The Nature of Hydrodesulfurization Active Sites

The need to meet more stringent standards limiting the sulfur content of gas oils urges a deeper understanding of the mechanism by which sulfur-containing compounds are destroyed over hydrodesulfurization (HDS) catalysts. Thiols and sulfides are readily treated by the Co- and Ni-promoted MoS2 catalysts currently employed, but satisfying the imminent restrictions will require the removal of the most refractory species, mainly alkyl-substituted polyaromatic thiophenes. Unfortunately, the nature and action of the catalyst’s active sites is not well known. We have undertaken density functional calculations on models of pure and Co-promoted MoS2 along with relevant organic substrates to determine (1) the structural and electronic factors indispensable to the “CoMoS” active phase, (2) the mechanisms by which thiophene and its derivatives react, and (3) how alkyl substituents promote and inhibit different branches of the reaction network.

Periodic Mo4-nConSx (n=0,1,2) models were used to study the structure and energy of Cosubstituted MoS2 edges. Systematically varying the amount of sulfur in the model edges determined the geometry and concentration of vacancies as a function of temperature and H2/H2S ratio. All models showed that the energy required to abstract a sulfur atom from the crystallite edge increases as more sulfur atoms are removed. As expected, the presence of cobalt promotes the desorption of sulfur. So, edges display an equilibrium concentration of vacancies which increases with temperature, H2/H2S ratio, and concentration of cobalt. Two hydrogen atoms were added to the Mo2Co2Sx model (as -SH groups bridging cobalt atoms in the (0101) edge) and the number of sulfur atoms varied again. The presence of adsorbed hydrogen had little effect on the predicted number of vacancies.

Step-by-elementary-step mechanistic studies are underway using cluster models of the MoS2 (0110) edge. Energies of adsorption calculated for dibenzothiophene, 4,6-dimethydibenzothiophene and hexahydro-4,6-dimethyldibenzothiophene correlate with their ease of desulfurization. The strengths of adsorption of pyridine and the sterically hindered 2,6-dimethylpyridine relative to the thiophenes support that only pyridine poisons HDS activity.

Personnel: Robert (Smith) Nielsen