Page 226 - Principles of Catalyst Development

CATALYST DEACTIVATION 215

I \
C C
\ /
C-CHHH·C C -- C
~~~~//~f/~~~~-L--~-~
/DEHYDROGENATION
/// CATALYST //
////////////,
Figure 8.22. Carbon formation on dehydrogenation catalysts.

dehydrogenation surface to fuel the reactions discussed for acid catalysts.
Both types of coke are present but in much lesser amounts than for cracking,
since acidities are lower and hydrogen from dehydrogenation helps keep
the catalyst clean. The carbon-forming tendencies of these catalysts correlate
well with hydrogenolysis activity, as expected, since C-C bonds rupture in
the presence of hydrogen. Therein lies the best possibility for control, since
we have seen many examples ofhydrogenolysis suppression through catalyst
modification.
The characteristics of dehydrogenation coking vary with the chemistry
of the catalytic process. Important features are best considered with three
important examples: (1) catalytic reforming, (2) hydro treating, and (3)
metal contamination.

8.3.8.2a. Catalytic Reforming. Catalytic reforming combines two car-
bon-producing functions: dehydrogenation and acidity. For many years,
processes that upgraded octane number with hydrodecyclization, isomeriz-
ation, and hydrocracking of straight chain paraffins and naphthenes used
dispersed platinum on acidified alumina. Much research has been done on
carbon formation by these metal crystallites, and a clearer insight is begin-
ning to emerge. (282) Carbon is produced through hydrogenolysis at sites
that correlate well with low index positions, such as faces, edges, and
corners. Carbon diffuses over the surface as a type of surface carbide and
through the bulk as carbon atoms reaching the interface between platinum
and support. Some of it progresses to acid sites to begin the polymerization
process leading to pseudographite. Thus the metal "feeds" the coke precur-
sor to acid sites and carbon forms in regions around the crystallites. In
extreme cases, ribbons of graphitic carbon grow, supponing the platinum
crystallite at its tip. Ultimately, these ribbons and patches cover the surface
and block pores. (283)
Early workers in catalytic reforming discovered that a small amount
of sulfur poisons hydrogenolysis sites and reduces coking. Studies with