Purification of bisphenol-A

A method for purifying water-crystallized bisphenol-A involving treatment with an aqueous alkaline solution. The concentration of base preferably ranges from 0.1 to 25 weight percent of crude bisphenol-A. The crude bisphenol-A is a solid and preferably at a temperature in the range of about 10.degree. to 80.degree. C.

CROSS REFERENCE TO RELATION APPLICATION 
Reference is made to copending application Ser. No. 443,344 filed Nov. 15, 
1982, assigned to the same assignee as the present invention. 
BACKGROUND OF THE INVENTION 
This invention is concerned with the purification of 
2,2-bis(4-hydroxyphenyl) propane (herein identified as "bisphenol-A" or 
"BPA"). More particularly, this invention is directed to a method of 
recovering bisphenol-A in a purified state from crude bisphenol-A by 
treatment with an aqueous alkaline solution. 
Crude bisphenol-A is the isolated product of commercial processes for 
preparing bisphenol-A. It is a mixture of bisphenol-A and impurities 
derived from a BPA synthesis reaction. An example of a BPA synthesis 
reaction is the acid-catalyzed condensation of phenol and acetone, where 
phenol and acetone react in the presence of an acidic material, such as 
sulfuric acid, hydrochloric acid, cation exchange resin, etc. 
The crude bisphenol-A produced contains undesirable impurities such as 
phenol, 2-(4-hydroxyphenyl)-2-(2-hydroxyphenyl) propane having the formula 
##STR1## 
a trishydroxyphenyl compound of the formula 
##STR2## 
small amounts of other impurities such as the two compounds having 
formulas 
##STR3## 
and some impurities which discolor the crude bisphenol-A with unknown 
structure (herein identified as color bodies). 
Since bisphenol-A is used in making polycarbonate resins by reaction of the 
former with either phosgene or diphenyl carbonate, or for making epoxy 
resins, both resins being used extensively in commercial applications 
involving molding, casting and sheet forming processes, it is highly 
important that the monomeric bisphenol-A used to make such resins be as 
pure as possible in order to avoid adverse effects on the properties of 
the polymers thus obtained. 
The preparation of bisphenol-A by the acid catalyzed reaction of phenol and 
acetone is usually carried out in excess phenol (2 or more moles per mole 
of acetone). This reaction mixture is either subjected to a series of 
distillation steps to remove substantially all phenol, acetone and water 
or the acetone and water are removed and the bisphenol-A product is 
crystallized in the presence of phenol, followed by stripping of the 
phenol. Both procedures provide crude bisphenol-A which is the starting 
point for making bisphenol-A of high purity. Distillation or stripping of 
the phenol from the reaction mixture can be carried out only to a limited 
extent on account of the thermal instability of the bisphenol-A product. 
A conventional method of recovering pure bisphenol-A product from crude 
bisphenol-A involves crystallization in the presence of an organic 
solvent. The crude molten bisphenol-A is first dissolved in a suitable 
organic solvent and the solution is then cooled to yield pure bisphenol-A 
crystals, which are recovered by filtration. However, this process suffers 
in that (1) the crystals produced are fine, powdery and needle-like and 
are difficult to handle, store and dry; (2) these crystals permit some 
impurities and organic solvent to occlude during crystallization, these 
impurities cannot be removed during the drying step and hence they are 
present in the polymerization process and (3) these processes require the 
use of organic solvent. 
A method which uses water as a crystallization medium for crude bisphenol-A 
is described in U.S. Pat. Nos. 3,326,986 and 3,277,183. According to the 
disclosure in U.S. Pat. No. 3,326,986, the isolated crude bisphenol-A in 
molten form is purified by first mixing with water and cooling the mixture 
to yield large, rhombic crystals of bisphenol-A. The crystallization in 
water does not provide purification; however, separation of these crystals 
from the mother liquor, followed by an organic solvent wash, results in 
purified bisphenol-A. Although the process described within the 
above-referenced patent avoids occlusion by an organic solvent during the 
crystal formation step and yields large, less needle-like crystals that 
are easy to handle, the purification obtained is limited and large 
quantities of organic solvent are required to wash the crystals. 
The disclosure in U.S. Pat. No. 3,277,183 teaches that crystallizing crude 
bisphenol-A in hot water containing sodium hydroxide provides purification 
of the bisphenol-A. A process described within the above-referenced patent 
does not require organic solvent to obtain a high degree of purification. 
However, the crystals obtained are needle-like and difficult to handle and 
permit occlusion of impurities. Therefore, the purification obtained by 
this process is limited. In addition, high temperatures are utilized in 
the presence of caustic, which is undesirable since bisphenol-A is subject 
to increased degradation or cracking at high temperatures. 
The process described in Ser. No. 443,344, referenced above, describes a 
washing procedure for purifying aqueous crystallized bisphenol-A with a 
water/organic solvent wash. Although the process provides large, purified 
crystals of bisphenol-A which are easy to handle, the process comprising 
this invention provides crystals of high purity and is much simpler to 
execute due to the absence of organic solvent. 
The process comprising this invention provides a method of obtaining large 
bisphenol-A crystals of high purity without utilizing an organic solvent 
and with less degradation of bisphenol-A product than the process 
described in U.S. Pat. No. 3,277,183. 
SUMMARY OF THE INVENTION 
A method of purifying water-crystallized crude bisphenol-A is provided 
comprising washing crude bisphenol-A with an aqueous alkaline solution. 
The concentration of base is typically within the range of 0.1 to 25 
weight percent of crude bisphenol-A and is most preferably in the range of 
1 to 10 weight percent of crude bisphenol-A. 
OBJECTS OF THE INVENTION 
An object of the present invention is to purify crude bisphenol-A without 
utilizing an organic solvent. 
Another object of the present invention is to obtain bisphenol-A crystals 
of high purity of a rhombic shape and large size. 
Another object of the present invention is to provide a process for 
purifying crude bisphenol-A utilizing an aqueous alkaline solution with no 
significant degradation of bisphenol-A product. 
Another object of the present invention is to provide a process for 
purifying crude bisphenol-A which comprises a relatively small number of 
simple steps and procedures. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The objects of the present invention, and other objects, are achieved by 
treating aqueous crystallized crude bisphenol-A with an aqueous alkaline 
solution. 
The crude bisphenol-A produced from a BPA synthesis reaction is typically 
in the form of a liquid residue as, for example, in the case of crude 
bisphenol-A produced from the acid-catalyzed condensation reaction with 
phenol and acetone after the excess phenol, acetone, and water is removed 
as described above. 
Typically the crude bisphenol-A is molten and crystallization can be 
effected by simply adding water and slowly cooling the mixture to a 
temperature in the range of 60.degree. to 70.degree. C. The quantity of 
water used is not critical, but preferably equal to 1 to 2 parts by weight 
of the quantity of crude bisphenol-A that is crystallized. The molten 
crude bisphenol-A and water exist in two phases with the liquid crude 
bisphenol-A phase resting below the water phase. As the mixture cools with 
agitation, large crystals form and a slurry of BPA crystals and water is 
produced. All the water immiscible impurities and color bodies adhere to 
the crystals during crystallization. Essentially only excess phenol is 
removed from the crystals in this step. It is these impure crystals with 
which this process is most useful. 
This process is also capable of purifying bisphenol-A obtained by other 
means, such as bisphenol-A which is crystallized from an organic solvent. 
Bisphenol-A which is crystallized in such a manner is substantially pure 
since the organic solvent removes a significant quantity of impurities. In 
such a situation, it is impractical to utilize this process since the 
purity of the bisphenol-A crystals will not increase significantly. This 
route to pure bisphenol-A crystals is undesirable due to the large 
quantities of organic solvent that are required to crystallize the molten 
crude bisphenol-A. 
After the molten crude bisphenol-A is crystallized in the presence of 
water, a water soluble base is introduced into the water/BPA crystal 
slurry. The base ionizes impurities, including phenol, isomeric diphenols 
and triphenols, color bodies, etc. found in crude bisphenol-A. The ionized 
impurities become soluble in the aqueous medium and are washed from the 
surface of the crude bisphenol-A crystals. 
Suitable alkaline substances which may be utilized to ionize the impurities 
and increase their water solubility are, for example, alkali metal 
hydroxides, alkali earth metal hydroxides, carbonates, bicarbonates, 
ammonium and tertiary amines of the formula R'.sub.3 N, where R' is a 
monovalent radical selected from the group consisting of hydrogen, alkyl 
radicals of from 1 to 8 carbon atoms and aryl radicals of from 6 to 12 
carbon atoms. The preferred alkaline substances include sodium hydroxide, 
potassium hydroxide, calcium hydroxide, ammonia, triethylamine, 
trimethylamine, and pyridine. 
The quantity of base utilized can range between about 0.1 to 25 weight 
percent of crude bisphenol-A. The quantity of base which obtains maximum 
purification is dependent on the temperature of the crude bisphenol-A and 
aqueous alkaline solution. Lower temperatures demand larger quantities of 
base to maximize purification than the higher temperatures. However, the 
maximum degree of purification obtained by this process is independent of 
temperature. The maximum purification which can be obtained at 70.degree. 
C. is approximately equal to the maximum purification which can be 
obtained at 20.degree. C. Therefore, to conserve base and maximize 
purification, high temperatures may be desired. However, degradation of 
the bisphenol-A will increase at higher temperatures. The quantity of base 
utilized is preferably in the range between about 1 and 10 weight percent 
of crude bisphenol-A at temperatures between 10.degree. C. to 80.degree. 
C. Smaller quantities of base will usually provide a lower degree of 
purification, particularly at lower temperatures. Utilizing excessive 
quantities of base will also reduce the purification obtained, especially 
when quantities in excess of 10 weight percent of crude bisphenol-A are 
utilized. Within the preferred range, slight variations in the 
concentration of base will slightly effect the purification obtained. 
Increasing or decreasing the concentration of base as little as 0.5 weight 
percent will effect purification. Where maximum purification is desired 
for a particular temperature of the crude bisphenol-A and aqueous alkaline 
solution, the quantity of base utilized should be within the preferred 
range. However, quantities within the preferred range may be excessive and 
reduce the degree of purification obtained at certain temperatures. For a 
temperature of about 60.degree. C., maximum purification is expected from 
quantities of base within the range of about 3.5 to 4.5 weight percent of 
crude bisphenol-A. For temperatures of about 70.degree. C., maximum 
purification is expected from quantities of base within the range of about 
3.0 to 4.0 weight percent of crude bisphenol-A. For temperatures of about 
50.degree. C., quantities of base in the range of 5.0 to 6.0 weight 
percent of crude bisphenol-A are expected to provide maximum purification. 
However, as indicated above, conditions which favor maximum purification 
with small quantities of base may not be desired if the extent of 
bisphenol-A degradation is high. 
The alkaline substance to be added is preferably in an aqueous solution. 
Adding the alkaline substance directly, such as ammonia gas, is suitable 
but requires the use of complex equipment. Organic solvents will interfere 
with the washing process and they also interfere with the ionization and 
solubilization of impurities. 
It is preferable to maintain a crude bisphenol-A as a solid suspended in 
water when washing with the aqueous alkaline solution. The crude 
bisphenol-A crystals are preferably in a temperature within the range of 
about 10.degree. C. to about 80.degree. C. Purification can be obtained at 
temperatures above the preferred range; however, significant degradation 
of the bisphenol-A product could result upon exposure to an oxidative 
atmosphere in the alkaline solution and the maximum degree of purification 
which can be obtained is reduced. To avoid significant degradation of the 
bisphenol-A product, an inert atmosphere, such as a nitrogen atmosphere, 
is maintained above the slurry of crude bisphenol-A and aqueous alkaline 
solution. The extent of bisphenol-A degradation within the preferred 
temperature range is substantially less than at temperatures above 
80.degree. C.; however, the use of an inert atmosphere is still desirable 
when operating within the preferred range. 
The quantity of water utilized is preferably within the range of about 1 to 
5 parts per part of crude bisphenol-A. Quantities outside of this range 
can be utilized; however, either the effectiveness of the purification is 
reduced or the efficiency of the process is reduced. Where the aqueous 
crystallized bisphenol-A is within the aqueous crystallization medium, a 
concentrated alkaline solution need only be added to the slurry without 
significantly increasing the volume of water. Where the aqeuous 
crystallized bisphenol-A is removed from the aqueous medium, a volume of 
water within the preferred range must be added to the crystals along with 
the base. 
Once the aqueous crystallized crude bisphenol-A is washed with the aqueous 
alkaline solution, the purified bisphenol-A crystals are removed by 
employing conventional solid/liquid separation equipment. The purified 
crystals are typically removed by a basket centrifuge or by filtration. 
The crystals obtained will be water-wetted with a very low alkaline 
content. As such, the crystals need not be washed and completely dried if 
used to form polycarbonate since water and base are utilized in the 
polymerization process. However, where it is desirable to remove these 
trace quantities of base, such as when the BPA will be stored for later 
use, the purified bisphenol-A crystals are washed with water, preferably 
in a quantity of about 1 to 5 parts per part of bisphenol-A. Washing the 
crystals with water can be achieved in the separation equipment, such as a 
filter or centrifuge, or it can be achieved by reslurrying the crystals. 
When reslurried, the bisphenol-A crystals can be removed from the slurry 
by applying the same procedure that is utilized to remove the aqueous 
alkaline solution. Upon separation, the crystals are dried to remove 
excess water. The slightly water wetted crystals need not be completely 
dried if used to form polycarbonate. Water is utilized in the 
polymerization process and its presence in the bisphenol-A starting 
material will not effect the finished product. Drying the purified BPA may 
be aided by the use of conventional equipment. 
Bisphenolic compounds other than the 2,2-bis-(4-hydroxyphenyl)propane which 
can be purified in accordance with the process of this invention are, for 
example, bis(hydroxy-aryl)-alkenes such as 4,4'-dihdroxydiphenyl-methane; 
1,1-bis(4-hydroxyphenyl)ethane, propane, butane, isobutane; 
2,2-bis(4-hydroxyphenyl)butane and bis(4-hydroxyphenyl) ether, sulfide, 
sulfoxide, and sulfone. 
The impurities and small quantities of bisphenol-A which are solubilized by 
the aqueous alkaline solution may be isolated and returned to the BPA 
synthesis reaction. After the purified crystals are separated from the 
aqueous alkaline solution, the solution may be neutralized by a 
stoichiometric quantity of acid. Suitable acids include hydrogen chloride, 
hydrogen bromide, hydrogen fluoride, acetic acid, formic acid, etc. The 
impurities and bisphenol-A precipitate out of the aqueous alkaline 
solution upon neutralization. The precipitate is removed from the 
neutralized solution and washed with water. The precipitate, which 
contains bisphenol-A and impurities may be returned to the acid catalyzed 
condensation reaction where bisphenol-A is produced. The neutralized 
solution is comprised of water and the salt corresponding to the base 
started with and the acid utilized in neutralization. The neutralized 
solution contains only minute quantities of organic species (impurities, 
phenol and BPA) and can usually be discarded or recycled. 
The process comprising this invention has the advantage that the entire 
purification process, including aqueous crystallization, can take place in 
one vessel. The crystallized crude bisphenol-A can be washed in the same 
vessel in which crystallization takes place. Another advantage to this 
process is its simplicity. Essentially only one step is required to purify 
the crystal, i.e., the addition of a base to the water/BPA crystal slurry. 
The aqueous alkaline solution need only be separated to recover purified 
bisphenol-A crystals. 
The purified bisphenol-A product from this process will be free of any 
organic washing solvent and will only be water wetted. By avoiding the use 
of organic solvent, the production of substantially pure polycarbonate 
will be simple and less hazardous. Another advantage to this process is 
that the crystals produced are large and rhombic in shape. These crystals 
are easy to handle in the equipment utilized to produce polycarbonate 
resin. In addition, these crystals approach 100% purity with no impurities 
being detected by liquid chromatograph analysis, melting point tests and 
absorbance values. Such crystals are highly desirable in the production of 
polycarbonate resin. 
EXPERIMENTAL 
The following experiments were made to illustrate and compare the processes 
described in U.S. Pat. Nos. 3,326,986 and 3,277,183 with that of the 
present invention. 
The following experiment demonstrates the process described in U.S. Pat. 
No. 3,326,986. Samples of aqueous crystallized bisphenol-A were obtained 
by introducing about 200 grams of crude bisphenol-A (96% pure, initial 
absorbance value=2.00) to about 400 grams of water in a 1000 milliliter 
flask and cooling the resulting slurry to about 65.degree. C. The flask 
was equipped with an agitator, a thermometer, baffles and condensor. The 
crystals were melted by heating the flask to around 100.degree. C. within 
an oil bath. A nitrogen blanket was maintained over the flask contents 
during heating. This was followed by cooling (with agitation) to about 
65.degree. C. to effect crystallization of the crude bisphenol-A. The 
contents of the flask were centrifuged in a basket centrifuge to separate 
the solid crystals. The separated crystals were washed with 400 grams of 
toluene and then analyzed to determine the quantity of impurities. High 
pressure liquid chromatography indicated that the crystals contained 0.2 
weight percent impurities. The initial absorbance value was measured to 
determine the quantity of color bodies by determining the light absorbance 
of a 10% BPA solution in a 10 centimeter cell. The BPA solution contained 
5 grams of the washed bisphenol-A crystals and 50 milliliters methanol. 
The wavelength of light utilized in the test was 350 nm. The initial 
absorbance value for the washed crystals was 0.25. 
The following procedure illustrates the process described in U.S. Pat. No. 
3,277,183. 100 grams of impure bisphenol-A (96% purity; initial absorbance 
value=2.0) was mixed with 150 grams water in a 1000 milliliter flask 
equipped with an agitator, overhead condensor, and a thermometer. The 
flask and its contents were heated to about 100.degree. C. in an oil bath. 
A nitrogen blanket was maintained over the flask contents during heating. 
Two liquid phases were observed, the top aqueous phase was clear and 
colorless and the bottom organic phase was highly colored and clear. While 
maintaining the temperature at about 100.degree. C., a 50% aqueous 
solution of NaOH was added slowly to the two phase mixture. No noticeable 
change in color or volume of the two phases was observed until the 
addition of about 6.4 grams of 50% aqueous NaOH. Further addition of 0.2 
grams 50% NaOH resulted in a one phase, clear, colored solution. This 
solution was cooled gradually while stirring. Small, needle-like crystals 
were observed at about 83.degree. C. and after cooling the mixture to 
75.degree. C., the crystals were separated from the solution with the help 
of a basket centrifuge. The crystals were washed with 80 grams water and 
spun to near dryness. A 65% yield of pure BPA product was obtained. The 
product had an initial absorbance value of 0.30, measured as described 
above, with 0.05 weight % impurities being detected by high pressure 
liquid chromatography. 
The following examples are provided in order that those skilled in the art 
may be better able to understand this invention. They are provided to 
illustrate the invention and are not intended to limit the scope of the 
invention to their contents.

EXAMPLE I 
To 100 grams of crude bisphenol-A (96% purity; initial absorbance 
value=2.00) were added 150 grams of water in a 1000 milliliter flask. The 
flask was equipped with an agitator, thermometer and an overhead 
condensor. The flask and its contents were heated to about 100.degree. C. 
under a nitrogen blanket in an oil bath until all the solids were melted. 
A two phase liquid mixture was obtained; the top aqueous phase being clear 
and colorless and the bottom organic phase being highly colored and clear. 
The mixture was cooled slowly with agitation, and large, rhombic crystals 
were observed at about 95.degree. C. When the mixture was further cooled 
to about 70.degree. C., about 6.6 grams 50% aqueous NaOH was added to the 
water/BPA crystal slurry. The mixture was agitated and the crystals were 
separated with the help of a basket centrifuge. The crystals were washed 
with 80 grams of water and then spun to near dryness. A 75% yield of pure 
BPA was obtained. The pure BPA product had an initial absorbance value of 
0.10, measured as described above, and no impurities were detected by high 
pressure liquid chromatography. 
EXAMPLES II-XI 
The following examples illustrate the extent to which purification is 
dependent on the quantity of base utilized at various temperatures. For 
each of examples II-XI the following procedure was followed to obtain each 
sample. 
To 100 grams of crude bisphenol-A (96% purity; initial absorbance 
value=2.00) were added 150 grams of water in a 1000 milliliter flask. The 
flask was equipped with an agitator, thermometer and an overhead 
condensor. The flask and its contents were heated to about 100.degree. C. 
under a nitrogen atmosphere in an oil bath until all the solids were 
melted. A two phase liquid mixture was obtained; the top aqueous phase 
being clear and colorless and the bottom organic phase being highly 
colored and clear. The mixture was cooled slowly and crystals were 
observed at about 95.degree. C. The mixture was then further cooled to the 
temperature indicated in Table I. At this temperature, a 50% aqueous NaOH 
solution was added in a quantity sufficiently high to provide the weight 
percentage of crude bisphenol indicated in Table I. 
The mixture was agitated and the crystals were separated with the help of a 
basket centrifuge. These crystals were washed with about 80 grams of water 
and spun to near dryness. The washed bisphenol-A samples obtained were 
then analyzed by high pressure liquid chromatography and the initial 
absorbance values were obtained. No impurities were detected by high 
pressure liquid chromatography and the initial absorbance values obtained 
appear in Table I. 
TABLE I 
______________________________________ 
Degree of Purification Obtained 
from Various Quantities of Base 
Example Temp (.degree.C.) 
Quantity of NaOH* 
I.A.** 
______________________________________ 
II 45 2.9 0.63 
III 45 4.3 0.14 
IV 45 5.8 0.11 
V 45 6.5 0.17 
VI 60 3.25 0.18 
VII 60 4.0 0.12 
VIII 60 5.25 0.16 
IX 80 1.75 0.19 
X 80 2.5 0.11 
XI 80 4.25 0.21 
______________________________________ 
*The quantity of NaOH is represented as the weight percentage of crude 
bisphenolA started with. 
**I.A. represents the initial absorbance value obtained for the sample an 
is an indicator of the quantity of color bodies (impurities) in the 
sample. 
EXAMPLES XII-XIII 
Two samples of crude bisphenol-A (100 grams, 96% purity, initial absorbance 
value=2.00) were added to 200 grams and 300 grams of water, respectively, 
within a 1000 ml flask equipped as were described in Examples II-XI. 
Each sample was heated to about 100.degree. C. under a nitrogen atmosphere 
until all the solids were melted. These mixtures were cooled slowly and 
crystals were observed at about 95.degree. C. Each mixture was then cooled 
to about 45.degree. C. and treated with the same quantity of base as 
utilized in Example V (13 weight percent of crude bisphenol-A). The 
mixture was agitated and the crystals were separated and washed with 80 gm 
of water and spun to near dryness. The dried crystals had initial 
absorbance values as indicated below in Table II. 
TABLE II 
______________________________________ 
Degree of Purification Obtained 
Utilizing Various Quanties of Water 
Example Quantity of Water* 
Quantity of Base** 
I.A. 
______________________________________ 
V 1.5 6.5 0.17 
XII 2 6.5 0.18 
XIII 3 6.5 0.17 
______________________________________ 
*The quantity of water is represented as a weight ratio of water to crude 
bisphenolA started with. 
**The quantity of base is represented as the weight percent of crude 
bisphenolA started with.