Page 94 - Principles of Catalyst Development
P. 94
CATALYTIC MATERIALS 81
RELATIVE
CRACKING
C 7 PARAFFIN RATE
1.00
0.52
0.38
0.09
Figure 4.26. Relative cracking rates of heptane isomers over ZSM_5. i140 )
cavities, so disproportionation is spatially hindered and selectivity
improved. Another example is aromatic formation from methanol. Although
this reaction occurs readily over several acidic zeolites, only ZSM-5 limits
product to high octane aromatics.( 140)
When metal components are added through ion exchange, subsequent
reduction gives new dimensions to these applications. Transition ions inside
cavities are often difficult to reduce and are stabilized against agglomeration.
This is an advantage, since extremely small, molecular-sized clusters of
metal atoms may be produced, yielding activities and selectivities often
found in organometallic homogeneous catalysis. (138) As more is known
about the properties of these small metallic particles, many innovative uses
will suggest themselves. Already, the principle of metal-zeolite combinations
is commercially proven in hydrocracking, which combines both the features
of hydrogenation and selective cracking to produce better gasoline com-
ponents with less coking.(57)
Zeolites, with their vast possibilities of existing and potential structures,
with shape and size selectivity, and with precise control of acid and other
TABLE 4.14. Relative Diffusivities of Xylenes in ZSM-S
Hydrocarbon Relative diffusivity at 350°C
p-Xylene 1000
m,o,m·Xylene