Page 53 - Principles of Catalyst Development
P. 53
STRUCTURE OF CATALYSTS 39
effectively "doping" the electronic structure and producing a "CoMoS"
active site. (73)
The promotion effect of the CoMoS site appears to occur through
increased hydrogen activation, which facilitates removal of sulfur atoms
after cleavage of C-S bonds on exposed molybdenum ions. (74) Cobalt
incorporated into the support may also act as a structural promoter by
enhancing dispersion of the sulfided species. (75)
With heavier feeds, potassium is added to poison acid sites and inhibit
coking. Phosphorous is also used, presumably to inhibit acidity but also to
promote dispersion of the molybdenum compounds. (76)
One additional feature occurs when this catalyst is used with extremely
heavy residua feedstocks. With proper preparational techniques, bimodal
pore distributions are produced in which constrictions at the openings
inhibit diffusion of large asphaltenic, coke-producing molecules, whereas
smaller sulfur compounds have easy access. (77) This effectively reduces
excessive coking which otherwise occurs. (69)
In this example, we see how the catalyst designer "tailors" the com-
ponents of the catalyst to meet process needs. This is not usually a spon-
taneous, creative event but one that evolves with experience. How this
occurs and how catalysts are developed is considered in the next chapter.