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The selective oxidation of isobutane over phosphomolybdic acid has been analyzed using a low-pressure steady-state technique. Observed products are methacrolein, 3-methyl-2-oxetanone (a lactone), acetic acid, carbon dioxide and water. These products form in two distinct fashions designated Categories 1 and 2. Category 1 is exclusively observed with methacrolein and is consistent with a Bulktype (II) process. The lactone and acetic acid are Category 2 products and these form by the Bulk-type (I) mechanism. A kinetic analysis of the distributions shows that the activation energy for Category 1 production (methacrolein) increases with experiment number (varying from 56.9 ± 1.5 to 149 ± 4 kJ mol-1) and indicates a temperature-induced structural degradation of phosphomolybdic acid. The Category 2 products are not similarly affected and this suggests that the structural degradation is restricted to the surface of phosphomolybdic acid. |
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