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Ind. Eng. Chem. Res. 2006, 45, 4156-4162
Optimum Ratio of Co/Mn in the Liquid-Phase Catalytic Oxidation of p-Xylene to Terephthalic Acid Youwei Cheng,* Xi Li, Lijun Wang, and Qinbo Wang Department of Chemical and Biochemical Engineering, Zhejiang UniVersity, Hangzhou, 310027 Zhejiang, People’s Republic of China
Liquid-phase catalytic oxidation of p-xylene to terephthalic acid was performed at 150-210 °C over CoMn-Br catalyst system. There was an interesting synergistic effect of cobalt and manganese catalyst. In the previous papers (Ind. Eng. Chem. Res. 2005,44, 261-266; 4518-4522; 7756-7760), a lumped kinetic scheme and a fractional kinetic model for the liquid-phase oxidation of p-xylene were proposed and tested, and the effects of catalyst concentration, water content, and guanidine catalyst additive were investigated. In this paper, the synergistic effect of cobalt and manganese on the oxidation kinetics was studied. Experiments of several levels of Co/Mn ratio and temperature were carried out in a semibatch oxidation reactor where the gas and liquid phases were well-mixed. The results showed that the variation of the Co/Mn ratio in the catalyst did affect the activity and selectivity of the catalyst system. For the main reaction, there existed an optimum Co/Mn ratio at which the rates of the latter two slower oxidation steps of p-toluic acid and 4-carboxybenzaldehyde reached the maximum, and the optimum Co/Mn ratio decreased with the increase of reaction temperature. However, the burning side reaction of reactant and solvent due to carbon dioxide and carbon monoxide formation increased remarkably with the increase of Co/Mn ratio. A possible mechanism that the optimum Co/Mn ratio presents in the oxidation was proposed eventually. 1. Introduction Liquid-phase oxidation of methyl aromatic hydrocarbons is of great scientific, technological, and commercial importance.1-3 One of the most successful commercial applications is the production of terephthalic acid (TA) by liquid-phase oxidation of p-xylene (PX) with air over a Co-Mn-Br catalyst system (cobalt acetate, manganese acetate, and hydrogen bromine) in acetic acid (HOAc) at 150-210 °C.1 As practiced, this reaction is known as the MC (Mid-Century) process. For the last several years, the kinetics of the reaction network and the process engineering aspects of the MC process of PX oxidation has been investigated in details.4-10 The lumped kinetic scheme widely adopted is shown in Figure 1, where p-tolualdehyde (TALD), p-toluic acid (PT), 4-carboxybenzaldehyde (4-CBA), and TA are the most important intermediates and final products. The free-radical chain mechanism, as a general framework within which the kinetics features of hydrocarbon oxidation in liquid phase are interpreted, is now generally accepted. Therefore, we can consider this reaction as a typical catalyst modified free-radical chain reaction. The efficacy of the Co-Mn-Br catalyst system is due to the synergistic effect of the coupled catalytic cycles of cobalt, manganese, and bromide. The alternative cycle of the catalyst valence is coupled with the oxidation process of the alkyl aromatic hydrocarbon, and the two processes proceed synchronously until the reaction ends.2,6 Wang and Cheng have developed a fractional kinetic model as shown in eq 1 from the above reaction mechanisms.4-7 According to the fractional kinetic model, the water and guanidine contents effects on the oxidation reaction kinetics have been discussed in the previous paper.9,10
kj
rj )
4
(
[Cj], j ) 1-4
(1)
di[Ci] + )βj ∑ i)1
* To whom correspondence should be addressed. Tel.: 86 471 87952210. Fax: 86 471 87951227. E-mail:
[email protected].
Figure 1. Lumped kinetic scheme for the oxidation of p-xylene to terephthalic acid.
Even though the free-radical chain mechanism can explain some kinetics features of PX oxidation, a better understanding of the chemical mechanism of the catalyst system is essential for achieving the MC process improvement. From combined UV-vis and ESR spectroscopic studies, Partenheimer and Chavan11,12 found that the Co-Mn-Br system exhibited high catalytic activity due to formation of heteronuclear cluster complexes, i.e., different coordination compounds. In the previous paper,8-10 the water and guanidine content effects on the role of these complexes in the oxidation reaction were discussed. The mechanism was interpreted by the competition between coordination effect, which had negative influence on the transfer rate of the electron, and the decrease of the redox potential of the cobalt ion, which had positive influence on the reaction rate. As Partenheimer reported, using p-toluic acid as the reagent, a 4,5-rate increase is obtained when half of the cobalt is replaced by manganese.2 Using p-xylene as the oxidation reactant at 191 °C, addition of Mn to Co at constant bromide and metals concentration [Co + Mn] resulted in a maximum rate of the main reaction at the mole ratio of 1.0.6 Chavan found that the system exhibited high catalytic activity and yield of terephthalic acid when heteronuclear cluster complexes such as Co2Mn(O)(OAc)x and CoMn2(O)(OAc)x present in significant concentration in the reaction mixture. Accordingly, there is an interesting synergistic effect of cobalt and manganese; and the variation of the Co/Mn ratio in the catalyst does affect the activity and selectivity. The major objective of the present study is to investigate the synergistic effect of cobalt and manganese on the main and side reaction of liquid-phase oxidation of PX to TA. The detailed
10.1021/ie060007a CCC: $33.50 © 2006 American Chemical Society Published on Web 05/12/2006
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006 4157 Table 1. Operating Conditions for the Experimental Runs
run
T (°C)
CCo (10-6 kg/ kgHAC)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
210 210 210 210 190 190 190 190 170 170 170 170 150 150 150 150
40 98 193 284 72 178 350 516 648 954 1192 1250 1295 1908 2384 2500
CMn (10-6 kg/ kgHAC)
CBr (10-6 kg/ kgHAC)
CCo+Mn (10-6 kg/ kgHAC)
[Co]/ [Co + Mn] (mol/mol)
332 274 179 88 604 498 326 160 602 296 58
