Aerobic oxidation of p-toluic acid to terephthalic acid over T(p-Cl)PPMnCl/Co(OAc)2 under moderate conditions

Article abstract of DOI:10.1007/s10562-009-0227-1

The aerobic oxidation of p-toluic acid to terephthalic acid over tetra(p-chlorophenylporphinato)manganese (T(p-Cl)PPMnCl)/cobalt acetate was reported for the first time. The co-catalysis of T(p-Cl)PPMnCl/Co(OAc) ₂ was also studied. The studies

Full text of DOI:10.1007/s10562-009-0227-1

Catal Lett (2010) 134:155–161
DOI 10.1007/s10562-009-0227-1
Aerobic Oxidation of p-Toluic Acid to Terephthalic Acid over
T(p-Cl)PPMnCl/Co(OAc) Under Moderate Conditions
2
Y. Xiao
Q. Liu
•
W. P. Luo
G. F. Jiang
•
•
X. Y. Zhang
Q. H. Li
•
C. C. Guo
•
•
Received: 24 October 2009 / Accepted: 12 November 2009 / Published online: 25 November 2009
Ó Springer Science+Business Media, LLC 2009
Abstract The aerobic oxidation of p-toluic acid to tere-
phthalic acid over tetra(p-chlorophenylporphinato)manga-
nese (T(p-Cl)PPMnCl)/cobalt acetate was reported for the
first time. The co-catalysis of T(p-Cl)PPMnCl/Co(OAc)2
was also studied. The studies indicated that the PTA oxi-
dation was influenced by the catalyst composition, catalyst
concentrations and reaction conditions. A preliminary
mechanism of this co-catalyzed oxidation reaction was
proposed based on experimental observations.
In fact, the difficulty of the aerobic oxidation of PX to
TPA lies in the oxidation of p-toluic acid (PTA) with air to
TPA. Chemically, the aerobic oxidation of PX to TPA
involves the aerobic oxidation of PX to PTA and the sub-
sequent oxidation of PTA with air to TPA. However, the
oxidation of PTA to TPA is much more difficult than the
oxidation of PX to PTA [5]. So the effective oxidation of
PTA to TPA with air becomes the key step of the aerobic
oxidation of PX to TPA. The methyl group of PTA will be
deactivated by the electron withdrawing effect of the
–COOH group once one methyl group of PX is oxidized, and
the reactivity of PTA is only about one-tenth of PX [2]. So it
is feasible to realize the effective oxidation of PX to TPA
through two steps. The work we have reported [6, 7] has
solved the step of the oxidation of PX to PTA, however, the
oxidation of PTA to TPA still remains a challenge to the
oxidation of PX to TPA. So it is essential to explore how to
oxidize PTA to TPA effectively under moderate conditions.
Since 1960s, people have been trying to discover better
catalytic ways for PTA oxidation and much progress has
been achieved. Higashijima and Nagayama [8] carried out
the PTA oxidation with molecular oxygen over ruthenium-
substituted heteropolyanion in water under the conditions
of 200 °C and 6 MPa. Toland et al. [9] developed the
method of PTA oxidation with aqueous base and sulfur at
high-pressure over 15 MPa. Emerson et al. [10] conducted
the PTA oxidation with oxygen in the presence of Fe O
Keywords Metalloporphyrin ꢀ Cobalt acetate ꢀ
co-Catalysis ꢀ p-Toluic acid ꢀ Oxidation ꢀ Terephthalic acid
1
Introduction
The preparation of terephthalic acid (TPA) via the aerobic
oxidation of p-xylene (PX) is a very important industrial
process from both economical and environmental aspects
[
1, 2]. In practice, the oxidation of PX with air to TPA is
now carried out in HOAc (80%) using Co(OAc)2/
Mn(OAc) /HBr as catalysts on an industrial scale world-
2
wide. The low concentration of the reactants and the use of
the bromide make the low efficiency and high environ-
mental impact and corrosion of reactors becoming the
major shortcomings [3, 4]. The development of new PX
oxidation processes fitting the requirements of green
chemistry still remains a challenge.
2
3
and K CO in KOH solution at high-pressure (700 psi) and
3
2
2
40 °C. Ishii et al. [11] developed a new bromide free
method for the oxidation of PTA in HOAc with 10% of
NHPI, 0.5% of Co(OAc) and 0.5% of Mn(OAc) as cat-
2
2
Y. Xiao ꢀ W. P. Luo ꢀ X. Y. Zhang ꢀ C. C. Guo (&) ꢀ Q. Liu ꢀ
G. F. Jiang ꢀ Q. H. Li
alyst at 100 °C to yield TPA. Givens et al. [12] reported
that the oxidation of PTA could be accomplished by cobalt
and N-hydroxysuccinimide co-catalyzed reactions with
dioxygen under the conditions of 25 °C and 233 psi in
College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, People Republic of China
e-mail: ccguo@hnu.cn
123

1
56
Y. Xiao et al.
2
4 h. Zuo et al. [13] made use of a unique combination of
were charged into the reactor; nitrogen was injected to
maintain the pressure of 1.2 MPa at first in order to heat the
mixtures to 190 °C and assure the non-occurrence of
reactions during the stage of heating; the mixtures were
heated to 190 °C, then air was injected at the speed of
160 L/h and the reaction system pressure was maintained
at 1.2 MPa. An automatic gas-measuring instrument was
Co(OAc)2 with zirconium tetrakis(acetylacetonate) to
realized the oxidation of PTA using ketones as promoters
under the conditions of 100 °C and 60 bar in 24 h. Saha
and Espenson [14] carried out the autoxidation of PTA
-
with the Co(OAc) /Mn(OAc) /Br catalyst in the presence
2
2
of trifluoroacetic acid. However, above achievements did
not agitate the industrial preparation of TPA catalyzed by
Co(OAc) /Mn(OAc) /Br and these catalyst system still
used to measure the O concentration of tail gas on-line.
2
After the reaction completed, the autoclave was cooled to
room temperature and the products were quantitatively
analyzed by HPLC as reported previously [7].
2
2
suffered from high catalyst concentration, the use of bro-
mide as a promoter, high solvent proportion, serious
decarboxylation and rigorous reaction conditions.
This paper describes the aerobic oxidation of PTA to TPA
2.2.2 Oxidation of PTA at Atmospheric Pressure
over T(p-Cl)PPMnCl/Co(OAc) under moderate reaction
2
conditions. The T(p-Cl)PPMnCl/Co(OAc) system has the
2
Oxidation of PTA at atmospheric pressure was carried out
according to the following procedure: 1.5 g of PTA and
25 mL of HOAc as well as precisely weighed catalyst were
placed in a 100-mL three-necked flask (in dark) equipped
with condenser and magnetic stirring. Oxygen (0.1 MPa)
was continuously introduced into the flask at flow rate of
advantage of low catalyst loading, friendly environmental
effects, moderate reaction conditions, and low proportion of
the solvent. Mechanism of this co-catalyzed oxidation
reaction was proposed based on experimental observations.
4.2 L/h while the temperature was kept at 391 K. The
2
Experimental
contents of hydroperoxides of the oxidation products were
determined by an iodometric method.
2
.1 Instruments and Reagents
MS spectra were determined on an Agilent 1100 LC–MS.
1
A Bruck 400 MHz spectrometer was used for HNMR
3 Results and Discussion
analysis of porphyrins in CDCl . IR spectra were recorded
3
3.1 Aerobic Oxidation of PTA over
on a PE-783 Spectrometer. UV–Vis spectra were obtained
on a ShimadzuUV-2450 spectrometer. A Perkin-Elmer
T(p-Cl)PPMnCl/Co(OAc)2
2
400 elementary analyzer was used for elemental analysis.
The products of PTA aerobic oxidation over T(p-
Cl)PPMnCl/Co(OAc) , identified by LC–MS and also by
The high-pressure reactor with high speed magnetic agi-
tating and CYS-1 automatic oxygen-measuring instrument
is similar to that reported previously [15]. Cobalt acetate
was synthesized according to a previously published pro-
cedure [16], and the conversion of cobaltous acetate to
cobaltic acetate as determined by iodometric titration was
2
HPLC co-injection of commercially available authentic
samples, consisted mainly of terephthalic acid (TPA) and
4-carboxybenzalaldehyde (4-CBA), as shown in Scheme 1.
The changes of the yield and selectivity of products with
the reaction time were shown in Fig. 1a and b, respectively.
The plots showed that the yield of TPA increased rapidly in
the initial 3 h, and the TPA selectivity increased from
76.2% of 0.5 h to 92.7% of 3 h, and the increase rate of
TPA slowed with the further increase of time. The yield of
4-CBA changed slowly after 0.5 h, and the selectivity of
4-CBA reduced rapidly from 23.8% (0.5 h) to 9.21% (2 h)
5
4%. T(p-Cl)PPMnCl was synthesized and purified
according to documented procedures [17]. The structure
1
was characterized by UV–Vis,IR,MS and HNMR, and
analytical data were consistent with the document [7]. PTA
was purified by recrystallization and analyzed by HPLC to
ensure the absence of impurities before use. Other reagents
were of analytical grade all.
2
.2 General Procedure for PTA Oxidation
CH3
COOH
COOH
T(p-Cl)PPMnCl / Co(OAc)2
+
O2
+
2
.2.1 Oxidation of PTA at Elevated Pressure
acetic acid T, P
COOH
PTA
COOH
CHO
-CBA
The procedures of PTA oxidation with air catalyzed by
manganeseporphyrin and cobalt acetate were carried out as
TPA
4
-
5
following: 100 g PTA, 100 g acetic acid, 1.19 9 10 mol
Scheme 1 Oxidation of p-toluic acid with air over T(p-Cl)PPMnCl/
Co(OAc)
-
4
T(p-Cl)PPMnCl and 4.02 9 10
mol Co(OAc) ꢀ4H O
2
2
2
1
23

Aerobic Oxidation of p-Toluic Acid to Terephthalic Acid over T(p-Cl)PPMnCl/Co(OAc)
2
157
The conversion was only 45.6% in the presence of
6.2 ppm Mn(OAc) and 548 ppm Co(OAc) for 7 h. To
1
2
2
our surprise, when the same amount of T(p-Cl)PPMnCl
was added to the reaction mixture over Co(OAc) , the
2
conversion reached 57.1% which was much higher than
that when the two components were used alone and the
selectivity of TPA increased to 95.5% in 7 h. Both the PTA
conversion and TPA yield could be increased significantly
when T(p-Cl)PPMnCl and Co(OAc) were used together.
2
The results suggest that co-catalysis between the two cat-
alysts may be operative in the aerobic oxidation of PTA.
3.3 Effect of Catalyst Concentration
3.3.1 Effect of Co(OAc) Concentration
2
One can see from Table 2 that when the concentration of
2
Co(OAc)2 increased from 1.37 9 10
ppm to
3
.10 9 10 ppm, the PTA conversion and the selectivity of
1
TPA increased first, and then decreased. The optimal
2
concentration of Co(OAc) was 5.48 9 10 ppm. When
2
the concentration of Co(OAc) was either higher or lower
2
than the optimal concentration, the PTA conversion and
selectivity of TPA all suffered. The PTA conversion
increased from 14.6% to 51.9% and the selectivity of TPA
increased from 81.8 to 94.5% with the increases of the
2
concentration of Co(OAc) from 1.37 9 10 ppm to
2
2
.48 9 10 ppm. However, the conversion decreased with
5
the further increase of the Co(OAc) concentration. It may
2
3
?
2?
be relative to the different Co /Co and the 4-carboxyl
benzolperoxy chain radicals concentration [18] at the dif-
ferent concentration of Co(OAc)2.
Fig. 1 Effect of reaction time on the products yield (a) and
selectivity (b). Reaction conditions: PTA, 100 g; HOAc, 100 g;
T(p-Cl)PPMnCl, 16.2 ppm; Co(OAc)
1
2
ꢀ4H
2
O, 548 ppm; air pressure,
-
1
3.3.2 Effect of T(p-Cl)PPMnCl Concentration
.2 MPa; 190 °C; air flow, 160 L h
The concentration of T(p-Cl)PPMnCl influenced the oxi-
dation of PTA as well. We can see from Table 3 that the
optimal concentration of T(p-Cl)PPMnCl is 16.2 ppm.
When the concentration of T(p-Cl)PPMnCl increased from
3.2 to 16.2 ppm, the PTA conversion increased from 45.3
to 51.9%. But the PTA conversion decreased with the
further increase of T(p-Cl)PPMnCl concentration when the
concentration of T(p-Cl)PPMnCl was more than 16.2 ppm.
Moreover, the PTA conversion was only 47.1% when
22.7 ppm T(p-Cl)PPMnCl was used as catalyst. This
behavior was identified as the ‘catalyst-inhibitor conver-
sion’ [19]. A possible reason related to this phenomenon
was that metalloporphyrin could react with oxo-radical RO
to produce the non-radical product, and the chain initiation
was terminated. In addition, the metalloporphyrins tend to
form catalytically inactive dimeric l-oxo- and/or l-peroxo-
bridged dimetal species when its concentration surpassed
the critical value, which was not in fovor of the catalysis
in the initial 2 h followed by a slow decrease in the next
h. The yield of TPA reached 54.5%, and 95.5% of
6
selectivity was produced at 7.0 h. The result showed that
the PTA could be effectively oxidized to TPA with air over
T(p-Cl)PPMnCl/Co(OAc)2.
3
.2 Comparison of Catalyst system Composition
The aerobic oxidation of PTA was carried out using four
different catalyst systems, i.e., T(p-Cl)PPMnCl, Co(OAc)2,
Mn(OAc) /Co(OAc) and T(p-Cl)PPMnCl/Co(OAc) , and
2
2
2
the results were summarized in Table 1.
When Co(OAc) was used as the catalyst at 190 °C
2
-
1
under 1.2 MPa and 160 L h airflow for 7 h, the reaction
conversion reached 45.1% while the selectivity of TPA was
94.3%. When the catalyst was switched to T(p-Cl)PPMnCl,
there was no TPA produced while 1.1% of PTA reacted.
123

1
58
Y. Xiao et al.
Table 1 Effect of catalyst
system composition on PTA
oxidation
Catalyst composition
Reaction time (h)
Conversion (%)
Selectivity (%)
TPA
CBA
Co(OAc)
Co(OAc)
Co(OAc)
2
2
2
3
5
7
7
3
5
7
7
39.0
43.7
45.1
1.1
92.0
94.1
94.3
0
8.0
5.9
Reaction conditions: PTA,
5.7
1
00 g; HOAc, 100 g;
T(p-Cl)PPMnCl, 16.2 ppm;
Co(OAc) O, 548 ppm;
ꢀ4H
Mn(OAc) O, 16.2 ppm;
T(p-Cl)PPMnCl
100
7.3
Co(OAc)
Co(OAc)
2
/T(p-Cl)PPMnCl
/T(p-Cl)PPMnCl
42.0
51.9
57.1
45.6
92.7
94.5
95.5
94.5
2
2
5.5
2
ꢀ4H
2
2
temperature, 190 °C; air
pressure, 1.2 MPa; air flow,
Co(OAc) /T(p-Cl)PPMnCl
2
4.5
Co(OAc)
2
/Mn(OAc)
2
5.5
-
1
1
60 L h
Table 2 Effect of the concentration of Co(OAc)
2
on PTA oxidation
Table 4 Effect of temperature on the PTA oxidation
Co(OAc)
2
conc. (ppm)
Conversion (%)
Selectivity (%)
Temperature (8C)
Conversion (%)
Selectivity (%)
TPA
CBA
TPA
CBA
2
1
2
5
1
.37 9 10
14.6
47.8
51.9
45.4
81.8
94.8
94.5
93.8
18.2
5.2
160
180
190
210
11.5
44.1
51.9
56.4
52.9
82.6
94.9
94.5
97.6
97.1
17.4
5.1
5.5
2.4
2.9
2
2
3
.74 9 10
.48 9 10
.10 9 10
5.5
6.2
2
20
Reaction conditions: PTA, 100 g; HOAc, 100 g; T(p-Cl)PPMnCl,
16.2 ppm; air pressure, 1.2 MPa; 190 °C; air flow, 160 L h ; reac-
tion time, 5 h
-
1
Reaction conditions: PTA, 100 g; HOAc, 100 g; T(p-Cl)PPMnCl,
1
6.2 ppm; Co(OAc)
2
ꢀ4H
2
O, 548 ppm; air pressure, 1.2 MPa; air flow,
-
60 L h ; reaction time, 5 h
1
1
increase in the PTA conversion and the TPA yield (11.5–
51.9% for conversion, 9.5–49.0% for the TPA yield) when
the temperature was changed from 160 to 190 °C. The
selectivity of TPA changed from 82.6 to 94.5% with the
increases of the temperature from 160 to 190 °C. These
indicated that the increase of temperature was helpful to
the oxidation of PTA. But when the temperature was higher
than 210 °C, the PTA conversion and TPA yield began to
decrease. It might be caused by decarboxylation of the
TPA at exorbitantly high temperature.
Table 3 Effect of the concentration of T(p-Cl)PPMnCl on PTA
oxidation
T(p-Cl)PPMnCl conc. (ppm) Conversion (%) Selectivity (%)
TPA
CBA
3
9
1
1
2
.2
45.3
46.8
48.9
51.9
47.1
94.1
94.3
94.3
94.5
93.9
5.9
5.7
5.7
5.5
6.1
.7
3.0
6.2
2.7
Reaction conditions: PTA, 100 g; HOAc, 100 g; Co(OAc)
5
2
ꢀ4H
2
O,
48 ppm; air pressure, 1.2 MPa; 190 °C; air flow, 160 L h ; reac-
tion time, 5 h
-
1
3.5 Effect of PTA Concentration
It could be seen from Table 5 that the optimized concen-
tration of PTA was up to 65% which was enormously
higher than the concentration of PTA in other new catalyst
systems, and the system gave the highest conversion 62.9%
and TPA selectivity 96.8%. When the PTA concentration
was below 65%, the conversion and TPA selectivity were
increasing with the increase of the PTA concentration. This
might be because the reduction of solvent led to the
increase of the catalyst concentration. But the conversion
and TPA selectivity decreased with the further increase of
PTA concentration. It might be relative to the changes of
reaction system’s viscosity and polarity [21, 22] and the
[
20]. Similar phenomena were also observed in our previ-
ous works [6, 15].
3
.4 Effect of Reaction Temperature
The influence of reaction temperature on the oxidation
reaction was investigated and the results were shown in
Table 4. When the temperature was below 210 °C, the
PTA conversion and TPA selectivity enhanced with the
increasing of the temperature, and there was a sharp
1
23

Aerobic Oxidation of p-Toluic Acid to Terephthalic Acid over T(p-Cl)PPMnCl/Co(OAc)
Table 5 Effect of the PTA concentration on PTA oxidation
2
159
PTA conc. (mass%)
Conversion (%)
Selectivity (%)
TPA
CBA
3
5
6
8
9
5
0
5
0
0
33.4
51.9
62.9
50.8
26.3
92.7
94.5
96.8
95.5
92.0
7.3
5.5
3.4
4.5
8.0
Reaction conditions: PTA, 100 g; T(p-Cl)PPMnCl, 16.2 ppm;
Co(OAc) O, 548 ppm; air pressure, 1.2 MPa; 190 °C; air flow,
ꢀ4H
60 L h ; reaction time, 5 h
2
2
-
1
1
shift of the metalloporphyrin’s redox electric potential [23,
2
4] in different amount of HOAc.
Fig. 3 Absorption spectrum of a solution (a) of peracetic acid
-
2
4.1 9 10 M, 3.5 mL) and a mixed solution (b) of peracetic acid
(
after adding Co(OAc) O (3.6 mg) (glacial acetic acid solvent)
3
.6 The Preliminary Mechanism of co-Catalysis
Between T(p-Cl)PPMnCl and Co(OAc)2
in the Aerobic Oxidation of PTA to TPA
2
ꢀ4H
2
much higher over T(p-Cl)PPMnCl than Co(OAc) . Results
2
suggest that metalloporphyrin excels Co(OAc)2 in pro-
ducing peroxides as catalyst.
The selective oxidation of hydrocarbons with oxygen over
metalloporphyrins is a biomimetic catalytic process mim-
icking the cytochrome P450 monoxygenase [25]. When
The peroxides formed in the hydrocarbon oxidation play
an important role in the initiation of the chain reaction of
the hydrocarbon oxidation [27]. In order to find out the
relationship between cobaltous acetate and peroxide mostly
stemmed from the catalysis of metalloporphyrin, spectro-
scopic investigations were conducted. The UV spectra of
the peracetic acid was shown as Fig. 3a. When Co(OAc)2
was added into peracetic acid representing for hydroper-
oxide, the solution turned green and the UV spectra was
shown as Fig. 3b. A new absorption peak at 250 nm where
2
,6-di-tertbutyl-p-cresol as a free inhibitor was added into
the oxidation system, the reaction was substantially quen-
ched. The result suggested that the reaction proceeded via a
radical process.
Hydroperoxides could be produced under moderate
conditions by the use of metalloporphyrin as catalyst [26].
The changes of peroxides content with reaction time in
different catalyst system were investigated and the results
were shown in Fig. 2. The concentration of peroxides was
the absorption of Co(OAc) and HOAc at 250 nm was
2
2
?
negligible appeared. This phenomenon indicated Co
could react with peracetic acid. The solution was then
evaporated at decompressed pressure, and the green prod-
uct obtained was dried. The UV–vis spectra of the green
product (Fig. 4) showed that there was one intense
absorption band of 254 nm. The charge-transfer band at
254 nm indicated the cobalt(III)-oxygen bonding [28].
Therefore, the green solid obtained was cobaltic acetate.
2
Results suggested that hydroperoxides could oxidize Co
?
3
into Co (see Eq. 1):
?
2
þ
þ
RCOOOH þ 2 Co þ 2H
3
þ
!
RCOOH þ 2 Co þ H O
ð1Þ
2
It was reported that the key step of cobalt-catalyzed
?
hydrocarbon oxidation was the reaction between the Co
3
with the hydrocarbon to form alkyl free radicals [29, 30]. In
3
?
Fig. 2 Changes of content of peroxides with reaction time in
different catalyst system. Reaction conditions (in dark): PTA, 1.5 g;
order to investigate the effect of Co concentration on the
PTA oxidation with air, the aerobic oxidation of PTA to
TPA over T(p-Cl)PPMnCl/Co(OAc)2 was run with the
Co(OAc) instead of Co(OAc) . The oxidation results were
HOAc, 25 mL; O
2
pressure, 0.1 MPa; temperature, 391 K; O
3
2
flow
mmol;
-
rate, 4.2 L/h; catalyst: T(p-Cl)PPMnCl, 8.7 9 10
-
3
Co(OAc)
2
ꢀ4H
2
O, 8.7 9 10 mmol
3
2
123

1
60
Y. Xiao et al.
aerobic oxidation of PTA to TPA is proposed (see
Scheme 2).
The LC–MS analysis of the PTA oxidation products
showed that there were no 4-(hydroxymethyl)benzoic acid
and esterification reaction products of the hydroxymethyl
group produced. It seemed that the formation of aralkyl
free radical included a deoxy process of the alkoxy free
radical in this system. For the co-catalysis between
metalloporphyrin and cobalt acetate, the function of
T(p-Cl)PPMnCl might be to produce more peroxides from
3
?
the PTA (step a in Scheme 2), and then more Co was
2
formed from the oxidation of the Co with peroxides (step
?
3
?
b). Moreover, Co promoted the PTA oxidation due to the
ease of the free radicals’ formation (step c). Further studies
of the mechanism are underway, with the precise details
yet unknown.
Fig. 4 Absorption spectrum of a solution of cobaltic acetate
(
-
2.4 9 10 M) (glacial acetic acid solvent)
5
2
listed in Table 6. When equal amount of Co substituted
?
4 Conclusions
3
for Co , the PTA conversion increased from 51.9% to
?
5
9
6.8% and the selectivity of TPA increased from 94.5 to
6.5%. The duplicate runs had been done, and the data was
The PTA was oxidized to TPA with air by the catalysis of
T(p-Cl)PPMnCl/Co(OAc) , The aerobic oxidation of PTA
2
consistent. It appears to be similar to that of cobalt acetate-
3
catalyzed autoxidation. The results showed that Co
was influenced by the catalyst composition and the con-
centrations of the catalyst as well as reaction conditions.
Both the PTA conversion and TPA yield increased sig-
nificantly when the a small amount of T(p-Cl)PPMnCl was
added to the PTA-O -Co(OAc) system. By the use of
?
2
?
obtained by the reaction of peroxides and Co
promote the PTA oxidation. This might be because Co
abstracted one hydrogen atom of the substrates to form free
could
3
?
2
2
3
?
radicals. Once the concentration of Co was increased,
more free radicals were produced and chain initiation was
accelerated.
548 ppm Co(OAc) and 16 ppm T(p-Cl)PPMnCl as the
2
catalysts, PTA oxidation under the optimum conditions of
-
1
190 °C, 1.2 MPa and 160 L h airflow produced TPA at
96.8% selectivity and 62.9% conversion of PTA in 5 h. For
the co-catalysis between metalloporphyrin and cobalt
acetate, the function of T(p-Cl)PPMnCl might be to
According to the above experimental results and
reported facts [26, 27, 29–33], a preliminary mechanism of
co-catalysis between T(p-Cl)PPMnCl and Co(OAc) in the
2
Table 6 Comparison of the catalytic power between Co(OAc)
3
2
and Co(OAc) in combination with metalloporphyrin
2
?
3?
Co (mol)
Catalyst
Co (mol)
Conversion (%)
Selectivity (%)
TPA
CBA
-
4
4
Co(OAc)
Co(OAc)
2
/T(p-Cl)PPMnCl
/Co(OAc) /T(p-Cl)PPMnCl
4.01 9 10
0
1.03 9 10
51.9
56.8
94.5
96.5
5.5
3.5
-
-4
2
3
2.98 9 10
-
1
Reaction conditions: PTA, 100 g; T(p-Cl)PPMnCl, 16.2 ppm; air pressure, 1.2 MPa; 190 °C; air flow, 160 L h ; reaction time, 5 h
Scheme 2 Preliminary
T(p-Cl)PPMnCl, O2
Co2+
mechanism of co-catalysis
between T(p-Cl)PPMnCl and
Co(OAc) in the aerobic
HOOC-Ph-CH3
HOOC-Ph-CH OOH
2
HOOC-PhCH3
a
2
-
c
-OH
b
oxidation of PTA to TPA
Co3+
HOOC-PhCH
O2
HOOC-PhCH O
2
2
-
1/2 O2
[
O]
Co2+
HOOC-Ph-COOH
HOOC-Ph-CHO
HOOCH-PhCH OO
2
+
H
1
23

Aerobic Oxidation of p-Toluic Acid to Terephthalic Acid over T(p-Cl)PPMnCl/Co(OAc)
2
161
produce more peroxides from the PTA, and then more
13. Zuo XB, Subramaniam B, Busch DH (2008) Ind Eng Chem
47:546
3
Co was formed from the oxidation of the Co with
?
2?
1
1
4. Saha B, Espenson JH (2005) J Mol Catal A Chem 241:33
5. Guo CC, Chu MF, Liu Q, Liu Y, Guo DC, Liu XQ (2003) Appl
Catal A Gen 246:303
3
?
peroxides. Moreover, Co promoted the PTA oxidation
due to the ease of the free radicals’ formation.
1
6. Heiba EI, Dessau RM, Koehl WJJ (1969) J Am Chem Soc
91:6830
17. Guo CC, He XT, Zou GY (1991) Chinese J Org Chem 11:416
Acknowledgments We thank the financial supports of National
Natural Science Foundation of China (Grants CN J0830415).
1
8. Nemecek AM, Hendriks CF, Van Beek HCA (1978) Ind Eng
Chem Prod Res Dev 17:133
1
2
9. Black JF (1978) J Am Chem Soc 100:527
0. Musie GT, Wei M, Subramaniam B, Busch DH (2001) Inorg
Chem 40:3336
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