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the indices were formally defined in a slightly different manner which, from
a chemical reaction engineering viewpoint, needed some re-definition.) Com-
pared to the Constraint Index, the Selectivity Ratio is a more flexible index,
since probe molecules can be chosen from a variety of alkanes to more closely
match the catalyst pore size to be characterized. This is an interesting ap-
proach, but more experimental data collected on a much broader variety of
zeolites including extra-large-pore materials is needed, before the usefulness
of SR can be assessed.
Flego and Perego [111] recently proposed the aldol condensation of ace-
tone as a more unusual test for characterizing both the acid site density
and the pore dimension of small-, medium- and large-pore zeolites in their
acidic forms. The products can be readily analyzed by UV/Vis spectroscopy.
Medium-pore zeolites were shown to give phorone as the final product,
whereas large-pore zeolites tend to favor the formation of isophorone. How-
ever, no quantitative criterion was coined, hence this test reaction appears
to need more refinement, before it can be recommended for a more general
application.
The last test reaction for acidic zeolites that has to be discussed is the
methanol conversion test : The acid-catalyzed Methanol-to-Hydrocarbons
(MTH) reaction is known to proceed in a stepwise manner [112, 113]: With
increasing severity, dimethyl ether, olefins and a mixture of aromatics plus
alkanes are successively formed. By suitable adjustment of the reaction con-
ditions, either the yield of light olefins or that of aromatics plus alkanes
can be maximized, the corresponding process variants being referred to as
Methanol-to-Olefins (MTO) and Methanol-to-Gasoline (MTG), respectively.
Large amounts of water are necessarily formed in the MTH reaction, and the
risk exists that this brings about an undesired steaming of the zeolites with
a concomitant framework dealumination. To minimize these effects, the zeo-
lites to be tested are to be used in a form with a sufficiently high nSi/nAl ratio,
yet a minimum amount of aluminum must be present in the framework since
it generates the Brønsted acidity and, hence, the catalytic activity.
The methanol conversion test relies on the finding that the selectivity of
the MTH reaction is strongly influenced by the zeolite pore geometry. Zeo-
140 Y. Traa et al.
lite structures with 8-membered-ring pore apertures, such as erionite (ERI)
or chabazite (CHA) are capable of converting methanol selectively to light
olefins. Aromatics, if formed at the catalytic sites, would be trapped inside
the large cavities existing in most of these small-pore zeolites. On medium-
pore zeolites, such as ZSM-5, aromatics do occur in the product, but due to
the space limitations inside the pores, the aromatics distribution terminates
at around C10 (durene is widely considered as the bulkiest aromatic hydro-
carbon occurring in an MTG product) with a maximum at around C8. In
zeolite mordenite (MOR), the bulky polymethylbenzenes tend to be the main
products, whereas in ZSM-12 (MTW) with its smaller effective pore diameter,
a broader aromatics distribution has been observed [112]. These and other
selectivity effects in the MTH reaction were exploited by Yuen et al. [114]
for the characterization of AFI and CHA molecular sieves. Webster et al. [9]
used the results from the test performed at different temperatures with zeolite
H-ZSM-5 to assess the change of the effective pore dimensionwithtempera-
ture. The authors concluded that the effective channel size of H-ZSM-5 is
æ%
between 0.662 and 0.727 nm at 300 C, the MIN-2 (cf. Sect. 2.1) dimensions
of the reaction products p-xylene (which is formed) and o-xylene (which is
æ%
not formed), and at least 0.764 nm at 370 C, the MIN-2 dimension of the re-
action product 1,2,3-trimethylbenzene. Upon increasing the temperature, the
dimensions of the channel intersections increase as well, namely from 0.817
by 0.888 nm to 0.908 by 0.909 nm [9].
Bendoraitis et al. [115] used a test reaction similar to methanol conversion
(i.e., based on mass transfer shape selectivity) but this focused on reactant
shape selectivity instead of product shape selectivity: These authors deter-
mined the catalytic pore sizes of ZSM-5, ZSM-23 (MTT) and mordenite on the
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