Highly selective transfer hydrogenation of biomass-derived furfural to furfuryl alcohol over a zirconium-melamine coordination catalyst without Bronsted acid sites

The transfer hydrogenation of furfural (FF) to furfuryl alcohol (FFA) via the Meerwein-Ponndorf-Verley (MPV) reduction reaction is a promising and appealing strategy for the valorization of biomass, in which designing high-efficiency and high-selectivity Lewis acid-base catalysts is the central task. Recently, zirconium-organic acid coordination catalysts have been broadly constructed, however, most of them contained a certain amount of Bronsted acid sites (BASs) in addition to Lewis acid-base sites (LABSs), which inevitably influenced the yield and selectivity of FFA. In this work, a zirconium-melamine coordination catalyst (Zr-MM) was synthesized by a convenient self-assembly process, in which melamine (MM) was used as a cheap organic ligand. Systematic experimental results indicated that Zr-MM possessed strong LABSs, and at the same time, lacked BASs, which could not only catalyze the transfer hydrogenation of FF and facilitate the formation of FFA, but also avoid the further transformation of FFA and improve the selectivity of FFA. Hence, it showed remarkable catalytic activity, leading to 100% FF conversion and 99.4% FFA yield in isopropanol at 150 degrees C for 4 h. More significantly, Zr-MM also displayed excellent catalytic stability and catalytic universality, and it could not only be reused for five reaction cycles without an apparent decrease in catalytic activity, but also be employed for the effective transfer hydrogenation of 5-methylfurfural, 5-hydroxymethylfurfural, phenylaldehyde, cyclohexanone, methyl levulinate and ethyl levulinate. All in all, this work provided some crucial references to prepare the BASs-free zirconium-based coordination catalysts for the upgradation of biomass-derived carbonyl compounds.