Biomass-assisted Catalytic Reduction of Carbon Dioxide to Value-added Products Under Hydrothermal Conditions

A key challenge for the present century is the advancement of sustainable methods of fuel and chemical production. In the UK the chemical sector is the second-largest CO2 emitting industry. In this work, sub-critical water was investigated as a reaction medium for hydrothermal conversion of CO2 to value added products with glucose as a reductant. Analytical methods for the analysis of gaseous, aqueous and organic soluble products were established to gain mechanistic insight into the reaction. The hydrothermal conversion of NaHCO3, a CO2 precursor, and glucose to formate was investigated between 225–300 °C. In uncatalyzed reactions, a maximum of 0.502 g l-1 HCOO- was obtained at 250 °C and 2 hours, while at higher temperature of 300°C product degradation occurs. At maximum HCOO- concentration, >95% of formate was formed from glucose degradation, and a mechanism for this process proposed. Metal powders were investigated as heterogeneous catalysts to enhance bicarbonate conversion, and nickel was found to simultaneously increase formate formed from both NaHCO3 and glucose. Formate concentration was shown to increase to 1.240 g l-1, with concentration from bicarbonate increasing from 0.022 g l-1 in
uncatalyzed reactions to 0.330 g l-1. Characterisation of Ni powder showed no changes in functionality, oxidation state, or morphology of particles, indicating stability under the investigated conditions. This study demonstrated that catalytic hydrothermal conversion is a promising route for simultaneous conversion of both CO2 and biomass to value-added products. Supported Ni catalysts were subsequently investigated, and it was concluded that metal particle size, oxidation state, and nature of the support strongly influence catalyst stability and degradation of formate to CO2 and H2. Bicarbonates, carbonates and gaseous CO2 were compared as precursors. Formate concentration in carbonate reactions was found to be enhanced by the addition of CO2 gas, proposedly due to lowering of the reaction pH. Sodium carbonate with CO2 perform comparatively to sodium bicarbonate, demonstrating that two different CO2
sources can be used in conjunction to produce value-added organic acids. CO2 in NaOH solution glucose system was further examined with heterogeneous catalysts. Copper-containing catalysts increased the yield of formate nearly two-fold, although characterization showed that sintering and leaching of active phase results in unstable catalysts. Design and development of stable catalysts for CO2 and biomass
valorisation are promising routes for further research in this area.