(289c) Semi-Continuous Gas-Liquid Catalyzed Reaction of O-Phenylene Diamine and Carbon Disulfide by Tertiary Amine

Phase-transfer catalysis (PTC) is an effective technique to synthesize the products from two or more immiscible reactants which exist in different phases. Usually, quaternary ammonium salts were used as the phase-transfer catalyst to enhance the reaction. By adding a small catalytic amount of quaternary salt, the reaction rate is dramatically accelerated and the conversion of reactant is increased. The main function of the quaternary salt is to bring nucleophiles (anions) from the aqueous phase to the organic phase for reacting with organic reactants in organic solution in a normal phase transfer catalytic reaction (NPTC). Recently, this technique is also extended to the synthesize specialty chemicals from homogeneous solution with success. However, the application of phase-transfer technique to gas-liquid system is seldom discussed. In the past, the procedure for synthesizing 2-mercaptobenzimidazole (MBI) involves the reaction of o-phenylene diamine and reactants in a mixture of methanol-water catalyzed by active carbon. However, the reaction was carried out at a relatively temperature to obtain the desired product for a long time period. In the later, quaternary ammonium salts were employed as a catalyst in the reaction of o-phenylene diamine and carbon disulfide to synthesizie MBI. However, the kinetics and mechanism of the reaction was not discussed. In addition, MBI was also obtained by other techniques in using various reactants. Recently, 2-mercaptobenzimidazole (MBI) was synthesized from the catalyzed reaction of o-phenylene and carbon disulfide in a batch homogeneous solution or two-phase solution by appropriating choice of the organic solvents. The most advantage of these processes is that the catalyzed reaction is carried out at moderate reaction temperature. Nevertheless, the boiling point of carbon disulfide limited the application of PTC to carry the reaction at higher temperature in the homogeneous solution or liquid-liquid two-phase solution. The boiling point of carbon disulfide at atmosphere is 46.2°C. Therefore, it is difficult to carry the liquid-liquid two-phase phase transfer catalytic reaction larger than 46.2°C. The reason is that a larger amount of CS2 will evaporate to the gas phase. In order to increase the reaction, a higher temperature is used. The catalyzed reaction of o-phenylene diamine and carbon disulfide by tertiary amine to synthesize MBI is carried out in a gas phase/liquid solution two-phase medium in this work. The most advantage of this new process is that a higher reaction temperature higher than boiling point of carbon disulfide is used to enhance the reaction. In order to fulfill this requirement, a semi-continuous reactor is employed to carry out the reaction in this work. Carbon disulfide (CS2) mixed with dichloromethane (CH2Cl2), which is used as the gas phase, is introduced to the reactor for reacting with o-phenylene diamine catalyzed by tertiary amine. Based on the experimental evidence, the reaction mechanism is proposed and the kinetics is developed. A pseudo first order rate law is sufficient to describe the kinetic behaviors. Effects of the reaction conditions, including ammonia, tertiary amines, amount of tertiary amines, concentration of tertiary amine, flow rate of gas phase, amount of o-phenylene diamine and temperature, on the conversion are investigated. In this work, the reaction, which is carried out under a semi-continuous reactor, can be operated at a temperature larger than the boiling point of carbon disulfide. First, a mixture of carbon disulfide (bp 46.2°C) and dichloromethane (bp 39.6°C) in gas phase is first prepared and then continuously introduced to the reactor. The concentration of carbon disulfide is controlled by adjusting the the flow rate of the two streams. The flow rate of feeding CS2/CH2Cl2 gas is controlled by the feed pump. Second, carbon disulfide is then dissolved in MeOH/H2O solution which contains o-phenylene diamine and tertiary amine. Thus, carbon disulfide reacts with tertiary amine to produce an active intermediate (R3N-CS2), which can further react with o-phenylene diamine to produced the desired product 2-mercaptobenzimidazole (MBI). In this work, a stream of CS2/CH2Cl2 mixture is continuously fed to the reactor preparing for reacting with o-phenylene diamine in the liquid phase solution. By feeding the gas-phase carbon disulfide, the reaction can be carried out at a higher temperature to enhance the reaction. The present process provides the feasible conditions for carrying the reaction at higher temperature. From the experimental result, the reaction follows the pseudo first order rate law in which the apparent rate constants kapp at various reaction conditions were obtained. Tributylamine (TBA) exhibits higher reactivity among the tertiary amine and ammonia. The dissolving rate of CS2 in organic solvent and the mass transfer rate in the organic-phase solution are both larger. Therefore, the effect of agitation speed on the conversion of o-phenylene diamine is insignificant. The conversion of o-phenylene diamine is increased with the increase in the concentration of tertiary amine, temperature, concentration of carbon disulfide and the flow rate of CS2/CH2Cl2. However, the conversion is decreased with the increase in the concentration of o-phenylene diamine.