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width, however, the formation of p-xylene is increasingly favored over the for-
mation of the bulkier o-xylene [85]. This effect is best interpreted in terms
of product shape selectivity, i.e., progressively hindered diffusion of o-xylene
molecules, as the pores become narrower.
Fig. 7 Principal reaction pathways of m-xylene over acidic catalysts
Characterization of the Pore Size of Molecular Sieves 133
Criterion (ii) is a quantitative expression of the observation that, with de-
creasing pore width, isomerization of m-xylene is more and more favored
æ%
over its disproportionation. At 350 C, the bimolecular disproportionation
is completely suppressed in the medium-pore zeolite ZSM-5, whereas iso-
merization (which is likely to proceed via the monomolecular mechanism
at these high temperatures) does not experience significant hindrance [85].
Later, Olson and Haag [30] demonstrated that, if zeolite catalysts with differ-
ent pore systems are used, a linear relationship results between the ratio of
rate constants of disproportionation and isomerization and the dimensions of
the largest zeolite cavities. One of the conclusions of this study was that the
suppression of disproportionation is due to restricted transition state shape
selectivity rather than to mass transfer effects.
Finally, criterion (iii) was deduced from the observation that the dispro-
portionation of m-xylene in the spacious cavities of zeolite Y and in the much
narrower channels of mordenite (MOR) resulted in significantly different dis-
tributions of the trimethylbenzene isomers: Of the product obtained from
H-mordenite, 94% of the trimethylbenzenes consisted of the 1,2,4-isomer. On
the other hand, 74% of the 1,2,4-isomer and 24% of the 1,3,5-isomer were
found in the product from H-Y. (On both zeolites, only negligible amounts
of 1,2,3-trimethylbenzene were formed, and this was attributed to its low
concentration in thermodynamic equilibrium [85]). To interpret the hin-
dered formation of 1,3,5-trimethylbenzene from m-xylene in mordenite, the
authors mainly invoked restricted transition state shape selectivity.
m-Xylene isomerization as a catalytic test reaction was later adopted by
a number of other groups [2, 67], and the criteria proposed by Gnep et al. [85]
were refined and applied to a broader structural variety of molecular sieves.
Dewing [86] introduced the so-called R value, defined as the ratio of rate con-
stants observed experimentally for the formation of o- and p-xylene from
m-xylene under conditions of diffusional limitations. Joensen et al. [87]
coined the term Shape Selectivity Index (SSI), which is defined as the yield
ratio of p- and o-xylene observed on a shape-selective zeolite and extrap-
olated to zero conversion minus the same ratio obtained under identical
experimental conditions on a catalyst with sufficiently large pores, i.e., in
the complete absence of shape selectivity in m-xylene isomerization. Neither
Dewing s R value nor the Shape Selectivity Index by Joensen et al. received
much attention by others.
A critical evaluation of the published data revealed [2] that a comparison
of the rates of isomerization and disproportionation, if applied quantitatively
to a larger number of zeolites, does not allowfor a reliable ranking of large-
pore molecular sieves. No correlation at all exists between criterion (ii) and the
window or pore size [2]. Therefore, criterion (ii) will not further be dealt with.
According to Martens et al. [88] there seems to be, however, very valuable
information on the pore architecture in the distribution of the trimethyl-
benzenes formed from m-xylene in large-pore zeolites. If disproportionation
134 Y. Traa et al.
occurs at all in 10-membered-ring zeolites, it gives 1,2,4-trimethylbenzene
exclusively, probably because the diphenylmethane-type carbocation inter-
mediate leading to this particular isomer can be best accommodated in their
narrow pores. Exceptions are zeolites ZSM-50 (EUO) and NU-87 (NES) [89] in
which 1,2,3- and 1,3,5-trimethylbenzeneareformedas well. This canberatio-
nalized if one assumes that the disproportionation takes place, at least in part,
in the spacious side pockets running perpendicular to the 10-membered-ring
channels (EUO) or in the 12-membered-ring channels (NES). For the zeolites [ Pobierz całość w formacie PDF ]