The solid-state thermal rearrangement of the Dawson anion [P2Mo18O62]6- into a Keggin-type [PMo12O40]3--containing phase and their reactivity in the oxidative dehydrogenation of isobutyraldehyde

The thermal and structural stability of the Dawson-type heteropolyoxometalate anion, [P2Mo18O62](6-), with varying counter cations [K+, Rb+, Cs+, NH4+ and (CH3)(4)N+] and water/solvent (1,4-dioxane) of crystallization has been examined using IR spectroscopy, XRD, TGA/DTA, TEM/SEM and P-31 solid-state NMR spectroscopy. At temperatures higher than about 260 degrees C, the K+, Rb+ and Cs+ salts undergo an irreversible thermal rearrangement to give the corresponding Keggin anion [PMo12O40](3-) as one product, along with a second phase which appears to be another phosphorous-containing polyoxomolybdate, perhaps [P2Mo6O26](6-) or a mixture of related species, Although the NH4+ and Me4N+ salts behave similarly, the second phase does not seem to be as thermally stable in these systems, and at higher temperatures, decomposition to MoO3 is observed. Gas-phase oxidative dehydrogenation of isobutyraldehyde to methacrolein was used to examine the effectiveness of the [P2Mo18O62](6-) salts and their thermally-rearranged phases as catalysts. The former are poor catalysts, undergoing thermal rearrangement even at 250 degrees C under catalytic conditions, which is related to the role of the catalyst in the reaction. The thermally-rearranged NH4+ and Me4N+ salts are highly active catalysts at 300 degrees C, even resulting in the formation of some methacrylic acid, while the thermally-rearranged phases in the K+, Rb+ and Cs+ systems show little activity. The latter likely results from the presence of the catalytically-inactive second phase(s) formed in the thermal rearrangement in each case, which forms a surface layer on the catalytically-active [PMo12O40](3-) phase. (C) 2000 Elsevier Science B.V. All rights reserved.