Simultaneous formic acid production by hydrothermal CO2 reduction and biomass derivatives conversion in a continuous reactor

Abstract

The reduction of CO2 to obtain formic acid in hydrothermal media can contribute to the reduction and valorization of CO2 emissions, but in order to apply it industrially, scalable continuous-flow reactors must be developed. In this work, a continuous flow reactor is developed that can process up to 1.2 L/h of bicarbonate solution. Glucose as model compound of the products of the hydrothermal decomposition of lignocellulosic biomass is used as reducing agent. Feed stream is suddenly heated at the inlet of the reactor by mixing it with a preheated water stream and instantaneously cooled after the reactor by decompression Joule-Thompson effect, allowing a precise control of the residence time and enabling to apply short residence times down to 1 s that are not feasible in batch reactors. Several operational parameters, as residence time, temperature and additives, were studied. Yields to formic acid up to 63 % were achieved at 300 °C with residence times lower than 1 min. In order to better understand the process, NaH13CO3 was used in selected experiments to check if the origin of formic acid was the reduction of bicarbonate or the hydrolysis of glucose It was found that almost 50 % of formic acid was produced due to the reduction of NaHCO3 after 10 min of reaction, a proportion significantly higher than that observed in batch experiments at similar reaction times. Furthermore, experiments performed without NaHCO3 or with other alkaline buffers demonstrate that NaHCO3 also promotes the transformation of glucose into formic acid, increasing the overall selectivity of the process and facilitating downstream processing.