The effect of cation type and H+ on the catalytic activity of the Keggin anion [PMo12O40]3- in the oxidative dehydrogenation of isobutyraldehyde

The oxidative dehydrogenation of isobutyraldehyde to methacrolein over [PMo12O40](3-)-containing catalysts has been shown to proceed through bulk catalysis-type II, which depends on the rates of diffusion of the redox carriers (H+ and e(-)) into the catalyst bulk. Variations in catalyst behaviour have been shown to change with the countercation and appear to be related to the polarizing ability of the cation, which can be represented by the ionic potential (charge/ionic radius). This, in turn, may indicate that the active site at the [PMo12O40](3-) ion is close to an attendant countercation. For the alkali metal ions Li+, Na+, K+, Rb+, and Cs+ as well as the (isoelectronic) ions of the series Cs+, Ba2+, La3+, and Ce4+, the studies have shown that conversion generally decreases with increasing ionic potential, while selectivity to methacrolein is less affected by changes in this property. However, the observed enhancement in selectivity to methacrolein and methacrylic acid in the case of Ce4+ is attributed to the ability of this cation to undergo reduction to Ce3+, with the Ce4+/Ce3+ couple likely acting to expedite the transfer of electrons between anions and hence into the bulk. Trends in the conversion and selectivities among the countercations Cs+, NH4+, and (CH3)(4)N+ show significant increases along this series, which is consistent With progressively increasing dissipation of the formal cationic charge. A mechanism for the oxidative dehydrogenation of isobutyraldehyde is proposed. The presence of H+ enhances the activity of salts of [PMo12O40](3-), most likely by protonation of the organic component, as little evidence from extended-Huckel molecular orbital calculations could be found for any reduction in the HOMO-LUMO gap of the anion upon protonation, which would make it easier to reduce the anion as part of the catalytic process. (C) 2000 Academic Press.