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The Chemistry and Technology of Petroleum
produce a paraffin and leave carbene/carboid-type species (Chapter 1) on the surface. This carbene/
carboid-type species can produce other products, but the most damaging product will be one that
remains on the catalyst surface and cannot be desorbed and results in the formation of coke or
remains in a noncoke form but effectively blocks the active sites of the catalyst.
A general reaction sequence for coke formation from paraffins involves oligomerization, cyclization, and dehydrogenation of small molecules at active sites within zeolite pores:
Alkanes → alkenes
Alkenes → oligomers
Oligomers → naphthenes
Naphthenes → aromatics
Aromatics → coke
Whether or not these are the true steps to coke formation can only be surmised. The problem with
this reaction sequence is that it ignores sequential reactions in favor of consecutive reactions. And it
must be accepted that the chemistry leading up to coke formation is a complex process, consisting
of many sequential and parallel reactions.
There is a complex and little-understood relationship between coke content, catalyst activity, and the chemical nature of the coke. For instance, the atomic hydrogen/carbon ratio of coke
depends on how the coke was formed; its exact value will vary from system to system (Cumming
and Wojciechowski, 1996). And it seems that catalyst decay is not related in any simple way to
the hydrogen-to-carbon atomic ratio of the coke, or to the total coke content of the catalyst, or any
simple measure of coke properties. Moreover, despite many and varied attempts, there is currently
no consensus as to the detailed chemistry of coke formation. There is, however, much evidence and
good reason to believe that catalytic coke is formed from carbenium ions that undergo addition,
dehydrogenation and cyclization, and elimination side reactions in addition to the main-line chain
propagation processes (Cumming and Wojciechowski, 1996).
16.2.3 deHydrogenAtIon
The common primary reactions of pyrolysis are dehydrogenation and carbon bond scission. The
extent of one or the other varies with the starting material and operating conditions, but because of
its practical importance, methods have been found to increase the extent of dehydrogenation and, in
some cases, to render it almost the only reaction.
Dehydrogenation is essentially the removal of hydrogen from the parent molecule. For example,
at 550°C (1025°F), n-butane loses hydrogen to produce butene-1 and butene-2. The development of
selective catalysts, such as chromic oxide (chromia, Cr 2 O 3 ) on alumina (Al 2 O 3 ), has rendered the
dehydrogenation of paraffins to olefins particularly effective, and the formation of higher molecular
weight material is minimized.
Naphthenes are somewhat more difficult to dehydrogenate, and cyclopentane derivatives form
only aromatics if a preliminary step to form the cyclohexane structure can occur. Alkyl derivatives
of cyclohexane usually dehydrogenate at 480°C–500°C (895°F–930°F), and polycyclic naphthenes
are also quite easy to dehydrogenate thermally. In the presence of catalysts, cyclohexane and its
derivatives are readily converted into aromatics; reactions of this type are prevalent in catalytic
cracking and reforming. Benzene and toluene are prepared by the catalytic dehydrogenation of
cyclohexane and methylcyclohexane, respectively.
Polycyclic naphthenes can also be converted to the corresponding aromatics by heating at 450°C
(840°F) in the presence of a chromia–alumina (Cr 2 O 3 –Al 2 O 3 ) catalyst.
Alkylaromatics also dehydrogenate to various products. For example, styrene is prepared by the
catalytic dehydrogenation of ethylbenzene.
Other alkylbenzenes can be dehydrogenated similarly; isopropyl benzene yields α-methyl styrene.
The Chemistry and Technology of Petroleum
produce a paraffin and leave carbene/carboid-type species (Chapter 1) on the surface. This carbene/
carboid-type species can produce other products, but the most damaging product will be one that
remains on the catalyst surface and cannot be desorbed and results in the formation of coke or
remains in a noncoke form but effectively blocks the active sites of the catalyst.
A general reaction sequence for coke formation from paraffins involves oligomerization, cyclization, and dehydrogenation of small molecules at active sites within zeolite pores:
Alkanes → alkenes
Alkenes → oligomers
Oligomers → naphthenes
Naphthenes → aromatics
Aromatics → coke
Whether or not these are the true steps to coke formation can only be surmised. The problem with
this reaction sequence is that it ignores sequential reactions in favor of consecutive reactions. And it
must be accepted that the chemistry leading up to coke formation is a complex process, consisting
of many sequential and parallel reactions.
There is a complex and little-understood relationship between coke content, catalyst activity, and the chemical nature of the coke. For instance, the atomic hydrogen/carbon ratio of coke
depends on how the coke was formed; its exact value will vary from system to system (Cumming
and Wojciechowski, 1996). And it seems that catalyst decay is not related in any simple way to
the hydrogen-to-carbon atomic ratio of the coke, or to the total coke content of the catalyst, or any
simple measure of coke properties. Moreover, despite many and varied attempts, there is currently
no consensus as to the detailed chemistry of coke formation. There is, however, much evidence and
good reason to believe that catalytic coke is formed from carbenium ions that undergo addition,
dehydrogenation and cyclization, and elimination side reactions in addition to the main-line chain
propagation processes (Cumming and Wojciechowski, 1996).
16.2.3 deHydrogenAtIon
The common primary reactions of pyrolysis are dehydrogenation and carbon bond scission. The
extent of one or the other varies with the starting material and operating conditions, but because of
its practical importance, methods have been found to increase the extent of dehydrogenation and, in
some cases, to render it almost the only reaction.
Dehydrogenation is essentially the removal of hydrogen from the parent molecule. For example,
at 550°C (1025°F), n-butane loses hydrogen to produce butene-1 and butene-2. The development of
selective catalysts, such as chromic oxide (chromia, Cr 2 O 3 ) on alumina (Al 2 O 3 ), has rendered the
dehydrogenation of paraffins to olefins particularly effective, and the formation of higher molecular
weight material is minimized.
Naphthenes are somewhat more difficult to dehydrogenate, and cyclopentane derivatives form
only aromatics if a preliminary step to form the cyclohexane structure can occur. Alkyl derivatives
of cyclohexane usually dehydrogenate at 480°C–500°C (895°F–930°F), and polycyclic naphthenes
are also quite easy to dehydrogenate thermally. In the presence of catalysts, cyclohexane and its
derivatives are readily converted into aromatics; reactions of this type are prevalent in catalytic
cracking and reforming. Benzene and toluene are prepared by the catalytic dehydrogenation of
cyclohexane and methylcyclohexane, respectively.
Polycyclic naphthenes can also be converted to the corresponding aromatics by heating at 450°C
(840°F) in the presence of a chromia–alumina (Cr 2 O 3 –Al 2 O 3 ) catalyst.
Alkylaromatics also dehydrogenate to various products. For example, styrene is prepared by the
catalytic dehydrogenation of ethylbenzene.
Other alkylbenzenes can be dehydrogenated similarly; isopropyl benzene yields α-methyl styrene.