Toward a hydrogen-free reductive catalytic fractionation of wheat straw biomass

The reductive catalytic fractionation (RCF) is an attractive method for the conversion of lignin toward valuable low-molecular weight aromatics during the pretreatment of lignocellulosic biomass. A prominent limitation to the upscaling of such technology is represented by the use of pressurized hydrogen gas. In this contribution, the role of hydrogen gas within the RCF of wheat straw biomass is investigated. The use of H2 was shown to enhance lignin depolymerization, by virtue of an improved hydrogenolysis and hydrogenation of lignin fragments, with a yield of phenolic monomers that increased from ~12 wt% of acid-insoluble lignin in the initial biomass under inert atmosphere, up to ~25 wt% under H2, for a reaction in methanol, at 250 °C, with Ru/C. The adoption of methanol, ethanol and isopropanol as hydrogen-donor solvents was also investigated in the absence of H2, and the use of ethanol was found to give the highest yield of monophenolics (up to ~20 wt%) owing to a better balance between solvolysis, hydrogenolysis, and hydrogenation of lignin. Nevertheless, a substantial loss of the carbohydrate fraction was observed for reactions performed at 250 °C, irrespective of hydrogen pressure and of the solvent employed. The use of a lower temperature of 200 °C in combination with H3PO4 resulted in an improved recovery of cellulose in the pulp and in the solubilization of hemicellulose and lignin, with the formation of monosaccharides (~14 wt% of polysaccharides in the initial biomass) and phenolic monomers (up to 18 wt%, in the absence of H2). Overall, the obtained results show that a tradeoff exists between the removal of H2 from the process and the production of low-MW phenolics during RCF, which can be improved by accurately tuning the process conditions.