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In recent years, 2,5-dihydroxymethylfuran (DHMF), which can be produced by the selective hydrogenation of biomass-derived 5-hydroxymethylfurfural (HMF), has attracted great attention and interest of many scientists because of its peculiar symmetrical structure and wide potential applications. At present, studies of the production of DHMF are quickly progressing, with productive approaches being increasingly developed, and many crucial achievements have been continually obtained. However, to date, a special and real-time review of this research area is still lacking. To gain more insight into the current research situation, this review comprehensively summarizes and discusses state-of-the-art advancements of the production of DHMF from HMF via various chemocatalytic pathways, such as conventional hydrogenation, transfer hydrogenation, electrocatalytic hydrogenation, photocatalytic hydrogenation, disproportionation reaction, and biocatalytic pathways. Meanwhile, this review also systematically outlines the latest results on the further transformation of DHMF into value-added derivatives via etherification, polymerization, and rearrangement.

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A simple and efficient process was presented for the selective hydrogenation of 5-hydroxymethylfurfural (HMF) into the high-quality liquid fuel 2,5-dimethylfuran (DMF) in the presence of tetrahydrofuran (THF). Among the employed metal catalysts, the relatively inexpensive carbon-supported ruthenium (Ru/C) displayed the highest catalytic performance, which led to 94.7% DMF yield with 100% HMF conversion at a relatively mild reaction temperature of 200 °C for only 2 h. Although Ru/ C had a little loss in the catalytic activity when it was used for five successive reaction runs, the partially deactivated Ru/C could be easily regenerated by heating at a mixed flow of H 2 and N 2 . Moreover, the plausible mechanism involving an aldehyde group, a hydroxyl group, and a furan ring for the selective hydrogenation of HMF into DMF was also proposed. Subsequently, DMF was separated from the crude hydrogenation mixture according to their various boiling points by the combination of atmospheric distillation and vacuum distillation, and then, the chemical structures and physical properties of the separated DMF were confirmed to be consistent with the authentic DMF. More gratifyingly, Ru/C and THF were also found to be a good combination for the direct hydrogenation of carbohydrate-derived HMF into DMF, which is very important for the practical production of DMF from a variety of biomass-derived carbohydrates such as fructose, glucose, sucrose, maltose, cellobiose, starch, and cellulose.

A green and efficient process was developed for the conversion of biomass-derived ethyl levulinate (EL) into c-valerolactone (GVL) using supercritical ethanol as the hydrogen donor and the reaction medium over low-cost and eco-friendly ZrO 2 catalysts, which were prepared by the precipitation method and characterized by BET, SEM, XRD, FT-IR, TGA-DTA, TPD-NH 3 and TPD-CO 2 . The results indicated that amorphous ZrO 2 with a high specific surface area and a large number of acid-base sites exhibited the highest catalytic activity, an excellent GVL yield of 81.5% with 95.5% EL conversion was achieved at 523 K over 3 h. In addition, combined with the results of poisoning experiments, a plausible mechanism of catalytic transfer hydrogenation (CTH) via a six-membered ring transition state was also presented.

The scientific community has been seeking cost-competitive and green solvents with good dissolving capacity for the valorization of lignocellulosic biomass. At this point, deep eutectic solvents (DESs) are currently emerging as a new class of promising solvents that are generally liquid eutectic mixtures formed by self-association (or hydrogen-bonding interaction) of two or three components. DESs are attractive solvents for the fractionation (or pretreatment) of lignocellulose and the valorization of lignin, owing to the high solubility of lignin in DESs. DESs are also employed as effective media for the modification of cellulose to afford functionalized cellulosic materials, such as cellulose nanocrystals. More interestingly, biomassderived carbohydrates, such as fructose, can be used as one of the constituents of DESs and then dehydrated to 5-hydroxymethylfurfural in high yield. In this review, a comprehensive summary of recent contribution of DESs to the processing of lignocellulosic biomass and its derivatives is provided. Moreover, further discussion about the challenges of the application of DESs in biomass processing is presented.

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