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Formation of 5‑(Hydroxymethyl)furfural by Stepwise Dehydration over TiO<sub>2</sub> with Water-Tolerant Lewis Acid Sites
No full textThe
reaction mechanism for the formation of 5-(hydroxymethyl)Âfurfural
(HMF) from glucose in water over TiO<sub>2</sub> and phosphate-immobilized
TiO<sub>2</sub> (phosphate/TiO<sub>2</sub>) with water-tolerant Lewis
acid sites was studied using isotopically labeled molecules and <sup>13</sup>C nuclear magnetic resonance measurements for glucose adsorbed
on TiO<sub>2</sub>. Scandium trifluoromethanesulfonate (ScÂ(OTf)<sub>3</sub>), a highly active homogeneous Lewis acid catalyst workable
in water, converts glucose into HMF through aldose–ketose isomerization
between glucose and fructose involving a hydrogen transfer step and
subsequent dehydration of fructose. In contrast to ScÂ(OTf)<sub>3</sub>, Lewis acid sites on bare TiO<sub>2</sub> and phosphate/TiO<sub>2</sub> do not form HMF through the isomerization–dehydration
route but through the stepwise dehydration of glucose via 3-deoxyglucosone
as an intermediate. Continuous extraction of the evolved HMF with
2-<i>sec</i>-butylphenol results in the increase in the
HMF selectivity for phosphate/TiO<sub>2</sub>, even in highly concentrated
glucose solution. These results suggest that limiting the reactions
between HMF and the surface intermediates improves the efficiency
of HMF production
