Chemistry Reference
In-Depth Information
Fig. 5 Illustration of zeolite shape selectivity a) reactant selectivity, b) restricted transition
state selectivity and c) product selectivity. Adopted from Ref. 1.
sites within the zeolite crystal.
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Only molecules that are small enough to
enter the pore openings of the zeolite can be converted at the active sites.
Restricted transition state selectivity (Fig. 5b) is encountered in
chemical reactions that involve transition states which are too bulky to be
accommodated inside the zeolite pores.
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In this case, products are
formed only from reactions with intermediates small enough to fit inside
the pores of the zeolite. In restricted transition state selectivity, neither
reactants nor potential products are hindered from diffusing in or out of
the zeolite crystal.
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Product shape selectivity (Fig. 5c) is encountered when certain prod-
uct molecules are too big to diffuse intact out of the zeolite pores.
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Some
zeolite structures have cavities which allow formation of both small and
bulky products. However, the apertures are small, and the bulky product
molecules must undergo further reactions to smaller molecules in order
to leave the zeolite crystal.
In general, both reactant and product shape selectivities occur due to
mass transfer limitations. In reactant shape selectivity all molecules that
diffuse to and from the active sites suciently fast will be converted, while
in product shape selectivity molecules with high mass transport limi-
tations remain in the adsorbed phase and continue to react for a longer
period of time than species less affected by mass transfer limitations.
Therefore, both reactant and product shape selectivities are affected by
crystal sizes, whereas restricted transition state selectivity, which is not
caused by mass transfer limitations, does not depend on crystal size.
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In brief, shape selective catalysis may contribute to limiting byproduct
formation and thus can make processes more environmentally friendly
and more cost-effective.
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1.3 Catalysts described in this work
In this Section, shape selectivity will be illustrated by a selection of small
(8-ring), medium (10-ring) and large (12-ring) pore zeolites. Table 1 pre-
sents a list of those materials with a description of their pore systems.
The pore systems of the zeolites are further illustrated in Figs. 6-9. In
these figures the framework is removed and only the channel structure
