Biomass-formic acid-hydrogen conversion process with improved sustainability and formic acid yield: Combination of citric acid and mechanocatalytic depolymerization

Sustainable and efficient hydrogen production from biomass resources have been highly desired. Here, we report a low-temperature hydrogen production process from lignocellulosic biomass passing through formic acid (the biomass-formic acid-hydrogen conversion process) with improved sustainability and formic acid yield. In the biomass oxidation/hydrolysis step, toxic sulfuric acid has been replaced by sustainable citric acid. In addition, mechanocatalytic depolymerization of biomass has been applied to the biomass pretreatment step, resulting in the remarkable improvement in formic acid yield. In the first step, the citric-acid-impregnated biomass was subjected to a mechanocatalytic depolymerization process for facilitating contact between the substrate and catalysts (MD-CIB). In the second step, the crude formic acid (CF) solution was obtained via combined hydrolysis-oxidation process of the MD-CIB in the presence of dimethyl sulfoxide (DMSO), and hydrogen peroxide (H2O2). The yield of formic acid were measured up to be 50.03%. In the third step, the lignin-free crude formic acid (LCF) solution was recovered by filtration, or distilled formic acid (DF) solution was obtained by distillation of the CF solution. In the final step, hydrogen was generated from LCF or DF solutions through dehydrogenation of formic acid over Pd catalysts supported on amine-functionalized mesoporous silica (KIE-6) with a sodium formate. The turnover frequency (TOF) at 10 min were measured up to be 279.24 mol H2 mol Pd-1h−1. The clean hydrogen, which is produced from biomass passing through formic acid, could be directly applied in fuel cell as an energy source. Our hydrogen production process is expected to be much more efficient, sustainable and economical process for hydrogen production from biomass. In addition, organic acids as a byproduct could be utilized as a platform chemical.