Liquid phase oxidation of p-xylene to terephthalic acid at medium-high temperatures: Multiple benefits of CO2-expanded liquids

Article abstract of DOI:10.1039/b920262e

The Co/Mn/Br catalyzed oxidation of p-xylene to terephthalic acid (TPA) is demonstrated in CO₂-expanded solvents at temperatures lower than those of the traditional Mid-Century (MC) process. As compared with the traditional air (N₂/O₂) oxidation system, the reaction with CO₂/O₂ mixture at 160 °C and using an additional inert gas (N₂ or CO₂) pressure of 100 bar increases both the yield of TPA and the purity of solid TPA via a more efficient conversion of the intermediates, 4-carboxybenzaldehyde and p-toluic acid. At the same time, the amount of yellow colored by-products in the solid TPA product is also lessened, as determined by spectroscopic analysis. Equally important, the decomposition or burning of the solvent, acetic acid, monitored in terms of the yield of the gaseous products, CO and CO₂, is reduced by ca. 20% based on labeled CO₂ experiments. These findings broaden the versatility of this new class of reaction media in homogeneous catalytic oxidations by maximizing the utilization of feedstock carbon for desired products while simultaneously reducing carbon emissions.

Full text of DOI:10.1039/b920262e

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Liquid phase oxidation of p-xylene to terephthalic acid at medium-high
temperatures: multiple benefits of CO2-expanded liquids
Xiaobin Zuo,a,b Fenghui Niu,a Kirk Snavely,a,c Bala Subramaniam*a,c and Daryle H. Busch*a,b
Received 29th September 2009, Accepted 18th November 2009
First published as an Advance Article on the web 15th January 2010
DOI: 10.1039/b920262e
The Co/Mn/Br catalyzed oxidation of p-xylene to terephthalic acid (TPA) is demonstrated in
CO2-expanded solvents at temperatures lower than those of the traditional Mid-Century (MC)
process. As compared with the traditional air (N2/O2) oxidation system, the reaction with CO2/O2
mixture at 160 ◦C and using an additional inert gas (N2 or CO2) pressure of 100 bar increases both
the yield of TPA and the purity of solid TPA via a more efficient conversion of the intermediates,
4-carboxybenzaldehyde and p-toluic acid. At the same time, the amount of yellow colored
by-products in the solid TPA product is also lessened, as determined by spectroscopic analysis.
Equally important, the decomposition or burning of the solvent, acetic acid, monitored in terms
of the yield of the gaseous products, CO and CO2, is reduced by ca. 20% based on labeled CO2
experiments. These findings broaden the versatility of this new class of reaction media in
homogeneous catalytic oxidations by maximizing the utilization of feedstock carbon for desired
products while simultaneously reducing carbon emissions.
limited by the even harsher reaction conditions (T = 300–400 ◦C,
1. Introduction
P > 200 bar).
The Mid-Century (MC) process is well known for the production
CO2-based media have attracted much attention in re-
of terephthalic acid (TPA), a very important intermediate in
cent years because they provide the possibility of using pure O2
the synthesis of polyethylene terephthalate and related polymers
under safe conditions while overcoming most of the aforemen-
that can be fabricated into excellent fibers, resins and films.1,2 In
tioned limitations of air oxidation systems, as follows.7-10 (1) The
this process, p-xylene is oxidized on a large scale by air at 180–
use of CO2 in a mixed solvent increases both the O2 solubility
225 ◦C and 15–30 bar in the presence of a catalyst composed of
and the mass transport of the reactants in the liquid phase, the
cobalt acetate, manganese acetate and hydrogen bromide. The
former because of the significant solubility of O2 in liquid CO2
optimized yield of TPA is greater than 95%, along with excellent
and the latter because of improved diffusivities in CO2 media.11
product purity as shown by various analytical techniques.3 This
This is especially the case in carbon dioxide expanded liquids
process, despite successful on-stream operation for over 50 years,
(CXLs). A rule of thumb is the nearly ten fold increase in oxygen
has the following distinct disadvantages. (1) High temperature
solubility when the volume of solvent is expanded by a factor of
operation decreases the solubility of O2, while increasing solvent
two, depending on the properties of solvent. (2) The presence of
and product destruction and causing safety concerns. (2) Air is
CO2 in the vapor phase reduces flammability hazards because of
used as the primary oxidant instead of pure O2 for safety reasons.
the great abundance of inert CO2 and its large heat capacity. (3)
This produces large amounts of contaminated vent gas and the
Replacing the N2 from air with a much greater amount of CO2
abundant unreacted N2 must be cleansed before venting into
eliminates the contaminated vent gas because the CO2 is more
the environment. (3) The combination of corrosive bromide as a
easily separated and recycled in the oxidation process. However,
catalyst component and a high operating temperature requires
this is counterbalanced by the required separation of O2
the use of highly resistant but expensive titanium reactors. A
from N2.
recent breakthrough made by Poliakoff et al.,4 Savage et al.5
The first successful application of CXLs in oxidation reactions
was reported in 2002.12,13 The Co(salen) catalyzed oxidation of
and Fulton et al.6 has shown that this reaction can be effectively
catalyzed by MnBr2 with pure O2 in a new reaction medium—
2,6-di-tert-butyl phenol to 2,6-di-tert-butyl-1,4-benzoquinone in
high temperature and supercritical water, thus circumventing the
CO2-expanded acetonitrile was 1–2 orders of magnitude more
use of acetic acid. However, the scope of such reaction systems is
active than in either the neat organic solvent or supercritical
CO2. We recently described the synergistic effect of co-catalyst
aCenter for Environmentally Beneficial Catalysis, University of Kansas,
zirconium◦and promoter ketone in the oxidation of toluene from
50 to 100 C.14 It was found that CO2 has a strong activation
Lawrence, KS 66047, USA. E-mail: bsubramaniam@ku.edu;
Fax: 785-864-6051; Tel: 785-864-2903
effect on the reaction, especially by shortening the induction
period. Under certain conditions, the Co/Zr(acac)4 catalyzed
oxidation did not work at all with N2/O2 but proceeded
effectively with CO2/O2, affording high yields of benzoic acid. In
bDepartment of Chemistry, University of Kansas, Lawrence, KS, 66047,
USA. E-mail: busch@ku.edu; Fax: 785-864-6051; Tel: 785-864-1644
cDepartment of Chemical and Petroleum Engineering, University of
Kansas, Lawrence, KS 66045, USA
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this work, we extend the CO2 effect to this important industrial
TPA process, which is extremely challenging because of much
higher temperatures and the complexity of intermediates. More
specifically, the TPA precipitates during the reaction, occlud-
ing with it 4-carboxybenzaldehyde and other by-products. In
addition, the decomposition, or burning of solvent, is more
significant at higher temperatures. To this end, in the present
work, we first measured the expansion of acetic acid, containing
dissolved catalysts and substrate, by dense CO2. Then, based on
these results, we investigated the semi-continuous oxidation of
p-xylene at lower temperatures as compared with the MC process
for a more significant CO2 effect, with the expectation that
CXLs could bring about some specific advantages for product
selectivity and purity in this complex reaction system.
2. Experimental
2.1 Materials
All the catalysts, additives, substrates and solvents were commer-
cially available and used without further treatment. Industrial
grade (≥ 99.9% purity, < 32 ppm H2O, < 20 ppm THC) liquid
CO2 and ultra high purity grade oxygen were purchased from
Linweld. Isotopic gas 13CO2 (99.5 atom% 13C) was purchased
from Aldrich Chemical Co., Inc.
Fig. 1 Jerguson cell in oven for high temperature studies of CXLs.
2.3.2 Product analysis. After the reaction, the reaction
mixture was cooled to room temperature and then treated as
follows:
The gas mixture was analyzed by GC (Shin Carbon ST
100/120 mesh) to determine the yield of CO and CO2 produced
by solvent burning. For isotopic experiments, the 12CO2/13CO2
ratio was measured by mass spectrometry.
The insoluble terephthalic acid was separated from the liquid
mixture by filtration and the solid was washed with methanol to
remove most of the soluble impurities. The resulting white solid
was dried at 100 ◦C for 2 hrs to remove absorbed solvent, after
which a 10 mg sample was dissolved in 50 mL of methanol by
sonication and analyzed by HPLC (C18 ODS-2 column). The
reactor was washed with methanol and DMF to scavenge the
residual TPA solid, which was combined with the filtrate that
was retained after isolation of the solid TPA and analyzed by
HPLC to determine the composition of liquids. The yields of
products were based on the sums of those determined for the
solid and liquid components.
The presence of yellow colored by-products in solid TPA was
determined from optical density measurements using a UV-
Vis spectrometer at 340 nm.15 Typically, 0.3 g solid sample
was dissolved in 5 mL 4N NH4OH. The optical density was
calculated as in eqn 2:
2.2 Volumetric expansion of acetic acid + catalyst mixtures by
dense CO2
The volumetric expansion by CO2 of acetic acid containing
dissolved catalysts was measured in a high-pressure Jerguson
view cell that was placed in a high temperature oven (Yamato
DKN-400, part of a Cambridge Viscosity ViscoPro 2000 system,
Fig. 1). For these expansion studies, a measured volume of acetic
acid containing a known amount of catalysts was loaded into
the view cell and heated to the experimental temperature, which
was followed by the addition of 20 bar N2 as the substitute for
O2. Then CO2 was gradually introduced into the cell until an
expansion ratio (DV/Vo, eqn 1) of ca. 1.0 was obtained.
DV V(P)−Vo(P )
o
=
(1)
Vo
Vo(P )
o
where Po refers to ambient pressure from which the solution
is expanded; and P refers to cell pressure at CO2 expansion
conditions.
2.3 Oxidation experiments and product analysis
2.3.1 Oxidation experiments. The semi-continuous oxida-
tion of p-xylene to terephthalic acid was studied in a 50 mL,
stirred, titanium Parr reactor (Fig. 2). Typically, N2 or CO2 was
first added to the reactor containing the solvent and catalysts and
heated to the reaction temperature; then O2 was added until the
selected final pressure was reached. p-Xylene was subsequently
pumped into the reactor at a pre-defined rate (0.08 mL/min) to
initiate the reaction. The total reactor pressure was maintained
constant by continuously supplying fresh O2 to compensate for
the oxygen consumed in the reaction.
ODl = Al/L
(2)
where Al refers to the absorbance at wavelength l (nm); and L
refers to the distance (cm) that light travels through the sample.
2.4 Safety
The amount of substrate used in the reaction studies was
such that the maximum adiabatic temperature rise for total
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Fig. 2 Schematic diagram of semi-continuous oxidation p-xylene to terephthalic acid.
combustion of the substrate (taking into account the heat
capacities of the reaction mixture and the solid reactor) was
25 ◦C. The actual temperature rise observed during reactions
was a few ◦C, at most. In addition, the reactor was equipped
with a safety release valve that exhausts the reactor contents
safely to the building vent by means of a rupture disk in the
event the set safe pressure (200 bar) is exceeded.
3. Results and discussion
3.1 CO2 expansion measurements at 80 and 120 ◦C
Fig. 3 Catalyst solution and its expansion with increasing CO2
pressure. (a) room temperature, (b) 120 ◦C with 20 bar N2 and 18 bar
CO2, (c) 120 ◦C with 20 bar N2 and 159 bar CO2; [Co] = 60 mM, [Mn] =
1.8 mM, [Br] = 60 mM prior to the expansion.
Since supercritical CO2 (TC = 31 ◦C) expands liquids most
effectively at temperatures up to 1.2 times the absolute critical
temperature, our previous work was focused on reactions
◦
performed at 60 C or less wherein it was possible to measure
the solvent expansion in a water bath.12,16,17 Here, to measure
the catalyst solubility in CO2-expanded solvent at much higher
temperatures, in experiments that are the first of their kind for
p-xylene oxidation, we placed the high pressure Jerguson view
cell in a high temperature oven that was originally designed for
viscosity measurement (Fig. 1). Though the oven temperature
is expanded significantly at the much higher CO2 pressure of ca.
160 bar (Fig. 3c).
Fig. 4a shows the dependence of the expansion ratio on CO2
pressure at 120 ◦C. The expansion ratio increases gradually and
reaches a value of 0.85 when CO2 partial pressure is elevated to
160 bar (total pressure 180 bar). Accordingly, the concentrations
of cobalt, manganese and bromide are diluted to 33 mM,
1.0 mM and 33 mM, respectively. It is noteworthy that no
catalyst precipitates during the CO2 expansion. This implies the
possibility of replacing up to at least 50 vol% of the organic
solvent by CO2 while maintaining the catalyst in solution.
Fig. 4b shows the expansion result at 80 ◦C. As compared
with 120 ◦C, the expansion ratio at 80 ◦C is much higher,
◦
can be maintained as high as 200 C, these studies did not go
beyond 120 ◦C because of the corroding capability of acetic acid
and hydrogen bromide toward the stainless steel Jerguson cell.
The catalyst solution, containing 60 mM cobalt acetate,
1.8 mM manganese acetate and 60 mM hydrogen bromide in
acetic acid, has a slight pink tint at room temperature (Fig. 3a)
which turns blue when heated to 120 ◦C (Fig. 3b). The solution
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Further, the expansion behavior is different from that at 120 ◦C,
displaying a sharp enhancement when CO2 pressure exceeds 100
bar. This is attributed to the higher compressibility of the CO2
with pressure at the lower temperature resulting in enhanced
dissolution into and volumetric expansion of the liquid phase.
3.2 Semi-continuous oxidation of p-xylene
3.2.1 Reactions in CO2-expanded acetic acid: CO2 vs. N2 as
inert gas. The CO2 effect on oxidation of p-xylene has been
reported by Yoo et al.18,19,20 However, only a modest increase
in the yield of TPA was achieved because of the high reaction
temperature (> 190 ◦C) and low CO2 partial pressure (< 14 bar).
In these reactions, the solvents were not expanded; i.e., CO2 was
not a significant component of the solvent.
We started the reaction at 80 ◦C, much lower than the
temperature used in the p-xylene oxidation process, to ensure
a substantial expansion at relatively low CO2 pressure. Accord-
ingly, a preliminary reaction was performed at a CO2 pressure
of 111 bar, where the expansion ratio was 0.75. However, as
shown in Table 1, the process was very sluggish, generating only
1% TPA after 22 h (Entry 1). We rationalize this result on the
basis of our understanding that the temperature was too low to
overcome the activation energy of the reaction.
In the next reaction, the temperature was raised to 120 ◦C to
accelerate the oxidation. As shown in Table 1, under a low inert
gas pressure of 45 bar, there is no obvious difference between N2
and CO2 in the yield of TPA (ca. 80% in 0.5 h) as well as those of
the by-products 4-carboxybenzaldehyde (4-CBA) and p-toluic
acid (PTA) (Entries 2 and 3). HPLC analysis of the solid TPA
quantified the expected major impurities, 4-CBA and PTA. As
compared with p-xylene, PTA is much less reactive than the
starting material due to the deactivating effect of the electron
withdrawing carboxyl group. During the reaction, 4-CBA tends
to co-precipitate with TPA and is very difficult to remove.
Since 4-CBA is an inhibitor in the subsequent polymerization
reaction, its concentration should be kept as low as possible.
In addition, the solid also contains trace amounts of yellow
colored by-products such as the derivatives of fluorenone and
anthraquinone that can affect the quality of polymers.21 The
presence of these compounds is detected by the optical density at
340 nm.15 The lower that optical density, the lesser the amounts
Fig. 4 Volumetric expansion of acetic acid by CO2 with Co/Mn/Br
catalysts. (a) T = 120 ◦C, (b) T = 80 ◦C; [Co] = 60 mM, [Mn] =
1.8 mM, [Br] = 60 mM prior to the expansion.
reaching an approximate value of one (1) when the CO2 partial
pressure is 130 bar. However, the catalysts precipitate upon
further increases in CO2 pressure; e.g., approaching 140 bar.
Table 1 Oxidation of p-xylene in CO2 expanded acetic acida
P inert
Entry Inert gas (bar)
Expansion Ratio Reaction Y TPA Y 4-CBA Y PTA TPA (s)b 4-CBA
CO/p-x
OD340 (mol/mol)
PO2 (bar) T (◦C) (DV/Vo)
Time (h) (%)
(%)
(%)
(wt%)
(s)c ppm
1
2
3
4
5
6
7
8
9
CO2
N2
CO2
CO2
CO2
N2
CO2
N2
CO2
111
45
45
107
145
30
30
100
100
28
45
45
45
20
30
30
30
30
80
0.75
0
0.17
0.42
0.69
0
0
0
0.15
22
1.0
0.5
15.5 ——
——
—— ——
120
120
120
120
160
160
160
160
0.5
0.5
2.0
2.0
0.5
0.5
0.5
0.5
78.6 10.5
79.0 10.0
77.6 6.9
73.4 8.6
94.6 1.3
94.6 1.3
93.8 1.2
95.1 0.8
4.7
4.3
4.2
5.7
0.7
0.7
0.7
0.6
85.7
86.2
89.1
85.4
98.4
98.4
98.5
99.1
104000
101000
72000
94000
11700
11500
10500
6400
0.69
0.58
0.66
1.30
0.011
0.0065
0
0
0.086 0.040
0.087 0.038
0.069 0.035
0.063 0.019
a Reaction conditions: [Co] = 33 mM, [Mn] = 1.0 mM, [Br] = 33 mM, VL (volume of liquid phase) = 35 mL (with or without CO2 expansion), 1.6 mL
p-xylene added at 0.08 mL/min, n (stirrer speed) = 1200 rpm. b TPA in dry isolated solid. c 4-CBA in dry isolated solid. The conversion of p-xylene
is above 99% in all the reactions except entry 1. In addition to 4-CBA and PTA, a few other byproducts were detected in small amounts during the
HPLC analysis. These compounds were not identified.
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of these yellow colored by-products. As shown in Table 1, when
the inert gas pressure is 45 bar, the amount of 4-CBA in the
TPA solid is quite similar for N2 and CO2, whereas the optical
density is lower for CO2. As CO2 pressure is increased to 107 bar,
the corresponding expansion ratio is 0.42, and the yield of TPA
approaches 80% (Entry 4), however, a relatively long reaction
time of 2 h is required. Further increase of CO2 pressure to
145 bar (expansion ratio = 0.69) and decrease of O2 pressure to
21 bar produces a lower yield of TPA (Entry 5). On the other
hand, the optical density is much higher, which indicates higher
concentrations of yellow colored by-products. The reason for
this phenomenon remains unclear. Based on HPLC analysis, the
yields of TPA and the common by-products (4-CBA and PTA) at
a CO2 pressure of 145 bar are 73.4%, 8.6% and 5.7% respectively
(Table 1, entry 5). Since the p-xylene conversion is above 99%,
the unknown by-products (peaks seen in the HPLC but not
positively identified) account for more than 10% of the p-xylene
converted. The TPA precipitate recovered from the reaction
mixture has a very dense yellow color (the optical density is
much higher as compared with that of the TPA solid produced
at lower CO2 pressure), which indicates significant formation
of derivatives of fluorenone and anthraquinone. Based on these
accompanied by higher burning rate.22 In sharp contrast, our
finding demonstrates that it is possible to reduce the production
of both these solid and gaseous by-products simultaneously by
employing high pressure CO2. In conclusion, a multi-beneficial
effect of CXL has been exhibited in the Co/Mn/Br catalyzed
oxidation of p-xylene to terephthalic acid.
3.2.2 Reaction optimization at temperatures less than 180 ◦C.
As reported in the literature, two methods have commonly been
used to adjust the catalyst to reduced reaction temperatures
while maintaining TPA yields that are close to those of the
current optimized industrial process: (1) The considerable
increase of cobalt and bromide concentrations while decreasing
the manganese concentration.23 The mole ratio of cobalt to
manganese can be as high as 100 to avoid the precipitation
of manganese in order to avoid grey or black TPA products.24
(2) The addition of other transition metals as co-catalysts,25-28
among which zirconium is the most efficient one that can further
lower the reaction temperature to about 100 ◦C. Due to the lack
of systematic data in these related systems, we◦have optimized the
reactions in the temperature range 120–170 C, with emphasis
placed on the influence of cobalt and manganese concentrations
and effect of zirconium on the yield of TPA, solid TPA purity,
optical density at 340 nm and yield of CO. For ease of operation
and comparison purposes, the pressure of CO2 was chosen as
30 bar. As a superior flame inhibitor to nitrogen, the use of CO2
makes it possible to run the reactions safely with an equimolar
amount of oxygen in the gas phase.29 Thus, 30 bar O2 was
introduced to ensure a large excess of oxygen over p-xylene.
As shown in Fig. 5a, the yield of TPA is significantly elevated
by doubling the concentration of cobalt from 33 mM to 66 mM
and adding zirconium, in the form of the acetate (Zr/Co =
1/6) at lower temperatures. However, this promoting effect
◦
results, it is reasonable to assume that at 120 C, regardless of
the use of CXLs at various O2 partial pressures, the kinetic rates
of the consecutive oxidation reactions of p-xylene to yield TPA
are not fast enough to avoid the intermediate oxidation products
and the other byproducts that adversely affect the TPA purity.
At 120 ◦C, the only positive effect of CO2-based media is
therefore the lower yield of the gaseous product CO, which falls
to zero when CO2 pressure is above 100 bar. This suggests that
CO2 might inhibit solvent burning. However, at this point we
have not determined the yield of the second gaseous product,
CO2 produced by burning, although such a differentiation
should be possible using labeled CO2 (vide infra).
◦
The catalytic performance is greatly improved as the tem-
perature is further increased to 160 ◦C, where two sets of
reactions were carried out to provide additional comparisons
between the behaviors in N2 and CO2 at even higher yields of
TPA. As shown in Table 1, under a low inert gas (N2 or CO2)
pressure of 30 bar, the reaction results are almost identical for
N2 and CO2 (Entries 6 and 7). When N2 pressure is increased
to 100 bar, there is little difference in the reaction results except
a modest decrease of optical density (Entries 6 and 8). The
results are completely different, however, when CO2 pressure is
increased to 100 bar. The expansion ratio is estimated to be
0.15, as extrapolated from the expansion data for 80–120 ◦C. In
addition to a minor enhancement in TPA yield as compared to
that for 100 bar of N2, the oxidation process in CXL benefits
a great deal in several aspects (Entries 6–9). (1) The yield of
by-products, especially that of 4-CBA, is markedly decreased.
This indicates the more complete oxidation of the intermediates
to TPA in CO2-expanded solvent that is possibly associated
with increased oxygen availability. (2) The purity of solid TPA
is enhanced by the reduction of 4-CBA concentration from
11000 ppm to 6400 ppm. (3) The optical density is the lowest
among the four sets of reaction conditions. (4) The yield of CO
is significantly decreased; i.e., by about 50%. In general, the
concentration of 4-CBA in TPA solid is inversely related to the
burning of solvent, i.e. lower 4-CBA concentrations are always
diminishes when the temperature is above 150 C. The highest
TPA yield of 97.5% is achieved with the combination of cobalt
and zirconium at 170 ◦C.
The effect on the purity of solid TPA parallels that on TPA
yield. As shown in Fig. 5b, increasing the cobalt concentration
and the use of zirconium enhances the purity of solid TPA,
an effect that is more pronounced at lower temperatures. Also
the highest purity is achieved with the combination of cobalt
and zirconium at 170 ◦C. This is not surprising because the
more complete conversion of the dissolved intermediates will
definitely decrease their concentration in solid TPA.
The effect on the optical density at 340 nm is somewhat in-
triguing. As shown in Fig. 5c, an increase in cobalt concentration
always helps lower the yield of yellow colored by-products within
the studied temperature ranges. However, the incorporation of
zirconium works well only at lower temperatures. The quality of
the optical density no longer benefits from zirconium when the
temperature is greater than 160 ◦C.
The effect on the yield of CO is more complicated. As shown
in Fig. 5d, the production of CO can be inhibited by doubling
the cobalt concentration and adding zirconium at temperatures
below 135 ◦C. This might be unexpected since higher cobalt
concentrations are considered to favor solvent burning.30 In
contrast, there is a surge in CO yield when the temperature
rises above 150 ◦C, especially with the use of zirconium.
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Fig. 5 Effects of cobalt concentration and zirconium on p-xylene oxidation. Reaction conditions: PCO2 = PO2 = 30 bar, [Mn] = 1.0 mM, [Br] =
33 mM, VL = 35 mL, 1.6 mL p-xylene added at 0.08 mL/min, t = 0.5 hr, n = 1200 rpm.
Fig. 6 shows the effect of manganese. The reactions were
carried out with a higher cobalt concentration of 66 mM at
170 ◦C. No zirconium was used because of the much higher
solvent burning at this temperature.
As shown in Fig. 6a and 6b, the yield of TPA and
solid TPA purity decrease monotonically with the increase
of manganese concentration. In the absence of manganese,
the yield and purity can be as high as 96.9% and 99.5%,
and these are decreased to 93.3% and 98.5% respec-
tively when the manganese concentration is increased to
8.0 mM.
Too much manganese is also disadvantageous from the
standpoint of the optical density. As shown in Fig. 6c, in the
complete absence of manganese, the TPA solid has a very low
optical density of 0.025. In comparison, this value quadruples
to ca. 0.1 when the manganese concentration is 8.0 mM.
In spite of all these negative effects, manganese still plays a
crucial role. As shown in Fig. 6d, the yield of CO is decreased by
ca. 30% at a very low manganese concentration of 1.0 mM. At
the same time, the yield of TPA is only slightly decreased from
96.9% to 96.6%. Thus, manganese is very effective in reducing
solvent burning.
3.2.3 Effect of CO2 on solvent burning—isotopic study.
Solvent burning is a very important side reaction in p-xylene
oxidation, which accounts for a significant loss of acetic acid
each year. It is estimated that 70% of acetic acid produced
worldwide is used to manufacture TPA, and about 5% of this
solvent is destroyed by burning.22,31 Therefore, solvent burning
is a significant factor in the economics of the MC process. The
burn rate is reported as the mole ratio of gaseous products,
CO and CO2, produced to the moles of p-xylene added. Our
previous work on the Shell catalyst (Co/Zr) suggests that
CO2 might reduce solvent burning by inhibiting the Co(III)-
catalyzed decarboxylation of acetic acid.14 The compelling issue
is expressed in the question “is it possible to reduce solvent
burning in the Co/Mn/Br catalyzed oxidation of p-xylene by
using CO2-based media?”
For this purpose, we chose the labeled compound 13CO2 as
the inert gas. By mass spectrometry, the CO2 produced by
burning can be differentiated from the CO2 added to the system
by chemical reaction(s). To compare the different behavior of
N2 and CO2 in solvent burning, three sets of reactions have
been carried out, among which two sets are based on a lower
temperature of 160 ◦C and a higher inert gas pressure of 45 b◦ar,
while the other set is based on a higher temperature of 191 C
and a lower inert gas pressure of 23 bar. The O2 pressure for
the latter case is controlled at 7 bar to mimic an MC process as
From the results described above, the optimized parameters
for the medium-high temperature oxidation of p-xylene are:
[Co] ≥ 60 mM, [Mn] ≤ 1.0 mM, [Zr] = 0 mM, T = 150–160 ◦C.
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Table 2 Effect of CO2 on solvent burning
Inert gas
T (◦C)
P inert (bar)
PO2 (bar)
Y TPA (%)
12CO (mmol)
12CO2 (mmol)
(12CO2+12CO)/p-x (mol/mol)
N2
160
160
160
160
191
191
45
45
45
45
23
23
45
45
12
12
7
94.6
95.1
94.1
94.3
84.7
94.1
0.52
0.32
0.37
0.23
0.45
0.46
1.30
1.11
0.89
0.76
0.68
1.13
0.14
0.11
0.095
0.075
0.085
0.12
13CO2
N2
13CO2
N2
13CO2
7
Reaction conditions: for 160 ◦C reaction [Co] = 33 mM, [Mn] = 1.0 mM, [Br] = 33 mM; for 191 ◦C reaction [Co] = [Mn] = [Br] = 5.5 mM; VL
=
35 mL, 1.6 mL p-xylene added at 0.08 mL/min, t = 0.5 hr, n = 1200 rpm. The conversion of p-xylene is above 99% in all the reactions. In addition to
4-CBA and PTA, a few other byproducts were detected in small amounts during the HPLC analysis. These compounds were not identified.
Fig. 6 Effects of manganese concentration on p-xylene oxidation. Reaction conditions: PCO2 = PO2 = 30 bar, T = 170 ◦C, [Co] = 66 mM, [Br] =
33 mM, VL = 35 mL, 1.6 mL p-xylene added at 0.08 mL/min, t = 0.5 hr, n = 1200 rpm.
operated by industry. As the data in Table 2 reveal, the reactions
yield 2–3 times as much CO2 as CO, regardless of the kind of
inert gas present during reaction. This is in good agreement with
the previous report that the CO2/CO mole ratio is around 3.32
Remarkably, theuseofCO2 cansuppressnotonlytheproduction
of CO, but also the production of CO2 at 160 ◦C, and it is
interesting to note that the yield of CO is reduced to a larger
extent. As a result, the burn rate is 0.14 for N2 and 0.11 for CO2
when O2 pressure is 45 bar, and 0.095 for N2 and 0.075 for CO2
when O2 pressure is 12 bar. In both cases, the burning is reduced
by ca. 20% when compared to N2 with similar yields (ca. 95%) of
TPA. Also, the yield of TPA is much better with CO2 at 191 ◦C,
suggesting the positive effect of overcoming the kinetic barrier.
While the yield of CO is nearly indistinguishable, the yield of CO2
is increased markedly at the higher temperature. Consequently,
the reactions should be performed at lower temperature and
higher CO2 pressure to inhibit solvent burning.
The formation of COx (CO plus CO2) during the MC process
involves a very complicated reaction mechanism. Apart from
the solvent burning that is the major source of COx, COx
can also be produced by the over-oxidation of p-xylene and
the decomposition of the TPA product. These mechanisms
are currently under investigation. From the point of view of
thermodynamics, it seems plausible that the equilibrium shifts
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to the left upon the addition of a large excess of CO2. In addition,
we also observed that the CO2 yield is decreased by adding an
appropriate amount of water, another by-product of burning.
This also suggests the leftward shift of the equilibrium.
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In summary, we have studied the Co/Mn/Br catalyzed medium-
high temperature oxidation of p-xylene to terephthalic acid
in CO2-expanded acetic acid based on the measurement of
solvent expansion in a Jerguson cell that is equipped with
a high temperature oven. The reactions have been optimized
by varying the reaction temperature, the concentration of
cobalt and manganese and the use of co-catalyst zirconium.
As compared with N2/O2, the use of CO2/O2 at sufficiently high
inert gas pressure and 160 ◦C significantly improved the catalytic
performance by decreasing the yield of 4-carboxybenzaldehyde,
p-toluic acid and other sources of yellow colored by-products. In
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requires compression power and this could nullify the energy
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This work was supported by the National Science Foundation
Engineering Research Centers Program, Grant EEC-0310689.
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