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142 Y. Traa et al.
sion, e.g., by increasing the residence time or the reaction temperature, the
monobranched alkylcarbenium ions undergo another rearrangement. The
resulting dibranched carbenium ions desorb and appear in the product as di-
branched isoalkanes. Upon further increasing the conversion, the dibranched
cations rearrange once more into tribranched ones. These can undergo the
very rapid type A ²-scission [120]. Its salient feature is that ²-scission starts
from a tertiary and leads again to a tertiary carbenium ion. Tertiary carbe-
nium ions are much more stable than secondary or primary carbenium ions.
Indeed, in the absence of shape selectivity, i.e., using a zeolite or any other
catalyst with sufficiently spacious pores, the mechanism of hydrocracking of
long-chain n-alkanes proceeds via this route: Several skeletal rearrangements
are followed by type A²-scission; besides, there is a smaller contribution of
type B ²-scission [120] of dibranched carbenium ions, involving tertiary and
the less stable secondary carbenium ions.
4.3.1
Isomerization and Hydrocracking of Long-Chain n-Alkanes
and Light (C7) Cycloalkanes  The Refined or Modified Constraint Index, CI"
Whereas hydrocracking of n-alkanes proceeds via highly branched and,
hence, relatively bulky alkylcarbenium ions in large- and extra-large-pore
zeolites, the tribranched precursor ions of the favorable type A ²-scission
cannot form under the steric constraints imposed in medium-pore zeolites.
The system is then forced into alternative routes via less bulky intermediates,
i.e., the narrower the pores, the higher will be the contributions of ²-scissions
involving less stable secondary or even primary carbenium ions. This shift
in the hydrocracking mechanism with decreasing pore width brings about
a large number of selectivity changes in the hydrocracked product.
Striking shape selectivity effects do not only occur in hydrocracking, but
also in skeletal isomerization, which precedes hydrocracking. If n-decane is
converted on a bifunctional catalyst with sufficiently spacious pores, such as
zeolite Pt/Ca-Y, the product mixture obtained at low conversion consists of
all possible isodecanes with one branch, i.e., 2-, 3-, 4- and 5-methylnonane,
3- and 4-ethyloctane as well as 4-propylheptane [121]. As the catalyst pores
become narrower, the bulkier isomers cannot be formed any more inside
the pores or cannot escape from there. On Pt/H-ZSM-5, for example, neither
4-propylheptane nor the two ethyloctane isomers are formed [120, 122].
An even more subtle shape selectivity effect concerns the formation of the
four isomeric methylnonanes. In the absence of shape selectivity and at low
conversions, i.e., in the kinetic regime, 2- and 5-methylnonane form about
half as fast as 3- and 4-methylnonane. This can be readily understood in
terms of the branching mechanism via protonated cyclopropanes, cf. [121].
2-Methylnonane is the kinetically preferred isomer in 10-membered-ring
zeolites. Based on the observation that, with decreasing pore width of the
Characterization of the Pore Size of Molecular Sieves 143
zeolite, the amount of 2-methylnonane formed from n-decane at low conver-
sions increases relative to the other methylnonanes, the Refined or Modified
Constraint Index, CI", was defined as
CI" a" Y2-methylnonane/Y5-methylnonane (atYisodecanes H" 5%) (5)
(Y =yield)
in the isomerization of n-decane [123]. It should be noted that the reaction
pathways, on which Mobil s original Constraint Index (cf. Sect. 4.2.1) and the
Modified Constraint Index are based, are entirely different. The only feature
both indices have in common is that their numerical values increase with
decreasing pore size of the zeolite [67].
CI" values taken from the literature are presented in Fig. 8. It is evident that
the CI" values for 10-membered-ring zeolites extend over a relatively broad
range, namely from ca. 2.7 to 15; hence, this is the range where the Modified
Constraint Index is particularly useful. On the other hand, only a very narrow
range, namely from ca. 1 to 2.3, is available for 12-membered-ring zeolites. It
should, however, be kept in mind when using this or other indices, that the ac-
curacy of indices determined from catalytic tests is limited. For example, for [ Pobierz całość w formacie PDF ]