A New Strategy for the Preparation of Terephthalic Acid by the Aerobic Oxidation of p-Xylene Using N-Hydroxyphthalimide as a Catalyst

Article abstract of DOI:10.1002/1615-4169(20010226)343:2<220::aid-adsc220>3.0.co;2-n

A new methodology for the production of terephthalic acid (5) by the aerobic oxidation of p-xylene (1) using a combined catalytic system of N-hydroxyphthalimide (NHPI)/Co(OAc)₂/Mn(OAc)₂ was developed. The oxidation of 1 under a dioxygen atmosphere in the presence of a catalytic amount of NHPI/Co(OAc)₂/Mn(OAc)₂ at 100°C for 14 h afforded terephthalic acid in 82percent yield. Removal of Mn(OAc)₂ from the catalytic system resulted in considerable reduction in the yield of 3. When the oxidation of 1 was carried out under a pressure of air (30 atm) at 150°C, the reaction was completed within 3 h to give 3 in 84percent yield. The oxidation of p-toluic acid (2), which can be prepared by the oxidation of 1 using the NHPI/Co(OAc)₂ system at room temperature, by the NHPI/Co(OAc)₂/Mn(OAc)₂ system under pressure of air (30 aim) at 150°C gave 3 in 95percent yield. N-Acetoxyphthalimide (NAPI) was found to require a lower catalyst loading than NHPI, but oxidation with NAPI was slower. Thus, the oxidation of 1 catalyzed by NAPI (5 mol percent)/Co(OAc)₂ (0.5 mol percent)/Mn(OAc)₂ (0.5 mol percent) under a dioxygen atmosphere (1 atm) in acetic acid at 100°C gave 3 in 80percent yield.

Full text of DOI:10.1002/1615-4169(20010226)343:2<220::aid-adsc220>3.0.co;2-n

FULL PAPERS
A New Strategy for the Preparation of Terephthalic Acid
by the Aerobic Oxidation of p-Xylene
Using N-Hydroxyphthalimide as a Catalyst
Yasutaka Tashiro, Takahiro Iwahama, Satoshi Sakaguchi, Yasutaka Ishii*
Department of Applied Chemistry, Faculty of Engineering & High Technology Research Center, Kansai University, Suita,
Osaka 564±8680, Japan
Fax: (+81) 6-63 39-40 26; e-mail: ishii@ipcku.kansai-u.ac.jp
Received June 26, 2000; Revised October 21, 2000; Accepted December 9, 2000
Keywords: C±H acti-
vation; cobalt; radi-
cals; manganese; oxi-
dation; oxygen
Abstract: A new methodology for the production of acid (2), which can be pre-
terephthalic acid (3) by the aerobic oxidation of p- pared by the oxidation of 1
xylene (1) using a combined catalytic system of using the NHPI/Co(OAc)2
N-hydroxyphthalimide (NHPI)/Co(OAc)2/Mn(OAc)2 system at room tempera-
was developed. The oxidation of 1 under a dioxygen ture, by the NHPI/Co(OAc)2/Mn(OAc)2 system
atmosphere in the presence of a catalytic amount of under pressure of air (30 atm) at 150 °C gave 3 in
NHPI/Co(OAc)2/Mn(OAc)2 at 100 °C for 14 h af- 95% yield. N-Acetoxyphthalimide (NAPI) was found
forded terephthalic acid in 82% yield. Removal of to require a lower catalyst loading than NHPI, but
Mn(OAc)2 from the catalytic system resulted in con- oxidation with NAPI was slower. Thus, the oxidation
siderable reduction in the yield of 3. When the oxi- of 1 catalyzed by NAPI (5 mol %)/Co(OAc)2 (0.5
dation of 1 was carried out under a pressure of air mol %)/Mn(OAc)2 (0.5 mol %) under a dioxygen
(30 atm) at 150 °C, the reaction was completed with- atmosphere (1 atm) in acetic acid at 100 °C gave 3
in 3 h to give 3 in 84% yield. The oxidation of p-toluic in 80% yield.
mine is used, vessels lined with titanium or other ex-
pensive metals are necessary. The crude TPA pro-
Introduction
duced is unsuitable as a feedstock for polyester,
primarily owing to the 4-carboxybenzaldehyde (4-
CBA) formed as a side product which acts as a chain
terminator in polymerization reactions that convert
PTA to its most important end products, polyester fi-
bers and PET resins. In order to obtain pure TPA in a
fiber grade, a purification process has jointly been de-
veloped by Scientific Design and Amoco, in which the
4-CBA is hydrogenated to p-toluic acid by hydrogen
with a carbon-supported palladium catalyst.[2,4] Thus,
PTA including less than 25 ppm of 4-CBA is now pro-
duced.
Direct oxidation of hydrocarbons with air (O2) is a
commercially important reaction for the production
of oxygen-containing compounds such as alcohols,
ketones and carboxylic acids from petroleum feed-
stock and natural gas.[1] Among the carboxylic acids
manufactured by the oxidation of hydrocarbons, tere-
phthalic acid (TPA) derived from p-xylene has re-
cently become an important, fast-growing chemical
as a raw material for polyethylene terephthalate
(PET).[2] In 1999, ca. 26 million tons of PETwere man-
ufactured worldwide and its growth is estimated at a
minimum of 5% annually to the year 2002.
Until the development of the Amoco purification
process of TPA, only dimethyl terephthalate (DMT)
was used as a raw material for PET production, since
DMT could be purified to the fiber grade by crystalli-
zation.[2] DMT is manufactured through the following
four steps developed by C. W. Witten and modified by
Hercules and Dynamit-Nobel. First, p-xylene is oxi-
dized to p-toluic acid. It then passes to an esterifica-
tion step to form methyl p-toluate. The methyl p-tolu-
ate is isolated and returned to the oxidation reactor,
Nowadays, about 70% of the TPA used worldwide is
produced by the catalytic oxidation of p-xylene dis-
covered by Scientific Design and Amoco.[2,3] The oxi-
dation is catalyzed by a mixture of heavy metals, most
commonly cobalt and manganese salts, and bromine,
typically in the form of HBr, NaBr, or tetrabromo-
ethane. In the conventional process, the reactor is
maintained at an operating temperature between
175±225 °C and pressure between 15±30 atm of air,
using acetic acid as a solvent. Because corrosive bro-
220
Ó WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2001
1615-4150/01/34301-220±225 $ 17.50-.50/0
Adv. Synth. Catal. 2001, 343, No. 2

FULL PAPERS
Table 1. Oxidation of p-xylene (1) catalyzed by NHPI[a]
giving monomethyl terephthalate, followed by esteri-
fication to DMT.
Run
Catalyst (mol %)
Yield (%)
Since the Amoco purification process for TPA was
developed, almost all of the new plants use the Amoco
oxidation process. This process, however, has some
disadvantages: i. e., the use of bromine as a promoter
which corrodes the reactor, and the requirement of a
high operating temperature. From economical and
environmental points of view, the development of a
new oxidation system, which overcomes these disad-
vantages for the production of PTA, is desired.
In our preceding papers, we showed that N-hydro-
xyphthalimide (NHPI) promotes the oxidation of a
variety of hydrocarbons with dioxygen in the pre-
sence or absence of transition metal salts.[5] In this
oxidation, the phthalimide N-oxyl radical (PINO) gen-
erated in situ from the NHPI and dioxygen abstracts
the hydrogen atom from the hydrocarbon to form an
alkyl radical, which is readily trapped by dioxygen, af-
fording oxygenated-compounds. Thus, the aerobic
oxidation of toluene to benzoic acid under normal
pressure and temperature (1 atm, 25 °C) has first
been achieved in high yield (81%) through a catalytic
process using NHPI combined with Co(OAc)2.[5b] In
this paper, we wish to report a new strategy for the
production of PTA by the NHPI-catalyzed aerobic oxi-
dation of p-xylene under halogen-free conditions.
NHPI
Co(OAc)2 Mn(OAc)2
2
3
1
2
20
20
20
20
15
10
10
20
20
20
0
±
0.5
±
±
±
12
15
54
4
31
72
31
13
21
51
< 1
n. d.
67
6
82
48
21
55
73
68
3
3
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
4[b]
5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
6
7[c]
8[d]
9[e]
10[f]
11
0
[a] Substrate 1 (2 mmol) was allowed to react with O2 (1 atm)
in the presence of catalyst in acetic acid (5 ml) at 100 °C for
14 h.
[b] Conversion and other side-products are shown in text.
[c] Acetic acid (2 mL) was used.
[d] The reaction was carried out at 90 °C.
[e] Propionic acid was used as a solvent.
[f] Benzonitrile was used as a solvent.
Mn(OAc)2 was employed. The yield of 3 was consider-
ably dependent on the amount of NHPI used (runs 5
and 6). The oxidation at 90 °C afforded 3 in slightly
lower yield (73%) (run 8). Several solvents were ex-
amined in the present oxidation. Among them, acetic
acid, commonly used in the industrial oxidation of
these substrates, was found to be the best solvent
(runs 9 and 10). Needless to say, almost no reaction
took place in the absence of NHPI under these condi-
tions (run 11).
Results and Discussion
Direct Oxidation of p-Xylene to Terephthalic Acid
(TPA)
The oxidation of p-xylene (1) with molecular oxygen
(1 atm) in the presence of a catalytic amount of NHPI
(20 mol %), Co(OAc)2 (0.5 mol %), and Mn(OAc)2
(0.5 mol %) using acetic acid as solvent at 100 °C for
14 h afforded terephthalic acid (3) in 82% yield and
p-toluic acid (2) (4% yield) together with small
amounts of 4-carboxybenzaldehyde (4-CBA), 4-car-
Scheme 1.
boxybenzyl alcohol, 1,4-diacetoxymethylbenzene, Since oxidation by use of air instead of O2 is very im-
and 4-acetoxymethylbenzoic acid as well as several portant in practical industrial processes, 1 was oxi-
unidentified species in 1±2% yields, respectively, at dized with air using the NHPI/Co(OAc)2/Mn(OAc)2
> 99% conversion (Scheme 1, Table 1, run 4). Table 1 system (Scheme 2, Table 2). The oxidation of 1 with
summarizes representative results for the oxidation 20 atm of air in the presence of NHPI (20 mol %),
of 1 with dioxygen (1 atm) under various reaction Co(OAc)2 (0.5 mol %) and Mn(OAc)2 (0.5 mol %) in
conditions. The oxidation of 1 by NHPI alone gave 2 acetic acid at 150 °C for 3 h gave 3 (65%) together
in low yield (12%) and low conversion (14%) of 1, with 2 (19%) and 4-CBA (4%) (run 1). When the reac-
but 3 was not formed at all (run 1). Substrate 1 was tion was carried out under pressure of air (30 atm),
converted into 3 in 67% yield by the use of NHPI com- the yield of 3 reached 84%. In this oxidation, the yield
bined with Co(OAc)2, while the same oxidation by of 4-CBA was less than 1% (run 2). When the amount
NHPI combined with Mn(OAc)2 led to 2 rather than 3 of NHPI was decreased to 15 mol %, 3 was formed in
as a major product (runs 2 and 3). Since the synergis- almost the same yield as that by 20 mol % (run 3).
tic effect of Co and Mn salts in the aerobic oxidation of Both o- and m-xylenes were also successfully con-
hydrocarbons such as p-xylene has been reported,[6] verted into the corresponding dicarboxylic acids un-
the NHPI catalyst combined with Co(OAc)2 and der these reaction conditions, affording isophthalic
Adv. Synth. Catal. 2001, 343, 220±225
221

asc.wiley-vch.de
Table 2. Oxidation of xylenes catalyzed by NHPI[a]
Run
Substrate
Air (atm) NHPI (mol %) Yield (%)
2
3
1
2
3
4
p-Xylene (1)
1
1
1
m-Xylene
o-Xylene
20
30
30
30
30
30
20
20
15
10
15
15
19
1
4
14
7
6
65
84
84
71
87
73
5
6[b]
[a] The reaction was carried out in 50 mL stainless autoclave.
Xylene (2 mmol) was allowed to react in the presence of
NHPI, Co(OAc)2 (0.5 mol %), and Mn(OAc)2 (0.5 mol %) un-
der pressure of air in acetic acid (3 mL) at 150 °C for 3 h.
Figure 2. Time-dependence curves for the aerobic oxida-
tion of 1 catalyzed by NHPI. (a) 1 (2 mmol) was allowed to
react with O2 (1 atm) in the presence of NHPI (20 mol %)
and Co(OAc)2 (0.5 mol %) in acetic acid (5 ml) at 100 °C. (b)
1 (2 mmol) was allowed to react with O2 (1 atm) in the pre-
sence of NHPI (20 mol %), Co(OAc)2 (0.5 mol %), and Mn
(0.5 mol %) in acetic acid (5 mL) at 100 °C.
[b]
2-Carboxybenzyl alcohol (8%) and 2-carboxybenzalde-
hyde (3%) were also produced (see Experimental Section).
acid and phthalic acid in 87% and 73% yields, respec-
tively (runs 5 and 6).
with that from the NHPI/Co(OAc)2/Mn(OAc)2 system
(Figure 2). With both catalytic systems, 1 was found to
be consumed within 0.5 h, and 2 was formed in the
range of 73% to 75% yields. Subsequently, 2 was grad-
ually converted into 3 in 82% yield by the NHPI/
Co(OAc)2/Mn(OAc)2 and in 67% yield by the NHPI/
Co(OAc)2 systems, respectively. In the conventional
autoxidation of p-xylene, a similar enhancement by
the Mn salt has been observed.[1±3] The synergistic ef-
fect of Co(II) and Mn(II) is reported to lead to a reduc-
tion of the induction period and to an enhancement of
the rate in the chain-propagation step.[7] Unfortu-
nately, in the oxidation by the NHPI/Co(II)/Mn(II)
system, the effect of Mn(II) on the rate enhancement
cannot be described at the present time.
Scheme 2.
As reported previously, toluene was smoothly oxi-
dized under normal temperature and pressure of di-
oxygen (25 °C, 1 atm of O2) by the combined catalytic
system of NHPI with Co(OAc)2. However, the oxida-
tion of 1 by the NHPI/Co(OAc)2 system produced 3 in
a lower yield than that obtained by the NHPI/
Co(OAc)2/Mn(OAc)2 system. Hence, the effect of the
Mn salt on the present oxidation was examined (Fig-
ure 1 and Figure 2). As shown in Figure 1, the yield of
3 by the oxidation of 1 using the NHPI/Co(OAc)2 cata-
lyst was 67%, but the addition of 0.1 mol % of
Mn(OAc)2 to the NHPI/Co(OAc)2 system resulted in 3
in 75% yield. The yield of 3 reached to 82% by the ad-
dition of 0.5 mol % of Mn(OAc)2, but no acceleration
was observed upon further addition of Mn(OAc)2.
Time-dependence curves for the aerobic oxidation
of 1 by the NHPI/Co(OAc)2 system were compared
Production of TPA via Two Steps from p-Xylene
As mentioned in the introduction, Dynamit-Nobel
(Witten) manufactures dimethyl terephthalate
(DMT) from p-xylene 1 according to the following
steps: p-xylene ® p-toluic acid ® methyl p-toluate ®
monomethyl terephthalate ® DMT. In a previous pa-
per, we showed that p-xylene 1 was oxidized by the
NHPI/Co(OAc)2 system under a normal pressure of
O2 even at room temperature to form p-toluic acid 2
in 85% yield (Scheme 3). The conversion of 1 to 2 at
room temperature is thought to be very interesting.
Therefore, we next tried the NHPI-catalyzed oxida-
tion of 2 and methyl p-toluate (4) derived from the
oxidation of 1 at room temperature.
Figure 1. Addition effect of Mn(OAc)2 to NHPI/Co(OAc)2
system. Conditions: Substrate 1 (2 mmol) was allowed to re-
act under O2 (1 atm) in the presence of NHPI (20 mol %) and
Co(OAc)2 (0.5 mol %) in acetic acid (5 mL) at 100 °C for 14 h.
Scheme 3.
222
Adv. Synth. Catal. 2001, 343, 220±225

FULL PAPERS
Table 3. Oxidation of p-toluic acid (2) or methyl p-toluate
(4) catalyzed by NHPI[a]
Table 4. Oxidation of p-xylene (1) catalyzed by NAPI[a]
Run
Catalyst (mol %)
NAPI
Yield (%)
Run
Catalyst (mol %)
Conversion Yield (%)[b]
(%)
Co(OAc)2 Mn(OAc)2
2
3
NHPI
Co(OAc)2 Mn(OAc)2
1
2
3
20
20
10
10
5
5
10
5
0.5
0.5
±
±
±
±
0.5
±
±
1
92
91
38
89
24
80
80
88
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
48
1
53
8
1[c]
2
3
10
5
3
10
10
5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
±
0.1
0.5
99 (99)
94 (95)
74
94
99
94 (92)
86 (88)
71
86
93
4[b]
5[b]
6[b]
7[c]
8[d]
4
5
6
4
6[d]
99 (99)
95 (93)
[a] Compound 2 (or 4) was allowed to react with O2 (1 atm) in
the presence of catalyst in acetic acid (5 mL) at 100 °C for
14 h.
[a]
Compound 1 (2 mmol) was allowed to react with O2
(1 atm) in the presence of catalyst in acetic acid (5 mL) at
100 °C for 14 h.
[b] The number in the parenthesis shows the conversion and
yield in the oxidation of 4.
[b] Acetic acid (2 mL) was used.
[c]
Compound 1 (10 mmol) and acetic acid (10 mL) were
[c] The amount of 4-CBA was less than 1%.
[d] Compound 2 (or 4) was allowed to react under pressure of
air (30 atm) in the presence of catalyst in acetic acid (3 mL)
at 150 °C for 3 h. Side products other than 3 (5) were mainly
4-CBA, 4-carboxybenzyl alcohol, and 4-acetoxymethylben-
zoic acid.
used.
[d]
Compound 1 (2 mmol) was allowed to react under pres-
sure of air (30 atm) in the presence of catalyst in acetic acid
(2 mL) at 150 °C for 3 h.
phthalimide and phthalic anhydride during the
oxidation. Our efforts were directed to the reduction
of the amount of the catalyst employed in this oxida-
tion, and N-acetoxyphthalimide (NAPI) was found to
be an efficient catalyst. Table 4 shows the results for
the oxidation of 1 with molecular oxygen by the use
of NAPI instead of NHPI. The oxidation of 1 in the pre-
sence of catalytic amounts of NAPI (20 mol %),
Co(OAc)2 (0.5 mol %), and Mn(OAc)2 (0.5 mol %) by
dioxygen (1 atm) in acetic acid at 100 °C for 14 h af-
forded 3 in excellent yield (92%) together with 2
(1%) and 4-CBA (1%) (run 1). Almost the same re-
sults were obtained when the oxidation was carried
out in the absence of Mn(OAc)2 (run 2). Interestingly,
1 was oxidized to 3 in high yield even by the use of
10 mol % of NAPI (run 4). The oxidation in the pre-
sence of NAPI (5 mol %), Co(OAc)2 (0.5 mol %), and
Mn (0.5 mol %) afforded 3 (80%), 2 (8%), and 4-CBA
(5%) (run 6). In a 10 mmol scale experiment under
the same reaction conditions as the run 4, 3 was ob-
tained in 80% yield (run 7).
p-Toluic acid (2) or methyl p-toluate (4) was al-
lowed to react under dioxygen atmosphere (1 atm)
in the presence of NHPI (10 mol %), Co(OAc)2
(0.5 mol %), and Mn(OAc)2 (0.5 mol %) in acetic acid
at 100 °C for 14 h to give terephthalic acid (3) or
monomethyl terephthalate (5) in 94 or 92% yield, re-
spectively, along with a small amount (< 1%) of 4-CBA
(Scheme 4 and Table 3, run 1). It is worthy of note that
the oxidation successfully took place even by the use
of 5 mol % of NHPI forming 3 in 86% yield (run 2). A
remarkable enhancement of the oxidation was ob-
served by the addition of an Mn salt to the NHPI/
Co(OAc)2 system (runs 4 and 5). When the reaction
was carried out under pressure of air (30 atm) at
150 °C, the oxidation of 2 was completed within 3 h to
afford 3 in excellent yield (run 6). Compound 4 was
also oxidized with ease under these reaction condi-
tions. Treatment of 4 under pressure of air (30 atm)
by the NHPI/Co(OAc)2/Mn(OAc)2 system in acetic
acid at 150 °C for 3 h gave 5 in 93% yield (run 6).
Figure 3 shows the time-dependence curves for the
aerobic oxidation of 1 catalyzed by NAPI combined
with Co(OAc)2. In contrast to the oxidation of 1 by
NHPI where 1 was completely consumed within 1 h
(Figure 2), the analogous oxidation of 1 by NAPI pro-
ceeded much more slowly. This observation suggests
that oxidation of 1 at the early stage of the reaction
triggered the decomposition of the NHPI to phthali-
mide and phthalic anhydride. Although it is rather dif-
ficult to explain the effect of the NAPI in this oxida-
tion, it was found that the NAPI gradually generates
NHPI by hydrolysis with H2O formed during the oxi-
dation as well as involving acetic acid. Therefore, the
NAPI may be suppressing the decomposition to
phthalimide or phthalic anhydride by gradually sup-
plying NHPI during the oxidation.
Scheme 4.
Oxidation of p-Xylene Catalyzed
by N-Acetoxyphthalimide
As shown in Table 1, ca. 20 mol % of NHPI must be
used to convert p-xylene 1 into terephthalic acid 3 in
satisfactory yield (over 80%). This is believed to be
due to the gradual decomposition of the NHPI to inert
Adv. Synth. Catal. 2001, 343, 220±225
223

asc.wiley-vch.de
NaHCO3 and then a buffer solution (20 mol/L of K2HPO4
and KH2PO4 in water) was added.
General Procedure for Oxidation of 1 or 2 under
Pressure of Air (30 atm)
The reaction was carried out in 50-mL stainless steel auto-
clave. An acetic acid solution (3 mL) of substrate (2 mmol),
NHPI
(5 mol %,
0.1 mmol),
Co(OAc)2
(0.5 mol %,
0.01 mmol), and Mn(OAc)2 (0.5 mol %, 0.01 mmol) was
placed in an autoclave. The autoclave was pressurized to 30
atm with air. The mixture was stirred at 150 °C for 3 h. The
work-up was performed as described above.
Figure 3. Time-dependence curves for the aerobic oxida-
tion of 1 catalyzed by NAPI. Conditions: 1 (2 mmol) was al-
lowed to react with O2 (1 atm) in the presence of NAPI (10
mol %) and Co(OAc)2 (0.5 mol %) in acetic acid (2 mL) at
100 °C.
Procedure for the Oxidation of o-Xylene under
Pressure of Air (30 atm)
After the oxidation of o-xylene as described above, the sol-
vent was evaporated under reduced pressure. Then the re-
action mixture was treated with NaOH (15 mmol) in H2O
(30 mL) at 90 °C overnight in order to hydrolyse phthalic an-
hydride, which was formed as a by-product, giving phthalic
acid isolated in 73% yield.
In conclusion, a new strategy for the aerobic oxida-
tion of p-xylene to terephthalic acid has been exam-
ined by the use of a combined catalyst system of NHPI
(or NAPI) with Co(OAc)2 and Mn(OAc)2. The present
method provides the direct oxidation of 1 to 3 and
two-step production to 3 through 2 from 1. These oxi-
dations could be carried out under relatively mild
conditions without any halogen ion.
Acknowledgements
This work was partially supported by Research for the Future
program JSPS and DAICEL Chemical Industries, Ltd.
Experimental Section
General
References and Notes
All starting materials and catalysts were purchased from
commercial sources and used without further treatment.
The yield of 3 was isolated yield from the reaction mixtures.
The yields of 2 and 5 were estimated from the peak areas
based on the internal standard technique by using HPLC.
GC analysis was performed with a flame ionization detector
using a 0.2 mm ´ 25 m capillary column (OV-1). 1H and 13C
NMR spectra were measured at 270 MHz and 67.5 MHz, re-
spectively, in CDCl3 with Me4Si as the internal standard.
GC-MS were obtained at an ionization energy of 70 eV. All
products were identified by comparison of the isolated pro-
ducts with authentic samples.
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General Procedure for Oxidation of 1 or 2 under
Dioxygen Atmosphere
An acetic acid solution (5 mL) of substrate (2 mmol), NHPI
(20 mol %, 0.4 mmol), Co(OAc)2 (0.5 mol %, 0.01 mmol),
and Mn(OAc)2 (0.5 mol %, 0.01 mmol) was placed in a
three-necked flask equipped with a balloon filled with O2.
Caution: A balloon filled with oxygen can be hazardous, even
with a small-scale reaction. The mixture was stirred at 100 °C
for 14 h. After the reaction, a white precipitate including
terephthalic acid (3) was formed. The solid was filtered off
under vacuum, and washed with acetic acid several times
to give almost pure terephthalic acid. For analysis by HPLC,
the reaction mixture was added to ca. 150 mL of aqueous
224
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