Background of the Invention This invention relates to a purification process for impure mixtures containing dihydroxydiphenyl sulfone (bis- phenol-S) isomers. More particularly, it is concerned with a purification process for producing high purity 4,4'-dihydroxydiphenyl sulfone from an impure mixture resulting from a preparation process comprising 4,4'-dihy- droxydiphenyl sulfone, 2,4'-dihydroxydiphenyl sulfone, and optionally, triphenol disulfone, the reaction product of three mols of phenol and two mols of sulfuric acid. 4,4 '-Dihydroxydiphenyl sulfone [sulfonyldi( 4- phenol)], due to the uniqueness of its diphenyl sulfone linkage, has heat resistance, resistance to oxidation, and stability to light, and, because of these characteristics, 4,4 '-dihydroxydiphenyl sulfone has been increasingly uti¬ lized as a substitute for bisphenol-A in plastics such as polyester resins, epoxy resins, and polycarbonate resins. Importantly, 4,4'-dihydroxydiphenyl sulfone has been prepared from phenol and a sulfonating agent such as con¬ centrated sulfuric acid, fuming sulfuric acid, or sulfur trioxide. Alternatively, it has been made from phenol and p-phenolsulfonic acid. Made according to these processes, however, it is impossible to avoid formation of 2,4'-dihydroxydiphenyl sulfone as a by-product, and it is not easy to isolate the 4,4'-dihydroxydiphenyl sulfone from the crude. Therefore, substantial amounts of 2,4^-isomer and other impurities can be contained in industrial products made with or from dihydroxydiphenyl sulfone.

Previous methods of preparation as described above, whether solventless or employing a solvent or mixed sol¬ vent, produce in addition to more than one isomer, signif¬ icant amounts of triphenol disulfone and colored impurities which must also be separated before tne 4,4'- dihydroxydiphenyl sulfone can be used commercially. The

solventless preparation procedures are particularly bad in producing colored impurities.

In addition to isomer and color purity, formation of small amounts of triphenol disulfone (trimer) is found in crude product from some bisphenol-S preparations. This material is made by reaction of bisphenol-S with another molecule of phenol, and it has not generally been recog¬ nized as an impurity in crude dihydroxydiphenyl products. As previously noted, 4,4'-dihydroxydiphenyl sulfone, is a compound which has many applications in the polymer industry as a substitute for bisphenol-A. However, high polymer prepared from 4,4 '-dihydroxydiphenyl sulfone con¬ taining the 2,4'-isomer, trimer, and possibly colored impurities has a lower molecular weight, color, and exhib¬ its deterioration in mechanical properties when compared with polymer prepared from 4,4'-dihydroxydiphenyl sulfone not containing trimer and 2,4'-isomer. The larger the content of trimer and 2,4'-isomer, the more noticeable these deficiencies become. For the most effective indus¬ trial utilization of 4,4'-dihydroxydiphenyl sulfone there¬ for, it is necessary to remove the 2,4'-dihydroxydiphenyl sulfone isomer and trimer from the crude reaction product as well as ridding the desired 4,4'-isomer of colored impurities.

In the past a number of purification processes and solvents have been suggested for purifying 4,4'-dihydroxy- diphenol sulfone. Interestingly, water recrystallization has been claimed effective in removing color impurities but not effective in reducing isomer impurities. See U.S. Patent Nos. 4,382,147 and 3,065,274. Similarly, 4,4'-dihydroxydiphenyl sulfone purification from colored impurities has included the technique of "heat treatment" in water (Japanese Kokai Pat. No. Sho 63 [19881-48261) to improve color.

In addition, recrystallization from aqueous mixtures including water and methanol (U.S. Patent No. 4,113,974;,

recrystallization from organic solvents (U.S. Patent No. 2,833,828), and washing with a hot aqueous solution con¬ taining an aliphatic higher alcohol at above 120°C (see Japanese Patent Publication No. 3005/67) have all been used for isomeric and/or color purification.

However effective these processes are for removing colored impurities and other substances contained in the crude dihydroxydiphenyl sulfone mixture, they are not very effective for removing the 2,4'-dihydroxydiphenyl sulfone isomer. Therefor, as a process for separating 2,4'-dihydroxydiphenyl sulfone, there has been proposed a separation process based on the formation of a calcium complex (see U.S. Patent No. 2,392,137), and a separation process based on the formation of a benzene adduct [see "Journal of Chemical Society" (1949), pp. 2854-2856] or a phenol adduct (U.S. Patent No. 4,382,147). These processes are industrially less suitable because they require complicated and expensive operations. As industrially suitable processes for the separation of 2,4'-dihydroxydiphenyl sulfone, there have been pro¬ posed a process using sym-tetrachloroethane as an extraction solvent (see Japanese Patent Publication No. 5274/63), a process using o-dichlorobenzene as an extraction solvent (see Japanese Patent Publication

No. 24660/68), and a process using mono-, di- and trial- kylphenols as extraction solvents (see Japanese Patent Publication No. 43936/72). However, the solubility of 2,4'-dihydroxydiphenyl sulfone in those solvents at ordi- nary temperature is so low that it is difficult to fully separate 2,4'-dihydroxydiphenyl sulfone from the isomer mixture at ordinary temperature. All of these processes use a high temperature treatment (100°-150°C) , such as a hot filtration for increasing the solubility of 2,4'-dihy- droxydiphenyl sulfone thereby improving its separation from the isomer mixture. Using elevated temperature and organic solvents, however, drawbacks such as the complex-

ity of operation, damage of filter materials, etc., and environmental damage caused by solvent vaporization are unavoidable. Thus, these are not considered to be fully satisfactory industrially.

Now a purification process to make low color, high isomeric purity 4,4'-dihydroxydiphenyl sulfone has been found which has distinct environmental and economic advan¬ tages in that it employs water as a recrystallization solvent and a carbonaceous material as a decolorant. It is also a process amenable to simple multi-stage oper¬ ations to further enhance product purity.

Description of the Drawing The Figure shows a plot of weight fraction of 4,4'- dihydroxydiphenol sulfone in water solvent versus temper¬ ature in degrees centigrade.

Summary of the Invention A process for purifying 4,4'-dihydroxydiphenyl sul¬ fone from an impure mixture of dihydroxydiphenyl sulfone isomers containing at least 90 wt.% of the 4,4'-isomer which comprises dissolving said impure mixture in water at a temperature above about 120°C to form a solution, con- tacting said solution with a carbonaceous adsorbent to form a decolorized solution, separating said decolorized solution from said adsorbent, and cooling said decolorized solution to precipitate a product with a 4,4'-dihydroxydiphenyl sulfone purity of 98.5 wt.% or greater having a molar absorptivity in methanol of less than 0.05 at 475 n .

In another aspect, the invention is directed to sucr. a recrystallization and decolorization process as described above followed by another recrystallization employing a second dissolution in water at a temperature above about 120°C.

In yet another aspect, the recrystallization solvent water can contain a small amount of base such as sodium hydroxide or ammonium hydroxide.

Detailed Description of the Invention The impure mixture used in the process of this invention comprises at least 90 wt.% of 4,4'-dihydroxydi¬ phenyl sulfone and less than 10 wt.% of 2,4'-dihydroxydi- phenyl sulfone isomer. More preferably, an impure mixture comprising at least 93 wt.% 4,4'-dihydroxydiphenyl sulfone and less than 7 wt.% of 2,4 '-dihydroxydiphenyl sulfone is used. Triphenol disulfone (trimer) and colored impurities are also generally present. The triphenol sulfone is gen- erally present in amounts of less than 2 wt.%, more pref¬ erably less than about 1 wt.%.

Dissolving the isomeric mixture in water is performed at a temperature usually above about 120°C. Heating at a temperature higher than 140°C for prolonged periods can lead to destruction of the product. Usually, a temper¬ ature between about 125 and 135°C is satisfactory. A sol¬ ubility plot in water for an impure mixture of bisphenol-S isomers is shown in the Figure which can be used to select the appropriate temperatures. The solution is then cooled below about 100°C, more preferably between about 80°C and about 100°C to bring the purified 4,4'-dihydroxydiphenyl sulfone out of solution. When aqueous base is used to keep the trimer in solution, the temperature of the cooled solution can be as low as about ambient.

In carrying out the process of this invention, dis¬ solution of the isomer mixture in water and the isolation by cooling is carried out under pressurized conditions, at least the dissolution part of the process which is above the boiling point of water.

After dissolving the impure mixture in water, the resulting solution is cooled, whereby purified 4,4'-dι-

hydroxydiphenyl sulfone crystallizes. Then, the superna¬ tant liquid is separated from the solid by any suitable means, e.g., suction filtration, pressure filtration, and centrifugal separation.

If the concentration of the slurry of crystals formed exceeds about 40% by weight, it becomes more difficult to separate the solid, so slurry concentrations greater than 40 wt.% are not preferred. The process can be repeated to form a product of higher purity, but a single recrystallization can lead to a 99 wt.% or greater 4,4'-isomer product.

Any one of a number of active carbons can be used for the decolorization including such commercial products like Nuchar WV-B, Nuchar SN, Norit A, Calgon CPG, Calgon GLP-LF, and the like. Preferred is the use of Norit. Particle size of the active carbon is important, and it should be of a mesh size which is conveniently handled in the decolorization and separation steps, as may be under- stood by one skilled in the art.

A small amount of basic material such as a Periodic Group la hydroxide, carbonate, bicarbonate, ammonium hydroxide, and the like may be added to the water solvent before or during recrystallization to assist in removal cf the trimer from the solid product. Preferred is the use of sodium or ammonium hydroxide. Generally, up to 0.10 mol of base per mol of crude dihydroxydiphenyl sul¬ fone is used depending upon the amount of trimer present. Preferably, decolorization is done prior to addition of the base.

The following Examples will serve to illustrate cer¬ tain embodiments of the herein disclosed invention. These Examples should not, however, be construed as limiting the scope of the novel invention as they are many variations which may be made thereon without departing from the spirit of the disclosed invention, as those of skill in the art will recognize.

EXAMPLES General The purity of the bisphenol-S produced is character¬ ized by two measurements: isomeric purity and color.

Isomer content including determination of triphenol disulfone (trimer) in a crude product mixture was deter¬ mined by high-performance liquid chromatography (HPLC). The bisphenol-S was analyzed by an external standards HPLC method using an acetonitrile/water (0.1% formic acid) mobile phase and a 5 μm Spherisorb ODS II, 250 X 4.6 mm column. A UV detector set at 254 nm was used as a detection system and an integrator for peak quantitation. A standard sample of bisphenol-S was analyzed by quantita¬ tive silation gas liquid chromatography for weight percent concentration of the 2,4 '-isomer and trimer concen¬ trations. This sample was used to calibrate the HPLC for an external standards method. For analysis, a 10.0 mg sample of bisphenol-S was weighed out and diluted to 10 ml with mobile phase. A 10 μl injection was made to the HPLC system via a closed loop autosampler, and the material analyzed for 2,4'-isomer and trimer content. The balance of the material was taken to be 4,4'-isomer. Color was determined by determining the absorbance at 475 nm in a 50 mm silica cell for a 5.0 wt.% solution in methanol using the formula A = log (Po/P) = abc in which a is molar absorptivity, b is path length in cm, and c is molar concentration.

Example 1 Crude bisphenol-S having an analysis of 94.55 wt.% 4,4'-isomer, 3.65 wt.% 2,4'-isomer and 1.8 wt.% trimer, and having a molar absorptivity of 0.59 was purified. A 60 g portion of the crude bisphenol-S was charged along with 140 g of water and 9 g of active carbon (Nuchar WV-B, 14-35 mesh, made by Westvaco Chemical Division) to

an agitated pressure vessel. The mixture was heated to 140°C with stirring and held for 1 hour to insure complete dissolution of the bisphenol-S and adsorption equilibrium with the active carbon. At the end of the hour, the slurry of active carbon in aqueous bisphenol-S solution was discharged under nitrogen pressure through a filter to remove the spent carbon. The filter and transfer lines were heated to prevent crystallization of the bisphenol-S during the filtration.

The carbon-free aqueous solution of the crude bisphe¬ nol-S was cooled to about 80°C to crystallize the 4,4'-bisphenol-S. The crystals recovered by filtration were substantially decolorized and enriched in the desired 4,4'-isomer, having an analysis of 98.8 wt.% 4,4'-isomer, 1.05 wt.% 2,4'-isomer and 0.15 wt.% trimer, and having a molar absorptivity of 0.032.

In order to further increase the purity of this first crop of crystals, they were recrystallized a second time from water. Three parts by weight crystals were mixed with 7 parts by weight water and heated to 140°C in an agitated pressure vessel. The resulting mixture was held for 1 hour to guarantee complete dissolution and mixing. At the end of 1 hour, the solution was cooled at about 80°C to crystallize purified 4,4'-isomer. The crystals recovered by filtration had an analysis of 99.76 wt.% ,4'-isomer, 0.24 wt.% 2,4'-isomer, no detectable trimer, and a molar absorptivity of 0.022.

Example 2

Crude bisphenol-S having an analysis of 94.55 wt.% 4,4'-isomer, 3.65 wt.% 2,4'-isomer, and 1.8 wt.% trimer, and having a molar absorptivity of 0.59 (as measured by our standard analytical procedures, described earlier) was purified by our procedure.

A 60 g portion of the crude bisphenol-S was charged along with 140 g of water and 9 g of active carbon (Nucr.ar

SN powder, Westvaco Chemical Division) to an agitated pressure vessel. The mixture was heated to 140°C with stirring and held for 1 hour to insure complete dissoi- ution of the bisphenol-S and adsorption equilibrium with the active carbon. At the end of the hour, the slurry of active carbon in aqueous bisphenol-S solution was dis¬ charged under nitrogen pressure through a filter to remove the spent carbon. The filter and transfer lines were heated to prevent crystallization of the bisphenol-S during the filtration.

The carbon-free aqueous solution of the crude bisphe¬ nol-S was cooled to about 80°C to crystallize the 4,4'-isomer. The crystals recovered by filtration were substantially decolorized and enriched in the desired

4,4'-isomer, having an analysis of 98.84 wt.% 4,4'-isomer, 0.91 wt.% 2,4'-isomer and 0.25 wt.% trimer, and having a molar absorptivity of 0.037.

In order to further increase the purity of this first crop of crystals, they were recrystallized a second time from water. Three parts by weight crystals were mixed with 7 parts by weight water and heated to 140°C in an agitated pressure vessel. The resulting mixture was held for 1 hour to guarantee complete dissolution and mixing. At the end of 1 hour, the solution was cooled to about 80°C to crystallize purified 4,4'-isomer. The crystals recovered by filtration had an analysis of 99.65 wt.% 4,4'-isomer, 0.31 wt.% 2,4'-isomer, and 0.04 wt.% trimer, and a molar absorptivity of 0.019.

ExamDle 3

Crude bisphenol-S having an analysis of 94.55 wt.% 4,4'-isomer, 3.65 wt.% 2,4'-isomer and 1.8 wt.% trimer, and having a molar absorptivity of 0.59 was purified. A 60 g portion of the crude bisphenol-S were charged along with 140 g water to an agitated pressure vessel. No active carbon was used. The mixture was heated to 140"C

with stirring and held for 1 hour to insure complete dis¬ solution of the bisphenol-S. At the end of the hour, the aqueous bisphenol-S solution was discharged under nitrogen pressure through a filter. The filter and transfer lines were heated to prevent crystallization of the bisphenol-S during the filtration.

The aqueous solution of the crude bisphenol-S was cooled to about 80°C to crystallize the 4,4'-isomer. The crystals recovered by filtration had an analysis of

98.3 wt.% 4,4*-isomer, 1.4 wt.% 2,4'-isomer and 0.3% trimer, and a molar absorptivity of 0.54.

ExarriDle 4 A decolorized 10 g sample of bisphenol-S containing

96.4 wt.% of the 4,4'-isomer, 2.1 wt.% of the 2,4'-isomer, and 1.5 wt.% of trimer was weighed into a Fischer-Porter bottle, and approximately 56.5 g of water or aqueous sodium hydroxide was added such that the total solution was 15% bisphenol-S by weight. The solution was purged with argon for 10 min and placed in an oil bath, heated to 149-150°C (oil bath temperature), stirred for 20 min at this temperature, allowed to cool to 90-94°C, and removed from the oil bath. The top was removed from the appara- tus, and the mixture was stirred until the slurry temper¬ ature reached 30°C (30-45 min). The solids were filtered, washed with two 10 g portions of water and dried overnight in a vacuum oven at 100°C.

The Table below displays the effect of increasing basicity on the separation.

Table 1

Composition of the

Crude bisphenol-S having an analysis of 94.55 wt.% 4,4'-isomer, 3.65 wt.% 2,4'-isomer and 1.8 wt.% trimer, and having a molar absorptivity of 0.59 was purified as below.

A 60 g portion of the crude bisphenol-S was charged along with 140 g of water and 9 g of an active carbon to an agitated pressure vessel. The mixture was heated to 140°C with stirring and held for 1 hr to insure complete dissolution of the bisphenol-S and adsorption equilibrium with the active carbon. At the end of the hour, the slurry of active carbon in aqueous bisphenol-S solution was discharged under nitrogen pressure through a filter to remove the spent carbon. The filter and transfer lines were heated to prevent crystallization of bisphenol-S during the filtration.

The carbon-free aqueous solution of the crude bisphe¬ nol-S was cooled to about 80°C to crystallize the 4,4'-isomer. The crystals recovered were substantially decolorized and enriched in the desired 4,4'-isomer. Var¬ ious active carbons were used, with the results as reported in Table 2 below.

Table 2

Molar

Active Crystal Purity (wt.%) Absom- Carbon Used 4,4'-isomer 2,4'-isomer Trimer tivity Nuchar WV-B ¹ 98.8 1.05 0.15 0.032 (14x35 mesh)

Nuchar S 98.8 0.9 0.3 0.037 (powder)

Norit A 99.0 0.9 0.13 0.08 (powder)

Calgon CPG 97.5 ^' 1.2 1.3 0.07 (12x40 mesh)

Calgon GLP-LF ^* 98.9 0.8 0.3 0.12 (12x40 mesh)

Fisher 97.3 ^' 1.15 1.55 0.11 (6x14 mesh)

Wood product-derived carbon made by Westvaco, Chemical

Div. 2

Coconut shell carbon made by American Noπte Co., Inc.

Coal-derived carbon made by Calgon Corp., Activated

Carbon Div.

Coal-derived carbon supplied by Fisher Scientific Co.

Filtration temperature was below 80°C, and a larger amount than desired of trimer precipitated.