Spectroscopic, electrochemical, thermodynamic and theoretical insights into solvent effects for the intensification of the modified OxFA process

Abstract

Producing short-chain carboxylic acids like formic acid (FA) from biomass in the OxFA process is a promising strategy for a green and sustainable chemical industry. Herein, we employed spectroscopic (NMR, UV-vis, and GC-MS), electrochemical (CV and SWV), and thermodynamic (gas solubility) measurements in combination with theoretical modelling by DFT for rationalizing the dominating effects of co-solvents on the catalytic efficiency of xylose oxidation to FA in the modified OxFA process catalyzed by H₈PV₅Mo₇O₄₀ (HPA-5) polyoxometalate. Specifically, the effect of oxygen solubility in different solvent mixtures in combination with the redox potential of the first reduction event of HPA-5 during the electrochemical treatment revealed the combined effect of both thermodynamic and catalytic properties on the effective reaction kinetics for xylose oxidation to FA. Therefore, the ease of xylose oxidation is directly linked to the redox potential observed in different co-solvent mixtures. Based on these insights, systematic optimization of the reaction parameters in the most promising water-acetonitrile solvent mixture using design of experiments (DoE) and a central composite design (CCD) achieved an FA yield of 90% at full xylose conversion, with only 5% CO₂ formation after 2 hours of reaction. The insights from this study provide a strong foundation for future process intensification in biomass valorisation technologies.