Researches On M-xylene Oxidation Catalyzed By Phosphotungstic Acid Loaded On Activated Carbon

Isophthalic acid is an important organic chemical product with wide application in alkyd resin, unsaturated polyester resin and acid-modified copolyester etc. In industry, IPA is regularly prepared from m-xylene through liquid phase air oxidation with acetic acid as solvent and Co2+/Mn2+/[Br]as catalyst. Although this technology results in high MX conversion and IPA yield, the equipment corrosion caused by [Br] at high temperature is serious. Instead of bromide, a new catalytic system Co/Mn+/H3PW12O40 for MX oxidation was developed by Haifeng Lv which can eliminate the erosion caused by [Br],but large usage of HPW and low reaction rate under gas-liquid two phases reaction system limit its application.Against these issues, the active carbon-supported HPW catalyst was prepared using impregnation method in this paper, and the structure of this supported catalyst was characterized by XRD which showed a uniform dispersion of HPW on the surface of AC when its load ratio<15%. Firstly a combined catalysts Co2+/[HPW/AC] with Mn2+ removed was optimized and some catalyst parameters such as load ratio,HPW/AC amount,activation temperature,Co2+amount,on the effect of MX oxidation were got through experiments, the results were listed as:(1)When HPW load,HPW/AC amount,Co2+ amount,activation temperature were increased, the reaction rate initially increased and then dropped; (2)Adding Mn2+into the reaction system could slow MX oxidation; (3)The optimal catalyst parameter values for m-xylene oxidation were obtained which were 7.5% HPW load ratio,g AC,0.064% (wt) Co2+,220℃activation temperature.Then some operating conditions such as reaction temperature,oxygen partial pressure,water content,air flow,rotating speed on the effect of MX oxidation were researched through experiments, the results showed that (1)When water content,rotating speed were increased, the reaction rate initially increased and then dropped; Increasing air flow,reaction temperature could fasten the reaction rate; (2)When po2<0.735MPa, it could caused larger error. After that, increasing oxygen partial pressure could fasten the reaction; (3)The best effect of m-xylene oxidation was obtained when T=220℃,P=3.5MPa,Vair=1.6L/min,r=450r/min,H2O=4%(wt); All above parameters on combustion side-reaction were also analyzed. At last, a suitable kinetic model of surface-catalyzed reactions, based on some assumptions, was proposed to describe the process of MX oxidation; According to calculated kinetics parameters, (1))The last two steps(M-TA to 3-CBA,3-CBA to IPA) of MX oxidation were found as the surface reaction rate determine steps; (2)The apparent activation energy of 3-IMA to M-TA was highest(82.29KJ/mol) which indicated the second step(3-IMA to M-TA) was most sensitive to reaction temperature;The apparent activation energy of M-TA to 3-CBA was second highest(29.91KJ/mol);The apparent activation energy of 3-CBA to IPA was third highest(24.50KJ/mol);The apparent activation energy of MX to 3-IMA was the lowest (14.32KJ/mol);(3 through which a difficulty of the second methyl oxidation in our experimental conditions was concluded to result from the electrophilic effect of carboxyl, not due to high activation energy of its oxidation.