Impregnation of alumina with copper chloride-modeling of impregnation kinetics and internal copper profiles

Abstract

The impregnation of previously wetted alumina pellets with aqueous solution of copper(II) chloride is studied experimentally and theoretically. Internal copper concentration profiles resulting from impregnation with different copper chloride solution concentrations and various impregnation times are determined by electron probe microanalysis and indicate a shell-progressive immobilization. Temperature-programmed reduction measurements conducted on impregnated alumina catalysts indicate two types of immobilized copper species with different reducibilities. An impregnation model is applied to describe the experimentally determined internal copper concentration profiles and the copper uptakes in terms of the measurable properties of the system. The model is based on slow diffusion and rapid irreversible immobilization of the copper ions on the support. It extends the well-known shell-progressive immobilization model to account for precipitation, which occurs during the drying process, of metal ions from solution trapped in the catalyst pores. Thus the model extends the scope of the shell-progressive model, which is applicable only for local metal loadings corresponding to the site density of immobilization sites, to the preparation of catalysts with higher local metal loadings. A tortuosity factor of 1.8 for the alumina support is calculated from the effective diffusivity of the copper chloride as estimated from the measured data and its molecular diffusivity as reported in the literature. The experimentally determined internal concentration profiles and copper uptakes agree with the theory to within experimental error.