Cellulosic glycols: an integrated process concept for lignocellulose pretreatment and hydrogenolysis

Lignocellulose is the most abundant source of saccharides and it is therefore a promising feedstock for glycols, such as ethylene-glycol, via catalytic hydrogenolysis of the polysaccharides that it contains. However, this catalytic hydrogenolysis step is hampered by the presence of lignin and other biomass contaminants, such as ash, which need to be removed in a pretreatment step. We propose an organosolv-like pre-treatment that can delignify and de-ash lignocellulose to a level that allows it to be upgraded to glycol with comparable yields to pure cellulose under demanding hydrogenolysis conditions. This work identifies the main design constraints of the integrated process and provides an initial experimental validation of it. Pretreatment of biomass in water/ethanol/acetic acid solutions at 180–200 °C can reduce the lignin content of the solid residue to ≤6 wt%. The addition of an organic acid, such as acetic acid, appears important to improve the removal of the recalcitrant Ca²⁺, which is a known inhibitor of the tungstate catalyst typically used in this process. The pretreatment medium is designed to use the by-product(s) of the process as organic solvent, to reduce the need for fresh solvent input. However, the process still needs a high solvent recovery (between 93.5 and 99.9 wt%). This can be achieved by selecting volatile component as organic solvent, as it allows recovery by evaporation from the dissolved lignin and ashes.