Synthesis of Furfural from Xylose, Xylan, and Biomass Using AlCl3•6H2O in Biphasic Media via Xylose Isomerization to Xylulose

Furfural was prepared in high yields (75?%) from the reaction of xylose in a watertetrahydrofuran biphasic medium containing AlCl3.6?H2O and NaCl under microwave heating at 140?degrees C. The reaction profile revealed the formation of xylulose as an intermediate en route to the dehydration product (furfural). The reaction under these conditions reached completion in 45 min. The aqueous phase containing AlCl3.6?H2O and NaCl could be recycled multiple times (>5) without any loss of activity or selectivity for furfural. Extension of this biphasic reaction system to include xylan as the starting material afforded furfural in 64?% yield. The use of corn stover, pinewood, switchgrass, and poplar gave furfural in 55, 38, 56, and 64?% yield, respectively, at 160?degrees C. Even though AlCl3.6?H2O did not affect the conversion of crystalline cellulose, moderate yields of the by-product 5-hydroxymethylfurfural (HMF) were noted. The highest HMF yield of 42?% was obtained from pinewood. The coproduction of HMF and furfural from biomass was attributed to the weakening of the cellulose network in the biomass, as a result of hemicellulose hydrolysis. The multifunctional capacity of AlCl3.6?H2O (hemicellulose hydrolysis, xylose isomerization, and xylulose dehydration) in combination with its ease of recyclability make it an attractive candidate/catalyst for the selective synthesis of furfural from various biomass feedstocks.