Catalytic Gas Phase Oxidation of Methanol to Formaldehyde

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Abstract
Two gas-phase catalytic cycles for the two-electron oxidation of primary and secondary alcohols were detected by multistage mass spectrometry experiments. A binuclear dimolybdate center [Mo2O6(OCHR2)]- acts as the catalyst in both these cycles. The first cycle proceeds via three steps: (1) reaction of [Mo2O6(OH)]- with alcohol R2HCOH and elimination of water to form [Mo2O6(OCHR2)]-; (2) oxidation of the alkoxo ligand and its elimination as aldehyde or ketone in the rate-determining step; and (3) regeneration of the catalyst via oxidation by nitromethane. Step 2 does not occur at room temperature and requires the use of collisional activation to proceed. The second cycle is similar but differs in the order of reaction with alcohol and nitromethane. The nature of each of these reactions was probed by kinetic measurements and by variation of the substrate alcohols (structure and isotope labeling). The role of the binuclear molybdenum center was assessed by examination of the relative reactivities of the mononuclear [MO3(OH)]- and binuclear [M2O6(OH)]- ions (M = Cr, Mo, W). The molybdenum and tungsten binuclear centers [M2O6(OH)]- (M = Mo, W) were reactive toward alcohol but the chromium center [Cr2O6(OH)]- was not. This is consistent with the expected order of basicity of the hydroxo ligand in these species. The chromium and molybdenum centers [M2O6(OCHR2)]- (M = Cr, Mo) oxidized the alkoxo ligand to aldehyde, while the tungsten center [W2O6(OCHR2)]- did not, instead preferring the non-redox elimination of alkene. This is consistent with the expected order of oxidizing power of the anions. Each of the mononuclear anions [MO3(OH)]- (M = Cr, Mo, W) was inert to reaction with methanol, highlighting the importance of the second MoO3 unit in these catalytic cycles. Only the dimolybdate center has the mix of properties that allow it to participate in each of the three steps of the two catalytic cycles. The three reactions of these cycles are equivalent to the three essential steps proposed to occur in the industrial oxidation of gaseous methanol to formaldehyde at 300−400 °C over solid-state catalysts based upon molybdenum(VI)-trioxide. The new gas-phase catalytic data is compared with those for the heterogeneous process.