can accept an electron pair, rather than a protonated hydrogen. Although plausible
hydrocracking mechanisms can be written for both Brønsted and Lewis sites,
Brønsted acidity is believed to be more desirable because Lewis acid sites may
catalyze coke formation.
Zeolites began to be used in hydrocracking catalysts because they provided high
activity due to their higher acidity compared to the ASA materials. Zeolites are
crystalline aluminosilicates composed of Al 2 O 3 and SiO 2 tetrahedral units that form
a negatively charged microporous framework structure enclosing cavities occupied
by large ions and water molecules, both of which have considerable freedom of
movement, permitting ion exchange and reversible dehydration. The silicon and
aluminum atoms are tetrahedrally coordinated with each other through shared
oxygen atoms. Compositionally zeolites are similar to clay minerals as they are
both aluminosilicates, but zeolites are crystalline and clays are not. If the mobile
cations are exchanged with NH 4
+ (ammonium ion), followed by calcination to
remove NH 3 , a Brønsted acid site is formed. With the right modification, zeolites
can be tuned to provide similar yields of the desired products as ASA materials at
higher activity. Mobile cations, which are not part of the framework but are part of
the zeolites, are readily exchanged.
Both Brønsted and Lewis acids are possible in zeolites. The number of acid sites
and the strength of the acid sites can be varied in various synthesis steps. These sites
are highly uniform, but each zeolite may have more than one type of site. The
following factors influence the number and strength of acid sites in zeolites:
• The types of cations occupying the ion exchange sites
• Thermal treatments of the zeolite
• The framework silica to alumina ratio in the zeolite
For example, Y zeolite can be treated to modify the Si/Al ratio; common
methods to accomplish this are either a thermal or a hydrothermal treatment. In
Fig. 16 is an image after hydrothermal treatment of stabilized Y zeolite. When
aluminum is removed, the effect is to decrease the total number of acid sites,
because each proton is associated with framework alumina. As can be seen in
Fig. 16, there is also a generation of mesoporosity in the zeolite. However, the
reduction of the alumina sites increases the strength of the remaining acid sites in
the zeolite. As a result, the total acidity of the zeolite, which is a product of the
number of sites and strength per site, peaks at an intermediate extent of
dealumination. The crystallinity of the zeolite can also be modified depending on
the treatment history. The acid site concentration and strength of the zeolite will
affect the final hydrocracking catalyst properties. The principal advantage of
zeolites for hydrocracking is their high acidity.
One zeolite used in hydrocracking, Y zeolite, is synthetic (Fig. 15). It has a
structure nearly identical to the naturally found zeolite faujasite (Baerlocher and
McCusker). The Y zeolite has both a relatively large free aperture, which controls
access of reactants to acid sites, and a three-dimensional pore structure, which
allows diffusion of the reactants in and products out with minimal interference. The
338
M. Bricker et al.
hydrocracking mechanisms can be written for both Brønsted and Lewis sites,
Brønsted acidity is believed to be more desirable because Lewis acid sites may
catalyze coke formation.
Zeolites began to be used in hydrocracking catalysts because they provided high
activity due to their higher acidity compared to the ASA materials. Zeolites are
crystalline aluminosilicates composed of Al 2 O 3 and SiO 2 tetrahedral units that form
a negatively charged microporous framework structure enclosing cavities occupied
by large ions and water molecules, both of which have considerable freedom of
movement, permitting ion exchange and reversible dehydration. The silicon and
aluminum atoms are tetrahedrally coordinated with each other through shared
oxygen atoms. Compositionally zeolites are similar to clay minerals as they are
both aluminosilicates, but zeolites are crystalline and clays are not. If the mobile
cations are exchanged with NH 4
+ (ammonium ion), followed by calcination to
remove NH 3 , a Brønsted acid site is formed. With the right modification, zeolites
can be tuned to provide similar yields of the desired products as ASA materials at
higher activity. Mobile cations, which are not part of the framework but are part of
the zeolites, are readily exchanged.
Both Brønsted and Lewis acids are possible in zeolites. The number of acid sites
and the strength of the acid sites can be varied in various synthesis steps. These sites
are highly uniform, but each zeolite may have more than one type of site. The
following factors influence the number and strength of acid sites in zeolites:
• The types of cations occupying the ion exchange sites
• Thermal treatments of the zeolite
• The framework silica to alumina ratio in the zeolite
For example, Y zeolite can be treated to modify the Si/Al ratio; common
methods to accomplish this are either a thermal or a hydrothermal treatment. In
Fig. 16 is an image after hydrothermal treatment of stabilized Y zeolite. When
aluminum is removed, the effect is to decrease the total number of acid sites,
because each proton is associated with framework alumina. As can be seen in
Fig. 16, there is also a generation of mesoporosity in the zeolite. However, the
reduction of the alumina sites increases the strength of the remaining acid sites in
the zeolite. As a result, the total acidity of the zeolite, which is a product of the
number of sites and strength per site, peaks at an intermediate extent of
dealumination. The crystallinity of the zeolite can also be modified depending on
the treatment history. The acid site concentration and strength of the zeolite will
affect the final hydrocracking catalyst properties. The principal advantage of
zeolites for hydrocracking is their high acidity.
One zeolite used in hydrocracking, Y zeolite, is synthetic (Fig. 15). It has a
structure nearly identical to the naturally found zeolite faujasite (Baerlocher and
McCusker). The Y zeolite has both a relatively large free aperture, which controls
access of reactants to acid sites, and a three-dimensional pore structure, which
allows diffusion of the reactants in and products out with minimal interference. The
338
M. Bricker et al.