Preparation of percarboxylic acids

Process for preparation of percarboxylic acids by the reaction of hydrogen peroxide and a water-miscible carboxylic acid, in the presence of a solvent capable of forming a heteroazeotrope with water, and in the presence of a metalloid oxide catalyst.

TECHNICAL FIELD 
The present invention concerns a procedure for preparation of percarboxylic 
acids which are particularly useful for selective oxidation of organic 
compounds. 
BACKGROUND ART 
Since the work of Ans et al. (Ber. 45, 1845, 1912), it has been known that 
hydrogen peroxide reacts with aliphatic carboxylic acids to form 
percarboxylic acids according to a reversible reaction: 
##STR1## 
Given the instability of the peroxyacids, this reaction is usually 
performed at a low temperature. Under these conditions, the state of 
equilibrium is attained only after several hours of reaction, and this 
reaction time is prohibitive for an industrial procedure. Thus, it is 
necessary to use a catalyst. Only one type of catalyst has been proposed 
up until now: strong mineral acids, such as sulfuric acid, methanesulfonic 
acid, the arylsulfonic acids, phosphoric acid, the acid phosphoric esters, 
trifluoroacetic acid, as well as acid cation resins such as Dowex 50 and 
Amberlite IR-120. 
This catalytic process has given rise to numerous studies (D. Swern, 
ORGANIC PEROXIDES, Wiley Interscience, 1970, Vol. 1, pages 313-369, and 
pages 428-439), from which it is clearly apparent that the first stage of 
the reaction is the protonation of the acid function, involving the 
formation of an oxonium structure capable of reacting with H.sub.2 
O.sub.2, leading, after dehydration, to percarboxylic acid, as per the 
model: 
##STR2## 
Hydrogen peroxide is most often used in the form of commercial aqueous 
solutions containing 30-70% water. Moreover, the reaction also produces 
water, and the state of equilibrium is thus attained well before the 
hydrogen peroxide is fully transformed. Under these conditions, the 
product of the reaction is in effect a mixture of acid, hydrogen peroxide, 
per-acid, water, and strong acid. Because of this, the use of such a 
mixture as a means of oxidation in organic chemistry produces rather 
average yields. 
To overcome this drawback, it has been proposed that the operation take 
place in the presence of a heavy excess of carboxylic acid, so as to shift 
equilibrium toward the right. In this way, by using 10 moles of acetic 
acid to one mole of hydrogen peroxide, one may obtain a conversion rate of 
90% of the hydrogen peroxide into peracetic acid. Use of such an excess 
allows one to obtain only very diluted solutions of per-acid, and often 
involves losses in yields due to side reactions, without including the 
problems of subsequent separation of the products of the reaction. 
The proposal has also been made, such as in U.S. Pat. Nos. 2,877,266 and 
2,814,641, to operate only with a very slight excess of carboxylic acid, 
but to operate in the presence of a strong mineral acid and an azeotropic 
entrainer, in order to eliminate the water and thus shift the equilibrium 
(I) to the right. Such a practice is in fact excellent in terms of yield 
of percarboxylic acid in comparison to the hydrogen peroxide used. 
Compared with the preceding techniques, one could expect that this 
technique produces high yields in oxidation reactions in organic 
chemistry. This is scarcely the case, and the yield may be even worse, 
since the strong-acid catalyst very often gives rise to side reactions. 
For example, it is well known that, in reactions of epoxidation of olefins 
by per-acids, the epoxide formed is easily opened and transformed into a 
mono- or di-ester under the effect of strong-acid catalysts. 
It is true that the strong acid may be advantageously neutralized, but then 
the corresponding salt is generally insoluble in the medium and poses 
separation problems which are not insignificant on the practical level. 
Sometimes, the salt is even as good a catalyst of side reactions as the 
acid itself. 
This is why a method has been proposed recently, as in French Pat. Nos. 
2,359,132 and 2,300,085, for preparation of organic solutions of 
percarboxylic acids in two stages, which consists of causing hydrogen 
peroxide (20-35% solution) to react with propionic acid in an aqueous 
solution containing 10-45% sulfuric acid, and then extracting the 
perpropionic acid with the aid of a solvent, such as benzene or 
dichloropropane. The aqueous phase must be concentrated in order to 
eliminate the water contributed by the H.sub.2 O.sub.2 solution and by the 
reaction. The organic phase is washed in order to eliminate H.sub.2 
SO.sub.4, then dried by, for example, azeotropic distillation. This 
solution makes it possible in effect to obtain an organic solution of 
perpropionic acid that is anhydrous and free of sulfuric acid. However, 
this is a technique which is difficult to put into practice, and 
consequently costly. 
DISCLOSURE OF THE INVENTION 
The applicant has discovered that it is possible to arrive at the same 
result, that is, obtain an anhydrous organic solution of percarboxylic 
acid untouched by any traces of strong mineral acid, by causing the 
carboxylic acid and the hydrogen peroxide to react in the presence of new 
catalysts constituted of a metalloid oxide and an azeotropic entrainer, so 
as to constantly eliminate from the reaction medium the water contributed 
by the aqueous solution of hydrogen peroxide, as well as the water 
resulting from the reaction. 
The metalloid oxides which fall within the scope of the present invention 
are those of selenium, tellurium, arsenic, antimony, bismuth, and boron. 
By way of non-restrictive examples, one may cite the following oxides: 
SeO.sub.2, TeO.sub.2, As.sub.2 O.sub.3, As.sub.2 O.sub.5, Sb.sub.2 
O.sub.5, Bi.sub.2 O.sub.3, and B.sub.2 O.sub.3. 
The carboxylic acids concerned in the invention are the water-soluble 
aliphatic carboxylic acids, such as formic, acetic, propionic, butyric 
acids. 
The azeotropic entrainer may be chosen advantageously from among the 
solvents having a boiling point lower than 100.degree. C. and forming a 
heteroazeotrope with water. By way of non-restrictive examples, one may 
cite the chlorinated solvents such as chloroform, carbon tetrachloride, 
methylene chloride, dichloro-1,2-ethane, dichloropropane, hydrocarbon 
solvents such as cyclohexane, benzene, toluene, esters such as the 
formates, acetates, propionates, butyrates, isobutyrates of methyl, ethyl, 
propyl, isopropyl, and n-butyl. 
Hydrogen peroxide may be used either in anhydrous form or in the form of 
commercial aqueous solution assaying from 30 to 70% by weight. 
The procedure according to the invention thus comprises placing in contact 
the carboxylic acid, the azeotropic entrainer, the catalyst, and the 
hydrogen peroxide, and constantly eliminating water from the reaction 
medium by azeotropic distillation. 
The temperature at which the reaction is performed falls between 40.degree. 
C. and 100.degree. C., preferably from 40.degree. C. to 70.degree. C. 
Depending on the temperature chosen and the reaction system used, the 
elimination of water may be accomplished by operating at atmospheric 
pressure or at a low pressure. The pressure may thus vary from 20 mm 
mercury to 760 mm mercury. 
The duration of the reaction depends on the nature of the catalyst, the 
nature of the carboxylic acid, and the nature of the azeotropic entrainer, 
and the temperature chosen. It may last from several minutes to several 
hours. The reagents may be used in equimolecular quantities, but a molar 
deficiency or excess of one of the reagents may also be used. As an 
illustration, one may use 0.1 to 10 moles of carboxylic acid per mole of 
hydrogen peroxide but it is preferable to use from 1 to 5 moles. 
The catalyst is used at the rate of 0.001 to 0.1 mole of metalloid oxide 
per mole of hydrogen peroxide. However, a molar ratio from 0.001 to 0.01 
mole per mole of hydrogen peroxide used is preferred. 
The amount of azeotropic entrainer solvent falls between 50 and 75% by 
weight of the reaction mixture, so that one may regulate as desired the 
boiling point of the mixture and effectively eliminate the water. 
The reagents may be used in their usual commercial form. The hydrogen 
peroxide in particular may be used in the form of commercial aqueous 
solutions assaying from 30 to 70% by weight. It may be advantageous to add 
to the reaction mixture a hydrogen-peroxide stabilizing product, such as 
polyphosphates, derivatives of ethylenediaminetetraacetic acid (EDTA), 
etc. 
The percarboxylic acid solution thus obtained may then be used to bring 
about oxidation of a large number of organic compounds, such as olefins, 
ketones, amines, aromatic compounds, sulfur-containing derivatives, etc., 
through a second operation. However, it is not always necessary to resort 
to that procedure, and the two operations may sometimes be accomplished 
advantageously at the same time, that is, synthesis of the per-acid and 
its immediate consumption by the molecule to be oxidated. This is a 
variant of the procedure according to the invention. In this way, when the 
organic compound one wishes to oxidize with percarboxylic acid forms a 
heteroazeotrope with water, it may be used as azeotropic entrainer and at 
the same time react with the percarboxylic acid as the latter is formed. 
By way of example, one may cite the epoxidation of cyclohexene or allyl 
chloride by peracetic acid or perpropionic acid. Such a procedure is quite 
simple to bring about and is particularly safe, since it allows avoidance 
of any accumulation of peracid in the reaction medium. 
Within the scope of that variant, if the compound to be oxidized does not 
form a heteroazeotrope with water, it is of course quite possible to 
operate in the presence of an azeotropic entrainer solvent.

BEST MODES FOR CARRYING OUT THE INVENTION 
The following examples illustrate in non-restrictive manner the present 
invention. 
EXAMPLES 1 TO 9 
In a 250 cm.sup.3 reactor equipped with a distillation column having 5 
Oldershaw plates, topped by a reflux condenser, place 50 g propionic acid, 
70 g azeotropic entrainer solvent, 0.2 g catalyst. This mixture is brought 
to refulx, then one introduces gradually 0.1 mole of hydrogen peroxide in 
the form of an aqueous solution, 70% by weight. The condenser is designed 
so that only the condensed organic phase is returned to the column, the 
decanted aqueous phase being withdrawn in a continuous manner. The 
reaction conditions and the results are set forth in the following table. 
__________________________________________________________________________ 
H.sub.2 O.sub.2 
PERACID 
PRESSURE 
DURATION 
REMAINING 
FORMED 
H.sub.2 O.sub.2 
EXAMPLE 
CATALYST 
SOLVENT T .degree.C. 
mm Hg mn m mole m mole 
DISTILLED 
__________________________________________________________________________ 
1 -- Cyclohexane 
93.degree. 
760 30 14.1 13 34.3 
2 As.sub.2 O.sub.5 
Benzene 94.degree. 
760 60 2 20 20 
3 " Dichlorethane 
94.degree. 
760 60 19 58 11 
4 B.sub.2 O.sub.3 
Cyclohexane 
88.degree. 
760 30 3 49.5 20.5 
5 " Dichlorethane 
94.degree. 
760 30 6 74 7 
6 " " 70.degree. 
350 60 4 80 4 
7 " " 50.degree. 
150 60 27 62.5 15 
8 SeO.sub.2 
" 94.degree. 
760 30 3 36 6 
9 Sb.sub.2 O.sub.5 
" 95.degree. 
760 60 20 65 22 
__________________________________________________________________________ 
EXAMPLE 10 
In a 500 cm.sup.3 reactor equipped with a distillation column having 10 
Oldershaw plates, topped by a reflux condenser of the same type as the one 
described above, place 125 g propionic acid, 175 g dichloro-1,2 ethane, 
0.5 g boron oxide B.sub.2 O.sub.3, and 0.1 g disodium phosphate. Bring to 
reflux at a pressure of 150 mm Hg. The temperature of the reaction medium 
is 50.degree. C. Add gradually 0.3 mole of hydrogen peroxide in the form 
of aqueous solution, 70% by weight. After two hours of reaction during 
which the water is eliminated in continuous manner by azeotropic 
distillation, one determines in the medium 0.24 mole perpropionic acid, as 
well as 0.027 mole hydrogen peroxide, while the distilled aqueous phase 
contains 0.032 mole hydrogen peroxide. 
EXAMPLE 11 
The same experiment is repeated as in Example 7, replacing the propionic 
acid with acetic acid. After 60 minutes of reaction, one determines in the 
reaction medium 0.07 mole peracetic acid and 0.026 mole of hydrogen 
peroxide; 0.004 mole hydrogen peroxide has passed to the aqueous phase of 
the distillate. 
EXAMPLE 12 
120 g of perpropionic acid solution, prepared according to Example 10, and 
containing 0.09 mole peracid, is reacted with 8.2 g of cyclohexene, at 
room temperature. After one hour of reaction, one determines, by gas 
chromatography, 0.08 mole cyclohexene epoxide. 
EXAMPLE 13 
In a reactor such as described in Example 1, place 50 g propionic acid, 50 
g allyl chloride, as well as 0.1 g arsenic oxide As.sub.2 O.sub.5. Bring 
to reflux and adjust the temperature of the reaction medium to 64.degree. 
C. In 15 minutes, add 0.053 mole hydrogen peroxide in the form of 70% 
aqueous solution, and eliminate continuously the water contributed by 
H.sub.2 O.sub.2, and that formed during the reaction. After one hour of 
reaction, one determines in the reaction medium 0.034 mole 
epichlorohydrin, 0.008 mole perpropionic acid, and 0.006 mole hydrogen 
peroxide. The distillate contains 0.004 mole hydrogen peroxide. 
EXAMPLE 14 
In a tubular reactor, 15 m long and 2 mm diameter, introduce continuously 
and simultaneously, after passage through a mixer, 100 g/hr perpropionic 
acid solution prepared according to Example 10, assaying 6.6% per-acid, 
and 0.25% hydrogen peroxide, as well as 21 g/hr propylene. The reactor 
temperature is kept at 50.degree. C. Pressure in the reactor is 8 bars. At 
the outlet of the reactor, the reaction mixture is decompressed in a 
continuous process. The gaseous phase is washed with water in a washing 
column to recover the propylene oxide entrained. The liquid phase is 
cooled. Analysis of the products of the reaction reveals that 0.011 mole 
per hour of perpropionic acid leaves the reactor, and that 4 g/hr 
propylene oxide is formed.