Reaction Dynamics and Engineering in a Tandem Guaiacol Hydrodeoxygenation and Glycerol Aqueous Phase Reforming System

Abstract

Hydrogenation of guaiacol, a model compound of lignin-derived bio-oil, coupled with internally produced hydrogen from glycerol aqueous-phase reforming (APR) over Raney Ni, was investigated to evaluate the role of reactor operation on hydrogen availability and product distribution. Reaction engineering strategies, including batch, semi-batch, and continuous glycerol feeding modes, were systematically compared. Full saturation of the guaiacol aromatic ring was achieved with a combined liquid-phase selectivity to cyclohexanol and cyclohexanone reaching 90% when both guaiacol and glycerol conversion reached 99%. A previously unreported benzene formation pathway via cyclohexanol dehydration and dehydrogenation was identified, leading to a benzene selectivity of 13.8% under hydrogen-deficient conditions. This study shows that hydrogen availability is highly dynamic and controlled by both glycerol consumption and the feeding strategy, governing the transition between hydrogenation and dehydrogenation pathways. Increasing glycerol input in batch mode did not significantly improve selectivity to oxygen-free products due to catalyst site competition and inefficient hydrogen utilization. In contrast, semi-batch and continuous feeding maintained lower glycerol concentrations, leading to sustained hydrogen production and mitigation of undesired dehydrogenation reactions. Slightly improved hydrogenation performance with a liquid-phase selectivity of 69.4% to cyclohexanol was achieved with continuous glycerol feeding. Besides, guaiacol hydrogenation with glycerol APR as the hydrogen source was comparable to that using 8.5 bar external hydrogen, which resulted in an 88% combined liquid-phase selectivity to cyclohexanol and cyclohexanone. These findings demonstrate that reactor operation is a key design parameter affecting hydrogen supply and needs to be carefully considered in tandem APR-hydrogenation systems for biomass upgrading.