Synergistic Hydrothermal Conversion of Aqueous Solutions of CO₂ and Biomass Waste Liquefaction into Formate

CO₂ utilization by conversion into useful chemicals can contribute to facing the problem of increasing CO₂ emissions. Among other alternatives, hydrothermal transformation stands out by the high conversions achieved, just using high-temperature water as the solvent. Previous works have demonstrated that several organic compounds with hydroxyl groups derived from biomass can be used as reductants of NaHCO₃ aqueous solutions as inorganic CO₂ sources. Formate was obtained as the main product as it was produced by conversion both of the inorganic carbon and of the organic reductants, whose transformation into formate was promoted by the addition of NaHCO₃. Based on these results, in this work, the hydrothermal conversion of NaHCO₃ is performed together with the liquefaction of lignocellulosic biomass (sugarcane bagasse and pine needles) in a one-pot process. Results show that yields to formate of 10% wt/wt (with respect to the initial concentration of biomass) are achieved by hydrothermal treatment of NaHCO₃ and lignocellulosic biomass at 250 °C with a residence time of 180 min. Other products, such as acetic acid and lactic acid, were also obtained. These results demonstrate the feasibility of the hydrothermal reduction of CO₂ combined with the hydrothermal liquefaction of residual biomass in a simultaneous process.