Novel Effect of Zinc Nitrate/Vanadyl Oxalate for Selective Catalytic Oxidation of α-Hydroxy Esters to α-Keto Esters with Molecular Oxygen: An In Situ ATR-IR Study

Selective oxidation of α-hydroxy esters is one of the most important methods to prepare high value-added α-keto esters. An efficient catalytic system consisting of Zn(NO(3))(2)/VOC(2)O(4) is reported for catalytic oxidation of α-hydroxy esters with molecular oxygen. Up to 99% conversion of methyl DL-mandelate or methyl lactate could be facilely obtained with high selectivity for its corresponding α-keto ester under mild reaction conditions. Zn(NO(3))(2) exhibited higher catalytic activity in combination with VOC(2)O(4) compared with Fe(NO(3))(3) and different nitric oxidative gases were detected by situ attenuated total reflection infrared (ATR-IR) spectroscopy. UV-vis and ATR-IR results indicated that coordination complex formed in Zn(NO(3))(2) in CH(3)CN solution was quite different from Fe(NO(3))(3); it is proposed that the charge-transfer from Zn(2+) to coordinated nitrate groups might account for the generation of different nitric oxidative gases. The XPS result indicate that nitric oxidative gas derived from the interaction of Zn(NO(3))(2) with VOC(2)O(4) could be in favor of oxidizing VOC(2)O(4) to generate active vanadium (V) species. It might account for different catalytic activity of Zn(NO(3))(2) or Fe(NO(3))(3) combined with VOC(2)O(4). This work contributes to further development of efficient aerobic oxidation under mild reaction conditions.