Studies On Nitrosation Of Cyclohexanecarboxylic Acid

Caprolactam is one of important petrochemicals with a white crystal style at normal temperature and may occur some reactions such as hydrolyzation, oxygenation and halogenation. Caprolactam is mainly used for manufacture of engineering plastic and fibre for public and industry. In 1899, two German scholars, S. Gabriel and T. A. Mass reported that they obtained caprolactam by heating up epsilon-6-aminocaproic acid. In 1937, P. Schlack from I. G. Farben Company in Germany succeeded in preparation of caprolactam polymer by opening ring and polymerization with using a salt of hydrochloric acid of 6-aminocaproic acid as catalyst. This process started an epoch of the manufacture and application of caprolactam. In 1943, the preparation of caprolactam achieved industrial scale.At present, there are three representative techniques for manufacture of caprolactam: the way of cyclohexanone-hydroxylamine, of SNIA and of PNC. In the method of cyclohexanone-hydroxylamine, caprolactam can be obtained from benzene after the main processes: such as hydrogenation of benzene, oxygenation of cyclohexane, oximation of cyclohexanone, rearrangement of cyclohexanone oxime and refinement of crude caprolactam. The PNC method also uses benzene as raw materials, in which cyclohexanone oxime can directly be prepared by the light-catalyzed nitrosation of cyclohexane. In the method of the SNIA, toluene is used as raw materials. It mainly consists of oxygenation of toluene, hydrogenation of benzoic acid, lactamization of cyclohexane-carboxylic acid with nitroso-sulfuric acid and so on. The main disadvantage of the SNIA is with too much byproduct, ammonium sulphate, from 3.8 to 4.5 tons per ton caprolactam. This may leads to a very high cost for production.Now, the researches of the preparation of caprolactam pay more attention to the following ways: (1) Seeking some new techniques or improvement to the existing technique with cheaper raw material. (2)Improving the manufacture technique for less byproduct,ammonium sulphate.The main task of ours is to improve the nitrosation process, expecting to reduce the quantity of byproduct. Firstly, according to some references, we discuss the influence on by such factors as reaction time, reaction temperature, concentration of oleum, and the premixing ratio of cyclohexane-carboxylic acid and oleum, with using one-factor experiment. Secondly, we carry out the experiments with orthogonal design, based on single-factor experiment. The result show that the favorable conditions are as follows: the reaction temperature is 73℃, the reaction time 15 minutes, the mass ratio of cyclohexane-carboxylic acid and nitrosyl sulfuric acid 2.4 and the concentration of SO₃ in the system 8%In order to reduce the byproduct, we propose a new technique for the nitrosation, i.e. the heterogeneous nitrosation technique, we find the favorable conditions: the reaction temperature 68℃ , the concentration of oleum 50% and the ratio of premixing 2:1. Under the condition, conversion of cyclohexane-carboxylic acid and selectivity of caprolactam can reach 22.3% and 83.4% respectively. We find that the conversion of cyclohexane-carboxylic acid with a higher selectivity by nitrosation is too low. Comparing the homogeneous and the heterogeneous techniques, we find that SO₃ as catalyst may not be enough in the heterogeneous nitrosation. Thus we find that the conversion increase with increasing the content of SO₃. For example, the conversion of cyclohexane-carboxylic acid and the selectivity of caprolactam can reach 60.3% and 73.7% respectively under the premixing ratio of cyclohexane-carboxylic acid and oleum of 4:1 with reaction time of 50 minutes and reaction temperature of 78℃...