Spatially resolved spectroscopy on the preparation of CoMo/Al2O3 hydrodesulphurization catalysts

Abstract

Supported catalysts form an important class of functional materials, since they are widely applied in oil refining and the manufacturing of both bulk and fine chemicals. In these systems, the active phase, consisting of a metal, metal-oxide or metal-sulfide is dispersed into the pore system of mm-sized support bodies. Due to the large scale on which these materials are used, the means employed for their preparation need to be reasonably straightforward. In general, support bodies are impregnated with a solution containing metal-ion complexes, after which drying, calcination and sometimes reduction or sulphidation is carried out to obtain the active catalyst. For the activity of these catalysts, both the macro-distribution and the nature of the active phase are important parameters Hydrodesulphurization catalysts, which are used to remove sulphur-containing compounds from fuel streams, constitute an important family of supported catalysts. In these systems, the active phase consists of MoS2, while Co is often added as a promotor. Enable to meet future demands concerning the maximum amount of sulphur in transportation fuels, the activity of these catalysts needs to be improved. This thesis describes the development and application of micro-spectroscopic techniques that allow one to characterize CoMo/Al2O3 catalyst bodies at the different stages of their preparation. With the aid of Raman micro-spectroscopy, it proved possible to determine the nature and distribution of Mo(VI)-complexes inside Mo/Al2O3 catalysts bodies throughout the preparation process. After impregnation, the transport of different Mo(VI)-complexes inside support bodies was envisaged. It was illustrated that after impregnation a long ageing time may be required to obtain a homogeneous distribution of Mo(VI)-complexes inside the support bodies. Furthermore, it was found that an irreversible reaction of Mo7O246- with the Al2O3 support resulted in the formation of crystalline Al(OH)6Mo6O183- after impregnation. It was shown, that the formation of this compound resulted in the formation of bulk MoO3 after calcination, a poorly dispersed MoS2 phase in the final catalyst and a decrease in the hydrodesulphurization activity. To monitor the speciation of Co(II)-complexes in the catalyst bodies, UV-Vis-NIR micro-spectroscopy was developed. With the aid of this technique, the distribution of a H2PMo11CoO405- heteropolyanion inside Al2O3 bodies could be controlled. Finally the limitations and possibilities of MRI in catalyst preparation studies were evaluated. The non-invasive character of this technique implies that samples no longer need to be bisected before analysis. Using MRI, it proved possible to derive quantitative distribution profiles of paramagnetic metal-ion complexes inside catalyst bodies. Ultimately, the distribution of Co(II)-complexes inside Co/Al2O3 extrudates could be varied from egg-shell, through egg-white and egg-yolk to homogeneous by simple means. The micro-spectroscopic techniques described in this thesis are generally applicable and provide the possibility to study the preparation of supported catalyst bodies in great detail. A better understanding of the physical and chemical processes that take during the preparation of these systems may lead to a more controlled synthesis of industrial supported catalysts.