We speculate that the reaction pathway of converting xylose to
lactic acid over the ZrO2 catalyst starts from the retro-aldol condensation to form glycolaldehyde and glyceraldehyde. In the pH
neutral hydrothermal media, we did not observe a high yield production of lactic acid (only 2.7%) without adding the ZrO2 catalyst,
while the yield of furfural (32.4%) is instead significantly higher
Thus we conclude that the catalytic effects
of the free hydroxyl ions from the auto-dissociation of subcritcal water are negligible towards the formation of lactic acid from
xylose. We tentatively ascribe the bi-functional Lewis acid/base
pair on the ZrO2 surface to catalyzing the retro-aldol condensation of xylose. As shown in