Catalysis of Organic..

116Oxidation of n-PentaneIn the mechanism illustrated in Figure 6, the combination of the redox and acidproperties of the catalyst determines the relative contribution for the formation ofMA and PA. It is generally accepted that the higher the crystallinity of the VPP, themore selective to PA is the catalyst (3,4,10-12,17,18). Poorly crystalline VPP, likethat one formed after the thermal treatment of the precursor (especially when it iscarried out under oxidizing conditions), is selective to MA, but non-selective to PA.On the contrary, a fully equilibrated catalyst, characterized by the presence of a wellcrystallizedVPP, yields PA with a good selectivity. The presence of dopants thatalter the crystallinity of VPP may finally affect the MA/PA selectivity ratio (19).Moreover, the surface acidity also influences the distribution of products (17); anincrease of Lewis acidity improves the selectivity to PA, while that to MA ispositively affected by Brønsted acidity (2).-H 2 O½ O 2-H 2 OC 5 H 8½ O 2½ O 2-6 - 2 HCO 2 O 2-2 CO6.5 O 22OOO-CO 23.5 O 2-3 H 2 OO-CO 2OOOFigure 6. Mechanism proposed for the one-pot oxidation of n-pentane to MA and toPA (3,17).Data reported in the present work demonstrate that the degree of crystallinityand the acid properties are related the amount of V 5+ present at the surface of VPP.When the VPP is not fully equilibrated, and hence may contain discrete amounts ofV 5+ , it is more selective to MA and less to PA. The reason is that in oxidizedcatalysts, the olefinic intermediate is preferentially oxidized to MA, rather then beingsubjected to the acid-catalyzed condensation with a second unsaturated molecule, toyield the precursor of PA. When instead the catalyst is more crystalline, and hence itdoes contain less oxidized V sites, its surface acid properties predominate over O-insertion properties, and the catalyst becomes more effective in PA formation. In thiscase, the selectivity to PA at ≈ 50% n-pentane conversion becomes comparable tothat one of MA.