CHEMICAL METHOD FOR THE ISOLATION OF TRIMETHYLAMINE FROM A MIXTURE OF METHYLAMINES

M. Yu. Mogilevsky

(Submitted by Professor, Doctor of Chemical Sciences B. V. Tronov)

By the method of simple rectification of a mixture of methylamines, it is possible only to separate from one another three binary azeotropes, each of which will contain one or another amount of trimethylamine. Further separation is possible through the use of other physical and chemical methods.

In the scheme that we have developed and implemented on a plant scale, its binary mixtures with mono- and dimethylamine are used for the isolation of trimethylamine. The azeotrope consisting of ammonia and a small amount of trimethylamine is returned to the cycle for preparing the initial working mixtures.

Separation of the mixture of amines with the aid of ammonium chloride is achieved as a result of an exchange reaction. In aqueous solutions, methylamine and dimethylamine rapidly displace ammonia from ammonium chloride. Trimethylamine practically does not enter into the exchange decomposition reaction with ammonium chloride (under the selected conditions).

The theoretical basis of these processes is the difference in basic properties associated with differences in the constants of electrolytic dissociation. Amines have more sharply pronounced basic properties than ammonia.

In an aqueous system consisting of the three methylamines and ammonia, the strongest base is dimethylamine; next in basic strength is monomethylamine.

With respect to the ability of salts to undergo hydrolysis, the amines are arranged in the reverse order. In a system consisting of aqueous solutions of the chloride salts of methylamines and ammonia, ammonium chloride is hydrolyzed most readily. Trimethylammonium chloride is subject to hydrolysis to a considerably lesser extent, and the chloride salts of mono- and dimethylamine still less. When there is a deficiency of anions in the system, the strongest bases will be chemically bound first, i.e., di- and monomethylamine. All that has been said, of course, applies to aqueous solutions.

If sulfate or nitrate salts (or any other salts) are taken instead of chloride salts, the essence of the interaction mechanism does not change, since it is determined by the properties of the organic bases themselves and not by the anions.

For the method of isolating trimethylamine from its mixture with methylamine with the aid of ammonium chloride, M. Yu. Mogilevsky and N. F. Alekseeva received an inventor’s certificate (No. 108685, with priority of November 19, 1956).

All the nodal questions of the technological scheme for the preparation and isolation of trimethylamine were first worked out under laboratory conditions. This work prepared the initial material for enlarged trials and for setting up industrial production.

Experimental Part

Procedure

The dry salt was mixed with an aqueous solution of pure methylamines or with their mixtures (equimolecular or industrial). The resulting reaction mixture was evaporated to dryness on a water bath, and the residue was analyzed.

The content of the individual components in the salt residue was determined by the following method:

a) ammonia was separated as a precipitate of ammonium chloride by washing the salt residue with butanol, in which the amine hydrochlorides dissolved;

b) monomethylamine was determined by decomposing it with nitrous acid, followed by measurement of the amount of nitrogen evolved (in the absence of ammonia in the salt residue). If ammonia was present in the salt residue, the mixture under analysis was first freed from it by dissolving the amine hydrochloride salts in butanol. The butanol was then evaporated, the salts of di- and trimethylamine were dissolved in chloroform, and monomethylamine hydrochloride separated as a precipitate;

c) dimethylamine was separated from the other components as dimethylnitrosamine upon interaction of the solution of the analyzed mixture with nitrous acid;

d) trimethylamine was separated from the analyzed solution after decomposition of ammonia and monomethylamine and binding of dimethylamine as nitrosoamine by nitrous acid.

Analysis of the starting products and of the residue showed in which cases, and how completely, the displacement reaction takes place.

Eight series of experiments were carried out.

1. Interaction of ammonium chloride with trimethylamine. Ammonia is displaced from the salt by trimethylamine by approximately 60%.

2. Interaction of ammonium chloride with monomethylamine. Ammonia is completely displaced by monomethylamine.

3. Interaction of trimethylamine chlorohydrate with monomethylamine. Trimethylamine is displaced by monomethylamine by approximately 75%.

4. Interaction of ammonium chloride with dimethylamine. Ammonia is completely displaced by dimethylamine.

5. Interaction between monomethylamine hydrochloride and dimethylamine, the latter taken in 10% excess over the stoichiometric amount. Monomethylamine is displaced from the salt by dimethylamine by only 6%.

6. Interaction between ammonium chloride and a mixture of mono-, di-, and trimethylamine.

All reagents were taken in equimolecular amounts. Ammonia is displaced completely; trimethylamine does not combine into the hydrochloride; the salt residue obtained consists of equal amounts of the hydrochlorides of mono- and dimethylamine.

1. Interaction between ammonium nitrate and an industrial mixture of methylamines of the following composition: MMA 12 parts by weight, DMA-9, TMA-10.

The mixture of amines was taken in large excess over the stoichiometric amount. Ammonia is displaced completely; the salt residue obtained consists of the nitrates of monomethylammonium (72%), dimethylammonium (25%), and trimethylammonium (3%).

1. Interaction between ammonium sulfate and an azeotropic mixture consisting of 85% monomethylamine and 15% trimethylamine.

The azeotrope was taken in excess. Ammonia is displaced completely; the salt residue obtained consists of the sulfates of monomethylammonium (99%) and trimethylammonium (1%).

Production experience in obtaining trimethylamine over 1 year

Industrial verification of the chemical method for obtaining trimethylamine was carried out on the basis of processing, as the initial mixture, the azeotrope “monomethylamine + trimethylamine.”

The synthesis of amines is carried out from a mixture of methanol with ammonia taken in large excess on an alumonickel catalyst at a temperature of \(410 \pm 5^\circ\text{C}\) and a pressure of 50 atm. The resulting reaction mixture was subjected to the following operations:

1) separation of ammonia in the form of an azeotrope with part of the trimethylamine by rectification (the ammonia azeotrope is sent for preparation of new portions of the starting mixture);

2) separation of reaction water from the mixture of methylamines by rectification (the water is discharged to the sewer);

3) separation of monomethylamine in the form of an azeotrope with part of the trimethylamine by rectification;

4) obtaining methylamine hydrochloride by reacting the azeotrope “monomethylamine + trimethylamine” with ammonia liquor in aqueous solution;

5) isolation of pure trimethylamine by rectification of the condensed reaction mixture obtained from the interaction of the “MMA + TMA” azeotrope with ammonia liquor.

Solely as a result of producing commercial methylamine hydrochloride on the basis of the “MMA + TMA” fraction instead of pure monomethylamine (as was the case before introduction of the new method), an economic effect of more than 900 thousand rubles per year was obtained.

Conclusions

In their ability to displace a base, the substances are arranged in the following order: monomethylamine, dimethylamine, trimethylamine, ammonia.

In displacement ability, monomethylamine is close to dimethylamine, and trimethylamine to ammonia. The first two bases (mono- and dimethylamine) differ very strongly in displacement ability from the second pair of bases.

Free methylamines are capable of displacing ammonia not only from ammonium chloride, but also from other salts (in particular ammonium sulfate and nitrate). The general principles of the observed phenomenon are the same as in the case of ammonium chloride.

A practically acceptable method for the industrial production of trimethylamine has been developed and introduced into production. In this method, along with trimethylamine (free base), salts of mono- and dimethylamine (chlorides or sulfates), or a salt of only one of them (when demand is limited), are obtained.

The synthesis of the mixture of methylamines is carried out under ordinary conditions with a considerable excess of ammonia on a promoted alumonickel catalyst (GIPKh-101).