Mechanistic study on low-temperature fast pyrolysis of fructose to produce furfural

In the low-temperature fast pyrolysis of fructose to produce 5-hydroxymethyl furfural (HMF), furfural (FF) is formed as an important by-product. In this work, pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) were used to reveal the FF formation mechanism from low-temperature fast pyrolysis of fructose. It was found that both the yield and the relative content of FF increased with increasing pyrolysis temperature up to 350°C, but a further increase in the pyrolysis temperature led to a decrease of the FF. The largest peak area of FF reached as high as 11.6%. Four possible pathways are proposed for production of FF from fructose based on the density functional theory (DFT) calculations. The pathway 2 was found to be the optimal route. In this way, the fructose first underwent a six-membered ring (MR) transition state. Then, the C5-C6 bond broke with a simultaneous dehydration of the H of OH at C6 and the OH at C4 to give a dihydrofuran intermediate species containing C4=C5 double bond together with formaldehyde and water. This dihydrofuran intermediate species transformed to an enol intermediate upon a further dehydration of the OH at C2 and the H at C1 through a 4-MR transition state. Finally, the enol intermediate was transformed into FF through a 6-MR transition state and another dehydration process of the OH at C3 and the enol H at C1.