Characterization of Molybdenum Based Systems in Solution and on Supports By Spectroscopic Methods and Activity Studies (Titania, Hydro-Desulfurization, Molybdenum-Oxalate, Fischer-Tropsch Reaction, Raman Spectroscopy).

Abstract

The aggregation behavior of ammonium heptamolybdate and sodium tungstate precursor solutions as a function of concentration and pH were investigated by Raman spectroscopy and Laser Light Scattering. The results agree with the aggregation scheme of simple (--->) hepta (--->) octa (--->) Mo(,36)O(,112)('-8) (--->) protonated polymeric species, for molybdate solutions. Tungstate solutions aggregate according to simple (--->) Y-polytungstate (--->) paratungstate-A (--->) paratungstate-B (--->) (PSI)-metatungstate. The structure of molybdenum oxalate complexes in solution and on alumina were studied by Raman and FTIR spectroscopy. The results show that the molybdenum oxalate complex changes from a dimeric bidentate structure to a monomeric unidentate structure in solution as pH increases. Molybdenum oxalate complexes are stable compared to isopolymolybdates when supported on alumina. The Fischer-Tropsch activity and selectivity of carbon-supported molybdenum catalysts were studied as a function of preparation conditions and potassium promotion. Catalysts prepared by equilibrium adsorption were better dispersed and showed a high activity as compared to catalysts prepared by the incipient wetness method. Potassium promotion increased the selectivity towards C(,2) and C(,3) hydrocarbons while reducing moderately the activity. The nature of molybdate species on the titania surface, as a function of preparative variables, was followed by Raman and FTIR spectroscopy. The distribution of monomeric tetrahedral species, polymeric species and bulk like molybdenum trioxide depends on the loading, pH of the starting solution, method of catalyst loading, and pretreatment conditions. Addition of cobalt to titania supported molybdenum catalysts suppresses the formation of bulk MoO(,3) and leads to the formation of a cobalt-molybdate phase. Hydrodesulfurization (HDS) activity and selectivity were determined at 350(DEGREES)C and 1 atmosphere for a series of molybdena-titania catalysts. For loadings at or below monolayer coverage, titania-supported Mo catalysts were found to be more active when compared with an industrial Co-Mo catalyst. Addition of cobalt increases the HDS activity modestly. The selectivity of these catalyst correlates well with the proposed model. Surface acidity of these molybdena-titania catalysts was determined by using pyridine as a probe molecule. From the relative concentrations of Lewis acid sites, Bronsted acid sites and hydrogen bonding sites, the adsorption and aggregation mechanisms of molybdates on the titania surface were inferred.