Page 211 - Principles of Catalyst Development

200 CHAPTER 8
Aromatic production from dehydrocyclization decreases with larger
crystallite diameter, dc, whereas isomerization increases and hydrocracking
remains approximately the same. Thus, selectivity to the high-octane product
decreases with sintering.
In conclusion, sintering is primarily a thermal process of pore collapse
and crystallite growth, although chemical interactions with reactants may
also play secondary roles. It is normally irreversible but may be effectively
controlled with catalyst promoters.

8.3.7. Poisoning

Poisoning is a chemical effect, although temperature may influence
sensitivity. A poison is any agent that reacts permanently with an active
site. Exceptions are coking and catalyst impurities. Coking has separate
characteristics and is considered as a distinct type of deactivation. The effect
of impurities in the catalyst is a thermal phenomenon and has been discussed
earlier. Poisoning may be reversible or irreversible, regenerable or not,
depending on the type of poison, catalyst, and service. Table 8.4 shows
reaction schemes which involve poisons.
Most poisons are type (1), i.e., independent compounds present in the
feed, perhaps in minute quantities, that deactivate the site with a mechanism
different from the main reaction. Examples are also found of types (2) and
(3), where either parallel or series reactions generate side products that
poison the sites. These mechanisms may also be classified as examples of
kinetic inhibition but are considered poisoning if adsorption on the site is
irreversible. In situations where multiple sites are involved (for example,
dual-functional catalytic reforming), poisoning patterns become more
complex.
Chemically, a poison is any compound resulting in strong adsorption.
I n practice, many different types are encountered. Examples are listed in
Table 8.5. Since poisoning is an act of adsorption, two features are important.

TABLE 8.4. Origin of Catalyst Poisoning"
---_ _------- ------------ ----- -----------
..
Type of reaction
(II R ~ P
X ~ sox
12 ) R -, 1',
R -.. p. -+ X -+ sox
(3) R -~ 1', ~ p, ~ X -+ sox

! R, rea(.'ranL P, product; X. poison; S -X, poisoned site.