Study On Purification Of Chloroactic Acid By Catalytic Hydrogenation

This paper studied on the purification of chloroacetic acid from mother liquor of chloroacetic acid by hydrogenation, using Pd/C as catalyst. 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 suitable concentration resulted in remarkable increase of 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 suitable preparation conditions of Pd/C catalysts are obtained as follows: 0.8 % by weight of palladium,impregnation pH value was 3, impregnation temperature was 80°C, impregnation time was 4hr, impregnation reductor was HCHO. The catalyst prepared under the suitable preparation conditions can be used as catalyst for hydrogenation of dichloroacetic acid (DCA) to chloroacetic acid.The concentration of residual DCA was less than 0.2 %.Purification of MCA by hydrogenation of DCA from mother liquor was studied in this paper. The results of orthogonal experiments demonstrated that the influences of factors were different. The influences were in the order: reaction temperature, reaction time, reaction pressure, flow velocity of H₂ and N₂. The optimum reaction conditions were determined by orthogonal experiments and signal factor experiments as follows: the reaction temperature was 145 155°C,reaction time was 2.5 2.67 hr, H₂ pressure was 0.2 0.26 MPa, flow velocity of H₂ was 90 ml/min,and flow velocity of N₂ was 5 ml/min. Under the optimum reaction conditions,the activity of the selected catalyst was stable,the quality of MCA was higher. The reason, which led catalysts to lose the activity, was determined by analysis of N₂ physical adsorption, XRD and SEM. The demonstrated that the lost of Pd metal and the growth of Pd particle size during the hydrogenation process were the main reasons,the envenomation of hydrochloric acid and carbon deposition also led the activity decrease of Pd/C catalyst.