Although bio-oils produced by pyrolysis and hydrothermal synthesis demonstrate potential toward building a sustainable society, large amounts of char generated as a byproduct and their thermal instability owing to high oxygen content hinder their applications. Hence, a novel approach for the production of high-grade bio-oil was proposed herein. In this approach, zerovalent Fe was used as an agent for generating hydrogen in situ in the hydrothermal liquefaction of oil palm empty fruit bunch (EFB), a lignocellulosic biomass source, affording bio-oil containing water-soluble (WS) and water-insoluble (WI) fractions in high yields. Hydrogen generated by the reaction between Fe and H2O efficiently converted unstable intermediates obtained from the degradation of EFB into stable compounds, resulting in reduced char formation. Hydroxyketones were detected as components characteristic of the WS fraction in the H2O/EFB/Fe system, which were stable under hydrothermal condition. WS fractions were treated with the HZSM-5 zeolite, affording light olefins (C2–C4), as well as benzene, toluene, and xylene. This conversion was more efficient with the WS fraction obtained in the presence of Fe. The liquefaction of EFB and the conversion of WS fractions into olefins via catalytic cracking were also achieved using recycled Fe.

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a b s t r a c tRecently, biomass resources have garnered significant attention as sustainable and renewable raw materials for the production of chemicals. Propylene glycol (PG) is a valuable chemical product that can be synthesized from biomass. Herein, the selective transformation of glucose into PG was investigated on a carbon-supported 5 wt% Ru catalyst (5%-Ru/C) combined with solid acid-base catalysts under low hydrogen pressures. The reaction conditions, namely the amount of ZnO, temperature, and hydrogen pressure, were also evaluated. At a hydrogen pressure of 0.4 MPa, a full conversion of glucose and 38% yield of PG were obtained using the ZnO + Ru/C system at 453 K for 20 h, while the yield of PG on Ru/C alone was only 9.3% under the same conditions. Studies on the reaction mechanism indicated that the transformation of glucose into PG consisted of the isomerization of glucose to fructose, retro-aldol reaction of fructose to triose (dihydroxyacetone and glyceraldehyde), dehydration of glyceraldehyde to pyruvaldehyde, and successive hydrogenation of pyruvaldehyde to PG via hydroxyacetone. The ZnO catalyst promoted both the isomerization and retro-aldol reaction steps, and the Ru catalyst promoted the hydrogenation steps. The retro-aldol reaction, which included the cleavage of C3 C4 bond in glucose, enabled the selective transformation of glucose into PG under low hydrogen pressures. These findings provide novel insights into the efficient synthesis of PG from glucose, which could be achieved by combining and optimizing the retro-aldol and hydrogenation steps.

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