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