REMOVAL OF LEAD FROM ORGANIC DIARYL CARBONATE
REACTION MIXTURES
BACKGROUND OF THE INVENTION
The present invention relates to a method for purification of the
reaction mixtures arising from the production of diaryl carbonates. In
particular, the present invention relates to removal of lead species from the
process streams arising from the production of diaryl carbonates.
Diaryl carbonates, and diphenyl carbonate in particular, are
valuable monomer precursors for the preparation of polycarbonates by melt
transesterification. An advantageous route for the synthesis of diaryl- *
carbonates is the direct carbonylation of aromatic hydroxy compounds by
carbon monoxide and an oxidant in the presence of a catalyst.
A wide range of catalysts may be used in this preparation of
diaryl carbonates. For example, U.S. Patent No. 4,187,242 to Chalk discloses
catalysts derived from Group VIIIB metals, i.e., metals selected from the
group consisting of ruthenium, rhodium, palladium, osmium, iridium and
platinum, or complexes thereof. U.S. Patent Nos. 5,231,210 to Joyce, et al.,
5,284,964 and 5,760,272 to Pressman et al., and 5,399,734 to King, Jr., et al.
further disclose the use of co-catalysts, including metal co-catalyst species
such as cobalt pentadentate complexes and complexes of cobalt with
pyridines, bipyridines, terpyridines, quinolines, isoquinolines, aliphatic
polyamines such as ethylenediamine, crown ethers, aromatic or aliphatic
amine ethers such as cryptands, and Schiff bases, in combination with organic
co-catalysts such as terpyridines and quaternary ammonium or phosphonium
halides.
U.S. Patent No. 5,498,789 to Takagi further discloses the use of a
catalyst system comprising at least one lead compound soluble in a liquid
phase, at least one halide selected from quaternary ammonium halides and
quaternary phosphonium halides, and optionally at least one copper
compound. As examples of suitable lead compounds, the publication
disclosed lead oxides such as PbO, Pb3O4, PbO2 and the like; inorganic lead
salts such as Pb(NO3)2, PbSO4 and the like and other lead compounds
generally represented by the formula Pb(OR)2, wherein R is an alkyl or aryl
group, for example Pb(OCH3)2, Pb(OC6H6)2; or wherein R is an acyl group, for
example acid salts of lead such as Pb(OC(O)CH3)2, Pb(OC(O)CH3)4,
Pb(OC(O)C2H5)2 and the like; and lead complex compounds such as
phthalocyanine lead.
As can be seen from the above brief review, the crude reaction
mixtures arising from the production of diaryl carbonates can contain
complex mixtures of catalyst and co-catalyst metals, and organic products and
by-products. The cost of commercially implementing direct oxidative
carbonylation depends heavily on a combination of the efficiency of the
catalyst system and on the ability to reclaim and recycle unconverted hydroxy
aromatic starting material and the expensive catalyst components. Even
reclamation of less expensive metals such as lead can lead to financial and
environmental advantages. Accordingly, there remains a need for efficient,
convenient methods for the removal of lead from the reaction mixtures
generated in the carbonylation of hydroxy aromatic compounds.
SUMMARY OF THE INVENTION
The above-discussed and other drawbacks and deficiencies of
the related art are alleviated by the present methods for the removal of lead
species from the organic process streams of diaryl carbonate synthesis. One
embodiment comprises extracting an organic reaction mixture arising from
the production of diaryl carbonates with an acidic, aqueous solution
comprising an anion selected from the group consisting of nitrate, halide, or
acetate. A second embodiment comprises forming a lead compound which is
at least partially insoluble in an organic reaction mixture arising from the
production of diaryl carbonates; and separating the insoluble lead compound
from the organic reaction mixture. The recovered lead-containing solid may
then be calcined to provide a calcined lead compound which is catalytically
active in the production of diaryl carbonates. Implementation of this method
reduces both economic and environmental concerns in the preparation of
diaryl carbonates.
DETAILED DESCRIPTION OF THE INVENTION
The present invention makes possible efficient removal of lead
species from organic process streams resulting from the production of diaryl
carbonates by catalytic carbonylation of aromatic hydroxy compounds. The
process may be described generally as treating an organic reaction mixture
arising from the production of diaryl carbonates so as to form a lead
compound which is at least partially insoluble in the organic reaction mixture;
and separating the insoluble lead compound from the organic reaction
mixture. In one embodiment, the method comprises contacting the reaction
mixture with an aqueous solution into which lead migrates at least partially,
relative to the crude reaction mixture. Preferably, this first embodiment
comprises treating an organic reaction mixture from the production of diaryl
carbonates with a neutral or slightlv acidic, aqueous solution of an anion
selected from the group consisting at acetate, nitrate, or a halide anions,
thereby extracting lead from the reaction mixture into the aqueous solution.
In a second embodiment, the method comprises precipitating the lead species
from an organic reaction mixture from production of diaryl carbonates by
the addition of oxalic acid, or an acidu mitral solution of oxalic acid to the
reaction mixture. The precipitated and is then separated, and may be
calcined to provide a calcined lead compound, which is catalytically active in
the production of diaryl carbonates, by catalytic carbonylation of aromatic
hydroxy compounds.
In accordance with the first embodiment, the lead in a crude or
pre-treated organic reaction mixture arising from the production of diaryl
carbonates by catalytic carbonylation of aromatic hydroxy compounds is
removed by solvent extraction using a neutral or slightly acidic aqueous
extractant. The pH of the aqueous solution is preferably from about 0 to
about 7, more preferably from about 1 to about 4, and most preferably from
about 1 to about 3. The solution further comprises at least one anion selected
from the group consisting of acetate, nitrate, or halide anions. Preferred
halides are chloride and bromide. Combinations of the foregoing anions may "
also be used. These anions result in removal of up to about 30%, and
preferably up to about 50% by weight of the lead per extraction, that is, per
contact of the organic reaction mixture with a single volume of extractant.
A second embodiment of the present method comprises treating
a crude or pre-treated organic reaction mixture from the catalytic
carbonylation of diaryl carbonates with solid oxalic acid or a solid oxalic acid
salt, or with a concentrated, neutral or slightly acidic solution of oxalic acid or
an oxalic acid salt. Treatment with a concentrated solution is preferred, as it
allows faster dissolution of the oxalic acid or oxalic acid salt.
Treatment of the reaction mixture with oxalate results in
precipitation of lead species from the reaction mixture. The precipitated lead
species is typically recovered by filtration, and may be disposed of or further
treated to recover the lead. In a particularly advantageous feature, calcination
of the precipitated lead species under air results in production of a calcined
lead compound that is catalytically active in the carbonylation of aromatic
hydroxy compounds to produce diaryl carbonates. Any lead remaining in the
treated organic phase may be extracted as described above with a neutral or
slightly acidic, aqueous solution of an anion selected from the group
consisting of acetate, nitrate, or a halide, thereby extracting lead from the
treated organic phase into the aqueous solution.
Without being bound by theory, it is hypothesized that during
extraction acetate, nitrate, and halide anions selectively complex with lead
present in the reaction mixtures, which increases the solubility of the lead in
aqueous extractant and/or decreases the solubility of lead in the organic
reaction mixture.- It is furthermore hypothesized that upon treatment with
aqueous oxalate solutions, lead-oxalate complexes are formed, which are
insoluble in the reaction mixture. Other anions may therefore be effective in
either extraction or precipitation, as long as such anions are effective to
extract or precipitate lead in an amount of up to about 30%, preferably up to
about 50%, more preferably up to about 80%, and most preferably up to about
90% by weight of the lead per extraction or precipitation. Preferably, such
anions do not extract or precipitate other metal anions or other undesired
species into the aqueous extract or solid precipitate, and do not themselves
pose a waste disposal problem.
The concentration of anion effective to form a lead compound
which is extractable or which precipitates lead is empirically determined, and
will depend on factors such as how the lead compound is formed, the nature
of the anion, the concentration of the anion, the pH of the solution, and the
like. In general, an effective concentration of the anion in an aqueous
extraction or precipitation solution is in the range from about 0.1 percent by
weight to about 20 percent by weight, and preferably in the range from about
1 percent by weight to about 10 percent by weight. A preferred aqueous
extraction solution comprises between about 7 percent by weight and about 9
percent by weight of sodium acetate and between about 5 percent by weight
to about 7 percent by weight of acetic acid. A preferred aqueous precipitation
solution comprises between about 0.1 percent by weight and about 2 percent
by weight oxalic acid or oxalate anion in the reaction mixture.
Suitable counterions for the above-mentioned anions include
hydrogen, sodium, other alkali metals, alkaline earth metals, and the like.
Suitable counter-ions preferably do not interfere with the extraction of lead,
cause the extraction of undesirable species into the aqueous layer, or
themselves present a disposal problem.
The lead-containing aqueous extract solutions obtained in
accordance with the first embodiment may be disposed of, or further treated
in order to isolate the lead, e.g., by selective precipitation. These precipitates,
as well as the precipitates obtained in accordance with the second
embodiment may then be disposed of, or further treated in order to recover
the lead. Recovery of lead from a precipitate to yield a catalytically active
lead compound or precursor is achieved by subjecting the precipitate to a
heating program effective to regenerate lead catalyst. Accordingly, the
precipitate is heated to a temperature in the range between about 400 °C and
about 600 °C.
The following examples are provided by way of example only,
and should not be read to limit the scope of the invention.
EXAMPLE 1
7.3 g of a crude reaction mixture arising from the carbonylation
of phenol and having a lead concentration of 1563 ppm was contacted with
7.6 g of 3% HC1, mixed for 3 minutes, and allowed to settle for 10 minutes.
The lead content of the organic phase after extraction was 18 ppm,
representing a removal efficiency of approximately 98.9%. The crude reaction
mixture arose from a carbonylation using 60.5676 g (643.6 mmol) of phenol in
the presence of 9.9 mg of Pd(acetylacetonate)2 (0.032 mmol), 212 mg of PbO
(0.950 mmol), and 3.2049 g of hexaethylguanidinium bromide (10.36 mmol)
for 2.5 hours at 100 °C.
EXAMPLE 2
A sample of a crude reaction mixture using a catalyst system
comprising 4.6 mg (0.0151 mmol) of palladium acetylacetonate, 52.1 mg
(0.2334 mmol) of lead (II) oxide and 3.2 g (10.359 mmol) of
hexaethylguanidinium bromide in 60.4226 g phenol reacted for 2 hours at 100
°C, and containing about 391 ppm lead, was extracted with 8.7 weight %
sodium nitrate solution in an approximately a 2:1 ratio by mass of organic to
aqueous components (13.7 g organic reaction mixture to 7.341 g aqueous
solution). After extraction, the concentration of lead in the organic phase was
234 ppm as measured by atomic absorption. This represents a removal
efficiency of 40.2 %.
EXAMPLE 3
1123.5 g of a crude reaction mixture containing 3180 ppm of lead
at the end of the reaction (as determined by atomic absorption spectroscopy)
was treated with an aqueous solution comprising 2.896 g of oxalic acid in 10.1
g of water. The mixture was mixed for ten minutes and then allowed to stand
after mixing at 85 °C for 2 hours, then filtered to recover a cake (8.21 g). The
filtrate was analyzed for lead by atomic adsorption spectroscopy, which
showed a lead content of 12.5 ppm. This represents a removal efficiency of
99.6% for the lead.
COMPARATIVE EXAMPLE 4
627.1 g of the crude reaction mixture from the same source as in
Example 3 was filtered and the filtrate analyzed by atomic absorption
spectroscopy, which showed that the filtrate contained 2625 ppm of lead
(compared to the initial lead concentration of 3180 ppm). The filter cake
collected weighed 3.917 g.(There was no oxalic acid or water added, for
comparison). This was intended only as a comparison to the previous case,
which started with 1123.5 g.
EXAMPLE 5
377 mg of the filtered solid obtained in Example 3 was subjected
to a heating program, wherein the sample was heated under air. The
temperature was increased from room temperature to 600 °C, at a rate of 20
°C per minute, and then held at 600 °C for 15 minutes (total time of heating of
about 45 minutes). The loss in weight of the sample was 28%, and had
reached that weight loss after about 20 minutes (about 450 °C).
The solid recovered was used as a lead co-catalyst in a catalytic
carbonylation of phenol (61.2543 g phenol, 5.5 mg of palladium
acetylacetonate, 103.2 mg of the solid recovered from the heat treatment
described above, and 2.1711 g tetraethylammonium bromide). The reaction
yielded 14.8% of diphenyl carbonate in 1.5 hours, clearly showing the catalytic
activity of the lead-containing solid. By comparison, without any lead species
present, less than 1% of diphenyl carbonate is obtained.
EXAMPLE 6
Four successive extractions of an organic diphenylcarbonate
reaction mixture were performed at SOoC, using a sodium acetate/acetic acid
aqueous solution (7.22% sodium acetate.5.26% acetic acid by weight). Results
are shown in Table 1 below.
While preferred embodiments have been shown and described,
various modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly, it is to be
understood that the present invention has been described by way of
illustration and not limitation.
WHAT IS CLAIMED IS:
1. A method of removing lead from an organic reaction
mixture arising from the production of diaryl carbonates comprising the
steps of
forming a lead-containing compound which is at least
partially insoluble in an organic reaction mixture arising from the
production of diaryl carbonates; and
separating the lead-containing compound from the
organic reaction mixture.
2. The method of claim 1, wherein forming the at least
partially insoluble lead-containing compound is by treating the organic
reaction mixture with a reagent selected from the group consisting of solid
oxalic acid, a solid oxalic acid salt, an aqueous solution of oxalic acid, an
aqueous solution of an oxalic acid salt, and a combination thereof.
3. The method of claim 2, wherein the amount of oxalic
acid or oxalic acid salt is about 0.1% by weight to about 2% by weight of
the reaction mixture.
4. The method of claim 2, wherein the reagent is solid
oxalic acid.
5. The method of claim 1, further comprising treating the
separated, lead-containing compound to yield a catalytically active lead
compound or precursor to a catalytically active lead compound.
6. The method of claim 5, wherein the treating is by
heating the lead-containing compound to a temperature between about
400 °C and about 600 °C
7. The method of claim 6, wherein the treating is under
air.
8. The method of claim 1, further comprising extracting
lead from the organic reaction mixture after separation of the lead-
containing compound with a neutral or acidic aqueous solution
comprising an anion selected from the group consisting of acetate anion,
nitrate anion, halide anion, and mixtures thereof.
9. The method of removing lead according to claim 1
comprising the steps of
treating the organic reaction mixture with a reagent
selected from the group consisting of solid oxalic acid, a solid oxalic acid
salt, an aqueous solution of oxalic acid, an aqueous solution of an oxalic
acid salt, and a combination thereof, thereby forming an at least partially
insoluble lead-containing compound;
separating the solid lead-containing compound from
the organic reaction mixture; and further comprising
calcining the separated, lead-containing compound to
yield a catalytically active lead compound or precursor to a catalytically
active lead compound.
10. A method of removing lead from an organic reaction
mixture arising from the production of diaryl carbonates comprising the
step of
contacting an organic reaction mixture arising from
the production of diaryl carbonates with a neutral or acidic aqueous
solution comprising an anion which forms a water-soluble salt with lead,
thereby'extracting lead into the aqueous solution.
11. The method of claim 10, wherein the anion is selected
from the group consisting of acetate anion, nitrate anion, halide anion, and
mixtures thereof.
12. The method of claim 10, wherein the pH of the
aqueous solution is in the range from about 0 to about 7.
13. The method of claim 10, wherein the pH of the
aqueous solution is in the range from about 1 to about 4.
14. The method of claim 10, wherein the pH of the
aqueous solution is in the range from about 1 to about 3.
15. The method of claim 10, wherein at least about 30% of
the lead is removed per extraction.
16. The method of claim 15, wherein at least about 50% of
the lead is removed per extraction.
17. The method of claim 10, further comprising treating
the aqueous solution with extracted lead to yield a lead-containing
precipitate.
18. A method of reclaiming lead according to claim 10
comprising the step of
contacting an organic reaction mixture arising from
the production of diaryl carbonates with a neutral or acidic aqueous
solution selected from the group consisting of acetate anion, nitrate anion,
halide anion, and mixtures thereof, thereby extracting lead into the
aqueous solution.

A method for efficient removal of lead co-catalyst species from
organic process streams arising from diaryl carbonate synthesis, by contacting
the organic reaction mixtures with an aqueous acid, salt, or acid/salt solution,
thereby extracting the treated mixture into an aqueous phase, or by treating
the organic reaction mixtures with solid oxalic acid or oxalic acid salt, or an
aqueous solution of oxalic acid or oxalic acid salt, thereby resulting in
precipitation of the lead. The precipitated lead may then be calcined to
provide a lead compound that is catalytically active in the carbonylation of
phenol to yield diaryl carbonates. Use of these methods will substantially
reduce both financial and environmental concerns for the preparation of
diaryl carbonates.