Role of homogeneous reactions in the control of the selectivity to maleic and phthalic anhydrides in the oxidation of n-pentane

Oxidation of n-pentane on vanadium phosphate catalysts was followed in a mixed heterogeneous-homogeneous regime and the effect of the modification of both reaction regimes on the ratio of phthalic (PA) to maleic anhydride (MA) was evaluated. The extent of the oxidation under homogeneous conditions was controlled either by varying the volume of the void section of the reactor or by adding methyl-ter-butylether (MTBE). Results show that the homogeneous reaction represses preferentially PA formation, increasing the MA/PA ratio. MTBE inhibits both the homogeneous and the heterogeneous reaction. Such behaviour is reversible, however recovery of the PA formation was substantially slower after stopping MTBE addition. It could be concluded that, in the study of the activation of n-pentane (and alkanes), the interaction of both (homogeneous and heterogeneous) processes must be taken into account.