Study On The New Joint Technology Of O-, P-Chlorobenzoic Acid

O-chlorobenzoic acid and p-chlorobenzoic acid are important fine chemical intermediates which are widely used in the fields of dye, pesticide and medicine etc.The synthetic methods and conditions of chlorobenzoic acids were reviewed in this paper. At present, o-, p-chlorobenzoic acids are synthesized by o-, p-chlorotoluene as raw material respectively. Because o-, p-chlorotoluene are made from the ring chlorination of toluene at the same time, the difference of boiling point between o-and p-chlorotoluene is very small(the boiling point of o-chlorotoluene is 159°C and the one of p-chlorotoluene is 162°C), a great deal of energy are consumed to separated them by rectification. If chlorotoluene without separation were used to synthesize chlorobenzoic acids directly, and chlorobenzoic acids were separated by dissociation extraction crystallization, then a lot of energy could be saved by changing raw material difficult to separate into products easy to separate. For these reasons, a new joint process of chlorotoluene mixture to be oxided -chlorobenzoic acids to be separated was put forward. Two key techniques in the new joint process were needed to be resolved. In this paper, the effort was made to obtain the suitable operation parmeters by systematically studing the two key technologies.The method of liquid oxygen oxidation of chlorotoluene mixture was choosen to synthesize chlorobenzoic acids, and the reactions were carried out under atmospheric pressure. O-chlorotoluene and p-chlorotoluene as raw materials were mixed in proportion of the ring chlorination of toluene . A stired tank was used as reactor. By the selection of catalysts, oil-soluble cobaltous naphthenate was choosen to be primary catalyst, 1,1,2,2-tetrabromoethane to be assistant catalyst. In order to increase use ratio of the reactor and decrease the corrosion protection demand of the equipment, acetic acid was avoided as organic solvent. The effects of the ratio between cobaltous naphthenate and 1,1,2,2-tetrabromoethane, the amount of each catalyst, temperature and oxygen flow on the yields of chlorobenzoic acids were investigated respectively.It was found that there was an optimal quantity and proportion between cobaltous naphthenate and 1,1,2,2-tetrabromoethane. The suitable quantity of cobaltousnaphthenate and 1,1,2,2-tetrabromoethane was 0.5g and 0.107g in lOOg raw material respectively. When temperature range was from 120 °C to 150°C, the raise of temperature made the yields of chlorobenzoic acids to increase.At 160°C, the catalyst deactived seriously and the yield of chlorobenzoic acids fallen sharply. The optimal reaction temperature was 150°C. The increase of oxygen flow could raise the yield of chlorobenzoic acids, but if the flow of oxygen was too high, the loss of raw material would increased. So the suitable oxygen flow was 9.65mL/s under the experimental conditions.The relationship between the yields of chlorobenzoic acids and the reaction time was also studied in this paper. The yields of chlorobenzoic acids increased with the extension of reaction time withen 8 hours. After that, some p-chlorobenzoic acid crystal precipitated in the reactor and the reaction stopped. So it was decided that the suitable reaction time was 4 hours. The feasibility of reaction by using circular mother liquid was studied, it was confirmed by the experiment that the yields of chlorobenzoic acid had little reduction by the reuse of circular mother liquid three times, then the reaction stopped because too many impurities existed in the solution.For the separation of o- and p-chlorobenzoic acid, it is hard to separate them by conventional methods, such as crystallization because of their high melting point. For an effective separation, the difference in the solubilities and dissociation constants of the two chlorobenzoic acids in water were based on, aikaline and acidic extractants were selected to separate O- and p-chlorobenzoic acid by dissociation extration crystallization. The concentration and addition amount of aikaline and acidic extractants, reaction temperature and water consumption to scatter chlorobenzoic acids were investigated. The suitable operation conditions in the dissociation extration of p-chlorobenzoic acid were determined that the concentration of aikaline extractants was 0.2M, the mole ratio of OH" and o-chlorobenzoic acid was 1.05:1, reaction temperature was 55 "C, water dispersant was 100 g v.s 6 g raw material. The suitable operation conditions of the purification of o-chlorobenzoic acid was: the mole ratio of H+ and p-chlorobenzoic acid was 1.10:1, reaction temperature was 5 5 °C. Thus, through dissociation extration and extration crystallization in a single stage respectively, o-and p-chlorobenzoic acid of high purity(99%) could be obtained. It was proved that dissociation extration crystallization was an effective method to separate mixtures of chlorobenzoic acids.