EFFECT OF ALKALI-METAL PROMOTERS ON NICKEL MOLYBDATE CATALYSTS AND ITS RELEVANCE TO THE SELECTIVE OXIDATION OF BUTANE

The role of alkali metal promoters (Li, Na, K, Cs) in the alpha and beta phases of NiMoO, for butane oxidative dehydrogenation (ODH) was studied. The catalysts were characterized by BET, ICP, X-ray fluorescence, AA, XRD, TG, DSC, IR, and XPS techniques. It was evidenced that the added alkali metals are deposited only on the catalyst surface and practically do not affect the bulk. An influence of the concentration and basicity of the promoters on the decrease of the catalyst surface area was observed. Reaction studies show that addition of promoters generally reduces the catalytic activity but the promoters induce a high selectivity to butenes and butadiene. The greater the basicity and the amount of promoter ion, the higher is the selectivity and this improvement arises largely by suppression of total oxidation. The combination of activity and selectivity effects leads to improvement of yields of C-4 hydrocarbons by addition of small amounts of alkali promoters up to an optimum loading. Overdoping deteriorates yields, specially for beta phases. The observed changes are understood by considering the physical and chemical effects related with the interaction of the alkali promoters with the catalyst. Basicity is shown to be fundamental to promote the selective ODH of butane in the NiO-MoO3 system.