The preparation of terephthalic acid by solvent-free oxidation of p-xylene with air over T(p-Cl)PPMnCl and Co(OAc)2

Article abstract of DOI:10.1016/j.cclet.2010.09.027

An attempt has been made to prepare terephthalic acid (TPA) by solvent-free oxidation of p-xylene (PX) with air over tetra(p-chlorophenylporphinato) manganese chloride (T(p-Cl)PPMnCl) and cobalt acetate. The co-catalysis between T(p-Cl)PPMnCl and Co(OAc)₂ has been discovered under solvent-free conditions. TPA yield could be increased significantly when T(p-Cl)PPMnCl and Co(OAc)₂ were used together. The addition of T(p-Cl)PPMnCl into the reaction mixture over Co(OAc)₂ significantly accelerated the rate-determining step of the oxidation process of PX to TPA. The effect of temperature on reaction was also investigated.

Full text of DOI:10.1016/j.cclet.2010.09.027

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 135–138
www.elsevier.com/locate/cclet
The preparation of terephthalic acid by solvent-free oxidation
of p-xylene with air over T( p-Cl)PPMnCl and Co(OAc)2
*
Yang Xiao, XiaoYan Zhang, Qin Bo Wang, Ze Tan, Can Cheng Guo ,
Wei Deng, Zhi Gang Liu, He Fei Zhang
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
Received 25 May 2010
Abstract
An attempt has been made to prepare terephthalic acid (TPA) by solvent-free oxidation of p-xylene (PX) with air over tetra( p-
chlorophenylporphinato)manganese chloride (T( p-Cl)PPMnCl) and cobalt acetate. The co-catalysis between T( p-Cl)PPMnCl and
Co(OAc) has been discovered under solvent-free conditions. TPAyield could be increased significantly when T( p-Cl)PPMnCl and
2
Co(OAc)
the rate-determining step of the oxidation process of PX to TPA. The effect of temperature on reaction was also investigated.
2010 Published by Elsevier B.V. on behalf of Chinese Chemical Society.
2 2
were used together. The addition of T( p-Cl)PPMnCl into the reaction mixture over Co(OAc) significantly accelerated
#
Keywords: Terephthalic acid; Solvent-free; p-Xylene; Oxidation; Manganese porphyrin; Cobalt acetate
Terephthalic acid (TPA) is an important raw material for the production of polyethylene terephthalate [1,2].
Nowadays 70% of TPA used worldwide is prepared by the catalytic oxidation of p-xylene (PX). Typically, the
oxidation of PX is catalyzed by a combination of Co(OAc) /Mn(OAc) /HBr using 80% HOAc as solvent in industry
2
2
[
3]. The corrosion of the reactors due to the use of HOAc as solvent and the serious environmental threat posed by
bromide have become the major shortcomings [4,5]. In order to find out a better way for PX oxidation, scientists have
studied many new catalytic processes [6–15] with particular interest in the development of solvent-free systems [6–9].
However, little TPA was formed in previously reported solvent-free methods [6–8]. Metalloporphyrins have been
attracting intensive interest because of its excellent catalytic performance for hydrocarbon oxygenation [16–19].
Owing to the fact that a titanium reactor is required and it is very costly to built and maintain since the corrosive HOAc
is employed as a solvent, much effort was devoted to achieve the PX oxidation without any solvent. Herein, solvent-
free oxidation of PX to TPAwith air over tetra( p-chlorophenyl-porphinato)manganese chloride (T( p-Cl)PPMnCl) and
Co(OAc) was reported.
2
In a typical procedure for the PX oxidation, 3 mol of PX, a catalytic amount of T( p-Cl)PPMnCl and 535 ppm of
Co(OAc) Á4H O were put into the 500 mL semi-autoclave reactor with a condenser and stirred continuously, and the
2
2
mixture was heated to the desired temperature under protection of nitrogen. Then air was injected at the speed of
1
60 L/h and the reaction system pressure was maintained at 1.2 MPa. After the reaction completed, the autoclave was
*
Corresponding author.
E-mail address: ccguo@hnu.edu.cn (C.C. Guo).
1001-8417/$ – see front matter # 2010 Published by Elsevier B.V. on behalf of Chinese Chemical Society.
doi:10.1016/j.cclet.2010.09.027

[()TD$FIG]
1
36
Y. Xiao et al. / Chinese Chemical Letters 22 (2011) 135–138
CH3
COOH
CH3
T(p-Cl)PPMnCl / Co(OAc)2
O
+
+
Other products
+
2
4
63 K, 1.2 MPa
CH3
PX
COOH
TPA
COOH
PTA
2
Scheme 1. Solvent-free oxidation of PX to TPA with air over T( p-Cl)PPMnCl/Co(OAc) .
cooled to room temperature. The products were quantified with an Agilent HPLC with UV (254 nm) detector and a
Zorbax SB-C18 (4.5 Â 250 mm) column. Eluent consisting of 25% CH OH, 20% CH CN and 55% H PO buffer
3
3
3
4
À1
solvent (1%) with a flow rate of 1.0 mL min was used. T( p-Cl)PPMnCl was synthesized and purified according to
documented procedures [20].
The products of solvent-free oxidation of PX over T( p-Cl)PPMnCl/Co(OAc) under the optimum conditions of
2
1
90 8C and 1.2 MPa, identified by LC–MS and also by HPLC co-injection of commercially available authentic
samples, consisted mainly of terephthalic acid (TPA), p-toluic acid (PTA), and a small amount of other products
including 4-carboxybenzalaldehyde (CBA), p-toluic aldehyde (TALD) and p-toluic alcohol (TALC), as shown in
Scheme 1.
Results of solvent-free oxidation of PX using different catalyst systems were summarized in Table 1. Indeed, PX
can be oxidized by molecular oxygen under the catalysis of Co(OAc) or T( p-Cl)PPMnCl, but the TPA yield is not
2
satisfying. To our delight, both the yield and selectivity of TPA could be increased significantly when T( p-Cl)PPMnCl
and Co(OAc) were used together. Moreover, TPAyield did not change much as Mn(OAc) was added into the reaction
2
2
mixture over Co(OAc) . Above results suggest that there is synergy between T( p-Cl)PPMnCl and Co(OAc) , and co-
2
2
catalysis may be operative under solvent-free conditions. T( p-Cl)PPMnCl/Co(OAc) can effectively catalyze the
2
oxidation of PX with air in absence of any solvent, and the reaction of PX oxidation catalyzed by T( p-Cl)PPMnCl
(
5.6 ppm) and Co(OAc) (535 ppm) at 190 8C gave 74% of PX conversion and 38.9% of TPA selectivity in 5 h.
2
Compared with other solvent-free methods, our catalytic process has the advantages of giving higher TPA yield,
operating under milder reaction conditions, being more environmental friendly and it do not require the use of any
additive or initiator.
Chemically, the oxidation of PX to TPA involves the oxidation of PX to PTA and the subsequent oxidation of PTA to
TPA. Thestepof PTA oxidation isthe rate-determiningstepof the oxidationprocess ofPXtoTPA [3,21]. Fig. 1showsthe
time evolution of yields for PTA and TPA in different catalytic reactions. T( p-Cl)PPMnCl/Co(OAc) catalyst system
2
gavehigher TPAyieldallthe time, moreover, the TPA slopeofthe tangentline for T( p-Cl)PPMnCl/Co(OAc) was higher
2
than that for Co(OAc) . So it is reasonable to conclude that the addition of T( p-Cl)PPMnCl into the reaction mixture over
2
Co(OAc) accelerated the formation rate of TPA derived from PTA. Moreover, two different catalyst systems have the
2
same PTAyield at 240 min, thereafter, PTAyield for T( p-Cl)PPMnCl/Co(OAc) was lower than that for Co(OAc) . So
2
2
one might conclude that PTA consumption rate was also accelerated by the addition of T( p-Cl)PPMnCl. The PTA
formation rate was equal to the PTA consumption rate when the PTAyield reached the maximum, and thereafter the PTA
consumption rate was faster than the PTA formation rate. The reaction time until PTAyield reached the maximum was
postponed for 90 min when the catalyst was changed from T( p-Cl)PPMnCl/Co(OAc) to Co(OAc) . From all analysis,
2
2
we can see that the addition of T( p-Cl)PPMnCl into the reaction mixture over Co(OAc) accelerated the whole reaction
2
especially for rate-determining step of the oxidation process of PX to TPA.
Table 1
a
Investigation of the catalyst system composition on solvent-free oxidation of PX.
Catalyst (ppm)
Co(OAc)
Conversion (%)
TPA yield (%)
Selectivity (%)
2
T( p-Cl)PPMnCl
Mn(OAc)
2
TPA
CBA
PTA
TALD
TALC
5
35
69.7
18.8
74.0
70.0
19.8
0.3
28.4
1.7
5.2
2.0
5.7
5.3
66.4
73.2
55.4
65.7
0
15.1
0
0
5
.6
8.0
0
5
5
35
35
5.6
28.8
20.3
38.9
29.0
5.6
0
0
a
Reaction conditions: PX, 3 mol; temperature, 190 8C; reaction time, 5 h.

[()TD$FIG]
Y. Xiao et al. / Chinese Chemical Letters 22 (2011) 135–138
137
Fig. 1. Time evolution of yields for PTA and TPA in solvent-free oxidation of PX catalyzed by different catalysts. PTA (~); TPA (*); (a) T
(
p-Cl)PPMnCl/Co(OAc)
2
; (b) Co(OAc)
2
. Reaction conditions: PX, 3 mol; T( p-Cl)PPMnCl, 5.6 ppm; Co(OAc)
2
2
Á4H O, 535 ppm; 190 8C.
Table 2
Effect of reaction temperature on PX oxidation.
a
Temperature (8C)
Conversion (%)
TPA yield (%)
Selectivity (%)
TPA
CBA
PTA
TALD
TALC
1
1
1
1
2
70
80
85
90
00
30.4
38.6
73.2
74.3
71.1
0.5
1.4
1.6
3.6
2.7
3.0
4.7
5.4
5.1
79.2
87.3
66.5
57.8
60.5
12.9
6.1
0.2
0
3.6
0
20.9
27.3
24.5
28.6
36.8
34.4
0
0
0
0
a
Reaction conditions: PX, 3 mol; T( p-Cl)PPMnCl, 4.0 ppm; Co(OAc)
2
2
Á4H O, 535 ppm; reaction time, 5 h.
We also investigated the effect of reaction temperature on overall reaction performances (see Table 2). Results
indicated that the increase of temperature was helpful to the oxidation of PX to TPA. There was little TA produced
when the temperature was below 180 8C. There was a sharp increase in PX conversion and TPA selectivity (38.6–
7
3.2% for conversion, 3.6–28.6% for TPA selectivity) when the temperature was changed from 180 8C to 185 8C.
Since PTA, as one of the main products, is solid when reaction temperature is below the melting point of PTA (182 8C),
the above phenomena may be related to the phase state of oxidation product. But when temperature was higher than
190 8C, both PX conversion and TPA selectivity declined. It may be caused by the oxidative destruction of
metalloporphyrin at exorbitantly high temperature [17].
In conclusion, T( p-Cl)PPMnCl/Co(OAc) can effectively catalyze the oxidation of PX with air under solvent-free
2
conditions. By using Co(OAc) (535 ppm) and T( p-Cl)PPMnCl (5.6 ppm) as catalysts, PX oxidation under air
2
À1
(
between T( p-Cl)PPMnCl and Co(OAc) was operative under solvent-free conditions. TPA yield could be increased
1.2 MPa, 160 L h ) at 190 8C in 5 h produced TPA in 28.8% yield and 74% of PX was converted. The co-catalysis
2
significantly when T( p-Cl)PPMnCl and Co(OAc) were used together. The step of PTA oxidation was the rate-
2
determining step of the process of PX to TPA, and the addition of T( p-Cl)PPMnCl into the reaction mixture over
Co(OAc) accelerated the rate-determining step. The increase of temperature was helpful to the preparation of TPA.
2
Compared with other solvent-free methods, our catalytic process produces more TPA, runs under milder reaction
conditions, and it does not require any additive or initiator.
References
[
[
1] A.K. Suresh, M.M. Sharma, T. Sridhar, Ind. Eng. Chem. Res. 39 (2000) 3958.
2] J.R. Sheehan, Ullmann’s Encyclopedia of Industrial Organic Chemicals, vol. 8, Wiley-VCH, Weinheim, 1999, p. 4573.

138
Y. Xiao et al. / Chinese Chemical Letters 22 (2011) 135–138
[
[
[
[
[
[
[
3] W. Partenheimer, Catal. Today 23 (1995) 69.
4] P. Raghavendrchar, S. Ramachandran, Ind. Eng. Chem. Res. 31 (1992) 453.
5] J.M. Br e´ geault, J. Chem. Soc. Dalton Trans. (2003) 3289.
6] W. Zeng, J.Z. Li, S.Y. Qin, Supramol. Chem. 16 (2004) 569.
7] S.B. Halligudi, B.M. Devassy, N.K. Kala Raj, et al. React. Kinet. Catal. Lett. 71 (2000) 289.
8] C.R. Jacob, S.P. Varkey, P. Ratnasamy, Appl. Catal. A: Gen. 182 (1999) 91.
9] A. Shaabani, K. Soleimani, A. Bazgir, Synth. Commun. 34 (2004) 3303.
[
[
[
[
[
[
[
[
[
[
[
[
10] M. Gupta, S. Paul, R. Gupta, et al. Tetrahedron Lett. 46 (2005) 4957.
11] J.B. Dunn, D.I. Urquhart, P.E. Savage, Adv. Synth. Catal. 344 (2002) 385.
12] M. Higashijima, K. Nagayama, Stud. Surf. Sci. Catal. 145 (2003) 551.
13] Q. Jiang, H.Y. Hu, C.C. Guo, et al. J. Porphyr. Phthalocyan. 11 (2007) 524.
14] Q. Jiang, Y. Xiao, Z. Tan, et al. J. Mol. Catal. A: Chem. 285 (2008) 162.
15] F. Yang, J. Sun, R. Zheng, et al. Tetrahedron 60 (2004) 1225.
16] X.G. Li, J. Wang, R. He, Chin. Chem. Lett. 18 (2007) 1053.
17] S. Evans, J.R. Lindsay Smith, J. Chem. Soc. Perkin Trans. 2 (2000) 1541.
18] C.C. Guo, M.F. Chu, Q. Liu, Appl. Catal. A: Gen. 246 (2003) 303.
19] C.C. Guo, X.Q. Liu, Q. Liu, et al. J. Porphyr. Phthalocyan. 13 (2009) 1250.
20] C.C. Guo, X.T. He, G.Y. Zou, Chin. J. Org. Chem. 11 (1991) 416.
21] Q. Wang, Y. Cheng, L. Wang, et al. Ind. Eng. Chem. Res. 46 (2007) 8980.