Manganese Dioxide with Different Crystalline Structures for Benzyl Alcohol Oxidation: A Density Functional Theory and Experimental Approach

In the chemical industry, oxygen activation relies on efficient catalysts. This study investigates four crystalline MnO2 phases (α-, β-, γ-, and δ-) for the aerobic oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH). Notably, δ-MnO2 exhibits superior catalytic activity, achieving a BzOH conversion of 94% and a BzH selectivity of 91% at 50 °C within 3 h. Experimental and theoretical analyses confirm that its crystalline structure governs oxygen activation efficiency. Mechanistically, the typical Mars–van Krevelen mechanism (MvK) proposes that oxygen vacancies (OVs) facilitate O2 activation. The developed MvK containing the Langmuir–Hinshelwood mechanism (L-H) further emphasizes the pivotal role of the Mn3+/Mn4+ redox cycle, after the oxidation of Mn3+ to Mn4+ by adsorbed O2, while substrate interactions reduce Mn4+ back to Mn3+, sustaining catalytic activity. The exceptional activity of δ-MnO2 arises from its optimal OV density and elevated Mn3+/Mn4+ ratio, highlighting the critical interplay between OVs and Mn3+ for designing nonprecious metal catalysts.