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This invention provides a process for the purification of diphenyl
carbonate (DPC) from crude products of diphenyl carbonate production, in
which process crude products are used which have a distillable fraction
consisting of over 70 wt. % diphenyl carbonate and they are fractionally
crystallised from the melt (fractionating melt crystallisation).
Diphenyl carbonate may, for example, be produced by reacting phenol with
phosgene in the presence of equimolar quantities of aqueous sodium
hydroxide solution, by phosgenating phenol in the presence of catalyst, by
transesterification of phenol with dimethyl carbonate or oxidative
carbonylation of phenol.
Diphenyl carbonate may be produced and purified by distillation, extraction
of phenol with water or by crystallisation (GB-A 1 096 936, U.S. Pat. No.
4,013,702, U.S. Pat. No. 5,239,106).
A crystallisation process is described, for example, in U.S. Pat. No.
5,239,106. This process exhibits various disadvantages:
Since it is not pure diphenyl carbonate, but an adduct with phenol, which
is obtained from crystallisation, the crystallised product must be
purified by distillation in a further processing stage. This process may
only be used to work up crude mixtures having a low solids content, i.e. a
low diphenyl carbonate content, because the mixtures are otherwise
impossible to handle.
The educt material for crystallisation may thus only have a diphenyl
carbonate content of less than 70 wt. %, preferably of no more than 50 wt.
% (loc. cit., column 2, lines 40 to 53 and example 5), so that the crystal
slurry may still be sufficiently separated from the mother liquor by
filtration.
This is all the more disadvantageous as present-day industrial syntheses
yield crude products which have distinctly higher diphenyl carbonate
contents.
A process has now been found for the purification of diphenyl carbonate
(DPC) from crude products of diphenyl carbonate production which have
elevated diphenyl carbonate contents of above 70 wt. %, relative to the
distillable fraction, by fractional melt crystallisation. Surprisingly,
the process according to the invention is suitable for the purification of
diphenyl carbonate crude products from various production processes,
although these contain impurities and by-products of an entirely different
nature.
The present invention provides a process for the purification of diphenyl
carbonate by crystallisation from crude products of diphenyl carbonate
production which have elevated diphenyl carbonate contents of above 70 wt.
%, relative to the distillable fraction, by fractionating melt
crystallisation, characterised in that the melt to be purified is cooled
in the range from 85.degree. to 45.degree. C., preferably from 80.degree.
to 48.degree. C., at a cooling rate of 20.degree. to 0.1.degree. C./h,
preferably of 10.degree. to 0.5.degree. C./h, a holding time of 0 to 100
minutes, preferably of 1 to 70 minutes, is maintained at the lowest
coolant temperature before separation of the residual melt, the
crystallised product is then melted by heating at a heating rate of 20 to
0.1.degree. C./h, preferably of 10.degree. to 0.5.degree. C./h, to a final
temperature of 70.degree. to 80.degree. C., preferably of 72.degree. to
79.5.degree. C. and, during heating, further fractions of the melt with
impurities are separated at pauses or without interruption of heating from
the pure DPC melt arising at a higher temperature.
The process according to the invention may be used for the purification of
diphenyl carbonate crude products from the processes listed below.
In the phase interface phosgenation of phenol, a crude product is produced
containing over 99 wt. % DPC, which contains chloroformic acid phenyl
ester as by-product.
A diphenyl carbonate crude product containing >95 wt. % DPC is obtained
from the direct catalytic phosgenation of phenol. Here too, chloroformic
acid phenyl ester and salicylic acid phenyl ester are produced as
by-products.
Transesterification of dimethyl carbonate (DMC) with phenol yields, for
example, reaction products containing 0 to 2 wt. % DMC, 1.5 to 10 wt. %
phenol, 1 to 20 wt. % methylphenyl carbonate (MPC), 70 to 98 wt. %
diphenyl carbonate and ppm concentrations of by-products such as salicylic
acid phenyl ester, salicylic acid methyl ester and anisole. This mixture
additionally optionally contains catalysts or residues thereof, i.e.
titanium or tin compounds.
Depending upon the reaction conditions, running times and catalyst
concentrations, oxidative carbonylation of phenol yields DPC crude
solutions containing 20 to 90 wt. % phenol, 10 to 80 wt. % DPC and,
additionally, low concentrations of by-products such as salicylic acid
phenyl ester, o-phenylphenol and diphenyl ether. These mixtures
additionally still contain components of the catalyst system, i.e. for
example tetrabutylammonium bromide, sodium phenolate, manganese, Co or Cu
and palladium compounds.
Crude solutions containing 70 to 85 wt. % DPC, with which the purification
processes according to the invention may advantageously be performed, may
also be obtained from the dilute solutions of this process by distilling
off the phenol or by simple precrystallisation.
The quantity of diphenyl carbonate in the distillable fraction of the crude
product is thus above 70 wt. %, preferably above 75 wt. %, particularly
preferably above 80 wt. %.
The products obtained from the crystallisation according to the invention
may be handled without problems; they are not phenol/diphenyl carbonate
adducts and consequently need not be transformed into the pure diphenyl
carbonate by additional distillation.
The process according to the invention of fractionating melt
crystallisation of impure diphenyl carbonate may be performed both
discontinuously and continuously as a single or multiple stage process.
The purification of diphenyl carbonate according to the invention may be
performed using crystallisation processes as are, for example, described
in Chem. Ing. Techn. 57 (1985), 91, Chem. Ing. Techn. 63 (1991), 881,
Ullmann's Encyclopedia, 4th edition, volume 2, page 672 et seq. and 5th
edition, volume 32, pages 3 et seq. or in the fractional crystallisation
product literature from the company Sulzer dated August 1992.
Static and dynamic suspension and film crystallisation processes,
preferably dynamic processes, may be used to purify diphenyl carbonate.
The process according to the invention may, for example, be performed in
multi-tube crystallisers or modified plate-type heat exchangers of various
designs with or without recirculation of the melt, with and without using
pulsations or with and without subdivision of the tubes into segments with
separate discharge. Falling film crystallisers of various designs, for
example those known from EP-A 218 545, may also be used. Further apparatus
which may be used comprises bubble column crystallisers, crystallising
rolls and belts. Further details concerning continuously operating,
suitable crystallisation devices may be found in EP-A 521 499.
Multi-tube crystallisers, plate-type heat exchangers of various designs
with and without recirculation of the melt or falling film crystallisers
are preferably used for the process according to the invention.
The crystallisation operation in the process according to the invention for
purification of a diphenyl carbonate produced in accordance with the
above-mentioned processes may be initiated both by spontaneous nucleation
and by controlled addition of crystallisation nuclei (seeding).
Crystallisation is preferably initiated by crystal nuclei.
The process according to the invention for obtaining purified diphenyl
carbonate by melt crystallisation may be associated with purification
processes involving distillation. It is thus possible, before using the
melt crystallisation, in particular when separating diphenyl carbonate
from reaction solutions optionally containing catalysts, to distil the
diphenyl carbonate and subsequently to perform melt crystallisation
according to the invention.
Moreover, on completion of crystallisation, it is also possible to remove
small quantities of low-boiling constituents from the purified diphenyl
carbonate by simple distillation.
Any catalysts present may largely be separated from the diphenyl carbonate
by precipitation and filtration.
The process according to the invention may also be combined with other
simple crystallisation processes, for example that described in U.S. Pat.
No. 5,239,106, wherein mixtures are obtained which have already been
concentrated in diphenyl carbonate, which are then further purified using
the process according to the invention.
If the process according to the invention is performed in multi-tube
crystallisers or plate-type heat exchangers, the melt to be purified is
cooled in the range from 85.degree. to 45.degree. C., preferably from
80.degree. to 48.degree. C. at a cooling rate of 20.degree. to 0.1.degree.
C./h, preferably of 10.degree. to 0.5.degree. C./h. During this cooling
phase, the crystallisation operation is initiated by spontaneous
nucleation or by controlled addition of nuclei (seeding), preferably by
seeding. At the lowest temperature of the coolant, a holding time of up to
100 minutes is optionally maintained before separation of the residual
melt. In both variants, the holding time is 0 to 100 minutes, preferably 1
to 70 minutes. The residual melt is then separated and the crystallised
product is further purified by heating at a heating rate of 20.degree. to
0.1.degree. C./h, preferably of 10.degree. to 0.5.degree. C./h, to a final
temperature of 70.degree. to 80.degree. C., preferably of 72.degree. to
79.5.degree. C.
In order to improve the effectiveness of purification, further pauses may
act during this heating phase to separate further melt fractions and
impurities melted prior to theses pauses may be removed together with the
melt. In another embodiment, the initially occurring melt with the
remaining exuded impurities may be separated during the heating operation
without interruption of heating and are thus be separated from the pure
DPC melt which arises at a higher temperature.
The diphenyl carbonate purified according to the invention may be used, for
example, for the production of transesterification products such as
polycarbonates (for example from bisphenol A).
EXAMPLES
Fractionating melt crystallisation operations are performed in the
following examples. The produced diphenyl carbonate melt is introduced
into a vertical, jacketed tube 150 cm in height and with an internal
diameter of approximately 3 cm and, starting from 80.degree. to 85.degree.
C., is cooled in accordance with a certain cooling rate (.degree. C./h).
The melt is then seeded with diphenyl carbonate crystals. Once a layer of
crystals has formed, the residual melt is drained away and "sweating" of
the crystals is then begun by heating at a certain rate, wherein further
melt drips out. This operation is terminated at a temperature of
45.degree. to 70.degree. C., heating is then continued, the crystals
remaining in the tube are melted and this purified diphenyl carbonate melt
is collected in a separate vessel.
Example 1
A reaction product arising from transesterification of phenol with dimethyl
carbonate and containing approximately
1 wt. % dimethyl carbonate
5 wt. % methylphenyl carbonate
5 wt. % phenol
84 wt. % diphenyl carbonate
5 wt. % titanium tetraphenolate
was changed into a composition of
94 wt. % diphenyl carbonate
6 wt. % titanium tetraphenolate
by distilling off the more readily volatile components, was introduced into
the tube crystalliser, initially maintained at 78.0.degree. C. and then
cooled at 2.0.degree. C./h. At 76.8.degree. C., the melt was seeded with
some crystals of diphenyl carbonate. When the melt reached 70.2.degree.
C., it was discharged and the heating medium then reheated at 2.degree.
C./h. Once a temperature of 78.1.degree. C. had been reached, the mass of
crystals remaining in the tube was melted and collected separately. After
this first crystallisation stage, the melt contained only 0.3 wt. % of
catalyst at a crystallisation yield of 55% and only 330 ppm of catalyst
after a second similar stage.
Example 2
The reaction product with the composition stated in example 1 was distilled
off from the catalyst and introduced into crystallisation with a
composition of approximately
89 wt. % diphenyl carbonate
5 wt. % methylphenyl carbonate
5 wt. % phenol
1 wt. % dimethyl carbonate
140 ppm phenyl salicylate.
Starting from 70.degree. C., cooling was performed at 5.degree. C./h. When
the melt reached 51.0.degree. C., it was discharged and reheating was then
performed at 3.degree. C./h. Once the discharging melt reached a
temperature of 75.7.degree. C., the crystal deposit remaining in the tube
was melted and collected separately. This amounted to 55.2% of the
introduced product and contained
98.65 wt. % diphenyl carbonate
0.61 wt. % methylphenyl carbonate
0.72 wt. % phenol
0.02 wt. % dimethyl carbonate
phenyl salicylate (<10 ppm)
The small quantities of low-boiling constituents may readily be reduced to
critical values of <0.02% by distillation.
Example 3
The reaction product with the composition stated in example 1 was cooled to
60.degree. to 70.degree. C. and the precipitated catalyst, titanium
tetraphenolate, was removed by suction filtration.
A mixture was obtained with a composition of approximately
89 wt. % diphenyl carbonate
5 wt. % methylphenyl carbonate
5 wt. % phenol
1 wt. % dimethyl carbonate
0.01 wt. % titanium tetraphenolate
121 ppm phenyl salicylate
which was introduced into crystallisation and cooled from 72.0.degree. C.
to 62.0.degree. C. at a rate of 2.degree. C./h. Once the mother liquor had
drained away, the tube was reheated at 2.degree. C./h until the
discharging melt was at a temperature of 78.0.degree. C. The crystallised
product remaining in the tube was then melted and collected separately.
This amounted to 42% of the introduced quantity and contained
99.50 wt. % diphenyl carbonate
0.18 wt. % methylphenyl carbonate
0.31 wt. % phenol and
10 ppm titanium tetraphenolate.
The product purified in this manner is crystallised once more using the
same process. A diphenyl carbonate is obtained with a purity of 99.98 wt.
% which contains below <3 ppm of titanium tetraphenolate.
Example 4
Example 3 was repeated with the difference that the cooling rate from
70.degree. C. to 50.9.degree. C. was 5.degree. C./h and the heating rate
from 50.9.degree. C. to 76.1.degree. C. was 3.degree. C./h, and that
seeding was performed with diphenyl carbonate at 68.5.degree. C. Once a
melt temperature of 76.1.degree. C. had been reached, the crystals
remaining in the tube were melted and the melt collected separately. This
amounted to 48.3 wt. % of the introduced material and contained
98.80 wt. % diphenyl carbonate
0.42 wt. % methylphenyl carbonate
0.81 wt. % phenol
0.02 wt. % dimethyl carbonate
13 ppm titanium tetraphenolate
phenyl salicylate not detectable (<10 ppm)
Comparative Example 1
Example 4 of U.S. Pat. No. 5,239,106 was replicated:
The mixture of 54.1 wt. % diphenyl carbonate, 44.6 wt. % phenol and 1.3 wt.
% phenyl salicylate was melted, homogenised and cooled from 100.degree. C.
to 44.degree. C. within 60 minutes while being stirred, wherein a thick
crystal slurry was produced. This was suction filtered through a sintered
filter maintained at exactly 44.degree. C. with a thermostat and very
thoroughly squeezed out. Once the mother liquor had stopped dripping out,
the crystals were weighed. 43.2 g of crystals were obtained. These were
remelted to ensure homogeneity and a sample of the melt was analysed.
It consisted of 30.63 wt. % phenol
69.11 wt. % diphenyl carbonate
0.26 wt. % phenyl salicylate.
The process of U.S. Pat. No. 5,239,106 thus produces distinctly less
favourable results.
Comparative Example 2
A product of the composition as described in U.S. Pat. No. 230,106 was
cooled from 70.degree. C. to 44.degree. C. at a rate of 2.degree. C./h in
a tube crystalliser in a similar manner to example 4 and left at this
temperature for 1 hour. The melt was then allowed to drain away completely
and the crystals in the tube were finally transformed into a melt. This
amounted to 36% of the quantity introduced and contained
65.1% diphenyl carbonate (44% of introduced quantity of diphenyl carbonate)
34.8% phenol
400 ppm phenyl salicylate.
It may be seen from the examples that:
using the process according to the invention, it is possible to handle even
highly concentrated mixtures containing above 70 wt. % of diphenyl
carbonate without problems and to purify them by crystallisation (examples
1 to 4).
even after single-stage crystallisation, very pure products are obtained
(example 3).
subsequent distillation of phenol is not required.
by-products (phenyl salicylate) are removed to below the detection limit
(examples 2 to 4; comparative examples 1 and 2).
even after single-stage crystallisation, catalysts are reduced to very low
levels and to below the detection limit after two-stage crystallisation
(example 3).
the yield of pure crystallised product is distinctly higher than according
to the prior art (compare example 2 with comparative examples 1 and 2).
Example 5
A reaction solution from direct carbonylation of phenol with a composition
of approximately
24 wt. % diphenyl carbonate
74 wt. % phenol
1.5 wt. % tetrabutylammonium bromide
0.4 wt. % sodium phenolate
0.1 wt. % manganese compound
200 ppm palladium compound
together with approximately 150 ppm of by-products such as salicylic acid
phenyl ester and o-ophenylphenol, was changed into a composition of
73.2 wt. % diphenyl carbonate
18.9 wt. % phenol
7.5 wt. % tetrabutylammonium bromide
0.4 wt. % sodium phenolate
76 ppm manganese compound
5 ppm palladium compound
together with approximately 500 ppm of by-products such as salicylic acid
phenyl ester and o-phenylphenol by firming out the solids and distilling
off the phenol, was introduced into the tube crystalliser, initially
maintained at a temperature of 70.degree. C. and cooled at 2.degree. C./h.
Once the melt had reached 61.degree. C., it was held at this temperature
for 30 minutes, then discharged and the heating medium was then reheated
at 2.degree. C./h. Once a temperature of 79.degree. C. had been reached,
the mass of crystals remaining in the crystalliser melted and was
collected separately. After this first crystallisation stage, it was of
the following composition:
97.5 wt. % diphenyl carbonate
2.0 wt. % phenol
0.5 wt. % tetrabutylammonium bromide
6 ppm sodium phenolate.
Pd and Mn compounds were no longer detectable. By-products such as
salicylic acid phenyl ester and o-phenylphenol were present at
concentrations of <10 ppm. After a further identical crystallisation
stage, the DPC content was >99.8 wt. %.
Example 6
A reaction solution from direct carbonylation of phenol with a composition
of approximately
75 wt. % diphenyl carbonate
23 wt. % phenol 1.5 wt. % tetrabutylammonium bromide 0.4 wt. % sodium
phenolate 0.1 wt. % manganese compound 200 ppm palladium compound
together with approximately 210 ppm of by-products such as salicylic acid
phenyl ester and o-phenylphenol, was changed into a composition of
75.8 wt. % diphenyl carbonate
23.3 wt. % phenol
0.7 wt. % tetrabutylammonium bromide
0.2 wt. % sodium phenolate
80 ppm manganese compound
6 ppm palladium compound
by filtering out the solids, was introduced into the tube crystalliser,
initially maintained at a temperature of 70.degree. C. and cooled at
2.degree. C./h. Once the melt had reached 61.degree. C., it was discharged
and the heating medium was then reheated at 2.degree. C./h. Once a
temperature of 79.degree. C. had been reached, the mass of crystals
remaining in the crystalliser melted and was collected separately. After
this first crystallisation stage, it was of the following composition:
98.6 wt. % diphenyl carbonate
1.1 wt. % phenol
0.3 wt. % tetrabutylammonium bromide
4 ppm sodium phenolate.
Pd and Mn compounds were no longer detectable. By-products such as
salicylic acid phenyl ester and o-phenylphenol were present at
concentrations of <8 ppm. The crystallisation product obtained in this
manner was distilled and then had a DPC content of >99.9 wt. %.
Example 7
A reaction product from the catalytic phosgenation of phenol containing
75.6 wt. % diphenyl carbonate
23.5 wt. % phenol
0.9 wt. % chloroformic acid phenyl ester
1090 ppm salicylic acid phenyl ester
was crystallised in a similar manner to example 2. In this manner, a
purified diphenyl carbonate of the following composition was obtained:
99.5 wt. % diphenyl carbonate
0.5 wt. % phenol
<0.01 wt. % chloroformic acid phenyl ester
50 ppm salicylic acid phenyl ester.
The remaining quantities of low-boiling constituents could be removed to
below the detection limit by simple distillation.
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Description |
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