Process for crystallizing an adduct of 2,2-di(4-hydroxyphenyl) propane and phenol

2,2-di(4-hydroxyphenyl) propane, i.e., diphenylolpropane, is purified by preparing a liquid mixture of raw diphenylolpropane, phenol and water and applying reduced pressure thereto corresponding to the vapor pressure of the mixture while simultaneously cooling same whereby an adduct of pure diphenylolpropane and phenol is crystallized.

The present application relates to disclosure document No. 029919 deposited 
on Apr. 5, 1974 which is expressly incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
The present invention relates to a process for crystallizing an adduct of 
2,2-di(4-hydroxyphenyl) propane and phenol in aqueous phenol, under 
vacuum. 
It is known that in order to obtain 2,2-di(4-hydroxyphenyl) propane 
(hereinafter referred to as diphenylolpropane or DPP) having a suitable 
purity for certain applications such as the preparation of polycarbonate 
resins, it is first necessary to purify the raw product. One such prior 
art method consists of crystallizing diphenylolpropane in a suitable 
solvent such as aqueous phenol. Thus, in U.S. Pat. No. 3,192,270, a 
process for the manufacture of pure di(hydroxyphenyl) alkanes is described 
which comprises dissolving these compounds in hot aqueous phenol and 
cooling the solution thereby precipitating crystals of the adduct of 
di(4-hydroxyphenyl) alkane and phenol and liberating the phenol from the 
adduct. 
From a practical standpoint, the aforementioned process comprises the 
following steps: melting of the mixture resulting from the condensation of 
phenol and a ketone under the action of an acid catalyst, which contains 
the impure phenol-diphenylolalkane adduct as well as phenol; neutralizing 
the mixture with a concentrated aqueous solution of an alkali; separating 
the resulting aqueous phase; adding sufficient water to the remaining 
organic phase to saturate all of the phenol contained therein; decanting 
the mixture into two phases; separating the aqueous phase; cooling the 
organic phase to room temperature to allow the purified adduct of 
diphenylolalkane-phenol to crystallize and, lastly, separating this adduct 
from the mother-liquor. This process typifies prior art processes for the 
crystallization and purification of diphenylolpropane and it is apparent 
that this prior art process is overly complicated and requires excessive 
amounts of water. 
Therefore, there exists a need for a more efficient and economical process 
for crystallizing diphenylolpropane. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide a 
process for crystallizing an adduct of 2,2-di(4-hydroxyphenyl) propane and 
phenol to prepare pure 2,2-di(4-hydroxyphenyl) propane which obviates the 
problems and disadvantages inherent in prior art processes. 
It is a further object of the invention to provide a process for purifying 
diphenylolpropane by the crystallization of a pure adduct of 
diphenylolpropane and phenol and to afford diphenylolpropane in a highly 
purified form and in good yield. 
These and other objects are accomplished according to the crystallization 
process of the present invention wherein a mixture containing impure 
diphenylolpropane, phenol and a minor amount of water is prepared at a 
temperature at which the mixture is substantially entirely liquid, 
applying reduced pressure to the mixture corresponding to the vapor 
pressure thereof and simultaneously cooling the mixture to effectuate the 
crystallization of a pure adduct of diphenylolpropane and phenol. 
Other objects, features and advantages of the present invention will become 
apparent to those skilled in the art from the accompanying FIGURE of 
drawing and the detailed description of the invention which follows.

DETAILED DESCRIPTION OF THE INVENTION 
According to the process of the present invention, a mixture containing 
impure diphenylolpropane, phenol and less than about 15% by weight of 
water is prepared at a temperature at which the mixture is entirely liquid 
and then cooled to a temperature less than about 60.degree. C. while 
applying reduced pressure to the mixture which pressure approximately 
corresponds to the vapor pressure of the reaction mixture whereby an 
adduct of pure diphenylolpropane and phenol is crystallized. 
Applicants have surprisingly discovered that if crystallization of the 
diphenylolpropane-phenol adduct is effectuated under vacuum that the 
amount of water in the starting mixture necessary for the crystallization 
process can be significantly reduced, e.g., to as little as 2% or less by 
weight with respect to the mixture, and further that the solubility of the 
adduct in the crystallization medium is enhanced which favors 
crystallization yield. 
The liquid mixture to be crystallized typically contains raw 
diphenylolpropane to be purified, phenol and water. Raw diphenylolpropane 
as utilized herein means the product resulting from the manufacture 
thereof which is generally constituted principally with 
2,2-di(4-hydroxyphenyl) propane and with various impurities, particularly 
isomers of diphenylolpropane and compounds such as 
2,2,4-trimethyl(4-hydroxyphenyl) chromane (also referred to as co-dimer) 
and the like. The raw diphenylolpropane which is supplied to the starting 
mixture of the invention may be obtained by any conventional method such 
as, for example, by the condensation of phenol and acetone in the presence 
of an acid catalyst such as hydrochloric acid. The amount of raw 
diphenylolpropane present in the liquid mixture is not in the least 
critical and may vary over a wide range up to the point of saturation 
which generally should not be exceeded. An amount of raw diphenylolpropane 
ranging between 10% and 55% based upon the total weight in the mixture has 
been found to be generally suitable. The amount of water present in the 
initial mixture is, however, somewhat critical and should be comprised 
between about 2 and 12 % by weight and, preferably, 4 to 8%. When the 
mixture is introduced into the crystallization zone, the temperature of 
the mixture is generally that at which all of the components in the 
mixture are substantially in the liquid state. A temperature of between 
about 70.degree. and 100.degree. C. has been found to be particularly 
satisfactory for maintaining the components in a liquid state. 
The starting mixture may be prepared by heating suitable amounts of phenol, 
water and raw diphenylolpropane to a predetermined temperature. According 
to a preferred embodiment of the present invention, when the manufacture 
of diphenylolpropane is carried out in the presence of a large excess of 
phenol which is generally consistent with most commercial processes, the 
mixture to be crystallized is directly prepared from the reaction medium 
utilized in the manufacture of diphenylolpropane without preliminary 
separatory steps except for the removal of the catalyst utilized in the 
reaction. Optionally, the reaction mixture to be crystallized is first 
concentrated by distillation of a portion of the phenol and a suitable 
amount of water is added to the liquid remaining. 
During the crystallization process of the present invention, the starting 
mixture described hereinabove is cooled to a temperature between about 
25.degree. and 60.degree. C., preferably 40.degree. to 50.degree. C., 
under reduced pressure essentially corresponding to the vapor pressure of 
the crystallization medium. The vapor phase is primarily comprised of 
water, but also contains a small amount of phenol. The vapor pressure 
observed varies with the relative ratio of water in the initial mixture 
and the cooling temperature selected. Generally, with the temperatures and 
water ratios utilized herein, the vapor pressure will be between about 20 
and 120 mmHg. As stated previously, the reduced pressure applied to the 
crystallization zone generally corresponds to the vapor pressure of the 
mixture and in view of the relatively low vapor pressures prevailing under 
the condition in the crystallization zone, the vacuum applied to the 
system is likewise minimal so that the process of the invention is easily 
carried out. 
Some examples of vapor pressures are indicated in Table 1 below for various 
initial mixtures and crystallization temperatures according to the present 
invention. 
TABLE 1 
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Initial Mixture Vapor pressure (mm Hg) 
Precentage 
Water for various 
of diphenylol 
ratio crystallization 
propane (% temperatures 
(% by weight) 
by weight) 
40.degree. C. 
45.degree. C. 
50.degree. C. 
60.degree. C. 
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10 5 33.5 
10 5 .about.77 
10 8 .about.38 
10 8 .about.89 
10 11 45.5 .about.112 
25 6 .about.35 
30 6.5 .about.42 
38 9 .about.58 
50 4 35 
50 5 40 
______________________________________ 
From the foregoing table, it is apparent that the vapor pressure varies 
independently of the diphenylolpropane concentration in the initial 
mixture and depends on the amount of water in the mixture as well as the 
crystallization temperature. 
According to a preferred embodiment of the present invention, the vapor 
phase may be extracted from the crystallization zone and all or part 
thereof advantageously condensed and returned to the crystallization zone 
either continuously or discontinuously. 
The crystals obtained from the crystallization zone consist of a pure 
adduct of diphenylolpropane and phenol containing approximately equimolar 
amounts of the respective components. The crystals are separated from the 
mother-liquor, utilizing conventional separatory techniques including 
washing and subsequently treated to eliminate the phenol component. Phenol 
may be removed from the adduct by any of the methods heretofore employed, 
such as extraction, distillation or steam stripping. 
The mother-liquor remaining after the separation of the crystals as 
described above still contains a minor amount of diphenylolpropane. The 
residual diphenylolpropane may be utilized without modification for the 
preparation of a new liquid mixture to be crystallized according to the 
present invention. Of course, the diphenylolpropane may be distilled in 
order to recover aqueous phenol therefrom which can also be recycled. 
However, according to a particularly advantageous embodiment of the 
present invention, the mother-liquor may be crystallized to obtain an even 
higher crystallization yield. For example, the mother-liquor may be 
concentrated up to a desired diphenylolpropane ratio by distilling a part 
of the phenol off and adjusting the water ratio so that a mixture is 
obtained which may be reheated to a substantially liquid form and then 
crystallized according to the process of the present invention. 
Diphenylolpropane which is crystallized from the mother-liquor as described 
immediately above may be used directly, after elimination of phenol, for 
various applications. However, if diphenylolpropane of the same high 
degree of purity as that obtained according to the crystallization process 
herein is desired, the adduct of diphenylolpropane-phenol or only 
diphenylolpropane collected from the mother-liquors is utilized in the 
preparation of a liquid mixture used as the feed in the instant 
crystallization process. 
The present process can be carried out with equivalent results either 
discontinuously or continuously. Carrying out the present crystallization 
process continuously is particularly advantageous. Even after several 
hours, the crystallization apparatus remains unobstructed. Moreover, the 
present crystallization process requires no stirring or only moderate 
stirring thus avoiding physical degradation of the crystals formed which 
is a serious disadvantage of heretofore utilized crystallization systems 
based on cooling at ordinary pressures which require strong stirring of 
the mixture to eliminate the calories produced which concomitantly causes 
the adduct crystals being formed to break apart. The diphenylolpropane 
obtained from the pure crystallized adduct is of such purity and whiteness 
that it can be directly employed in applications wherein high purity of 
the diphenylolpropane starting material is essential, such as in the 
manufacture of polycarbonate resins. The crystallization yield is very 
high and generally exceeds 90%. 
The crystallization process of the present invention may be typically 
integrated in a process for the manufacture of diphenylolpropane. The 
overall integrated process will generally include the following steps 
described with reference to the accompanying schematic diagram of FIG. 1. 
In zone 1, acetone is reacted with a large excess of phenol, for example, 6 
to 10 moles of phenol for each mole of acetone in the presence of gaseous 
hydrochloric acid and a predetermined amount of water, such as 3 to 10% by 
weight at a temperature from about 20.degree. to 50.degree. C. until all 
of the starting acetone has been converted. Generally the initial reaction 
is carried out in several reactors. 
The reaction mixture from zone 1 is next subjected to two distillations in 
columns 2 and 3 to first remove hydrochloric acid and water and then a 
sufficient amount of phenol so that the distillation bottoms contain the 
desired amount of raw diphenylolpropane and phenol for subsequent 
crystallization. The first distillation is, preferably, carried out under 
reduced pressure, for example, between 30 and 50 mm Hg and the distillate 
head from column 2 comprises an aqueous solution of hydrochloric acid 
containing a small amount of phenol. This solution is then treated to 
recover the respective constituents, for example, according to the method 
described in U.S. Pat. No. 3,829,509, filed Nov. 24, 1969 and incorporated 
herein by reference. The second distillation is likewise advantageously 
effected under a reduced pressure which is approximately the same as that 
in the first column. The distilled phenol can be recovered and recycled to 
any stage of the process. 
The bottoms from column 3 are drawn off and an amount of water equal to 
about 2 to 12% by weight with respect to the total mixture is added via 
inlet 4 to prepare a mixture in zone 5 from which the pure 
diphenylolpropane-phenol adduct is crystallized according to the present 
invention. 
The crystals are separated from the mother-liquor in zone 6 by filtration 
or centrifugation, for example, and drawn off at 7 and subsequently 
treated to eliminate phenol therefrom as described hereinabove. 
In column 8, the mother-liquor from zone 6 is distilled to remove an amount 
of phenol sufficient to provide a distillation bottom containing the 
desired proportions of raw diphenylolpropane and phenol for subsequent 
crystallization. The distilled phenol entrains the water present in the 
mother-liquor and may, for example, be recycled for condensation with 
acetone. 
The bottoms of the preceding distillation are drawn off and an amount of 
water comprising between about 2 to 12% by weight with respect to the 
total mixture is added thereto at inlet 9 and a fresh crystallization of 
diphenylolpropane-phenol adduct is effected from this mixture in zone 10. 
Again, the crystals are separated from the mother-liquor in zone 11 and 
collected in zone 12 and either processed for the removal of phenol or 
introduced into the mixture of the initial crystallization zone. 
Complete distillation of the phenol present in the mother-liquor obtained 
in zone 11 is carried out in column 13, preferably under reduced pressure. 
The phenol entrains the water present in the mother-liquor and it may be 
used for condensation with acetone. The final distillation bottom in 
column 11 is generally comprised of unusable impurities and may be 
discarded. 
The integrated process described above including the various recycling and 
product recovery stages provided for therein permits the continuous 
obtention of diphenylolpropane of extremely high purity in a simple, 
efficient process which is totally acceptable from a commercial production 
standpoint. 
The following non-limitative examples further illustrate the present 
invention. 
EXAMPLE 1 
A crystallization zone is continuously fed at a rate of 50 l./hour at 
approximately 85.degree. C. with a liquid mixture comprising (by weight): 
raw diphenylolpropane: 50% (with 3% of impurities) 
water: 5% 
phenol: 45% 
In the crystallizer, the temperature is maintained at about 45.degree. C. 
and the vapor pressure is 40 mm Hg. and the residence time is one hour. 
The vapor phase extract is condensed in a heat exchanger the cooling water 
of which is at 20.degree. C. and the condensate is recycled entirely to 
the crystallizer. 
The crystallization slurry is continuously drawn off from the 
crystallization zone and dried in a centrifuge at 1000 g. After 60 hours, 
1800 kg. of adduct crystals are collected and subsequently washed and 
dried. 1600 kg. of diphenylolpropane-phenol adduct are obtained containing 
70% by weight of diphenylolpropane and 30% by weight of phenol. The 
diphenylolpropane contains less than 0.2% by weight of impurities. The 
crystals of adduct have a coloration corresponding to 30 A.P.H. after 
melting. 
EXAMPLE 2 
The procedures of Example 1 are repeated. The mother-liquors of 
crystallization are concentrated by distillation under 200 mm Hg. and 
160.degree. C. to obtain a bottom fraction comprised of phenol and 55% by 
weight of raw diphenylolpropane. Four percent by weight of water is added 
to this mixture and the mixture heated to maintain a temperature of about 
90.degree. C. The mixture is then crystallized at 50.degree. C. under 35 
mm Hg. After drying and washing, 40 parts of adduct are collected from 100 
parts of the initial mixture with the adduct having 2% by weight of 
impurities relative to the weight of diphenylolpropane. 
The above examples and disclosure are set forth merely for illustrating the 
mode and the manner of the invention and, while various modifications and 
embodiments can be made by those skilled in the art, in the light of this 
invention, they are made without departing from the spirit of the 
invention. It is intended, therefore, that the invention be limited only 
by the scope of the claims which follow.