Metal Chloride Mediated Efficient Conversion of Hydroxymethylfurfural (HMF) into Long-Chain Levulinate Ester

Levulinate esters are a class of promising alternative fuels derived from biomass. The synthesis of levulinate esters from carbohydrates generally undergoes a route with hydroxymethylfurfural as an intermediate product. The conversion of hydroxymethylfurfural is an intrinsic limiting step for the formation of levulinate esters from carbohydrates. Recent work focuses more on the synthesis of short-chain levulinate esters from biomass, but long-chain levulinate esters have superior properties as fuels. In this work, the synthesis of n-hexyl levulinate, a representative long-chain levulinate ester, was investigated with a series of metal chloride catalysts. Iron chloride (FeCl3) and copper chloride (CuCl2) were found to be the most effective amongst the tested catalysts. An n-hexyl levulinate yield of approximately 65% with 100% hydroxymethylfurfural conversion could be achieved at a temperature of 160 degrees C after 240 min. This system was also applied to the conversion of hydroxymethylfurfural and other hexanols such as 2-ethyl-1-butanol, cyclohexanol, 2-hexanol, and 3-hexanol. The catalyst in the system was reusable in a consecutive batch mode. Mechanistic study indicated that the production of levulinate ester from hydroxymethylfurfural probably went through levulinic acid as the intermediate.