Process for making peroxycarboxylic acid

A method of making a peroxycarboxylic acid is described in which hydrogen peroxide is reacted with carboxylic acid in the presence of a catalyst acid to produce a reaction mixture containing peroxycarboxylic acid, unreacted carboxylic acid and the catalyst acid. This reaction mixture then is treated with ammonia to neutralize the catalyst acid, and the ammonium salt formed is removed in an aqueous phase. The mineral acid-free peroxycarboxylic acid reaction mixture may then advantageously be utilized as an oxidant. The peroxy-acids readily add oxygen to unsaturated compounds to produce epoxide compounds.

Various methods have previously been proposed for the preparation of 
peroxyacids. Peroxyacids (peracids) are acyl hydroperoxides and are most 
commonly produced by the acid-catalyzed esterification of hydrogen 
peroxide. Early peroxycarboxylic acid indirect oxidation techniques 
recognized the use of a catalyst acid, e.g., a mineral acid, such as 
sulfuric acid, to promote the reaction of hydrogen peroxide with 
carboxylic acid to obtain a peroxycarboxylic acid-containing mixture. The 
peroxycarboxylic acid reaction mixture is useful as an oxidant. Typically, 
peracids are known to readily add oxygen to unsaturated compounds to form 
three-membered oxirane ring compounds, generally known as epoxides. For 
example, epoxidation of highly unsaturated oils with peroxyacids results 
in products useful as plasticizers for vinyl resins and in the formulation 
of epoxy resins. 
In a particular utility, the preparation of alkylene oxides, one method 
often suggested in the literature is the indirect oxidation of alkylenes 
via peroxycarboxylic acid techniques. However, relatively low alkylene 
oxide yields and the production of undesirable amounts of various 
by-products during epoxidation rendered these peroxyacid route techniques 
economically uncompetitive. In time, it was appreciated that the actual 
indirect oxidation reaction mixture should be substantially free of the 
acid catalysts from the peroxyacid-forming reaction. These catalyst 
contaminants serve to catalyze esterification and hydrolysis side 
reactions during desired epoxidation (see U.S. Pat. No. 3,341,556). One of 
the developments which the prior art literature describes in the 
preparation of peroxycarboxylic acid is the use of various ion exchange 
resins as peroxycarboxylic acid production catalysts in place of mineral 
acid catalysts. The use of these resins sometimes enhances 
peroxycarboxylic acid production and, in turn, minor increases in 
subsequent epoxidation yields are sometimes observed. Thus, U.S. Pat. Nos. 
3,140,312; 3,330,207; 2,976,265; and 2,910,504, for example, all suggest 
the use of various ion exchange resin catalysts in the production of 
peroxycarboxylic acids by the reaction of hydrogen peroxide with 
carboxylic acid. These ion exchange resins are commercially available 
products which have functional acidic groups, such as sulfonic, 
carboxylic, phosphoric and phosphorous groups and combinations of these, 
directly attached to the carbon structure resins, e.g., styrene divinyl 
benzene-based structures. 
Although minor increases in peroxyacid and epoxide, such as alkylene oxide, 
yields are sometimes achieved in substituting ion exchange resins for 
catalyst acids in the peroxyacid reaction, the use of these resins creates 
its own problems. For example, because the acidic ion exchange resin is 
being depleted of its acid sites as it is being used in the reaction of 
hydrogen peroxide and carboxylic acid to produce peroxyacid, the amount of 
catalysis may decrease as a function of time and eventually may lead to 
undesirable levels of productivity and/or yield. Thus, the ion exchange 
resins may have to be reactivated periodically and this means shut-downs 
or by-passing with auxiliary equipment. Either corrective approach is very 
expensive on a commercial scale. 
Now, it has been discovered that mineral acid-free yields of 
peroxycarboxylic acid can be achieved through use of an improved method 
including an acid catalyzed peroxycarboxylic acid-forming step, after 
which the acid catalyst is neutralized by reaction with ammonia to form an 
ammonium salt which is then extracted prior to subsequent use of the 
peracid as an oxidant, such as in an epoxidation reaction involving the 
peroxycarboxylic acid and an alkylene oxide. 
The peroxycarboxylic acid-forming reaction generally can be characterized 
as an acid catalyzed esterification of hydrogen peroxide. A peroxyacid is 
formed when a carboxylic acid is mixed with hydrogen peroxide in the 
presence of an acid catalyst, typically a mineral acid, such as sulfuric 
acid, as follows: 
##STR1## 
wherein R is hydrogen, a substituted or unsubstituted alkyl having 1 to 4 
carbon atoms or a substituted or unsubstituted aryl having up to 10 carbon 
atoms. Preferably, R is hydrogen, an unsubstituted alkyl having 1 to 2 
carbon atoms or an unsubstituted aryl having up to 7 carbon atoms. Among 
the most useful carboxylic acids are those in which R is hydrogen, methyl, 
ethyl or phenyl. Thus, the carboxylic acid starting material may be a 
relatively lower molecular weight acid, such as acetic acid, propionic 
acid, and the like, or it may be a higher acid such as phthalic acid. 
Acetic acid is most preferred. 
The hydrogen peroxide reacted with the carboxylic acid in the peroxyacid 
production is preferably dissolved in a solvent which is inert to all of 
the principal materials used in making the peroxyacid and the alkylene 
oxide. Among these solvents are those represented by the formula R'COOR" 
in which R' and R" are each independently selected from substituted and 
unsubstituted alkyls having 1 to 5 carbon atoms. Preferred solvents are 
esters of the above formula in which R' is methyl and R" is ethyl, 
n-butyl, n-propyl, isopropyl, sec-butyl, t-butyl or n-pentyl and those 
esters in which R' is ethyl and R" is ethyl, n-propyl or isopropyl. A most 
advantageous solvent for the hydrogen peroxide is isopropyl acetate or 
n-butyl acetate. 
In the preferred reaction scheme, a carboxylic acid of the formula RCOOCH, 
wherein R is defined above, is combined with hydrogen peroxide, in a 
proportion generally of about 1.0 mole to about 3.0 moles of carboxylic 
acid, preferably about 1.3 to about 2.0 moles of carboxylic acid, and most 
preferably about 1.5 to 1.7 moles of carboxylic acid, per mole of hydrogen 
peroxide. The hydrogen peroxide may, as mentioned, preferably be in a 
solvent solution which may contain about 2 to about 20% of hydrogen 
peroxide, preferably about 10 to about 15%, based on the weight of the 
solution. 
The hydrogen peroxide and carboxylic acid are reacted in a reactor which is 
inert to the reactants and products involved, including the catalyst acid. 
The hydrogen peroxide may be fed to the reactor in a separate stream or 
may be combined with the carboxylic acid just prior to being fed to the 
reactor. The catalyst acid may likewise be fed to the reactor by itself or 
may be combined with one or more of the reactants prior to being fed to 
the reactor. A preferred technique involves combining the catalyst acid 
with the carboxylic acid outside the reactor and then combining this 
mixture with the hydrogen peroxide to form a single reactant stream which 
is fed to the reactor. 
The catalyist acid employed in the method of the present invention is any 
acid which may be used to promote the formation of the peroxyacid. Among 
these catalyst acids are organic acids, such as toluene sulfonic acid, and 
methane sulfonic acid, or mineral acids, such as phosphoric acid and 
sulfuric acid. The amount of catalyst acid employed is the conventional 
amount used to produce the peroxyacid. For purposes of illustration, 
generally about 0.1 to about 2.0, and preferably about 0.2 to about 0.5% 
sulfuric acid typically is used to form the peroxyacid, based on the 
weight of the carboxylic acid feed, although more acid may be employed 
with significantly detrimental effects. This amount of sulfuric acid is 
based on dry weight although aqueous sulfuric acid may be used. The 
specific amount of catalyst acid used, of course, depends upon the 
particular acid chosen. For example, if phosphoric acid is used instead of 
sulfuric acid, about 5 to about 20% by weight is generally used. The 
peroxyacid reactants are maintained in the peroxyacid reactor in the 
presence of the catalyst acid until a desirable amount of peroxyacid is 
produced. Generally, the peroxyacid reactor is maintained at a temperature 
of about 40.degree. C. to about 65.degree.C., preferably about 50.degree. 
C. to about 60.degree.C. The reactor is evacuated to a pressure of about 
0.10 to about 0.7 atmospheres, preferably about 0.2 to about 0.5 
atmospheres, most preferably about 0.2 to about 0.3 atmospheres. Reduced 
pressures are generally preferred and may vary depending upon the choice 
of solvent used. 
When the hydrogen peroxide reacts with carboxylic acid, a reaction mixture 
is produced which contains water, peroxycarboxylic acid, unreacted 
carboxylic acid and catalyst acid, as well as the hydrogen peroxide 
solvent, if used. Since water is a product of the reaction above, its 
removal from the reaction mixture will enhance the formation of the 
peroxyacid. For this reason, the peroxyacid reactor is maintained at 
pressure and temperature levels so as to distill off substantially all of 
the water by-product at it is produced. If an ester solvent is also 
present, some of it may also evaporate off in the form of an azeotrope 
with the water. At any rate, the reactor is maintained under temperature 
and pressure conditions as described above so as to assure the removal of 
substantially all of the water from the reactor to produce a reaction 
mixture containing peroxycarboxylic acid and catalyst acid. 
The reaction mixture containing the peroxycarboxylic acid, carboxylic acid 
and catalyst acid generally contains about 5 to about 40%, preferably 
about 20 to about 30%, peroxycarboxylic acid; about 2 to about 50%, 
preferably about 8 to about 20%, carboxylic acid; and about 0.1 to about 
5.0%, preferably about 0.2 to about 1.0%, catalyst acid, e.g., when 
sulfuric acid is used. These percentages are based on the solvent-free 
weight of the reaction mixture removed from the peroxacid reactor. When 
phosphoric acid or methane or toluene sulfonic acid is used, a 
proportionate amount of catalyst acid to the amount fed into the 
peroxyacid reactor is obtained. If solvent is present in the reaction 
mixture, generally about 10 to about 95%, e.g., about 25 to about 55% 
solvent may be present based on the total weight of the reaction mixture, 
although the amount will vary depending upon the particular solvent 
chosen. 
The peroxycarboxylic acid reaction mixture, containing carboxylic acid and 
catalyst acid, has been used, as is, to convert an alkylene compound to 
its corresponding alkylene oxide. It is known that peroxyacids readily add 
oxygen to these unsaturated compounds to produce alkylene oxides with a 
three-membered oxirane ring, commonly known as epoxides. However, presence 
of the acid catalyst from the peroxycarboxylic acid reaction mixture 
serves to catalyze side reactions during epoxidation. By serving to 
catalyze esterification and hydrolysis side reactions, presence of these 
acid catalysts significantly reduce the yield of the desired alkylene 
oxide products. 
Therefore, according to the present invention, the peroxycarboxylic 
reaction mixture first is treated to remove the acid catalyst prior to the 
subsequent reaction of the peroxycarboxylic acid as an oxidant. In order 
to effectively remove the acid catalyst, the peroxycarboxylic reaction 
mixture is treated with ammonia, in vapor phase or anhydrous form, to 
neutralize the acid. Gaseous ammonia is preferred, because of its ease of 
handling. 
The ammonia is added in an amount sufficient to neutralize the catalyst 
acid in the reaction mixture obtained from the peroxyacid reactor. For 
example, when sulfuric acid is the catalyst acid, about 1.8 to about 4.0 
moles and preferably about 2.0 to about 2.2 moles of ammonia per mole of 
catalyst acid should be used. 
Neutralization of the acid with the ammonia base results in the formation 
of an ammonium salt which precipitates from the organic reaction mixture. 
For example, when a sulfuric acid catalyst is used, an insoluble ammonium 
sulfate precipitate forms from the reaction with the ammonia. 
Use of ammonia to neutralize the catalyst acid according to the present 
invention results in increased alkylene oxide yield in subsequent 
epoxidation reactions involving the peroxycarboxylic acid reaction 
mixture. Use of other neutralizing bases has been found to involve 
handling problems or undesirable decomposition of the peroxyacid. For 
example, use of a base, such as sodium hydroxide, a water-producing base, 
has been found to initiate hydrolysis of the peroxyacid to the carboxylic 
acid and hydroperoxide. Use of any strong base dictates the exercise of 
caution, since the peroxyacid exhibits unstability at pH levels of about 6 
or higher. Bases selected from ammonium, alkali metal, and alkali earth 
metal acetates and propionates have been found to be useful neutralization 
agents, but these salts are inconvenient to physically handle and transfer 
and, thus, introduce processing problems on a commercial scale. On the 
other hand, neutralization with ammonia, according to the present 
invention, creates no transfer problems and, for example, can readily be 
introduced into a reaction mixture merely by bubbling the gas through a 
neutralization reactor. 
Once the acid catalyst, e.g., sulfuric acid, has been completely 
neutralized by the ammonia, the resulting ammonium salt, e.g., ammonium 
sulfate, should be removed in order to avoid mechanical difficulties in 
ensuing reactions wherein the peroxycarboxylic acid is used. As described 
above, the ammonium salt is insoluble in the organic reaction mixture and 
accordingly forms as a precipitate in the peroxyacid reaction mixture. 
Removing the precipitate can be achieved by common means, such as 
filtering or decanting. However, such standard methods may sometimes by 
unsatisfactory, as they may result in product loss through incomplete 
separation and/or add time-consuming, cost-increasing steps which 
complicate the overall process. 
Therefore, in accordance with the preferred embodiments of the present 
invention, the ammonium salt precipitate can conveniently be removed by 
allowing it to settle from the organic peroxycarboxylic acid reaction 
mixture into an underlying aqueous phase in which the salt readily 
dissolves. This removal procedure is complicated, however, since water, as 
noted above, cannot be allowed to come in contact with the reaction 
mixture itself. Water causes the equilibrium of the peroxycarboxylic acid 
reaction mixture to shift and revert back to favor the carboxylic acid and 
hydroperoxide. In order to resolve this critical problem, the present 
invention introduces an interface barrier layer to separate the organic 
peroxycarboxylic reaction mixture from the underlying aqueous layer. 
Accordingly, the ammonium salt precipitate is allowed to pass from the 
peroxyacid reaction mixture, through the barrier layer, to the underlying 
aqueous layer where it dissolves for ready removal. The interface barrier 
layer may be any organic solvent which is inert to the components of the 
peroxyacid reaction mixture, immiscible with water, and a non-solvent for 
the ammonium salt. Among useful solvents are the same solvents that are 
described above for use in dissolving the hydrogen peroxide for 
introduction into the peroxyacid-forming reaction. It is most advantageous 
to use the same solvent as an interface barrier that was used as the 
hydrogen peroxide solvent. Isopropyl acetate or n-butyl acetate is 
particularly preferred. 
After ammonium salt has settled from the peroxyacid reaction mixture, this 
top organic layer can be drawn off with no loss of product and with the 
introduction of a minor amount of additional interface solvent. This 
peroxycarboxylic acid reaction mixture now advantageously can be used for 
alkylene oxide production. 
The peroxycarboxylic acid reaction mixture produced according to the 
present invention is particularly suited for use in the indirect oxidation 
of alkylenes. The alkylene oxides produced by the method of the present 
invention are those derived from unsubstituted, monoolefinically bonded, 
lower alkylenes having 2 to about 15 carbon atoms, including mixtures of 
these alkylenes. Among the preferred unsubstituted, mono-olefinically 
bonded, lower alkylenes of the present invention are those having 3 to 6 
carbon atoms. Propylene is particularly preferred. 
The unsubstituted, mono-olefinically bonded, lower alkylene is combined 
with the reaction mixture containing peroxycarboxylic acid and carboxylic 
acid after the catalyst acid therein has been neutralized and the ammonium 
precipitate removed. These reactants are combined inside an epoxidation 
reactor or just prior to being fed to an epoxidation reactor. The reaction 
mixture and the alkylene are proportioned so as to employ at least a 
stoichiometric amount or an excess of alkylene as compared to the 
peroxycarboxylic acid. Generally, about 1.0 mole to about 4.0 moles, and 
preferably about 1.5 to about 2.0 moles of alkylene are used per mole of 
peroxycarboxylic acid in the reaction mixture. The preferred method of 
epoxidation is to contact the unsubstituted, mono-olefinically bonded, 
lower alkylene with the peroxycarboxylic acid-containing reaction mixture 
under conditions which maintain the alkylene in the liquid phase, although 
gaseous alkylene, e.g., propylene, may be employed. 
The epoxidation reaction is carried out at temperatures generally ranging 
from about 37.degree. C. to about 100.degree. C., preferably about 
48.degree. C. to about 60 .degree. C., although the specific temperatures 
chosen will depend upon the particular alkylene or mixture of alkylenes, 
being epoxidized. Pressures will also vary depending upon the reactants, 
but generally a pressure of about 2.5 to about 30 atmospheres, e.g., about 
5 to about 15 atmospheres, may be used. It should be noted that, in any 
case, epoxidation temperature and pressure conditions should be chosen so 
as to enhance the epoxidation reaction without being so extreme as to 
cause explosion or carboxylic acid decomposition. 
The alkylene oxide obtained by the epoxidation reaction is readily produced 
without the need for an epoxidation catalyst. The alkylene oxide is 
withdrawn from the epoxidation reactor in a mixture containing unreacted 
alkylene, carboxyic acid and minor amounts of by-products. Any of the 
conventional recovery techniques may be employed to isolate the alkylene 
oxide, as are well known in the art. Methods such as are described in U.S. 
Pat. Nos. 3,654,094 and 3,580,819 may be employed. For example, a series 
of distillations may be used to isolate each of the components in the 
mixture withdrawn from the epoxidation reactor and the unreacted alkylene 
and carboxylic acid may be recycled and used as feed in the overall 
process. The degree of purification of the alkylene oxide obtained 
ultimately depends upon the subsequent use of the product and is a matter 
of choice. 
Reference is now made to the following examples which show preferred 
embodiments of the present invention, but do not limit the scope of the 
invention thereto. All parts and percentages are by weight unless 
otherwise specified. 
COMATIVE EXAMPLE A 
To establish the improved results using peroxycarboxylic acid prepared by 
the method of the present invention over such peroxyacid prepared by prior 
art methods, the following run is made which represents the prior art in 
which peracetic acid, including sulfuric acid as the catalyst acid, is 
used as an oxidant in the preparation of propylene oxide. 
One hundred grams of a solution of peracetic acid in isopropyl acetate 
containing 11.45% peracetic acid, 6% acetic acid, 0.75% sulfuric acid, by 
weight, remainder being isopropyl acetate, is charged to a pressure 
reactor. Next, 28.1 grams of propylene, containing about 10% by weight of 
propane, is fed to the reactor in the liquid phase at a pressure of about 
11 atmospheres for about two hours, after which the excess propylene is 
vented. A product mixture is obtained weighing 99.8 grams containing 
5.0-5.05% by weight of propylene oxide. The yield of propylene oxide is 
determined to be about 57.2%.

EXAMPLE I 
A peracetic acid reaction mixture containing 22.4% by weight peracetic 
acid, 26.5% by weight acetic acid, 0.75% by weight sulfuric acid, and the 
balance isopropyl acetate was prepared by reacting hydrogen peroxide, in 
isopropyl acetate solution, with acetic acid, using sulfuric acid as a 
catalyst, in a preacid reaction vessel. 
In order to neutralize the sulfuric acid catalyst in the mixture, 0.39%, by 
weight, a slight stoichiometric excess, of gaseous ammonia was introduced 
into the reaction mixture. This H.sub.2 SO.sub.4 -neutralized peracetic 
acid solution was then pumped into a decant device and successive portions 
first of isopropyl acetate and then of water were forced into the base of 
the decant device. The decant device was arranged so that the interface 
between the organic barrier layer (isorpopyl acetate) and the water could 
be visibly observed. As the precipitated ammonium sulfate from the ammonia 
neutralization of the sulfuric acid catalyst in the peracetic acid mixture 
was allowed to settle from the reaction mixture, it was observed to pass 
through the isopropyl acetate barrier layer into the underlying aqueous 
layer where it dissolved and went into solution. The top organic reaction 
mixture, free of precipitated solids and much more easily handled, was 
decanted off. 
The peracetic acid mixture was then fed into a pressure reactor, at 240 
psig and 154.degree.-157.degree. F., at the rate of 20.36 lbs/hr. 
Propylene, at 90% purity (balance propane), was also introduced 
continuously at the rate of 5.89 lbs/hr. At these feed rates, the average 
residence time in the reactor was 94 minutes. 
The reaction product effluent from the reactor was analyzed to contain 2.0% 
peracetic acid and 10.8% propylene oxide. This indicates a yield of 
propylene oxide of about 92.9%.