A CO-57 MOSSBAUER EMISSION SPECTROMETRIC STUDY OF SOME SUPPORTED COMO HYDRODESULFURIZATION CATALYSTS

A suite of 11 CoMo/Al2O3 (and one COMO/SiO2) catalysts has been prepared employing four preparation routes, viz. one sequential-impregnation route and three different coimpregnation routes. Speciation of the Co present in the oxidic precursors (octahedral vs tetrahedral Co) and in the activated, sulfided catalysts (CoMoS, Co9S8, and unsulfided Co) was effected with the aid of 57Co Mössbauer emission spectroscopy (MES). A linear relation between the thiophene-hydrodesulfurization (HDS) activity and wt% Co-in-CoMoS was observed for each preparation route, but no unique correlation was found to exist. This was traced to the fact that the preparation routes differ in the amount of CoMoS I and CoMoS II they produce in the activated catalyst. Although these two phases differ in specific activity, CoMoS 11 being twice as active in thiophene HDS as CoMoS I, they cannot be distinguished on the basis of their Mossbauer parameters. It appears that octahedral Co is easier to sulfide than tetrahedral Co, but a substantial fraction of the latter is also found to be capable of entering CoMoS upon sulfidation. The reduced effectiveness of high-loading catalysts is traced to their being prone to COMoO4 formation in the calcination step. A rationalization of this behavior is offered. © 1992.