Note: Descriptions are shown in the official language in which they were submitted.
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A process for preparing peroxycarboxylic acid solutions,
in particular eguilibrium peracetic and
perpropionic acids
Description
The invention provides a process for preparing
peroxycarboxylic acid solutions, in particular so-called
equilibrium peroxycarboxylic acid solutions which contain
peroxyacetic acid or peroxypropionic acid. The process is
based on reaction of the corresponding carboxylic acid with
an aqueous hydrogen peroxide solution in the presence of
polyphosphoric acid as catalyst.
Peroxycarboxylic acid solutions, often also called
percarboxylic acid solutions, including solutions of
peroxyacetic acid and peroxypropionic acid and in
particular so-called equilibrium solutions which contain,
apart from the peroxycarboxylic acid, also the parent
carboxylic acid, hydrogen peroxide and water in an
equilibrium condition, are used in a wide variety of
applications. These types of solutions are used, for
example, in detergents, bleaches and cleansing agents and
also in microbiocidal compositions for disinfectant
purposes and also for destroying lower plant and animal
organisms.
To prepare an aqueous peroxycarboxylic acid solution, the
carboxylic acid is usually reacted, in the presence or
absence of an organic solvent, with hydrogen peroxide,
normally using an aqueous hydrogen peroxide solution in the
presence of a strongly acidic catalyst. Provided reaching
the equilibrium point is not affected by external factors,
such as azeotropic dewatering, when using an organic
solvent, the reaction proceeds to the equilibrium point and
the reaction mixture obtained is called the equilibrium
peroxycarboxylic acid solution. The time required to reach
the equilibrium point depends on the concentration of the
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reaction partners and the amount and strength of the acid
added as catalyst. Since stability-impairing factors during
the preparation, storage and handling of aqueous
peroxycarboxylic acid solutions can never be excluded in
practice, one or more stabilisers, preferably a synergistic
combination of stabilisers, are added in an amount of
normally less than 0.1 wt.%, with respect to the solution,
during and/or after preparation.
German patent DE 195 17 465 discloses peroxycarboxylic acid
solutions, including equilibrium peroxyacetic acid and
equilibrium peroxypropionic acid, the preparation of which
takes place in the presence of a polyphosphoric acid of the
formula Hn,~Pa03n~1, in which n is greater than or equal to
2.3. The polyphosphoric acid is used in an amount of 0.2 to
10 wt.%, in particular 0.5 to l0 wt.%. Expediently, the
solutions also contain one or more stabilisers from the
group consisting of chelating agents, pyrophosphoric acid
and its salts, radical traps and tin compounds. The amount
of stabiliser added is generally less than 0.5 wt.%, with
respect to the solution, in particular in the range l0 to
1000 ppm. A particularly effective stabiliser combination
is known from WO 91/13058 and this combination consists of
dipicolinic acid and 1-hydroxyethane-1,1-diphosphonic acid.
Polyphosphoric acid, which is extremely hygroscopic, is
present in the form of a solidified melt at room
temperature, so this product has to be either crushed or
melted, involving some costs, prior to use; the melting
point is usually about 80°C in the case of an average
degree of polymerisation. An alternative form of supply of
polyphosphoric acid is that of the molten material. The
requirement for heatable delivery and storage containers
and of heatable pipes from the storage container to the
point of use makes the preparation of peroxycarboxylic acid
solutions more costly.
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CA 02377243 2001-12-12
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German patent DE 195 17 465 discloses adding the catalyst
polyphosphoric acid either to the carboxylic acid to be
reacted or to the aqueous hydrogen peroxide prior to
reaction; as an alternative to this, the catalyst may also
be added to the reaction mixture of carboxylic acid and
aqueous hydrogen peroxide. A substantial disadvantage of
this process is that the handling of polyphosphoric acid is
very difficult due to its high degree of hygroscopicity and
its solidification point, which is above room temperature.
l0 Although commercially available polyphosphoric acid can be
added as a liquid by heating it to about 80°C, the
production costs increase as a result. Another problem when
preparing stabilised peroxycarboxylic acid solutions using
powdered dipicolinic acid or salts thereof, on their own or
in combination with a chelating agent based on hydroxy and
aminophosphonic acids, such as 1-hydroxyethane-1,1-
diphosphonic acid, comprises the low solubility of
dipicolinic acid and its salts which means that the
reaction mixture consisting of the carboxylic acid, aqueous
hydrogen peroxide solution, catalyst and stabilisers has to
be agitated for several hours.
There is, therefore, interest in i~roving the process for
preparing lower peroxycarboxylic acid solutions using
polyphosphoric acid as catalyst, to the effect that the
problems involved when metering out the polyphosphoric acid
need to be solved. The invention is also aimed at enabling
preparation of the peroxycarboxylic acid solution in a
simple manner at the point of use in order to avoid
transporting highly concentrated peroxycarboxylic acid
solutions over long distances, and thus the risks involved
in transport. In addition, there is interest in any case in
increasing the space-time yield of the process in which a
stabiliser system which contains dipicolinic acid is used.
It was found that solutions of acetic acid or propionic
acid with polyphosphoric acid are surprisingly stable,
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wherein the expression stability does not refer to chemical
stability but refers to the fact that the catalytic
activity of the mixture is the same as or greater than that
of pure polyphosphoric acid. On the basis of this finding,
it is possible to prepare solutions of polyphosphoric acid
in acetic acid or propionic acid immediately after the
process for preparing the polyphosphoric acid, by
dissolving the latter in the carboxylic acid, preferably
before it solidifies; this solution (= catalyst solution)
can be stored and delivered to the site of preparation of
the peroxycarboxylic acid solution with no problem. In
principle, solid, for example, solidified, polyphosphoric
acid can also be dissolved in the carboxylic acid and the
solution stored.
Thus, the invention provides a process for preparing a
peroxycarboxylic acid solution from the group consisting of
peroxyacetic acid and peroxypropionic acid solutions, in
particular equilibrium peroxycarboxylic acid solutions;
comprising reacting a carboxylic acid from the group
consisting of acetic acid and propionic acid with an
aqueous hydrogen peroxide solution in the presence of a
catalyst based on polyphosphoric acid, which is
characterised in that an anhydrous solution which has been
obtained by dissolving 0.1 to 99 wt.% (with respect to the
solution) of polyphosphoric acid in the carboxylic acid
which is to be converted into a peroxycarboxylic acid is
used as catalyst. The subclaims provide preferred types of
embodiments. Preparation of the catalyst solution is
preferably performed at the site of preparation of the
polyphosphoric acid.
The catalyst solution to be used in the process according
to the invention contains the carboxylic acid and
polyphosphoric acid or secondary products formed therefrom
in an amount of preferably 1 to 35 wt.% and in particular
of 5 to 20 wt.%, calculated as polyphosphoric acid. It is
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assumed that a mixture results from the mixture of
polyphosphoric acid and acetic acid or propionic acid, when
prepared with the use of a hot, molten polyphosphoric acid
and/or after storage for one or more days, which contains,
5 apart from the starting components, the carboxylic
anhydride, polyphosphoric acid with a reduced degree of
polymerisation and/or mixed anhydrides of the carboxylic
acid and polyphosphoric acid. In accordance with a
preferred embodiment of the process, a solution with an
amount of polyphosphoric acid such that this corresponds
fully to the concentration of catalyst required in the
reaction mixture and also to the required concentration of
carboxylic acid is used. Obviously, the catalyst-containing
carboxylic acid solution may be diluted further with the
corresponding carboxylic acid prior to use, or the latter
may be added to the reaction mixture of catalyst-containing
carboxylic acid and aqueous hydrogen peroxide.
The degree of polymerisation of the polyphosphoric acid to
be used is greater than 2, in particular 2.3 to 10. The
principle in accordance with the invention may also be used
to prepare other peroxycarboxylic acids, including those of
higher aliphatic and aromatic carboxylic acids, which may
also be substituted.
The solution of a carboxylic acid and polyphosphoric acid
to be used according to the invention is liquid at room
temperature and the solution can be transported and stored
for an extended period in containers for liquids with no
problem. Due to being in a liquid state, the solution can
also be metered out with no problem by using conventional,
simple devices for metering out liquids. The
peroxycarboxylic acid solution can be prepared either in a
conventional reaction tank or else directly in a transport
tank for the peroxycarboxylic acid solution to be prepared.
Containers made of a plastics material or stainless steel
are suitable as reaction and transport tanks. The
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polyphosphoric acid or secondary products thereof contained
in the peroxycarboxylic acid solution act as corrosion
inhibitors.
To prepare the peroxycarboxylic solution, the catalyst
solution, undiluted or diluted with more carboxylic acid,
is reacted with aqueous hydrogen peroxide. If required,
water and/or carboxylic acid may also be added to the
reaction system. A person skilled in the art can determine
the amounts of feedstocks to use to prepare a solution with
the desired concentrations of peroxycarboxylic acid,
carboxylic acid and non-reacted hydrogen peroxide in
preliminary trials. The equilibrium point is reached by
allowing the reaction mixture to stand at room temperature
or a slightly elevated temperature.
As a result of the step according to the invention of using
a catalyst.solution prepared off-site and consisting of
polyphosphoric acid and a carboxylic acid from the group
consisting of acetic acid and propionic acid, the
corresponding peroxycarboxylic acid solutions, in
particular equilibrium solutions, can be prepared in a
simple manner and without incurring high costs. As already
mentioned, no special devices are required for handling and
metering out the carboxylic acid solutions which contain
polyphosphoric acid which are prepared off-site, usually at
the site for preparing the polyphosphoric acid. This
simplifies the method of preparation. The simplicity of the
process enables the preparation of peroxycarboxylic acid
solutions in simple devices at the location where they are
needed, which means that coats and problems which are
associated with the transport of in particular highly
concentrated peracetic acid and perpropionic acid solutions
are avoided.
One or more stabilisers from the group consisting of a)
chelating agents based on hydroxy and aminophosphonic
acids, amino and hydroxycarboxylic acids, N-heterocyclic
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carboxylic acids and salts of the acids mentioned, b)
pyrophosphoric acid and its salts, c) radical traps based
on alkylated hydroxyaromatic compounds and d) tin compounds
are usually added in effective amounts during or after
preparation of a peroxycarboxylic acid solution. It is
particularly expedient to use a synergistic combination of
stabilisers consisting of dipicolinic acid or a salt of
same and at least one chelating agent from at least one of
the previously mentioned classes. The problem described at
the beginning which stems from the very low solubility of
dipicolinic acid and its salts can be overcome in a simple
manner by adding the dipicolinic acid or a salt of same and
a chelating agent which is synergistic therewith, in the
form of a single aqueous solution, to the reaction mixture
of polyphosphoric acid-containing carboxylic acid and
aqueous hydrogen peroxide at the beginning of, during or
after preparation of the peroxycarboxylic acid solution.
The aqueous stabiliser solution, which is storage stable
per se, contains 0.1 to 2 wt.% of dipicolinic acid or a
salt of same and 0.1 to 10 wt.% of a chelating agent from
the group consisting of hydroxy and aminophosphonic acids,
amino and hydroxycarboxylic acids, N-heterocyclic
carboxylic acids and salts of the acids mentioned. In
accordance with a particularly suitable embodiment, the
aqueous stabiliser solution contains dipicolinic acid and
1-hydroxyethane-1,1-diphosphonic acid. The stabiliser
solution is added to the system in an effective amount. In
accordance with a particularly preferred embodiment, the
Stabiliser solution is added to the reaction mixture in an
amount such that this contains 5 to 200 ppm of dipicolinic
acid or an alkali metal or ammonium salt of same and 50 to
1000 ppm of one or more chelating agents. Due to the use of
an aqueous solution of a synergistic stabiliser
combination, the stirring or recirculation of the reaction
mixture, which has hitherto been conventional, can be
omitted so that the time required to prepare the
peroxycarboxylic solution can be considerably shortened.
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It is also possible, in accordance with another
alternative, to add the stabiliser dipicolinic acid or a
salt of same directly to the catalyst solution during
preparation of this solution. By adding, for example,
molten polyphosphoric acid heated to 80°C and powdered
dipicolinic acid or a salt thereof to the carboxylic acid,
a clear and colourless solution is obtained after stirring
for a short time.
By using a solution, prepared off-site, of polyphosphoric
acid and a carboxylic acid from the group consisting of
acetic acid and propionic acid to be used according to the
invention and which optionally also already contains
dipicolinic acid or a salt of same, the process for
preparing stabilised equilibrium peracetic acid and
equilibrium perpropionic acid solutions can be considerably
simplified. If the catalyst solution does not contain any
stabiliser from the group consisting of dipicolinic acid
and salts of same, it is advantageous to add an aqueous
solution of one of the previously mentioned synergistic
stabiliser combinations to this within the scope of
preparing the peroxycarboxylic acid. The production costs
can be reduced by using easy-to-meter solutions and the
time of preparation can also be reduced. The process can be
performed very simply at the location where the
peroxycarboxylic acid solution is needed.
The invention is explained further by means of the
following example.
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Example
a) Preparing the catalyst solution
16.8 g of polyphosphoric acid, heated to 80°C, and 0.7 g of
dipicolinic acid were introduced into 83.0 g of acetic
acid. After stirring the mixture for two hours at about
30°C, this was a clear, colourless solution. (Stage a)
corresponds to "off-site preparation" of the catalyst
solution).
b) Preparing a 5 wt.% strength peroxyacetic acid
380 g of hydrogen peroxide (50 wt.%) and 220 g of water
were added to the catalyst solution prepared in accordance
with a). The mixture was allowed to stand and reach
equilibrium after stirring once.
