Page 92 - Principles of Catalyst Development
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CATALYTIC MATERIALS 79
TABLE 4.12. Effect of Faujasite
Cation on Activity for
Cumene Dealkylation"
Cation Relative activity
Na+ 1.0
BaH 2.5
Sr H 2.0 x 10
Ca 2 + 5.0 x 10
Mg2+ 1.0 X 10'
NiH 1.1 X 10 3
La' 9.0 X 10 3
H+ 8.5 X 10 3
SiOr AI,03 = 1.0
" Reference 141.
There are three possible types of shape and size selectivity effects, as
shown in Fig. 4.25. First, the reactant molecules may be too large to enter
the cavities. Comparison of Tables 4.11 and 4.13 shows that all of the
molecules access faujasite structures. Only molecules larger than penta-
methyl benzene are excluded. Early examples of shape and size selectivity
were almost completely limited to small openings and normal versus
branched paraffins. For example, n-hexane was selectively cracked in the
presence of 3-methylpentane over zeolite A catalysts. Other, more subtle,
effects may occur. The diffusivity of trans-butane-2 is 200 times larger than
that of cis-butene-2 in zeolite CaA. By adding Pt to CaA, selective hydroge-
nation of trans-butene-2 occurs.(t34)
TABLE 4.13. Critical Diameter of Hydrocarbons
Hydrocarbons Size (nm)
n-Paraffins 0.45
Methyl-paraffins 0.57
Dimethyl paraffins
Benzene
Toluene 0.63
p-Xylene
o,m-Xylene
1,2,4-Tri-methyl benzenes
1,2,4,5-Tetra-methyl benzene naphthalene 0.69
1,3,5-Tri-methyl benzene
Penta-methyl benzene 0.78