The sustainable room temperature conversion of: P -xylene to terephthalic acid using ozone and UV irradiation

Article abstract of DOI:10.1039/c9gc02095k

Current industrial processes utilize Co/Mn bromides as catalysts to catalyze the oxidative conversion of para-xylene to terephthalic acid (TA) in acetic acid at high temperatures (>200 °C, air, 15-30 atm.). The decomposition of metallo-catalysts and solvents at high temperatures as well as a subsequent hydropurification process releases thousands of millions of tons of wastewater, global warming gas (CO₂) and ozone depleting gas (CH₃Br) into the global environment per year, causing global warming, ozone depletion, dramatic climate change, huge economic losses, and many other environmental problems. Herein, we report an alternative sustainable process with low energy demand for the room temperature oxidative conversion of p-xylene to terephthalic acid, with 96% TA yield and 98% selectivity, via ozone treatment and concurrent UV irradiation and without the generation and release of greenhouse gas (CO₂), ozone depleting gas (CH₃Br), and wastewater, or the need for a high energy-demand hydropurification process. The reaction mechanism involves the singlet O(¹D)- and hydroxyl radical-mediated selective C-H functionalization of p-xylene.

Full text of DOI:10.1039/c9gc02095k

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DOI: 10.1039/C9GC02095K
ARTICLE
Sustainable Room Temperature Conversion of p-Xylene to
Terephthalic Acid using Ozone and UV Irradiation.
Received 00th January 20xx,
Accepted 00th January 20xx
Kuo Chu Hwang*, Arunachalam Sagadevan, and Pradip Kundu
DOI: 10.1039/x0xx00000x
The current industrial process utilizes Co/Mn bromides as catalysts to catalyze oxidative
conversion of para-xylene to terephthalic acid in acetic acid at high temperatures (>200 C, air, 15-
30 atm.). Decomposition of metallo-catalysts and solvent at high temperatures as well as a later
hydropurification process release thousands-million tons of wastewater, global warming gas (CO2)
and ozone depleting gas (CH3Br) per year to the earth environment, causing global warming, ozone
depletion, dramatic climate changes, huge economic loss, and many other environmental problems.
Herein, we report an alternative low energy demanding sustainable process for room temperature
oxidative conversion of p-xylene to terephthalic acid, with 96% TA yield and 98% selectivity, via
ozone treatment and concurrent UV irradiation without generation and release of greenhouse gas
(CO2), ozone depleting gases (CH3Br), and wastewaters, and high energy demanding
hydropurification processes. The reaction mechanism involves singlet O(1D)- and hydroxyl
radical-mediated selective C-H functionalization of p-xylene.
acid7. In the hydropurification process, it was reported that 3~10
tons of wastewater were produced per ton purified TA (pTA)8.
Introduction
At present, very few large scale industrial productions of major
chemical intermediates are based on low energy-demanding green
processes. Terephthalic acid (TA) is one of the largest quantities of
key industrial intermediates, and is a raw material for the synthesis
of polyethylene terephthalate (PET), polyester, plasticizers, and
Kevlar fibers. In 2018, the global production of TA is ~80 million
These values are corresponding to a release of 8 million tons of CO2,
~2.56 kilo-tons of CH3Br, as well as 240~800 million tons of
wastewater to the earth environment per year for annual
production of 80 million tons of (ortho-, meta- and para-) phthalic
acid. In addition, replenishment of ~5.6 million tons of fresh acetic
acid is required per year, which increases the production cost of TA
significantly. The wastewater contains various kinds of organic
residues, including, acetate, benzoate, terephthalate, p-toluate, etc.
4-Carboxyl benzaldehyde (4-CBA) is the major impurity co-existing
with the final terephthalic acid product, and is a terminator in the
later polymerization process for the synthesis of PET9. In the
industrial purification process of crude TA, 4-CBA was catalytically
reduced back to p-toluic acid by 0.5 wt% Pd/C under hydrogen
atmosphere (>70 atm.) at 270~290 C in hot water steam10.
Deactivation of Pd catalyst via high-temperature sintering, sulfur
and lead poisoning occurs commonly, leading to shortened catalyst
lifetime (in general, less than a year)10-12. In 2000, the demand for
the expensive Pd/C catalyst was estimated to be ca. 1000 tons per
tons per year with a ~5% annual increasing rate per year1-5
. The
polymers derived from TA are widely used in daily life, including
soft drink bottles, liquid crystal displays, polyester films/clothes,
reinforced glass, bulletproof windows, boats, aircraft, etc. The
industrial production of TA is conducted via direct air oxidation of p-
xylene in corrosive acetic acid at 200 °C in the presence of Co2+,
Mn2+, and bromide catalysts under 15~30 air atmosphere pressure,
i.e., the AMOCO-MC process (Figure 1(a)). Due to the high reaction
temperature, decarboxylation or “burning” of solvents and
products occurs severely, leading to increase in the energy
consumption, production cost, generation/emission of global
warming gas (CO2) and ozone depleting gas (CH3Br), as well as
environmental pollution. It was estimated that production of 1 kg
TA is accompanied with a loss of 0.05~0.07 kg acetic acid via
combustion or burning at high reaction temperatures.6.7, which
leads to the generation of ~0.1 ton of global warming CO2 gas and
~0.000032 ton of ozone-depleting CH3Br gas per ton of terephthalic
year10,11
. To avoid corrosion by acetic acid and bromide at high
temperature, expensive titanium reactors have to be used.
Although many efforts have been devoted to developing more
energy-saving and greener synthetic conditions to avoid
decomposition of metallic catalysts and solvents, the harsh reaction
condition and production of several hundred million tons of wastes
per year were rarely avoided4-6,13,14. The high energy consumption
for the production of crude TA, decomposition of metallo-catalysts,
and the release of a huge amount of wastes contribute significantly
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan, ROC. E-
mail: kchwang@mx.nthu.edu.tw; Fax: (+886) 35711082.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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(a) Industrial AMOCO-MC process
to the global warming, ozone depletion, and environmental
soil/atmospheric pollution. In recent years, it was also reported
production of pTA by E. coli-assisted conversion of p-xylene or
View Article Online
DOI: 10.1039/C9GC02095K
air, 15~30 atm
Co/Mn/Br-,
CH3CO2H
CO2H
HO2C
CO2H
>98% conversion
~95% selectivity
major impurity: 4-CBA
p-TA
enzymatic conversion of biomass15-17
. However, these bio-based
200 o
C
processes are also facing their own problems/challenges, including
availability of huge quantities of multiple enzymes, scalability,
conversion efficiencies, products production rates, tedious product
purification processes, and still have a long way to go before they
can compete or replace the current fossil fuel-based industrial
AMOCO-MC pTA production process. It is urgently needed to
develop a green alternative process for large scale production of
pTA and to cut down the energy consumption, resolve these
metallo-catalysts associated problems, generation/release of
hundreds million tons of wastes/global warming gas/ozone
depleting gas every year to the environment, without the need of
energy-consuming 4-CBA hydropurification process, and the use of
expensive Pd/C catalyst.
"hydropurification" step
CO2H
CHO
H2,>70 atm,0.5% Pd/C
H2O, >270 oC
HO2C
p-TA
+
HO2C
HO2C
in crude TA
4-CBA (~ 7 ppm)
(b) Current O3-UV process
HO
O
HO
O
H
HO
O
O3, 1 atm, 25 ¢XC
100 W Hg Lamp
+
SM
+
+
O
OH
O
p-xylene:CH3CN:H2O
96%
2%
~160 ppm
= 1:3:2
pH= 4.5
98% conversion
98% selectivity
very trace 4-CBA
Solvent-wash purification step
CO2H
CH3CN-H2O
TA
purified
(4-CBA ~6 ppm)
washing at RT
HO2C
in crude TA
In this paper, we report an alternative sustainable process to
resolve the above-mentioned pollution problems associated
with the metal bromide catalysts-based industrial process for
the oxidative conversion of p-xylene to terephthalic acid at
room temperature. Our green process utilizes ozone and UV
light irradiation to generate highly reactive oxygen atom O(1D)
and hydroxyl radical, which render room temperature
oxidative conversion of p-xylene to terephthalic acid become
possible.
Fig. 1. Comparison of processes for production of terephthalic acid.
(a) Industrial AMOCO-MC process, and (b) the O3-UV process.
~80% of p-xylene was converted to solid precipitates, which are
composed of p-toluic acid (60 mol%) and TA (20 mol%). Upon
bubbling with ozone in the dark for 20 h, 35% p-xylene could also
be oxidized and converted to p-toluic acid (30 mol%) with few
amounts of TA (5 mol%, Table 1). The structure of p-toluic acid and
1
TA were confirmed by H/ 13C NMR spectra as well as single-crystal
X-ray crystallography (see the experimental section in
supplementary materials and Fig. S2 and S3 for X-ray
crystallography of p-toluic acid and ethyl terephthalate,
respectively). Control experiments show that p-toluic acid and 4-
CBA can dissolve in acetonitrile-water (3:2 volume ratio, pH= 4.5)
co-solvent, and can be nearly quantitatively oxidized and converted
to TA upon ozone treatment and concurrent UV irradiation at room
temperature (Table 1). Therefore, it is very likely that co-
precipitation of a large quantity of p-toluic acid solid with TA solid is
due to polarity mismatch and thus poor solubility of p-toluic acid
and 4-CBA in neat p-xylene. Precipitation of p-toluic acid and 4-CBA
as solids will make these reaction intermediates far less accessible
by oxidants. To accelerate oxidation of p-toluic acid and 4-CBA by
ozone, singlet O(1D) atom, and hydroxyl radical (vide infra), we
therefore add acetonitrile-water as a co-solvent to p-xylene in order
to dissolve and keep the major reaction intermediates (i.e., p-toluic
acid and 4-CBA) in the solution phase, rendering them available for
further oxidation. After 20 h ozone treatment and UV irradiation of
a p-xylene-acetonitrile-water (5:3:2 volume ratio) solution white
solid precipitates were still formed, but they are composed of less
p-toluic acid (32 mol%) and more TA (65 mol%) in addition to 160
ppm of 4-CBA (see HPLC chromatographs shown in Fig. S4). In a
case of p-xylene-acetonitrile-water (1:3:2) solution, we are able to
obtain 98% conversion with 96 mol% of TA (that is, ~ 98%
selectivity, see Table 1, entry 4) after 26 h O3-UV irradiation. Since
TA is far less soluble in acetonitrile-water co-solvent than p-toluic
Results
Inspired by the previous observation that ozone-UV irradiation is
able to achieve selective methylene C-H bond oxidation and
conversion of cyclohexane to adipic acid18, we investigate the
possibility of using ozone-UV irradiation to oxidatively convert p-
xylene to terephthalic acid. The industrial nitric acid oxidation
method works for the oxidation of cyclohexane to adipic acid, but
cannot be applied to oxidize pX to TA, since nitration of aromatic
ring occurs preferentially over oxidation of the benzylic protons19.
Likewise, it is not clear whether the ozone-UV irradiation-induced
oxidation will occur preferentially at the benzylic C-H bonds over
the aromatic C=C and C-H bonds of pX. In addition, the problems
involved in the industrial production of pTA are quite different from
those associated with the industrial production of adipic acid.
Selective C-H bond functionalization of hydrocarbons is known to
be very difficult, especially in the absence of any catalysts20-22. It still
means a lot even if a similar ozone-UV irradiation process can be
developed or modified to conquer the above-mentioned problems
associated with industrial production of pTA.
In a typical experiment, neat p-xylene was bubbled with ozone gas
and concurrently irradiated with a 100 W Hg lamp under an
exceptionally mild condition, i.e., room temperature (Figure 1(b)).
During irradiation, solid precipitates were gradually formed (see
optical pictures in supplementary fig. S1). After 20 h irradiation,
2 | J. Name., 2012, 00, 1-3
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acid and 4-CBA, the solid precipitates collected from the first run of followed by second insertion of singlet O(1D) atom to another
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DOI: 10.1039/C9GC02095K
photo irradiation were washed with acetonitrile-water (3:2 v/v, pH= benzylic C-H bond, the weakest C-H bond among all, to form
4.5) to dissolve and remove p-toluic acid and 4-CBA from the solid
TA. After 3 times of washing using an equal volume of acetonitrile-
water co-solvent, the final washed solid contains 97 mol% of TA and
Table 1. Substrates scope of oxidative C-H functionalization of
xylenes and reaction intermediates. The reactions were carried out
either under irradiation using a 100 W Hg lamp (200 mW/cm2 at
310 nm) reported first below or in the dark (reported below in
parentheses) at room temperature.
3
mol% of p-toluic acid and 6 ppm of 4-CBA (see HPLC
chromatographs shown in supplementary fig. S4). The final level of
6 ppm 4-CBA is better than that (25 ppm) obtained from the current
industrial high temperature hydropurification (0.5 wt% Pd/C, H2 >70
atm, 270~290 C in hot water steam) process, but without the need
of high energy consumption and the generation of huge amount of
wastewater. The washed acetonitrile-water solution is not a waste,
and can be recycled. Upon exposure to ozone treatment and UV
irradiation, p-toluic acid and 4-CBA in the recycled acetonitrile-
water solution can be nearly quantitatively oxidized and converted
to solid TA precipitate. By the current low-temperature reaction
condition and simple acetonitrile-water solvent washing process,
the high energy consuming and costly industrial hydropurification
process can be avoided. Meanwhile, the generation of several
hundred million tons of wastewater can be substantially reduced.
Most importantly, the formation of global warming gas (CO2) and
ozone depleting gas (CH3Br), which were formed via high
temperature decomposition of solvents/products, can be
completely avoided.
Mass
Selectivity
Total
conv (%)
Time
(h)
Products yield (%)
Solvent
Substrate
balance (%)
3
of (%)
CO2H
3a
CO2H
2a
25(14)a
85(80)
neat
20
80(35)
+
20(5)
CO2H
1a
1a
60(30)
67(33)a
10a
97(85)
92
2a
97(45)
50
3a
20
20
p-xylene:MeCN:H2O
= 5:3:2 (pH=4.5)
+
+
32(30)
65(15)
p-xylene:MeCN:H2O
= 5:3:2 (pH=4.5)
5 mol% BHTb
1a
1a
2a
3a
5
45
98a
98
p-xylene:MeCN:H2O
3a
2a
+
26
8
98
=1:3:2 (pH=4.5)
96
2
CO2H
CO2H
99(99)a
99(99)
95(15)
1 M in MeCN:H2O (2:1)
(pH= 4.5)
95(15)
CO2H
CO2H
2a
CO2H
1 M in MeCN:H2O (2:1)
(pH= 4.5)
98(97)
5
99(99)
99(99)
98(97)
CO2H
2a'
CHO
CO2H
CO2H
CO2H
1b
:MeCN:H2O
= 5:3:2 (pH=4.5)
47(14)
55(25)
95(85)
96(85)
20
95(35)
96(40)
+
1b
2b
3b
50(30)
CO2H
45(5)
CO2H
Under a similar condition, ortho- and meta-xylenes can also be
converted to phthalic acid and isophthalic acid, respectively (Table
1). Due to electronic and steric hindrance effects, ortho- and meta-
xylenes are less easy to be oxidized by ozone, singlet O(1D) atom,
and hydroxyl radical (vide infra) as compared to p-xylene, leading to
relatively lower yields of phthalic acids under the same condition
(Table 1). By the same way of ozone treatment and UV irradiation,
neat toluene, ethylbenzene and diphenylmethane all can be
oxidatively converted to benzoic acid, acetophenone, and
benzophenone, respectively, with 75~95% yields at room
1c
:MeCN:H2O
= 5:3:2 (pH=4.5)
20
+
CO2H
42(30) 3c 53(10)
1c
2c
a. p-Toluic acid and 4-CBA are intermediates and can be further
converted to terephthalic acid. The only final product observed is
terephthalic acid. Therefore, the true selectivity should be ~100% for all
processes. b. BHT= butylated hydroxytoluene, a known inhibitor for
peroxidation chain reactions.
germinal diol, which is commonly known to be unstable and will
rapidly undergo dehydration to form p-methylbenzaldehyde (Fig.
2(a))27, which can be easily oxidized and converted to carboxylic
acid functionality upon exposure to ozone in the dark. Note that
the bond strengths of benzylic C-H, aromatic C-H, and O-H bonds
are ~90, ~110, and ~105 kcal/mol, respectively28. Direct insertion of
singlet O(1D) atom to the weakest benzylic C-H bond requires
cleavage of one benzylic C-H bond and formation of two bonds (i.e.,
the C-O and O-H bonds), which is exothermic and
thermodynamically allowed/favored.
temperature (Table 2).
Benzoate, benzophenone, and
acetophenone are food preservative, photo-initiator, and important
intermediates in the pharmaceutical industry for the synthesis of
various types of medicines/drugs, respectively.
It was reported that photo-irradiation by short UV light ( < 330
nm), ozone molecule will decompose into singlet O(1D) atom and
1
singlet oxygen (1O2, g) with a quantum yield of 0.7923,24. Control
experiments show that prolonged photo irradiation of neat p-
xylene in the presence of a singlet O2(1g) photosensitizer at room
temperature generates only trace amount of TA, suggesting that
singlet O(1D) atom is probably the key oxidant responsible for the
oxidative conversion of p-xylene to p-toluic acid and terephthalic
acid. Singlet O(1D) atom is known to be able to directly insert into
C-H and O-H bonds of hydrocarbons in the gas phase with the
conservation of total spin angular momentum25,26
. In the current
process, it is likely that singlet O(1D) atom selectively inserts into
the benzylic C-H bond of p-xylene to generate benzylic alcohol,
In the case of p-xylene-acetonitrile-water system, the reaction
mechanism is likely to be different from the above neat p-xylene
system, since both ozone and singlet O(1D) are known to react with
water molecule to generate hydroxyl radical (see, equations (3) &
(4) in Fig. 2(b))29,30. The formation of hydroxyl radical was confirmed
by EPR measurements (Fig. S5). Hydroxyl radical is known to be able
to abstract a hydrogen atom from saturated hydrocarbons and
initiate a peroxidation chain reaction, in the presence of molecular
oxygen, to generate hydroperoxides31. In literature, it was reported
that hydroxyl radical can abstract a benzylic hydrogen atom from
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aqueous p-xylene with a rate constant32 of ~8.4 x 108 M-1s-1 . The H
atom abstraction rate of hydroxyl radicals was shown to become
much slower in dipolar or aprotic solvent environment31. In the
current p-xylene-acetonitrile-water system, a similar selective H-
atom abstraction at the benzylic position might occur to generate p-
methylbenzyl radical. Note that the benzylic C-H bond strength
(~90 kcal/mol) is much weaker than the aromatic C-H bond (~110
kcal/mol) (Fig. 2(b)), which favors selective H-atom abstraction by
hydroxyl radical from the benzylic position. The formation of p-
methylbenzyl radical was detected and confirmed by EPR
measurements (see, supplementary Fig. S6). The reaction of benzyl
radicals with molecular oxygen results in the formation of
benzylperoxyl radical, which can also abstract hydrogen atom from
another p-xylene molecule to form p- methylbenzyl hydroperoxide
and re-generate p-methylbenzyl radical (see equation (5) in Fig.
2(b)). In this peroxidation chain reaction, it involves the formation
of one O-H bond (releasing ~105 kcal/mol bond energy) to form p-
methylbenzyl hydroperoxide and cleavage of a benzylic C-H bond
(absorbing ~90 kcal/mol bond energy) to form p-methylbenzyl
radical, which is also exothermic and thermodynamically favored.
p-Methylbenzyl hydroperoxide is not stable, especially in the
presence of trace amounts of metal ions, and will decompose to
generate p-methylbenzaldehyde. The formation of p-
methylbenzaldehyde was reported and detected in the gas phase
reaction of p-xylene with hydroxyl radical33. The p-
methylbenzaldehyde can be easily oxidized by ozone to generate p-
toluic acid. A similar reaction scheme might occur to convert the
methyl group of p-toluic acid and generate dicarboxylic acid
product, i.e., terephthalic acid (see, equation (6) in Fig. 2(b)). The
existence of the hydroxyl radical-initiated peroxidation chain
reaction is supported by the observation that addition of an
inhibitor of peroxidation chain reaction, i.e., butylated
hydroxytoluene (BHT)34, dramatically suppresses the yield of TA
(Table 1). In the industrial high temperature air oxidation
processes, it is commonly observed that the formation of the first
electron-withdrawing carboxylic acid functionality decreases the
electron density of the para-methyl group, strongly suppresses the
reactivity of the para-methyl group in p-toluic acid, and thus hinders
further
Total conv
(%)
Mass
Vie
ivityw Article Online
DOI: 10.1039/C9GC02095K
Time
(h)
Select
of 3 (%)
Substrate
Products yield, %
Solvent
balance (%)
CHO
5(5)
CO2H
1d
:MeCN:H2O
= 5:3:2 (pH=4.5)
+
75(45)
90(55)
94(89)
98(97)
80(65)
95(85)
8
1d
2d
3d
70(40)
O
8
8
1e
:MeCN:H2O
= 5:3:2 (pH=4.5)
3e
3f
90(55)
O
1e
1f
Ph
1f
:MeCN:H2O
95(70)
98(97)
98(98)
Ph
95(70)
= 5:3:2 (pH=4.5)
neat p-xylene + O3 + UV
(a)
O3
O(1D)
+
1O2 (1Dg)
(1)
1
1
O
O
( D)
( D)
H
O
H
O
H
H
O
H
H
H
O
H
H
H
insertion
insertion
H2O
geminal-diol
O3
CO2H
CO2H
CO2H
(2)
O(1D)
O3
CO2H
p-xylene-CH CN-H O + O3 + UV
CHO
(b)
3
2
O3 + H2O
O(1D) + H2O
(3)
(4)
2 OH + O2
2 OH
H
OH
H
H
H
O
H
H
H
H
OOH
H
H-abstraction
(5)
H2O
H2O
O2
H
H
H
H
OO
O3
CO2H
OH/O2
(6)
Table 2. Substrates scope of oxidative C-H functionalization of
monosubstituted benzenes. The reactions were carried out either
under irradiation using a 100 W Hg lamp (200 mW/cm2 at 310 nm)
reported first below or in the dark (reported below in parentheses)
at room temperature.
CO2H
CO2H
O3
CO2H
CHO
Figure 2. Plausible reaction mechanisms. (a) Neat p-xylene, and (b)
p-xylene-CH3CN-H2O with ozone upon UV light irradiation.
oxidative conversion of p-toluic acid (by molecular oxygen) to
terephthalic acid35. Such a phenomenon is well understood, since a
benzylic C-H bond with lower electron density will disfavor an
oxidation process by oxidants, such as O2, metal oxides with poor
electron densities. To overcome such a problem, esterification of
the p-toluic acid to reduce electron withdrawing ability was
performed by addition of methanol to the high temperature
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reactor. Therefore, terephthalate dimethyl ester, instead of catalysts, no TA was obtained by using ozone alone37. In other
View Article Online
DOI: 10.1039/C9GC02095K
terephthalic acid, was obtained at the end. Alternatively, a higher words, transition metal catalysts are absolutely needed in these
reaction temperature was adopted to promote the air oxidation of three papers for the oxidative conversion of p-xylene to TA.
the second methyl moiety in p-toluic acid, which is at the expense Overall, these earlier work did not resolve the above-mentioned
of more energy consumption, decarboxylation/burning loss of key problems involved in the current industrial production of TA. In
solvent/products, environmental pollution, and production cost. In the current work, we use a combination of ozone treatment and UV
the current system, the problem of difficulty in the oxidation of the irradiation at room temperature/ 1 atm atmosphere to oxidatively
second methyl group in p-toluic acid does not exist at all, since the convert pX to TA without the need of any transition metal catalysts,
formation of the first electron-withdrawing carboxylic acid group, in minimized production of global warming gas, ozone depleting gas
fact, weakens the para-methyl C-H bonds and thus favors/promotes and wastewater, as well as less high energy consumption. Large
direct benzylic C-H bond insertion by singlet O(1D) and H-atom parts of the problems involved in the current industrial AMOCO-MC
abstraction by hydroxyl radicals, which is key or the rate processes are resolved.
determining step in the current O3-UV irradiation process. In other
It was reported in the literature that ozone and hydroxyl radical
were potent oxidants for the removal of organic pollutants in both
the atmosphere and polluted water41,42. Both ozone and hydroxyl
radical-mediated oxidation reactions were never adopted to
synthesize useful organic compounds in organic solutions. The
current process is very simple, green and sustainable with low
energy consumption, and has great potential to replace the harsh
industrial processes (Co2+, Mn2+, bromide, acetic acid, 200 °C, air,
15~30 atom, and hydropurification of 4-CBA at >270 °C, H2 >70 atm.
on Pd/C catalyst) for the synthesis of o-/m-/p-phthalic acids,
benzoic acid, acetophenone and benzophenone. From the Green
Chemistry Metric point of view, the current process has an E factor
of ~0.118, which is substantially smaller than the value of
3.14~10.14 for the industrial AMOCO-MC process (Table 3, and also
the section of “Green Chemistry Metric Evaluation” section in the
supplementary information for details of calculations). Besides the
Green Chemistry Metric values, the energy consumption of the
current process is also another important factor to be considered
when comparing to the industrial AMOCO-MC process. To avoid
distraction of the main focus of the current new chemical process,
we leave the detail energy and cost analysis of terephthalic acid
production from the current new process to a future manuscript.
words, the current process does not rely on direct “oxidation” of
benzylic C-H bond by oxidants, and therefore the reaction was not
disfavoured/suppressed by the formation of the first electron-
withdrawing benzoic acid functionality. Experimentally, p-toluic acid
and 4-CBA in the solution can be oxidized and nearly quantitatively
converted to PTA at room temperature (Table 1). When the
carboxylic acid group is located at ortho- or meta- position, the
electron-withdrawing (or benzylic C-H bond weakening) effect on
the methyl group becomes less effective, leading to slightly lower
yields of dicarboxylic acid products than the para isomer under the
same condition (Table 1). The co-precipitation of large amount p-
toluic acid (as well as some 4-CBA) in the final crude solid
precipitates is most probably due to poor solubility of polar p-toluic
acid and 4-CBA in non-polar p-xylene. Addition of acetonitrile-
water co-solvent to p-xylene can help dissolve and keep p-toluic
acid and 4-CBA intermediates in the solution phase, allowing easier
access by oxidants and further oxidative conversion of, otherwise,
insoluble p-toluic acid and 4-CBA solid to PTA. Experimentally, we
1
did not observe (by H NMR) any products deriving from hydroxyl
radical attack at the benzene ring of p-xylene, which is most
probably due to the much less stability of the ring-opened
hexadiene-OH adducts, as compared to the p-methylbenzyl radical
generated via hydrogen atom abstraction at the benzylic position.
In addition, much weaker benzylic C-H bond than the aromatic C-H
bond also favors selective H atom abstraction by hydroxyl radical at
the benzylic position. In the literature, it was reported that the
ozone consumption rate in aqueous p-xylene solution is 70 fold
larger than that in aqueous benzene, suggesting that attack of
hydroxyl radical at the benzylic protons is the major/dominant
process over that at the benzene ring moiety36.
Table 3. Side-by-side comparison of Green Chemistry Metric values
for the industrial AMOCO process and the current work.
Current
process
Green metrics values
Industrial
process
No
0.118
3.14~10.14
E-factor
1
2
Atom economy
Carbon efficiency
82%
82%
90.2%
92.2%
3
4
5
75.6%
98%
74.1%
95%
Reactiion mass efficiency
In the literature, it was ever reported that ozone was used as an
additional oxidant, besides oxygen/air, in the oxidative conversion
of p-xylene to terephthalic acid.37-40. The common features of these
earlier work are that transition metal catalysts, high temperature
(90~100 C), and corrosive acetic acid and bromides have to be
used in the reaction. The main function of ozone there was to
shorten the induction period of time for onset oxidation of p-xylene
and to lower down the reaction temperature a little bit, from 150 to
90 C. The conversion of p-xylene to TA mainly relies on the
transition metal catalysts. In the absence of transition metal
Selectivity of terephthalic acid
Conclusions
We have presented an unprecedented low energy demanding
green process to resolve problems associated with metal bromide
catalysts-based industrial process, and to oxidatively convert pX to
TA at an exceptionally mild condition of room temperature and 1
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atmosphere by ozone treatment and concurrent UV irradiation of
pX. Experimental results clearly demonstrate that singlet O(1D)
atom and hydroxyl radical-mediated oxidation preferentially occurs
at the benzylic C-H bonds over the aromatic C=C and C-H bonds in
pX. We have also developed a simple solvent washing process for
Acknowledgements
View Article Online
DOI: 10.1039/C9GC02095K
The authors are grateful for the financial support from the Ministry
of Science and Technology, Taiwan.
purification of crude TA, which prevents the necessity of the high Notes and references
energy demanding industrial “hydropurification” process and
consequently prevention of producing huge amount of wastewater.
The current method is greener and more sustainable than the
current industrial process for production of pTA (E factor 0.118 vs.
3.14-10.14), has great potential to replace the industrial pTA
production processes to prevent generation/release of several
hundred million tons of wastewater/ global warming gas (CO2)/
ozone-depleting gas (CH3Br) per year, reduces substantially the
energy consumption, as well as avoids environmental air/soil
pollution to the only earth we have.
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Conflicts of interest
There are no conflicts of interest to declare.
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DOI: 10.1039/C9GC02095K
Table of Contents
COOH
O3, uv light
room temperature
1 atm, air
COOH
p-Xylene was oxidatively converted to terephthalic acid at room
temperature with ~98% selectivity in the absence of any
catalysts via ozone treatment with concurrent UV irradiation
without productions of global warming gases.
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