Selective isobutane oxidation over polyoxomolybdate Keggin-type structures

Abstract

Keggin-type heteropoly compounds, such as H3[PMo12O40], show unique catalytic activity due to their acidity and redox properties. Such catalysts are used in industry for the oxidation of low-cost alkanes to more valuable alkenes, alcohols, aldehydes and carboxylic acids [1]. The activity and selectivity of polyoxomolybdates can be adjusted by varying the counter-cation [2]. Reported products from isobutane oxidation over Keggin polyoxometalates are isobutene, methacrolein, methacrylic acid and carbon oxides [1]. In the case of polyoxomolybdates, high yields of the aldehyde and carboxylic acid indicate that Mo(VI) can abstract hydrogen and add oxygen to the alkane. Recently we have developed a novel low-pressure technique to determine the kinetics of heterogeneous catalytic processes [3]. Molecular flow conditions and temperature-programming are used to accurately calculate activation energies and determine primary products for the rate-determining reaction. A quadrupole mass spectrometer monitors all gaseous species that effuse from the low-pressure reactor. Observed major products following exposure of isobutane to both H3[PMo12O40] and Cs2.5H0.5[PMo12O40] located in the low-pressure reactor were acetic acid, methacrolein, carbon dioxide and 3-methyl-2-oxetanone. Neither isobutene nor methacrylic acid was observed. Acetic acid formed and desorbed from the catalyst surface at low temperatures (<300°C), while carbon dioxide and methacrolein are the dominant products and evolved from the catalyst with increasing rate as the temperature rose to 450°C. The rate of methacrolein formation is less pronounced over the caesium-containing catalyst. The lactone is a rare molecule that has not previously been reported as a product of isobutane oxidation. Temperature-programmed profiles of this lactone reach a maximum at 340°C for the caesium-containing catalyst and at 425°C for the pure acid catalyst. The formation of acetic acid and lactone intermediates is is an indicator of the mechanism for methacrolein formation.