transfer of pyruval dehyde in water by Meerwein Ponndorf Verley
(MPV) reduction under mild reaction conditions [37]. We thus propose that the transformation of pyruvaldehyde to lactic acid is via the intramolecular Cannizzaro reaction catalyzed by Lewis acid
sites on ZrO2 instead of a strong base, as shown in Scheme 3. Free
OH− and H3O+ ions from the auto-dissociation of water are prevaiing in subcritical water [38,39]. It is also reported that water can
be dissociated on ZrO2 [40,41]. We thus speculate that the cycle of
intramolecular Cannizzaro reaction is closed with the aid of free or
surface bound hydroxyl and hydronium ions.