Surface distribution and heteroatom removal activity of equilibrium adsorption prepared, doubly promoted (Zn,Co)Mo/Al2O3 catalysts

Abstract

Adsorption isotherms of Zn²⁺ and/or Co²⁺ ions on an 8 wt.% MoO₃/alumina catalyst have been obtained. The Langmuir parameters derived from regression of the results indicate that preimpregnation of alumina with molybdate solutions notably decreased the adsorption of Zn²⁺ and Co²⁺ ions. In solutions containing both Zn²⁺ and Co²⁺ ions, the saturation capacity of Zn²⁺ was practically unaffected, while that of Co²⁺ was significantly reduced owing to the higher intrinsic affinity of Zn²⁺ ions for alumina sites. Adsorption of Co²⁺ ions occurs on two type of sites, i. e. those on which Co²⁺ ions are loosely and strongly bound; the Co²⁺ on the latter type is not affected by the presence of Zn²⁺ ions. After calcination, the non-adsorbed Co, retained in the pores, was deposited heterogeneoulsy as a surface cobalt oxide bound loosely to the Al₂O₃. However, the homologous Zn was more homogeneously deposited, owing to its higher reactivity with Al₂O₃. These differences result in an additional promotional effect in the gas-oil hydrodesulfurization (HDS) reaction, which has been observed on the catalysts prepared by simultaneous incorporation of Zn and Co. This effect is explained in terms of changes in the distribution and dispersion of promoters. These changes appear to be detrimental in hydrodenitrogenation (HDN).