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 [PMo 12O40](3-)-containing catalysts has been shown to proceed through bulk cata lysis-type II, which depends on the rates of diffusion of the redox carrier s (H+ and e(-)) into the catalyst bulk. Variations in catalyst behaviour ha ve 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 acti ve site at the [PMo12O40](3-) ion is close to an attendant countercation. F or the alkali metal ions Li+, Na+, K+, Rb+, and Cs+ as well as the (isoelec tronic) ions of the series Cs+, Ba2+, La3+, and Ce4+, the studies have show n that conversion generally decreases with increasing ionic potential, whil e selectivity to methacrolein is less affected by changes in this property. However, the observed enhancement in selectivity to methacrolein and metha crylic 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 e xpedite the transfer of electrons between anions and hence into the bulk. T rends in the conversion and selectivities among the countercations Cs+, NH4 +, and (CH3)(4)N+ show significant increases along this series, which is co nsistent With progressively increasing dissipation of the formal cationic c harge. 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 evidenc e from extended-Huckel molecular orbital calculations could be found for an y 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) 2 000 Academic Press.