Selective oxidation of methyl glycolate to methyl glyoxylate with molecular oxygen catalyzed by VOx/γ-Al2O3

The selective oxidation of methyl glycolate (MG) to methyl glyoxylate (MGO) using molecular oxygen in a fixed-bed reactor represents a greener and more efficient alternative to the traditional batch processing for producing this important fine chemical and intermediate. Despite its potential, detailed understanding of this catalytic reaction is still limited. In our study, VO_x/γ-Al₂O₃ catalysts were identified as effective for the reaction, but their performance strongly depended on the structure of VO_x species. The optimal catalyst with nearly monolayer dispersed VO_x achieved a high MG conversion of 91% and an MGO selectivity of 71%, alongside maintaining stability for 300 h without noticeable deactivation. At a low V loading of 0.6%, isolated VO_x species were formed, exhibiting the highest turnover frequency (TOF) but the lowest MGO selectivity. Increasing the V loading to 3.9% resulted in a blend of less polymeric and isolated VO_x species, which decreased the TOF but enhanced MGO selectivity to 59%. With the V loading ranging from 5% to 6.2%, monolayer dispersed VO_x became dominant, yielding a lower TOF and the highest MGO selectivity. A further increase in V loading caused the emergence of crystalline V₂O₅, which seemed to act as a bystander. Moreover, the redox cycles of V⁴⁺ and V⁵⁺ over the VO_x/γ-Al₂O₃ catalyst played an important role in the selective oxidation of MG to MGO, while the overoxidation of MG to CO₂ might take place through the V³⁺/V⁴⁺ redox pairs, the extent of which was determined by the structure of VO_x species. These insights are crucial for developing high-performance VO_x-based catalysts for the production of MGO through the selective oxidation of MG.