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a propyl side chain (4-, 5- and 6-propylundecane) and even one with a butyl
Characterization of the Pore Size of Molecular Sieves 145
side chain (5-butyldecane) among the monobranched product isomers and
that such bulky isomers may be suitable for detecting subtle differences in the
pore systems. Indeed, such differences were found: 5-butyldecane was com-
pletely lacking in the product obtained on Pd/H-ZSM-20 at low conversion
(Xn-tetradecane H" 2%), while it did appear at the same conversion on zeolite
Pt/Ca-Y. Moreover, the three isomeric propylundecanes appeared in signifi-
cantly different distributions on both bifunctional catalysts [128].
Martens and co-workers [129, 130] went one step further and suggested the
use of n-heptadecane as probe molecule for the characterization of large-pore
zeolites. In this case, however, the information was acquired from the product
selectivity of hydrocracking, rather than from the distribution of isomeriza-
tion products of varying bulkiness, as in the test with n-tetradecane proposed
by Weitkamp et al. [128].
Very recently, the isomerization of methylcyclohexane was proposed as
a test reaction to simultaneously probe the acid site density, the acid site
strength and the pore size [131]. The reaction observed is the ring contraction
of methylcyclohexane to ethylcyclopentane and the five isomeric dimethylcy-
clopentanes. The authors claim that the selectivity ratio of the bulkier trans-
1,2-dimethylcyclopentane to the less bulky trans-1,3-dimethylcyclopentane is
a measure for the pore size due to product shape selectivity effects. The ratio
was found to vary in the range of 0.1 to 0.5 for medium-pore zeolites and from
1.2 to 2.4 for large-pore zeolites and amorphous solid acids, the equilibrium
value being 1.8 to 1.9.
At higher reaction temperatures, Raichle et al. [132] demonstrated that the
selectivity for ethane, propane and n-butane (the mixture of these alkanes is
a high-quality feedstock to steamcrackers for the manufacture of ethene and
propene) during the hydroconversion of methylcyclohexane on acidic zeolites
increases significantly with increasing spatial constraints inside the zeolite
pores and correlates well with the Modified Constraint Index. This example
shows that the Modified Constraint Index data are capable of providing valu-
able quantitative information concerning the shape-selective performance of
zeolites with 10-membered-ring pores in a completely different reaction. This
is particularly noteworthy in view of the fact that the CI" data have been
collected on bifunctional forms of the zeolites, whereas Raichle et al. [132]
employed their zeolite catalysts in their monofunctional acidic forms.
In conclusion, shape-selective test reactions based on the isomerization
and hydrocracking of long-chain n-alkanes are now well established for char-
acterizing the pore width of medium-pore zeolites. While a variety of quan-
titative criteria have been proposed, the most popular one by far is the
Modified Constraint Index CI", which is based on the selectivity of n-decane
isomerization at low conversion on a bifunctional form of the zeolite. Among
the features that render the determination of CI" so straightforward is the
lack of catalyst deactivation. Expectedly, CI" is particularly sensitive in a cer-
tain range of pore widths only, i.e., where the pore width strongly influences
146 Y. Traa et al.
the selectivity of isomerization and hydrocracking. As clearly shown in Fig. 8,
this is the region of 10-membered-ring zeolites. By contrast, CI" is of little use
in the range of 12-membered-ring zeolites. Fortunately, there is another index
based on a different test reaction, which is ideally complementary to CI". This
is the Spaciousness Index, SI.
4.3.2
Hydrocracking of C10 Cycloalkanes such as Butylcyclohexane 
The Spaciousness Index, SI
Isomerization and hydrocracking of cycloalkanes follow essentially the same
mechanistic rules as the reactions of alkanes. The cycloalkane undergoes sev-
eral steps of skeletal isomerization until a structure is reached from which
the favorable type A ²-scission can start. In comparison with alkanes, how-
ever, the carbon-carbon bond rupture inside the cycloalkane ring proceeds
more sluggishly, which is best interpreted in terms of an unfavorable or- [ Pobierz całość w formacie PDF ]