Simoultaneous valorization of CO2 and biomass by means of hydrothermal media: Green alternative for the obtention of formic acid

Abstract

In the last decades, the depletion of fossil fuels and the imbalance of carbon cycle in the atmosphere have raised some concerns about the socioeconomic and environmental consequences. In order to minimize the possible effects, different technologies have been investigated to, not only limit the concentration of CO2 in the atmosphere, but also to reduce the anthropogenic dependence in non-renewable sources. In this way, hydrothermal processes have been envisioned as a promising technology for the obtention of platform chemicals and fuels from lignocellulosic biomass, which is sometimes considered residue. In addition, hydrothermal conversion of CO2 is an unbeknownst method for its transformations in useful products, such as formic acid and methanol, which are Liquid Organic Hydrogen Carriers that can play an important role in the Green Hydrogen economy. This thesis aims to investigate the possibility of combining CO2 and biomass valorisation by means of hydrothermal treatment to obtain useful products. Different organic model compounds and biomass residues were used as biomass examples, and sodium bicarbonate (NaHCO3) was utilised as CO2 source. Yields to formic acid up to 65% mol/ mol organic compound were achieved at 300 °C, observing that the NaHCO3 reduction need of long reaction times and high temperature to take place. According to the results, a plausible mechanism has been proposed, being the formic acid production favoured by the alkaline specie of CO2. Further experiments were performed with biomass residues, showing the synergetic effect of NaHCO3 for the liquefaction of biomass and the obtention of target products. Finally, the process was implemented in a continuous pilot plant in order to study the potential industrial applications of the process. This thesis represents a first stage for the formic acid production by means of hydrothermal processes using residues as feedstock, resulting in an attractive approach to obtain chemicals and fuels from renewable sources.