Process for producing sorbic acid

A process for producing sorbic acid from a polyester obtained by reaction of crotonaldehyde with ketene using an organic acid zinc salt as catalyst is provided, which process comprises dissolving the polyester in an aromatic hydrocarbon solvent having an azeotropic temperature of the solvent with water of 92.degree. to 100.degree. C., followed by washing the polyester solution with water or a mineral acid water to remove zinc matter and continuously contacting the solution with a strongly acidic porous ion exchange resin, the process being simple, commercially profitable, making possible a long term operation of the polyester decomposition apparatus, capable of easily purifying sorbic acid and affording sorbic acid with a good yeild.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for producing sorbic acid. More 
particularly, it relates to a process for sorbic acid commercially and 
advantageously, from a polyester obtained from the reaction of 
crotonaldehyde with ketene (hereinafter referred to merely as polyester). 
2. Description of the Related Art 
Sorbic acid or its salts either have superior fungiresistance; hence they 
have been preferably used as a food preservative. 
The process for producing sorbic acid includes those of reacting 
crotonaldehyde with ketene to obtain a polyester via .beta.-lactone as an 
intermediate, followed by subjecting the polyester to acidolysis, thermal 
decomposition or alkaline decomposition to produce sorbic acid, and among 
these processes, acidolysis process has generally been often carried out. 
As the acidolysis process, a mineral acidolysis process using a mineral 
acid such as hydrochloric acid or sulfuric acid as a catalyst, a 
decomposition process with an ion exchange resin using as a catalyst, an 
acidic giant reticulated ion exchange resin (MR) having pores of at least 
200 .ANG. on an average and a water content of 20% by weight or less 
(Japanese patent publication No. Sho 57-47,655), or the like processes are 
known. 
According to the mineral acidolysis process, there are advantages that the 
polyester is not only decomposed into sorbic acid, but also a byproduced 
cis-isomer is isomerized into sorbic acid, but a problem is raised that a 
black-brown byproduct is formed simultaneously with the polyester 
decomposition, whereby crystals of sorbic acid are contaminated to make 
their purification difficult. Further, an additional problem is raised 
that catalyst-mineral acid water is contaminated to make its circulation 
use difficult; hence the mineral acidolysis process cannot always be 
regarded as a commercially advantageous production process. 
On the other hand, the decomposition process with an ion exchange resin is 
superior to the mineral acidolysis process in that the quantity of the 
black-color byproduct formed is very small, but the present inventors 
found as a result of the tracing test of the former process, a problem 
that the ion exchange resin loses its catalytic activity within such an 
unexpectedly short time that the catalyst cannot be practically used. 
In order to solve the above problem, we attempted to restore the catalytic 
acitivity by adding a mineral acid, as carried out in the prior art, but 
the restoring effect was small and to the contrary, various new problems 
occurred such as a problem of formation of a balck-brown byproduct due to 
the mineral acid, a problem of corrosion of equipment, and a problem that 
since the added mineral acid mixes in crystals of sorbic acid, it is 
necessary to remove the acid. The decomposition process with an ion 
exchange resin has a problem in that it is difficult to control the water 
content of the ion exchange resin used. 
Namely, in general, ion exchange resins have been commercially availed in a 
hydrous state of a water content in the vicinity of 50%, and when the 
water contained in the ion exchange resins is removed by means of 
azeotropic distillation, etc. as much as possible, in order to enhance the 
catalytic acitivity, then the catalytic activity is entirely lost, while 
when the water content is increased to the contrary, the catalytic 
activity is lost after a certain time. Thus, the subject to be solved by 
the invention is to provide a more advanced process for producing sorbic 
acid, having overcome the above-mentioned problems. 
The present inventors have examined the reduction in the catalytic activity 
as one of the above problems in the case of the decomposition process with 
an ion exchange resin, and as a result have found that the reduction is 
deeply related to impurities contained in the polyester, particularly to 
zinc, that is, when the polyester is prepared from crotonaldehyde and 
ketene, an ion exchange occurs between a zinc salt and the exchange group 
of the exchange resin to reduce the acid concentration of the ion exchange 
resin and thereby lose the catalytic function of the resin and as a 
result, reduce the percentage of decomposition of the polyester into 
sorbic acid. 
In order to recover the lost catalytic function of the ion exchange resin, 
it is necessary to wash the resin with about 5 to 10% hydrochloric acid, 
sulfuric acid or the like to dissolve away zinc, but as a result, since 
the ion exchange resin contains a large quantity of water, a cumbersome 
matter occurs that the resin should be dehydrated up to an adequate water 
content. 
Thus, the present inventors have made extensive research in order to find a 
process of not damaging the catalytic life of the ion exchange resin, and 
as a result have found that when the polyester is first dissolved in a 
solvent, followed by washing the resulting solution with a mineral acid 
water and, if the zinc salt is water-soluble, washing it with water in 
advance, the object can be easily achieved. 
Next, the present inventors have made extensive research into the catalytic 
acitivity of the ion exchange resin, and as a result, have found that when 
the ion exchange resin is treated with, preferably washed with a lower 
alkyl alcohol to replace a portion of water by the lower alkyl alcohol, 
the object can be easily achieved without the above difficult control of 
the water content. 
Namely, the ion exchange resin retaining a lower alkyl alcohol in the pores 
of the resin has a superior affinity to the solution of the polyester 
dissolved in the solvent, as compared with the ion exchange resin 
retaining water in the pores of the resin; hence the former resin easily 
contacts with the solution of the polyester dissolved in the solvent so 
that decomposition of the polyester raises no obstacle. 
This washing process with a lower alkyl alcohol removes alcohol-soluble 
impurities present in the ion exchange resin, and at the same time the 
process is applicable to a process for regenerating the ion exchange resin 
in the case where impurities such as ketene polymer, crotonaldehyde 
polymer, etc. remaining in the polyester adhere to the ion exchange resin 
during decomposition of the polyester to reduce the catalytic activity. 
The present inventors have solved these problems and have completed the 
present invention. 
SUMMARY OF THE INVENTION 
As apparent from the foregoing, an object of the present invention is to 
provide a novel, improved process for producing sorbic acid by decomposing 
a polyester with an ion exchange resin. Another object of the present 
invention is to provide an ion exchange resin catalyst for the polyester 
decomposition, treated with a lower alkyl alcohol, and a process for 
regenerating the catalyst. 
The present invention has the following constitutions (1) to (5): 
(1) In the process for producing sorbic acid by decomposing a polyester 
obtained by reacting crotonaldehyde with ketene in the presence of an 
organic acid zinc salt, the improvement which comprises dissolving said 
polyester in an aromatic hydrocarbon solvent having an azeotropic 
temperature thereof with water of 92.degree. to 100.degree. C., followed 
by washing the resulting polyester solution with water or a mineral acid 
water to remove the zinc matter contained therein, and continuously 
contacting the resulting solution with a strongly acidic porous type ion 
exchange resin. 
(2) The improvement according to item (1), wherein said strongly acidic 
porous type ion exchange resin is treated with a lower alkyl alcohol in 
advance, followed by continuously contacting said polyester solution with 
the treated resin at 60.degree. to 95.degree. C. 
(3) The improvement according to item (2), wherein said lower alkyl alcohol 
is at least one member selected from the group consisting of methanol, 
ethanol, propanol and isopropanol. 
(4) A catalyst for decomposing a polyester obtained by the reaction of 
crotonaldehyde with ketene in the presence of an organic acid zinc salt, 
which catalyst comprises a strongly acidic porous type ion exchange resin 
treated with a lower alkyl alcohol to replace a portion or the total of 
water contained in said resin by said lower alkyl alcohol. 
(5) A catalyst according to item (4), wherein said lower alkyl alcohol is 
at least one member selected from the group consisting of methanol, 
ethanol, propanol and isopropanol. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The constitutions of the present invention will be described in more 
detail. 
Polyester: 
The polyester used in the present invention is first prepared by reacting 
crotonaldehyde with ketene in the presence of an organic acid zinc salt 
catalyst, followed by heating the resulting polyester under reduced 
pressure to distil off low boiling matters of crotonaldehyde, a solvent 
used for the reaction of crotonaldehyde with ketene, diketene, etc. The 
resulting polyester is used in the present invention. 
Dissolution of the polyester: 
As the solvent for dissolving the polyester, water-insoluble aromatic 
solvents which dissolve the polyester in an optional proportion and have 
an azeotropic temperature thereof with water of 92.degree. to 100.degree. 
C. are used. Examples of such solvents are xylene, ethylbenzene, 
mesitylene, etc. 
Use of benzene or toluene has no inconvenience upon dissolution of the 
polyester, but when the polyester is decomposed at 85.degree. to 
90.degree. C., such solvents form an azeotrope with water; hence in the 
system wherein an ion exchange resin is filled in a column and the 
polyester is decomposed into sorbic acid by passing it through the filled 
resin layer, the operation is impossible unless a countermeasure such as 
pressurizing is applied. 
Further, in the case of a solvent which forms no azeotrope with water and 
has a boiling point of 100.degree. C. or higher, when sorbic acid is 
purified, the cost of removing the solvent attached to sorbic acid is so 
high that use of such a solvent is undesirable. 
The proportion of the polyester to the solvent depends upon the solubility 
of sorbic acid in the solvent, and a proportion of 3 to 8 parts by weight 
of the solvent per one part by weight of the polyester is usually 
employed. 
Purification of the polyester: 
Purification of the polyester solution depends upon the physical properties 
of the organic acid zinc salt as the catalyst at the time of preparation 
of the polyester, and in general, the purification is carried out 
sufficiently contacting the solution with a mineral acid water to convert 
the zinc salt into a water-soluble salt or precipitate and separating and 
removing them. 
Thus, in the case of a water-soluble salt such as zinc acetate, even when a 
mineral acid is not particularly used, it is possible to separate and 
remove the zinc salt into aqueous layer only by extraction with water. 
The acid concentration in the mineral acid water is generally 1% or higher, 
preferably 5% or higher. Further, even when the concentration of the 
mineral acid water is elevated, the purification effect is not 
particularly increased; hence the concentration is preferably 50% or lower 
in the case of sulfuric acid water or phosphoric acid water and preferably 
15% or lower in the case of hydrochloric acid. 
Ion exchange resin: 
As to the ion exchange resin used in the present invention, among 
styrene-divinylbenzene system sulfonated strongly acidic porous type 
resins, resins having an affinity to non-polar solvents are preferably 
used. Resins having no affinity to non-polar solvents have almost no 
catalytic function. 
Such resins having an affinity to non-polar solvents have been commercially 
availed in Japan genrally in a hydrous state in the tradenames of 
Highporous resin (made by Mitsubishi Kasei Co., Ltd.), MP resin (made by 
Organo Co., Ltd., Sumitomo Kagaku Co., Ltd.), etc. 
Such ion exchange resins are filled in a column and used, and when the 
polyester is decomposed, the ion exchange resins filled in the column 
together with water or a lower alkyl alcohol are continuously treated i.e. 
washed by passing the lower alkyl alcohol therethrough, followed by 
successively washing with the solvent used for dissolving the polyester 
and using the resulting resin. Such washing conditions (temperature, time) 
are not particularly limited, but they are usually 10.degree. to 
80.degree. C., preferably room temperature, and 10 minutes to 10 hours, 
preferably 4 to 5 hours. As the lower alkyl alcohol, lower alkyl alcohols 
such as methanol, ethanol, propanol, isopropanol, etc. are usable. 
According to this process, it is unnecessary to use anhydrous lower alkyl 
alochol in order to make the water content in the ion exchange resin as 
low as possible, but 0.5 to 5% hydrous commercially available product is 
usable. 
The quantity of the lower alkyl alcohol used for the washing has no 
particular limitation, but usually, these ion exchange resins contain 
impurities soluble in the lower alkyl alcohol and the solvent and often 
make the subsequent purification of sorbic acid difficult; thus the 
washing is carried out regarding as a measure, that any colored substance 
dissolved out of the resins is not seen. 
Further, an ion exchange resin commercially availed as a dried product may 
also be alike treated and used. 
The alcohol containing water in the thus obtained ion exchange resin may be 
purified by distillation and reused by circulation. 
Decomposition of polyester: 
The decomposition of polyester is carried out preferably according to a 
continuous system of passing a polyester dissolved in an aromatic organic 
solvent through an ion exchange resin layer filled in a column. The 
decomposition is possible even by way of an agitation vessel system, but 
since breakage of the ion exchange resin may occur, such a system is 
undesirable as an industrial method. 
The temperature of the column layer in the case of the polyester 
decomposition according to the continuous system is set within a 
relatively narrow temperature range of 60.degree. to 95.degree. C., 
preferably 65.degree. to 90.degree. C. 
If the decomposition temperature is set to the azeotropic temperature of 
water and a solvent or higher, the water and solvent remaining in the 
resin forms an azeotrope to make a stabilized operation difficult, and 
also in the case where water is lost from the ion exchange resin due to 
the formation of the azeotrope, a serious state of losing its catalytic 
function occurs. On the other hand, if the temperature is 55.degree. C. or 
lower, the decomposition speed is too low; hence such temperatures are 
practically undesirable. 
The ion exchange resin catalyst having been reduced in the activity at the 
time of its use is regenerated according to a process of washing it with a 
lower alkyl alcohol same as that described in the above item of ion 
exchange resin. The solution obtained by passing through the column and 
decomposing the polyester into sorbic acid, is subjected to solid-liquid 
separation to obtain crystals of sorbic acid. 
The solvent in the filtrate is circulated as it is and used for dissolving 
the polyester. 
Effectiveness of the Invention 
According to the process of the present invention, a long term operation of 
a polyester decomposition apparatus becomes possible and sorbic acid 
capable of being easily purified is obtained with a high yield, by way of 
a simple process of choice of solvent, preferably use of the polyester and 
treatment of an ion exchange resin with a lower alkyl alcohol. 
The present invention will be described in more detail by way of Examples, 
but it should not be construed to be limited thereto.

EXAMPLE 1 
A hydrous ion exchange resin (RCP-150, tradename of product made by 
Mitsubishi Kasei Co., Ltd.) was dispersed in hot water at 80.degree. C., 
followed by filling it into four stainless columns (diameter: 5 cm), each 
equipped with a jacket and in a resin layer height of 200 cm, washing off 
a brown eluate from the ion exchange resin with flowing methanol, and 
further flowing xylene at a temperature of the jacket of the column of 
65.degree. C. to prepare a methanol-substituted ion exchange resin column. 
Three columns among the four were connected in series and the temperature 
of the jackets was set to 75.degree. C. Another of the columns was used as 
a preliminary column. On the other hand, a polyester was prepared using 
zinc sorbate as catalyst and low-boiling matters were removed therefrom. 
The resulting polyester (zinc content: 0.2%) (one part by weight) was 
dissolved in xylene (5 parts by weight) to prepare a polyester-xylene 
solution (preparation rate: 3.6 Kg/Hr). 
This polyester-xylene solution was fed at a rate of 3.6 Kg/Hr, into an 
extraction column being operated at 70.degree. C., while feeding 5% 
hydrochloric acid at a rate of 0.1 l/Hr in counterflow to the above 
solution to contact the polyester-xylene solution with the 5% hydrochloric 
acid and thereby extract zinc sorbate into the aqueous layer, followed by 
further washing the resulting polyester-xylene layer with water and then 
feeding it into the above-mentioned ion exchange columns (three in series; 
jacket temperature 75.degree. C.). 
Successively, the decomposition fluid flowing out of the ion exchange resin 
columns was continuously cooled and filtered to separate crystals of 
sorbic acid and circulate the filtrate for dissolving the polyester. 
Continuous operation was carried out for 240 hours under the above 
conditions to obtain crystals of sorbic acid (136.5 Kg, net, after 
correction), that is, 94.8% of the used polyester was decomposed into 
sorbic acid (hereinafter the weight of net crystals of sorbic acid per 100 
g of the polyester will be referred to as decomposition percentage). 
After lapse of the above 240 hours, the decomposition fluids at the exits 
of the columns were analyzed to give a decomposition percentage of 95.5%. 
No abnormality occurred. 
The operation was further continued. As a result, while crystals of sorbic 
acid were quantitatively obtained, the decomposition percentage at the 
exit of the first column lowered since lapse of about 600 hours. Thus, the 
first column was separated and the above-mentioned preliminary column was 
connected to the last column to continue the oepration. The separated 
column was washed with methanol till the washing methanol became uncolored 
to regenerate it, followed by replacing methanol by xylene to obtain a 
preliminary column. 
As described above, when the decomposition percentage at the first column 
lowers, the column is separated, a preliminary column is connected to the 
last column and the separated column is regenerated to prepare a 
preliminary column, to carry out continuous operation, and during this 
operation, the quantity of xylene corresponding to its loss is 
supplemented. As a result, operation over 6,000 hours was effected. 
Comparative example 1 
Example 1 was repeated except that washing with 5% hydrochloric acid was 
not carried out. As a result, the decomposition percentage lowered down to 
85% after 120 hours; thus further operation was given up. Further, 
regeneration of the ion exchange resin by washing it with methanol was 
almost not effective. 
EXAMPLE 2 
Example 1 was repeated except that 5% hydrochloric acid aqueous solution 
used for purifying the polyesterxylene solution was replaced by 40% 
sulfuric acid aqueous solution. As a result, the decomposition percentage 
even after lapse of 4,000 hours was in the vicinity of 95%, that is, 
quantitative. 
EXAMPLE 3 
Example 1 was repeated except that the polyester (zinc content: 0.2%) was 
prepared using zinc acetate as catalyst and 5% hydrochloric acid water was 
replaced by water. The ion exchange resin used was &he same as in Example 
1. As a result, even after lapse of 5,000 hours, the decomposition 
percentage was in the vicinity of 95% and quantitative. 
EXAMPLE 4 
Example 3 was repeated except that the ion exchange resin was replaced by 
RPC-170 (tradename of product made by Mitsubishi Kasei Co., Ltd.). As a 
result, even after lapse of 4,000 hours, the decomposition percentage was 
in the vicinity of 95%. 
EXAMPLE 5 
Example 2 was repeated except that the ion exchange resin was replaced by 
Amberlist 15 (water content: 6%) (tradename of product made by Organo Co., 
Ltd.), and this material was dispersed in methanol and the resulting 
dispersion was filled in the columns. As a result, even after 1,000 hours, 
the decomposition percentage was in the vicinity of 95%. 
Comparative example 2 
The same ion exchange resin as in Example 5 was dispersed in xylene and the 
resulting dispersion was filled in three columns same as in Example 5. 
The same polyester (one part by weight) as in Example 1 was dissolved in 
xylene (3.5 parts by weight) and the solution was passed through the above 
columns set to a jacket temperature of 75.degree. C. at a rate of 3.6 
Kg/Hr. As a result, the decomposition percentage measured after lapse of 
10 hours was in the vicinity of 93%, but that measured after lapse of 40 
hours lowered down to 81%. 
Thus, conc. hydrochloric acid was fed at a rate of 0.01 Kg/Hr and the 
temperature was raised up to 85.degree. C., but the effect was small and 
the decomposition percentage lowered gradually so that the operation was 
stopped after lapse of 80 hours. This ion exchange resin could not be 
regenerated by washing with methanol, but when 5% hydrochloric acid was 
continuously passed through the resin to dissolve out zinc, the original 
catalyst performance was restored.