Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 81

Figure 4.22: First-order plots; ( )HDV,( ) hydrodenickelization (HDNi), ( )
hydrodeasphalting (HDAs) [From ref. 207 . Reprinted with permission].
its separation from the porphyrin skeleton. Thus, in spent catalysts and fresh presulfided
catalysts, only a partial sulfidation, with the V O entity still being present, was observed
[51,234] . The V O entity can interact both with the uncovered support and catalytically active
metals deposited on the support [254] . According to Loos et al. [255] , the V O was still
present, essentially unaltered in spent NiMo/Al 2 O 3 catalyst, although additional four sulfur
atoms contributed to the average coordination polyhedron of V. This suggests that the
sulfidation of V was incomplete compared with that of Mo and Ni/Co. The same was
confirmed by Janssen et al. [256] . Thus, during the sulfidation of the Al 2 O 3 impregnated with
the ammonium metavanadate, the complete conversion to V 2 S 3 required temperature of
1273 K, whereas at 673 K, most of the V was still present as an oxysulfide.
In an extreme case, even the unconverted porphyrins in deposits on catalyst surface may be
present. This is supported by several potential forms of the interaction of porphyrins with
catalyst surface, which have been identified. For example, they may include a donor-acceptor
bonding, in which the system of the porphyrin ring is the donor and the Bronsted and/or
Lewis sites are acceptors [257] . It should be noted that this information was obtained under
low temperature conditions. There is little experimental evidence confirming the presence of
unconverted porphyrins in coke. It is believed that because of the complex nature of deposits, a
convincing identification of porphyrin structures in coke on the spent catalysts from
hydroprocessing of heavy feeds would be a rather challenging task.