Note: Descriptions are shown in the official language in which they were submitted.
2032362
""'" 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
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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
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3 24325-174
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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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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':
203236
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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
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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.
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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
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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
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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
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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,
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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.
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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.
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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
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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.
