A Sustainable Route to a Terephthalic Acid Precursor

Article abstract of DOI:10.1002/cssc.201600166

A new synthetic pathway for the production of p-toluic acid has been developed starting from reagents derived from renewable resources. A Diels-Alder reaction between sorbic and acrylic acids is followed by a combined dehydrogenation/ decarboxylation process, providing p-toluic acid in high yields. This route permits to use milder conditions compared to other Diels-Alder approaches reported in the literature, and therefore can contribute to a more sustainable terephthalic acid production.

Full text of DOI:10.1002/cssc.201600166

DOI: 10.1002/cssc.201600166
Communications
A Sustainable Route to a Terephthalic Acid Precursor
Maria Barbara Banella, Claudio Gioia,* Micaela Vannini, Martino Colonna, Annamaria Celli,
[a]
and Alessandro Gandini
A new synthetic pathway for the production of p-toluic acid
has been developed starting from reagents derived from re-
newable resources. A Diels–Alder reaction between sorbic and
acrylic acids is followed by a combined dehydrogenation/
decarboxylation process, providing p-toluic acid in high yields.
This route permits to use milder conditions compared to other
Diels–Alder approaches reported in the literature, and there-
fore can contribute to a more sustainable terephthalic acid
production.
that comprises the DA reaction of isoprene with acrylic acid
using different catalysts. A mixture of the meta and para iso-
mers was thus obtained, with a 23:1 excess of the para isomer
in the case of a reaction performed at room temperature with
TiCl4 as catalyst. Then, the two isomers must be separated
before aromatization in order to obtain pure TPA. Berard
[
16]
et al. recently reported the synthesis of p-toluic acid by the
reaction of sorbic acid with ethylene. Sorbic acid is a very
promising biobased DA diene, which can be (i) directly extract-
ed from non-edible berries (e.g., those produced by Sorbus Au-
[
17]
cuparia), (ii) synthesized starting from ethanol (via acetalde-
[
17]
Current industrial and academic studies are increasingly orient-
ed towards the preparation of chemicals and materials starting
from renewable resources, in order to avoid the environmental
hyde and sorbaldehyde), or (iii) prepared from triacetic acid
[
18]
[19]
lactone, a compound enzymatically derived from glucose.
Its market demand has increased consistently during the last
[
1–4]
À1 [16]
issues associated with the use of oil-based counterparts.
Ar-
several years, with a current production of 30 kta . The DA
omatic building blocks are of great interest in polymer science
owing to the better mechanical and thermal performances of
the according aromatic macromolecular backbones, compared
to those of aliphatic structures. Terephthalic acid (TPA), charac-
reaction between sorbic acid and ethylene displays a high se-
lectivity, but needs more than 40 h to reach high conver-
[
16]
sions. Moreover, a high pressure of ethylene (40 bar) is re-
quired and toluene has to be used as solvent, in combination
with a high temperature (1808C). The second step of aromati-
zation also needs significant improvements as a selectivity in
À1
terized by a production of approximately 40 Mta and with
an estimated growth of 5% until 2020, is the most widely used
[
5]
[16]
aromatic monomer. TPA is currently prepared on a large
p-toluic acid of only 41% was obtained.
[
6]
scale, using non-renewable resources, from p-xylene via
It follows that a process that requires milder conditions and
provides higher yields with respect to the results previously re-
[
7]
p-toluic acid. Recently, different processes have been devel-
oped obtain alternative synthetic routes for a sustainable TPA
production. For example, terephthalic acid and its derivatives
can be recovered from the depolymerization of poly(ethylene
[
16]
ported would be welcome. With this purpose in mind, the
present Communication reports a synthetic route for the pro-
duction of p-toluic acid in high yields using an activated dieno-
phile, that is, acrylic acid.
[
8,9]
terephthalate) (PET).
Several patents and papers have de-
[
20]
scribed the possibility to obtain TPA from biobased chemi-
Alder et al. studied the reaction between sorbic acid and
acrylic acid in 1950. The reaction led to two isomers consisting
of six-membered aliphatic rings, which were dehydrogenated
using sulfur. This aromatization process led to an isophthalic
[10]
[11]
[12]
[13]
cals, such as muconic acid, isobutanol, limonene, or
[
14]
isoprene. In particular the Diels–Alder (DA) reaction consti-
tutes a very promising route for the preparation of cyclic mole-
cules (Scheme 1). The Draths corporation developed a system
based on the use of DA reactions of muconic acid, which can
be produced by yeast fermentation of glucose, with different
[
20]
acid-like and a phthalic acid-like molecule. The procedure
presented here consists of a modern adaptation of Alder’s
route associated with the determination and characterization
of the resulting mixture of cyclic aliphatic structures via
[11]
dienophiles such as ethylene or acrylic acid. However, mu-
conic acid can be obtained only using genetically modified
1
H NMR. Furthermore, this approach differs from Alder’s
[
15]
[20]
strains and its synthetic process still needs to be optimized.
work in that the cycloaddition reaction is followed by an ar-
Indeed, dienes other than muconic acid can be used for the
preparation of aromatic dicarboxylic acid precursors. Frost
omatization step characterized by a selective decarboxylation
of the carboxylic groups present on the cycloadduct. Acrylic
acid is also a biobased reagent, as it can be easily obtained
[
14]
et al. developed a method for the synthesis of p-toluic acid
[
21,22]
from renewable resources such as lactic acid
or 3-hydroxy-
[
23]
[24]
propionic acid,
which can be prepared from glycerol.
[
a] M. B. Banella, Dr. C. Gioia, Dr. M. Vannini, Prof. Dr. M. Colonna,
Prof. Dr. A. Celli, Prof. Dr. A. Gandini
Therefore, a fully sustainable intermediate for TPA production
can be readily obtained.
Department of Civil, Chemical, Environmental and Materials Engineering
University of Bologna
Via Terracini 28, 40131, Bologna (Italy)
The DA reaction was carried out starting from acrylic and
sorbic acids or the corresponding esters (Scheme 2). The reac-
tion temperature was chosen according to the type of reactant
used. In particular, in the case of sorbic acid, the reaction must
E-mail: claudio.gioia2@unibo.it
Supporting Information for this article can be found under
http://dx.doi.org/10.1002/cssc.201600166.
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Communications
[
11]
[14]
[16]
Scheme 1. TPA synthesis from (a) muconic acid and ethylene, (b) isoprene and acrylic acid, (c) sorbic acid and ethylene.
Table 1. Reaction conditions, conversion, and ortho/meta ratio of the DA
reactions.
1
R
R
Catalyst
/
T
Reaction Conversion ortho/meta ratio
[
8C] time [h] [%]
CH
CH
CH
CH
CH
CH
CH
H
3
3
3
3
3
3
3
H
H
H
CH
CH
H
RT
4
4
0
3
–
n.c.
3
CH COOH RT
LiClO
4
RT
RT
80
80
140
140
4
7
n.c.
3
/
/
/
/
/
4
0
–
3
40
4
4
0
–
Scheme 2. Diels–Alder reaction between sorbate and acrylate.
79
100
100
64/36
64/36
61/39
H
H
4
be performed above its melting temperature (1388C), whereas
for reactions performed using the methyl ester of acrylic acid,
the temperature must be kept below its boiling temperature
acrylate (808C), using an excess of 2 equivalents of acrylate,
did not show any conversion even after 40 h. On the contrary,
the reaction between methyl sorbate and acrylic acid at the
same temperature afforded the cycloadduct with a 79% con-
(
(
808C). In all of these cases, an excess of the dienophile
2 equivalents with respect to sorbic acid or its ester) was
used. A purification step to remove the resulting excess was
not necessary, because its presence did not influence the
second step of the process (aromatization). Furthermore, in an
industrial process, acrylic acid or its methyl ester can be easily
recovered by distillation from the reaction mixture. Table 1
gives the reaction conditions in terms of reagents, temperature
and time, as well as the corresponding product yields.
1
version of methyl sorbate (calculated by H NMR analysis on
the crude product). The reaction between methyl sorbate and
acrylic acid was also conducted at 1408C obtaining a complete
conversion of the limiting reagent. Notably, a complete conver-
sion into the DA adduct was also obtained using sorbic acid
and acrylic acid at 1408C. These results suggest an acid-pro-
moted mechanism because carboxylic acids appear to be di-
rectly responsible for the activation of acrylates, increasing
their dienophilic character toward cycloaddition reactions.
Indeed, acidic species can lower the energy of the lowest un-
occupied molecular orbital (LUMO) level of the dienophile in-
The reactions between methyl sorbate and acrylic acid and
between methyl sorbate and methyl acrylate were performed
at room temperature. In both cases no conversion was ob-
served. The use of 10 mol% of catalysts (Table 1) did not signif-
icantly improve the conversion that was very limited even after
[
25]
4
h of reaction. For this reason the reactions have been per-
creasing its tendency to react with the diene.
formed at higher temperatures. In particular, the reaction be-
tween the two esters at the boiling temperature of the methyl
Figure 1 shows all the possible isomers resulting from the
DA reaction between sorbic and acrylic acid consisting of the
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Communications
two structural isomers (ortho and
meta diacids), each of which com-
prises two different diastereoiso-
mers.
p-toluic acid. This result indicates that the selective decarboxy-
lation of both carboxylic acids, at the ortho or meta position
with respect to the methyl group, occurred during the aroma-
tization process (the proposed mechanism is reported in the
Supporting Information) and therefore no intermediate separa-
tion or purification of the DA reaction products was needed.
When using a less-concentrated sulfuric acid (47.6% w/w),
a complete conversion was achieved in 40 min, whereas nei-
ther aromatizations nor other reactions were observed when
using solutions with even lower concentrations of sulfuric acid
(30.2% w/w and 14.2% w/w).
1
H NMR bidimensional analysis
conducted on the products ob-
tained from a selective precipita-
[20]
tion
permitted to evaluate the
composition of the isomeric mix-
ture. In particular, the ortho iso-
mers were separated from the
meta isomers by exploiting their
different tendencies to precipitate
in acetonitrile. The first white solid,
obtained from the acetonitrile so-
Figure 1. Structure of the
isomers formed by the DA
reaction of sorbic acid with
acrylic acid.
[14]
The approach is similar to those of Frost et al. and Wang
[
27]
[14]
[27]
et al. to produce toluic acid. Both Frost and Wang fo-
cused on monocarboxylic cyclohexene acids, and in both stud-
ies decarboxylation reactions were not observed during the ar-
[
27]
lution after 3 days, was crystallized in ethyl acetate. Bidimen-
omatization process. Wang observed a partial decarboxyla-
tion only when dealing with maleic anhydride cyclohexene de-
rivatives. In agreement with our results, p-toluic acid was ob-
1
sional g-COSY H NMR spectroscopy showed that this product
corresponds to the ortho isomer. The second white solid, pre-
cipitated at longer times, showed NMR peaks ascribable to
both ortho and meta isomers, On the basis of the NMR spectra
of the ortho and meta isomers, we determined the ortho/meta
ratios reported in Table 1.
[
27]
tained in that case, also.
Here the process offers the
advantage of combining the aromatization of the cyclohexene
ring with the selective decarboxylation of the carboxylic acid
group that is either at ortho or meta position with respect to
the methyl group.
The reaction mixture based on acrylic and sorbic acid had
a molar ratio of ortho/meta of 61/39, whereas the reaction
mixture based on acrylic acid and methyl sorbate led to an
ortho/meta molar ratio of 64/36. Such preferential formation of
the ortho diacids can be explained by a slightly more effective
orbital overlapping of the electron-poor C3 of acrylic acid with
the electron-rich C5 of sorbic derivative. NOESY experiments al-
lowed us to assign the proton signals of the regio- and stero-
isomers which were present in the crude mixtures resulting
from the reactions reported in Table 1. Regio- and stereo-iso-
mers ratios were calculated for each reaction and they are re-
ported in the Supporting Information.
In conclusion, we report a synthetic procedure for the prep-
aration of p-toluic acid, the most-often-used precursor for the
preparation of terephthalic acid (TPA), starting from renewable
resources (sorbic and acrylic acids). The process involves two
steps: a Diels–Alder reaction between sorbic acid and acrylic
acid in mild conditions followed by a combined dehydrogena-
tion/decarboxylation reaction that affords p-toluic acid in high
yields. Notably, the products obtained in the first step
(Figure 1) can be also used for the production of other diacids
such as isophthalic-like acids. This route allows milder condi-
tions as compared to Diels–Alder approaches previously re-
ported in the literature, and therefore contributes to a more
sustainable production of terephthalic acid.
The crude mixture afforded after the DA reaction was used
to obtain an aromatic compound. The procedure developed
here (Scheme 3) involves the addition of concentrated sulfuric
Experimental Section
Experimental details are described in the Supporting Information.
Keywords: Diels-Alder · environmental chemistry · polymers ·
renewable resources · synthetic routes
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Published online on April 13, 2016
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