Mechanistic insights on catalytic conversion fructose to furfural on beta zeolite via selective carbon-carbon bond cleavage

The mechanism for the formation of furfural by dehydration of D-fructose via selective carbon-carbon (C - C) bond cleavage was investigated over beta zeolite (H beta). Several different pore size zeolites were employed to determine the shape selectivity of fructose isomers. Zeolitic pore sizes that smaller than the kinetic diameter of cyclic fructose also produced yield furfural (similar to 22.5%), while the high yield (66.0%) of hydroxymethylfurfural (HMF) can be achieved over large pore zeolite (USY zeolite), which could accommodate the cyclic fructose. These results indicated that the furfural formation likely began with acyclic fructose. In situ C-13 NMR and GC - MS studies, using labeled C-13-1-fructose as substrate, suggested that the conversion'of fructose to furfural involved with splitting of the C5-C6 bond. Furthermore, the C1 compound from the cleavage of C-C bond was identified as formaldehyde, inferring that the selective scission of C-C bond was ascribe to the retro-aldol reaction. Interestingly, in situ NMR studies implied that the acyclic fructose mainly derived from pyranose forms. In addition, compared with glucose, fructose directly converted to furfural at the higher yield and reaction rate.