CA2032362C - Process for the preparation of n,n-dimethylamine - Google Patents

Abstract

The present invention relates to a process for the preparation of N,N-dimethylamines by reaction of an aldehyde with dimethylamine and hydrogen under pressure and at elevated temperature in the presence of a hydrogenation catalyst containing Ni, Co, Cu, Mn, Fe, Rh, Pd and/or Pt. After unconverted dimethylamine and excess hydrogen have been separated off, formaldehyde or a formaldehyde-forming substance is added, and distillation is carried out.

Description

""'" Process for the preparation of N,N-Dimethylamines The present invention relates to a process for the preparation of N,N-dimethylamines by reaction of an aldehyde with dimethylamine and hydrogen under pressure and at elevated temperature in the presence of a hydrogenation catalyst.
The reaction results in the exchange of the oxgyen atom in the aldehyde group by a dimethylamine radical, during which water is formed. In this reaction an enamine can be formed as an intermediate, which is converted to the corresponding- dimethylamine derivative by means of hydrogen. In what follows, N,N-dimethylamines are under-stood to mean tertiary amines containing, apart from two methyl groups, any desired additional radical, which results from the incorporation of the aldehyde. Tertiary amines are compounds of industrial importance. They can serve as polymerization and curing catalysts for the preparation of plastics based on epoxides and urethanes.
Furthermore, they are suitable as corrosion inhibitors and absorbents for the washing liquid of synthesis gas.
This is in particular true of the easily producible N,N-dimethylamines.
US Patent 4,207,260 describes the preparation of tertiary amines by reaction of an aldehyde with hydrogen and an amine in the presence of a rhodium carbonyl or ruthenium-carbonyl or halogenocarbonyl as catalyst.
US Patent 4,229,374 relates to a method for the pre-paration of an amine by reaction of an alcohol, aldehyde or ketone with ammonia, a primary or secondary amine in a reducing atmosphere by means of a supported catalyst containing copper, tin and, if desired, an alkali metal or alkaline earth metal.
The reaction of aldehydes with dimethylamine and hydrogen - also known under the name reductive amination of aldehydes - is a technically feasible route for the zo~z~sz preparation of N,N-dimethylamines.
Details of this process can be found in Houben-Weyl, Methoden der organischen Chemie (Methods of Organic Chemistry), Volume IV/lc, pages 423 to 425, 4th edition (1980), Georg Thieme Verlag, Stuttgart-New York and in S.B. Cavitt et al., J. Org. Chem. 27, 1211 (1962).
The reaction can be carried out batchwise or contin-uously. In the continuous procedure, pressure-resistant tubular reactors containing the hydrogen catalyst in the form of pellets are usually used. The starting materials dimethylamine, aldehyde and hydrogen are fed into the tubular reactor either at the bottom or the top. Depend-ing on the type of additicn, the procedure is called downward flow or liquid phase procedure.
However, it is also possible to carry out the reaction in the presence of a suspended hydrogenation catalyst either batchwise or continuously.
In the course of the reaction, various by-products are formed. In particular the formation of secondary N-methylamines, which are possibly formed as a result of the cleavage of dimethylamine and already formed N,N-dimethylamines, interferes in the reaction. These secon-dary N-methylamines cannot be separated off by distil-lation from the desired valuable product, the N,N-dimethylamines, to the required extent, because of the closely spaced boiling points.
For a number of fields of application, fairly high demands on the purity of the N,N-dimethylamines to be used are made. Since in these cases even small amounts of secondary N-methylamines prevent the use of the N,N-dimethylamines, there was a need for a process which is not only easy to carry out but also allows the use of conventioal hydrogenation catalysts and, at- the same time, avoids not only the abovementioned disadvantages caused by the presence of secondary N-methylamines, but also gives the desired valuable product in a high yield.
These ob~ects are achieved by a process for the preparation of N,N-dimethylamines by reaction of an aldehyde with dimethylamine and hydrogen under pressure and at a temperature of from 50 to 150°C in the presence of a hydrogenation catalyst. It comprises separating off unconverted dimethylamine, then adding 0.1 to 25~ by weight of formaldehyde or a formaldehyde-forming substance, relative to N,N-dimethylamine, and distilling the mixture.
The process according to the invention can be carried out - as described at the beginning - using a hydrogenation catalyst which is present in finely divided suspended form or in the form of pellets either batchwise or continuously. It is particularly suitable for a continuous process.
If the reaction is carried out in the presence of a suspended hydrogenation catalyst, the catalyst suspension is, for example, initially introduced into a stirred container, and starting materials are added to the stirred suspension.
If a hydrogenation catalyst in the form of pellets is :_zsed, it is recommended to arrange it in a form of a bed in a pressure-resistant reaction tube, and passing the starting anaterial through this bed. The starting materials aldehyde and ~imethylamine can be used in the reaction either in gaseous or liquid form. The process according to the invention is particularly easy when carrying out the reaction in liquid phase.
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2o323s2 3a 24325-174 Suitable aldehydes are any desired aromatic, araliphatic, cycloaliphatic and aliphatic, in particular araliphatic, cycloaliphatic and aliphatic, preferably ~:ycloaliphatic and aliphatic, particularly preferably aliphatic, aldehydes and derivatives thereof.
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Representatives of aromatic aldehydes are benzaldehyde, p-hydroxybenzaldehyde, those of araliphatic aldehydes are phenylacetaldehyde, phenylpropionaldehyde, those of cycloaliphatic aldehydes are formylcyclohexane and those of aliphatic aldehydes are straight-chain and branched aldehydes of 2 to 20, in particular 2 to 15, preferably 2 to 10 carbon atoms. Examples of suitable aliphatic aldehydes are propanal, n- and i-butanal, n- and i-pentanal, such as 3-methylbutanal, n- and i-hexanal, heptanals, octanals, for example 2-ethylhexanal, nonanal, decanal, undecanals, dodecanals, tridecanals, aldehydes of 14 to 20 carbon atoms, such as hexadecanlas, but also unsaturated aldehydes, such as acrolein, crotonaldehyde, pentenals, hexenals, octenals, such as 2-ethylhexenal.
The aldehyde can be used either in dilute or in pure form. In a large number of cases, it is advantageous to omit the use of a solvent and to use the aldehyde in undiluted form.
The dimethylamine required for the reaction can be used either in dissolved form or as an undiluted substance. A
suitable solvent is water. However, it is also possible to use pure dimethylamine in undiluted form for the reaction. Because of its low boiling point, it has to be stored in suitable pressurized containers.
The aldehyde and the dimethylamine can be combined before their use in the reaction and used in the reaction as a mixture. However, both starting materials can also be introduced separately for the reaction. The hydrogen required for the reaction is usually fed into the re-action vessel containing the hydrogen catalyst via a separate line, which may contain devices for the uniform distribution of the hydrogen.
The aldehyde is used together with the dimethylamine and the hydrogen in a molar ratio of 1 : ( 1 to 10 ) : ( 1 to 50), in particular 1 . (1 to 8) : (1 to 30), preferably 1 : (1 to 5) : (1 to 10). It has proven suitable to use dimethylamine and hydrogen in excess, relative to the aldehyde.
In general, an amount of 50 mold of dimethylamine and 100 mold of hydrogen in excess of the stoichiometric requirement is sufficient. The reaction is carried out at 1 to 20, in particular 3 to 15, preferably 7 to 12, MPa and 50 to 150, in particular 70 to 130, preferably 100 to 110,°C.
The hydrogenation catalysts which can be used are the conventional hydrogenation catalysts. The pressure and the temperature are to a certain extent also dependent on the type of hydrogenation catalyst and must therefore be adapted to one another.
The hydrogenation catalyst can be a support-free metal catalyst or a supported catalyst containing A1203, Si02, siliceous earth, activated carbon or pumice, in particular Si02, siliceous earth, or activated carbon, as support, preferably Si02, siliceous earth. The hydro-genation catalyst contains Ni, Co, Cu, Mn, Fe, Rh, Pd and/or Pt, in particular Ni, Co, Cu, Rh, Pd and/or Pt, preferably Ni, Co and/or Rh, particularly preferably Ni and in addition, if desired, conventional additives and promotors, such as alkaline earth metal oxides, Si02, A1203, Mn02--and/or Crz03~
Support-free catalysts which can be used are Raney catalysts, such as Raney nickel or Raney cobalt, but also those based on palladium, rhodium or platinum.
The process according to ,the invention is particularly simple if supported hydrogenation catalysts containing 10 to 75, in particular 20 to 70, preferably 40 to 65, ~ by weight of Ni, Co, Cu, Mn and/or Fe, in particular Ni, Co, Cu and/or Mn, preferably Ni, Co and/or Cu, relative to the entire catalyst weight, are used.

Noble metal catalysts make it possible to carry out a reaction under particularly mild conditions. They are usually accommodated on a support and have a metal content of 0.1 to 20, in particular 0.2 to 15, preferably 0.5 to 10, $ by weight of Pd, Rh and/or Pt, relative to the entire catalyst weight. Recommended supports are shaped materials based on A1203, SiOz, siliceous earth, activated carbon and/or pumice.
The catalyst determines the reaction conditions, in particular the reaction temperature and the pressure.
Usually, the reaction can be carried out quite success-fully at 50 to 150, in particular 70 to 130, preferably 100 to 110, °C and 1 to 20, in particular 3 to 15, preferably 7 to 12, MPa.
If a hydrogenation catalyst in the form of pellets is used, the reaction can be carried out either by direct flow or with the use of any reaction product cycle. The starting materials are introduced into the reactor -usually an upright tubular reactor - in which the catalyst is present in the form of a bed, at the top or bottom. According to a specific embodiment, the starting materials are introduced into the reactor at the bottom, and the reaction is carried out by direct flow, i.e:
without circulation of the reaction product. The reaction mixture leaves the reactor at the top. In a separator downstream from the reactor, hydrogen together with any unconverted dimethylamine is removed from the reaction mixture under superatmospheric pressure. Unconsumed hydrogen and unconverted di.methylamine can be used again in the reaction:
For the process according to the invention to succeed, it is important to remove thoroughly any still present dimethylamine from the reaction mixture. Usually, this separation is carried out in two or more steps, in which in the first step distillation is carried out under a pressure of 0.5 to 1.0 MPa and in the subsequent steps _ 7 _ 2032362 at 0.05 to 0.15 MPa. In most cases, a two-step separation of the di.methylamine is sufficient.
Formaldehyde can be used either in pure form or in solution. Suitable solvents are aliphatic alcohols of 1 to 4 carbon atoms and/or water. The process according to the invention can be carried out in a particularly simple manner by using aqueous solutions containing, for example, 10 to 60, in particular 15 to 55, preferably 20 to 50, $ by weight of formaldehyde.
As a formaldehyde-forming substance, paraformaldehyde as a pure product or in suspended or dissolved form is the logical choice. Useful solvents for preparing paraformal-dehyde suspensions are water, methanol and ethanol. If the use of a paraformaldehyde solution is intended, methanol and water, in particular water, are recommended solvents. Formaldehyde or the formaldehyde-forming substance is added to the reaction mixture only after the dimethylamine has been separated off.
An amount of 1 to 10, in particular 1.1 to 5, preferably 1.5 to 2, ~ by weight of formaldehyde or formaldehyde-forming substance, in each case relative to N,N-dimethyl-amine, is usually sufficient to guarantee the desired result.
The reaction product is exposed to the formaldehyde under a pressure of 0.05 to 1.0 MPa, in particular 0.1 to 0.5 MPa and at 50 to 250, in particular 70 to 200, preferably 80 to 180°C. A very simple procedure is to introduce the formaldehyde or the formaldehyde-forming substance, after the dimethylamine has been separated off, to the reaction product, for example before its use in the distillation or preferably directly at the bottom of the first distillation column.
In general, the conditions which prevail in the bottom of the column are sufficient for removing the undesired ~o3z3sz secondary N-methylamines. It is possible that a reaction between the secondary N-methylamine and the formaldehyde, which surprisingly leads to a product which can be purified without difficulties, is responsible for this.
If the secondary N-methylamine content in the reaction product is known, this value can be used as orientation for the formaldehyde addition. It is recommended to use formaldehyde in a sufficient excess, relative to the secondary N-methylamine. Usually, 1 to 10, in particular 1.2 to 5, mol, preferably 1.5 to 2 mol of formaldehyde are added per mole of secondary N-methylamine. In this manner, it is not only possible to remove larger amounts (> 2$ by weight) but also fairly small amounts, for example 2~ by weight and less, of secondary N-methylamine almost completely.
To remove the water formed in the N,N-dimethylamine synthesis, an organic solvent can be added to the re-~action mixture as entrainer. Suitable solvents are aliphatic hydrocarbons, hexene, cyclohexane, benzene, toluene, in particular hexene, cyclohexane and preferably hexene.
The water is separated off by distillation in one or, advantageously two or several steps. The choice of organic solvent as well as the number of distillation steps is also dependent on the type of N, N-dimethylamine .
The preparation of pure N,N-dimethylamine of > 99, in particular > 99.5, ~ by weight, by means of fractional distillation usually causes no difficulty. When using a column having 20 to 30 theoretical plates, the N,N-dimethylamine pure product is~obtained as the 4th or 5th fraction. The removal of the water can be carried out in a particularly favorable manner by separating the hetero-geneously formed water from the distillate removed as the top product and recycling the entrainer into the column.
Formaldehyde or a formaldehyde-forming substance can in - 9 _ 202362 general be used for removing secondary N-methylamines from mixtures containing secondary N-methylamines.
Formaldehyde or the formaldehyde-forming substance is added, and the batch is heated to 50 to 250°C with mixing. Even after a short period of time, the secondary N-methylamine content has substantially dropped. If a further reduction is desired, it is recommended to add formaldehyde again. Usually, an amount of 1.5 to 5.0, in particular 1.5 to 3.0, preferably 1.5 to 2.0, mol of formaldehyde is sufficient per mole of secondary N-methylamine to be removed, for reducing the secondary N-methylamine content to the required degree.
After the treatment with formaldehyde, the reaction mixture is subjected to fractional distillation, in which first any water present is separated off by distillation with the aid of a suitable entrainer. After the forerun products have been separated off, it is in most cases possible to obtain the N,N-dimethylamine as 4th or 5th fraction in a purity of > 99, in particular > 99.5, ~ by weight.
The examples described below illustrate the invention, without limiting it.
Experimental section The reactor vessel consists of a pressure-resistant tube having an inside diameter of 28 mm. The lower part of the pressure tube is filled with Raschigrings having a diameter of 3 mm up to a height of 800 mm. This zone serves as preheating zone in which the starting materials are heated to the predetermined temperature. Above this preheating zone, a fixed bed of the hydrogenation catalyst in the form of tablets is present. The desired temperature is adjusted by means of a heating mantle surrounding the reactor.
At the beginning of the reaction, the reactor is filled with N,N-dimethylamine. The starting materials aldehyde, ~~32362 dimethylamine and hydrogen are then introduced into the reactor at the bottom, flow through the preheating zone filled with Raschigrings and enter the catalyst zone, where the reaction takes place. After leaving the catalyst zone, the reaction mixture is removed at the top of the reactor and transferred to a pressurized separator, in which liquid reaction product is separated off from unconverted dimethylamine and hydrogen. The reaction mixture is then passed through a column suitable for distillation under super atmosperhic pressure and freed from residual dimethylamine and, if necessary, hydrogen. The reaction mixture removed from the bottom of this column serves as starting material for the fractional distillation. The formaldehyde is added to the bottom product formed during the separation of the forerun by distillation. The entrainer which facilitates the separation of water is then added, as a result of which two water-containing fractions are obtained before, after separating off the intermediate run, the valuable 'product is distilled off.
Example 1 Continuous preparation of N,N-dimethyl-n-butylamine 300 ml of a Ni catalyst (- 50 to 53~ by weight of Ni and about 25 to 30~ by weight of kieselguhr as support;
commercial product from Hoechst AG: RCH Ni 52/35) in tablet form are present in the reactor vessel described above.
The reactor is filled with N,N-dimethy~-n=butylamine.At 105 to 110°C and an HZ pressure of 8 MPa, 56 ml of n-butanal/h, 200 ml of dimethylamine/h and 34 N1 of hydrogen/h are then introduced into the reactor from the bottom. The resulting reaction mixture is removed at the top of the reactor vessel and separated off from excess dimethylamine and hydrogen in the downstream pressurized separator, which is under the same pressure as the reactor.

zo3z~sz -~~-The distillation under pressure which follows is carried out at 0.7 MPa and a temperature of about 130°C at the bottom, during which residual dimethylamine and any still present hydrogen are removed. The distillation is carried out at atmospheric pressure by means of a column having 30 theoretical plates.
The first fraction is obtained at a temperature of 80°C
at the top and a temperature of about 93°C at the bottom.
About 3~ by weight of formaline solution (37$ aqueous solution)/h, relative to N,N-dimethyl-n-butylamine, are fed into the bottom product.
The second fration is obtained at a temperature of 66°C
at the top and a temperature of 85°C at the bottom with the addition of hexene, and the third fraction is obtained at a temperature of 75°C at the top and a temperature of - 95°C at the bottom likewise with the addition of hexene, which serves as entrainer to remove the water. The water/hexene mixture is separated, the water is removed, and the hexene is recycled into the column.
The fraction of the intermediate run comes over at the top at - 90-92°C, while the temperature at the bottom is about 96-98°C. The pure valuable product is obtained at a temperature of 93°C at the top and a temperature of 110-112°C at the bottom. The composition of the valuable product can be seen from Table 1.
Comparative experiment The procedure as described in Example 1 is repeated, except that the addition of formaldehyde is omitted.
The composition of the valuable product can be seen from Table 1.

zo3z3s2 Table 1 Valuable Valuable product from product from Example 1 Comparative Experiment 1 Composition Hydrocarbon 0.11% by weight 0.17% by weight Forerun 0.14% by weight 0.15% by weight N,N-Dimethyl-n-butylamine 99.65% by weight 98.09% by weight N-methyl-n-butylamine 0.02% by weight 1.22% by weight After-run 0.07% by weight 0.37% by weight As can be seen from the above analysis determined by gas chromatography (calculated as anhydrous content), the addition of formaldehyde causes a very considerable reduction of the sec. N-methylamine (here N-methyl-n-butylamine) content.
Example 2 Use of formaldehyde for removing N-methyl-n-butylamine In a 2-1 three-neck flask equipped with a stirrer, a thermometer and a reflux condenser, 1,000 g of N,N-dimethyl-n-butylamine (content 98.3% by weight) obtained by fraction distillation and contaminated with 1.5% by weight of N-methyl-n-butylamine no longer separ-able by distillation are initially introduced. 30 g of formalise solution (37% by weight of formaldehyde in water) corresponding to 1.1% by weight of formaldehyde, relative to the amine, are then added.
The mixture is heated to the reflux temperature with stirring, the reaction is allowed to proceed for 30 minutes, and the mixture is then cooled. According to analysis by gas chromatography (calculated as anhydrous zo3z36z s.~- - 13 -content), the product formed has now 99.6 by weight of N,N-dimethyl-n-butylamine. Secondary N-methylamine (here N-methyl-n-butylamine) can no longer be detected by gas chromatography.

Claims (11)

1. A process for the preparation of N,N-dimethylamines by reaction of an aldehyde with dimethylamine and hydrogen under pressure and at a temperature of from 50 to 150°C in the presence of a hydrogenation catalyst, which comprises separating off unconverted dimethylamine, then adding 0.1 to 25% by weight of formaldehyde or a formaldehyde-forming substance, relative to N,N-dimethylamine, and distilling the mixture.

2. The process as claimed in claim 1, wherein the aldehyde is reacted with dimethylamine and hydrogen in a molar ratio of 1 : (1 to 10) : (1 to 50), at 1 to 20 MPa and 70 to 130°C.

3. The process as claimed in claim 1, wherein the hydrogenation catalyst is a support-free metal catalyst or a supported catalyst containing Al2O3, SiO2, siliceous earth, activated carbon or pumice, as support,

4. The process as claimed in any one of claims 1 to 3, wherein the hydrogenation catalyst contains Ni, Co, Cu, Mn, Fe, Rh, Pd and/or Pt.

5. The process as claimed in any one of claims 1 to 3 wherein unconverted dimethylamine is separated off by distillation first at 0.5 to 1.0 MPa and then at 0.05 to 0.15

6. The process as claimed in any one of claims 1 to 3 wherein 1 to 10% by weight of formaldehyde or a formaldehyde-forming substance, relative to N,N-dimethylamine, are added.

7. The process as claimed in any one of claims 1 to 3 wherein formaldehyde is added at 0.05 to 1.0 MPa and 50 to 250°C.

8. The process as claimed in any one of claims 1 to 3 wherein the distillation is carried out in the presence of a water-entraining solvent.

9. A process according to claim 4 wherein the hydrogenation catalyst further comprises a promotor selected from alkaline earth metal oxides, SiO2, Al2O3, MnO2 and Cr2O3.

10. A process according to claim 8 wherein the distillation is carried out in a plurality of steps.

11. A method of removing a secondary N-methylamine from a mixture containing a secondary N-methylamine and a N,N-dimethylamine, which method comprises adding to the mixture 0.1 to 25% by weight of formaldehyde or a formaldehyde-forming substance, relative to N,N-dimethylamine, at 50 to 250°C and 0.05 to 1.0 MPa and removing secondary N-methylamine.