Direct hydrogenation of CO2-rich scrubbing solvents to formate/formic acid over heterogeneous Ru catalysts: A sustainable approach towards continuous integrated CCU

Abstract

Ru(III) modified melamine formaldehyde polymers were synthesized by simple hydrothermal method. Extensive characterization of the materials by FT-IR, XRD, XPS, SEM, TEM, EDX, 13C solid NMR, TGA, CO2-TPD, CO2-physiorptaion and N2-physisorption showed that they exhibited a macro/mesoporous were structure with Ru(III) sites coordinated to the electron rich/weakly basic N-species. These low-cost polymeric materials presented excellent activity as heterogeneous catalysts for selective hydrogenation of CO2 derived carbonate/bicarbonates present in CO2-rich scrubbing solvents (amine and as well as inorganic base derived) to formate/formic acid. The low-cost polymeric catalysts outperformed various benchmark catalysts (Ru/LDH, Ru/MgO, Ru/AC, Ru/NAC, Ru/gC3N4, Pd/ACSigma etc.) affording near stoichiometric formate yields (0.51–1.84 M) under relatively mild conditions (120 °C, 3 MPa, 5–10 h). The polymeric catalysts were robust and demonstrated excellent reusability in batch mode, as well as stable performance for continuous formate/formic acid production in a lab scale fixed-bed reactor (verified during ca. 270 h time on stream, 80 °C, 5.5 MPa, WHSVfeed: 7.2 h−1, feed: CO2 saturated aq. triethylamine). The excellent activity, selectivity and stability of the polymeric catalyst was attributed to the presence of stable Ru(III) sites coordinated to the robust and hydrothermally stable melamine formaldehyde framework. Most importantly, this work represents the first example of continuous integrated CO2 capture and conversion to formic acid using a solid catalyst.