Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 55

stable to the treatment at 623 K, slightly unstable at 673 K and remarkably destroyed above
773 K. It was suggested that at high temperatures, part of the Co atoms is detached from the
Co-Mo-S structure, leaving some MoS 2 edge sites vacant, while the detached Co atoms form
catalytically inactive Co-sulfide clusters. The catalyst support influenced the thermal stability
of Co-Mo-S structure; Co-Mo-S structure supported on Al 2 O 3 was thermally more stable than
that supported on either boron modified Al 2 O 3 or SiO 2 . Quantitative calculations suggested
that at 873 K about 30% of the Co-Mo-S structure supported on Al 2 O 3 was decomposed after a
2-h treatment in 10% H 2 S/H 2 stream, in contrast to about 50% of that supported on
boron-modified Al 2 O 3 and SiO 2 .
Recently, Guichard et al. [159] investigated the stability of Co-Mo-S and Ni-Mo-S phases in
working state using a variety of techniques such as X-ray photoelectron spectroscopy (XPS),
TEM, energy dispersive X-ray (EDX), density functional theory (DFT), calculations and
catalytic measurements. They concluded that part of Co and Ni were destabilized and
segregated from the edges of the Co-Mo-S and NoMoS crystallites under the reaction
conditions (high temperature and highly reductive environment).
The chemical composition of the original active phase on catalyst may be gradually modified
by the metals deposited from heavy feeds during the operation. The effect of deposited V and
Ni on the catalyst activity is rather complex. Moreover, the deactivating patterns will change
with progressive deposition with time on stream. For example, the deposits had beneficial
effects on HDM reaction as it was demonstrated by a gradual increase in catalyst activity up to
maximum attained between 15 and 20 wt.% of the deposited metals [154,160] . Then, the
HDM activity began to decline with further increase in the metal deposition. Almost certainly,
the activity decline resulted from the change in pore size distribution, which affected the
diffusion of reactant molecules into the catalyst pores. Therefore, for an active HDM catalyst,
porosity may be at least as important parameter as is its chemical composition. Thus, industrial
experience showed that about 5 wt.% of MoO 3 in the absence of promoter on the -Al 2 O 3
support possessing suitable porosity resulted in the active catalyst for HDAs and HDM. Such
catalysts have been used industrially. Other catalyst functionalities, e.g., HDS and HDN, were
influenced by the metal deposits differently than HDM. This resulted from the transformation
of the Co(Ni)MoS phase into the VMoS phase, which was less active than the former [161] .
Moreover, it was reported that the unsupported V 2 S 3 -sulfide alone exhibited some activity for
hydrogenation (HYD) and HDS [162-164] . However, this was only demonstrated for model
compounds rather than for heavy feeds.
4.2 Deactivation by Coke and Nitrogen Bases
For distillate feeds, coke deposition and poisoning by N-bases are the main causes of catalyst
activity decline. To various extends, the deactivation by coke and N-compounds occur in
parallel. For N-compounds, deactivation results from their strong adsorption on the catalytic