Page 19 - Principles of Catalyst Development

CATALYTIC FUNCTIONS 5
selectively into methane (Nil AI 20 3 ),(4) paraffinic hydrocarbons (Fe/kiesel-
guhr),(S) alcohols, aldehydes, and acids (Co/Th0 2 ),(6) or methanol
(Cui ZnO).(7) The catalyst becomes a useful tool for manipulating selectivity.
The last point concerns the question of permanent involvement by the
catalyst. Ideally, the catalyst is unchanged by the reaction. In practice this
is not true. Since it is itself a reacting substance, the catalyst suffers from
irreversible chemical and physical changes, which decrease its ability to
perform. Within the time frame of the reacting molecules, these changes
are small. But as the process time continues and the catalyst experiences
many billions of these events, deactivation becomes significanL (8 )
We have extracted from our definition an appreciation for three catalytic
functions: activity, selectivity, and deactivation. Which is the most impor-
tant? This is difficult to answer generally since each application has its own
set of specific needs.
Certainly, for the reaction to proceed, the catalyst must have chemical
activity. Beyond that, increasing activity can have several benefits:

1. Higher rates for the same conditions.
2. Equivalent rates but with higher throughputs or smaller reactors.
3. Equivalent rates at lower temperatures or pressures where equili-
brium yields increase, operations become easier, deactivation
becomes less, or selectivity improves.

Selectivity becomes a factor in the presence of mUltiple reactions. These
are generally of the types
k
Parallel: R~D (1.5)

k
R~U

Or consecutive:

k" k, k" k"
R --+ 0 --+ U and R --+ U --+ 0 (1.6 )

With catalyst control, the ratio kDI ku may be increased to optimize
the desirable product D. Benefits are obvious and include greater yields
of D and less extensive separation operations. An especially important case
occurs when U is a deactivating agent such as "coke" or carbonaceous
deposits.