The solid-state thermal rearrangement of the Dawson anion [P 2Mo 18O 62] 6− into a Keggin-type [PMo 12O 40] 3−-containing phase and their reactivity in the oxidative dehydrogenation of isobutyraldehyde

Abstract

The thermal and structural stability of the Dawson-type heteropolyoxometalate anion, [P 2Mo 18O 62] 6−, with varying counter cations [K +, Rb +, Cs +, NH 4 + and (CH 3) 4N +] and water/solvent (1,4-dioxane) of crystallization has been examined using IR spectroscopy, XRD, TGA/DTA, TEM/SEM and 31 P solid-state NMR spectroscopy. At temperatures higher than about 260°C, the K +, Rb + and Cs + salts undergo an irreversible thermal rearrangement to give the corresponding Keggin anion [PMo 12O 40] 3− as one product, along with a second phase which appears to be another phosphorous-containing polyoxomolybdate, perhaps [P 2Mo 6O 26] 6− or a mixture of related species. Although the NH 4 + and Me 4N + salts behave similarly, the second phase does not seem to be as thermally stable in these systems, and at higher temperatures, decomposition to MoO 3 is observed. Gas-phase oxidative dehydrogenation of isobutyraldehyde to methacrolein was used to examine the effectiveness of the [P 2Mo 18O 62] 6− salts and their thermally-rearranged phases as catalysts. The former are poor catalysts, undergoing thermal rearrangement even at 250°C under catalytic conditions, which is related to the role of the catalyst in the reaction. The thermally-rearranged NH 4 + and Me 4N + salts are highly active catalysts at 300°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 [PMo 12O 40] 3− phase.