Development of molybdena catalysts supported on γ-alumina extrudates with four different Mo profiles: Preparation, characterization, and catalytic properties

Abstract

The main goal of this work is to prepare, characterize, and determine the catalytic properties of molybdena supported γ-alumina extrudates with four Mo profiles. Concerning preparation, a procedure was established allowing us to prepare (both axially and radially) egg-shell, egg-white, egg-yolk, and uniform profiles with the same, relatively high, Mo loading. The preparation of the egg-shell and uniform profiles was achieved by impregnating γ-alumina extrudates with acidic and alkaline ammonium heptamolybdate solution, respectively. Moreover, it was found that an egg-white (egg-yolk) profile may be achieved from an egg-shell (uniform) profile by successive pore volume or simple nondry impregnations of the Mo-supported extrudates with NH 4OH aqueous solutions. A qualitative mechanism, based on the relative rates of desorption and diffusion of the MoO 4 2− ions occuring in the NH 4OH impregnation, was developed to interpret the influence of desorption time, concentration of NH 4OH solution, drying, and mode of impregnation on the characteristics of the egg-white and egg-yolk profiles obtained. The physicochemical characterization of the Mo/γ-alumina extrudates for which radial profiles have been achieved, done by using a number of techniques, showed a progressive increase of the “Mo supported-support” interactions and Mo dispersion following the order egg-yolk < uniform < egg-white < egg-shell. A similar trend (uniform < egg-yolk < egg-white < egg shell) was obtained for the number of active sites as estimated by the amount of the adsorbed NO. The trends mentioned above were related with the values of the preparative parameters used for obtaining each profile. Kinetic experiments were done, using the hds of thiophene as a probe reaction, over the characterized samples being in the form of extrudates and powders produced by crushing the extrudates. It was found that hds activity was mainly determined by the number and the quality of active centers and not by the type of Mo profile, though these suggested the presence of no significant diffusional effects. The relative yield of butane, produced by the hydrogenation of the unsaturated hydrocarbons formed during hds, increased with the distance of the maximum of the Mo profile from the periphery of the extrudate to its center. Finally, it is important to note that the most active radial profile proved to be the egg-white for hds and the egg-yolk for hydrogenation.