Page 218 - Principles of Catalyst Development

CATALYST DEACTIVATION 207
Furthermore, the surface can be cleaned with sulfur-free purge gas. Regen-
erative processes for chlorine- and sulfur-poisoned low t1emperatures shift
catalysis are available, but there is some doubt as to desirability.
In general, the best approach to poisons of this type is to pretreat the
feed. This is done in three ways: (1) chemical treating, which is expensive
and may introduce other contaminants; (2) catalytic processing, which is
effective for poisons with organic origins; and (3) guard chambers. In guard
chambers, a cheaper adsorbent precedes the main reactor. Sometimes this
is a separate unit, such as sulfur-removing zinc oxide beds in natural gas
reforming. On other occasions it may be part of the reactor bed and the
front section is sacrificed as a scavenger, using the same catalyst or a cheaper
substitute. Some very ingeneous guard applications are found in automobile
exhaust catalysts, as discussed in previous sections. (255)

8.3.7.2. Oxides

Semiconducting oxide catalysts owe their activity to electron accepting
or donating surface sites, with specific surface geometry and favoring redox
reactions. Any molecule which adsorbs strongly const;itutes a potential
poison. Unfortunately, not too much attention has been given to this subject
in the literature. (256) For example, poisoning of hydrogenation or hydro-
genolysis activity in supported oxides and sulfides has not been considered
to any extent. Other types of deactivation take precedence. Other than these
general remarks, very little information can be added.

8.3.7.3. Solid Acids
Poisoning of acid sites is straightforward. Basic constituents are
required to neutralize acidity. This is found in alkali and alkaline earth
compounds and in basic organic molecules. Process poisoning by alkaline
and alkaline earths is rare. These materials are added as deliberate promoters
to remove acidity but are not normally encountered in process streams in
the basic form. One exception is Na + ions, encountered in steam used for
stripping cracking catalysts and other purposes.
Basic organics are primarily nitrogen-containing molecules that are
abundant in petroleum feedstocks. In a typical straight run stock, 25% -35%
of the nitrogen compounds are basic. Basic and nonbasic types are shown
in Table 8.8.
The susceptibility of solid acids to these poisons correlates well with
the basicity of nitrogen compounds, as shown in Fig. 8.16. In practical
feeds, a wide range and variation of types are present.