Process for purifying boric acid for hydrocarbon oxidation

CONSTRUCTION The present invention relates to an industrial purification process in the process of manufacturing alcohols and ketones by oxidation of hydrocarbon wherein after removing by extraction organic impurities, formed as by-products of an oxidation reaction, from aqueous boric acid solution obtained by hydrolysis of an oxidation reaction solution containing boric acid ester, obtained by performing an oxidation reaction in the presence of boric acid, in order to essentially regenerate boric acid and prevent lowering of the reaction rate, selectivity and excessive soiling of the apparatus in the case of cyclically using said regenerated boric acid in an oxidation reaction, boric acid is regenerated and cyclically used again in said hydrocarbon oxidation reaction. EFFECT The present invention is a process for purifying boric acid for hydrocarbon oxidation which reduces the amount of harmful impurities to an extent that allows circulating aqueous boric acid solution to be reused in a continuing oxidation reaction, while also maintaining the selectivity of the oxidation reaction at a level of selectivity obtained with fresh boric acid.

BACKGROUND OF THE INVENTION 
The present invention relates to an industrial purification process in the 
process of manufacturing alcohols and ketones by oxidation of hydrocarbon 
wherein after removing by extraction organic impurities, formed as 
by-products of an oxidation reaction, from aqueous boric acid obtained by 
hydrolysis of an oxidation reaction solution containing boric ester, 
obtained by performing a hydrocarbon oxidation reaction in the presence of 
boric acid with gas containing molecular oxygen, using an organic polar 
solvent having an oxygen atom within its molecule, boric acid is 
regenerated and cyclically used again in said hydrocarbon oxidation 
reaction. As examples of usage of alcohols and ketones, in the case of 
cyclododecanol and cyclododecanone, they are used for the intermediate 
starting material of laurolactam, a monomer used in the manufacturing of 
12-Nylon and for the starting material of a dodecane dibasic acid, and in 
the case of cyclohexanol and cyclohexanone, they are used for the 
intermediate starting material of caprolactam, a monomer used in the 
manufacturing of 6-Nylon and for the starting material of adipic acid. 
Various methods of the prior art are known for purifying boric acid used in 
a process wherein boric acid that has been regenerated from aqueous boric 
acid, obtained by hydrolysis of an oxidation reaction solution obtained in 
a hydrocarbon oxidation reaction performed in the presence of boric acid, 
is used cyclically. Examples of known methods for purifying aqueous boric 
acid obtained by hydrolysis include steam stripping the aqueous boric acid 
obtained by hydrolysis of an oxidation reaction solution to remove organic 
impurities contained therein (see U.S. Pat. No. 3,423,571). However, the 
above-mentioned known steam stripping method has the disadvantage of being 
unable to remove a majority of the organic impurities, which have a 
detrimental effect on the oxidation reaction, even if a considerably large 
amount of steam is used. 
In addition, examples of known methods wherein aqueous boric acid obtained 
by hydrolysis is purified with solvent include a method wherein aqueous 
boric acid obtained by hydrolysis is removed by extraction with saturated, 
linear or cyclic hydrocarbons such as cyclododecane, cyclohexane, 
cyclopentane, n-decane, n-heptane and n-hexane (see U.S. Pat. No. 
3,679,751). However, as nearly all of the organic impurities contained in 
said aqueous boric acid are polar compounds, methods which use nonpolar 
solvents like the above-mentioned hydrocarbons for the extracting solvent 
have the disadvantage of being unable to achieve a satisfactory removal 
rate during removal of those impurities by extraction. 
Moreover, examples of known methods wherein aqueous boric acid obtained by 
hydrolysis is crystallized include a method wherein boric acid crystals 
are removed from aqueous boric acid obtained by hydrolysis by 
crystallization, and organic impurities are removed by wet oxidation of 
the mother liquor formed as a result of said crystallization (see Japanese 
Patent Publication No. 4524/1977). This proposed method has the 
disadvantage of requiring high temperature and high pressure apparatus 
resulting in increased apparatus complexity and high equipment costs. In 
addition, another example of a known method wherein aqueous boric acid 
obtained by hydrolysis is crystallized is a crystallization purification 
method wherein only crystals obtained by removing boric acid crystals in 
the first stage of crystallization are cyclically used for an oxidation 
reaction as a result of two-stage crystallization from aqueous boric acid 
obtained by hydrolysis (see Japanese Patent Publication No. 39250/1970 and 
Japanese Unexamined Patent Publication No. 29299/1978). However, 
accumulation of organic substances is prevented by redissolving and 
returning the crystals obtained in the second stage of crystallization 
either to the first stage crystallization process or the hydrolysis 
process, and discarding a portion of the mother liquor of second stage 
crystallization while a portion of the second stage crystallization 
outside the system. Thus, this method has the disadvantage of being 
undesirable in terms of costs and the environment as a portion of the 
aqueous boric acid must be discarded. Thus, all of the methods of the 
prior art were not able to be satisfactory in terms of industrial use. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a process for purifying 
an oxidation assistant wherein organic impurities contained in aqueous 
boric acid obtained by hydrolysis of an oxidation reaction solution, 
obtained in a hydrocarbon oxidation reaction, are removed in order to 
cyclically use boric acid, used in said hydrocarbon oxidation reaction, 
again in said oxidation reaction. 
When manufacturing alcohols and ketones using a hydrocarbon oxidation 
reaction, it is known that the presence of boric acid is able to 
drastically increase the selectivity of the target alcohol and ketone 
products. However, as the occurrence of high-order oxidation cannot be 
completely prevented, organic impurities such as organic acids are 
constantly formed by the oxidation reaction resulting in the oxidation 
reaction solution containing a plurality of types of impurities. 
The boric acid used in said oxidation reaction includes ortho- and 
meta-boric acid, B.sub.2 O.sub.3 and B.sub.4 O.sub.5 boric acid anhydrides 
or their mixtures. 
As the formed alcohols and boric acid in the oxidation reaction solution 
are present as boric esters, when hydrolysis is performed to isolate the 
boric acid, an organic substance containing unreacted hydrocarbon, the 
alcohol and ketone of the target compound, and aqueous boric acid are 
obtained. The ortho-boric acid, meta-boric acid or anhydrous boric acid 
obtained by additional dehydration having a lower degree of hydration or 
their mixture that is obtained by removal of water from the aqueous boric 
acid are cyclically used again in the oxidation reaction. However, as 
organic impurities are distributed in this aqueous boric acid, if 
regenerated boric acid for which dehydration was performed to remove water 
from aqueous boric acid is used again in the oxidation reaction, adhesion 
of regenerated boric acid occurs in the reaction vessel. In addition, this 
also has the detrimental effect of remarkably reducing the efficiency of 
the oxidation reaction in terms of the conversion rate and selectivity of 
the alcohols and ketones. Thus, in order to industrially perform this 
oxidation reaction in the presence of boric acid, an industrial process is 
required wherein organic impurities in the boric acid to be reused 
cyclically are removed. As the various known technologies of the prior art 
have disadvantages as mentioned above, and as the industrial cyclic use of 
regenerated boric acid for performing an oxidation reaction remarkably 
decreases yield and is disadvantageous as a stable procedure with a high 
degree of selectivity while also having a considerable effect on economic 
feasibility, the inventors of the present invention invented process in 
order to solve these problems that allows aqueous boric acid obtained by 
isolation to be efficiently purified following hydrolysis of said 
oxidation reaction solution. 
The present invention relates to a process for purifying boric acid for 
hydrocarbon oxidation wherein after removing by extraction organic 
impurities, formed as by-products of an oxidation reaction, from aqueous 
boric acid obtained by hydrolysis of an oxidation reaction solution, 
obtained in a hydrocarbon oxidation reaction performed in the presence of 
boric acid, using for said extraction an organic polar solvent having an 
oxygen atom within its molecule, boric acid is regenerated and used 
cyclically in said oxidation reaction. 
In the case of removing by extraction organic impurities, formed as 
by-products of an oxidation reaction, from aqueous boric acid obtained by 
hydrolysis of an oxidation reaction solution using for said extraction an 
organic polar solvent having an oxygen atom within its molecule, a portion 
of the boric acid is dissolved in the resulting extraction solvent. 
However, by washing said extraction solvent with water and returning the 
resulting water containing boric acid to the extraction process using an 
organic polar solvent, the amount of boric acid lost can essentially be 
reduced to zero. This method may be performed by dividing between two 
extraction columns consisting of solvent extraction column 3 and water 
washing column 5 as illustrated in FIG. 1, or may be performed with a 
single extraction column 31 wherein the upper stage is the washing portion 
and the lower stage is the solvent extraction portion as illustrated in 
FIG. 2. In other words, the present invention provides a process for 
purification of boric acid wherein the amount of harmful impurities is 
reduced to an extent that allows the boric acid to be reused in the 
oxidation reaction, maintains the selectivity of the oxidation reaction at 
a level of selectivity obtained with fresh boric acid, and virtually 
eliminates the loss of boric acid in order to cyclically use said boric 
acid in an oxidation process.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following provides a detailed explanation of the present invention. 
It is widely known that industrially important oxygenated organic 
derivatives are formed when hydrocarbons are oxidized with a gas 
containing molecular oxygen. The boric acid used in this oxidation 
reaction plays the role of forming a boric acid ester with the target 
alcohol and preventing high-order oxidation reactions. It is also known to 
be used as an oxidation assistant that effectively increases the 
selectivity by which the target monoalcohol or ketone derivatives are 
converted. In the case of performing said process on an industrial basis, 
it is necessary to purify the boric acid by isolation and regeneration so 
that it can be cyclically reused in the oxidation reaction for economic 
reasons as well. 
In the case of cyclododecane for example, the hydrocarbon oxidation 
reaction of the present invention is carried out under the reaction 
conditions described to follow. More specifically, the reaction is 
suitably carried out at a reaction temperature within the range of 
140.degree.-200.degree. C., and preferably 150.degree.-180.degree. C. In 
addition, it is suitably carried out at a reaction pressure within the 
range of atmospheric pressure to 10 Kg/cm.sup.2 G, and preferably 
atmospheric pressure to 5 Kg/cm.sup.2 G. The oxidation reaction is carried 
out on cyclododecane by supplying boric acid under the above-mentioned 
reaction conditions at a suitable oxygen concentration of 2%-50%, and 
preferably 4%-25%, diluted with air containing molecular oxygen, and/or an 
inert gas such as nitrogen gas. 
In addition, the reaction is carried out under the following reaction 
conditions in the case of oxidation of cyclohexane. More specifically, the 
reaction is suitably carried out at a reaction temperature within the 
range of 140.degree.-200.degree. C., and preferably 
150.degree.-180.degree. C. In addition, it is suitably carried out t a 
reaction pressure within a range of 6-20 Kg/cm.sup.2 G, and preferably 
8-12 Kg/cm.sup.2 G. The oxidation reaction is carried out on cyclohexane 
by supplying boric acid under the above-mentioned reaction conditions at a 
suitable oxygen concentration of 2%-50%, and preferably 4-25%, diluted 
with air containing molecular oxygen, and/or an inert gas such as nitrogen 
gas. 
As a result of the oxidation reaction, the target alcohol is present in the 
resulting reaction solution in the form of boric ester. In addition, 
ketone is formed at a weight ratio of 20:1 to 10:2 with respect to the 
alcohol. As the boric acid ester of the alcohol contained in the oxidation 
reaction solution disassociates as alcohol, a hydrolysis reaction is 
performed by adding water to the oxidation reaction solution. 
In order to hydrolyze the boric ester of the alcohol, an amount of water 
sufficient to dissolve the boric acid is added at a weight ratio of 
suitably 5%-30%, and preferably 10%-20%, with respect to the oxidation 
solution. An organic solution containing the target alcohol and ketone to 
be formed, and unreacted hydrocarbon, as well as aqueous boric acid are 
obtained. In the case of performing this process industrially, the 
hydrolysis reaction is performed by bringing the oxidation solution and 
water in contact by counterflow. Suitable examples of apparatus which 
perform hydrolysis by counterflow contact include perforated plate column, 
packed column and rotary disk column types as well as mixer-settler type 
apparatus. In addition, the operation method is not limited, with either 
continuous type or batch type able to be used. 
When the aqueous boric acid solution obtained by hydrolysis is separated, 
organic impurities formed as by-products in the oxidation reaction are 
distributed within the aqueous boric acid solution (examples of which 
include organic acids such as mono- and dicarboxylic acids, alcohols, 
hydroxy carboxylic acids and high-order oxidation products). The amount of 
these organic impurities contained is 2000 ppm to 8000 ppm in terms of 
total organic carbon (refer to the definition of TOC). 
As the above-mentioned organic impurities are contained within the 
regenerated boric acid obtained by dehydration of aqueous boric acid, when 
regenerated boric acid is used cyclically in the oxidation reaction, 
remarkable scaling occurs in the oxidation reaction vessel. As this 
shortens the operating period, this has a considerably detrimental effect 
on the ease of operation. In addition, as this also leads to a remarkable 
decrease in the selectivity and conversion rate to the target alcohol and 
ketone, the reaction results of the oxidation reaction, the cyclical use 
of boric acid after removing organic impurities by treating the aqueous 
boric acid following the oxidation reaction is both necessary and 
important in order to improve industrial economic feasibility. 
Furthermore, the concentration of organic impurities was measured in the 
form of total organic carbon (TOC) with the Shimadzu TOC-500 Total Organic 
Carbon Counter. As indicated above, TOC is the abbreviation for Total 
Organic Carbon. 
As the process of the present invention is a method wherein organic 
impurities in aqueous boric acid are extracted by solvent extraction in 
order to remove said organic impurities from said aqueous boric acid, 
selection of the solvent is important. As stated above, the organic 
impurities in aqueous boric acid solution to be extracted are nearly all 
polar compounds. If the extraction solvent is a nonpolar solvent as stated 
previously, it is difficult to satisfactorily remove organic impurities 
from aqueous boric acid. 
Thus, those solvents that can be used industrially are preferably organic 
solvents that are able to extract the by-products of the above-mentioned 
oxidation reaction from aqueous solution to the organic solvent as a 
result of organic impurities being highly soluble in said solvent. 
Examples of solvents used in the present invention are saturated aliphatic 
ketones or alicyclic ketones having an oxygen atom and 3-20, and 
preferably 4-15, carbon atoms within their molecules. In addition, 
saturated aliphatic or alicyclic alcohols as well as aromatic alcohols 
having 5-15 carbon atoms can also be used. Moreover, esters similarly 
having 4-15 carbon atoms can also be used. In other words, the present 
invention provides a solvent optimum for extracting polar impurities from 
a strongly polar aqueous solution. 
Examples of extraction solvents that satisfy these conditions include 
hydrocarbon solvents containing ketone groups of saturated aliphatic 
ketones such as methyl isobutyl ketone, methyl ethyl ketone, methyl 
isopropyl ketone, methyl propyl ketone, diethyl ketone and methyl butyl 
ketone; and, alicyclic ketones such as cyclohexanone, methyl cyclohexanone 
and cyclododecanone; or, methods using mixtures of these solvents. 
In addition, examples of alcohols used in the present invention include 
aliphatic alcohols such as amyl alcohol, isoamyl alcohol, n-hexanol, 
n-heptyl alcohol, octyl alcohol and capryl alcohol; aromatic alcohols such 
as benzyl alcohol; and, alicyclic alcohols such as cyclopentanol and 
cyclohexanol. Examples of esters that are used include formic acid ester, 
acetic acid ester and propionic acid ester. 
As a condition of the extraction process using the above-mentioned solvent 
of the present invention, the reaction is performed at a solvent ratio 
within a range of 0.2-4 weight parts solvent to 1 weight part aqueous 
boric acid solution. The temperature during extraction is within a range 
of room temperature to 90.degree. C. Examples of extraction apparatus that 
can be used include a perforated plate extraction column, packed 
extraction column, mixer-settler extraction apparatus, centrifugal 
extraction apparatus or rotary disk column. 
In addition, as a small amount of boric acid dissolves in the solvent used 
to extract organic impurities, this is economically disadvantageous if 
left uncorrected as the boric acid loss will increase. In order to prevent 
this loss of boric acid, boric acid is extracted by washing the extraction 
solvent with water. As the water used to extract the boric acid contains a 
portion of the organic impurities, this is returned to the apparatus which 
extracts aqueous boric acid solution with solvent. As a result, organic 
impurities can be removed by extraction from aqueous boric acid while 
reducing boric acid loss to essentially zero. 
The extraction apparatus used for this purpose can be an apparatus in which 
the above-mentioned solvent extraction portion and water washing portion 
are divided into two typical extraction columns. In addition, an apparatus 
may also be used in which counterflow extraction is performed in a single 
extraction column in which the solvent extraction portion is provided in 
the bottom of the column and solvent is introduced from the bottom stage 
of the column while introducing aqueous boric acid following hydrolysis 
from the intermediate stage of the column. Moreover, this apparatus may 
also wash the boric acid in the solvent rising in the column and water 
introduced from the upper stage of the column. A water washing portion may 
be provided in the top of the extraction apparatus. 
The following provides a schematic explanation of an industrial process 
according to the present invention with reference to the attached 
drawings. 
In FIG. 1, the reaction solution and water are brought in contact by 
counterflow by introducing an oxidation reaction solution, in which a 
hydrocarbon such as cyclododecane is reacted with an oxygen-containing gas 
such as air in the presence of boric acid such as meta-boric acid, from 11 
to the bottom portion of hydrolysis column 2 and introducing water from 
the upper portion 9. 
As unreacted hydrocarbon, the target alcohol and ketone along with reaction 
by-products in the form of impurities are contained in the organic 
reaction solution from which organic impurities have been extracted by 
counterflow extraction following hydrolysis of boric ester by water, in 
order to remove these additional impurities, the quality of the organic 
phase is attempted to be improved by performing contact treatment with 
alkali in saponification reaction process 8. Next, as the saponified 
organic reaction solution contains unreacted hydrocarbon and the target 
alcohol and ketone, said organic reaction solution is sent to the 
separation process by 21. 
The aqueous solution containing boric acid and organic impurities formed as 
by-products in the oxidation reaction is introduced from the lower portion 
of hydrolysis column 2 to the upper portion of solvent extraction column 
3. An amount of solvent such as methyl isobutyl ketone required for 
extraction is introduced from solvent tank 4 to the lower portion of 
solvent extraction column 3 via 13 where counterflow extraction takes 
place. As a result, organic impurities in the boric acid are extracted 
into the solvent after which the solution is discharged via 14 to water 
washing column 5 where the accompanying boric acid is removed. 
Consequently, washing is performed by introducing water (using the water 
separated in 6) via 16. The washing water is taken out via 15 and returned 
to solvent extraction column 3. 
Next, the extraction solvent of water washing column 5 is separated by 
distillation into extracted impurities and solvent in solvent recovery 
process 7 after being discharged from washing column 5 via 19. The 
distillation residue is removed via 7 and suitably processed for disposal. 
The separated solvent is removed via 20 and returned to solvent tank 4 for 
cyclical use. 
The purified aqueous boric acid, which is the heavy solution of solvent 
extraction column 3, is removed via 17. In order to remove accompanying 
solvent, a small amount of which has dissolved in the above-mentioned 
solution, the aqueous boric acid discharged from solvent extraction column 
3 is seam stripped by injection of steam in stripping process 6. This 
results in the obtaining of purified aqueous boric acid from 18. The 
separated water removed by the stripping process is removed via 16 and 
used cyclically for supplying water to water washing column 5. 
The extraction purified aqueous boric acid 18 is used in the oxidation 
reaction after performing suitable dehydration treatment. 
Solvent extraction of the aqueous boric acid, the above-mentioned heavy 
liquid of hydrolysis, and washing of the extract with water is 
accomplished by the means indicated in FIG. 2. FIG. 2 indicates an 
apparatus which simplifies the solvent extraction purification process. 
The apparatus consists of extraction column 31 in which solvent extraction 
column 3 and water washing column 5 explained in FIG. 1 are integrated 
into a single column. The lower portion of this single column is the 
solvent extraction column while the upper portion is the water washing 
portion. Thus, this extraction apparatus is able to perform two processes 
simultaneously. Aqueous boric acid following hydrolysis of the oxidation 
reaction solution is introduced into the intermediate stage of the column 
via 32. Counterflow extraction is then performed in the lower solvent 
extraction portion with solvent 33 introduced into the lower stage of the 
tower via 33. Organic impurities contained in the aqueous boric acid are 
thus extracted into the solvent. As a result, purified aqueous boric acid 
is obtained from the bottom of the extraction column. The aqueous boric 
acid at this point corresponds to the operation starting at 12 in FIG. 1. 
Next, extraction solvent rises from the lower portion and starting at the 
intermediate stage, counterflow washing is performed in order to wash 
boric acid accompanying the extraction solvent with water introduced via 
34 into the upper stage in the water washing portion of the upper portion. 
As a result, extraction solvent into which organic impurities have been 
extracted from aqueous boric acid is discharged from the top of the 
column. The process after this point corresponds to the operations of 19 
and beyond explained in FIG. 1. As such, solvent extraction and water 
washing of the boric acid accompanying the solvent can be performed 
simultaneously with a single column in order to simplify solvent 
extraction. 
EXAMPLES 
The following provides a detailed explanation of the present invention by 
indicating reference examples, working examples and comparative examples. 
Reference Example 1 
300 g of cyclododecane and 140 g of ortho-boric acid (H.sub.3 BO.sub.3) in 
the form of an 18% aqueous solution used as an oxidation assistant were 
charged into 1l autoclave . Removal of free water and dehydration of boric 
acid were performed while heating and using nitrogen as the water-removing 
gas. At that time, the exhaust gas is removed outside the system after 
passing through a condenser mounted on the upper outlet of the autoclave 
and decompressing to atmospheric pressure via a valve. As the condensate 
is collected in a water separation tank provided in the lower portion of 
the condenser thereby separating into an aqueous phase and organic phase, 
only the organic phase is refluxed to the autoclave. Using a similar 
operation performed separately, the content of meta-boric acid in the 
boric acid at this time was 97 mol%. Next, oxidation is performed by 
introducing air at 2 Kg/cm.sup.2 G and 170.degree. C. at a flow rate of 
0.3 Nl/min. The supply of air is stopped after 90 minutes and returned to 
atmospheric pressure. The contents of the autoclave are transferred from 
the autoclave into a flask followed by the addition of 120 g of water and 
stirring for 10 minutes at 90.degree. C. to perform hydrolysis. After 
allowing to stand undisturbed, the aqueous boric acid of the heavy 
solution phase drained from the cock of the bottom portion, said solution 
phase contained 4600 ppm of organic impurities as total organic carbon 
(TOC). 50 g of a 10% aqueous NaOH were added to the light solution phase 
remaining in the flask followed by stirring for 30 minutes to separate 
after allowing to stand undisturbed. As a result of analysis of the 
resulting organic phase by gas chromatography, the cyclododecane 
conversion rate was 17.9%, and the total selectivity of cyclododecanol and 
cyclododecanone was 86.1%. 
##EQU1## 
Comparative Example 1 
As a result of performing oxidation according to a procedure similar to 
Reference Example 1 with the exception of using the aqueous boric acid 
(TOC: 4600 ppm) obtained in the hydrolysis of Reference Example 1 as the 
oxidation assistant, the cyclododecane conversion rate was 15.4% and the 
total selectivity of cyclododecanol and cyclododecanone was 78.2%. 
EXAMPLE 1 
280 g of methyl isobutyl ketone (abbreviated as MIBK) saturated with water 
at 80.degree. C. were added to 140 g of aqueous boric acid obtained by 
hydrolysis in the same manner as Reference Example 1. After extraction by 
stirring and allowing to stand undisturbed, 280 g of MIBK were again added 
to the heavy solution followed by extraction. The MIBK phase was separated 
by allowing to stand undisturbed. Next, 28 g of water were added to the 
separated heavy solution phase. The 28 g of water along with the dissolved 
MIBK were distilled by heating after which aqueous boric acid having a TOC 
of 330 ppm was obtained as the distillate. 
The TOC removal rate of this extraction purification was 93%. In addition, 
although 0.65% of boric acid was dissolved in the MIBK extract, the boric 
acid concentration in MIBK was lowered to 0.02% by extracting twice with 
10 g of water to 100 g of MIBK extract. As a result of performing 
oxidation similar to Reference Example 1 with the exception of using 140 g 
of this purified aqueous boric acid for the oxidation assistant, the 
conversion rate of cyclododecane was 18.4% and the total selectivity of 
cyclododecanol and cyclododecanone was 86.4%. Thus, results were obtained 
that are equivalent to those using fresh ortho-boric acid as in Reference 
Example 1. 
Comparative Example 2 
With the exception of using 280 g of cyclododecane instead of the MIBK of 
Working Example 1, extraction was performed twice using a similar method. 
The TOC concentration of the resulting aqueous boric acid was 2,070 ppm, 
and the TOC removal rate was 55%. Moreover, as a result of performing 
oxidation in the same manner as Reference Example 1 with the exception of 
using this aqueous boric acid for the oxidation assistant, the 
cyclododecane conversion rate was 16.0% and the total selectivity of 
cyclododecanol and cyclododecanone was 81.0%. 
Reference Example 2 
400 g of cyclohexane and 200 g of ortho-boric acid (H.sub.3 BO.sub.3) in 
the form of an 18% aqueous solution used as an oxidation agent were 
charged into 1l autoclave. Removal of free water and dehydration of boric 
acid were performed while heating and using nitrogen as the water-removing 
gas. At that time, the exhaust gas is removed outside the system after 
passing through a condenser mounted on the upper outlet of the autoclave 
and decompressing to atmospheric pressure via a valve. As the condensate 
is collected in a water separation tank provided in the lower portion of 
the condenser thereby separating into an aqueous phase and organic phase, 
only the organic phase is refluxed to the autoclave. Using a similar 
operation performed separately, the content of meta-boric acid in the 
boric acid at this time was 94 mol%. 
Next, oxidation is performed by introducing a gas containing 4% oxygen 
diluted with nitrogen at 9 Kg/cm.sup.2 G and 165.degree. C. at a flow rate 
of 1 Nl/min. The supply of gas is stopped after 4 hours after which 170 g 
of water are added to the autoclave followed by stirring for 10 minutes at 
120.degree. C. to perform hydrolysis. After allowing to stand undisturbed, 
the aqueous boric acid of the heavy solution phase drained from the cock 
of the bottom portion contained 3500 ppm of organic impurities as total 
organic carbon (TOC). 50 g of a 10% aqueous NaOH were added to the light 
solution phase remaining in the flask followed by stirring for 30 minutes 
to separate after allowing to stand undisturbed. As a result of analysis 
of the resulting organic phase by gas chromatography, the cyclohexane 
conversion rate was 10.3%, and the total selectivity of cyclohexanol and 
cyclohexanone was 86.9%. 
Comparative Example 3 
As a result of performing oxidation using a procedure similar to Reference 
Example 2 with the exception of using the aqueous boric acid (TOC: 3500 
ppm) obtained in the hydrolysis of Reference Example 2, the conversion 
rate of cyclohexane was 8.0% and the total selectivity of cyclohexanol and 
cyclohexanone was 77.9%. 
EXAMPLE 2 
400 g of methyl isobutyl ketone (abbreviated as MIBK) saturated with water 
at 80.degree. C. were added to 200 g of aqueous boric acid obtained by 
hydrolysis in the same manner as Reference Example 2. After extraction by 
stirring and allowing to stand undisturbed, 400 g of MIBK were again added 
to the heavy solution followed by extraction. The MIBK phase was separated 
by allowing to stand undisturbed. Next, 40 g of water were added to the 
separated heavy solution phase. The 40 g of water along with the dissolved 
MIBK were distilled by heating after which aqueous boric acid having a TOC 
of 280 ppm was obtained as the distillate. The TOC removal rate was 92%. 
As a result of performing oxidation similar to Reference Example 2 with 
the exception of using 200 g of this purified aqueous boric acid as the 
oxidation assistant, the conversion rate of cyclohexane was 10.5% and the 
total selectivity of cyclohexanol and cyclohexanone was 86.8%. Thus, 
results were obtained that are equivalent to those using fresh ortho-boric 
acid as in Reference Example 2. 
Comparative Example 4 
With the exception of using 400 g of cyclohexane instead of the MIBK of 
Working Example 2, extraction was performed twice using a similar method. 
The TOC concentration of the resulting aqueous boric acid was 1,790 ppm, 
and the TOC removal rate was 49%. Moreover, as a result of performing 
oxidation in the same manner as Reference Example 2 with the exception of 
using this aqueous boric acid for the oxidation assistant, the cyclohexane 
conversion rate was 9.4% and the total selectivity of cyclohexanol and 
cyclohexanone was 81.5%. 
EXAMPLE 3 
280 g of n-hexanol saturated with water at 80.degree. C. were added to 140 
g of aqueous boric acid obtained by hydrolysis in the same manner as 
Reference Example 1. After extraction by stirring and allowing to stand 
undisturbed, 280 g of n-hexanol were again added to the heavy solution 
followed by extraction. The n-hexanol phase was separated by allowing to 
stand undisturbed. Next, 28 g of water were added to the separated heavy 
solution phase. The 28 g of water along with the dissolved n-hexanol were 
distilled by heating after which aqueous boric acid having a TOC of 410 
ppm was obtained as the distillate. The TOC removal rate by this n-hexanol 
extraction purification is 91%. 
As a result of performing oxidation similar to Reference Example 1 with the 
exception of using 140 g of this purified aqueous boric acid as the 
oxidation assistant, the conversion rate of cyclododecane was 17.5% and 
the total selectivity of cyclododecanol and cyclododecanone was 85.7%. 
EXAMPLE 4 
280 g of butyl acetate saturated with water at 80.degree. C. were added to 
140 g of aqueous boric acid obtained by hydrolysis in the same manner as 
Reference Example 1. After extraction by stirring and separation by 
allowing to stand undisturbed, 280 g of butyl acetate were again added to 
the heavy solution followed by extraction. The butyl acetate phase was 
separated by allowing to stand undisturbed. Next, 28 g of water were added 
to the separated heavy solution phase. The 28 g of water along with the 
dissolved butyl acetate were distilled by heating after which aqueous 
boric acid having a TOC of 370 ppm was obtained as the distillate. The TOC 
removal rate is 92%. 
As a result of performing oxidation similar to Reference Example 1 with the 
exception of using 140 g of this purified aqueous boric acid as the 
oxidation assistant, the conversion rate of cyclododecane was 17.9% and 
the total selectivity of cyclododecanol and cyclododecanone was 85.9%. 
Effect of the Invention 
The present invention has the following advantages: 
(1) Organic impurities contained in aqueous boric acid obtained by 
hydrolysis of an oxidation solution can be removed in a simple apparatus 
at ambient temperature (25.degree. C.) and atmospheric pressure, the 
removal rate of said impurities is high, and aqueous boric acid purified 
by the process of the present invention can be used cyclically in an 
oxidation process following dehydration treatment without having any 
effect whatsoever on the reaction. 
(2) The recovery rate of boric acid is high and there is essentially no 
loss. 
(3) The purification process of the present invention can also be applied 
as a process for cycling boric acid to an oxidation process in the case of 
a simple and superior process which does not require a crystallization 
procedure wherein aqueous boric acid is dehydrated within the hydrocarbon 
to be oxidized to form meta-boric acid which is then supplied to the 
oxidation process together with hydrocarbon, thereby eliminating the 
complexity of transporting solid state crystals. 
In the case of comparing the effect of the process of the present invention 
with that of the known technology by comparing the removal rates of 
organic impurities in boric acid for the cyclododecane extraction solvent 
of the known technology and the methyl isobutyl ketone extraction solvent 
of the process of the present invention at a solvent ratio of 2, although 
the removal rate in the case of cyclododecane extraction solvent is 55%, a 
removal rate of 90% or more is obtained in the case of methyl isobutyl 
ketone extraction solvent. 
Thus, the present invention is carried out to provide an advantageous 
extraction purification process for removing organic impurities formed as 
by-products in an oxidation reaction that are contained in circulating 
aqueous boric acid in order to industrially perform an oxidation reaction 
in the presence of boric acid. In other words, the present invention 
provides a process for purification of boric acid which reduces the amount 
of harmful impurities to an extent that allows circulating aqueous boric 
acid to be reused in a continuing oxidation reaction, while also 
maintaining the selectivity of the oxidation reaction at a level of 
selectivity obtained with fresh boric acid.