fuel cells due to its much better electrochemical oxidation.This paper focused on the design and employment of three ruthenium-based heterogeneous catalysts,denoted as Ru-DBU/Al₂O₃,Ru-PPh₃/Al₂O₃,and Ru/Fe-BTC,to promote the CO₂ conversion to formic acid via hydrogenation.A turnover frequency of 239 h^-1 for formic acid produced from CO₂ hydrogenation over the selected Ru-DBU/Al₂O₃ was achieved in the presence of highly polar solvent dimethylsulphoxide,Lewis organic base triethylamine and protonic additive KH2PO4.The FT-IR,XRD and XPS characterization of Ru-DBU/Al₂O₃ illustrated that DBU formed an amorphous Ru?III?-DBU species and dispersed uniformly on the carrier,further promoting the formation of carboxyl through accelerating the nucleophilic attack of the H ligand to CO₂.The superb reactivity of Ru-PPh₃/Al₂O₃?1:9?made it a leading candidate for a catalyst for hydrogenation of CO₂ to formic acid.Ru-PPh₃/Al₂O₃ was demonstrated that the electron-donation ligand PPh₃ was coupled with the active center Ru,increasing the catalytic activity of Ru-PPh₃/Al₂O₃ via providing electrons to the Ru center.Ru-PPh₃/Al₂O₃?1:9?,possessing the optimal atomic ratio n PPh₃/nRu,offered a TOF of 751 h^-1 in the presence of KH2PO4 and PPh₃ as additives.A probable catalytic mechanism for CO₂ hydrogenation to formic acid over Ru-PPh₃/Al₂O₃ was proposed.The PPh₃ ligands coordinated with the Ru center,acting as strong donor ligands,promoted the H-H bond breaking by elevating the nucleophilicity of Ru.The proton source KH2PO4 accelerated CO₂ insertion to Ru-H bondenormously through donating protons to the formate ligand.Lewis organic base triethylamine was adopted as the reaction solvent,shifting the reaction equilibrium to the desirable direction by reaction with formic acid.Ru/Fe-BTC,which possessed a uniform distribution of Ru nanoparticles on Fe-BTC,was further exploited and applied to catalyze the conversion of CO₂ through hydrogenation.Ru/Fe-BTC exhibited impressive stability and good catalytic performance for formic acid formation.The as-processed and reused catalyst was characterized in terms of the content of active center Ru,specific surface area and morphology.It was demonstrated that the catalyst was nearly unchanged after six recycles.And the turnover frequencies of formic acid were all around 690 h^-1.