Synthesis Of 3,3'-Dichlorobenzidine By Catalytic Hydrogenation

2,2'-dichlorohydrazobenzene was prepared from ortho-nitrochlorobenzene, using Pd/C as catalyst in the base media, then 3,3'-dichlorobenzidine was received by rearrangement of 2,2'-dichlorohydrazobenzene in the acid media. The results of elementary selective experiments of active carbon showed that palladium catalysts supported on the active carbon with large surface area and high concentration of surface acidic groups usually exhibited higher activity for hydrogenation reaction of ortho-nitrochlorobenzene to prepare 2,2'-dichlorohydrazobenzene. The effects of surface modification of active carbon by pretreatment with HNO₃ on behaviors of Pd/C catalysts were investigated. The active carbon supports and the catalysts were characterized by chemical titration of surface groups, N₂ physical adsorption and SEM. The results showed that active carbon supports treated by nitric acid of different concentration did not remarkable changed the surface structure of active carbon, but resulted in remarkable increase in the surface acidic groups. These led to higher dispersion of palladium and enhanced the activity of the catalysts prepared by impregnation of PdCl₂ aqueous solution. Optimum results were obtained by treatment with 30% HNO₃. The activity of the catalysts supported on active carbon pretreated by 30% HNO₃ is 2.0 times to that of the catalysts supported on active carbon without pretreatment. The yield of 2,2'-Dichlorohydrobenzene was increased a little. The reused times of this catalyst was significantly increased. The suitable preparation conditions of Pd/C catalysts are obtained as follows: impregnation pH value is 2.5; impregnation concentration is 1.2 g.mL^-1; impregnation temperature is 80℃; impregnation time is 2h; dosage of 36%HCHO 4ml; reduction condition is that maintain the temperature 40℃for 2h and then 80℃for 1h. The catalyst prepared under the suitable preparation conditions can be used as catalyst for hydrogenation of ortho-nitrochlorobenzene to 2, 2'-dichlorohydrazobenzene. The rate of H₂ consumption and the yield of DHB was up to 3370mL.min^-1.g^-1, 89.72ï¼…respectively.Synthesis of DHB by hydrogenation of ortho-nitrochlorobenzene was studied in this paper. The results of orthogonal experiments demonstrated that the influences of factors on the yield of 2, 2'-Dichlorohydrobenzene were different. The influences were in the order: sodium hydroxide's mass fraction, weight ratio of Pd/C catalyst, weight ratio of co-catalyst, reaction temperature. The optimum reaction conditions were determined by orthogonal experiments and signal factor experiments as follows: the weight ratio of Pd/C catalyst to o-nitrochlorobenzene was 4.5ï¼…, the mass of sodium hydroxide fraction was 25ï¼…, the weight ratio of 2, 3-dichioro-1, 4-naphthopuinone to ortho-nitrochlorobenzene was 0.267ï¼…, the reaction temperature was 65℃, H₂ pressure 0.4Mpa, mass ratio of toluene to ortho-nitrochlorobenzene was 0.87: 1. Pd/C catalyst could be recycled at least eight times under the optimum conditions, and the yield of DHB was between 90ï¼…and 93ï¼…. Some parameters including reaction temperature, sulfur acid mass concentration, dosage of the sulfur acid and reaction time etc., were discussed through different rearranged-conditions. Under the optimum reaction conditions, the yield of DCB from ortho-nitrochlorobenzene was up to 80～83ï¼…, the ammonia group value was above 95.0ï¼….