624
The Chemistry and Technology of Petroleum
of the zeolite produced depends on the composition of the solution. Aqueous alkali metal hydroxide
solutions favor zeolites with relatively high aluminum contents, while the presence of organic molecules such as amines or alcohols favors highly siliceous zeolites such as silicalite or ZSM-5. Various
tetra-alkyl ammonium cations favor the formation of certain specific zeolite structures and are
known as template ions, although it should not be supposed that the channels and cages form simply
by the wrapping of aluminosilicate fragments around suitably shaped cations.
Zeolite catalysts have also found use in the refining industry during the last two decades. Like
the silica–alumina catalysts, zeolites also consist of a framework of tetrahedral usually with a silicon atom or an aluminum atom at the center. The geometric characteristics of the zeolites are
responsible for their special properties, which are particularly attractive to the refining industry
(DeCroocq, 1984). Specific zeolite catalysts have shown up to 10,000 times more activity than the
so-called conventional catalysts in specific cracking tests. The mordenite-type catalysts are particularly worthy of mention since they have shown up to 200 times greater activity for hexane cracking
in the temperature range 360°C–400°C (680°F–750°F).
Other zeolite catalysts have also shown remarkable adaptability to the refining industry. For
example, the resistance to deactivation of the type Y zeolite catalysts containing either noble or
non-noble metals is remarkable, and catalyst life of up to 7 years has been obtained commercially in
processing heavy gas oils in the Unicracking-JHC processes. Operating life depends on the nature
of the feedstock, the severity of the operation, and the nature and extent of operational upsets.
Gradual catalyst deactivation in commercial use is counteracted by incrementally raising the operating temperature to maintain the required conversion per pass. The more active a catalyst, the
lower is the temperature required. When processing for gasoline, lower operating temperatures have
the additional advantage that less of the feedstock is converted to iso-butane.
Any given zeolite is distinguished from other zeolites by structural differences in its unit cell,
which is a tetrahedral structure arranged in various combinations. Oxygen atoms establish the four
vertices of each tetrahedron, which are bound to, and enclose, either a silicon (Si) or an aluminum
(Al) atom. The vertex oxygen atoms are each shared by two tetrahedrons, so that every silicon atom
or aluminum atom within the tetrahedral cage is bound to four neighboring caged atoms through an
intervening oxygen. The number of aluminum atoms in a unit cell is always smaller than, or at most
equal to, the number of silicon atoms because two aluminum atoms never share the same oxygen.
The aluminum is actually in the ionic form and can readily accommodate electrons donated from
three of the bound oxygen atoms. The electron donated by the fourth oxygen imparts a negative, or
anionic, charge to the aluminum atom. This negative charge is balanced by a cation from the alkali
metal or the alkaline earth groups of the periodic table. Such cations are commonly sodium, potassium, calcium, or magnesium. These cations play a major role in many zeolite functions and help
to attract polar molecules, such as water. However, the cations are not part of the zeolite framework
and can be exchanged for other cations without any effect on crystal structure.
Zeolites provide the cracking function in many hydrocracking catalysts, as they do in fluid catalytic cracking catalysts. The zeolites are crystalline aluminosilicates, and in almost all commercial
catalysts today, the zeolite used is faujasite. Pentasil zeolites, including silicalite and ZSM-5, are
also used in some catalysts for their ability to crack long chain paraffins selectively. Typical levels
are 25%–50% by wt. zeolite in the catalysts, with the remainder being the hydrogenation component
and a silica (SiO 2 ) or alumina (Al 2 O 3 ) binder. Exact recipes are guarded as trade secrets.
Crystalline zeolite compounds provide a broad family of solid acid catalysts. The chemistry
and structures of these solids are beyond the scope of this book. What is important here is that the
zeolites are not acidic as crystallized. They must be converted to acidic forms by ion-exchange
processes. In the process of doing this conversion, the chemistry of the crystalline structure is often
changed. This complication provides tools for controlling the catalytic properties, and much work
has been done on understanding and applying these reactions as a way to make catalysts with higher
activities and more desirable selectivity.
The Chemistry and Technology of Petroleum
of the zeolite produced depends on the composition of the solution. Aqueous alkali metal hydroxide
solutions favor zeolites with relatively high aluminum contents, while the presence of organic molecules such as amines or alcohols favors highly siliceous zeolites such as silicalite or ZSM-5. Various
tetra-alkyl ammonium cations favor the formation of certain specific zeolite structures and are
known as template ions, although it should not be supposed that the channels and cages form simply
by the wrapping of aluminosilicate fragments around suitably shaped cations.
Zeolite catalysts have also found use in the refining industry during the last two decades. Like
the silica–alumina catalysts, zeolites also consist of a framework of tetrahedral usually with a silicon atom or an aluminum atom at the center. The geometric characteristics of the zeolites are
responsible for their special properties, which are particularly attractive to the refining industry
(DeCroocq, 1984). Specific zeolite catalysts have shown up to 10,000 times more activity than the
so-called conventional catalysts in specific cracking tests. The mordenite-type catalysts are particularly worthy of mention since they have shown up to 200 times greater activity for hexane cracking
in the temperature range 360°C–400°C (680°F–750°F).
Other zeolite catalysts have also shown remarkable adaptability to the refining industry. For
example, the resistance to deactivation of the type Y zeolite catalysts containing either noble or
non-noble metals is remarkable, and catalyst life of up to 7 years has been obtained commercially in
processing heavy gas oils in the Unicracking-JHC processes. Operating life depends on the nature
of the feedstock, the severity of the operation, and the nature and extent of operational upsets.
Gradual catalyst deactivation in commercial use is counteracted by incrementally raising the operating temperature to maintain the required conversion per pass. The more active a catalyst, the
lower is the temperature required. When processing for gasoline, lower operating temperatures have
the additional advantage that less of the feedstock is converted to iso-butane.
Any given zeolite is distinguished from other zeolites by structural differences in its unit cell,
which is a tetrahedral structure arranged in various combinations. Oxygen atoms establish the four
vertices of each tetrahedron, which are bound to, and enclose, either a silicon (Si) or an aluminum
(Al) atom. The vertex oxygen atoms are each shared by two tetrahedrons, so that every silicon atom
or aluminum atom within the tetrahedral cage is bound to four neighboring caged atoms through an
intervening oxygen. The number of aluminum atoms in a unit cell is always smaller than, or at most
equal to, the number of silicon atoms because two aluminum atoms never share the same oxygen.
The aluminum is actually in the ionic form and can readily accommodate electrons donated from
three of the bound oxygen atoms. The electron donated by the fourth oxygen imparts a negative, or
anionic, charge to the aluminum atom. This negative charge is balanced by a cation from the alkali
metal or the alkaline earth groups of the periodic table. Such cations are commonly sodium, potassium, calcium, or magnesium. These cations play a major role in many zeolite functions and help
to attract polar molecules, such as water. However, the cations are not part of the zeolite framework
and can be exchanged for other cations without any effect on crystal structure.
Zeolites provide the cracking function in many hydrocracking catalysts, as they do in fluid catalytic cracking catalysts. The zeolites are crystalline aluminosilicates, and in almost all commercial
catalysts today, the zeolite used is faujasite. Pentasil zeolites, including silicalite and ZSM-5, are
also used in some catalysts for their ability to crack long chain paraffins selectively. Typical levels
are 25%–50% by wt. zeolite in the catalysts, with the remainder being the hydrogenation component
and a silica (SiO 2 ) or alumina (Al 2 O 3 ) binder. Exact recipes are guarded as trade secrets.
Crystalline zeolite compounds provide a broad family of solid acid catalysts. The chemistry
and structures of these solids are beyond the scope of this book. What is important here is that the
zeolites are not acidic as crystallized. They must be converted to acidic forms by ion-exchange
processes. In the process of doing this conversion, the chemistry of the crystalline structure is often
changed. This complication provides tools for controlling the catalytic properties, and much work
has been done on understanding and applying these reactions as a way to make catalysts with higher
activities and more desirable selectivity.