Solid-State NMR Investigation of Ethylbenzene Reactions over HMOR and Pt-HMOR Catalysts

Article abstract of DOI:10.1006/jcat.1997.1582

Ethylbenzene reactions on HMOR and Pt-HMOR have been studied by means of solid-state MAS NMR. These experimental findings clearly indicate that disproportionation via the intermediacy of diphenylethane structure constitutes the first steps of this catalytic process over both HMOR and Pt-HMOR. In addition, ethylbenzene is suggested to undergo nonbranching rearrangements via the intermediacy of a two-electron-three-center bond leading to a ¹³C-label transfer from α- to β-position of the side chain. This appears to be a major catalytic event over both catalysts. At later stages, disproportionation of ethylbenzene via dealkylation/alkylation steps accompanied by secondary reactions of ethylene proceeds freely on HMOR, whereas it is very much suppressed on Pt-HMOR. While dealkylation is very pronounced over Pt-HMOR, alkylation is very much limited by rapid hydrogenation of ethylene to ethane on Pt sites. Based on the spectroscopic evidence of this investigation, a number of schematic representations are illustrated to address the mechanistic approaches taken in this contribution.

Full text of DOI:10.1006/jcat.1997.1582

JOURNAL OF CATALYSIS 167, 266–272 (1997)
ARTICLE NO. CA971582
Solid-State NMR Investigation of Ethylbenzene Reactions
–
over HMOR and Pt HMOR Catalysts
Andreas Philippou and Michael W. Anderson
Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, United Kingdom
Received September 20, 1996; revised December 16, 1996; accepted December 16, 1996
–
whereas ethylene, ethane, C3 C8, and xylenes are produced
–
in very small amounts. It follows that disproportionation of
ethylbenzene is the main reaction pathway accompanied
by dealkylation. Ethylene resulting from dealkylation un-
Ethylbenzene reactions on HMOR and Pt HMOR have been
studied by means of solid-state MAS NMR. These experimen-
tal findings clearly indicate that disproportionation via the inter-
mediacy of diphenylethane structure constitutes the first steps of
–
dergoes secondary reactions leading to a variety of C3 C8
–
this catalytic process over both HMOR and Pt HMOR. In ad-
products. The effect of platinum on this reaction is that it de-
creases the rate of disproportionation and increases the rate
of dialkylation. Furthermore, ethylbenzene is suggested (2–
5) to isomerize over a dual function catalyst via the inter-
mediacy of cycloalkenes, formed by hydrogenation, which
in turn isomerize and undergo dehydrogenation to form
xylenes. Hydrocracking of these highly reactive intermedi-
ates is an undesirable secondary reaction which limits the
rate of ethylbenzene isomerization, yielding a variety of
dition, ethylbenzene is suggested to undergo nonbranching rear-
rangements via the intermediacy of a two-electron–three-center
bond leading to a 13C-label transfer from ꢀ- to ꢁ-position of the
side chain. This appears to be a major catalytic event over both
catalysts. At later stages, disproportionation of ethylbenzene via
dealkylation/alkylation steps accompanied by secondary reactions
of ethylene proceeds freely on HMOR, whereas it is very much sup-
–
pressed on Pt HMOR. While dealkylation is very pronounced over
Pt HMOR, alkylation is very much limited by rapid hydrogenation
–
of ethylene to ethane on Pt sites. Based on the spectroscopic evi-
dence of this investigation, a number of schematic representations
are illustrated to address the mechanistic approaches taken in this
c
–
C1 C7 hydrocarbons.
The potential of MAS NMR for monitoring hydrocar-
bon conversions proceeding inside the catalyst pores using
sealed sampleshasbeen widelydemonstrated (6–18). In our
studies the reaction of ethylbenzene over both HMOR and
contribution.
ꢀ 1997 Academic Press
–
Pt HMOR was investigated by means of MAS NMR. A
INTRODUCTION
large number of organic species were detected at interme-
diate stages of this catalytic process and successfully iden-
tified, producing vital information as to the reaction mech-
anism of this process.
In a modern refinery, the C8 stream obtained from pyrol-
ysis gasoline and reformate contains o-, m-, and p-xylenes
and ethylbenzene. p-Xylene is required in large quanti-
ties for the synthesis of terepthalic acid which is in turn
used for the manufacture of poly (ethylene terepthalate)
fibres. There is yet a little commercial demand for the
other two xylene isomers and additionally, ethylbenzene,
although a major petrochemical intermediate, is mostly
made by synthesis from ethylene and benzene. It follows
EXPERIMENTAL
Sample Preparation
–
Zeolites HMOR and Pt HMOR were provided by Shell
that conversion of the mixture of o- and m-xylenes and Research and Technology Centre in Amsterdam. Chemi-
ethylbenzene into p-xylene is a significant process in the cal analysis of the as-synthesized samples gives a Si/Al ra-
petrochemical industry. Isomerization of xylenes and ethyl- tio of 5 and a sodium content of ca 0.08 w/w% . The Pt
–
benzene is carried out in the presence of hydrogen on a content of Pt HMOR is 0.4 w/w% and TEM shows a fine
dual function catalyst consisting of a hydrogenation compo- dispersion of Pt over the whole sample. Furthermore, these
nent (e.g., platinum) and an acid catalyst (e.g., acidic zeolite catalysts were fully characterized by powder X-ray diffrac-
1
HMOR).
tion (XRD), scanning electron microscopy (SEM), and H
Isomerization of ethylbenzene over both acidic and and 27Al MAS NMR. SEM images picture crystal sizes of
Pt-modified acidic zeolites has been broadly studied (1–5). 75–300 nm, while the 27Al MAS NMR spectra of both cata-
Over monofunction acid catalyst (e.g., HMOR), ethylben- lysts depict that 15% of aluminum sits in octahedral coor-
zene is mainly converted into diethylbenzenes and benzene, dination.
266
0021-9517/97 $25.00
c
Copyright ꢀ 1997 by Academic Press
All rights of reproduction in any form reserved.

NMR INVESTIGATION OF ETHYLBENZENE REACTIONS
267
Catalysis-MAS NMR
Ethyl[ꢀ-13C]benzene (99% 13C-enriched) was purchased
from CDN isotopes. HMOR catalyst (50 mg) was loaded
into specially designed Pyrex capsules which were in turn
connected to a vacuum line. This catalyst was activated un-
der vacuum at 400ꢁC for 24 h and then loaded with a con-
–
trolled quantity of the reactant. Pt HMOR was reduced in
H2 flow at 500ꢁC for 5 h and then transferred to the vacuum
line in Pyrex capsules where they were vigorously evacu-
ated to assure removal of physisorbed H2 prior to load-
–
ing. Experiments on both catalysts HMOR and Pt HMOR
were carried out with the loading of 1.6 ꢂ 0.1 moles of ethyl-
benzene per unit cell. Experiments with higher loading
were not feasible as condensation of the reactant within the
capsule was unavoidable. The sample capsules were sealed
(6, 7) and heated at various reaction temperatures for a pe-
riod of 4 min. All the 13C MAS NMR spectra were recorded
at room temperature on a Bruker MSL 400 spectrometer
using a Chemagnetics APEX 400 pencil probe. Most of the
experimentswere carried out bymeansofsingle pulse direct
excitation with high-power proton decoupling. Pulse delays
of 5 s were found adequate to yield quantitatively reliable
spectra. Where necessary spectra were recorded with cross-
polarization to enhance weak resonances and contact times
in these cases were 2 ms. Chemical shifts are referenced
to external tetramethylsilane. In our spectra, asterisks de-
note spinning side bands, whereas background signals are
marked and result from the silicon rubber sleeve at 1.2 ppm
and impurities at 3.9 and 40 ppm.
RESULTS
13C MAS NMR spectra depicted in this section demon-
strate the progression of this catalytic process at different
reaction temperatures. During the course of this investi-
gation, only broad and featureless signals, resulting from
coke, were detected in the olefinic and aromatic region of
the spectrum by means of cross-polarization and therefore,
only the aliphatic region is shown.
FIG. 1. 13C MAS NMR spectra of ethyl[ꢀ-13C]benzene reaction over
HMOR (Backgr, background; Xyl, xylenes; Tol, toluene).
Figure 1 shows the 13C MAS NMR spectra of ethylben-
zene reaction over HMOR catalyst at a range of tempera-
tures. Initially at room temperature (RT), one resonance is from the ꢀ- to the ꢁ-position in the side chain appears to
observed at 28.8 ppm which is assigned to methene group be a major catalytic event in this process and interpretation
–
–
( CH2 ) in the side chain of ethylbenzene. The strong inten- of such an observation merits a great deal of significance
sity of this signal results from the selective 13C enrichment in this investigation. The spectrum corresponding to 350ꢁC
at this position. Reaction commences at ca 200ꢁC with the indicates a considerable increase in the intensity of the line
appearance of two signals at 28.2 and 25 ppm. Both of these at 15.2 ppm and exhibits two additional weak signals at
lines are assigned to diethylbenzenes retaining the label at 20 and 53 ppm. The line at 20 ppm is assigned to methyl
the ꢀ-position of the side chain. The line at 28.2 ppm is groups in toluene or xylene molecules, whereas the reso-
attributed to p-diethylbenzene and the one at 25 ppm to nance at 53 ppm could result from the formation of surface
o-diethylbenzene. At 300ꢁC a third strong line at 15.2 ppm bound alkoxy species and most likely ethoxy groups. The
appears in the spectrum which is assigned to both ethylben- formation of alkoxides has already been reported (8, 10,
zene and diethylbenzene isomers accommodating the label 19–21), and in several contributions (8, 21) NMR signals in
at the ꢁ-position of the side chain. The 13C label transfer the region of ca 50–55 ppm were assigned to ethoxy groups.

268
PHILIPPOU AND ANDERSON
Furthermore, rapid interchange of the two carbons in the
ethyl group within the NMR time scale has been suggested
(21) to average the two carbons, giving rise to one signal
in the range of 50–55 ppm. At 400ꢁC, a large number of
new resonances are observed in the corresponding spec-
trum while the intensity of the signal at 15.2 ppm keeps on
increasing. The lines at ca 13–14 ppm are attributed to ter-
–
minal methyl groups of hydrocarbon isomers C4 C6 with
their internal carbons giving rise to lines in the range of
22–25 ppm. Propane also forms at this reaction stage as
evidenced by the lines at ca 15.6 and 15 ppm which are as-
signed to propane in adsorbed and gas phase, respectively.
The spectrum of 450ꢁC exhibits mainly the same pattern as
the spectrum above although now the conversion is by far
more pronounced.
Figure 2 illustrates the 13C MAS NMR spectra of ethyl-
–
benzene reaction over Pt HMOR catalyst at a range oftem-
peratures. At first, one resonance is observed at 28.8 ppm
–
–
which is assigned to methene group ( CH2 ) in the side
chain of ethylbenzene. The strong intensity of this signal
results from the selective 13C enrichment at this position.
Reaction begins at ca 200ꢁC with the appearance of sig-
nals at 25, 28.2, 30.4, and 45 ppm. Both lines at 25 and
28.2 ppm are assigned to diethylbenzenes retaining the la-
bel at the ꢀ-position of the side chain as observed in the
previous series of spectra. In addition, a weak interaction
between ethylbenzene and Pt is suggested to account for the
downfield shift (30.4 ppm) of the ethylbenzene methene
group observed in this spectrum. Of great interest is the
appearance of two weak lines at ca 45 ppm which could
a
–
be attributed to the side-chain methyne group ! CH of
diphenylethane isomers, intermediates in ethylbenzene dis-
proportionation. The spectrum of 300ꢁC displays a consid-
erable line-broadening arising from a continuously inten-
sifying interaction between the different organic species
and Pt. In this spectrum, the presence of two signals at
5.6 and 15.2 ppm becomes apparent. The line at 5.6 ppm
is attributed to ethane whereas the resonance at 15.2 ppm
is assigned to both ethylbenzene and diethylbenzene iso-
mers accommodating the label at the ꢁ-position of the side
chain. A large number of catalytic events are pictured in
the spectrum of 350ꢁC. The increasing interaction of or-
ganic molecules with Pt coupled with coke formation may
account for both the substantial line-broadening and the
loss in signal intensity. The lines at ca 3, 5.6, and 7 ppm
are assigned to ethane in gas, adsorbed, and strongly ad-
sorbed phases, respectively. In the same spectrum, the line
at 13.4 ppm is associated with terminal methyl groups of hy-
FIG. 2. 13C MAS NMR spectra of ethyl[ꢀ-13C]benzene reaction over
–
Pt HMOR (Backgr, background; DPE, diphenylethane isomers; DEB,
diethylbenzenes; EB, ethylbenzene; Xyl, xylenes).
(350ꢁC) is recorded with cross-polarization. This spectrum,
although not quantitative, clearly illustrates a number of
intensified broad resonances observed in the above spec-
trum. The main resonances depicted in this spectrum arise
from ethylbenzene, diethylbenzenes, xylenes, and strongly
adsorbed ethane, all sharing a substantial lack of mobil-
ity. At 400ꢁC, the signal intensity decreases dramatically
to an extent that only cross-polarization spectra could fea-
sibly obtain. In the cross-polarization spectrum of 400ꢁC,
the broad signal at ca 35 ppm is ascribed to side chains
of aromatic coke species, whereas the new line at ꢃ5 ppm
is attributed to methane in adsorbed phase. Furthermore,
–
drocarbon isomers C4 C6 with their internal carbons giving
rise to lines in the range of 22–25 ppm. The broad lines in
the region of 14–17 ppm are attributed to both propane
and diethylbenzene isomers while the resonance at 20 ppm
is characteristic of methyl groups in xylene isomers. The
next spectrum corresponding to the same reaction stage

NMR INVESTIGATION OF ETHYLBENZENE REACTIONS
269
ethane, propane, diethylbenzenes, and xylenes are still ob-
served at this late stage of this catalytic process.
Summary of MAS NMR Results
In the above sections, we have seen how the reaction
proceeds inside the pores of these catalysts loaded with a
certain amount of reactant and heated at various temper-
atures. For the two catalysts these results are summarized
below.
HMOR
—Reaction commences at 200ꢁC with the formation of
diethylbenzenes as primary products retaining the 13C label
at the ꢀ-position of the side chain.
—The amount of diethylbenzenes increases markedly up
to the temperature of 400ꢁC. A major catalytic event in this
reaction process is the 13C label transfer from the ꢀ- to
the ꢁ-position in the side chain of both ethylbenzene and
diethylbenzene.
SCHEME 1
stages including the onset of a catalyzed reaction process.
In these experiments, the reaction commences at 200ꢁC
with the formation of diethylbenzenes as primary prod-
ucts. At this reaction stage, the 13C label is retained at
the ꢀ-position in the side chain of both ethylbenzene and
diethylbenzene and this appears to be vital information
as to the mechanistic pathway of disproportionation. This
observation strongly suggests that disproportionation, at
this early reaction stages, proceeds via the intermediacy of
diphenylethane structure (1–5) as shown in Scheme 1. In
such a mechanistic route the 13C label is necessarily retained
at the ꢀ-position of the side chain.
—In the same temperature region, 200–400ꢁC, a small
amount of toluene/xylene and oligomerization products
–
(C3 C6) are also produced. Of great interest is the appear-
ance of a weak line at ca 54 ppm which could be assigned
to alkoxy species (most likely ethoxy groups).
—At higher temperatures, the amount of diethylben-
zenes tend to decrease, whereas an increase in the lines
–
assigned to toluene/xylenes and C3 C6 products becomes
apparent.
–
Pt HMOR
Conversely, disproportionation via dealkylation/alkyla-
tion steps (4) would necessarily lead to a statistical distribu-
tion of the 13C label in the side chain of diethylbenzene iso-
mers as shown in Scheme 2. In this scheme, methene group
is labeled A and observed at 28.2 and 25 ppm, whereas
methylgroup ismarked asB and observed at 15.2ppm in the
side chains of diethylbenzenes. Based on this reaction route,
one would expect to observe a ratio of A/B = 3 in the NMR
spectra of this process. The complete absence of signal B at
this reaction stage confirms our mechanistic approach pic-
tured in Scheme 1. Furthermore, dealkylation/alkylation
of ethylbenzene entails the formation of primary ethyl
—Reaction commences at 200ꢁC with the formation of
diethylbenzenes retaining the label at the ꢀ-position as
above. At the same time, ethylbenzene starts interacting
with Pt, whereas diphenylethane isomers, intermediates in
ethylbenzene disproportionation, are detected.
—At 300ꢁC, the 13C label transfer from the ꢀ- to the
ꢁ-position in the side chain of both ethylbenzene and di-
ethylbenzene is observed , whereas ethane is also seen as a
main product at this reaction stage.
—At 350ꢁC, the strong interaction of Pt with hydrocar-
bons coupled with coke formation makes lines extremely
–
broad and weak. However, ethane C3 C6, xylenes, and di-
ethylbenzenes lines are still observed.
—In the spectrum corresponding to the last stage of this
reaction process (400ꢁC), the main resonances are assigned
to methane, ethane, propane, xylenes, diethylbenzenes, and
aromatic coke.
DISCUSSION
Ethylbenzene on HMOR
The first reaction stages. Investigation of a catalytic
process by MAS NMR yields spectra picturing different
SCHEME 2

270
PHILIPPOU AND ANDERSON
carbenium ions and must be rejected on energetic grounds,
considering the relatively low reaction temperature
(200ꢁC).
The 13C label transfer from ꢀ to ꢁ-position. Of great in-
terest is the 13C label transfer to the ꢁ-position of the side
chain as observed in the next reaction stage (300ꢁC), and
this turns out to be a major catalytic event throughout this
reaction process. Methyl groups in the side chain of both
ethylbenzene and diethylbenzene isomers come very close
at ca 15.2 ppm and separation of these lines is not feasible.
Hence, the line at 15.2 ppm in our spectra (signal B) is as-
signed to methyl groups in both ethylbenzene and diethyl-
benzenes. The question now remained to address is how
does the 13C label move to the ꢁ-position of the side chain
in such a high rate. As Scheme 2 illustrates, disproportion-
ation via dealkylation/alkylation steps could account for
this 13C label transfer. However, such a mechanistic path-
way would produce A and B signals in a ratio of A/B = 3.
Bearing in mind the intensity of A signals in the spectrum
corresponding to the previous reaction stage (200ꢁC), one
would now expect to observe a ratio of A/B > 3. In the spec-
trum of 300ꢁC, a ratio of A/B = 1.5 is observed and thus
dealkylation/alkylation steps fail to explain the strong in-
tensity of the signal B. Additionally, dealkylation/alkylation
processes are often accompanied by secondary reactions of
SCHEME 4
the original ꢀ-position of the side chain. Such a mechanis-
tic route could explain both the 13C label transfer, yielding
signal B, and thus the relatively low ratio of A/B signals at
this reaction stage.
Naturally, after the labelistransferred to the ꢁ-position in
ethylbenzene molecules, these may undergo disproportion-
ation via Scheme 1 to produce diethylbenzenes retaining
the label in the ꢁ-position of the side chains. Hence, signal
B may result from both ethylbenzene and diethylbenzene
accommodating the label in the ꢁ-position, whereas signal
A is entirely derived from diethylbenzenes. Considering the
limited growth rate of signal A (from 200 to 300ꢁC) which
parallels that of disproportionation via Scheme 1, one could
see that the monomolecular nonbranching rearrangement
of ethylbenzene is very much favored over the bimolecular
disproportionation at this reaction stage. Bearing in mind
the low loading of the catalysts in these experimental pro-
cedures (0.2 moles/acid site), the high nonbranching rear-
rangement/disproportionation ratio observed above seems
to be entirely reasonable.
–
ethyl carbenium ions, leading to a variety of C3 C6 hydro-
–
carbons (4, 5). In our spectra, lines assigned to C3 C6 prod-
ucts are observed only at later reaction stages. This veri-
fies even further the argument that disproportionation via
dealkylation/alkylation steps has not started contributing to
this catalytic process as yet. Since the 13C label transfer can
be accounted for by neither of the two disproportionation
pathways, a third mechanistic approach should be consid-
ered. Nonbranching rearrangement of ethylbenzene via the
intermediacy of a two-electron–three-center bond (22, 23),
as shown in Scheme 3, is suggested to take place at this re-
action stage. This nonskeletal rearrangement simply entails
1, 2 shift of the phenyl group via the intermediacy shown
in Scheme 3. The resulting ion may be stabilized by 1, 2
hydride shifts (ꢄH:), and finally desorb as ethylbenzene by
hydride transfer. It is worth noting that 50% of the 13C label
involved in this process, will be located at the ꢁ-position,
giving rise to signal B, whereas the rest will return back
Disproportionation via dealkylation/alkylation steps.
At higher temperatures (350–450ꢁC), dealkylation/alkyla-
tion of ethylbenzene accompanied by secondary reactions
–
of ethylene leading to a variety of C3 C6 hydrocarbon
products, as shown in Scheme 4, start contributing to this
catalytic process. Moreover, the energy barrier of primary
ethyl carbenium ions, involved in this case, may now be
overcome at these elevated reaction temperatures. Follow-
ing the dealkylation reaction (Scheme 2a), ethyl carbenium
ions may attack the aromatic ring to yield diethylben-
zenes (Scheme 2b). Alternatively, ethyl carbenium ions
may desorb as either ethylene, which in turn undergoes
oligomerization/isomerization reactions, yielding a variety
–
of C3 C6 products (Schemes 4a and 4b) or ethylate the
zeolitic framework forming alkoxides (Scheme 4c).
As evidenced by the strong intensity of signal B, non-
branching rearrangement of ethylbenzene is still a major
catalytic event. A statistical 13C label distribution in dis-
proportionation via dealkylation/alkylation of ethylben-
zene accommodating the label in ꢁ-position is shown in
Scheme 5.
As shown in this scheme, such a reaction pathway would
yield a signal ratio of A/B = 0.333. Therefore, nonbranching
SCHEME 3

NMR INVESTIGATION OF ETHYLBENZENE REACTIONS
271
SCHEME 5
rearrangement of ethylbenzene (Scheme 3) coupled
with disproportionation via dealkylation/alkylation steps
(Scheme 5) may account for the strong intensity of the sig-
nal B at these late stages of this reaction process.
SCHEME 7
Toluene/Xylenes formation. Over and above, toluene/
xylenes are formed at this reaction stage. On HMOR, ex-
tensive secondary reactions of ethylene have been previ-
ously suggested (4, 5) to lead to toluene/xylenes formation.
If this was so, one would expect the 13C label to alternate po-
sitions in xylene isomers as shown in Scheme 6. Of course,
such a distribution would produce signals in both the aro-
matic as well as the aliphatic regions of the spectrum. The
absence of signals in the aromatic region strongly suggests
that xylenes do not form by extensive secondary reactions
of ethylene.
The results of this work suggest the reaction pathway pic-
tured in Scheme 7, where a primary oligomer such as butyl
carbenium ion may attack the aromatic ring and then un-
dergo cracking at the side chain to form a highly reactive
cyclotriene. The last may be protonated and then depro-
tonated to form toluene which in turn could form xylenes
or ethyltoluene via disproportionation/transalkylation re-
actions. Interestingly, the 13C label is now remained in the
side chain and does not move to the aromatic ring.
suppresses ethylbenzene alkylation by rapid hydrogena-
tion of the highly reactive ethylene. In this way, dispro-
portionation via dealkylation/alkylation steps in very much
limited.
This observation adds to the argument that the 13C label
transfer to the ꢁ-position and thus the signal at 15.2 ppm is
not derived from dealꢁkylation/alkylation steps. As shown
–
in the spectrum of 300 C (Pt HMOR), while alkylation of
ethylbenzene is very much suppressed by rapid hydrogena-
tion of ethylene, the intensity of the line at 15.2 ppm arising
from 13C label transfer to the ꢁ-position is still very strong.
Therefore, nonbranching rearrangement of ethylbenzene
as shown in Scheme 3 is proved once more to be a func-
–
tioning reaction pathway in both HMOR and Pt HMOR
catalysts.
Xylenes formation. In the next reaction stages (350–
400ꢁC), the increasing interaction of organic species with
Pt coupled with coke formation lead to an extensive loss
of signal intensity as well as a substantial line-broadening
of the detected resonances. At these stages, ethane is a ma-
jor product formed by dealkylation/hydrogenation steps. A
–
Ethylbenzene on Pt HMOR
–
number of C1 C6 hydrocarbon products are also produced
The first reaction stages—Effect of Pt. The early staꢁges
most probably by hydrocracking of the highly reactive cy-
cloalkenes (2), intermediates in ethylbenzene skeletal iso-
merisation to xylenes (Scheme 9a).
–
of the reaction of ethylbenzene on Pt HMOR (200–300 C)
resemble those of HMOR (Schemes 1 and 3). Hence, dis-
proportionation via Scheme 1 coupled with the nonbranch-
ing rearrangement of ethylbenzene (Scheme 3) constitute
the first stages of this catalytic process. Moreover, ethyl-
benzene starts interacting with Pt, whereas the interme-
diacy of diphenylethane structure in disproportionation
reactions (Scheme 1) is now spectroscopically confirmed.
Ethane is also observed at these early reaction stages result-
ing from ethylene hydrogenation on Pt sites (Scheme 8).
A comparison of the spectrum at 300ꢁC with the corre-
sponding one of HMOR catalyst indicates that Pt pro-
motes ethylbenzene dealkylation and at the same time
It has long been proposed (2) that ethylbenzene may be
isomerised to xylenes via the intermediacy of cycloalkenes
formed by partial hydrogenation of ethylbenzene, as shown
in Scheme 9. It becomes clear that in such an isomerization
route the 13C label is expected to move from the side chain
to the aromatic ring (Scheme 9b). Thus, one would expect
to observe resonances in the aromatic region of the spec-
trum. The NMR spectra corresponding to these late reac-
tion stages do exhibit lines assigned to methyl groups of
xylene isomers, but due to line-broadening and loss in sig-
nal intensity, the aromatic region of the spectrum becomes
SCHEME 6
SCHEME 8

272
PHILIPPOU AND ANDERSON
drocracking of the highly reactive cycloalkenes, intermedi-
ates in ethylbenzene to xylene isomerization, may account
–
for the formation of C1 C6 hydrocarbon products over
–
Pt HMOR.
ACKNOWLEDGMENTS
We acknowledge Shell Research and Technology Centre in Amster-
dam for financial support and for providing the catalysts as well as the
13
C-enriched ethylbenzene.
REFERENCES
SCHEME 9
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CONCLUSION
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The resultsofthiswork provide spectroscopicevidence as
to the reaction mechanisms operating in these catalytic pro-
–
cesses. Over catalysts HMOR and Pt HMOR, the reaction
commences with the formation of diethylbenzenes retain-
ing the label at the ꢀ-position of the side chain.This obser-
vation coupled with simple statistical calculations, strongly
suggests that disproportionation, at these early stages, pro-
ceeds via the intermediacy of diphenylethane structure. The
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–
this reaction over both HMOR and Pt HMOR catalysts,
13. Munson, E. J., Kheir, A. A., Lazo, N. D., and Haw, J. F., J. Phys. Chem.
is accounted for by the nonbranching rearrangement of
ethylbenzene entailing the intermediacy of a two-electron–
three-center bond. Over HMOR catalyst, disproportiona-
96, 740 (1992).
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4732 (1993).
tion via dealkylation/alkylation reactions accompanied by 15. Nowak, A. K., Wilson, A. E., Roberts, K., and Datema, K. P., J. Catal.
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secondary reactions of ethylene leading to a variety of
16. Philippou, A., and Anderson, M. W., J. Am. Chem. Soc. 116, 5774
–
C3 C6 hydrocarbon products becomes apparent at later
(1994).
reaction stages. At the same time, the 13C label transfer
17. Ivanova, I. I., and Derouane, E. G., Stud. Surf. Sci. Catal. 85, 357 (1994).
18. Philippou, A., and Anderson, M. W., J. Catal. 158, 385 (1996).
to the ꢁ-position via the nonbranching rearrangement of
ethylbenzene appears to be a predominant process. Over 19. Van den Berg, J. P., Wolthuizen, J. P., Clague, A. D. H., Hays, G. R.,
Huis, R., and van Hoof, J. H. C., J. Catal. 80, 130 (1983).
–
Pt HMOR catalyst, dealkylation of ethylbenzene followed
20. Lange, J.-P., and Gutsze, A., Appl. Catal. 45, 345 (1988).
21. Karge, H. G., Darmstadt, H., Gutsze, A., Vieth, H. M., and
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22. Olah, G. A., J. Am. Chem. Soc. 94, 808 (1972).
by hydrogenation of the highly reactive ethylene to ethane
is very much promoted by Pt, whereas alkylation of ethyl-
benzene is suppressed. In the meantime, xylenes as well as
–
a variety of C1 C6 hydrocarbon products are formed. Hy- 23. Daage, M., and Fajula, F., J. Catal. 81, 405 (1993).