Catalysis of Organic..

114Oxidation of n-Pentanethe presence of P-OH groups arising from the transformation of pyrophosphate intoorthophosphate groups.Catalytic tests were carried out following a protocol that allowed establishrelationships between the characteristics of fresh and spent catalysts and the catalyticperformance. Specifically, within the first 5-10 hours reaction time, the catalyticperformance was not stable, due to variations in samples characteristics. Therefore,two different sets of results were taken, the first after less than 1 hour (representativeof fresh samples), and the second one after 3-4 hours (representative of spentsamples). Only in the case of the equilibrated sample, no difference was foundbetween the characteristics of samples eq and eqsp (Tables 1 and 2); for this catalyst,also the catalytic performance did not vary at all during catalytic tests.Figure 4 shows the catalytic performance of sample ox1sp; the conversion of n-pentane, and the selectivity to the main products, MA, PA and carbon oxides, arereported as functions of the reaction temperature. The selectivity to MA increased onincreasing the reaction temperature, and correspondingly the selectivity to PAdecreased. The overall selectivity to PA and MA was approximately constant up to400°C, but then decreased, due to the preferred formation of carbon oxides.100n-pentane conversion80conversion, selectivity %604020PA selectMA select.COx select0330 360 390 420reaction temperature, °CFigure 4. Effect of temperature on catalytic performance. Catalyst ox1sp.Figure 5 summarizes the results obtained for fresh and spent samples; theselectivity to MA and that to PA at approximately 43-50% n-pentane conversion areplotted as a function of the average oxidation state of V. The most oxidized thecatalyst was, the most preferred was the formation of MA with respect to that of PA,with a MA/PA selectivity ratio equal to 7 at 50% n-pentane conversion for sampleox3. The opposite was true for most reduced samples, with a MA/PA selectivity ratio