Molybdena/alumina acidity

Surface molybdenum species and acid sites on nitrided molybdena-alumina catalysts... 

Dehydrogenation over Molybdena—Alumina. Detailed results have been reported previously (12) for molybdena-alumina catalysts. It appears that freshly prepared molybdena-alumina catalysts are strongly acidic and exhibit very high activity but poor selectivity towards monoolefin formation initially. After about 24 hours, these catalysts become... 

The high acidity of molybdena-alumina composites appear to be derived mainly not from the intrinsic acidity of the alumina but from active sites on a surface phase containing molybdenum atoms. The amount of LB and DTA formation that is observed initially over molybdenum-alumina is at least 10 times greater than observed over the bare support when the latter reacts with 8% 1-dodecene. In this case, the bare support was alumina B listed in Table II. 

Indeed, Lund and Dumesic (5-8) reported that the water-gas shift activity of an iron oxide catalyst is reduced by several orders of magnitude when supported on silica. Strong interactions between molybdena and alumina have been documented for the calcined states of hydrotreating catalysts (e.g., 9-11). Also, interaction is manifested in many mixed oxides by enhanced acidity, compared to the acidities of the pure component oxides (12-14). 

Apart from the degree of reduction affecting overall performance, the nature of the support is also crucial in determining final activity. For supported molyb-dena catalyst, alumina and titania support materials provide best performance. Silica, zirconia, chromia, and zinc oxide are also good support materials, although they produce less active catalysts. Inactive catalysts can be readily synthesized by supporting molybdena upon cobalt oxide, nickel oxide, magnesium oxide, or tin oxide. To date, no correlation between the acidity of the support material and cataljdic activity has been found (304). 

The benefical effects of phosphorus has stimulated research on its influence on molybdena based catalysts. However, most of the above mentioned literature essentially focused on the influence of phosphorus on the catalytic properties of the modified system and its effect on the dispersion on the active phase deposited on the alumina surface has not yet been investigated. Also, very few works deal with the influence of the sequence of phosphorus incorporation during the preparation step of the MoP/Al,0j catalysts on the surface acidity, dispersion and distribution of the supported phases. As the phosphate ions strongly interact with alumina, competing with molybdate ions, a factor of possible importance is the preparation procedure. 

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