Dawson-type polyoxometalates in Liquid phase isobutane hydroxylation

Abstract

Dawson polyoxometalates (POMs) have been reported as efficient catalysts for gas- phase oxidative dehydrogenation of isobutane to isobutene [1] and isobutyric acid to methacrylic acid with oxygen molecular as oxidant [2]. In this paper, we have studied the catalytic activity of a series of Dawson-type POMs, as potassium salts, namely K6P2W12Mo6O62, K7P2Mo5VW12O62, P2W10Mo5V3O62 and P2W12V6O62, in the isobutane hydroxylation in acetonitrile solvent with hydrogen peroxide as oxidant. POMs were characterized by FTIR, 31P NMR and BET. Terbutyl alcohol (tert-BuOH), isobutyl alcohol (iso-BuOH), acetone, isopropyl alcohol (iso-PrOH) and isobutyric acid (isoBuOOH) are the products detected in isobutane hydroxylation at 60°C. The distribution of these products depends of POM composition and the framework symmetry. Thus, V free POM didn‟t catalyse liquid isobutane oxidation while V containing POMs promoted the formation of oxygenated products at low temperature. The first major product is isobutiric acid and the second one is tertiobutanol. The investigation of isobutanol and tertiobutanol transformation in equivalent reaction conditions (with or without catalyst) evidenced the formation of isobutyric acid and C3 oxygenated products. This suggests that tertiobutanol and isobutanol would the primary products, while acetone, isopropanol and isobutyric acid would result from successive transformations of the C4 alcohols. The results shown that the presence of vanadium atoms favoured the formation of oxygenated compounds in the isobutane oxidation. The distribution of reaction products depends on the polyanion composition.