Genesis of zinc–cobalt promoted hydrotreating catalysts: adsorption isotherms of Zn and/or Co on Al2O3

Abstract

The adsorption isotherms of Zn2+ and/or Co2+ ions from aqueous solutions on γ-Al2O3 have been obtained, and analysed by the Langmuir model. The results show that adsorption of both ions occurs predominantly on the same type of site, but alumina exerts a higher electrostatic attraction for Zn2+ than for Co2+ species. In solutions containing both Zn2+ and Co2+ ions the adsorption of Zn2+ was practically unaffected, whereas the adsorption of Co2+ was markedly inhibited and only a small fraction of Co2+ ions, probably as a different complex, was adsorbed. Characterization by X-ray diffraction, diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and metal-extraction and zeta-potential measurements of the calcined forms of selected samples of the three Co-, Zn- and CoZn-Al2O3 series showed that the adsorbed and occluded Co2+ species at Co loadings below 1.2 wt %(the saturation coverage, derived from the adsorption isotherm) interact strongly with alumina forming a surface spinel. Above the Co saturation coverage both XPS and zeta-potential measurements indicated a significant change in Co dispersion, suggesting formation of small Co3O4 clusters, which, according to the DRS results, is consistent with a high relative proportion of octahedrally coordinated Co. In contrast, the Zn species in the Zn-Al2O3 calcined samples appeared uniformly dispersed, predominantly as a surface spinel, throughout the explored composition range. Results of selective extraction of Zn by HCl and ammonia solutions for samples calcined at different temperatures showed clearly that the non-adsorbed Zn filling the pores upon calcination at 623 K gives rise to ZnO, and then at 823 K undergoes a solid-state reaction with the free alumina surface.