Removal of cyclohexanol

Cyclohexanol is removed from aqueous solutions containing it and aromatic sulfonic acids by extraction with one or more cyclohexyl ethers liquid under extraction conditions and having the formula ##STR1## where R is alkyl of from 1 to 16 catbon atoms, cycloalkyl of from 5 to 8 carbon atoms, aralkyl of from 7 to 10 carbon atoms or phenyl, which may additionally have substituents which are inert under reaction conditions.

European Patent Application 123,713 discloses a process wherein 
cyclohexanol is obtained by hydrating cyclohexene at from 50.degree. to 
200.degree. C. in from 5 to 80% strength by weight solutions of aromatic 
sulfonic acids in water. To prevent corrosion, the reaction is carried out 
for example in the presence of heteropoly acids, salts or oxides of 
molybdenum, tungsten or vanadium. The cyclohexanol formed by hydration of 
cyclohexene predominantly dissolves in the aqueous phase, which contains 
the aromatic sulfonic acid, and is mostly not removed therefrom even by 
excess cyclohexene. 
JP-A2-103,033/1987 recommends aliphatic, cycloaliphatic and aromatic 
hydrocarbons for extracting cyclohexanol from aqueous solutions of 
aromatic sulfonic acids. However, appreciable amounts of extractants are 
necessary to obtain effective extraction, Industrially, this is very 
expensive. 
In a process described in European Patent Application 227,931, the 
hydration of cyclohexene is carried out in aqueous solutions of aromatic 
sulfonic acids with one or more hydroxyl groups on the aromatic ring and 
the cyclohexanol is subsequently extracted with aromatic hydrocarbons. It 
is true that with this process it is possible to reduce the amount of 
extractant, but the use of hydroxyl-containing aromatic sulfonic acids 
substantially reduces the yield of and selectivity for cyclohexanol. 
It is an object of the present invention to improve the removal of 
cyclohexanol from aqueous aromatic sulfonic acid solutions as obtained in 
the hydration of cyclohexene in the presence of aromatic sulfonic acids. 
We have found that this object is achieved by a process for removing 
cyclohexanol from aqueous solutions containing it and aromatic sulfonic 
acids by extraction using as the extractant a cyclohexyl ether liquid 
under extraction conditions and having the formula 
##STR2## 
where R is alkyl of from 1 to 16 carbon atoms, cycloalkyl of from 5 to 8 
carbon atoms, aralkyl of from 7 to 10 carbon atoms or phenyl, which may 
each additionally have substituents which are inert under reaction 
conditions. 
The novel process has the advantage that the amount of extractant to be 
used is appreciably reduced and a better extraction result is obtained. 
This is even true if hydroxyl-free sulfonic acids are used as hydrating 
agents in the preparation of cyclohexanol, which additionally increases 
the yield of cyclohexanol. 
According to the invention, the starting point is an aqueous solution which 
contains cyclohexanol and an aromatic sulfonic acid in solution. A 
solution of this type is obtained in the hydration of cyclohexene to 
cyclohexanol by reacting cyclohexene in an aqueous solution which contains 
from 5 to 80% by weight of an aromatic sulfonic acid, in particular a 
benzene- or naphthalene-sulfonic acid, which may be substituted, such as 
benzenesulfonic acid, toluenesulfonic acid or naphthalenesulfonic acid, or 
dodecylbenzenesulfonic acid, at from 50.degree. to 200.degree. C., in 
particular from 70.degree. to 150.degree. C., under from 1 to 10 bar. It 
is advantageous to use in addition molybdenic acid or a salt thereof, 
vanadium oxide or a vanadate in an amount of for example from 0.001 to 5% 
by weight, based on the aromatic sulfonic acid. In addition, it is also 
advisable to use a heteropoly acid such as phosphorusmolybdenic acid, 
phosphorustungstic acid, phosphorusmolybdenictungstic acid or 
phosphorusmolybdenicvanadic acid. Suitable processes are described for 
example in European Patent No. 123,713 and European Patent Application 
206,631. The resulting reaction mixture separates into an organic phase 
comprising excess cyclohexene, which contains minor amounts of 
cyclohexanol. The aqueous phase obtained is typically composed for example 
of from 5 to 10% by weight of cyclohexanol, from 40 to 70% by weight of 
aromatic sulfonic acids and from 20 to 50% by weight of water. The 
cyclohexene phase can be separated off, by example by decanting, and 
worked up separately. However, it is not absolutely necessary to separate 
off the cyclohexene before the extraction since it can be utilized as an 
additional extractant. 
According to the invention, the aqueous solutions containing cyclohexanol 
and aromatic sulfonic acids are extracted with one or more cyclohexyl 
ethers of the formula I where R is alkyl of from 1 to 16 carbon atoms, 
cycloalkyl of from 5 to 8 carbon atoms, aralkyl of from 7 to 10 carbon 
atoms or phenyl, which may each have substituents which are inert under 
the reaction conditions. The cyclohexyl ethers of the formula I used as 
extractants should be liquid under extraction conditions. It will be 
readily understood that they are substantially or completely 
water-immiscible. The radicals R may contain for example up to two inert 
substituents such as chlorine atoms or alkoxy groups. Preferably, the 
radical R has a hydrocarbon structure. 
Suitable ethers are for example cyclohexyl benzyl ethers, cyclohexyl alkyl 
ethers and cyclohexyl aryl ethers. It is particularly advantageous to use 
cyclohexyl ethers such as cyclohexyl methyl ether, cyclohexyl ethyl ether, 
cyclohexyl n-propyl ether, cyclohexyl i-propyl ether, cyclohexyl n-butyl 
ether, cyclohexyl i-butyl ether or phenyl cyclohexyl ether. 
In preferred cyclohexyl ethers of the formula I, R is alkyl of from 1 to 12 
carbon atoms or cyclohexyl. Dicyclohexyl ether has become particularly 
important. 
For every part by volume of the aqueous solution of aromatic sulfonic acid 
and cyclohexanol to be extracted it is advantageous to use from 0.1 to 50 
parts by volume, in particular from 1 to 10 parts by volume, of 
extractant. The extraction in general is carried out at from 0 to 
200.degree. C., advantageously from 10.degree. to 100.degree. C., in 
particular from 10.degree. to 50.degree. C. 
The extraction of the aqueous solution containing cyclohexanol and aromatic 
sulfonic acid can be carried out batchwise by mixing the aqueous solution 
intensively with the extractant and subsequently separating the phases. 
Advantageously, the extraction is carried out continuously in 
countercurrent in suitable extraction columns such as stirred disk 
columns, pulsed columns or single- or multi-stage mixer settlers. The 
aqueous phase obtained, which contains aromatic sulfonic acids, is reused 
for the hydration of cyclohexene, while the cyclohexanol-containing 
extractant phase is worked up by distillation. If small amounts of 
aromatic sulfonic acids end up in the cyclohexanol-containing extract, 
they are easily removable by washing with a little water and are 
advantageously added to the aqueous hydrating solution. 
Particularly advantageously, the cyclohexyl ethers I used as extractants 
have a higher boiling point than cyclohexanol. As a consequence, it is 
only necessary to separate the cyclohexanol from the extractant by 
distillation. The extractant obtained as the bottom phase is 
advantageously recycled into the extraction stage. In a preferred 
embodiment, the extraction is carried out using the excess cyclohexene 
phase obtained in the hydration. The subsequent workup thus gives first a 
cyclohexene fraction, which is reused for the hydration, a cyclohexanol 
fraction and, as the bottom product, the extractant. 
The cyclohexanol obtainable by the process of the invention is suitable for 
preparing cyclohexanone, an important starting material for caprolactam.

The process according to the invention is illustrated by the Examples 
below. The parts by weight bear the same relation to parts by volume as 
the kilogram to the liter. 
EXAMPLE 1 
17.4 parts by volume of an aqueous solution of 54% parts by weight of 
p-toluenesulfonic acid, 10% parts by weight of cyclohexanol and 36% parts 
by weight of water were intensively mixed with 21.5 parts by volume of 
dicyclohexyl ether in a vessel at room temperature for 5 minutes. After 
phase separation it was found by gas chromatography that 19.4% by weight 
of the cyclohexanol used had been extracted from the aqueous phase. 
COMATIVE EXAMPLES 1-5 
The extraction was carried out as described in Example 1, except that in 
each case the hydrocarbons indicated in the table were used for the 
extraction. The results are shown in the table: 
TABLE 
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Extracted 
Comparative cyclohexanol 
Example Extractant [% by weight] 
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1 cyclohexene 13.7 
2 toluene 16.4 
3 tetralin 14.4 
4 decalin 5.2 
5 4-tert-butyltoluene 
11.6 
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