ROLE OF SURFACE-PROPERTIES OF THE VANADYL PYROPHOSPHATE IN THE FORMATION OF MALEIC AND PHTHALIC ANHYDRIDES BY N-PENTANE OXIDATION

The mechanism of synthesis of maleic and phthalic anhydrides by the ga s phase oxidation of n-pentane over VPO-based catalysts was studied. T he oxidation of the paraffin was carried out under conditions which ma y lead to the isolation of possible reaction intermediates: very low r esidence time and high n-pentane/oxygen ratio. Under these conditions a number of by-products were identified, which were attributed to the oxidation of intermediate pentene and pentadiene, as well as other pro ducts with more than 5 C atoms, among which were alkylaromatics. Alkyl aromatics also were found to be formed in the anaerobic transformation of pentadiene. A mechanism is proposed, which takes into account the oxidehydrogenation of the paraffin to pentene and pentadiene, followed by two parallel pathways: (i) dimerization, dehydrocyclization and ar omatization of the diolefin, followed by oxidation of the alkylaromati c to phthalic anhydride, and (ii) multi-step oxidation of pentadiene t o maleic anhydride.