Isothermal reduction behaviour of some metal molybdates. Selective light alkane oxydehydrogenation and/or olefins partial oxidation

The reduction profile of several unpromoted and promoted metal molybdate catalysts was investigated correlating their reducibility with the reactivity in catalysis. Using the stoichiometric alpha- and beta -nickel molybdate compounds it was observed that the reduction rate was significantly affected by the nature of the phase. The results show that beta -NiMoO4 phase led to a significant increase in the reduction rate with respect to alpha phase. The increased resistance to reduction by hydrogen due to the structure of the catalytic system is reported. It was found that there is a relationship between the reducibility of the catalysts and selectivity to dehydrogenation products, indicating that the lattice oxygen plays an important role in the reaction. The effect of MoO3, TeO2 and Te2MoO7 added to NiMoO4 systems on the reducibility of the catalyst and on the propylene oxidation were also studied. It was observed that the reduction rate was significantly affected by the nature of the doping element. The results show that NiMoO4-MoO3 combination led to a significant increase of the reduction resistance of the nickel molybdate while TeO2 or Te2MoO7 addition increases its oxygen depletion rate. Ni-Mo-O systems (Mo/Ni >1) were found to favour low COx selectivity, high selectivity to C3H4O and C3H4O2 and good propylene conversion. In presence of TeO2 and Te2MoO7 doped Ni-Mo-O system both acrolein and propylene conversion were increased with respect to the undoped system. Ni-Mo-Te-O catalysts have been found to have a reducibility trend which fits well with the acrolein and acrylic acid formation from propylene oxidation in presence of molecular oxygen.