Hydroprocessing of Petroleum - Spent Hydroprocessing Catalysts - page 26

both edges. The mechanism involving brim sites in HYD allows the understanding of many
observations, which were difficult to explain using previous models. Since brim sites are fully
coordinated sites, they do not adsorb H 2 S. This explains lack of inhibition of HYD reactions
by H 2 S. The brim site model also explains the lack of steric hindrance of alkyl substituents in
the HYD pathway of molecules, such as 4,6-DMBT. The brim sites are very open sites and,
therefore, they allow the adsorption of the refractory sterically hindered molecules, which
need to be removed in the ultra low sulfur diesel (ULSD) production. The brim sites and their
neighboring protons can interact strongly with basic N-containing molecules. This interaction
is stronger than the interaction with simple aromatic compounds like benzene [66] . In this way,
the observed strong inhibition of the HYD pathway by basic N-compounds may be explained.
It should be noted that the introduction of “brim” sites model represents a highlight of
hydroprocessing catalysis in recent years, although most of the observations were made for a
simple molecule, such as thiophene. However, because of unique approach, the authors [62,63]
were able to describe the most early stages and intimate state of the reactions of thiophene.
This has never been achieved before. The information on the reactions of more complex
molecules is desirable to enhance the validity of the “brim” sites model. Nevertheless, it
appears almost certain that during hydroprocessing, several types of active phase may facilitate
catalytic reactions occurring either in parallel or consecutively.
3.3.1.3 Co-Mo-C(S) Phase
The presence of carbon on catalyst and conditions encountered during hydroprocessing
support the presence Co(Ni)-Mo(W)-S phase. The same was supported by the study of
Wenetal. [67] who showed that formation of the Mo 27 S x C y cluster was thermodynamically
favorable. In this case, the edge sulfur atom on MoS 2 could be readily replaced by carbon
atom. Similarly, Chianelli and Berhault [68] suggested that carbon could play an important
role in stabilizing active phase. They proposed that the excess of sulfur on the surface of MoS 2
could be replaced by carbon to give stoichiometric MoS x C y phase. The clusters with three
different S/C (i.e., 1.83, 1.68, and 8.27) were proposed [69] . According to Kasztelan [70] , the
replacement of sulfur with carbon on the edge of MoS 2 can be accommodated
crystallographically. Therefore, the Co(Ni)-Mo(W)-S-C phase may be part of the overall
hydroprocessing catalysis, particularly for the carbon supported catalysts. In this regard, the
recent article published by Kibsgaard et al. [71] should be noted. These authors used STM
spectroscopy to study the MoS 2 nanoclusters supported on graphite. A limited dispersion of
MoS 2 clusters was achieved on pure graphite. However, a high dispersion was observed after
introduction of small density defects. It is speculated that some form of bonding with the
surface, presumable involving Mo C bonds, was responsible for the increased dispersion.
During operation, a modifying effect of carbon from coke on catalytically active phase cannot
be ruled out. This indicates on the coexistence of the Co(Ni)-Mo(W)-S-C phase and
Co(Ni)-Mo(W)-S phase and potentially other phases (e.g., brim sites). Therefore, because of