Regeneration of Spent Iron Molybdate Catalysts via Ammonia Leaching for Catalytic Methanol Oxidation to Formaldehyde

The iron molybdate (FeMo) catalyst, due to its excellent catalytic activity, has been widely used in the oxidation of methanol to produce formaldehyde, while the partial loss of active molybdenum species leads to a short lifetime. The spent FeMo catalyst is generally discarded as solid waste; however, it still contains a significant amount of molybdenum species. Thus, the recovery and regeneration of molybdenum are greatly needed, yet it remains a great challenge. In this study, ammonia leaching was applied to recover molybdenum species from the spent FeMo catalyst, achieving a high recovery efficiency of 95.3%. Based on the results from XRD, Raman, and UV–vis analyses, the spent FeMo catalyst was found to consist of MoO3, Fe2(MoO4)3, and segregated FeOx, and then, a possible process for Mo leaching was proposed: Mo species leached rapidly from MoO3 at low pH values (<6.3), followed by leaching from Fe2(MoO4)3 at pH > 6.3. Meanwhile, segregated FeOx remained completely insoluble. Correspondingly, Mo species were leached as Mo3O102–, MoO42–, and Mo7O246–, which depend on the pH values during the leaching process. Finally, the leached Mo species were applied to prepare fresh FeMo catalysts for methanol oxidation. It showed 99.9% methanol conversion and 89.3% formaldehyde selectivity, which was comparable to that over the industrial FeMo catalyst. It also showed excellent catalytic stability in an ∼510 h continuous life test. These findings offer a potential pathway for large-scale industrial recovery and reuse of spent FeMo catalysts and other molybdenum-containing catalysts.