Furan and benzene ring formation in cellulose char: the roles of 5-HMF and reducing ends

The authors previously proposed that 5-hydroxymethylfurfural (5-HMF) can be produced from the reducing ends of cellulose as a key intermediate during carbonization. The present work investigated the mechanisms by which furan and benzene rings are formed in cellulose char based on carbonization at 280 °C using ¹³C-labeled 5-HMF together with ¹³C-labeled glucose (as a model for the cellulose reducing ends). Glycerol was added to the 5-HMF to prevent the formation of stable glassy polymers. The resulting char was subjected to pyrolysis gas chromatography/mass spectrometry (764 °C, 5 s hold time) and the incorporation of ¹³C in furan-, phenol-, benzofuran- and benzene-type fragments was assessed. The apparent formation mechanisms include a direct rearrangement of the six carbons of 5-HMF to phenols, Diels-Alder reactions of furan rings with double bonds to give benzofurans and a more random process involving reactive fragments producing benzene rings. On the basis of these results, the roles of 5-HMF and reducing ends during cellulose carbonization are discussed herein.