623
Hydrocracking
to the base metal sulfides is palladium sulfide (PdS). Although expensive, palladium sulfide
forms the basis for several very active catalysts.
Clays have been used as cracking catalysts particularly for heavy feedstocks (Chapter 19) and
have also been explored in the demetallization and upgrading of heavy crude oil. The results indicated that the catalyst prepared was mainly active toward demetallization and conversion of the
heaviest fractions of crude oils.
The cracking reaction results from attack of a strong acid on a paraffin chain to form a carbonium ion (a carbon cation, e.g., R + ) (Dolbear, 1998). Strong acids come in two fundamental types,
Brønsted and Lewis acids. Brønsted acids are the familiar proton-containing acids; Lewis acids are
a broader class including inorganic and organic species formed by positively charged centers. Both
kinds have been identified on the surfaces of catalysts; sometimes both kinds of sites occur on the
same catalyst. The mixture of Brønsted and Lewis acids sometimes depends on the level of water
in the system.
Examples of Brønsted acids are the familiar proton-containing species such as sulfuric acid
(H 2 SO 4 ). Acidity is provided by the very active hydrogen ion (H + ), which has a very high positive
charge density. It seeks out centers of negative charge such as the pi electrons in aromatic centers.
Such reactions are familiar to organic chemistry students, who are taught that bromination of aromatics takes place by attack of the bromonium ion (Br + ) on such a ring system. The proton in strong
acid systems behaves in much the same way, adding to the pi electrons and then migrating to a site
of high electron density on one of the carbon atoms.
In reactions with hydrocarbons, both Lewis and Brønsted acids can catalyze cracking reactions.
For example, the proton in Brønsted acids can add to an olefinic double bond to form a carbon cation. Similarly, a Lewis acid can abstract a hydride from the corresponding paraffin to generate the
same intermediate (Dolbear, 1998). Although these reactions are written to show identical intermediates in the two reactions, in real catalytic systems, the intermediates would be different. This is
because the carbon cations would probably be adsorbed on surface sites that would be different in
the two kinds of catalysts.
Zeolites and amorphous silica–alumina provide the cracking function in hydrocracking catalysts
(Sherman, 1998). Both of these have similar chemistry at the molecular level, but the crystalline
structure of the zeolites provides higher activities and controlled selectivity not found in the amorphous materials. Zeolites (Greek: zeo, to boil, and lithos, stone) consist primarily of silicon, aluminum, and oxygen and to host an assortment of other elements. In addition, zeolites are highly porous
crystals veined with submicroscopic channels. The channels contain water (hence the bubbling at
high temperatures), which can be eliminated by heating (combined with other treatments) without
altering the crystal structure (Occelli and Robson, 1989).
Typical naturally occurring zeolites include analcite (also called analcime) Na(AlSi 2 O 6 ) and faujasite Na 2 Ca(AlO 2 ) 2 (SiO 2 ) 4 .H 2 O, which is the structural analog of the synthetic zeolite X and zeolite
Y. Sodalite (Na 8 [(Al 2 O 2 ) 6 (SiO 2 ) 6 ]Cl 2 ) contains the truncated octahedral structural unit known as
the sodalite cage that is found in several zeolites. The corners of the faces of the cage are defined
by either four or six Al–Si atoms, which are joined together through oxygen atoms. The zeolite
structure is generated by joining sodalite cages through the four Si–Al rings, so enclosing a cavity
or super cage bounded by a cube of eight sodalite cages and readily accessible through the faces of
that cube (channels or pores). Joining sodalite cages together through the six Si–Al faces generates
the structural frameworks of faujasite, zeolite X, and zeolite Y. In zeolites, the effective width of the
pores is usually controlled by the nature of the cation (M+ or M 2+ ).
Natural zeolites form hydrothermally (e.g., by the action of hot water on volcanic ash or lava), and
synthetic zeolites can be made by mixing solutions of aluminates and silicates and maintaining the
resulting gel at temperatures of 100°C (212°F) or higher for appropriate periods (Swaddle, 1997).
Zeolite-A can form at temperatures below 100°C (212°F), but most zeolite syntheses require hydrothermal conditions (typically 150°C/300°F at the appropriate pressure). The reaction mechanism
appears to involve dissolution of the gel and precipitation as the crystalline zeolite and the identity
Hydrocracking
to the base metal sulfides is palladium sulfide (PdS). Although expensive, palladium sulfide
forms the basis for several very active catalysts.
Clays have been used as cracking catalysts particularly for heavy feedstocks (Chapter 19) and
have also been explored in the demetallization and upgrading of heavy crude oil. The results indicated that the catalyst prepared was mainly active toward demetallization and conversion of the
heaviest fractions of crude oils.
The cracking reaction results from attack of a strong acid on a paraffin chain to form a carbonium ion (a carbon cation, e.g., R + ) (Dolbear, 1998). Strong acids come in two fundamental types,
Brønsted and Lewis acids. Brønsted acids are the familiar proton-containing acids; Lewis acids are
a broader class including inorganic and organic species formed by positively charged centers. Both
kinds have been identified on the surfaces of catalysts; sometimes both kinds of sites occur on the
same catalyst. The mixture of Brønsted and Lewis acids sometimes depends on the level of water
in the system.
Examples of Brønsted acids are the familiar proton-containing species such as sulfuric acid
(H 2 SO 4 ). Acidity is provided by the very active hydrogen ion (H + ), which has a very high positive
charge density. It seeks out centers of negative charge such as the pi electrons in aromatic centers.
Such reactions are familiar to organic chemistry students, who are taught that bromination of aromatics takes place by attack of the bromonium ion (Br + ) on such a ring system. The proton in strong
acid systems behaves in much the same way, adding to the pi electrons and then migrating to a site
of high electron density on one of the carbon atoms.
In reactions with hydrocarbons, both Lewis and Brønsted acids can catalyze cracking reactions.
For example, the proton in Brønsted acids can add to an olefinic double bond to form a carbon cation. Similarly, a Lewis acid can abstract a hydride from the corresponding paraffin to generate the
same intermediate (Dolbear, 1998). Although these reactions are written to show identical intermediates in the two reactions, in real catalytic systems, the intermediates would be different. This is
because the carbon cations would probably be adsorbed on surface sites that would be different in
the two kinds of catalysts.
Zeolites and amorphous silica–alumina provide the cracking function in hydrocracking catalysts
(Sherman, 1998). Both of these have similar chemistry at the molecular level, but the crystalline
structure of the zeolites provides higher activities and controlled selectivity not found in the amorphous materials. Zeolites (Greek: zeo, to boil, and lithos, stone) consist primarily of silicon, aluminum, and oxygen and to host an assortment of other elements. In addition, zeolites are highly porous
crystals veined with submicroscopic channels. The channels contain water (hence the bubbling at
high temperatures), which can be eliminated by heating (combined with other treatments) without
altering the crystal structure (Occelli and Robson, 1989).
Typical naturally occurring zeolites include analcite (also called analcime) Na(AlSi 2 O 6 ) and faujasite Na 2 Ca(AlO 2 ) 2 (SiO 2 ) 4 .H 2 O, which is the structural analog of the synthetic zeolite X and zeolite
Y. Sodalite (Na 8 [(Al 2 O 2 ) 6 (SiO 2 ) 6 ]Cl 2 ) contains the truncated octahedral structural unit known as
the sodalite cage that is found in several zeolites. The corners of the faces of the cage are defined
by either four or six Al–Si atoms, which are joined together through oxygen atoms. The zeolite
structure is generated by joining sodalite cages through the four Si–Al rings, so enclosing a cavity
or super cage bounded by a cube of eight sodalite cages and readily accessible through the faces of
that cube (channels or pores). Joining sodalite cages together through the six Si–Al faces generates
the structural frameworks of faujasite, zeolite X, and zeolite Y. In zeolites, the effective width of the
pores is usually controlled by the nature of the cation (M+ or M 2+ ).
Natural zeolites form hydrothermally (e.g., by the action of hot water on volcanic ash or lava), and
synthetic zeolites can be made by mixing solutions of aluminates and silicates and maintaining the
resulting gel at temperatures of 100°C (212°F) or higher for appropriate periods (Swaddle, 1997).
Zeolite-A can form at temperatures below 100°C (212°F), but most zeolite syntheses require hydrothermal conditions (typically 150°C/300°F at the appropriate pressure). The reaction mechanism
appears to involve dissolution of the gel and precipitation as the crystalline zeolite and the identity