Catalysis of Organic..

Bacchini et al . 115around 1. The decrease of selectivity to PA, for increasing values of V oxidationstate, was much more relevant than the corresponding increase of MA; therefore, theoverall selectivity to the products of partial oxidation (MA+PA) decreased forincreasing extents of V oxidation, from 80% for sample ox1sp to 59% for sampleox3sp. The exception was sample ox3 (the most oxidized one), which gave anoverall selectivity to MA+PA equal to 68%.80ox3selectivity, %6040ox1speqox2spox1ox2ox3spMA selectivity20PA selectivity03,9 4 4,1 4,2 4,3 4,4 4,5Vanadium oxidation state, n+Figure 5. Selectivity to MA and to PA at 43-50% n-pentane conversion as functionsof the average oxidation state of V in fresh and spent samples.The mechanism for the formation of PA in n-pentane oxidation has been widelydiscussed in the literature. The main controversial point is whether MA and PA areformed through parallel reactions starting from a common intermediate, or if, on thecontrary, PA is consecutive to the formation of MA. In regard to the formerhypothesis, it has been proposed that n-pentane is first oxydehydrogenated topentene and pentadiene; the latter can either be oxidized to MA (with the loss of oneC atom), or be transformed to cyclopentadiene, which then dimerizes to a cyclictemplate and is finally oxidized to PA (15,16). An alternative route to PA is throughan acid-assisted dimerization and dehydrocyclization of intermediate pentadiene toyield a cyclic 6C-membered ring aliphatic C 10 hydrocarbon, precursor of analkylaromatic compound, that is finally oxidized to PA and 2 CO x (3,17). This lattermechanism is illustrated in Figure 6. Other authors suggest that n-pentane is eithertransformed to isopentane (precursor of citraconic anhydride), or to butene andbutadiene; the latter is oxidized to MA, and finally a Diels-Alder reaction betweenMA and adsorbed butadiene leads to the formation of PA (2). In this case, therefore,PA is consecutive to the formation of MA.