This study presents a synergistic catalytic system using the combination of Brønsted hydrothermal carbon-based acid (HTCG-SO3H) and Lewis acid catalysts for one-pot conversion of cellulose to levulinic acid (LA). Chromium chloride (CrCl3), among a number of other Lewis acidic metal chlorides, was found to give the highest LA yields, and was therefore used in combination with the HTCG-SO3H, for cellulose conversion to LA. With the appropriate amount of HTCG-SO3H, the formation of side products could be reduced, resulting in improved selectivity of LA. Compared with that obtained by CrCl3 alone, at 5 wt % HTCG-SO3H, 0.015 M CrCl3, 200 °C and 5 min reaction time, the LA yield was considerably enhanced from 30 wt % to 40 wt %. Since HTCG-SO3H is a heterogeneous catalyst that can be easily prepared from biomass at moderate temperature, its use in such combined catalyst system offers economic and environmental benefits, thus making large-scale implementation of such process potentially feasible.

An acetal protection strategy for 5-hydroxymethylfurfural (HMF) was used to obtain 2,5-diformyfuran (DFF) using concentrated HMF solutions and a γ-Al 2 O 3 -supported Ru catalyst (Ru/γ-Al 2 O 3 ). The HMF-acetal with 1,3-propanediol can be oxidized to DFFacetal with a yield of 84.0 % at an HMF conversion of 94.2 % from a 50 wt % solution. In contrast, aerobic oxidation of nonprotected HMF using a 10 wt % solution afforded DFF only in a moderate yield (52.3 %). Kinetic studies indicated that the six-membered ring acetal group not only prevents side reactions but also accelerates aerobic oxidation of the À CH 2 OH moiety to À CHO under retention of the acetal functionality. Organic deposits formed during the reaction explained the significant decrease in the activity of the Ru/γ-Al 2 O 3 catalyst, which could be recovered neither by washing in water or organic solvents, nor by a calcination-reduction treatment. Sonication of the used Ru/γ-Al 2 O 3 catalyst in an aqueous NaOH solution successfully removed the deposits and allowed reuse of the catalyst for at least four times without activity loss.