Conversion of carbohydrates at concentrated feeds represents highly desirable for the industrial deployment of biobased fuels and chemicals but challenging. One key bottleneck is that the excessive formation of polymeric humins greatly diminishes the utilization rate of feedstocks and the destination product yield. We report that the use of ethylene glycol as co-solvent for acid-catalyzed conversion of concentrated sugars enhances desirable alkyl levulinate (AL) production compared to reactions carried out in single monohydric alcohols (e.g., methanol, ethanol). Ethylene glycol served not only as a solubilizer of sugars in the reaction medium to lessen their tendency to polymerize by protecting reactive hydroxyl groups in sugars with alcohols, but also as a supporter to restrain the condensation of furan intermediates. With 10 vol% ethylene glycol as the co-solvent of ethanol, an improved yield of ethyl levulinate (EL) from 45% to 56% was accomplished from concentrated feeds of glucose (200 g/L). In particular, high space time yield and EL concentration respectively up to 30 kg/m 3 ·h and 90 g/L were obtained in a batch reactor. The solvents and catalyst could be isolated from the products, and showed good reusability. This contribution opens a reliable avenue for converting highly concentrated feeds of biomass-related sugars to oxygenated liquid fuels and versatile chemicals.
Graphical Zr oxo cluster bifunctional catalyst : The Zr oxo cluster [Zr 6 O 4 (OH) 4 (HSCH 2 CH 2 COO) 12 ] 2 has been developed, and then the molecular catalyst with Lewis acid and Brönsted acid sites were attained by the oxidation of the sulfhydryl group of the Zr oxo cluster with H 2 O 2 . The resulting bifunctional catalyst showed a superior catalytic activity for alcoholysis of furfural to alkyl levulinates. A novel dodecanuclear Zr oxo cluster [Zr 6 O 4 (OH) 4 (HSCH 2 CH 2 COO) 12 ] 2 (ZrO-SH-10) has been constructed under the room temperature, followed by oxidation of the sulfhydryl group with H 2 O 2 to achieve a bifunctional catalyst with Lewis acid and Brönsted acid sites. The characterization of catalysts indicated that {Zr 6 O 4 } cluster core can be stabilized with a shell of carboxylate ligands, and the resulting ZrO-SO 3 H was formed as a discrete molecular catalyst, exhibiting superior activity and recyclability for the cascade conversion of furfural to alkyl levulinate by the integration of transfer hydrogenation and alcoholysis in n-butanol. The yield of n-butyl levulinate can achieve as high as 91 % under the optimum conditions. Meanwhile, no leaching of the active species was found, which confirmed the structure of the Zr oxo cluster was air and moisture-stable. On the basis of the studies on the reaction kinetics and isotope tracking, the reaction mechanism was proposed accordingly.
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