Page 48 - Principles of Catalyst Development
P. 48
34 CHAPTER 2
There are situations where support acidity has a positive influence,
influencing the main reaction. The support adds dual functionality to the
overall catalysis, as best demonstrated with catalytic reforming. (56) The
objective in this process is to convert low octane components of naphtha,
typically normal paraffins and naphthenes, into high-octane iso-paraffins
and aromatics. Low loadings of Pt-type metals on AI 20) are used for this
purpose. Metallic Pt dehydrogenates naphthenes to aromatics but cannot
isomerize or cyclize normal paraffins. This is accomplished through the
acidic function of the support, as shown for n-hexane:
(2.4)
H'
n-C;; - i-C;;
AI 2 0 J
Neither Pt or Al 20 3 promotes paraffin isomerization, but AI20)· acid
sites isomerize normal olefins. In a three step sequence, n-C 6 is dehydroge-
nated by Pt to n-C;;, which migrates to the AI 20) and isomerizes to i-C;,
then hydrogenated by the Pt to i-Co. Platinum and AI 20) must be in intimate
contact and the AI 20) have sufficient acidity. Chlorine is added during
activation to generate the needed acidity. (56) Dual functionality is indeed
an elegant demonstration of catalyst design. Another important example is
hydrocracking. (57)
Catalytic chemists have long suspected that the properties of the active
component may be affected by contact with the support. Indeed, as we shall
see, there are so many ways in which supports exert influences that it is
not surprising that these were noted. It was not until recently that researchers
were able to unravel some of the these effects with more sophisticated
characterization methods. For example, measured areal rates of ethane
hydrogenolysis showed the patterns in Table 2.4. t5K )
The most direct influence of the support is on dispersion and
morphology. t5~1 It is well known that nickel is better dispersed on Si0 2 than
AI 20), and the shape of crystallites may also be affected. Data in Table 2.4
do not address the question of crystallite size dependence, which has a
much firmer foundation. t60 ) Surface contamination is always present. Com-
mercial reagents used in support preparation may contain impurities or
provide ions which remain with the oxide. Subsequent deposition of active
components may incorporate these ions as poisons. Acidic sites vary on
different supports and during initial exposure to reagents produce car-
bonaceous deposits which in turn deactivate nickel 1(1 ) sites. A possibility
that cannot be overlooked is spillover, a phenomena in which a reactive