Challenges in lignin valorization during biomass hydrogenolysis for ethylene glycol production

Abstract This study examined the extent to which lignin upgrading is possible when maximizing ethylene glycol (EG) production during the hydrogenolysis of lignocellulosic biomass over bifunctional tungsten–nickel catalysts. Reductive catalytic fractionation (RCF) was selected as a benchmark lignin valorization process because its reaction conditions resemble those of biomass hydrogenolysis. Evaluation of the effects of hydrogen pressure, temperature, catalyst loading (Raney Ni and sodium polytungstate), and pH on EG and lignin depolymerization showed that complete lignin solubilization was not obtained under the tested conditions. Under the optimal conditions for maximizing EG yield (34.3 wt%) – 260 °C, 60 bar H 2 (room temperature), pH ~ 3.3, with 10 wt% biomass loading – approximately 45% of the initial lignin was solubilized, and only approximately 5% (gel permeation chromatography area integration) of lignin‐derived monomers and dimers formed. Identified monomers included guaiacol, 4‐methyl guaiacol (4‐MethG), 4‐ethyl guaiacol (4‐EthG), 4‐propylguaiacol (4‐PropG), and homovanillic acid (HVac). Hydrogen pressure emerged as a critical parameter affecting EG production, solubilization, and lignin depolymerization. Higher hydrogen pressure hindered the solubilization of lignin, although the average molecular weight of solubilized lignin was higher. Despite the various experimental conditions and types of biomass tested – both softwood and hardwood – the lignin depolymerization remained limited with monomer plus dimer yields below 20 wt% based on solubilized lignin. These findings highlight the limitations in lignin valorization under one‐pot hydrogenolysis conditions relative to more specialized approaches for lignin valorization such as RCF.