Reaction mechanism of low-temperature fast pyrolysis of fructose to produce 5-hydroxymethyl furfural

Low-temperature fast pyrolysis of fructose offered a promising way to produce 5-hydroxymethyl furfural (HMF) together with furfural (FF) as an important by-product. In this work, pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) measurements were performed to investigate the product distribution from fast pyrolysis of fructose; the effects of pyrolysis temperature on the HMF formation behaviors were considered. The results indicated that HMF is the predominant product from the fast pyrolysis of fructose; the product mixture with highest content of HMF (81.2%, determined by the gas chromatography peak areas) is obtained at 250 ℃, while the maximal yield of HMF is achieved at 350 ℃. Five possible pathways of HMF formation from fructose were considered by density functional theory (DFT). The DFT calculation results suggested that pathway 1 is most energetically favored, i.e. fructose molecule first undergoes a dehydration process by losing -OH at C2 and -H at C1 and then it is subjected to subsequent dehydrations involving -OH at C3 and -H of hydroxyl group at C1 as well as -OH at C4 and -H at C5, to form HMF.