Synthesis of p-xylene from ethylene

Article abstract of DOI:10.1021/ja307612b

As oil supplies dwindle, there is a growing need to develop new routes to chemical intermediates that utilize alternative feedstocks. We report here a synthesis of para-xylene, one of the highest volume chemicals derived from petroleum, using only ethylene as a feedstock. Ethylene is an attractive alternative feedstock, as it can be derived from renewable biomass resources or harnessed from large domestic shale gas deposits. The synthesis relies on the conversion of hexene (from trimerization of ethylene) to 2,4-hexadiene followed by a Diels-Alder reaction with ethylene to form 3,6-dimethylcyclohexene. This monoene is readily dehydrogenated to para-xylene uncontaminated by the ortho and meta isomers. We report here a selective synthesis of para-xylene, uncontaminated by the ortho or meta isomers, using ethylene as the sole feedstock.

Full text of DOI:10.1021/ja307612b

Communication
pubs.acs.org/JACS
Synthesis of p‑Xylene from Ethylene
Thomas W. Lyons, Damien Guironnet, Michael Findlater, and Maurice Brookhart*
†
†,‡
§
Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
S Supporting Information
*
produce the normal mix of ortho, meta, and para isomers6,
requiring separation and purification as outlined above.
Additionally, routes to PX from 2,5-dimethylfuran (DMF),
derived from carbohydrates, have also been reported. Potential
disadvantages of these methods include production of
unwanted side products and low availability of DMF.
Here we report the synthesis of PX using ethylene as the sole
feedstock. Ethylene is an attractive raw material for two reasons.
First, it can be readily derived from ethanol and is therefore
biorenewable. Braskem has commercialized a process using
ABSTRACT: As oil supplies dwindle, there is a growing
need to develop new routes to chemical intermediates that
utilize alternative feedstocks. We report here a synthesis of
para-xylene, one of the highest volume chemicals derived
from petroleum, using only ethylene as a feedstock.
Ethylene is an attractive alternative feedstock, as it can be
derived from renewable biomass resources or harnessed
from large domestic shale gas deposits. The synthesis relies
on the conversion of hexene (from trimerization of
ethylene) to 2,4-hexadiene followed by a Diels−Alder
reaction with ethylene to form 3,6-dimethylcyclohexene.
This monoene is readily dehydrogenated to para-xylene
uncontaminated by the ortho and meta isomers. We report
here a selective synthesis of para-xylene, uncontaminated
by the ortho or meta isomers, using ethylene as the sole
feedstock.
7
“
bioethylene” derived from sugar cane, to produce 200 000 tons
8
of polyethylene per year. Second, and most significantly, the
vast reserves of natural gas in the Marcellus and other shale
deposits (shale gas) contain substantial fractions of ethane (up
to 16%), which is readily cracked to ethylene. It is generally
agreed that ethylene derived from shale gas will become a major
(
nonpetroleum based) source of this feedstock in the near
9
future. To that end, several petrochemical firms have
announced their intention to increase U.S. ethylene production,
including Shell’s proposed ethane cracker in western
ith diminishing oil reserves, the production of
9c−e
W
commodity chemicals will increasingly rely on alter-
native feedstocks such as ethylene. In this regard, new routes to
para-xylene (para-dimethylbenzene, PX) are of particular
interest, as PX is one of the highest volume chemical
Pennsylvania.
Our strategy for implementing the synthesis of PX from
ethylene is outlined in Scheme 1. Hexene can be produced via
chromium-catalyzed selective trimerization of ethylene, a
1
10
intermediates derived from petroleum. The primary use of
commercial process for preparing this monene. Catalytic
PX is for the production of the dimethyl ester of terephthalic
acid, which is copolymerized with ethylene glycol to produce
polyethylene terephthalate (PET). The ca. 11 billion pounds of
PET consumed annually in the U.S. are used to manufacture a
wide variety of common products including plastic bottles,
disproportionation of hexene via transfer dehydrogenation with
another equivalent of substrate yields hexadiene and hexane.
Subsequent Diels−Alder reaction of 2,4-hexadiene (2,4-HXD)
with ethylene affords 3,6-dimethylcyclohexene, which, upon
dehydrogenation, produces PX and hydrogen. Significantly, this
synthetic route results in complete selectivity for PX,
uncontaminated by the OX and MX isomers and with only
minor amounts of ethylbenzene (EB). Details of the develop-
ment and optimization of this process are described below.
We began our studies by exploring the conversion of 1-
hexene to hexadienes. Previous work had shown catalytic
2
polyester fibers, and films such as Mylar.
Traditionally, PX is produced by catalytic reforming of
various crude oil streams, which results in a mixture of benzene,
toluene, and ortho- (OX), meta- (MX), and para-xylene (so-
called BTX). For the synthesis of PET, high purity PX is
required and thus several technologies have been developed to
3
transfer dehydrogenation of n-alkanes (C , n > 5) with iridium
purify and maximize PX yields. The remarkably close boiling
n
pincer complexes using 4 equiv of the sacrificial alkene acceptor
TBE (tert-butylethylene) ultimately resulted in formation of
points of the xylene isomers (ortho, meta, para: 144.0, 139.1,
138.5 °C) make distillation an unappealing separation method.
1
1
cyclic aromatic products. Aromatics are produced via
formation of linear, conjugated trienes followed by ring closure
to cyclohexadienes and further transfer dehydrogenation to the
aromatic. We expected that the disproportionation of hexene
Low temperature crystallization and surface adsorption are
often used, taking advantage of the difference in melting points
of the three isomers (ortho, meta, para: −25.0, −47.4, 13.2
3
a
°
C). While recent, significant progress has been made in
4
5
(
in which 1 equiv of hexene acts as the acceptor for
developing new isomerization and separation technologies, a
direct, selective route to PX from petroleum sources has yet to
be developed.
dehydrogenation of a second equivalent of hexene) may be
terminated at the diene stage prior to benzene formation. This
expectation was based on the fact that olefin and diene
Alternative approaches to xylenes from biomass feedstock
have recently been reported; however these methods still rely
on traditional hydrocarbon reforming technology. For example,
hydrocarbons produced from plant sugars can be used to
Received: August 2, 2012
Published: August 30, 2012
©
2012 American Chemical Society
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Journal of the American Chemical Society
Communication
Scheme 1. Route to PX from Ethylene
dehydrogenation; further dehydrogenation can only occur from
the thermodynamically much less favored 1,3-diene isomers.
Figure 1 shows four iridium pincer catalysts that were
screened for disproportionation of 1-hexene into hexadienes
and hexane. A 0.04 mol % catalyst loading was used, and the
degassed hexene solution was heated in a closed, glass vessel at
1
80 °C for 3.5 h (eq 1).
Conversions were monitored by gas chromatography. The
turnover frequencies (TOFs) shown were calculated based on
3
0 min conversions, and the turnover numbers (TONs) were
based on product formation after 3.5 h. Surprisingly, the more
12
13
hindered tert-butyl-substituted catalysts, 3 and 4, exhibited
almost no activity in contrast to the remarkably high activity
these complexes display in transfer dehydrogenation of
cyclooctane using TBE as an acceptor.
isopropyl-substituted systems, 1
1
0d,11
The less hindered
11,14
11
and 2, showed good
activities, with the anthraphos-based system, 1, exhibiting a
TOF and TON of ca. four times that of 2.
Figure 1. Catalysts screened for hexene disproportionation.
The kinetic profile of 1-hexene disproportionation catalyzed
by 1 (0.04 mol %, 180 °C) was followed by GC and is
displayed in Figure 2. The iridium pincer complex, 1, catalyzes
the isomerization of hexenes and hexadienes much faster than
transfer dehydrogenation under these conditions. Thus 1-
hexene is rapidly converted to a thermodynamic mixture of
hexenes prior to significant hydrogen transfer and the
hexadienes produced appear as the thermodynamic ratio of
dienes. As is evident from Figure 2, the reaction reaches
equilibrium at ca. 3 h under these conditions with a Keq ≈ 1.
The thermodynamic ratios of the hexene and hexadiene
isomers are illustrated in Figure 3, which shows the full
product distribution at equilibrium.
Next, conditions for the desired Diels−Alder reaction with
ethylene were explored. Literature reports suggested that the
cycloaddition reaction of pure (2E,4E)-HXD with ethylene
required high temperatures and pressures (185 °C, 4300 psi)
for the cycloaddition to occur. The 2E,4Z and 2Z,4Z isomers
Figure 2. 1-Hexene disproportionation profile with 1.
15
were unreactive under these conditions. We were able to
achieve high conversion of (2E,4E)-HXD (96%) and (1,3)
HXDs (100%) in the crude reaction mixture to Diels−Alder
products after 48 h at 600 psi and 250 °C. Under these
conditions selective formation of the Diels−Alder adduct of the
(
2E,4E)-HXD isomer is observed, with minimal conversion of
the (2E,4Z)- and (2Z,4Z)-HXD isomers. As shown by the
results summarized in Table 1, only the concentration of the
reactive dienes has changed significantly to produce A and B,
illustrating the selectivity observed in this reaction. Further-
more, the GC trace for this reaction only shows one signal for
isomer A with no other signal that could be assigned as the
other stereoisomer (Figure S2).
Figure 3. Product distribution at equilibrium from disproportionation
of 1-hexene with 1.
In the final step of the synthesis of PX we sought to
dehydrogenate cis-3,6-dimethylcyclohexene, the Diels−Alder
adduct. The overall reaction shown in eq 2 involves the loss of
isomerizations are fast relative to dehydrogenation and thus the
major diene isomers produced from hexene should be 2,4-
hexadienes (2,4-HXDs). These dienes cannot undergo transfer
2 equiv of H and formation of a stable aromatic structure.
2
1
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Journal of the American Chemical Society
Communication
a
Table 1. Diels−Alder Reaction
hexane
hexenes
1,3E-HXD
1,3Z-HXD
2E,4E-HXD
2E,4Z-HXD
2Z,4Z-HXD
A
B
start
33.9
31.4
34.8
31.1
1.6
0
0.3
0
15.1
0.6
11.7
7.5
2.3
1.7
0
0
final
23.9
2.9
a
Numbers shown are in mol % of solution.
Scheme 2. Dehydrogenation of Diels−Alder Adducts
2). Importantly, EB is another valuable aromatic, as it is a
precursor to styrene.
Thus far, a stepwise approach to PX using ethylene as the
sole feedstock has been outlined. The efficiency and utility of
this sequence could be improved if ethylene were to serve as
the hydrogen acceptor for 1-hexene dehydrogenation and drive
the conversion of hexene to higher yields of hexadienes.
Heating 1-hexene at 250 °C with 600 psi of ethylene for 24 h in
the presence of 0.32 mol % 1 resulted in conversion of 1-
hexene into HXD (35.1% HXD, Table 2, entry 2). Significantly,
the dienes produced underwent a subsequent Diels−Alder
reaction with ethylene in situ, providing the 3,6-dimethylcyclo-
hexene (16.9% A, entry 2) in one pot. This remarkable result
demonstrates a significant step forward in this approach to PX.
The yield of HXD (and ultimately PX) is no longer limited by
the equilibrium disproportionation of 1-hexene into HXD and
hexane, making this one-pot approach to 3,6-dimethylcyclohex-
ene a more attractive route for PX synthesis from ethylene.
Further heating of the reaction vessel resulted in nearly
complete conversion with respect to hexenes (93% after 192 h,
entry 6). Only trace amounts of hexane are observed, indicating
ethylene has served as the dominant acceptor. To our
knowledge this is the first example of ethylene serving as a
hydrogen acceptor in alkane dehydrogenation. The selective
conversion of 1-hexene to A and B also suggests that the
unreactive HXDs are isomerized to reactive HXDs under these
reaction conditions. Thus a high conversion to A and B is
observed for the Diels−Alder reaction with minimal HXD
remaining. Moreover, the Diels−Alder adducts are further
dehydrogenated under these reaction conditions resulting in
PX and EB (Table 2, aromatics) as well as partially
The high positive entropy offsets the positive enthalpy (ΔH
+18.4 kcal/mol) and renders the reaction highly exergonic at
=
temperatures above 200 °C, suggesting this dehydrogenation
can be accomplished “acceptorless” with a heterogeneous
1
6
catalyst.
Indeed, dehydrogenation of cyclohexene to benzene over
17
heterogeneous catalysts has ample literature precedent. In this
study, commercial catalysts Pd/C, Pt/C, and Pt/Al O were
2
3
screened. Crude reaction mixtures of the Diels−Alder adducts
were passed in the vapor phase using nitrogen as the carrier gas
over a bed of these catalysts at 300−500 °C, and the product
mixtures were trapped and analyzed by GC. In initial screening,
all these catalysts gave good conversion (>80%) of the starting
Diels−Alder adduct to PX. Upon further optimization, Pt/
Al O at 400 °C was observed to give the highest yield (93%)
2
3
of PX. Ethylbenzene (EB) was also observed in high yield from
this reaction (88%) with a final ratio of 8.5:1, PX/EB (Scheme
a
Table 2. One-Pot Diene Formation/Diels−Alder Reaction
b
c
entry
time
hexane
hexenes
HXD
A
B
aromatics
other
1
2
3
4
5
6
24
48
0.4
0.6
0.5
0.5
0.4
0.2
42.1
30.4
24.6
21.6
14.2
7.5
35.1
23.5
14.9
10.4
2.0
16.2
33.4
44.6
48.7
59.7
65.5
4.7
6.7
1.5
5.4
72
8.3
7.1
96
8.8
7.8
2.2
3.8
4.3
144
192
10.2
12.2
9.7
0.2
10.3
a
b
Time in h; other numbers shown are in mol % of solution. Reaction monitored by GC and recharged with ethylene after sampling. Aromatics
c
consist of PX, EB, and trace benzene. Others consist of partially dehydrogenated products from A and B which are readily converted to aromatics.
1
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Journal of the American Chemical Society
Communication
dehydrogenated A and B (other). Importantly, these partially
dehydrogenated species as well as A and B can easily be
dehydrogenated to aromatics via Pt-catalyzed dehydrogenation.
In summary, we have demonstrated a selective route to para-
xylene using ethylene as the sole feedstock. The route features a
transfer dehydrogenation of hexene to generate hexadienes
using ethylene (or hexene itself) as the hydrogen acceptor.
When ethylene is used, a subsequent Diels−Alder reaction
occurs with hexadienes to yield, in one pot, 3,6-dimethylcyclo-
hexene as the major product along with some 3-ethyl-
cyclohexene. Thermal dehydrogenation over Pt/Al O yields
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3
para-xylene and ethylbenzene.
(
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ASSOCIATED CONTENT
Supporting Information
■
(
*
S
Experimental procedures, gas chromatographs, and complete
ref 16. This material is available free of charge via the Internet
at http://pubs.acs.org.
(
Goldman, A. S. J. Am. Chem. Soc. 2009, 131, 15827.
(14) (a) Romero, P. E.; Whited, M. T.; Grubbs, R. H. Organometallics
2
008, 27, 3422. (b) Haenel, M. W.; Oevers, S.; Angermund, K.; Kaska,
AUTHOR INFORMATION
Corresponding Author
mbrookhart@unc.edu
Present Addresses
BASF Corp., 11501 Steele Creek Rd., Charlotte, NC 28273.
Department of Chemistry and Biochemistry, Box 41061,
Texas Tech University, Lubbock, TX, 79409-1061.
Author Contributions
W. C.; Fan, H.; Hall, M. B. Angew. Chem., Int. Ed. 2001, 40, 3596.
(
(
■
15) Bartlett, P. D.; Schuller, K. E. J. Am. Chem. Soc. 1968, 90, 6071.
16) Calculated for 3,6-dimethylcyclohexene using the Gaussian 09
program. For details see Supporting Information.
17) (a) Tsai, M.; Friend, C. M; Muetterties, E. L. J. Am. Chem. Soc.
1982, 104, 2539. (b) Borade, R. B.; Zhang, B.; Clearfield, A. Catal.
Lett. 1997, 45, 233. (c) Aramendía, M. A.; Borau, V.; García, I. M.;
Jimenez, C.; Marinas, A.; Marinas, J. M.; Urbano, F. J. J. Mol. Catal. A
000, 151, 261.
(
‡
§
́
́
2
†
T.W.L. and D.G. contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
■
This material is based on work supported by the National
Science Foundation under the auspices of the Center for
Enabling New Technologies through Catalysis (CHE-
0
650456). T.W.L. gratefully acknowledges support from the
NSF-American Competitiveness in Chemistry postdoctoral
fellowship (CHE-1136951). We thank Prof. Alan. S. Goldman
and Prof. Tomas Baer for helpful discussions. We also thank
Peng Kang for help with computational experiments and Sabuj
Kundu for help with experimental design.
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