Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2038~2~
O.Z. 41479
METHOD OF IMPROVING THE QUALITY
OF CRUDE DIAMINODIPHENYLMETHANES
The present invention relates to a method of improving
the quality of crude diaminodiphenylmethanes.
The industrial mixture coming from the acid-catalyzed
reaction of aniline with formaldehyde and containing various
condensates (diaminodiphenylmethanes, normally referred to as
'MDA') mainly consists of the binuclear p,p'-isomer
H2N~CH2 ~NH2
MDA is chiefly used for the synthesis of corresponding
diphenylmethane diisocyanates ('MDI') by reacting it with
phosgene. MDI itself is one of the most important intermediates
in the preparation of polyurethanes.
It is well known, e.g. from DE-A 3,329,124 and US-A
3,479,384, that phosgenation of MDA leads to highly undesirable
discoloration of the product, which discoloration remains when
the MDI is processed to polyurethanes.
The proposals of the above patents involve subjecting
the crude MDI to certain purifying extraction processes. These
processes, however, suffer from the drawback that the production
of undesirable components causing discoloration is not avoided,
so that such components have to be removed in an additional
process step and collected for disposal.
US-A 4,465,639 proposes a method of brightening MDI
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comprising treating the reaction solution with a little water
immediately after the MDA has been reacted with phosgene but
before the excess phosgene has been removed. It is then not
necessary to work up the solution by extraction, but the hydrogen
chloride formed leads to corrosion problems.
It is thus an object of the present invention to avoid,
from the outset, any discoloration of the product obtained when
MDA is treated with phosgene and thus to make any further,
expensive purification of the crude MDI unnecessary.
Accordingly, we have found a method of improving the
quality of crude diaminodiphenylmethanes comprising the treatment
thereof with hydrogen in the presence of a hydrogenation
catalyst.
The aqueous reaction mixture produced by the reaction
of aniline and formaldehyde to form MDA can be immediately
subjected to the hydrogenation treatment, and the excess aniline,
water and other low-boiling components can then be distilled off.
However, we recommend that the treatment be effected on crude MDA
from which aniline and water have been removed. Such MDA usually
has the following composition:
50-95% w/w of 4,4'-diaminodiphenylmethane,
1-40% w/w of 2,4'-diaminodiphenylmethane,
0.1-10% w/w of 2,2'-diaminodiphenylmethane,
10-40% w/w of tri- or quadri-nuclear MDA homologues and
1-10% wow of higher molecular weight MDA homologues and
unidentified substances.
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The conditions used during the hydrogenation treatment
are not critical, although it is obvious that severe conditions
causing damage to the aromatic ring are to be avoided.
In general, a hydrogen pressure of from 1 to 300 bar,
preferably from 10 to 200 bar and more preferably from 20 to 60
bar, and a temperature of from 20 to 400C, preferably from 700
to 320C, are to be recommended, so that the crude MDA remains
liquid even at atmospheric pressure.
Suitable hydrogenation catalysts are any of those
conventionally used for catalytic hydrogenation, in the form of
solid catalysts or supported catalysts.
A particularly suitable active material is a platinum
metal, especially platinum and palladium. In addition, all
conventional hydrogenation catalysts are suitable, for example
Raney nickel, a metal, an oxide of tungsten or molybdenum, or a
mixture of metals such as a mixture of nickel and molybdenum.
Suitable supports are, for example, aluminum oxide,
silica gel, silicates, active carbon and zeolites.
The amount of catalyst used is advantageously from
0.001 to 10% w/w of active material, based on the weight of crude
MDA.
The process may be carried out continuously or
batchwise. In the latter case, the time required until no more
hydrogen is absorbed is usually from 10 minutes to 2 hours.
The reaction is advantageously carried out without the
addition of solvent, but it is possible to use additional
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solvent, for example a hydrocarbon such as n-heptane and
cyclohexane, an alcohol such as methanol and ethanol, or an ether
such as tetrahydrofuran and 1,4-dioxane, in a concentration of
from 0.05 to 5 kg per kg of MDA. Particularly good results are
obtained when the hydrogenation treatment is effected in the
presence of from 0.5 to 20% w/w of water.
MDI prepared from MDA which has been treated by
hydrogenation is pale to dark yellow and can be used in the
preparation of polyurethanes without further purification.
example 1
Batchwise treatment of crude MDA by hydrogenation
500 g of crude MDA, which had been prepared by
condensation of aniline and formaldehyde under acid conditions
and from which water and excess aniline had been removed, and
which contained 55% w/w of 4,4'-diaminodiphenylmethane, 3% w/w of
2,4'-diaminodiphenylmethane, 1% w/w of 2,2'-
diaminodiphenylmethane, 25% w/w of tri-nuclear MDA compounds, 10
w/w quadri-nuclear MDA compounds and 6% w/w of MDA compounds
having a higher number of nuclei and unidentified substances,
were subjected to hydrogenation treatment for 2 hours at 80~C
under a hydrogen pressure of 50 bar and in the presence of 5 g of
a powdered supported catalyst consisting of 5% wjw of palladium
and 95~ w/w of aluminum oxide, after which the catalyst was
separated off.
E~pl~ 2
Batchwise treatment of crude MDA
203~:L26
To an anhydrous and aniline-free MDA mixture of the
composition stated in Example 1 there were added 25 g of water.
This mixture was subjected to hydrogenation treatment for 2 hours
at 100C under a hydrogen pressure of 50 bar and in the presence
of 5 g of a powdered supported catalyst consisting of 5% w/w of
palladium and 95% w/w of aluminum oxide, after which the catalyst
was separated off.
example 3
Continuous treatment of crude MDA by hydrogenation
50 g/h of crude MDA as used in Example 1 were passed
continuously through 50 g of a fixed bed catalysts consisting of
3% w/w of palladium on aluminum oxide while hydrogenation was
effected under a hydrogen pressure of 50 bar at 150C.
Example
Continuous treatment of crude MDA by hydrogenation
50 g/h of a water-containing and aniline-containing MDA
mixture containing 42% w/w of 4,4'-diaminodiphenylmethane, 2% w/w
of 2,4'-diaminodiphenylmethane, 0.5% w/w of 2,2'-
diaminodiphenylmethane, 19% w/w of trinuclear MDA compounds, 8
w/w of quadrinuclear MDA compounds, 5% w/w water, 20% w/w of
aniline and 3.5~ w/w of NDA compounds having more than 4 nuclei
and unidentified substances were continuously passed through 50 g
of a fixed bed catalyst consisting of 5% w/w of nickel and 12.7%
w/w of molybdenum on alumina while hydrogenation was effected
under a hydrogen pressure of 30 bar at 300C.
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Example 5
Quality tests on the pure MDA obtained in Examples 1 to 4
The MDA obtained in Examples 1 to 4 was distilled to
remove any water and~or aniline present and then treated with
chlorobenzene saturated with phosgene under a phosgene pressure
of 2 bar to produce MDI by reaction with phosgene. Samples of
the MDI were diluted with 5 times their volume of chlorobenzene,
and the color indices of the resulting solutions were determined
as specified by DIN 6162.
For control purposes, the color index was determined on
an MDI prepared by anhydrous, aniline-free crude MDA which had
not been subjected to hydrogenation treatment. The results of
these tests are given in the Table below.
TABLE
MDI prepared Color Color Index
from MDA (determined visually) (determined by DIN 6162)
of the invention:
Example 1 pale yellow 40
Example 2 pale yellow 20
Example 3 pale yellow 20
Example 4 pale yellow 50
for comparison:
with no hydrogena-
tion treatment dark brown 180
