It has been known for a long time (Ber. 45, 1845, 1912) that hydrogen peroxide reacts with aliphatic carboxylic acids to form percarboxylic acids according to the following reaction: ##STR1##
It is also known (D. Swern, ORGANIC PEROXIDES, Wiley Interscience, 1970, Vol. I) that although a few rare peracids, particularly performic acid, can be prepared in the absence of a catalyst, it is generally necessary for the majority of them to employ a catalyst to reduce the reaction time.
In fact, because of the instability of percarboxylic acids, this reaction is generally performed at a moderate temperature. Under these conditions, a state of equilibrium is not reached until after several hours of reaction time, and such a duration is not acceptable in an industrial process. Therefore, it is necessary to resort to the use of a catalyst.
The catalysts that have been proposed and used up to now are strong mineral or organic acids, such as phosphoric acid, sulfuric acid, hydrochloric acid, alkylor arylsulfonic acids, such as methanesulfonic acid, paratoluenesulfonic acid, trifluoroacetic acid, as well as acid cationic resins such as "Dowex 50" or "Amberlite IR-120" commercial resins.
This catalytic process had been the object of many studies (D. Swern, ORGANIC PEROXIDES, Wiley Interscience, 1970, Vol. I, pages 313-369 and pages 428-439). It has been shown that the first step of the reaction corresponds to the protonation of the carboxylic acid function resulting in the formation of an oxonium structure capable of reacting with H.sub.2 O.sub.2 to lead to percarboxylic acid according to this scheme: ##STR2##
Hydrogen peroxide is most often employed in the form of commercial aqueous solutions containing 30 to 70% water. Since the reaction also supplies one molecule of water per molecule of percarboxylic acid, it is clear that the state of equilibrium (1) is reached well before the hydrogen peroxide is completely converted. Under these conditions, the reaction product is actually a mixture of carboxylic acid, hydrogen peroxide, percarboxylic acid, water and strong acid catalyst.
Many means have been proposed to shift the equilibrium to the right, in order to utilize the hydrogen peroxide as completely as possible or to obtain a peroxide free of some of the other constituents of the equilibrium, frequently troublesome for the use intended. In fact, although it is sometimes possible to use the mixture as is and to complete the conversion of the hydrogen peroxide in situ, during certain epoxidations for example, this simple process is often inadequate because of the side reactions that can be caused by the other constituents of the mixture, such as oxidation by the hydrogen peroxide, hydrolysis by the water, and harmful action of the strong acid catalyst.
Most of the methods described use known means of dehydration of the organic media. Thus, it has been proposed that the reaction be conducted in the presence of a strong acid catalyst and of a sufficient quantity of dehydrating agents to combine with the water that is present or formed during the reaction. But the agents advocated up to now are not entirely satisfactory and, in addition, produce problems of separation that are difficult to solve from a practical standpoint. Thus, in French Pat. No. 1,492,059, metaboric acid is proposed as a stoichiometric fixing agent for water, in the presence of strong acid catalysts. This method does make it possible to attain very high levels of conversion of the hydrogen peroxide, but necessitates separating the orthoboric acid resulting from the hydration of the metaboric acid and which is insoluble in the medium. The necessity of handling a solid that may be impregnated with peracid and then having it undergo dehydration at high temperature makes this process tricky and difficult to carry out.
It has also been proposed, as in U.S. Pat. No. 2,877,266 and No. 2,814,641, to operate only with a very slight excess of carboxylic acid, but in the presence of a strong mineral acid catalyst and an azeotropic entraining agent to eliminate the water and shift the equilibrium (1) to the right. Such a practice is indeed excellent with respect to the yield of percarboxylic acid in relation to the hydrogen peroxide involved.
However, all of these methods have a major drawback in common: while the solutions of peracids are being used, the strong acid catalyst needed to accelerate the reaction process most often produce parasitic reactions that result in significantly lower yields. It is well known, for example, that in reactions of epoxidation of olefins by peracids, the epoxide formed is easily opened and converted to a mixture of mono and diester under the effect of the 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 this poses problems of separation that are not unappreciable from a practical standpoint. Sometimes this salt is even as good a catalyst of parasitic reactions as the acid itself.
This is why, as in French Pat. No. 2,359,132 and No. 2,300,085, a two-stage manufacturing method for organic solutions of percarboxylic acids was proposed, which consists of allowing an aqueous solution containing from 10 to 45% sulfuric acid serving as a dehydrating agent and a catalyst and 20 to 35% hydrogen peroxide to react with propionic acid, and then extracting the perpropionic acid that has formed with a solvent such as benzene or dichloropropane. The aqueous phase must be concentrated to eliminate the water supplied by the hydrogen peroxide and the water formed during the reaction. The organic phase is washed with water to eliminate H.sub.2 SO.sub.4, and then dried by azeotropic distillation. This solution does make it possible to obtain, with a good yield on the hydrogen peroxide, an organic solution of anhydrous perpropionic acid that has been rid of strong acid catalyst. However, this is a very cumbersome and therefore very costly method to use.