Chapter 24: Hydrothermal CO₂ Reduction Using Metals and Biomass Derivatives as Reductants

At hydrothermal conditions (temperatures above 120 °C), carbon dioxide can be converted into valuable organic compounds such as formic acid, using metals such as Al or Zn or organic compounds such as alcohols as reductants. Compared to alternative CO₂ conversion technologies, the hydrothermal reduction achieves high conversion yields of up to 60% in short reaction times of 2–3 h. In this chapter, an overview of the hydrothermal conversion of CO₂ with metals or with organic reductants derived from lignocellulosic biomass is presented. First, an overview of the mechanisms and kinetics of the conversion of bicarbonate to formic acid using metal reductants is presented. Then, alternative inorganic carbon sources, corresponding to the components obtained by absorption of carbon dioxide in aqueous solutions of ammonia or amines, are tested. Next, organic reductants are considered, beginning with a screening analysis of the main compounds obtained by hydrolysis and fractionation of lignocellulosic biomass in hydrothermal media. The mechanisms of reduction of sodium bicarbonate with glucose are assessed. Finally, a continuous flow reactor for the hydrothermal reduction of CO₂ with biomass derivatives is presented.