Recovery of hydroxyaromatic compound from aqueous extract solutions

A method for efficiently recovering a hydroxyaromatic compound from aqueous extract streams of diaryl carbonate reaction mixtures, comprises contacting the aqueous extracts with a suitable solvent, preferably diphenyl carbonate or anisole. The aqueous stream may then be further recycled or reclaimed, and the hydroxyaromatic compound isolated from the solvent for disposal or further use.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for recovering a hydroxyaromatic 
compound from aqueous extract streams. In particular, this invention 
relates to methods for recovering a hydroxyaromatic compound from aqueous 
extract streams generated in 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 hydroxyaromatic 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. Pat. 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. Pat. 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. In U.S. Pat. No. 5,498,789 to Takagi et al., the 
catalyst system consists of a palladium compound, at least one lead 
compound, at least one halide selected from quaternary ammonium halides 
and quaternary phosphonium halides, and optionally at least one copper 
compound. 
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 is heavily dependent on a combination of the efficiency of 
the catalyst package and on the ability to reclaim and recycle the 
expensive catalyst components and unconverted hydroxyaromatic starting 
material, in particular phenol. 
It has been found that aqueous solvent extraction of crude diphenyl 
carbonate mixtures gives rise to an aqueous extract stream containing from 
about 0.5 to about 20% phenol, and between about 80% to about 99% water. 
The aqueous stream may further comprise metals, water-soluble organic 
materials, including other hydroxyaromatic compounds in addition to 
phenol, and any extractants (e.g., salts, acids, or complexing agents) 
added to the original aqueous extract. The presence of phenol in these 
aqueous extract streams may interfere with subsequent manipulations of the 
stream, for example by impeding phase separation. The presence of phenol 
starting material and other hydroxyaromatic compounds formed as 
by-products or present as impurities further complicates disposal of the 
stream. Methods for the recovery of a hydroxyaromatic compound from such 
aqueous extract streams would therefore be both financially and 
environmentally desirable. 
SUMMARY OF THE INVENTION 
The above-discussed and other drawbacks and deficiencies of the art are 
alleviated by the method of the present invention for the recovery of a 
hydroxyaromatic compound from an aqueous extract stream generated in 
diaryl carbonate production, comprising extracting the aqueous extract 
stream with a solvent essentially immiscible with water and in which a 
hydroxyaromatic compound is soluble, thereby effecting transfer of 
hydroxyaromatic compound from the aqueous extract stream into the solvent. 
Implementation of the method of this invention substantially reduces both 
economic and environmental concerns in the preparation of diaryl 
carbonates. The process is particularly useful for the recovery of phenol 
from an aqueous extract stream generated in diphenyl carbonate production. 
DETAILED DESCRIPTION OF THE INVENTION 
Hydroxyaromatic compound as used in the present invention refers to at 
least one hydroxyaromatic compound present in a process stream from diaryl 
carbonate production. The hydroxyaromatic compound comprises the 
hydroxyaromatic starting material used in the diaryl carbonate production 
process and any hydroxyaromatic compounds which may be present in a 
process stream from diaryl carbonate production as by-products or 
impurities. Such by-products and impurities may include, but are not 
limited to, coupled hydroxyaromatic compounds comprising compounds with 
more than one aromatic ring, and halogenated hydroxyaromatic compounds and 
halogenated coupled hydroxyaromatic compounds. The hydroxyaromatic 
starting material preferably comprises at east 90%, more preferably 
greater than 95%, and most preferably greater than 99% by weight of the 
mixture of hydroxyaromatic compounds present in the aqueous extract stream 
arising from any diaryl carbonate production process. 
The present invention makes possible efficient recovery of hydroxyaromatic 
starting material from aqueous extracts of crude mixtures resulting from 
the production of diaryl carbonates, preferably diphenyl carbonate. The 
method comprises treating an aqueous extract stream from the production of 
diaryl carbonates with a suitable solvent by liquid--liquid extraction. 
The aqueous stream may then be further recycled or reclaimed, and the 
hydroxyaromatic starting material may be isolated from the solvent for 
further use. 
Solvent extraction of crude diphenyl carbonate mixtures with an aqueous 
solution gives rise to an aqueous stream containing between about 0.5 to 
about 20% phenol starting material and between about 80% and about 99% 
water. Depending on extraction conditions, the aqueous extract stream may 
further comprise extracted catalyst and co-catalyst components, as well as 
water-soluble organic components, including other hydroxyaromatic 
compounds, and added extractants such as acids, bases, salts, surfactants, 
or chelating agents. In accordance with the method of the present 
invention, phenol is removed from these or other phenol-containing aqueous 
extract streams of the diphenyl carbonate process by solvent extraction 
using a suitable solvent. 
Depending upon the reaction conditions in diphenyl carbonate production, 
minor amounts of other hydroxyaromatic compounds may be present in the 
aqueous extract stream along with phenol starting material. These other 
hydroxyaromatic compounds, such as biphenols and halogenated phenols, may 
also be recovered along with phenol in the solvent extraction process, 
depending upon their relative solubilities in water and the extracting 
solvent. It should be understood that phenol starting material is the 
predominant hydroxyaromatic compound in any mixture of hydroxyaromatic 
compounds obtained in an aqueous extract stream from diphenyl carbonate 
production. Phenol preferably comprises at least 90%, more preferably 
greater than 95%, and most preferably greater than 99% by weight of the 
total amount of hydroxyaromatic compounds present in the aqueous extract. 
Suitable solvents for extraction of aqueous extract streams from diphenyl 
carbonate production include those which are essentially immiscible with 
water and which have a high solubility for phenol at the extraction 
temperature. Preferably, phenol is 95% or more soluble in the solvent, and 
water is less than 5% soluble in the solvent at the extraction 
temperature. Even more preferably, phenol is completely soluble in the 
solvent, and water is less than I % soluble in the solvent at the 
extraction temperature. The temperature of the extraction may be adjusted 
in order to increase the solubility of phenol in the solvent. The solvents 
are preferably stable in the presence of acid, most preferably stable in 
the presence of an aqueous solution having between about 1% and about 20% 
of hydrochloric acid by weight (or a solution having equivalent acidity). 
Such solvents include, but are not restricted to anisole and diphenyl 
carbonate. Diphenyl carbonate is preferred for extractions at temperatures 
above 80.degree. C., and is preferably used for extraction at temperatures 
between about 80.degree. C. and about 110.degree. C. Above 80.degree. C., 
diphenyl carbonate is completely miscible with phenol, whereas water 
solubility of diphenyl carbonate is less than 1 percent. Anisole is 
preferred for extractions at temperatures between about 5.degree. C. and 
about 80.degree. C. or higher (including room temperature), because of its 
low freezing point (-37.3.degree. C. for pure anisole). Anisole is 
miscible with phenol in the temperature ranges of interest, and water 
solubility in anisole is about 1000 ppm at room temperature. 
Methods of liquid-liquid extraction are well known in the art, being 
described, for example in "Science and Practice of Liquid--Liquid 
Extractions," Vols. 1 and 2, edited by John D. Thornton, Clarendon Press, 
Oxford (1992), particularly volume 1, pp. 492-589 which are incorporated 
by reference herein. Generally, liquid-liquid extraction is effected by 
washing through an aqueous extract that amount of solvent effective to 
cause phenol to migrate out of the aqueous phase and into the extractant 
(solvent) phase, employing any of a number of contacting devices, such as 
a mixer-settler unit, an agitated column, or similar device, in a batch, 
semi-continuous, or continuous process. The amount of aqueous stream 
extracted, amount of solvent used, number of washings, and length of time 
required for each wash are empirically determined, depending on such 
factors as the solubility of phenol in the solvent, the miscibility of the 
solvent with water, the temperature of the extraction, the cost of the 
solvent, disposal requirements of the solvent, the required degree of 
phenol removal from the aqueous stream, the mixing efficiency between the 
two phases, the ease of phase separation, and like concerns. Balancing of 
such considerations to maximize efficiency and/or removal, in addition to 
transfer of this information from laboratory testing to a continuous or 
batch operation on a large scale, taking into account phase densities and 
fluid dynamic considerations, is well within the skills of a practitioner 
in the art. 
While the above method is directed to the removal of phenol from aqueous 
extract streams derived from reaction mixtures arising from diphenyl 
carbonate production, it is to be recognized that the method may be 
employed to recover different hydroxyaromatic compounds from aqueous 
extract streams derived from other diaryl carbonate processes, provided 
that the requisite conditions of miscibility of hydroxyaromatic compounds 
with the extracting solvent, and immiscibility of the extracting solvent 
with water are met. Preferably, the hydroxyaromatic starting material is 
95% or more soluble in the solvent, and water is less than 5% soluble in 
the solvent at the extraction temperature. Even more preferably, the 
hydroxyaromatic starting material is completely soluble in the solvent, 
and water is less than 1% soluble in the solvent at the extraction 
temperature. 
Such other diaryl carbonate processes may employ as starting materials 
hydroxyaromatic compounds including monocyclic, polycyclic or fused 
polycyclic aromatic monohydroxy or polyhydroxy compounds having from 6 to 
30, and preferably from 6 to 15 carbon atoms. Illustrative hydroxyaromatic 
compounds include, but are not limited to, phenol, cresol, xylenol, 
resorcinol, hydroquinone, naphthol, catechol, cumenol, the various isomers 
of dihydroxynaphthalene, 
bis(4-hydroxyphenyl)propane-2,2,.alpha.,.alpha.'-bis(4-hydroxyphenyl)p-dii 
sopropylbenzene, and bisphenol A. Aromatic organic monohydroxy compounds 
are particularly preferred, with phenol being the most preferred. 
The following examples are provided by way of example only, and should not 
be read to limit the scope of the invention. All phenol % values are 
weight % values based on the liquid phase in which the phenol is present.

EXAMPLE 1 
55.2 g of aqueous extract of a crude diphenyl carbonate reaction mixture, 
containing 4.06% phenol and no diphenyl carbonate, was extracted with 11.8 
g of anisole at 25.degree. C. After the single extraction, the aqueous 
layer contained 1.14% phenol as determined by liquid chromatography. 
EXAMPLE 2 
937.4 g of aqueous extract of a crude diphenyl carbonate reaction mixture, 
containing 4.13% phenol and no diphenyl carbonate, was extracted with 
300.5 g of anisole at 25.degree. C. After extraction was complete, with 
the two phases still in contact with each other, the phenol level in the 
aqueous phase was 0.8%, while the anisole phase contained 10.87% phenol. 
Then, 811.6 g of untreated aqueous extract was added to the system, 
allowed to equilibrate, and then phase separated (1688.62 g total aqueous 
phase, 360.88 g anisole phase). The aqueous phase was then separated and 
extracted with 301.6 g of fresh anisole. The mass of the second anisole 
phase after this extraction was 314.5 g, and that of the aqueous phase was 
1673.4 g. The aqueous phase after this extraction contained 0.19% phenol, 
while the anisole phase contained 4.12% phenol. 
EXAMPLE 3 
196.2 g of aqueous extract of a crude diphenyl carbonate reaction mixture, 
containing 8.14% phenol and no diphenyl carbonate, was extracted with 66.1 
g of diphenyl carbonate at 85.degree. C. After extraction, the phenol 
level in the aqueous extract was 2.07%, while the phenol level in the 
diphenyl carbonate phase was 15.63% as determined by liquid 
chromatography. 
The extraction solvent containing one or more hydroxyaromatic compounds may 
be disposed of by standard methods, for example incineration. Also, the 
hydroxyaromatic starting material contained in the extraction solvent may 
be isolated from the solvent by standard methods for further use. Such 
methods may include evaporation of the solvent, crystallization of the 
hydroxyaromatic starting material, distillation, chromatography, and like 
isolation and purification methods. In the course of isolation and 
purification the starting hydroxyaromatic compound may be substantially 
freed of any other hydroxyaromatic compounds which may be present at the 
end of the diaryl carbonate production process as by-products or 
impurities, such as coupled hydroxyaromatic compounds and halogenated 
hydroxyaromatic compounds. 
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.