Note: Descriptions are shown in the official language in which they were submitted.
CA 02305282 2000-03-28
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METHOD FOR PURIFYING 4-AMINO-PIPERIDINES
The present invention relates to a process for purifying
2,2,6,6-tetrasubstituted 4-aminopiperidines which have been
prepared from the corresponding piperidin-4-ones with subsequent
distillation.
In particular, the invention relates to a process for purifying
4-aminopiperidines of the formula I
NH2
I,
R1 N~ R3
R2 H R4
where R1 to R4 are C1-C6-alkyl, R1 and R2 and/or R3 and R4 together
form a CHZ chain having from 2 to 5 carbon atoms, which have been
prepared from the corresponding piperidin-4-ones of the formula
II
0
II,
R1 N~ R3
2 0 R2 H R4
where the radicals R are as defined above, with subsequent
distillation.
The 4-aminopiperidines which, owing to 2,2,6,6-substitution, eg.
tetraalkyl substitution, are designated as sterically hindered,
are used in a variety of applications. In particular, they serve
as intermediates in the preparation of UV stabilizers for
synthetic polymers (see, for example: R. Gachter, H. Miiller
(editors): Taschenbuch der Kunststoffadditive, Carl Hanser
Verlag, Munich, 1979; F. Gugumus, Polym. Degrad. Stabil. 44
(1994), 299 - 322) and as chain regulators and stabilizers in the
30 preparation of polyamides (cf., for example: w0 952 84 43, DE-A-
4 413 177).
0050/48395 CA o23os2s2 2000-o3-2s
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It is of particular importance that the sterically hindered
4-aminopiperidines have not only a high chemical purity but also
no or very little intrinsic color and that no discoloration takes
place over a storage time of a number of months. This applies
particularly when the 4-aminopiperidine is used for the
preparation of stabilizers or employed directly as additive,
since the product quality of these and thus also the quality of
the stabilized polymers depends decisively on the chemical purity
and, in particular, the color quality.
The sterically hindered 4-aminopiperidines are generally prepared
industrially from dialkyl ketones, eg. acetone or acetone
derivatives. The piperidines I can be obtained in a single step
from the compounds
R1 O R3
R R4
by a catalytic ring-closure reaction in the presence of ammonia
and hydrogen (DE-A-2 412 750) or from piperidin-4-ones II
O
II,
R1 N~ R3
R2 H R4
by aminative hydrogenation in one or two stages, for example
catalytically. (See, for example DE-A-35 25 387, DE-A-2 040 975,
DE-A-2 349 962, DE-A-26 21 870, EP-A-33 529, EP-A-42 119,
Ep-A-303 279, EP-A-410 970, EP-A-611 137, EP-A-623 585 and
DE-A-42 10 311).
The sterically hindered 4-aminopiperidines produced industrially
are generally purified by distillation. (See, for example EP-A-
33 529).
However, troublesome amounts of substances which give a color are
still present, or are formed after a short time, in the
4-amino-2,2,6,6-tetraalkylpiperidines which are prepared using
conventional processes.
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According to DE-A-195 55 58, EP-A-28 555, US 3 819 710 and JP
01,160,947 CChem. Abstr. 111: 232081r), color-producing compounds
in certain aminoalcohols, which compounds are formed in the
preparation of the aminoalcohols from ethylene oxide and an
appropriate amine, can be converted into less color-producing
compounds by catalytic hydrogenation.
JP 06 25,410 CChem. Abstr. 12I: 56988n), JP-OS 48-52708 CChem.
Abstr. 80: 36697y), JP 05,345,821 CChem. Abstr. 120: 272327t), US
5 362 914 and EP-A-262 562 disclose that discolorations in
certain polyamines can be reduced by treatment with hydrogen in
the presence of hydrogenation catalysts.
DE-A-22 05 958 discloses a process for purifying tertiary amines
which have been obtained by reacting primary alcohols with
ammonia, by hydrogenation of the impurities before the final
distillation of the tertiary amine.
The abovementioned compounds are different classes of substances
from those of the sterically hindered 4-amino-piperidines of the
present invention. Furthermore, the origin of the color producing
impurities in the abovementioned compounds lies in the specific
preparative processes and starting materials employed in each
case.
Japanese Patent Application JP-OS 02-311 457 describes the
purification of 2,2,6,6-tetraalkyl-4-piperidinones by treatment
with hydrogen in the presence of a hydrogenation catalyst.
According to the examples given in this application, the process
product (triacetoneamine, TAA) is so color-unstable that it has a
much poorer color quality than the starting material after just
one week's storage at 60°C.
The development of preparative methods for chemically pure
sterically hindered 4-aminopiperidines having low discoloration
has taken a completely different route:
DD-A-266 799 teaches reacting 4-amino-2,2,6,6-tetramethyl-
piperidine in acetone/water solution with C02, separating off the
precipitate, washing with acetone, subsequently decomposing
thermally and purifying the product by distillation.
SU 18 11 527 describes a different purification process for
sterically hindered 4-aminopiperidines I. In that process, the
contaminated crude product is dissolved in an aprotic solvent,
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reacted with ethylene glycol, the reaction product is isolated,
the 4-aminopiperidine is set free using aqueous alkali and is,
after removal of the aqueous phase, fractionally distilled.
Both processes are extraordinarily complicated and costly.
Finally, the earlier German Application No. 19622269.9 describes
a distillation process for purifying crude 4-amino-piperidines I
by' in a first step, removing high-boiling substances and
possibly water from the crude piperidines by distillation, in a
second step adding from 0.01 to 5~ by weight, based on the
product of the first step, of a reducing agent, for example
sodium borohydride, and, in a third step, isolating the
plperidines I by distillation.
It is an object of the present invention to provide an
inexpensive process which is simple to carry out and by means of
which colorless, color-stable and highly pure sterically hindered
4-aminopiperidines, eg. of the formula I, can be made available.
This means products whose very low intrinsic coloration is
maintained for a prolonged period and which have, at the same
time, a low by-product content and contain no stabilizing
auxiliaries. The reference to the 4-aminopiperidines I as
"colorless" includes a minimum discoloration, namely up to a
color number of at most 40 APHA (measured in accordance with
DIN-ISO 6271).
We have found that this object is achieved by a process for
purifying 2,2,6,6-tetrasubstituted 4-aminopiperidines which have
been prepared from the corresponding piperidin-4-ones with
subsequent distillation, which comprises reacting the distilled
2,2,6,6-tetrasubstituted 4-aminopiperidine with hydrogen in the
presence of a hydrogenation or dehydrogenation catalyst and then
again separating the 4-aminopiperidine from the reaction mixture.
In the sterically hindered 4-aminopiperidines I, the radicals R1,
RZ, R3 and R4, independently of one another, are preferably
C1-C3-alkyl, for example ethyl or methyl, in particular methyl.
The pure but discolored and/or color-unstable 4-amino-piperidine,
eg. of the formula I, which has been obtained according to known
methods by reaction of the corresponding piperidin-4-one, for
example by aminative hydrogenation, and subsequent fractional
rectification, is, according to the present invention, reacted
with hydrogen in the presence of a hydrogenation catalyst, for
example at from 20 to 200~C. Higher temperatures are also
0050/48395 ca o23os2s2 2000-o3-2s
possible. Preference is given to a reaction temperature in the
range from 50 to 150~C.
The reaction of the 4-aminopiperidine with hydrogen can be
5 carried out at atmospheric pressure or preferably under
superatmospheric pressure, for example from 0.1 to 50 MPa. Higher
pressures are also possible. Preference is given to a reaction
pressure of from 1 to 30 MPa, in particular from 1 to 20 MPa.
The reaction of the 4-aminopiperidine with hydrogen can be
carried out in the presence of a solvent or a solvent mixture
which is inert under the reaction conditions, or preferably in
the absence of solvent. When using a solvent, for example
tetrahydrofuran, dioxane, 1,2-dimethoxyethane, methanol or
ethanol, the reaction conditions are selected such that the
solvent is present in liquid form in the reaction mixture.
As regards the hydrogenation catalysts which can be used, there
are no restrictions. Dehydrogenation catalysts can also be used.
Suitable hydrogenation catalysts are, for example, catalysts
which comprise copper, silver, gold, iron, cobalt, nickel,
ruthenium, rhodium, palladium, rhenium, osmium, iridium,
platinum, chromium, molybdenum or a mixture thereof.
The hydrogenation catalyst used can be homogeneously dissolved in
the 4-aminopiperidine or in the mixture of 4-aminopiperidine and
solvent, for example when using tris(triphenylphosphine)rhodium
chloride, or, particularly preferably, be present in undissolved
form, ie. heterogeneous.
Suitable heterogeneous catalysts are support-free catalysts such
as Raney nickel or Raney cobalt, with the latter being present as
a suspension in the reaction mixture, or supported catalysts
which are preferably arranged as a fixed bed in the reactor. In
the case of the supported catalysts, suitable support materials
for the catalytically active metals or metal compounds are, for
example, carbon, aluminum oxide, silicon dioxide, titanium
dioxide, zirconium dioxide, zinc oxide, magnesium oxide, silicon
carbide, zeolites or mixtures thereof.
Examples which may be mentioned are supported catalysts
comprising copper and/or cobalt and/or nickel and aluminum oxide
and/or zirconium dioxide.
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For example, the two supported catalysts having the following
compositions can be employed:
76% by weight of A1, calculated as A1203, 4% by weight of Cu,
calculated as CuO, 10% by weight of Co, calculated as CoO, and
10% by weight of Ni, calculated as Ni0 (as described in DE-A-
1 953 263)
°r
31.5% by weight of Zr, calculated as Zr02, 50% by weight of Ni,
calculated as NiO, 17% by weight of Cu, calculated as CuO, and
1.5% of Mo, calculated as Mo03 (as described in EP-A-696 572).
The necessary residence time of the 4-aminopiperidine over the
catalyst is determined, inter alia, by the degree of
discoloration of the distilled 4-aminopiperidine and by the
desired decolorization and/or color stability of the piperidine.
As a rule, the residence time is longer, the higher the degree of
discoloration of the 4-aminopiperidine used in the process step
of the present invention and the higher the color quality
requirements for the product.
Depending on the reaction conditions selected, residence times of
from 10 minutes to a few hours are generally sufficient.
The process step of the present invention can be carried out
either continuously, for example in tube reactors, stirred
vessels or cascades of stirred vessels, or batchwise, for example
in stirred vessels.
After carrying out the process step of the present invention, the
4-aminopiperidine is separated from the hydrogenation catalyst
and from any solvent used.
Hydrogenation catalysts dissolved in the reaction product are
separated off by distillation, with any solvent used and the
4-aminopiperidine being taken off in succession at the top in a
customary manner.
Hydrogenation catalysts used in suspension are separated off by
decantation and/or filtration. The reaction product can
subsequently be distilled, which is in any case necessary if a
solvent is used.
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When using a hydrogenation catalyst arranged as a fixed bed, the
reaction product is advantageously filtered in order to remove
any abraded catalyst present. The reaction product can
subsequently be distilled, which is in any case necessary if a
solvent is used.
The process of the present invention gives, in a simple,
inexpensive manner, pure sterically hindered 4-aminopiperidines
which have virtually no intrinsic color and remain color-stable
on storage, so that the addition of stabilizers is superfluous.
Examples
The color of the sterically hindered 4-aminopiperidines was
determined by measuring the APHA color number in accordance with
DIN-ISO 6271.
In all examples, the storage of the TAD samples was carried out
under comparable conditions (room temperature, in the dark).
Example 1 (Comparative Example):
1.632 kg of crude synthetic product having the composition (in %
by weight)
85.6% of 4-amino-2,2,6,6-tetramethylpiperidine
(triacetonediamine, TAD)
9.0% of Hz0
about 0.7% of NH3
1.4% of 4-hydroxy-2,2,6,6-tetramethylpiperidine
(triacetoneaminoalcohol,TAA-ol)
3.3% of by-products including high boilers,
which had been obtained by aminative hydrogenation of
2,2,6,6-tetramethylpiperidin-4-one (triacetoneamine, TAA) [GC
purity: 99%; APHA color number: about 370 (measured as a 1%
strength solution in ethanol)] at 165°C, 12 MPa HZ, in the
presence of a heterogeneous catalyst comprising 76% by weight of
A1, calculated as A1203, 4% by weight of Cu, calculated as CuO,
10% by weight of Co, calculated as CoO, and 10% by weight of Ni,
calculated as Ni0 (as described in DE-A-1 953 263), was rectified
in a distillation apparatus comprising a 2.0 1 still pot with
double-wall heating, a 70 cm mirrored glass column filled with 3
mm V2A wire helices (3.5 cm internal column diameter) and a
column head with liquid splitter.
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As main fraction, 1.191 kg of slightly yellowish TAD having a
purity of 99.9 (according to GC) were obtained at a pressure of
40-42 hPa and a temperature at the top of 99-102~C.
The TAD obtained became increasingly yellow during the following
days, ie. it was not color-stable.
Storage time in days APHA color number
5 145
70 268
Example 2:
In a 300 ml autoclave with stirrer and basket insert, about 70 g
(0.45 mol) of yellowish TAD (purity according to GC:> 99.9%; APHA
color number: 72), which had been obtained as described in
Example 1, was treated with hydrogen at a pressure of 10 MPa for
6 hours at 90~C in the presence of a heterogeneous catalyst
comprising 76% by weight of A1, calculated as A1z03, 4% by weight
of Cu, calculated as CuO, 10% by weight of Co, calculated as Co0
and 10% by weight of Ni, calculated as Ni0 (4 mm extrudates),
which was present as a fixed bed in the basket insert.
The reaction product was then filtered. The purity of the
colorless TAD obtained was 99.9% according to GC.
The APHA color number of the TAD was < 1 after a storage time of
50 days. Even after a storage time of 147 days, the TAD appeared
colorless to the eye, ie. the APHA color number was < 10.
Example 3:
Carrying out the experiment as in Example 2 but using 5 g of
Raney nickel as heterogeneous catalyst (suspended) gave, after
filtration, colorless TAD having a purity of 99.9% according to
GC and an APHA color number of 17 after a storage time of 50
days. After a storage time of 148 days, the APHA color number was
23.
45
