Study On Reaction, Mechanism And Kinetics Of Biomimetic Catalytic Oxidation Of P-Xylene Over Metalloporphyrin

Terephthalic acid (TPA) is an important raw material for the production of polyethylene terephthalate (PET). Nowadays70%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)2/Mn(OAc)2/HBr using80%HO Ac as solvent in industry. 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. In order to find out a better way for PX oxidation, scientists have studied many new catalytic processes. However, above achievements did not agitate the industrial preparation of TPA catalyzed by Co(OAc)2/Mn(OAc)2/HBr. The development of new PX oxidation processes fitting the requirements of green chemistry still remains a challenge.In this paper, solvent-free oxidation of PX to TPA with air catalyzed by tetra(p-chlorophenylporphinato)manganese chloride (T(p-Cl)PPMnCl)/Co(OAc)2was reported for the first time, and the co-catalysis between T(p-Cl)PPMnCl and cobalt acetate was discovered. Mechanism and kinetics studies on the oxidation of p-xylene to terephthalate acid cocatalyzed by metalloporphyrin and cobalt acetate were conducted. The main work of this paper was as follows:Firstly, an attempt has been made to prepare TPA by solvent-free oxidation of PX with air over T(p-Cl)PPMnCl and cobalt acetate. The co-catalysis between T(p-Cl)PPMnCl and Co(OAc)2has been discovered. TPA yield could be increased significantly when T(p-Cl)PPMnCl and Co(OAc)2were used together. Studies indicate that the p-toluic acid (PTA) oxidation step is the rate-determining step of the oxidation process of PX to TPA. The addition of T(p-Cl)PPMnCl into the reaction mixture over Co(OAc)2significantly accelerated the rate-determining step. Effects of catalyst concentrations and reaction conditions on the reaction were also investigated.Secondly, the aerobic oxidation of PTA to TPA over T(p-Cl)PPMnCl and Co(OAc)2was studied. The co-catalysis of T(p-Cl)PPMnCl/Co(OAc)2was operative in the catalytic oxidation of PTA. The studies indicated that the PTA oxidation was influenced by the catalyst composition, catalyst concentrations and reaction conditions.Thirdly, the mechanism of co-catalysis between T(p-Cl)PPMnCl and Co(OAc)2 was investigated. The study of aerobic oxidation of PTA catalyzed by Co3+indicated that the function of T(p-Cl)PPMnCl might be to produce more peroxides from the substrate; The Co3+, obtained by the reaction of peroxides and Co2+, could promote the PTA oxidation. Effects of p-toluic alcohol (TALC) product on the structure and catalytic performance of T(p-Cl)PPMnCl in aerobic oxidation of PX were also studied. Axial coordination of TALC to T(p-Cl)PPMnCl was confirmed by spectroscopic analysis. The molecular TALC could axially coordinate to the T(p-Cl)PPMnCl in the ratio1:1, and the coordination is positively related with TALC concentration. The catalytic performance of T(p-Cl)PPMnCl dropped sharply when T(p-Cl)PPMnCl was coordinated with TALC. A preliminary mechanism of this co-catalyzed oxidation reaction was proposed based on these experimental results.Fourthly, a fractional-like kinetic model was derived from the assumed mechanism via the application of steady state approximation. We applied the experimental data to regress the kinetic model parameters in order to obtain those model parameters. Our kinetic model could explain and correlate the experimental observations well. Based on the fact that the kinetic model’s well fitting of experimental data, we speculate that the aerobic oxidation of PX has two competing reaction paths, i.e. the radicals escape way and the radicals capture way.Compared with other solvent-free catalyst systems, the reaction system in this paper has the advantages of giving higher TPA yield, operating under relatively milder reaction conditions, being more environmental friendly and it do not require the use of any additive or initiator.