In SituStudy of Redox and of p-Type Semiconducting Properties of Vanadyl Pyrophosphate and of V–P–O Catalysts during the Partial Oxidation ofn-Butane to Maleic Anhydride

Abstract

In situelectrical conductivity measurements have been performed on vanadyl pyrophosphate samples as a function of their preparation temperature and on V–P–O catalysts as a function of their activation time. In agreement with X-ray diffraction results, electrical conductivity has shown that heat treatment of vanadyl pyrophosphates between 450 and 800°C results in a less conducting state corresponding to a very crystalline form. Such a stabilized form can already be reached at a temperature between 600 and 700°C. For V–P–O catalysts, electrical conductivity decreased as a function of the activation time,tact, under the reaction mixture, reaching a steady value fortact≥84 h. Both types of solids were found to be p-type semiconductors with positive holes as the main charge carriers. During subsequent exposures ton-butane and to oxygen at reaction temperature (400°C), it was confirmed that vanadyl pyrophosphate constitutes the main phase of efficient V–P–O catalysts working with some excess of surface oxygen probably in relation with the presence of V5+species. The recorded changes in conductivity upon exposure to reactants provide illustrative evidence of a redox (Mars and van Krevelen) mechanism. The long activation period necessary to produce a high yield of maleic anhydride corresponds to structural and chemical changes of the surface which accommodates the reaction mixture and gives rise to active and selective sites able to convertn-butane to maleic anhydride.