N, N-dimethylaniline is an important intermediate for the preparation of dyes, rubber vulcanization accelerators, explosives and pharmaceuticals. Liquid phase method and gas phase method are often used in industry. In order to reduce the cost and increase the yield, methanol is usually used as N-alkylation reagent.
The industrial production of N, N-dimethylaniline uses methanol as raw material instead of monochloromethane because methanol is easily available and the production cost is low. Although the reaction goes through the alkaline hydrolysis process of quaternary ammonium salt, the actual yield is also very high.
Synthesis of N, N-dimethylaniline
The N-alkylation reaction using methanol as alkylation reagent is a reversible series of reactions.
Industrial Production
Aniline was added to the autoclave and sulfuric acid was used as catalyst. N-alkylation was carried out with methanol at 225oC with equilibrium constants K1=9.2x102 and K2=6.8x105. K1 and K2 are both larger, and K2 is 739 times larger than K1. Therefore, aniline can be completely converted to N, N-dimethylaniline by using proper excess methanol.
For the production of N, N-dimethylaniline by liquid phase method, aniline, methanol and sulfuric acid can be added into the autoclave at the molar ratio of 1:3.56:0.1, and the excess methanol and by-product dimethyl ether can be evaporated at 210~215oC, 3~3.3MPa, after 4 hours of heat preservation and pressure release. The alkylation solution is neutralized with sodium hydroxide, and the water layer is still and layered. The water layer reacts for 3h at 165oC and 1.6MPa in an autoclave to hydrolyze the contained N, N, N-trimethylaniline hydroxide into N, N-dimethylaniline and methanol. Industrial N, N-dimethylaniline can be obtained from the oil layer by vacuum distillation, and the theoretical yield of aniline can reach 96%. However, the liquid phase method requires corrosion-resistant autoclave, and the equipment investment is large.
The production of N, N-dimethylaniline by gas-solid contact method (gas phase method) is to pass the mixed vapor of aniline and methanol through sulfate / glass catalyst at 320oC under atmospheric pressure, the contact time is 6s, the conversion of aniline is 99.5%, and the theoretical yield of the product can reach 98%. The metal used in the catalyst can be one or more of nickel, iron, aluminum, manganese, copper and chromium.
Gas phase method can make the reaction proceed under close to normal pressure, continuous operation, large production capacity, less side reaction, high yield, less waste water and high product purity. The key to the success of gas phase method is the selection and preparation of catalyst. For the production of N, N-dimethylaniline, the latest catalyst is spherical alumina. It is reported that its activity is very high and its service life is more than 5000h.
