A geometrical model of the active phase of hydrotreating catalysts

A geometrical model of sulphided unpromoted and promoted molybdenum or tungstensupported on alumina catalysts has been developed on the basis of two hypotheses which can be deduced from literature data: these are the presence of small patches of supported MoS2(WS2) and the importance of the edge sites located on these patches. The activity and specific activity for a given reaction, specific sulphur content (S/Mo), amount of chemisorbed oxygen (O/Mo) and the promotor effect could be related to composition parameters by simple algebraic expressions. The predictions of the model have been compared with experimental results previously published by different authors. The good fit obtained indicates (i) the importance of the geometric factors and (ii) the edge site specificity on unpromoted and promoted MoS2 (WS2) based catalysts, and leads to a description of the supported active phase as a distribution of very small (edge decorated when promoted) two-dimensional MoS2 (WS2) patches (rhombohedral or hexagonal shape) with a size depending on the carrier loading. The hydrodesulphurization of thiophene and hydrogenation of propene, cyclohexene or toluene rate limiting steps as well as 02 chemisorption are found to occur only on edge sites.