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Applied Catalysis, 67 (1991) 307-324 Elsevier Science Publishers
B.V.. Amsterdam
307
Methylcyclohexane and methylcyclohexene cracking over zeolite Y
catalysts
Avelino Corma*, F. Mocholi and V. Orchilles Instituto de
Catalisis y Petroleoquimica, C.S.I.C. Serrano 119,28006 Madrid
(Spain)
and
Gerald S. Koermer* and Rostam J. Madon Engelhard Corporation,
Menlo Park, CN 40, Edison, NJ 08818 (U.S.A.), (tel. (+
l-908)2055011, fax. (+ l-908)2055300
(Received 8 June 1990, revised manuscript received 24 August
1990)
Abstract
Naphthenes are an important class of molecules in fluid
catalytic cracking. The cracking behavior of the model naphthenes,
methylcyclohexane and methylcyclohexene was investigated over rare
earth Y and USY zeolite catalysts. Initial products from
methylcyclohexane are formed by a combination of protolytic and
/?-scission cracking plus isomerization, H- transfer, H+ transfer
and dehydrogenation reactions. Methylcyclohexane is a sensitive
probe for characterizing the chemistry occurring on solid acid
surfaces. Methylcyclohexene is the key intermediate in the
formation of aromatics from methyl- cyclohexane. Methylcyclohexene
cracks at a slower rate than methylcyclohexane but overall
conversion is higher because hydride transfer reactions are
fast.
Keywords: zeolites, cracking catalysts, methylcyclohexane
cracking, methylcyclohexene cracking, na- phthene cracking.
INTRODUCTION
Naphthenes are important constituents of fluid catalytic
cracking (FCC) feedstocks and products [ 11. Despite this, the
cracking of alkanes, alkenes and short chain alkyl aromatics have
been studied much more than naphthenes. Relatively little has been
reported about the cracking chemistry of naphthenes over zeolite
catalysts [ 2-51. Unsaturated naphthenes have been studied even
less [6,7].
Currently, the aromatics content of FCC gasoline fractions is an
important issue for both gasoline octane and reformulated
gasolines. In this regard, naph- thenes are important in cracking
chemistry as precursors for aromatics and coke [ 81 and as key
agents in hydrogen transfer chemistry [ 2,3]. Thus a better
understanding of naphthene cracking chemistry is important for
controlling aromatics yields.
0166.9834/91/$03.50 0 1991 Elsevier Science Publishers B.V.
-
Nevertheless, the cracking chemistry of naphthenes is expected
to be com- plex because a number of competing reactions such as
cracking, hydride trans- fer, ring opening and isomerization can
occur simultaneously. However this rich chemistry should make
naphthenes a sensitive probe for catalyst proper- ties. In fact,
there have been recent publications using cyclohexene as a test
molecule for hydrogen transfer [ 9,101. However this work was done
at tem- peratures significantly lower than commercial cracking
temperatures.
We have investigated the cracking behavior of the model
naphthenes, meth- ylcyclohexane (MCHA) and methylcyclohexene
(MCHE), over zeolite Y cat- alysts. We report here (but see also
refs. 11 and 12) the cracking chemistry of these compounds and the
utility of MCHA as a probe molecule. The cracking chemistry
provides insights into naphthene reactivity for both higher molec-
ular weight and gasoline range naphthenes. Reaction product ratios
allow us to assess subtle differences among zeolite catalysts.
EXPERIMENTAL
Materials
Methylcyclohexane (99% ) and 1-methyl-1-cyclohexene (97% ) were
pur- chased from Aldrich and used without further purification.
Ultrastable Y (USY, Catalyst I) was prepared by standard
hydrothermal techniques [ 131 from particles of approximately 80%
zeolite Y in a low surface area, low activity y-alumina binder.
After hydrothermal calcination and am- monium exchange, the zeolite
unit cell size (UCS) was 24.53 A and the catalyst Na,O content was
0.69%.
Rare earth Y (Catalyst III) was prepared by ion exchanging mixed
rare earth ions into approximately 80% zeolite Y bound with a low
surface area alumina. The rare earth (RE,O,) content of the
catalyst was 7.2 wt.-%, the Na,O con- tent was 1.41% and the UCS
was 24.70 A.
Catalyst I was steamed in a fluidized bed at 732 C for 8 h at
one bar of 100% steam to give Catalyst II which had a UCS of 24.31
A. Steaming Catalyst III in a fluidized bed at 732C for 8 h in 1
bar of 100% steam reduced the UCS to 24.45 A (Catalyst IV).
Steaming reduced the absolute zeolite content of the unsteamed
material by lo-15%.
Catalysis
Catalytic results were obtained using a fixed bed glass tubular
reactor [ 141 with a 32-mm internal diameter. Reactions were done
at atmospheric pressure and 500. Data was generated by two methods:
(i) by holding time-on-stream constant and changing the amount of
catalyst (i.e. changing the catalyst-to- oil ratio) to vary
conversion and (ii) at constant cat/oil with a variable time-
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309
on-stream. Standard optimum performance envelope techniques
described previously [ 2,151 were used. Initial selectivities were
calculated from the ini- tial slopes of plots of mol-% yield vs.
percent conversion of feed.
Analysis
Product analysis was done by gas chromatography. Gaseous
products were analyzed using a Poropak Q plus silica column. Liquid
products were analyzed using a 60-m SE 39 capillary column. Product
identification was done by re- tention time comparison to known
samples and by gas chromatography-mass spectrometry (GC-MS) using a
Shimadzu Model QP-1000 GC-MS equipped with a 100-m Supelco Petrocol
DH capillary column.
REACTION CHEMISTRY
Methylcyclohexane (MCHA) cracking
MCHA has one primary, one tertiary and five secondary carbon
atoms. In principle, MCHA cracking could occur by both protolytic [
21 and p-scission mechanisms [ 2,3] via secondary and tertiary
carbocation intermediates.
Protolytic cracking: C-C bond cleavage *
+ H- c
0
+ ii -0
+ CH,
(1)
(2)
The intermediate carbocations formed from protolytic cracking of
ring C-C bonds can react in several ways:
- desorption as heptene by returning H+ to the catalyst (proton
transfer from the carbenium ion).
- desorption as heptane after abstraction of H- from another
hydrocarbon (hydride transfer).
- isomerization to branched carbenium ions, followed by
desorption as de- scribed above.
- cracking before or after isomerization to give an alkene and
an adsorbed carbenium ion. This carbenium ion can desorb by proton
transfer or hydride transfer to give an alkene or alkane
respectively.
Therefore, one expects heptanes, heptenes and products with less
than seven carbon atoms as primary products from protolytic
cracking of the MCHA ring. If however, the terminal methyl group is
cleaved, methane and the products from the cyclohexyl carbenium ion
result. These products will include cyclo-
-
hexene, cyclohexane, methylcyclopentane, methylpentanes,
methylpentenes and products with less than six carbons.
Protolytic cracking: C-H bond cleavage The processes discussed
above involve C-C bond cleavage. Alternatively
strong Bronsted sites can protonate a C-H bond to form hydrogen
[ 161. This process is less favorable than C-C bond cleavage but is
still feasible at cracking temperatures for molecules like MCHA
that contain tertiary hydrogens [ 171. Protonation of the tertiary
hydrogen in MCHA gives hydrogen and methyl- cyclohexyl carbocation
(eq. 3 ). The methylcyclohexyl
(3)
carbocation can isomerize, lose a proton to give
methylcyclohexene (MCHE ) or crack by the j?-scission mechanism to
give the products described below.
Cracking by j%scission
(4)
Once a methylcyclohexyl carbocation is formed, it can crack by
the conven- tional /3-scission process (eqn. 4 ) . This gives
heptadiene, heptenes and prod- ucts with less than seven carbons,
primarily Cqs and C,s.
Isomerization Methylcyclohexyl carbocation can also isomerize to
give alkyl cyclopentyl
carbocations [5]. These species can either desorb to give
alkylcyclopentenes and alkylcyclopentanes or crack giving mostly
branched C, carbocations. Since the isomerization of cyclohexyl
carbenium ions is fast, monoalkylcyclopen- tanes and
monoalkylcyclopentenes should also occur as products.
Dehydrogenation As described above, protolytic cracking of a C-H
bond can lead to dehydro-
genation of MCHA. Another dehydrogenation route involves
hydrogen trans- fer. Here a surface carbenium ion abstracts a
hydride from a donor such as MCHA, thereby generating a
methylcyclohexyl carbocation. This species can lose H+ and desorb,
thereby regenerating the protonic site and giving meth-
ylcyclohexene (MCHE). MCHE can continue to lose hydrogens through H
transfer [ 181 giving methylcyclohexadiene and finally toluene.
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311
RESULTS AND DISCUSSION
Initial cracking products
Table 1 shows the observed initial products for MCHA cracking
over cata- lysts I-IV at 500C. Fig. 1 contains plots of mol-% yield
versus MCHA con- version for selected products from Catalyst I. In
the following discussion, for brevity, unless otherwise noted,
Catalyst I results are used.
Inspection of Table 1 indicates that the products can be
rationalized by a combination of protolytic and j$scission
cracking. In addition, both dehydro- genation mechanisms noted
above seem to occur.
Additional information can be extracted from the data. For
example, the amount of methane is slightly higher than the amount
of cyclohexane. This can be explained by considering that
cyclohexyl carbocation, before desorbing
TABLE 1
Initial molar selectivities for methylcyclohexane cracking over
catalysts I-IV at 500C
Catalyst I II III IV
Methane 0.017 0.020 0.012 0.009 Ethane 0.005 0.004 0.005 0.003
Ethene 0.030 0.023 0.034 0.021 Propane 0.073 0.054 0.090 0.073
Propene 0.106 0.146 0.125 0.113 Butanes 0.186 0.131 0.205 0.194
Butenes 0.083 0.100 0.066 0.097 Pentanes 0.060 0.045 0.063 0.058
Pentenes 0.007 0.007 0.010 0.010 Cyclohexane 0.011 0.018 0.008
0.010 2-Methylpentane 0.027 0.012 0.034 0.020 3-Methylpentane 0.022
0.017 0.021 0.018 Hexane 0.005 0.004 0.004 0.004 Methylcyclopentane
0.002 0.001 0.001 0.001 Dimethylcyclopentanes 0.042 0.040 0.051
0.035 Dimethylpentane 0.004 0.003 0.005 0.005 2Methylhexane 0.012
0.016 0.014 0.017 3-Methylhexane 0.019 0.030 0.021 0.023 n-Heptane
0.152 0.156 0.146 0.167 Heptenes 0.219 0.258 0.206 0.250
Methylcyclohexenes 0.092 0.122 0.106 0.080 Methylcyclohexadienes
0.005 0.002 0.005 0.006 Toluene 0.058 0.035 0.052 0.040 Xylenes
0.033 0.018 0.028 0.022 Hydrogen 0.073 0.066 0.027 0.024 Coke 0.006
0.003 0.003 0.003
-
Mole % Yield
6 (A)
o- 0 10 20 30
Methylcyclohexane Mole % Yield
6i@)
4+
40 50
Conversion
_, 3-
2- / /
I- ,d
/
/
Ok
0 I
10 20 30 40 50
Methylcyclohexane Conversion
60
60
Fig. 1. Plots of mol-% yield vs. methylcyclohexane conversion
over Catalyst I at 500C. (A) propene; (B) butenes; (C) heptane; (D)
heptenes; (E) substituted cyclopentanes; (F) cyclo- hexane; (G)
methylcyclohexenes; (H) toluene.
can react further to give methylcyclopentane, plus branched
hexanes and hex- enes derived from ring opening. Indeed, a marked
instability of cyclohexane is apparent in Fig. 1F even at
conversion as low as 5%. If one adds the initial selectivities of
all C!, products detected, the ratio of C,/C, is lower than one,
indicating that C6 products could also be formed by reaction other
than crack-
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313
Mole % Yield
OS&- 0 10 20 30
% Methylcyclohexane Mole % Yield .^
40 50 60
Conversion
0 IO 20 30 40 50 60
% Methycylcohexane Conversion
ing of MCHA to methane and Cs products. One way to generate
excess C6 products without methane could be transalkylation of
methyl groups between two molecules of MCHA (eqn. 5 ) . Indeed,
small amounts of dimethyl and even trimethylcyclohexanes were
identified in the products by GC-MS spectral analysis. Moreover, it
has been shown that when cracking heptane, the C,/C, ratio is also
lower than one [ 141. Disproportionation reactions have been used
to account for this observation [2]. Disproportionation reactions
could also occur in the MCHA system and would produce some extra C,
products. This may account for the low Cl/C6 ratio observed.
-
Mole % Yield
0 10 20 30 40 50
Methylcyclohexane Conversion Mole % Yield
0.16 ! 0.12 c
/
0.08 I 0.04
0 Y 0
_1
60
10 20 30 40 50 60
Methycyclohexane Conversion
Fig. 1.
20-b+ 0 (5) The open chain C7 products are also informative.
Heptanes and heptenes
appear as primary products. Within experimental error, both
heptane and hep- tenes are stable. Instability, if it occurs, is
seen only at high MCHA conver- sions. This means that cracking
products (
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315
Mole % Yield
*I 0
Methylcyclohexane Mole % Yield
J
40 50 60
Conversion
7 (H) 6-
5
4
3 1
2-
l-
ok 0
A
10 20 30 40 50 60
% Methylcyclohexane Conversion
readsorption of heptanes or heptenes. If readsorption followed
by cracking oc- curred extensively, heptane and heptenes would show
marked curvature in the selectivity curves, Figs. 1C and D. This is
not observed. In addition, it seems clear that little or no heptane
is formed from saturation of heptenes by hydro- gen transfer
reactions but mostly by hydride transfer from MCHA to C!, car-
benium ions derived from ring opening of MCHA. Of course heptanes,
hep- tenes and cracked products can also be formed from
dimethylcyclopentanes. Our study does not address this or whether
dimethylcyclopentanes crack more readily than MCHA. Regardless of
the precise precursor for the ring opened
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316
C, carbenium ion, the fate of this cation can be approximated by
looking at the amount of cracking products and the amounts of
heptenes and heptanes. Cracked products come from J?-scission.
Heptanes will reflect the amount of H- transfer to the carbocation
and heptenes reflect the amount of H+ transfer from the ion to the
catalyst. We will discuss the implications of these results in the
next section dealing with the use of MCHA as a probe molecule.
Aromatics formation is also an important process. The
selectivity to hydro- gen is not sufficient to account for the
yields of MCHE, methylcyclohexadiene and toluene by direct
dehydrogenation. However, as a first approximation, hy- dride and
proton transfer between saturated naphthenes, unsaturated na-
phthenes, carbenium ions and the catalyst surface seem to be
responsible for most of the unsaturated naphthenes and aromatics.
We note that MCHE (Fig. lG), and methylcyclohexadiene are unstable
primary products. Whereas tol- uene, (Fig. 1H) is a stable primary
and secondary product. While MCHE is easily rationalized as a
primary product, methylcyclohexadiene and toluene are much more
difficult to explain. The formation of toluene by hydrogen transfer
requires three consecutive bimolecular reactions with intermediate
desorption and readsorption of MCHE and methylcyclohexadiene. Thus,
as observed, MCHE and methylcyclohexadiene are unstable products.
This multi- step bimolecular process for aromatics formation should
have a very low fre- quency factor. The surprising appearance of
toluene even at very low conver- sion could be due to the confined
space in the zeolite cavities that produces high concentrations of
reactants and relatively long residence times for the molecules in
the pores. Also one must take into account the faster rate of de-
hydrogenation of MCHE and methylcyclohexadiene compared with the
initial dehydrogenation of MCHA to MCHE. This can be deduced from
the magni- tude of Catalyst I initial selectivities to MCHE (0.092
), methylcyclohexadiene (0.005) and toluene (0.058). In any case
MCHE is clearly a key intermediate in the reaction chemistry of
MCHA. The importance of MCHE as an inter- mediate will be discussed
further in the section on MCHE reactions.
Methylcyclohexane as a catalyst probe molecule
Our results suggest that initial product ratios from MCHA
cracking can be used as a sensitive probe to characterize and
quantify the chemistry occurring on a catalyst surface. Based on
the previous discussion, the ratio of cracking products to (MCHE +
2 (methylcyclohexadiene) + 3 (toluene ) + 3 (xylenes) + 6 (coke)
-hydrogen) should reflect cracking relative to hydride transfer. In
addition, the ratio of cracking products to heptenes should mirror
cracking relative to H+ transfer to the catalyst. Finally, the
ratio of heptenes to heptane should give the relative rate of
proton transfer to hydride transfer for surface heptenium ions
formed from either C5 or C, ring opening.
If the above ratios are indeed diagnostic for catalyst
performance, these ra-
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317
tios should vary in reasonable ways as the unit cell size (UCS)
of Y zeolite in the catalyst is changed. We assessed this by
determining the ratios for catalysts I through IV. The zeolite UCS
varied from 24.70 to 24.31 A. The results are shown in Figs.
2-4.
Fig. 2 plots the ratio of cracking/H- transfer vs. UCS. The
ratio increases as unit cell size decreases. This is consistent
with previous work showing that
Cracking/Hydride Transfer
3-P
,i- ~ I A_ 24.3 24.4 24.5 24.6 24.7
Unit Cell Size (Angstroms)
Fig. 2. Cracking/ (MCHE+2(methylcyclohexadiene) + 3 (toluene) +6
(coke) - hydrogen) vs. zeolite unit cell size.
Cracking/Heptenes 3.5
2.5 -
2 1 I 24.3 24.4 24.5 24.6 24.7
Unit Cell Size (Angstroms) Fig. 3. Cracking/heptenes vs. zeolite
unit cell size.
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318
Heptenes/Heptane 1.3
, i_ 1 24.3 24.4 24.5 24.6
Unit Cell Size (Angstroms)
Fig. 4. Heptenes/heptane vs. zeolite unit cell size.
24.7
as zeolite UCS decreases cracking becomes more important and
hydrogen transfer decreases [ 19,201.
Fig. 3 plots cracking/heptenes (which reflects cracking relative
to H+ trans- fer) vs. UCS. This plot decreases with UCS, indicating
that as UCS decreases, the rate of H+ transfer becomes steadily
more important relative to cracking.
Finally, heptenes/heptane should increase as the H- transfer
ability of the zeolite decreases with UCS [ 19,201. This indeed is
observed (Fig. 4). So the ratio of heptane/heptenes is a unique
measure of the partitioning of the C, ion between H- transfer to
the ion and H+ transfer from the ion to the catalyst.
Methylcyclohexene (MCHE) reactions
As discussed above, MCHE is a key intermediate in understanding
the cracking chemistry of MCHA and in forming aromatics from MCHA.
MCHE cracking over Catalyst I at 500 C gives the initial molar
selectivities shown in Table 2. Selected plots of mol-% yield vs.
conversion are in Fig. 5. Comparison of the initial selectivities
for MCHE and MCHA from the same catalyst (I ) at 500 C (Table 1)
shows some striking differences. The major products from MCHE are
MCHA and aromatics (toluene and xylenes ) . In contrast, MCHA
conversion gives primarily cracking products (C&s, C4s) and
ring opened prod- ucts (C,s). Clearly MCHE must have an alternate
reaction pathway besides simple protonation of the double bond to
give methylcyclohexyl carbocation.
The reason for the differences in product selectivities between
MCHA and MCHE can be understood from Table 3 which gives the
approximate initial rate constants for the appearance of cracking,
ring opening, hydrogen transfer
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319
TABLE 2
Initial molar selectivities for methylcyclohexene cracking over
Catalyst I at 500C
Product I.S.
Methane 0.010 Ethane 0.003 Ethene 0.020 Propane 0.006 Propene
0.088 Butanes 0.040 Butenes 0.060 Pentanes 0.010 Pentenes 0.013
Hexanes 0.028 Hexenes 0.012 Methylcyclopentanes 0.003
Dimethylpentanes 0.001 Dimethylcyclopentanes 0.013 Cyclohexane
0.034 Heptane 0.050 Heptenes 0.087 Methylcyclohexane 0.290
Methylcyclohexadiene 0.020 Toluene 0.245 Xylenes 0.052 Hydrogen
0.004 Cg aromatics 0.002 Coke 0.029
and isomerization products from MCHA and MCHE. These rates were
gen- erated from plots of time-on-stream vs. the sum of yields for
products of each class [ 151.
Clearly the rate of cracking and ring opening for MCHE is less
than the rate of hydrogen transfer. In addition, the rate of MCHE
cracking is slower than that of MCHA. The relative cracking rates
are surprising since alkenes gen- erally crack as much as two
orders of magnitude faster than their saturated analogs [ 211.
However, the total rate of reaction of MCHE exceeds that for
MCHA be- cause the transformation of MCHE to aromatics,
methylcyclohexadiene and MCHA is very fast. Also noteworthy is the
relatively high coke yield from MCHE and the lower hydrogen yield
compared to MCHA.
The key to understanding MCHE cracking chemistry is the hydrogen
trans- fer reaction. Rapid hydrogen transfer occurs for several
reasons. First, MCHE is both an alkene and a naphthene, thus it can
fulfill two roles. MCHE can act as both a hydrogen donor and
acceptor. Protonation of MCHE leads to a high
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320
Mole % Yield 0.5
(n)
0.4 I
% Methylcyclohexene Conversion
Mole % Yield
I2 (6)
10 -
8-
8-
0 10 20 30 40 50 60
% Methylcyclohexene Conversion
Fig. 5. Plots of mol-% yield vs. percent methylcyclohexene
conversion over Catalyst I at 500C. (A) methylcyclohexadiene; (B )
toluene.
concentration of the hydride acceptor, methylcyclohexylcarbenium
ion on the catalyst surface. Hydrogen transfer then occurs between
this ion and MCHE which is present in high concentration since it
is the feed. The net result is MCHA plus methylcyclohexenyl
carbocation. Thus the bimolecular hydride transfer reaction is
facilitated by a high concentration of both acceptor and donor.
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321
TABLE 3
Initial rate constants (s-l) over Catalyst I at 500C
Methylcyclohexane Methylcyclohexene
Cracking 0.104 0.049 Isomerizationa 0.006 B
Hydrogen transfer 0.045 0.107 Ring opening 0.032 0.032
Formation of substituted cyclopentyl compounds. ?Not
determined.
Another reason for rapid hydride transfer in this system is the
formation of a relatively stable [ 221 allylic delocalized
carbocation from MCHE.
Thirdly, the axial hydrogens adjacent to the double bond in MCHE
are held by the cyclohexenyl ring in a conformation that makes them
coplanar with a p orbital of the methylcyclohexene double bond.
This stabilizes the developing positive charge in the C-H sp3-s
bond as the hydride is being transferred. The net result is a
relatively low activation energy for hydride transfer from an
unsaturated naphthenic ring.
The resulting allylic carbenium ion loses a proton to give
methylcyclohex- adiene which quickly again transfers a hydride and
then eliminates a proton to give toluene. The high reactivity of
methylcyclohexadiene can be seen from Fig. 5. Toluene formation is
thermodynamically favorable and toluene is com- paratively very
stable. Thus the overall process is fast and essentially
irreversible.
A simplified reaction pathway for MCHE reaction is shown in Fig.
6. MCHA+ (I) is formed by protonation of MCHE. A hydride is then
transferred to MCHA+ from MCHE to give the allylic carbenium ion II
and MCHA. Ion II then loses a proton to the surface to give
methylcyclohexadiene. Methylcy- clohexadiene gives up a hydride to
another MCHA+ to form intermediate III, which quickly loses a
proton to give toluene. Since /z3 >> k_,; k, >> k_,;
and k6 >> k_6, the process is irreversible. Cracking, ring
opened products and ring isomerization products arise from MCHA or
I.
The role of allylic carbenium ions in strong acid zeolite
catalysis has been invoked [ 181 but not well studied. However
there is evidence for allylic car- benium ion formation from
propene oligomers in mordenites and HNaY zeo- lites [ 231. In
naphthene systems, allylic carbenium ions seem to play a more
important role than in straight chain alkanes or alkenes.
It is interesting to compare the cracking chemistry of MCHE to
its acyclic C, analogue, heptene. Heptene cracking chemistry has
been reported for HY and ZSM-5 zeolites [ 24,251. These reports
indicate that heptene cracking gives no aromatics or cyclic
hydrocarbons as initial products. In contrast, aromatics
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322
CRACKING 6 ISOMERIZATION
Fig. 6. Simplified reaction scheme for inital reaction of
methylcyclohexene over zeolite catalysts.
and dehydrogenation products dominate MCHE reactivity. The lack
of aro- matics from heptene suggests that the first and most
difficult step in the aro- matization of acyclics is the
cyclization of the carbenium ion [ 261. The absence of dialkenes as
initial products from acyclic alkenes [ 261 suggests that allylic
carbenium ions, while they may form, are of much less consequence
in acyclic systems than in naphthenic systems.
Our data suggest hydride transfer cannot be the slow step that
controls cracking at least in the MCHE system. Previous reports
[27] have suggested that chain transfer via hydrogen transfer is
the slow step in alkane cracking. In the MCHA/MCHE system, MCHE has
a much higher hydrogen transfer rate than MCHA. Nevertheless, the
cracking rate is lower for MCHE than for MCHA. This means that
hydrogen transfer does not control cracking. As Fig. 6 illustrates,
for MCHE feed, hydrogen transfer actually suppresses cracking by
converting methylcyclohexyl carbocation to less reactive MCHA. MCHA
cannot compete with MCHE for adsorption sites and thus is
relatively stable at low conversions of MCHE.
CONCLUSIONS
Most of the cracking chemistry of methylcyclohexane can be
explained by a combination of j&scission, and protolytic
cracking plus isomerization, H-
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323
transfer, H+ transfer and dehydrogenation reactions. MCHA can be
used as a probe of acid catalyst behavior; specifically one can
estimate the amount of ring opening, H+ transfer, H+ transfer vs.
H- transfer and protolytic cracking occurring on the catalyst
surface.
Mono-unsaturated naphthenes are key intermediates for aromatics
forma- tion from saturated naphthenes. Once the mono-ene is formed,
aromatics for- mation is relatively facile.
Methylcyclohexene is more highly reactive than methylcyclohexane
but cracks more slowly than methylcyclohexane. This occurs because
an alterna- tive reaction pathway, hydrogen transfer, competes
effectively with cracking.
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