Production of Diethyl Terephthalate from Biomass-Derived Muconic Acid

Article abstract of DOI:10.1002/anie.201509149

We report a cascade synthetic route to directly obtain diethyl terephthalate, a replacement for terephthalic acid, from biomass-derived muconic acid, ethanol, and ethylene. The process involves two steps: First, a substituted cyclohexene system is built through esterification and Diels-Alder reaction; then, a dehydrogenation reaction provides diethyl terephthalate. The key esterification reaction leads to improved solubility and modulates the electronic properties of muconic acid, thus promoting the Diels-Alder reaction with ethylene. With silicotungstic acid as the catalyst, nearly 100 % conversion of muconic acid was achieved, and the cycloadducts were formed with more than 99.0 % selectivity. The palladium-catalyzed dehydrogenation reaction preferentially occurs under neutral or mildly basic conditions. The total yield of diethyl terephthalate reached 80.6 % based on the amount of muconic acid used in the two-step synthetic process.

Full text of DOI:10.1002/anie.201509149

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International Edition: DOI: 10.1002/anie.201509149
German Edition: DOI: 10.1002/ange.201509149
Biomass Conversion
Hot Paper
Production of Diethyl Terephthalate from Biomass-Derived
Muconic Acid
Rui Lu, Fang Lu,* Jiazhi Chen, Weiqiang Yu, Qianqian Huang, Junjie Zhang, and Jie Xu*
Abstract: We report a cascade synthetic route to directly obtain
diethyl terephthalate, a replacement for terephthalic acid, from
biomass-derived muconic acid, ethanol, and ethylene. The
process involves two steps: First, a substituted cyclohexene
system is built through esterification and Diels–Alder reaction;
then, a dehydrogenation reaction provides diethyl terephtha-
late. The key esterification reaction leads to improved solubility
and modulates the electronic properties of muconic acid, thus
promoting the Diels–Alder reaction with ethylene. With
silicotungstic acid as the catalyst, nearly 100% conversion of
muconic acid was achieved, and the cycloadducts were formed
with more than 99.0% selectivity. The palladium-catalyzed
dehydrogenation reaction preferentially occurs under neutral
or mildly basic conditions. The total yield of diethyl tereph-
thalate reached 80.6% based on the amount of muconic acid
used in the two-step synthetic process.
a challenge to directly convert cellulose-based platform
chemicals into aromatic compounds because of the absence
of benzene rings in the molecular structure. C5 and C6 sugars
can be converted into a BTX (benzene, toluene, and xylenes)
stream by bioforming technology, which is based on aqueous-
phase reforming and conventional chemical processing and
was developed by Virent.[5] Furan-based compounds, which
are important platform chemicals and can be produced from
glucose and other biomass feedstock, have a conjugated diene
structure.[6] Recently, several groups applied Diels–Alder
reactions between a conjugated diene and an alkene to form
substituted cyclohexenes and further dehydration or dehy-
drogenation to obtain aromatic compounds. For example,
furfural,[7] 2,5-dimethylfuran,[8] and oxidized variants of
5-hydroxymethylfurfural[9] have been used to construct aryl
rings. Biomass-derived ethylene,[10] isoprene,[11] methyl cou-
malate,[12] and sorbic acid[13] can also be used as starting
materials in Diels–Alder reactions. However, further oxida-
tion is required to form the two carboxylic acid groups.
Muconic acid has garnered significant interest owing to its
potential as starting material for the synthesis of adipic acid,
a bulk chemical for the production of Nylon-6,6.[14] It contains
a conjugated diene and two carboxylic acid moieties, which
render it an attractive candidate for the synthesis of tereph-
thalic acid. Frost et al. have described the formation of
terephthalic acid through a Diels–Alder reaction between
muconic acid and ethylene and further dehydrogenation in
a patent.[15] This method constitutes a very promising route to
produce renewable terephthalic acid. However, the solubility
of muconic acid is very poor in many solvents, and its
molecular structure contains two electron-withdrawing car-
boxylic acid groups, rendering its reaction with a dienophile
difficult.[9,13]
Herein, we propose a synthetic route to diethyl tereph-
thalate (DET), using trans,trans-muconic acid (TTMA),
ethanol, and ethylene as the reactants, through a cascade
process combining esterification, Diels–Alder cycloaddition,
and dehydrogenation (Scheme 1). The key esterification
reaction improves the solubility of the reaction products in
ethanol and modulates the electronic properties of TTMA,
thus promoting the Diels–Alder reaction with ethylene.
Various metal catalysts were examined for the dehydrogen-
ation reaction, and palladium catalysts showed the most
efficient performance under neutral or mildly basic condi-
tions. This work serves as a proof of concept for utilizing
biomass molecules by rational design of the functional groups
present in the platform chemicals.
S
ynthetic polyesters play an essential and ubiquitous role in
the area of polymer materials. Purified terephthalic acid
(PTA) is used mainly as a monomer for the synthesis of
polyesters, especially for polyethylene terephthalate (PET).
The global consumption of PTA was over 50 million tons in
2014, which utilizes huge amounts of fossil oil.[1] As an
abundant and renewable resource, biomass has the potential
to serve as a feedstock for the production of liquid fuels and
chemicals.[2] Therefore, methods for the efficient conversion
of biomass into PTA and its derivatives are of great
importance for the development of the biopolymer industry.[3]
Terephthalic acid comprises a benzene ring with dicar-
boxylic acid groups at the 1- and 4-position. Some important
strategies have been proposed to construct aromatic struc-
tures by catalytic biorefinery routes. One of these is based on
the fact that lignocellulosic biomass can undergo a fast
catalytic pyrolysis process in the presence of zeolites as the
catalyst to produce aromatic compounds.[4] However, it is
[*] R. Lu, Prof. Dr. F. Lu, J. Chen, Dr. W. Yu, Q. Huang, J. Zhang,
Prof. Dr. J. Xu
State Key Laboratory of Catalysis
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
Dalian National Laboratory for Clean Energy
457 Zhongshan Road, Dalian 116023 (PR China)
E-mail: lufang@dicp.ac.cn
xujie@dicp.ac.cn
Homepage: http://www.orgcatal.dicp.ac.cn
R. Lu, J. Chen, Q. Huang, J. Zhang
University of Chinese Academy of Sciences
Beijing 100049 (PR China)
The esterification and the Diels–Alder reaction of TTMA
with ethanol and ethylene, respectively, took place at 2008C
in the presence of a silicotungstic acid catalyst. The crude
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201509149.
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increased. Compounds 1 and 2 were
not detected at an ethylene pressure of
3.0 MPa, and the cycloadducts were
formed with an overall selectivity of
> 99.0%. Among the three cycload-
ducts, cis product 3 was the major
product because of the stereospecificity
of the Diels–Alder reaction.[16] There-
fore, a suitably high ethylene pressure
promotes the Diels–Alder reaction.
To further investigate the reaction
of TTMA with ethanol and ethylene,
we let the reactions run for different
periods of time at an ethylene pressure
of 2.0 MPa (Figure 2). TTMA reacted
rapidly in the initial stage of the
reaction, and its conversion reached
up to 87.9% in the first twelve minutes.
At this point, 1 and 2 had been formed
with selectivities of 37.8% and 37.0%,
Scheme 1. Cascade process for the production of diethyl terephthalate (DET) from trans,trans-
muconic acid (TTMA).
reaction mixture was directly used as the feedstock of the
dehydrogenation reaction (Scheme 1). Typically, the two
primary ethyl esters of TTMA and three cycloadducts were
detected by GC-MS, including the (2E,4E)-hexadienedioic
acid monoethyl ester (1), (2E,4E)-hexadienedioic acid diethyl
ester (2), cis-2-cyclohexene-1,4-dicarboxylic acid diethyl ester
(3), trans-2-cyclohexene-1,4-dicarboxylic acid diethyl ester
(4), and 1-cyclohexene-1,4-dicarboxylic acid diethyl ester (5;
see the Supporting Information, Figure S2a,b). The desired
product, DET (6), and other side products of the dehydrogen-
ation reaction could be also identified (Figure S8a,b).
respectively. Upon prolonging the reaction time to
30 minutes, the amount of 1 decreased while the amount of
2 slightly increased, implying that the formation rate of 2 was
greater than its consumption rate within the first 30 minutes.
Upon further extending the reaction time, the amounts of
1 and 2 decreased while those of the cycloadducts increased
gradually.
The ethylene pressure exerts a great influence on the
product selectivity of the esterification and Diels–Alder
reaction (Figure 1). At an ethylene pressure of 0.1 MPa,
mainly the products of the esterification reaction were
present. Monoester 1 and diester 2 were formed with
selectivities of 4.2% and 92.1%, respectively. With increasing
ethylene pressure, the amount of 1 and 2 dropped signifi-
cantly, whereas the amount of the cycloadducts (3, 4, and 5)
Figure 2. Time course of the esterification and Diels–Alder reaction of
TTMA with ethanol and ethylene. Reaction conditions: TTMA
(0.5 mmol), ethylene (2.0 MPa), ethanol (20 mL), silicotungstic acid
(0.005 mmol), 2008C.
The combined esterification and Diels–Alder reaction of
TTMA with ethanol and ethylene was studied by real-time
in situ FT-IR spectroscopy (Figure 3). Owing to the limita-
tions of our spectrometer, relatively low temperature and
ethylene pressure (1258C and 0.4 MPa) were selected. After
background correction (see Figure S15), TTMA (reactant),
(2E,4E)-hexadienedioic acid diethyl ester (esterification
product), and cis-2-cyclohexene-1,4-dicarboxylic acid diethyl
ester (primary cycloadduct) gave rise to characteristic
absorptions at 1690 cmÀ1, 1715 cmÀ1, and 1740 cmÀ1 (stretch-
Figure 1. The effect of ethylene pressure on the selectivity of the
esterification and Diels–Alder reaction of TTMA with ethanol and
ethylene. Reaction conditions: TTMA (0.5 mmol), ethanol (20 mL),
silicotungstic acid (0.005 mmol), 2008C, 240 min.
=
ing vibrations of C O), which could be used to measure
250
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Figure 3. Real-time in situ FT-IR spectroscopic analysis of the esterifi-
Figure 4. Dehydrogenation of the liquid products from the esterifica-
cation and Diels–Alder reaction of TTMA with ethanol and ethylene.
tion and Diels–Alder reaction of TTMA with ethanol and ethylene
using various catalysts. Reaction conditions: 0.1 MPa initial N2 pres-
sure, 2008C, 360 min; 30 mol% Ni or Cu, 2.5 mol% Ru, Pt, or Pd with
respect to TTMA. “Others” refers to a combination of diethyl adipate,
1,4-cyclohexanedicarboxylic diethyl ester, and (E)-2-hexenedioic acid
diethyl ester.
The peaks at 1690 cmÀ1 and 1715 cmÀ1 correspond to the C O
=
stretching vibrations of TTMA and 2, respectively. Reaction conditions:
TTMA (1.8 mmol), ethylene (0.4 MPa), ethanol (10 mL), silicotungstic
acid (0.018 mmol), 1258C.
changes in the reaction process. The intensity of the peak at
1690 cmÀ1 was very low at the beginning owing to the poor
solubility of TTMA in ethanol at room temperature (0.18 g
per 100 mL ethanol, Table S1). The reaction of TTMA with
ethanol leads to diester 2, which has a better solubility (9.86 g
per 100 mL ethanol), promoting the esterification reaction.
Therefore, the intensity of the peak at 1715 cmÀ1 increased
rapidly. However, no peak was observed at 1740 cmÀ1 peak
over the whole reaction, revealing that the cycloadduct had
not been formed. Therefore, the Diels–Alder reaction
requires more rigorous conditions to proceed.
The reaction time course data and integration of the real-
time in situ FT-IR results indicate that the esterification and
Diels–Alder reaction take place sequentially: First, TTMA
reacts with ethanol to form the monoester and diester in
a stepwise process, which then undergo the Diels–Alder
reaction to form the cycloadducts. We surmise that two factors
are crucial in determining the reaction sequence, namely that
cycloaddition is preceded by esterification: 1) Ethanol serves
as both the solvent and the esterification reagent. The product
of esterification, 2, is significantly more soluble in ethanol
than the starting material. 2) Diesters possess two electron-
donating ethoxy groups, which facilitate the reaction with
a dienophile in a Diels–Alder process compared to TTMA.
To construct stable aromatic compounds, cyclohexenes
need to eliminate 2 equiv of H2. We tested different metals
supported on activated carbon (Ni/C, Cu/C, Ru/C, Pt/C, and
Pd/C) as dehydrogenation catalysts. The crude liquid product
mixture of the first step was used as the feed for the
dehydrogenation reaction. Typically, the reaction was con-
ducted at 2008C in nitrogen atmosphere. As illustrated in
Figure 4, Ni/C and Ru/C were inactive, and Cu/C showed only
1.0% selectivity towards the formation of DET. For Pt/C and
Pd/C, the selectivity increased to 11.9% and 33.7%, respec-
tively. The distribution of the cycloadducts changed after the
reaction. In particular, the amount of 5 increased to 70.2%
and 53.8% in the reactions catalyzed by Pt/C and Pd/C
whereas the amount of 5 was only 2.2% in the feed solution.
These findings imply that isomerization and dehydrogenation
reactions proceed simultaneously in the presence of these
metal catalysts.[17] Owing to the absence of an additional
hydrogen acceptor, other hydrogenated products were
detected, including diethyl adipate, 1,4-cyclohexanedicarbox-
ylic diethyl ester, and (E)-2-hexenedioic acid diethyl ester
(see Figures S10–S12).
Further studies were performed on the palladium-cata-
lyzed dehydrogenation reaction (Table 1). The yield of DET
increased from 34.0% to 75.5% upon increasing the amount
of Pd from 2.5 to 10 mol% with respect to TTMA (entries 1–
3). However, doubling the Pd amount from 5 mol% to
10 mol% only resulted in a slight increase in DETyield while
Table 1: Palladium-catalyzed dehydrogenation of the liquid products
from the esterification and Diels–Alder reaction of TTMA with ethanol
and ethylene.[a]
Entry
Pd
OHÀ/H+[c]
Specific rate[d]
Yield
[%][e]
[mol%][b]
[molDETmolPdÀ1 hÀ1
]
1
2
3
2.5
5
10
5
5
5
5
5
5
–
–
–
–
1
2
4
8
11
2.3
2.4
1.3
0.6
2.7
2.7
2.3
2.2
1.7
34.0
70.5
75.5
19.1
80.6
80.1
69.0
66.6
50.3
4[f]
5
6
7
8
9
[a] Reaction conditions: 0.1 MPa initial N2 pressure, Pd/C, 2008C,
360 min. [b] The amount of metallic palladium with respect to TTMA.
[c] The protons result from the silicotungstic acid added in the first step;
entries 1–4: no base; entries 5–9: OHÀ added as KOH. [d] The specific
rate is defined as the number of moles of DET produced by one mole of
the catalyst per hour. [e] The yield of DET was determined by HPLC
analysis; for entry 4, the yield of terephthalic acid is given. [f] The feeds
are the reaction mixtures containing cyclohexene-1,4-dicarboxylic acid
that were prepared by the reaction of TTMA (0.5 mmol) and ethylene
(2.0 MPa) in deionized water (20 mL) for 360 min at 2008C.
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the specific rate was reduced by almost one half. Therefore,
we selected a Pd amount of 5 mol% for the following
experiments. Using water as the solvent, the overall yield of
terephthalic acid was only 19.1% (entry 4), compared to
70.5% DET when ethanol was used as both reactant and
solvent under otherwise identical reaction conditions
(entry 2). This is due to the fact that the solubility of TTMA
in deionized water is rather low at room temperature (0.02 g
per 100 mL water, see Table S1). As TTMA reacts with
ethanol, diester 2 is formed, which has a very high solubility in
ethanol and can thus easily undergo the Diels–Alder reaction
with ethylene. Therefore, the esterification reaction is neces-
sary to obtain DET in high yield.
We also conducted several experiments to study the
influence of the acidity/basicity of the reaction mixture by
adding base to the solution (Table 1, entries 5–9). When the
same amount of hydroxide ions as that of protons resulting
from the silicotungstic acid used in the first step is added, the
yield of DET increased to 80.6% (entry 5). Doubling the
OHÀ/H+ molar ratio resulted in a slight decrease in the yield
to 80.1% (entry 6). However, upon further increasing the
basicity of the reaction system, the yield decreased rapidly
(entries 7–9). Obviously, the palladium-catalyzed dehydro-
genation reaction preferentially proceeds under neutral or
mildly basic conditions.
esters from biomass-derived mucic acid in excellent yield
(99.0%).[18] The production of bioethanol has been
commercialized, and ethylene can be manufactured by
the dehydration of bio-ethanol on large scale. Addition-
ally, unreacted ethanol and ethylene can be recycled in our
process.
In conclusion, a synthetic route to directly obtain diethyl
terephthalate (DET) from biomass-derived trans,trans-
muconic acid (TTMA) through a cascade process combining
esterification, Diels–Alder cycloaddition, and dehydrogen-
ation in a single reactor has been developed. A palladium-
based catalyst gave the best overall yield of DET, namely up
to 80.6% based on the amount of TTMA. The key esterifi-
cation reaction significantly increases the solubility of the
diester product in ethanol and promotes the Diels–Alder
reaction by modulating the electronic properties of TTMA.
The dehydrogenation reaction preferentially occurs under
neutral or mildly basic conditions. This work provides an
example of producing high-value aromatic chemicals by fully
utilizing the existing specific structures of biomass-derived
feedstock. By rational design of the functional groups, the
utilization of biomass can thus become more economic and
environmentally friendly.
The block flow diagram for the production of DET
starting from biomass-derived TTMA, ethylene, and ethanol
is depicted in Figure 5.
Acknowledgements
The whole process can be conducted in a single reactor
followed by one separation system. Compared to the reported
routes for the production of biomass-derived PTA (see
Figures S21 and S22), our process has the following features:
1) The process enables the direct formation of DET,
a replacement for terephthalic acid. The commonly
employed oxidation step is avoided, and the diethyl
terephthalate product could be separated and purified
by a common distillation method.
This work was financially supported by the National Natural
Science Foundation of China (21203183, 21233008, and
21473188).
Keywords: biomass · catalysis · cycloaddition · muconic acid ·
terephthalic acid
How to cite: Angew. Chem. Int. Ed. 2016, 55, 249–253
Angew. Chem. 2016, 128, 257–261
2) TTMA has a very low solubility at room temperature in
many solvents. Subjecting TTMA to a key esterification
reaction with ethanol leads to a significant increase in
solubility. Furthermore, the diester of TTMA possesses
two electron-donating ethoxy groups, facilitating the
reaction with a dienophile.
3) The raw materials used in this process can all be derived
from biomass. More specifically, TTMA can be produced
from d-glucose and lignin by microbial synthesis.[14a,c]
Furthermore, Toste et al. and Zhang et al. have proposed
catalytic chemical routes to synthesize TTMA and its
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Published online: November 23, 2015
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