Synthesis Of P-Amionphenol By Catalytic Hydrogenation

This paper studied on the preparation of p-aminophenol by catalytic hydrogenation of nitrobenzene in the presence of aqueous sulfuric acid and a surfactant to assist in dispersing the nitrobenzene throughout the reaction medium, using Pt/C as catalyst. Four commercial activated carbons were used as the supports of the catalysts, the results of nitrobenzene hydrogenation experiments showed that the actived carbon with moderate surface area, low ratio of micropore volume and little ash was appropriate support for platinum catalysts. The effects of the purification and functionalization treatments by HNO3 of the activated carbon on the performance for nitrobenzene hydrogenation reaction of Pt/C catalysts were investigated. The actived carbon supports and the catalysts were characterized by chemical titration of surface groups, N2 adsorption and scanning electron microscope (SEM). The results displayed that the functionalization treatments of purified carbon with the HNO3-treatment had litter effect of surface structure,but resulted in remarkable increase in the content of some surface acidic groups. The acidic sites with moderate HNO3-treatment showed strong interaction with H2PtC16 during impregnation. The HNO3-treatment would favoure the Pt dispersion on the carbon surface and the catalytic behaviour. The optimum conditions of HNO3-treatment were determined as followed: 30%HNO3(wt) solution, 90℃, reflux 3h. The catalysts supported on the actived carbon with HNO3-treatment exhibited higher activity for hydrogenation reaction than the catalysts supported on actived carbon without pretreatment. The reused times of the catalysts were significantly increased. The preparation conditions of Pt/C catalysts had positive effects both on the Pt dispersion and the hydrogenation capacity of the catalyst.The optimum preparation conditions of Pt/C catalysts were obtained by experiments as followed: impregnation concentration was 3.0g/mL, impregnation pH was 3.5, impregnation temperature was 80℃, impregnation time was1.5h, dosage of 36%HCHO was 4mL, reduction pH was 8~9, reduction temperature was 80℃. The catalysts prepared under the optimum preparation conditions were used in the reaction of hydrogenation of nitrobenzene to p-aminophenol, the rate of H2 consumption and the productivity of p-aminophenol were up to 24.5mL/min, 6975 gPAP/gPt·h.The process of nitrobenzene hydrogenation was analyzed detailedly. The results showed that the reaction should be stopped when nitrobenzene was consumed 75～80%, this can kept the catalyst dispersed in the organic phase. The organic phase with the catalyst could be reused directly after the reaction of hydrogenation. The results of signal factor experiments showed that the optimum ratio (wt) of nitrobenzene to platinum was 24000 and the suitable ratio (mol) of nitrobenzene to sulfuric acid was 1.05. The results of orthogonal experiments demonstrated that the influences of factors on the productivity of p-aminophenol were different. The influences were in the order: reaction temperature, the concentration of sulfuric acid, reaction pressure and the amount of surfactant. The optimum reaction conditions were determined by orthogonal experiments and signal factor experiments as followed: nitrobenzene was 20mL, amount of catalyst was 100mg, sulfuric acid was 8.6mL, reaction temperature was 85℃, concentration of sulfuric acid was 15% , reaction pressure was 20cm water , amount of surfactant was 0.1g. The catalyst could be recycled at least ten times directly under the optimum reaction conditions. The catalysts with less activity could be also recycled four times after treated by HNO3. The reasons led catalysts to lose the activity were determined by analysis of N2 adsorption, Atomic absorption, X-ray diffraction (XRD) and SEM. The results demonstrated that the organic coverings of the catalysts surface was the main reason, the lost of Pt metal and the growth of Pt particles during the hydrogenation process also led the activity decrease of Pt/C catalyst.The side products such as the 4,4'-diaminodiphenyl ether, aniline and so on were produced in the hydrogenation reaction. High purity p-aminophenol used in pharmaceutical compounds with 4,4'-diaminodiphenyl ether preferably less than 10 ppm was obtained by the treatment of crude p-aminophenol. The first step of treatment was extraction with a mixture of nitrobenzene and aniline (the ratio of nitrobenzene to aniline is 2:1 by volume) to eliminate 4,4'-diaminodiphenyl ether, the second step was extraction with toluene to remove aniline and nitrobenzene. The effects of favourite process parameters were tested. The optimum extraction conditions were obtained as followed: the time of extraction equilibrium was 5min, the suitable pH was 4.5~5.0, the extraction temperature was 40℃~ 60℃. The products were characterized with infrared ray (IR) and nuclear magnetic resonance (NMR). The purity was tested by high performance liquid chromatography (HPLC) and melting point. The purity of the production was up to par after five times of extraction with the mixture of nitrobenzene and aniline. The wastewater treated by suitable method could reach the second grade of national wastewater discharge standards. The platinum could be recovered after burning off the support carbn. So, this method has remarkable economic considerations and social advantage.