437
Refining Chemistry
crystalline aluminosilicate solids, and the acid treatment develops acidic sites by removing aluminum from the structure. The acid sites also catalyze the formation of coke, and the development of a
moving bed process that continuously removed the cooked beads from the reactor reduced the yield
of coke; clay regeneration was achieved by oxidation with air.
Clays are natural compounds of silica and alumina, containing major amounts of the oxides of
sodium, potassium, magnesium, calcium, and other alkali and alkaline earth metals. Iron and other
transition metals are often found in natural clays, substituted for the aluminum cations. Oxides of
virtually every metal are found as impurity deposits in clay minerals.
Clays are layered crystalline materials. They contain large amounts of water within and between
the layers (Keller, 1985). Heating the clays above 100°C can drive out some or all of this water;
at higher temperatures, the clay structures themselves can undergo complex solid-state reactions.
Such behavior makes the chemistry of clays a fascinating field of study in its own right. Typical
clays include kaolinite, montmorillonite, and illite (Keller, 1985). They are found in most natural
soils and in large, relatively pure deposits, from which they are mined for applications ranging from
adsorbents to papermaking.
Once the carbonium ions are formed, the modes of interaction constitute an important means
by which product formation occurs during catalytic cracking; for example, isomerization either by
hydride ion shift or by methyl group shift, both of which occur readily. The trend is for stabilization
of the carbonium ion by movement of the charged carbon atom toward the center of the molecule,
which accounts for the isomerization of α-olefins to internal olefins when carbonium ions are produced. Cyclization can occur by internal addition of a carbonium ion to a double bond which, by
continuation of the sequence, can result in aromatization of the cyclic carbonium ion.
Like the paraffins, naphthenes do not appear to isomerize before cracking. However, the naphthenic hydrocarbons (from C 9 upward) produce considerable amounts of aromatic hydrocarbons
during catalytic cracking. Reaction schemes similar to that outlined here provide possible routes
for the conversion of naphthenes to aromatics. Alkylated benzenes undergo nearly quantitative
dealkylation to benzene without apparent ring degradation below 500°C (930°F). However, polymethylbenzenes undergo disproportionation and isomerization with very little benzene formation.
Catalytic cracking can be represented by simple reaction schemes. However, questions have
arisen as to how the cracking of paraffins is initiated. Several hypotheses for the initiation step
in catalytic cracking of paraffins have been proposed (Cumming and Wojciechowski, 1996). The
Lewis site mechanism is the most obvious, as it proposes that a carbenium ion is formed by the
abstraction of a hydride ion from a saturated hydrocarbon by a strong Lewis acid site: a tricoordinated aluminum species. On Brønsted sites, a carbenium ion may be readily formed from
an olefin by the addition of a proton to the double bond or, more rarely, via the abstraction of a
hydride ion from a paraffin by a strong Brønsted proton. This latter process requires the formation of hydrogen as an initial product. This concept was, for various reasons that are of uncertain
foundation, often neglected.
It is therefore not surprising that the earliest cracking mechanisms postulated that the initial
carbenium ions are formed only by the protonation of olefins generated either by thermal cracking
or present in the feed as an impurity. For a number of reasons this proposal was not convincing, and
in the continuing search for initiating reactions, it was even proposed that electrical fields associated with the cations in the zeolite are responsible for the polarization of reactant paraffins, thereby
activating them for cracking. More recently, however, it has been convincingly shown that a pentacoordinated carbonium ion can be formed on the alkane itself by protonation, if a sufficiently strong
Brønsted proton is available (Cumming and Wojciechowski, 1996).
Coke formation is considered, with just cause to a malignant side reaction of normal carbenium
ions. However, while chain reactions dominate events occurring on the surface, and produce the
majority of products, certain less-desirable bimolecular events have a finite chance of involving the
same carbenium ions in a bimolecular interaction with one another. Of these reactions, most will
Refining Chemistry
crystalline aluminosilicate solids, and the acid treatment develops acidic sites by removing aluminum from the structure. The acid sites also catalyze the formation of coke, and the development of a
moving bed process that continuously removed the cooked beads from the reactor reduced the yield
of coke; clay regeneration was achieved by oxidation with air.
Clays are natural compounds of silica and alumina, containing major amounts of the oxides of
sodium, potassium, magnesium, calcium, and other alkali and alkaline earth metals. Iron and other
transition metals are often found in natural clays, substituted for the aluminum cations. Oxides of
virtually every metal are found as impurity deposits in clay minerals.
Clays are layered crystalline materials. They contain large amounts of water within and between
the layers (Keller, 1985). Heating the clays above 100°C can drive out some or all of this water;
at higher temperatures, the clay structures themselves can undergo complex solid-state reactions.
Such behavior makes the chemistry of clays a fascinating field of study in its own right. Typical
clays include kaolinite, montmorillonite, and illite (Keller, 1985). They are found in most natural
soils and in large, relatively pure deposits, from which they are mined for applications ranging from
adsorbents to papermaking.
Once the carbonium ions are formed, the modes of interaction constitute an important means
by which product formation occurs during catalytic cracking; for example, isomerization either by
hydride ion shift or by methyl group shift, both of which occur readily. The trend is for stabilization
of the carbonium ion by movement of the charged carbon atom toward the center of the molecule,
which accounts for the isomerization of α-olefins to internal olefins when carbonium ions are produced. Cyclization can occur by internal addition of a carbonium ion to a double bond which, by
continuation of the sequence, can result in aromatization of the cyclic carbonium ion.
Like the paraffins, naphthenes do not appear to isomerize before cracking. However, the naphthenic hydrocarbons (from C 9 upward) produce considerable amounts of aromatic hydrocarbons
during catalytic cracking. Reaction schemes similar to that outlined here provide possible routes
for the conversion of naphthenes to aromatics. Alkylated benzenes undergo nearly quantitative
dealkylation to benzene without apparent ring degradation below 500°C (930°F). However, polymethylbenzenes undergo disproportionation and isomerization with very little benzene formation.
Catalytic cracking can be represented by simple reaction schemes. However, questions have
arisen as to how the cracking of paraffins is initiated. Several hypotheses for the initiation step
in catalytic cracking of paraffins have been proposed (Cumming and Wojciechowski, 1996). The
Lewis site mechanism is the most obvious, as it proposes that a carbenium ion is formed by the
abstraction of a hydride ion from a saturated hydrocarbon by a strong Lewis acid site: a tricoordinated aluminum species. On Brønsted sites, a carbenium ion may be readily formed from
an olefin by the addition of a proton to the double bond or, more rarely, via the abstraction of a
hydride ion from a paraffin by a strong Brønsted proton. This latter process requires the formation of hydrogen as an initial product. This concept was, for various reasons that are of uncertain
foundation, often neglected.
It is therefore not surprising that the earliest cracking mechanisms postulated that the initial
carbenium ions are formed only by the protonation of olefins generated either by thermal cracking
or present in the feed as an impurity. For a number of reasons this proposal was not convincing, and
in the continuing search for initiating reactions, it was even proposed that electrical fields associated with the cations in the zeolite are responsible for the polarization of reactant paraffins, thereby
activating them for cracking. More recently, however, it has been convincingly shown that a pentacoordinated carbonium ion can be formed on the alkane itself by protonation, if a sufficiently strong
Brønsted proton is available (Cumming and Wojciechowski, 1996).
Coke formation is considered, with just cause to a malignant side reaction of normal carbenium
ions. However, while chain reactions dominate events occurring on the surface, and produce the
majority of products, certain less-desirable bimolecular events have a finite chance of involving the
same carbenium ions in a bimolecular interaction with one another. Of these reactions, most will