Mechanistic considerations in acid-catalyzed cracking of olefins

Article abstract of DOI:10.1006/jcat.1996.0027

The relative rates of cracking and resultant product distributions for cracking C₅-C₈ olefins over ZSM-5 at 510°C were quantified and rationalized in terms of carbenium ion mechanisms. Conditions were chosen to minimize bimolecular reactions. Cracking rates increase more dramatically with carbon number for olefins than for monomolecular cracking of paraffins, as more energetically favorable modes become available for β-scission of the carbenium ion formed by proton donation to the olefin. Product distributions were used to determine the relative rates of various modes of β-scission, as classified by the types of carbenium ions involved. For hexene and heptene feeds, the most-favorable β-scission mode available (C-type, involving just secondary carbenium ions, for hexene feed; B-type, involving secondary plus tertiary carbenium ions for heptene) accounted for 70-80% of the cracking. Product distribution was independent of which hexene or heptene isomer was fed, since double-bond and skeletal isomerization precedes significant cracking. For 1-octene feed, however, the olefin was nearly all cracked via secondary-tertiary and tertiary-secondary β-scission (after isomerizing to a dimethylhexene) before it isomerized further to the 2,4,4-trimethylpentene isomer, which would be required to undergo the most energetically favored (tertiary-tertiary) form of cracking. A semiquantitative prediction of rates and product distribution for 1-octene cracking could be made, using rates for the various types of β-scission calculated from results with C₆-C₇ feeds.

Full text of DOI:10.1006/jcat.1996.0027

JOURNAL OF CATALYSIS 158, 279–287 (1996)
ARTICLE NO. 0027
Mechanistic Considerations in Acid-Catalyzed Cracking of Olefins
J. S. Buchanan, J. G. Santiesteban, and W. O. Haag
Mobil Research and Development Corporation, P. O. Box 1026, Princeton, New Jersey 08543
Received April 17, 1995; revised September 22, 1995; accepted September 22, 1995
branching (1). Paraffin cracking, isomerization, and dehy-
drocylization can also proceed via olefinic intermediates.
Cracking of olefins may be desired or not desired, de-
pending on the process. Selection of catalysts and operating
conditions can be assisted by knowledge of olefin crack-
ing patterns.
The relative rates of cracking and resultant product distribu-
tions for cracking C5–C8 olefins over ZSM-5 at 510؇C were
quantified and rationalized in terms of carbenium ion mecha-
nisms. Conditions were chosen to minimize bimolecular reac-
tions. Cracking rates increase more dramatically with carbon
number for olefins than for monomolecular cracking of paraf-
fins, as more energetically favorable modes become available
for ␤-scission of the carbenium ion formed by proton donation
to the olefin. Product distributions were used to determine the
relative rates of various modes of ␤-scission, as classified by
the types of carbenium ions involved. For hexene and heptene
feeds, the most-favorable ␤-scission mode available (C-type,
involving just secondary carbenium ions, for hexene feed;
B-type, involving secondary plus tertiary carbenium ions for
heptene) accounted for 70–80% of the cracking. Product distri-
bution was independent of which hexene or heptene isomer
was fed, since double-bond and skeletal isomerization precedes
significant cracking. For 1-octene feed, however, the olefin was
nearly all cracked via secondary-tertiary and tertiary-secondary
␤-scission (after isomerizing to a dimethylhexene) before it
isomerized further to the 2,4,4-trimethylpentene isomer, which
would be required to undergo the most energetically favored
(tertiary-tertiary) form of cracking. A semiquantitative predic-
tion of rates and product distribution for 1-octene cracking
could be made, using rates for the various types of ␤-scission
Olefin interconversion and cyclization at temperatures
around 200–400ЊC have been the subject of numerous stud-
ies, especially in connection with the conversion of metha-
nol and light olefins to gasoline over ZSM-5 (2–6). Crack-
ing of higher olefins over acid catalysts at high temperature
(Ͼ400ЊC) presents experimental challenges due to high
reaction rates, and has received less attention in the litera-
ture than paraffin cracking. Haag et al. (7) compared crack-
ing rates at 538ЊC of linear and branched C6–C9 paraffins
and C6 olefins over ZSM-5 crystallites of different sizes to
distinguish between shape selectivity caused by diffusion
limitation and by constraint on the size of the transition
state complex. Abbot and Wojciechowski (8) found that
pentene and hexene cracked over ZSM-5 at 404ЊC via a
dimerization-cracking route, while cracking of C7–C9 ole-
fins was much faster and appeared to be a monomolecular
process. They related the observed rates of reaction of
linear and branched octene feeds over H–Y at 300ЊC to
differing strengths of adsorption (9). Buchanan (10) deter-
mined olefin cracking rates and selectivities over a steamed
ZSM-5 FCC additive at 538ЊC. At this temperature, hexene
cracking was predominantly monomolecular. Nayak and
Moffat (11) cracked C6–C8 olefins over various ZSM-5
calculated from results with C6–C7 feeds.
1996 Academic Press, Inc.
INTRODUCTION
Olefins are produced in the FCC unit from the cracking catalysts at around 400ЊC, and found that the activity per
of feed molecules. Many of the primary olefin products aluminum atom increased with a decrease in the Al content
are large enough (C5 ) that they undergo further cracking. of the zeolite.
ϩ
This secondary olefin cracking, which proceeds over the
It is widely accepted that olefin cracking over catalysts
Y-containing base catalyst and over additives such as with Brønsted acidity involves the protonation of the dou-
ZSM-5, has a major impact on the yield and composition ble bond to form a tricoordinate carbenium ion, with subse-
of the FCC gasoline and C4-gases. Olefin reactions over quent scission of a carbon–carbon bond in the beta posi-
medium-pore zeolites are also at the heart of many other tion, to form a free olefin and a smaller carbenium ion.
fuel-related processes. These include MTG and MTO for While paraffin cracking at high temperature and low pres-
converting methanol to gasoline and to olefins, respec- sure occurs by a monomolecular process involving a pro-
tively; MOGD for converting light olefins to gasoline and tonated, pentacoordinated carbonium ion intermediate,
distillate; and processes for olefin interconversion, for shift- paraffin cracking can also proceed via tricoordinate carbe-
ing 1-butene to 2-butene, and for increasing C4–C5 nium ions, especially at lower temperatures, higher paraffin
279
0021-9517/96 $12.00
Copyright 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

280
BUCHANAN, SANTIESTEBAN, AND HAAG
TABLE 1
pressure, and over catalysts with large pores (12). In carbe-
nium ion cracking, extensive skeletal isomerization and
shifting of the position of the carbenium center may pre-
cede the bond scission. The carbenium ion remnant do-
nates a proton to the catalyst to desorb as a smaller olefin.
Protonation of olefins, and the reverse reaction, the de-
protonation of carbenium ions, are extremely fast reac-
tions. This is indicated by the fact that the rate of double-
bond isomerization of olefins exceeds that of other olefin
conversion reactions by orders of magnitude. Thus, the
carbenium ions can be considered to be in equilibrium
with the corresponding olefins and, for equilibrated olefins,
with each other. The detailed nature of the intermediate
species, often depicted simply as a free carbenium ion, is
the subject of continuing investigation (13). The ground
state is proposed to be covalently bound to an anionic
lattice oxygen, forming an alkoxy species; higher energy
moieties are an ion pair, and a more or less ‘‘free’’ carbe-
nium ion which resembles the transition state. Thus, it is
the concentration and reactivity of the latter that deter-
mines the reaction rate. The stability, and hence relative
concentrations, of carbenium ions increases strongly in the
order 1Њ Ͻ 2Њ Ͻ 3Њ, so processes which involve the forma-
tion of a primary cation are energetically unfavorable.
Carbenium ion intermediates are involved in isomeriza-
tion and cracking of paraffins as well as olefins over acid
catalysts and dual-function (metal/acid) catalysts. Martens
and Jacobs (14) discuss carbenium ion mechanisms in de-
tail, drawing on results from both low-temperature (195
K) reactions in superacids and paraffin hydrocracking over
dual-function solid catalysts. They point out the impor-
tance of protonated cyclopropane intermediates in skeletal
isomerization.
In treating the isomerization and hydrocracking of C9–
C16 paraffins over Pt/ZSM-5, Weitkamp et al. (15) intro-
duced terminology which is useful in organizing the various
types of carbenium ion scission. ͱ-scission, where the initial
carbenium ion is tertiary and the carbenium ion remaining
after scission is also tertiary, is denoted as type A. Type
B1 scission starts with a secondary ion and finishes with a
tertiary ion, while type B2 moves from a tertiary to second-
ary ion. Type C involves a secondary to secondary transfor-
mation, and type D is secondary to primary. We adopt this
terminology in the discussion below, but find it necessary to
increase the number of categories to include, for instance,
primary-tertiary transformations, which we designate as
type E.
Cracking Rate Constants for Hydrocarbons over HZSM-5
at 510؇C and 10 Torr Partial Pressure
Rate constant k,
1
secϪ
Ratio of rate constants,
Carbon number
Paraffin
Olefin
olefin/paraffin
4
5
6
7
8
0.08
0.30
0.84
1.49
2.25
—
9.5
231.0
1823.0
5732.0
—
32
275
1220
2550
diffusion effects (7). For each feed, all of the plausible
carbenium ion beta-scission steps were enumerated and
then related to the observed reaction rates and product
distributions, in order to obtain estimates of the relative
contribution of each mode of beta-scission. This work with
C5–C8 olefin cracking may be compared with a detailed
study of hydrocracking of C8–C10 paraffins over Pt/USY
where olefins are involved as intermediates (16, 17).
METHODS
Cracking reactions were run at atmospheric pressure in
an 8 mm i.d. quartz reactor with a central thermowell. The
reactor was heated by a tubular furnace. Nitrogen carrier
gas passed through a bubbler containing the feed hydrocar-
bon, and then through a temperature-controlled condenser
kept at a lower temperature than the bubbler. A second
stream of diluent nitrogen was used to adjust the hydrocar-
bon partial pressure to the desired value (typically 10–100
Torr). For olefin cracking, powdered catalyst (5–30 mg)
was diluted with about 1 cc of 50/80 mesh quartz chips.
For paraffin cracking, around 0.5 g of 14/30 mesh catalyst
was used. The conversion of the various feeds was main-
tained within a kinetically useful range by adjusting the
amount of catalyst and the flow rate. On-line analysis of
products was done using a Varian 3700 gas chromatograph
with a 60 m, 0.25 mm i.d. nonpolar capillary column.
Experiments were carried out using a ZSM-5 catalyst
with a molar silica : alumina ratio of 650 : 1. First order rate
constants were calculated using a contact time in seconds,
defined as (gas flow at reaction temperature, cc/sec) (cata-
lyst density, 1.78 g/cc)/(catalyst loading, g). The catalyst
density here is the density of the zeolite crystals.
In the present study, an effort is made to quantify the
relative rates of the various types of beta-scission involved
in cracking of olefins at elevated temperatures. Cracking
rates for C5–C8 olefins over ZSM-5 at 510ЊC were deter-
mined, using low hydrocarbon partial pressures (10–100
RESULTS
First-order rate constants for monomolecular cracking
Torr) to minimize bimolecular reactions. ZSM-5 of high of C4–C8 paraffins and C5–C8 olefins over ZSM-5 at 510ЊC
SiO2/Al2O3 and hence of low activity was used to minimize are listed in Table 1, and values are plotted in Fig. 1. As

ACID-CATALYZED OLEFIN CRACKING
281
tern when the conversion is varied. A small amount of
skeletal isomerization of the butene product is observed
with hexene cracking. Since C7 and C8 cracking occur at
much shorter contact time, secondary butene isomerization
is not significant with these feeds. Thus, the olefinic prod-
ucts from cracking of C6–C8 olefins are kinetically deter-
mined and mechanistically significant.
The equilibrium percentage of isobutene in total butenes
is about 43% at 510ЊC. The percentage of isobutene in
butenes from 1-hexene cracking (around 30%) falls well
below the equilibrium value. Cracking of 1-heptene pro-
duces mainly isobutene (about 75%), even though the feed
molecule is linear.
FIG. 1. Cracking rule constants for linear hydrocarbons over 650 : 1
ZSM-5 at 510ЊC.
DISCUSSION
These results can be rationalized in terms of carbenium
ion mechanisms. Shown in Figure 2 and Figures 4–6 are
possible carbenium ion structures and types of beta-scis-
sion for C5 , C6 , C7 and C8 species. Transformations which
would involve both initial and final primary carbenium
ions or (except for pentene cracking) the formation of a
methyl carbenium ion are omitted, because of unfavorable
energetics, but reactions involving formally primary carbe-
nium ions had to be recognized to account for the cracked
products. The nomenclature of Weitkamp et al. (15) is
followed in the main, but several new categories have been
added. Secondary-to-primary scissions are referred to as
type D2, rather than the original D, and primary-to-second-
ary processes are termed D1. It was felt that primary–
secondary processes should not be barred from consider-
ation, unless they can be shown to be much less probable
than secondary–primary scissions. The population of sec-
ondary carbenium ions is greater than that of primary
carbenium ions, which would tend to favor secondary–
primary processes, but this may be balanced by the greater
energy barrier for the actual scission, relative to primary–
secondary cracking. Primary–tertiary (E1) and tertiary–
primary (E2) processes are introduced, partly to explain
the formation of isobutene during hexene cracking. The
cracking patterns of the individual olefin feeds are dis-
cussed in detail below.
expected, paraffins crack much more slowly than olefins.
The rate of paraffin cracking increases only moderately
with carbon number. At the relatively high temperature
and low pressure chosen for these experiments, paraffin
cracking proceeds mainly via proton donation to form a
pentacoordinated carbonium ion (12). Since the true acti-
vation energies and transition states in monomolecular
cracking are essentially independent of carbon number
(18), this weak dependence of the rate on carbon number
is mostly due to the increase in sorption constants. By
contrast, the rate of olefin cracking increases dramatically,
especially for C5 through C7 . In addition to an increase
in the sorption constants, which is similar to that of the
corresponding paraffins, increasing the size of the olefin
can allow much more facile cracking modes of the carbe-
nium ion intermediate, as discussed below.
Product distributions are listed in Table 2, along with
percent branching of the butenes. Product selectivities ob-
tained from C6–C8 olefin cracking typically changed little
with level of conversion. Selectivities towards paraffins
were less than 1 mol%, except for pentene, which had a
methane selectivity of about 3 mol% (Table 2). The product
distribution for 2-pentene (mainly ethylene and propylene)
indicates monomolecular cracking dominates under our
experimental conditions, although some dimerization-
cracking is indicated by the butene product and the excess
of propylene over ethylene. The formation of other prod-
ucts such as pentane and cyclopentene indicates the occur-
rence of significant secondary reactions during pentene
cracking.
Pentene Cracking
Figure 2 shows three modes of ͱ-scission of a C5 carbe-
nium ion via D-type cracking, all involving a primary cat-
For hexene and larger olefin feeds, however, over 99% ion. This accounts for the slow rate of monomolecular
of the cracking occurs by a single cracking step to produce cracking of pentene. A fourth mode, primary-to-primary,
two smaller olefin molecules per feed molecule cracked. is expected to be much more difficult and will not be consid-
Under our reaction conditions, secondary bimolecular re- ered in the following. The slow rate of monomolecular
actions of the products are negligible, as indicated by the cracking allows dimerization-cracking and various side re-
good molar balances between pairs of expected fragments, actions to become significant with pentene feed. Some of
the absence of paraffins which might result from bimolecu- these side reactions are shown in Fig. 3.
lar reactions, and the constancy of the fragmentation pat-
The third cracking type, D3, results in a methyl cation,

282
BUCHANAN, SANTIESTEBAN, AND HAAG
FIG. 2. Modes of ͱ-scission of C5 carbenium ion.
which is energetically highly unfavorable. Nevertheless,
the formation of about 3% methane may result from this
type of cracking. The presence of the anionic zeolite frame-
work can stabilize methyl or primary carbenium ions, such
that the energy differences among the different types of
carbenium ions is less than indicated by measurements of
the heats of formation of free, gas-phase carbenium ions
(19). Instead of eliminating a proton, the methyl carbenium
ion can add to pentene to form a C6 species, or abstract a
hydride ion (hydrogen transfer), preferably from the most
abundant hydrocarbon, pentene. The resulting pentenyl
cation can undergo cyclization to cyclopentene (Fig. 3I).
Additional hydrogen transfer between two pentene mole-
cules yields pentane and additional cyclopentene (Fig. 3II).
FIG. 4. Modes of ͱ-scission of C6 carbenium ion.
than its cracking rate (8, 10), so the rate of isomerization
to form the 4-methyl-2-pentyl cation probably does not
limit the cracking rate. For a 1-hexene feed, significant
amounts of the other normal isomers (2-hexene and 3-
hexene, both cis and trans) appear at short contact times
(less than 5% conversion to C5-cracking products), fol-
lowed by branched isomers at longer contact time. This
double bond shift is much faster than skeletal isomerization
(14). A near-equilibrium hexene isomer distribution can
be obtained at 26% conversion to C5-cracking products
(10). These observations indicate that adsorption (with
formation of a carbenium ion) and desorption of olefins
on an acid catalyst are fast processes. These data rule out
a mechanism whereby a given carbenium ion resides on
an acid site long enough to undergo double bond shift,
formation of a methyl side chain, methyl shift, and eventual
cracking, with no desorption intervening. The appearance
in the gas phase of the hexene isomers prior to appreciable
cracking indicates, rather, than a C6 entity undergoes nu-
merous adsorption and desorption events on Brønsted acid
sites, and only during a few of its adsorption episodes does
a methyl branch form or shift to give skeletal isomers.
Cracking occurs even less frequently.
Hexene Cracking
For the hexyl cation (Fig. 4), there are six modes of ͱ-
scission which involve a primary cation, and one C-type
scission, where only secondary cations are involved. The
availability of this C-type scission of the 4-methyl-2-pentyl
carbenium ion is mainly responsible for the dramatic in-
crease in cracking rates in going from pentene to hexene.
The rate of hexene skeletal isomerization is much faster
Order of magnitude estimates of the relative rates of
the various modes of ͱ-scission shown in Fig. 4 can be
made by relating them to the observed rates of formation of
propylene, n-butene, and isobutene. Propylene, n-butene,
and isobutene are produced from 1-hexene in proportions
of about 42 : 3 : 1, from the selectivities listed in Table 2.
Iso-butene is produced only by the E1 and E2 scissions,
n-butene by the D1 and two D2 scissions, and propylene
FIG. 3. Hydrogen transfer and diene cyclization as side reactions in
pentene cracking.

ACID-CATALYZED OLEFIN CRACKING
283
TABLE 2
Olefin Cracking over HZSM-5 at 510؇C and 10 Torr
Partial Pressure
Selectivity, mol%
Feed
Conv. wt%
C2ϭ
C3ϭ
C4ϭ
C5ϭ
i-C4ϭ/C4ϭ%
a
2-C5ϭ
1-C6ϭ
1-C6ϭ
1-C6ϭ
1-C7ϭ
1-C8ϭ
7.7
20.1
34.0
65.5
94.1
85.0
31.7
7.0
7.4
7.5
0.9
38.3 11.9
—
35.4
26.0
28.3
34.2
74.2
44.0
b
83.6
83.2
83.4
7.9
8.0
7.9
0.6
0.7
0.7
0.9
47.3 47.7
trace 27.5 44.4 28.0
a Other observed products include methane 3.2 mol%, ethane 0.3%,
iso- and n-pentane 3.7%, cyclopentene 7.2%, methylcyclopentene 2.7%,
cyclohexene 0.2%, and hexadiene 0.7%.
b Measurement at 5 Torr hydrocarbon partial pressure.
by one D2 and the C scission. Since hexene isomerizes
much faster than it cracks, it is reasonable to assume that
all seven forms of the hexyl carbenium ion can exist in
contact with the acid site of the catalyst. If the further
assumption is made that the rates of all D1 and D2 reac-
tions (lumped as ‘‘D’’) are equal, and likewise that the
rates of the E1 and E2 (‘‘E’’) reactions are equal, a set of
three equations can be formulated and solved to obtain
the relative rates of the C, D, and E scissions as 40 : 2 : 1,
respectively. The C scission accounts for about 80% of the
total hexene cracking rate.
Heptene Cracking
FIG. 5. Modes of ͱ-scission of C7 carbenium ion.
Heptene has two B and two C scission modes, compared
to only one C mode available for hexene. Figure 5 shows,
for heptene cracking, five D or E type scissions leading to
ethylene plus pentene, and four D or E scissions leading to
propylene plus butene. Two C scissions produce propylene
Octene Cracking
The isomerization of 1-octene leads to a large number
and n-butene, while two B-type scissions give propylene of isomers. The structural isomers consist of linear octene,
and isobutene. Using the same set of assumptions de- three methylheptenes, five dimethylhexenes, four trimeth-
scribed above, the iso-butene/total butenes ratio listed in ylpentenes, and two isomers with ethyl side chains. The
Table 2 for heptene cracking (74.2%) indicates that, on predominant beta scission structures for octenes of differ-
average, a B-type scission is about three times faster than ent degrees of branching are depicted in Fig. 6, together
a C-type. Thus, the heptyl cation, with 2 C and 2 B scission with the resulting cracked products. The possibility of un-
modes available, might be expected to crack around eight dergoing a more facile type of scission (D Ͻ C Ͻ B Ͻ A)
times faster than the hexyl cation, whose cracking is domi- increases with the degree of branching. Dimethylhexenes
nated by a single C-type scission. This corresponds closely and most trimethylpentenes crack via B-type scission. For
to the experimental ratio of heptene to hexene cracking only one isomer, 2,4,4-trimethyl-2-pentene, is tertiary-ter-
rates of 7.9, from the data in Table 1.
tiary or A-type scission possible. Six B-type beta-scissions
Cracked over ZSM-5 at 400ЊC, 2,3-dimethyl-2-pentene are possible, three of which lead to propylene plus pentene,
and 4,4-dimethyl-2-pentene feeds gave the same product with the other three each giving one iso-butene plus one
distribution as that shown in Table 2 for 1-heptene. This n-butene. Of the eight C-type scissions, six yield propylene
suggests that with heptene, as with hexene, isomerization plus pentene, and two each give two n-butenes. In general,
is rapid compared with cracking.
D- and C-type cracking yields only linear products; B-type

284
BUCHANAN, SANTIESTEBAN, AND HAAG
TABLE 3
Hexene Cracking over HZSM-5 at 510؇C
Relative rates of
formation (moles)
Cracking
rate type
Rate per
type
No. of
structures
Rel.
rate
C2ϭ C3ϭ n-C4ϭ i-C4ϭ
E 2/i-4
D 2/n-4
D 3/3
1
2
2
40
2
3
1
1
2
6
2
40
50
2
6
—
—
8
—
—
4
80
84
—
6
—
—
6
2
—
—
—
2
C 3/3
Rate totals:
Distribution, mol%
Calculated
Observed
8.0 84.0 6.0
7.5 84.4 5.8
2.0
2.3
for selectivity differences found in hydrocracking of large
paraffins in ZSM-5 versus large pore catalysts. For ZSM-5,
we neglect A-type cracking of 1-octene in our analysis of
the relative contributions of different cracking modes.
Prediction of Cracking Selectivities and Rates from
Cracking Rates of Individual Carbenium Ions
It is possible to describe the overall rate of cracking of an
olefin as the sum of the various cracking rates of individual
carbenium ions
r total ϭ k[olef] ϭ (␯BkB ϩ ␯CkC ϩ ␯DkD ϩ ␯EkE) [olef]
FIG. 6. Predominant ͱ-scission structures and cracking types and
resulting products for octene cracking.
where ␯ ϭ number of carbenium ion structures cracking
via a particular scission mode, and ki ϭ relative cracking
rate constant of ith mode.
i
By the procedure outlined above, relative ki values for
E-, D-, and C-type cracking have been obtained from hex-
ene cracking. From heptene cracking, we obtain an addi-
tional value for kB . These relative rates of cracking are
as follows:
produces one linear olefin and one branched olefin, and
A-type scission gives two branched olefins.
If the A-type scission were dominant, the products would
be mostly isobutene, since that is the exclusive product of
the A-type scission. However, for 1-octene cracking over
ZSM-5 at 510ЊC (Table 2), the selectivity to propylene plus
pentene exceeds the selectivity to total butenes, and the
percent isobutene in butenes is only 44%. Either the A-type
cracking is not appreciably faster than B-type cracking, or
the octyl cations crack before they isomerize to the one
tribranched isomer involved in A-type scission. The latter
explanation is more likely. Abbot and Wojchiechowski
(8) noted that the rate of cracking of olefins increased
Relative rate constant
Cracking type
for ZSM-5 at 510ЊC
E
D
C
B
1
2
40
120
Using these values for ki , and the corresponding abun-
dramatically with carbon number in going from pentene dance factors ␯ obtained from the detailed listing of all
i
to octene, but the rate of skeletal isomerization hardly the relevant carbenium ion structures (e.g. Figs. 3–4), one
changes. Moreover, the formation of the highly branched can calculate an expected product distribution for cracking
2,4,4,-trimethylpentyl cation may be greatly restricted of C6–C8 olefins. The procedure and the results are listed
within the ZSM-5 pores. Weitkamp et al. (15) present this in Tables 3–5. The relative rates of reaction given in Tables
constraint on formation of highly branched carbenium ion 3–5 are scaled to a hexene cracking rate of 50 mol per unit
precursors to A-type cracking in ZSM-5 as an explanation time. For example, the sum of the relative rates of the

ACID-CATALYZED OLEFIN CRACKING
285
TABLE 4
Heptene Cracking over HZSM-5 at 510؇C
Relative rates of formation (moles)
Cracking
type
Rate per
type
No. of
structures
Rel.
rate
C2ϭ
C3ϭ
n-C4ϭ
i-C4ϭ
C5ϭ
E 2/5
1
1
2
2
40
3
1
3
3
2
2
3
1
6
6
80
3
—
6
—
—
—
9
1.3
0.9
—
1
—
6
80
240
327
48.7
48.8
—
—
—
6
80
—
86
12.8
12.7
—
1
—
—
—
240
241
35.9
36.6
3
—
6
—
—
—
9
1.3
0.9
E 3/i4
D 2/5
D 3/n4
C 3/n4
B 3/i4
120
Rate totals:
240
336
Calculated
Observed
Distribution, mol%:
various hexene cracking modes listed in Table 3 sum to tivity to isobutene; however, at conversions higher than
50 moles hexene cracked per unit time, while the relative about 10%, isomerization to other skeletal isomers with
rate of products formed from hexene cracking sum to 100 B- and C-type cracking begins to contribute. Since A-type
mol per unit time. The product distributions for Tables cracking is intrinsically faster than other types, the rate of
3–5 were renormalized to take into account only C2–C5 cracking with 2,4,4-trimethyl-2-pentene feed ought to be
olefins. It is apparent that a satisfactory agreement between appreciably faster than the rate of 1-octene cracking, in
the calculated and the observed product distribution exists. the absence of diffusional limitations. However, 1-octene
It is noteworthy that the calculated product distribution cracked more than 50 times faster than 2,4,4-trimethyl-
for 1-octene cracking in Table 5 is not based on data from 2-pentene, due to the severely limited transport of the
octene cracking (other than the insight that A-type crack- tribranched isomer in the ZSM-5 channels. Preliminary
ing can be neglected for 1-octene feed), but is predicted results with cracking n-octene over ultrastable Y (USY)
from the relative rates of B- and C-type beta-scissions as catalyst at about 500ЊC show product distributions similar
determined using the heptene feed.
to those seen with ZSM-5, indicating that, even for the
In Table 6 are summarized the contributions of the vari- large-pore USY, B-type and C-type cracking largely drain
ous cracking types to the overall cracking rate for C6–C8 away the C8 species before they can isomerize to the 2,4,4-
olefins. About 70–80% of the cracking process follows the isomer and crack via A-type cracking. Restriction on the
easiest cracking path available. The values for 1-octene formation or transport of the tribranched species cannot
apply to essentially all octene isomers as feed except for be invoked as the reason for this cracking pattern in USY,
2,4,4-trimethylpentene. With ZSM-5 at 510ЊC, this isomer since a 2,4,4-trimethyl-2-pentene feed cracked much faster
cracks at low conversion almost exclusively by the tertiary– than n-octene over USY. Thus, at high temperature over
tertiary A-type scission, as evidenced by nearly 100% selec- solid acids, cracking of monobranched and dibranched C8
species can compete effectively with isomerization to the
TABLE 5
TABLE 6
Octene Cracking over HZSM-5 at 510؇C
Percent Cracking by Cracking Type 510؇C,
Relative rates of formation
HZSM-5
(moles)
Cracking Rate per
No. of
structures rate
Rel.
Feed
1-C8ϭ
type
type
C3ϭ
n-C4ϭ i-C4ϭ
C5ϭ
Cracking type
1-C6ϭ
1-C7ϭ
2,2,4-TMPϭ
C 3/5
C n4/n4
B 3/5
40
40
120
6
2
3
3
240 240
80
360 360
360
1040 600
—
160
—
360
520
—
—
—
360
360
240
—
360
—
—
A
B
C
D
—
—
80
16
4
—
71
24
4
—
69
31
—
—
100
—
—
—
—
B n4/i4
120
—
Rate totals:
600
Distribution, mol%:
Calculated
Observed
28.8
27.5
25.0
24.9
17.3
19.5
28.9
28.0
E
1
Totals:
100
100
100
100

286
BUCHANAN, SANTIESTEBAN, AND HAAG
TABLE 7
counted for 35% of the octane cracking (17), whereas with
1-octene cracking, essentially all the cracking was via B-
and C-type beta scission. Since the slower cracking modes
are expected to have higher activation energies, the greater
contribution by these cracking modes observed in the pres-
ent study is likely due, in large part, to the higher reaction
temperature employed.
Relative Rates of Olefin Cracking over HZSM-5 at 510؇C
No. of
Relative rate
constant, ki
structures,
Relative
rate
Olefin feed
C6ϭ
Cracking type
ni
C
D
E
40
2
1
1
4
2
40
8
2
CONCLUSIONS
50
240
80
12
4
C7ϭ
B
C
D
E
120
40
2
2
2
6
4
The relative rates of cracking and resultant product dis-
tributions for cracking C5–C8 olefins over ZSM-5 catalyst
were quantified and rationalized in terms of carbenium
ion mechanisms. Cracking rates increase more dramati-
cally with carbon number for olefins than for paraffins, as
more energetically favorable modes become available for
ͱ-scission of the carbenium ion formed by proton donation
to the olefin. For instance, cracking of pentene involves
formation at some point of a primary carbenium ion,
whereas octene cracking can proceed via tertiary carbe-
nium ion intermediates. The appearance of isomers in
abundance prior to appreciable cracking of olefin feeds
indicates that an olefin typically undergoes numerous
chemisorption and desorption events before cracking.
The product distribution from cracking of C6–C7 olefins
was used to estimate the relative contributions of various
modes of ͱ-scission, as classified by the types of carbenium
ions involved. From these results, it was possible to esti-
mate both the overall rate and product distribution for
cracking of 1-octene, by summing the expected contribu-
tions of the different types of ͱ-scission involved in octene
cracking. For hexene and heptene feeds, the most-favor-
able ͱ-scission mode available (involving just secondary
ions for hexene, and secondary plus tertiary for heptene)
accounted for 70–80% of the cracking. For 1-octene feed,
however, the olefin was nearly all cracked via secondary–
tertiary and tertiary–secondary ͱ-scission before it iso-
merized to the 2,4,4-trimethylpentene isomer required to
undergo the most energetically favored (tertiary–tertiary)
form of cracking.
1
336
720
320
1040
C8ϭ
B
C
120
40
6
8
2,4,4-trimethylpentyl isomer which cracks by the energeti-
cally favorable tertiary–tertiary (A-type) ͱ-scission.
Using the same rate parameters (obtained from analysis
of C6–C7 cracking selectivities) which were used above to
match the product distribution, relative rates of the overall
cracking of the parent C6–C8 olefins have been obtained, as
shown in Table 7. The calculated rates of overall cracking
increase in the ratios 1 : 7 : 21 for hexene, heptene and oc-
tene, respectively, as compared to the observed ratios of
1 : 8 : 25. The agreement between calculated and observed
relative reaction rates is satisfactory, considering that the
calculated octene rate is based solely on carbenium ion
scission rates estimated from hexene and heptene cracking.
The results of this study at 510ЊC involving pure olefins
as feeds and products show similarities to the study of
Martens et al. (16, 17) involving hydrocracking of C8–C10
paraffins to light paraffins at 200ЊC or less with a dual-
functional catalyst containing an active metal (Pt), where
olefins are believed to be reaction intermediates. In both
cases, isomerization of linear feed molecules to branched
isomers precedes cracking. The nature of the products and
their relative abundance can be accounted for by applying
the rules of carbenium ion chemistry. However, some dif-
ferences are apparent. In contrast to paraffin cracking, it
was found necessary, with light olefin cracking at 510ЊC,
to invoke the participation of cracking paths involving pri-
mary carbenium ions (D- and E-type cracking). The frac-
tion of cracking occurring by these paths ranges from over
60% for C5 , to 20% for C6 , and 5% for C7 . Also, the ratios
of cracking by the most energetically favored paths are
different. For C7 , hydrocracking proceeded 99% by path
B, and only 1% by path C (16); for olefin cracking, B-type
cracking accounted for only about 70% of the total crack-
ing. With linear C8 feed, isomerization to the 2,4,4-trimeth-
ylpentyl isomer with subsequent A-type cracking ac-
REFERENCES
1. Chen, N. Y., Garwood, W. E., and Dwyer, F. G., ‘‘Shape Selective
Catalysis in Industrial Applications,’’ Dekker, New York, 1989.
2. Haag, W. O., Lago, R. M., and Rodewald, P. G., J. Mol. Catal. 17,
161 (1982); Haag, W. O., in Proceedings Sixth Int. Zeolite Conf.,
Reno, Nev. (D. H. Olson and A. Bisio, Eds.), p. 466, Butterworth,
Guildford, Surrey, 1984.
3. Dejaife, P., Vedrine, J. C., Bollis, V., and Derouane, E. G., J. Catal.
63, 331 (1980).
4. Dessau, R. M., and LaPierre, R. B., J. Catal. 78, 136 (1982).
5. Chang, C. D., Catal. Rev. Sci. Eng. 25, 1 (1983).
6. Quann, R. J., Green, L. A., Tabak, S. A., and Krambeck, F. J., Ind.
Eng. Chem. Res. 27, 565 (1988).
7. Haag, W. O., Lago, R. M., and Weisz, P. B., Faraday Disc. 72, 317
(1982).

ACID-CATALYZED OLEFIN CRACKING
287
8. Abbot, J. and Wojciechowski, B. W., Can. J. Chem. Eng. 63, 462 15. Weitkamp, J., Jacobs, P. A., and Martens, J. A., Appl. Catal. 8,
(1985).
123 (1983).
9. Abbot, J. and Wojciechowski, B. W., J. Catal. 108, 346 (1987).
10. Buchanan, J. S., Appl. Catal. 74, 83 (1991).
11. Nayak, V. S. and Moffat, J. B., Appl. Catal. 60, 87 (1990).
12. Haag, W. O. and Dessau, R. M., ‘‘Proc. 8th Int. Congr. Catalysis,’’
Vol. II, p. 305. Verlag Chemie, Weinheim, 1984.
13. Kazansky, V. B., Acc. Chem. Res. 24, 379 (1991).
14. Martens, J. A. and Jacobs, P. A., in ‘‘Theoretical Aspects of Heteroge-
neous Catalysis’’ (J. B. Moffat, Ed.), Van Nostrand Reinhold, New
York, 1990.
16. Martens, J. A., Jacobs, P. A., and Weitkamp, J., Appl. Catal. 20,
239 (1986).
17. Martens, J. A., Jacobs, P. A., and Weitkamp, J., Appl. Catal. 20,
283 (1986).
18. Haag, W. O., in ‘‘Zeolites and Related Microporous Materials: State
of the Art 1994’’ (J. Weitkamp, H. G. Karge, H. Pfeifer, and W.
Holderich, Eds.), Stud. Surf. Sci. Catal., Vol. 84, p. 1375, Elsevier,
Amsterdam, 1994.
19. Lossing, F. P. and Semeluk, G. P., Can. J. Chem. 48, 955 (1970).