Hydrothermal reduction of CO2 into formate in a semicontinuous plant with soft wood as reducing agent

There is a growing interest in producing valuable products from renewable materials such as biomass and CO₂. One of the current challenges in this field is developing processes that can be scaled up to match the large volumes of CO₂ emissions. In this study, a process is developed that consists of the simultaneous conversion of biomass and reduction of CO₂ into formic acid in hydrothermal media. Experimental results from a semi-continuous plant, using NaHCO₃ as inorganic CO₂ source and soft wood as biomass reducing agent, are presented. In addition to formic acid, the other main products are acetic acid and lactic acid. Nuclear Magnetic Resonance analyses revealed that acetic acid and lactic acid are derived exclusively from the oxidation of biomass, while formic acid is the main product originating from the inorganic carbon source. It was determined that at 300 °C, 37 % of the total production of formic acid comes from the reduction of inorganic CO₂. Experiments with ramp heating with temperature increments of 5 °C/min showed that low reaction temperatures (150, 200 °C) favor the production of acetic acid from biomass, while higher temperatures (300 °C) promote the production of formic acid and lactic acid from both biomass and CO₂. According to these results, a staged heating can lead to a fractionation of these products. These results validate the hydrothermal reduction technology for the scalable, semicontinuous conversion of CO₂ using lignocellulosic biomass reductants, and offer a new approach for the downstream processing of the effluent, based on a prior fractionation in the reactor.