Characterization of the Pore Size of Molecular Sieves Using Molecular Probes

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While the Modified Constraint Index is now widely employed for charac-
terizing 10-membered-ring molecular sieves, the precise origin and nature of
the shape selectivity effects on which it is based have not yet been fully elu-
cidated. From the finding that the diffusion coefficients of the four isomeric
�%
methylnonanes in H-ZSM-5 and H-ZSM-11 at 80 C, i.e., at a sub-catalytic tem-
perature, were practically identical [122], the strongly preferred formation of
2-methylnonane was attributed to restricted transition state shape selectivity
rather than to mass transfer shape selectivity. Essentially the same conclusion
was drawn by Martens and Jacobs [126] from n-decane isomerization studies
with Pt/H-ZSM-5 of different crystal size. Increasing the crystal size from 1 to
ca. 15 �m brought about only a minor increase in CI" from 7.5 to 9.4, and this
was considered to be essentially consistent with the earlier interpretation of the
favored formation of 2-methylnonane in 10-membered-ring zeolites.
Fig. 8 Refined or Modified Constraint Indices for various zeolites. Data from [93, 123,
124]. The numbers above the arrow denote the numerical value of CI"
144 Y. Traa et al.
Webb III and Grest [127] recently used molecular dynamics simulations to
estimate the self-diffusion coefficients of n-decaneandthemethylnonaneiso-
�%
mers at what they considered a catalytically relevant temperature of 327 C
(actually, catalytic studies on the isomerization of long-chain n-alkanes such
�%
as n-decane are often performed at temperatures around 250 C). These
authors arrived at the conclusion that the diffusion coefficients of the methyl-
nonane isomers were clearly different in all medium-pore zeolites studied.
The diffusion coefficients were minimal for different isomers in the various
zeolites. This was explained by effects related to the correspondence between
the molecular structure and the voids present at intersections of the main
channels. Molecular motion of the monobranched nonanes was regarded as
being dominated by competition amongst CH3 groups to place themselves at
energetically favorable positions along the channel, and diffusion down the
channel was accomplished by jumps between successive pocket mouths. This
means that molecules that are incommensurate with the void system diffuse
faster, because these molecules are less efficiently locked to intersection pos-
itions. The relevant result was that, for five of the seven zeolites included in
the study, the order of CI" values was the same as their order of the ratios
of diffusion coefficients for 2-methylnonane and 5-methylnonane. From this,
Webb III and Grest concluded that the shape selectivity effect underlying the
Modified Constraint Index CI" does have to do, at least in part, with different
mass transfer rates of the isomeric methylbranched isomers [127].
In summary, the literature on diffusion coefficients of isomeric alkanes in
zeolites is very scant. The available information on diffusion coefficients of
methylnonanes is contradictory. We expect a clarification in this respect by
future research work.
The Modified Constraint Index exploits the selectivity occurring in the first
step of the n-alkane reaction network. Numerous additional shape-selectivity
effects are encountered in the consecutive reactions, namely the formation of
dibranched isomers and hydrocracking. After a careful inspection of all these
effects, Martens et al. [123, 126] defined a total of eight quantitative criteria. For
a thorough discussion of these criteria and their usefulness for characterizing
the pore width of microporous materials, the reader is referred to the pertinent
literature [2, 123, 126]. While the Modified Constraint Index has been well ac-
cepted for characterizing the pore width of medium-pore zeolites, the seven
other criteria did not reach any significant popularity.
In certain cases, for instance, if zeolites with very large pore sizes or void
dimensions are to be characterized, it can be advantageous to use a probe
molecule with more than ten carbon atoms. For example, an attempt was un-
dertaken to detect differences between the pore systems of zeolite Y (FAU)
and zeolite ZSM-20 (FAU/EMT), which is an intergrowth of zeolites FAU and
EMT [127]. For this purpose, the isomerization of n-tetradecane was chosen [ Pobierz całość w formacie PDF ]

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