Enhancing polyol/sugar cascade oxidation to formic acid with defect rich MnO2 catalysts

Oxidation of renewable polyol/sugar into formic acid using molecular O2 over heterogeneous catalysts is still challenging due to the insufficient activation of both O2 and organic substrates on coordination-saturated metal oxides. In this study, we develop a defective MnO2 catalyst through a coordination number reduction strategy to enhance the aerobic oxidation of various polyols/sugars to formic acid. Compared to common MnO2, the tri-coordinated Mn in the defective MnO2 catalyst displays the electronic reconstruction of surface oxygen charge state and rich surface oxygen vacancies. These oxygen vacancies create more Mnδ+ Lewis acid site together with nearby oxygen as Lewis base sites. This combined structure behaves much like Frustrated Lewis pairs, serving to facilitate the activation of O2, as well as C-C and C-H bonds. As a result, the defective MnO2 catalyst shows high catalytic activity (turnover frequency: 113.5 h-1) and formic acid yield (>80%) comparable to noble metal catalysts for glycerol oxidation. The catalytic system is further extended to the oxidation of other polyols/sugars to formic acid with excellent catalytic performance.