Note: Descriptions are shown in the official language in which they were submitted.
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1
PROCESS FOR THE PRODUCTION OF
HYDROGEN PEROXIDE AND CHLORATE.
The present invention relates to a process for the production of alkali metal
chlorate in a divided electrochemical cell.
Alkali metal chlorate, and especially sodium chlorate, is an important
chemical,
particularly in the pulp and paper industry as a raw material for the
production of chlorine
dioxide that is widely used for bleaching. Alkali metal chlorate is
conventionally produced
by electrolysis of alkali metal chlorides in open non-divided electrolytic
cells provided with
hydrogen evolving cathodes. The overall chemical reaction taking place in such
cells is
MCI + 3H20 -> MCIO3 + 3H2
where M is an alkali metal. The process is very efficient although the
hydrogen gas
evolved at the cathode create some over-voltage increasing the power
consumption.
Hydrogen peroxide is also widely used in the pulp and paper industry and is
generally produced by the anthraquinone process involving alternate
hydrogenation and
oxidation of anthraquinones and/or tetrahydro anthraquinones in a working
solution.
Although very efficient, this process is complicated to operate and requires
extensive
equipment. Alternative processes have so far not been proved competitive
unless under
very special circumstances.
WO 2004/005583 discloses production of alkali metal chlorate in an
electrolytic
cell divided by a cation selective separator into an anode compartment in
which an anode
is arranged and a cathode compartment in which a gas diffusion electrode is
arranged.
Oxygen is introduced to the cathode compartment and electrolysed to produce
alkali
metal hydroxide.
Electrochemical production of alkaline hydrogen peroxide solution by reducing
oxygen on a cathode is disclosed in e.g. US 6322690.
Electrochemical production of alkaline hydrogen peroxide solution by reducing
oxygen on a cathode and simultaneous production of sodium chlorate is
disclosed in E. E.
Kalu and C. Oloman, "Simultaneous electrosynthesis of alkaline hydrogen
peroxide and
sodium chlorate", Journal of Applied Electrochemistry 20 (1990), 932-940.
E.L. Gyenge and C.W. Oloman disclose in "Electrosynthesis of hydrogen peroxide
in acidic solutions by mediated oxygen reduction in a three-phase
(aqueous/organic/gaseous) system Part I: Emulsion structure, electrode
kinetics and
batch electrolysis", Journal of Applied Electrochemistry (2003), 33(8), 655-
663 and
"Electrosynthesis of hydrogen peroxide in acidic solutions by mediated oxygen
reduction
in a three-phase (aqueous/organic/gaseous) system. Part I I: Experiments in
flow-by fixed-
bed electrochemical cells with three-phase flow", Journal of Applied
Electrochemistry
(2003), 33(8), 665-674, production of hydrogen peroxide by electroreduction of
2-ethyl-
9,10-anthraquinone to the corresponding anthrahydroquinone dissolved in an
organic phase
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emulsified in water. The anthrahydroquinone is reacted with gaseous oxygen to
obtain
hydrogen peroxide.
JP 61-284591 and US 4067787 disclose production of hydrogen peroxide by
reduction of a water soluble anthraquinone derivate in an aqueous solution
followed by
reaction with oxygen.
A. Huissoud and P. Tissot disclose in "Electrochemical reduction of 2-ethyl-
9,10-
anthraquinone on reticulated vitreous carbon and mediated formation of
hydrogen
peroxide" Journal of Applied Electrochemistry (1998), 28(6), 653-657,
electrochemical
reduction of 2-ethyl- 9,10-anthraquinone in dimethoxyethane comprising 5%
water and 0.1
mole/litre of tetraetyl ammonium tetrafluoroborate.
Electrochemical reduction of oxygen on a carbon cathode grafted with
anthraquinone is disclosed in e.g. WO 02/02846, Mirkhalaf, Fakhradin;
Tammeveski,
Kaido; Schiffrin, David J., "Substituent effects on the electrocatalytic
reduction of oxygen
on quinone-modified glassy carbon electrodes", Phys. Chem.Chem.Phys.(2004),
6(6),
1321-1327, and Vaik, Katri; Schiffrin, David J.; Tammeveski, Kaido;
"Electrochemical
reduction of oxygen on anodically pre-treated and chemically grafted glassy
carbon
electrodes in alkaline solutions", Electrochemistry Communications (2004),
6(1), 1-5.
Vaik, Katri; Sarapuu, Ave; Tammeveski, Kaido; Mirkhalaf, Fakhradin; Schiffrin,
David J. "Oxygen reduction on phenanthrenequinone-modified glassy carbon
electrodes
in 0.1 M KOH", Journal of Electroanalytical Chemistry (2004), 564(1-2), 159-
166,
discloses use of a cathode grafted with phenanthrenequinone.
WO 03/004727 discloses electrosynthesis of organic compounds by
electrochemical transformation of a compound in the presence of an electrolyte
comprising a room temperature ionic liquid and recovering the product.
It is an object of the invention to provide an efficient process for the
production of
alkali metal chlorate.
It is another object of the invention to provide a process enabling
simultaneous
production of alkali metal chlorate and hydrogen peroxide.
The invention concerns a process for the production of alkali metal chlorate
comprising:
providing an electrochemical cell comprising an anode and a cathode in
separate anode
and cathode compartments;
contacting the cathode with a catholyte comprising at least one organic
mediator and one
or more organic or mineral acids;
reacting the organic mediator at the cathode to form at least one reduced form
of the
mediator;
reacting the at least one reduced form of the mediator with oxygen to form
hydrogen
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peroxide;
contacting the anode with an anolyte comprising alkali metal chloride;
reacting chloride at the anode to form chlorine that is hydrolysed; and,
reacting the hydrolysed chlorine to form chlorate.
Preferably the hydrogen peroxide is separated from the catholyte as a product
or
used as a reactant for the production of other chemicals without separation
from the
electrolyte. However, in the case hydrogen peroxide is not needed at the
production site it
is also possible simply to destruct the hydrogen peroxide, for example by
catalytic
decomposition.
The anolyte may have a concentration of chloride ions up to saturation,
preferably from about 30 to about 300 g/l, more preferably from about 50 to
about 250 g/l,
most preferably from about 80 to about 200 g/l, calculated as sodium chloride.
The
anolyte usually also comprise at least some chlorate, preferably from about 1
to about
1200 g/l, calculated as sodium chlorate. In one embodiment the preferred
content is from
about 300 to about 650 g/l, most preferably from about 500 to about 650 g/l,
calculated as
sodium chlorate. In another embodiment the preferred content is from about 1
to about 50
g/l, most preferably from about 1 to about 30 g/l, calculated as sodium
chlorate.
In order to stimulate the hydrolysis of chlorine the pH in the bulk of the
anolyte is
preferably at least about 4, most preferably from about 4 to about 10. It is
to be
understood that there may be local variations of the pH and particularly that
there may be
local zones having considerably lower pH as protons are formed.
In order to suppress undesired side reaction it is possible to include alkali
metal
chromate, dichromate or another suitable pH buffer into the anolyte. If
chromate or
dichromate is present in the anolyte the content thereof is preferably from
about 0.01 to
about 10 g/l, most preferably from about 0.01 to about 6 g/l, calculated as
sodium
chromate. However, it is also possible to operate in the substantial absence
of chromate
in the anolyte.
Unless otherwise stated, all concentrations in g/I refer to the volume of the
actual
solution.
The temperature of the anolyte is preferably from about 20 to about 100 C,
most
preferably from about 40 to about 90 C.
Suitably, most of the chlorine generated in the anode compartment is dissolved
in the anolyte solution. Dissolved chlorine spontaneously undergoes partial
hydrolysis to
form hypochlorous acid according to the formula:
CI2 +H2O -> HCIO+HCI
The hypochlorous acid undergoes disproportionation (in some literature
referred to as
autoxidation) to chlorate according to the overall formula:
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2 HCIO + CIO- -> CI03 + 2 CI- + 2 H+
The disproportionation may take place in the cell, in a separate chlorate
reactor or a
combination thereof.
Preferably, anolyte from the anode compartment, also referred to as chlorate
electrolyte and usually comprising chlorine and/or hypochlorous acid, is
brought to a
chlorate reactor for proceeding with the disproportionation reactions to
produce chlorate.
The chlorate reactor may be of any conventional kind, for example as described
in
US 5419818, and may comprise one or several chlorate vessels.
When the disproportionation is completed to a sufficient degree the chlorate
electrolyte may be brought to a crystalliser for precipitating and withdrawing
solid alkali
metal chlorate. The mother liquor from the crystallisation, usually containing
unreacted
chloride ions, hypochlorite, and chlorate, is then preferably recirculated to
the anode
compartment of the electrochemical cell to form an anolyte together with
freshly added
alkali metal chloride. It is also possible to use the chlorate electrolyte as
such without
crystallisation, for example as a feed to a chlorine dioxide generator. This
embodiment is
advantageous in an integrated process where depleted generator liquor from the
chlorine
dioxide production is recycled back to the anode compartment of the
electrochemical cell
for the chlorate production.
The production of alkali metal chlorate may be performed continuously,
batchwise or a combination thereof.
The organic mediator in the catholyte is preferably dissolved in a
predominantly
aqueous or a predominantly organic continuous liquid phase, preferably having
an
electrical conductivity under process conditions of at least about 0.1 S/m,
more preferably
at least about 1 S/m, most preferably at least about 3 S/m. However, it is
also possible for
the organic mediator to be dissolved in a predominantly organic phase
emulsified in a
continuous predominantly aqueous phase.
The organic mediator is a substance capable of being electrochemically reacted
at a cathode to yield one or several reduced forms, which in turn are capable
of reacting
with preferably molecular oxygen and be converted back to the original form,
thus
enabling a cyclic process. The reaction of the reduced forms of the mediator
with oxygen
preferably take place in the presence of protons. However, in the absence of a
suitable
proton source it is possible to form peroxide salts, for example Na202, which
subsequently may be hydrolyzed to yield hydrogen peroxide. Without being bound
to any
theory it is believed that the reaction scheme yielding hydrogen peroxide
comprises the
transfer of two electrons and two protons taking place in separate or combined
simultaneous reactions and is believed to involve as intermediate species O2
=, HOO=,
and HOO-.
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Examples of classes of organic substances forming redox systems and useful as
mediators include quinones, flavoquinones, pyridine derivates such as
nicotineamides,
and ketones.
Useful quinones include molecules containing a (benzo)quinone-moiety (orto- or
5 para- forms), of which anthraquinones, tetrahydro anthraquinones,
naphtoquinones,
benzoquinones and derivates thereof are preferred. Anthraquinones,
naphtoquinones
and benzoquinones are preferably substituted, for example alkyl substituted
like 2-alkyl-
9,10-anthraquinones. Specific examples include 2-ethyl-9,10-anthraquinone, 2-
tert-butyl-
9,10-anthraquinone, 2-hexenyl-9,10-anthraquinone, eutectic mixtures of alkyl-
9,10-
anthraquinones, mixtures of 2-amyl-9,10-anthraquinones, all of which having
high stability.
Specific examples of alkyl substituted napthoquinones include 2-methyl-1,4-
naphthoquinone, 2-ethyl-1,4-naphthoquinone, 2-propyl-1,4-naphthoquinone, 2-
tert-butyl-
1,4-naphthoquinone, 2-tert-amyl-1,4-naphthoquinone, 2-iso-amyl-1,4-
naphthoquinone,
2,3-dimethyl-1,4-naphthoquinone. Other examples of substituents useful for
controlling
reactivity and solubility of quinones include -S03H/-S03 ,-P02R-, -OP03R-, -
NO2, -OCH3, -
S02CH3, -OPh, -SPh, -SO2Ph, -COOH/-COO-, -CN, -OH, -COCH3 ,-F, -Cl, -Br, -CF3,
-
NH2/-NH3+, -NRH/-NRH2+, -NR2/-NR2H+, -NR3+, -PH2/-NH3+, -SR2+, -PRH/-PRH2+, -
PR2/-PR2H+ and -PR3+, R preferably being, independently of each other,
optionally
substituted alkyl, alkenyl or aryl, or hydrogen. Anthraquinones may be singly
or multiply
substituted with a combination of the above and/or other substituents. It is
also possible to
use quinone derivates having common charge bearing substituents imposing an
ionic
character of the molecule. Specific examples of non-alkyl substituted quinones
derivates
include anthraquinone-2-sulfonate, 5,6,7,8-tetrahydro-9-10-anthraquinone-2-
sulfonate,
anthraquinone-2,6-disulfonate, naphthoquinone-2-sulfonate, 2-methoxy-1,4-
naphthoquinone, 2-ethoxy-1,4-naphthoquinone, 2-amino-anthraquinone, 2-amino-
naphtoquinone, 2-(alkyl amino)-anthraquinone, 2-(dialkyl amino)-anthraquinone,
2-(trialkyl
ammonium)-anthraquinone, 2-(alkyl amino)-naphtoquinone, 2-(dialkyl amino)-
naphtoquinone, 2-(trialkyl ammonium)-naphtoquinone. Naphtoquinones may, e.g.
be
substituted at any position on the lateral ring, e.g. naphtoquinone-6-
sulphonate or 6-
trialkylammonium naphtoquinone. One substituent on each ring can also be
advantageous,
such as 6-amyl-naphtoquinone-2-sul phonate or 6-ethyl-2-triethylammonium
naphtoquinone.
Corresponding examples for benzoquinone are benzoquinone-2-sul phonate and 2-
(ethyl,dimethyl)ammonium. Anthraquinones and naphtoquinones with the lateral
rings
partially hydrogenated, e.g. 1,2,3,4-tetrahydro anthraquinone, 5,6,7,8-
tetrahydro-2-ethyl-
anthraquinone, 5,6,7,8-tetrahydronaphtoquinone, could also be used. This also
applies to
substituted anthra- and naphto-quinones, including those corresponding to the
kinds
mentioned above.
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In the case a quinone is substituted and comprise one or more optionally
substituted alkyl, alkenyl or aryl groups, it is preferred that these groups
independently from
each others, have from 1 to 12 carbon atoms, most preferably from 1 to 8
carbon atoms. If
of more than one such group is present, they are preferably of mixed chain
length. Alkyl,
alkenyl and aryl groups may also be substituted, e.g. with one or more
hydroxyl group.
Quinones, including anthraquinones, tetrahydro anthraquinones,
naphtoquinones, benzoquinones and derivates thereof, can be reduced to
corresponding
hydroquinones by successive addition of two electrons and two protons. Next to
the
quinone and the hydroquinone a number of intermediate forms are believed to be
present
and active, like the semi-quinone radical and the semiquinone anion, as well
as the base
forms of the acidic hydroquinone. All these reduced forms may react with
oxygen and
contribute to the overall reaction yielding hydrogen peroxide and the original
quinone.
Other mediator systems capable of reducing oxygen to superoxide and
subsequently hydrogen peroxide include flavoquinones, e.g. flavin (see e.g. H.
Tatsumi et
al in "Mechanistic study of the autooxidation of reduced flavin and quinone
compounds" in
Journal of Electroanalytical Chemistry (1998), 443, 236-242) and pyridine
derivates like
nicotinamide and derivates thereof.
Further mediator systems are formed by ketones and their corresponding
alcohols. The ketone can be electrochemically reduced to the corresponding
alcohol,
which reacts with oxygen to form hydrogen peroxide and the original ketone.
Secondary
alcohols are preferred and particularily phenylic ones. Useful alcohols
include isopropyl
alcohol, benzyl alcohol, diphenylmethanol, methylphenylmethanol. Secondary
alcohols
also containing a charge bearing group can also be used.
The content of organic mediator, including the reduced forms, in the catholyte
is
preferably at least about 0.1 wt%, more preferably at least about 1 wt%, most
preferably
at least about 3 wt%. It is limited upwards only by the solubility, which
depends on the
mediator used and the composition of the liquid phase, but in many cases may
be as
much as about 10 wt% or about 20 wt% or even higher. In an embodiment where a
significant part of the hydrogen peroxide is formed outside the cell the
content of organic
mediator is preferably at least about 1 wt% to, more preferably at least about
3 wt%, most
preferably at least about 10 wt%.
If the catholyte comprises a predominantly aqueous continuous phase dissolving
the organic mediator, this phase preferably comprises at least about 50 wt%,
most
preferably at least about 80 wt% water and other inorganic components. It is
then
preferred that the mediator is selected from those with high solubility in
water, for
example quinones comprising one or more hydrophilic group such as -S03H/-SO3, -
NO2,
-COOH/-COO-, -OH, -NH2/-NH3+, -NRH/-NRH2+, -NR2/-NR2H+, -NR3+, -PH2/-NH3+, -
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PRH/-PRH2+, -PR2/-PR2H+ and -PR3+, R preferably being, independently of each
other,
hydrogen or optionally substituted alkyl or alkenyl.
If the catholyte comprises a predominantly organic continuous phase dissolving
the organic mediator, this phase preferably comprises at least about 50 wt%,
most
preferably at least about 80 wt% organic components, and may in extreme cases
be
substantially free from inorganic components. However, it is preferred that at
least about
5 wt%, most preferably at least about 20 wt% of inorganic components are
included.
Such inorganic components may, for example, be inorganic ions from salts made
up of
both organic and inorganic ions.
The continuous liquid phase of the catholyte preferably comprises an at least
partially organic salt, comprising at least one kind of organic cation and/or
organic anion.
The content thereof in the continuous liquid phase is preferably from about 20
wt% to
about 99 wt%, more preferably from about 40 wt% to about 95 wt%, most
preferably from
about 60 wt% to about 90 wt%.
The continuous liquid phase of the catholyte preferably has a pH below 7. If
the
liquid phase is predominantly organic, the pH refers to the pH obtained in
water kept in
equilibrium with the predominantly organic liquid phase.
The at least partially organic salt may be selected from the group of salts
referred to as ionic liquids, a diverse class of liquids substantially
consisting of ions. An
ionic liquid can be simple and contain a single kind of anions and a single
kind of cations,
or may be complex and contain a mixture of different anions and/or different
cations.
Some ionic liquids have a low melting point and negligible vapour pressure
near or below
room temperature and are often referred to as room temperature ionic liquids.
Such ionic
liquids usually remain liquids over a large temperature range.
The at least partially organic salt may also be selected from salts that alone
are
not classified as ionic liquids but have such properties when present together
with a
neutral co-solvent such as water or a low molecular alcohol like methanol,
ethanol or
propanol, of which water is preferred. The weight ratio salt to co-solvent is
preferably from
about 1:1 to about 1000:1, more preferably from about 2:1 to about 100:1, most
preferably from about 5:1 to about 20:1.
It is preferred to use an at least partially organic salt that in itself or in
combination with a neutral co-solvent forms a liquid phase at atmospheric
pressure below
about 130 C, preferably below about 100 C, most preferably below about 80 C.
Further,
the partial pressure of the salt at 100 C is preferably below about 10 kPa,
more
preferably below about 1 kPa, most preferably below 0.1 kPa (excluding the
partial
pressure from an optional neutral co-solvent).
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A liquid with suitable physical properties may comprise one or a mixture of
two or
more at least partially organic salts, preferably in combination with one or
more neutral
co-solvents, of which water is preferred. It may also comprise anions and
cations that
alone do not form salts with suitable properties.
The at least partially organic salt may be formed from various combinations of
cations and anions, among which at least one kind of ion is organic. The ions
are
preferably monovalent. Examples of cations include 1-alkyl-3-methyl
imidiazolium, 1-
butyl-3-methyl imidazolium [BMIM], 1-ethyl-3-methyl imidazolium [EMIM], 1,2,3-
trimethyl
imidazolium, N-alkylpyridinium, N-butyl pyridinium [BPY], pyrrolidinium,
guanidinium and
alkyl guanidinium, isouronium, PR4+, NR4+, SR3+, tetramethylammonium, choline,
cocomonium, and mixtures thereof, R preferably being, independently of each
other,
optionally substituted alkyl, alkenyl or aryl, or hydrogen. Other examples
include substituted
quinones here denoted [Q-NR3+] and [Q-PR3+], where Q represents a quinone such
as
anthraquinone, naphtoquinone or benzoquinones and R being as above. Examples
of
anions include hexafluorophosphate [HFP], tetrafluoroborate [TFB],
fluorosulfonate,
hexafluoroantimonate hexafluoroarsenate, chloroaluminate, bromoaluminate,
bis(trifluoromethylsulfonyl)imide, tris(trifluoromethylsulfonyl)methide,
tricyanomethide,
dicyanamide, nonafluorobutanesulfonate, trifluoromethane sulfonate, 2,2,2-
trifluororethanesulfonate, nitrate, sulphate, phosphate, RP042-, R2P04 , R2P02
(e.g. a
dialkylphosphinate), perchlorate, actetate, al kylsul phonate, bis(2-
ethylhexyl)sodium
sulfosuccinate, diethyleneglycolmonomethylethersulfate,
alkyloligoethersuitFate, pivalate,
tetraalkylborate, propionate, succinate, saccharinate, glycolate, stearate,
lactate, malate,
tartrate, citrate, ascorbate, glutamate, benzoate, salicylate,
methanesulfonate,
toluenesulfonate, and mixtures thereof, R being as above. Other examples
include
substituted quinones here denoted [Q-(O)-S03 ] and [Q-(O)-P03R-], where Q
represents a
quinone such as anthraquinone, naphtoquinone or benzoquinones, (0) denotes an
optional oxygen (e.g. sulphate/sulphonate and phosphate/phosphonate) and R
being as
above.
In the case any cation or anion comprise one or more optionally substituted
alkyl,
alkenyl or aryl groups, it is preferred that these groups independently from
each others, have
from 1 to 12 carbon atoms, most preferably from 1 to 8 carbon atoms. If of
more than one
such group is present, they are preferably of mixed chain length. Alkyl,
alkenyl and aryl
groups may also be substituted, e.g. with one or more hydroxyl group.
Examples of salts useful for the present invention include any combination of
the
following cations; [1,3-dialkyl imidazolium], [trialkylammonium],
[tetraalkylammonium],
[trial kylphosphonium], [tetraalkylphosphonium], [alkylpyridinium], [choline],
[Q-NR3+] and
[Q-PR3+] in combination with any of the following anions; [sulphate],
[phosphate], [alkyl
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sulphate], [alkyl sulphonate], [dialkyl phosphate], [alkyl phosphonate], [Q-
(O)-S03 ] and
[Q-(O)-P03R-], where Q, (0) and R are defined as above.
Specific combinations of groups include [1,3-dialkyl imidazolium] [alkyl
sulphonate] such as any one of [1-butyl-3-methyl imidazolium][methyl-SO3], [1-
ethyl-3-
methyl imidazolium][ethyl sulphonate], [1-hexyl-3-methyl
imidazolium][tosylate], [1-butyl-
3-methyl imidazolium][anthraquinone-2-sulphonate] or [1-butyl-3-methyl
imidazolium][5-
tert-amyl-naphtoquinone-2-sul phonate]; [tetraalkylammonium][Q-(O)-SO3] such
as any
one of [methyl, tri-ethyl ammonium], [5-tert-amyl-naphtoquinone-2-sulphonate],
[methyl,di-ethyl,butyl ammonium][anthraquinone-2-sul phonate] or [choline][5-
amyl-
bezonquinone-2-sulphonate]; or [Q-NR3+][alkyl sulphonate] such as [5,6,7,8-
tetrahydro
anthraquinone-2-aminium, N,N,N-(methyl,diethyl)][methylsulphonate];
[tetraalkylphosphonium][dialkylphosphate] such as any of [ethyl tributyl
phosphonium][diethyl phosphate], [phenyl triethyl phsophonium][diisobutyl
phosphate].
Not being bound to specific combinations of groups a multitude of combinations
are possible, such as any one of [triisobutyl(methyl) phosphonium][tosylate],
[trihexyl(tetradecyl)phosphonium][bis 2,4,4-trimethylpentyl phosphinate]
[tetrabutylammonium][methanesulhponate][1-ethyl-3- methyl imidazolium] [HFP],
[tripentyl
sulphonium][dipentyl, benzyl ammonium], [benzoquinone-2-aminium-N,N,N-
diethyl,phenyl][5,6,7,8-tetrahydro-9,10-antraquinone-2-sulphonate],
[choline][5-ethoxy-
1,4-naphtoquinone-6-sulphate],[N-propyl-pyridinium][saccharinate].
In addition to those mentioned above, also other kinds of commercially
available
or otherwise known ionic liquids or salts having such properties in
combination with a
neutral co-solvent may be used.
It may also be possible to use a salt where at least one of the ions also
function
as a mediator that is reacted at the cathode to a reduced form and thus
participates in the
cyclic process for generation of hydrogen peroxide. In this case the mediator
used may
partly of fully consist of ions from such a salt. Examples include salts
comprising a cation
or an anion of a substituted quinone or a nicotinamide derivate such as those
mentioned
above.
The use of an at least partially organic salt as described above in the
continuous
phase of the electrolyte involves the advantages of combining high solubility
of organic
mediators like quinones with good electric conductivity. Another advantage is
the very low
flammability allowing reaction with oxygen to be carried out safely at higher
oxygen
concentrations and higher temperature than would be the case for conventional
flammable solvents. It is also easy to separate hydrogen peroxide therefrom,
for example
by evaporation or extraction, and thereby obtaining hydrogen peroxide either
of high
purity or in a mixture with a selected compound for further processing, for
example water.
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Another example is a methanol/hydrogen peroxide mixture for use as reagent in
an
epoxidation reaction. Use of ionic liquids to form a medium suitable for
carrying out
reactions involving hydrogen peroxide has been disclosed in B. Chhikara et al.
in
"Oxidation of alcohols with hydrogen peroxide catalyzed by a new imidazolium
ion based
5 phosphotungstate complex in ionic liquid", Journal of Catalysis 230 (2005,
436-439).
The catholyte further comprises one or more organic or mineral acids like
formic
acid, acetic acid, monochloro acetic acid, benzoic acid, sulphonic acids,
phosphonic
acids, nitric acid, sulphuric acid, hydrochloric acid, hydroiodic acid,
hydrobromic acid,
perchloric acid or phosphoric acid.
10 The catholyte may also comprise further components. For example, a
predominantly organic continuous liquid phase may comprise a neutral co-
solvent as
earlier mentioned, preferably water. The amount of neutral co-solvent is
preferably up to
about 50 wt%, most preferably from about 1 to about 20 wt%. A particularly
preferred
content of neutral co-solvent may, for example, be from about 1 to about 5 wt%
or from
about 5 to about 10 wt%. Examples of other optional additives include hydrogen
peroxide
stabilisers, emulsifiers, corrosion inhibitors, anti-foaming agents, buffers,
conductivity
enhancers, viscosity reducers, etc. Examples of hydrogen peroxide stabilisers
include
those commonly used such as phosphoric acid, phosphonic acid based complexing
agents, protective colloids like alkali metal stannate and radical scavengers
like pyridine
carboxylic acids. Examples of phosphonic acid based complexing agents include
1-
hydroxyethylidene-1, 1 -diphosphonic acid, 1 -aminoethane-1, 1 -diphosphonic
acid, aminotri
(methylenephosphonic acid), ethylene diamine tetra (methylenephosphonic acid),
hexamethylene diamine tetra (methylenephosphonic acid), diethylenetriamine
penta
(methylenephosphonic acid), diethylenetriamine hexa (methylenephosphonic
acid), 1-
aminoalkane-1,1-diphosphonic acids (such as morpholinomethane diphosphonic
acid,
N,N-dimethyl aminodimethyl diphosphonic acid, aminomethyl diphosphonic acid),
reaction products and salts thereof, preferably sodium salts.
It is preferred that a predominantly organic liquid phase in the catholyte has
a
viscosity at operating conditions below about 100 mPas, more preferably below
about 30
mPas, and most preferably below about 10 mPas. Furthermore, due to the
inherent risks
of handling substantially pure hydrogen peroxide, the product recovered is
preferably a
mixture of hydrogen peroxide with water or low molecular alcohols, for example
methanol.
The partial pressure at 100 C of liquid components that do not form part of
the product
mixture should preferably be below about 10 kPa, more preferably below about 1
kPa,
most preferably below 0.1 kPa.
The electrochemical cell may comprise only one compartment for the anode and
one for the cathode or further comprise one or several compartments in-
between, for
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11
example an electrodialysis stack enabling any known electrodialysis to be
performed. The
means for separating the compartments may be a non-selective physical barrier,
e.g. a
porous membrane or diaphragm, or it may be selectively permeable for certain
species
such as cations or anions. Also a combination of membranes may be used, such
as
bipolar membranes enabling splitting of water to protons and hydroxide ions.
Non-
selective barriers may, for example, be made from asbestos, ceramics, glass,
polyolefines, PTFE, PVC, etc. Cation selective membranes may, for example, be
made
from organic polymers such as PTFE, polystyrene, styrene/divinylbenzene or
vinylpyridine/divinylbenzene modified with acid groups like sulphonate,
carboxylate or
phosphonate. Anion selective membranes may, for example, be made from organic
polymers such as PTFE, polystyrene, styrene/divinylbenzene or
vinylpyridine/divinylbenzene modified with basic groups like quaternary
ammonium. A
bipolar membranes may comprise an anion permeable membrane and a cation
permeable membrane laminated together, optionally with a catalyst layer in-
between. Ion
selective and bipolar membranes are commercially available, for example under
the
trademarks NafionTM, FlemiumTM , Neosepta bipolar .
The electrolyte in the cathode compartment may contain one, two or more liquid
phases. In a single liquid phase system there is only a predominantly organic
or a
predominantly aqueous liquid electrolyte phase. In a system with two liquid
phases, the
non-continuous phase may be emulsified or simply mixed into the continuous
liquid
phase. If there are more than a single liquid phase, the components in the
electrolyte will
be distributed between the phases depending on their solubility properties. In
addition to
the liquid phase or phases there may also be gas and/or solids present.
In the cathode compartment the temperature and the pressure may be the same
or different from the anode compartment and are preferably set so the
catholyte is liquid.
A high temperature favours low viscosity, high electrical conductivity and
high mass
transfer rates, while a low temperature favours the stability of hydrogen
peroxide and
components in the electrolyte. Normally the temperature is preferably from
about 0 to
about 200 C, more preferably from about 40 to about 150 C, most preferably
from about
60 to about 100 C. The pressure is preferably from about 10 to about 30000
kPa, more
preferably from about 80 to about 2000 kPa, most preferably from about 100 to
about 800
kPa.
The reaction of the one or more reduced forms of the mediator and oxygen to
yield hydrogen peroxide may take place inside the cell or in a separate vessel
or as a
combination of the two, usually resulting in formation of hydrogen peroxide in
the
catholyte and reformation of the mediator to take part in another reaction
cycle. Normally
molecular oxygen is added to the electrolyte comprising reduced mediator, but
part of it
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12
may come from oxygen generated in anodic reactions and transported through the
electrolyte in the cell, optionally via a membrane, or be isolated as a
separate stream and
reintroduced into the cell. Molecular oxygen may be added dissolved in a
liquid or in the
form of any oxygen containing gas such as air, oxygen enriched air or
substantially pure
oxygen. Adding at least part of the oxygen as a gas directly into the cell
involves the
advantage of improving the agitation and may also create a gas-lift for
transporting
electrolyte out of cell, alternatively contribute to stripping of hydrogen
peroxide from the
electrolyte. Adding oxygen directly to the cell may enable the full catalytic
cycle of the
mediator to be completed inside the cell, substantially eliminating the need
for
withdrawing a stream comprising a reduced form of the mediator and feeding a
stream
comprising a mediator. The reactions to yield hydrogen peroxide are
facilitated by the
presence of protons that may originate from any available source, such as
water,
hydroquinone, protons generated at the anode or any acid that has been added
to the
electrolyte. If the reaction with oxygen takes place in a separate vessel, the
conditions
like temperature, pressure etc. may be the same or different from what is
prevailing in the
cell. The temperature is preferably from ambient, e.g. about 20 C, to an upper
limit
determined either by the flammability of the solvent or the stability of the
hydrogen
peroxide, for example up to about 70 C. The pressure is preferably from about
atmospheric up to about 5 barg. Generally it is preferred to use a bubble
column, either
packed or with sieve plates. Preferably oxygen containing gas is fed at the
bottom and
the liquid flows either upwards or downwards.
Various methods may be used for separating hydrogen peroxide from the
electrolyte, such as evaporation, extraction or membrane-based technologies.
The
separation may take place in the cell, in separate equipment from which the
remaining
electrolyte then is recycled back to the cell, or a combination thereof.
In one embodiment hydrogen peroxide is evaporated from a predominantly
organic phase of the electrolyte, preferably together with water and
optionally other
volatile substances that might be present. The evaporation may be effected
directly from
the cell or from a separate vessel, for example, by stripping with any gas,
e.g. oxygen, air
or nitrogen, or by distillation at atmospheric or sub-atmospheric pressure. A
low vapour
pressure of the at least partially organic salt and other organic species
optionally present
in the electrolyte and not forming part of the desired product mixture
facilitates the use of
evaporation techniques for separating hydrogen peroxide. In this embodiment is
possible
to obtain a hydrogen peroxide containing product stream of high purity without
extensive
purification steps.
In another embodiment hydrogen peroxide is extracted from a predominantly
organic liquid phase by any suitable solvent such as water or methanol. All
commonly
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13
used extraction technologies may be used, such as one or several mixer-
settlers, sieve-
plate columns, packed bed columns. If an electrolyte also comprising a
predominantly
aqueous phase is used, hydrogen peroxide will automatically be enriched in
that phase,
which may be withdrawn as a product, alternatively be subjected to
distillation or other
kind of purification and recycled back to the cell.
In a further embodiment membrane based separation is used. Examples of such
processes include membrane extraction, pervaporation and nanofiltration.
In still another embodiment the mediator and its reduced forms are dissolved
in a
predominantly aqueous phase. If also the oxidation of the reduced forms of the
mediator
takes place in the predominantly aqueous phase the hydrogen peroxide may be
separated by e.g. distillation. Another option is to keep the predominantly
aqueous phase
substantially free from oxygen and separate, e.g. by extraction, the reduced
forms of the
mediator to a predominantly organic phase and then effect the oxidation.
In still a further embodiment hydrogen peroxide is not withdrawn from the
electrolyte but is used directly as a reactant in the production of other
chemicals.
Electrolyte remaining after such reactions may then be recycled to the cell.
The production of hydrogen peroxide is preferably operated continuously,
either
with electrolyte flowing through the cell or by continuously separating
hydrogen peroxide
from the electrolyte in the cell. It is preferred to serve for adequate
agitation, particularly
around the cathode, for example by gas blow, mechanical agitation, circulation
of
electrolyte, or combinations thereof. Gas blow is preferably done with oxygen
or oxygen
containing gas such as air. In a cell with an essentially vertical flow, gas
blowing may also
creates a gas-lift enhancing the transport of electrolyte through the cell
alternatively
stripping of hydrogen peroxide, optionally together with water or any other
component
that is volatile at the temperature and pressure of operation.
In order to avoid detrimental accumulation of impurities from feed chemicals
or
degradation products formed in side reactions it may in some cases be
advisable to bleed
off part of the electrolyte from the system and/or purifying with various
methods like
electrodialysis, adsorbtion, recrystallization, precipitation, washing, ion-
exchange,
evaporation or stripping using a carrier gas, reactive regeneration with
acid/base or
reductive/oxidative steps.
As hydrogen gas may be formed as a side reaction on the cathode it may be
appropriate to include a gas analyzer and a device for flushing with inert
gas.
The temperature may be controlled by any suitable means, e.g. by heat
exchangers at any appropriate flow. Cooling can also be effected by
evaporation, e.g. in
the electrochemical cell, and subsequent condensation of the vapour. If
evaporative
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14
cooling is effected by water it may be appropriate to add water specifically
for this
purpose.
The anode can be made of any suitable material, e.g. titanium titanium alloy.
The
anode is preferably coated with e.g. RuO2/TiO2, RuO2/TiO2/IrO2 or Pt/Ir.
Particularly
preferred anodes are DSATM (dimension stable anode).
Various kinds of cathodes may be used. It is preferred that the cathode is
made
of a material suppressing parasitic reactions like hydrogen evolution, direct
oxygen
reduction to water and destruction of the organic mediator, the ionic liquid
or the
hydrogen peroxide formed. In most cases it is preferred to use a cathode with
a
hydrophobic surface. Examples of materials for the cathode include carbon
based
materials like boron doped diamond, graphite, glassy carbon, highly oriented
pyrolytic
graphite, reticulated carbon and conductive polymers. Examples of conductive
polymers
include poly(para)phenylene, polypyrrole, polythiophene and polyaniline. The
conductive
polymer can be applied as a thin film, with a preferred thickness from about
0.1 to about
100 pm, on any suitable substrate, such as Pt or stainless steel. The polymer
film can be
prepared by chemical synthesis or preferably by electrosynthesis. A specific
example is a
cathode obtained electrosynthesis of a polypyrrole film on stainless steel.
Other examples
cathode materials include metals like iron, steel, lead, nickel, titanium or
platinum, or
conductive metal oxides such as Pb02, Ni02, Ti407, NiCo2O4 or Ru02. Still
further
examples include electrocatalytic cathodes of a material like titanium or
titanium alloy
coated, fully or partially, with particles of noble metals like gold,
platinum, palladium or
grafted with catalysts for anthraquinones.
The cathode and the anode can be made in various geometrical shapes and
may, for example, take the form of a flat sheet or plate, a curved surface, a
convoluted
surface, a punched plate, a woven wire screen, an expanded mesh sheet, a rod,
or a
tube. However, the anode and cathode preferably have a planar shape, most
preferably
in the form of a sheet, mesh or plate.
Any conventional cell design can be used, preferably with as short distance as
possible between the anode and cathode. A divided cell may, for example, be of
the "zero
gap" type where at least one of the electrodes is pressed against a membrane
dividing
the cell.
A typical production plant includes a multitude of cells to achieve the
desired
production rate. The cells can be arranged in a monopolar or bipolar way in an
electrolyser according to any conventional design.
Some embodiments of the invention will now be further described in connection
with the appended schematic drawings. However, the scope of the invention is
not limited
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to these embodiments. Fig. 1 shows a schematic configuration of the cathodic
process
part, while Figs. 2, 3 and 4 show schematic designs of various electrochemical
cells.
Referring to Fig. 1, an electrochemical reduction of the mediator takes place
in a
predominantly organic continuous phase of an electrolyte in a cell compartment
1, that
5 may be a cathode compartment or a single compartment cell. A feed stream 14
provides
the cell compartment 1 with any substances that are consumed in the process,
such as
oxygen, or withdrawn in any product stream not recycled, such as water or an
inert gas
like nitrogen. If oxygen is present a reaction between the reduced form or
forms of the
mediator and oxygen to hydrogen peroxide or alkali metal peroxide may also
take place
10 in the cell compartment 1. If this reaction proceeds to a sufficiently
large extent it is
sufficient to remove the hydrogen peroxide together with e.g. water in a
stream 6. If the
reaction to hydrogen peroxide or alkali metal peroxide is incomplete
electrolyte is
withdrawn and the reaction completed to the extent desired in an oxidation
reactor 2
where additional oxygen 15 may be supplied. A resulting stream 7 contains
hydrogen
15 peroxide or an alkali metal peroxide in one or several forms depending on
the conditions
used, for example as a vapour or dissolved in a liquid phase. If both a gas
and at least
one liquid phase is present they are brought to a gas liquid separator 3 from
which a gas
stream 8 is brought to a condenser 4. Hydrogen peroxide product 10 is
withdrawn from
the condenser 4 while remaining gas 13, e.g. oxygen, steam and other optional
components, is either recycled to any point where oxygen can be used, such as
the cell
compartment 1 or the oxidation reactor 2, or bleed off via 16. A liquid stream
9 from the
separator 3 is recycled to the cell compartment 1. If the liquid stream 9
contains hydrogen
peroxide it is first brought to a separator 5, which, for example, may be an
extraction unit
or a membrane separation unit. Here the stream 5 is separated into a hydrogen
peroxide
containing product stream 11 and a recycle stream 12 comprising the
predominantly
organic electrolyte.
The various units illustrated in Fig. 1 can be combined in a multitude of
ways.
For example, oxygen may be introduced in the cell compartment 1 in various
ways, for
example separately or together with any liquid feed or recycled stream. Oxygen
may also
be introduced at a position above the electrodes in order to separate the
electrochemical
reactions and the oxidation. The oxidation reactor 2 and the gas liquid
separator 3 may
be combined, for example by using a bubble column. If the operation conditions
are set
so no gas forms and only a liquid phase is withdrawn from the cell compartment
1, the
gas liquid separator 3 and the condenser 4 may be omitted.
Referring to Fig. 2, an electrochemical cell operated according to the
invention
comprises an anode 21 in an anode compartment 23 and a cathode 22 in a cathode
compartment 24. The cell also comprises a middle compartment 25 separated from
the
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16
anode and cathode compartments 23, 24 by ion selective membranes 26, 27. In
one
embodiment, the membrane 26 is anion permeable and the membrane 27 is cation
permeable. In another embodiment both membranes 26, 27 are cation permeable.
The
cathode compartment 24 holds a catholyte comprising an organic mediator
according to
the invention. Oxygen containing gas is fed through inlet stream 28 to the
cathode
compartment 24 and an outlet stream 29 comprising hydrogen peroxide and/or
reduced
mediator is brought to a unit 30 where further processing takes place. Such
further
processing may include oxidation of reduced mediator to obtain hydrogen
peroxide and
separation thereof, resulting in a product stream 31 comprising hydrogen
peroxide and
optionally other species, such as water that may remain in the final product
and others
that may be separated later, and a recycle stream 32 comprising e.g. catholyte
with an
organic mediator obtained by oxidation of the reduced forms thereof. The anode
compartment 23 is fed with an inlet stream 33 that may have various
compositions
depending on the desired reactions. Anolyte, including reaction products, are
withdrawn
in an outlet stream 34 to a product separator 35 from which a product 36 is
withdrawn
and remaining electrolyte 37 recycled to the anode compartment 23. The middle
compartment 25 is fed through an inlet stream 38 with a preferably aqueous
solution, the
composition of which depends on the desired overall reactions. An outlet
stream 39 from
the middle compartment 25 may be recycled or used in any other way.
If the cell of Fig. 2 is used in an embodiment where the membrane 26 is anion
selective, the membrane 27 is cation selective and chloride is reacted at the
anode to
form chlorine that may be hydrolysed further to form hypochlorous acid and
finally
chlorate, the anolyte is preferably an aqueous solution comprising NaCI,
NaCIO3, or the
corresponding potassium salts KCI or KCIO3, and optionally a buffer such as
chromate,
dichromate or any other suitable salt. A product stream 34 of anolyte is
withdrawn and
brought to product separator 35 where alkali metal chlorate is crystallised
and withdrawn
36 while remaining electrolyte is recycled 37 to the anode compartment 23. It
is also
possible to provide a unit (not shown), e.g. a chlorate reactor, for further
reactions to
form chlorate by disproportionation of hypochlorous acid in the withdrawn
product stream
34 before the crystallisation. In one option the middle compartment 39 is
preferably fed
through inlet stream 38 with a solvent like water containing HCI or NaCI and
chloride ions
are transferred through the anion selective membrane 26 to the anode
compartment 23
where they are consumed at the anode 21 to form chlorine in a first step. Then
Na+ or K+
are fed to the cathode compartment 23 through inlet stream 33 for example in
the form of
NaOH or KOH. In another option the middle compartment 25 is fed through inlet
stream
38 with OH-, for example as NaOH or KOH, the hydroxide ions will be
transferred through
the anion selective membrane 26 and chloride ions are then fed through inlet
stream 33,
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17
for example as NaCI. In either option cations, normally Na+ or H+, are
transferred from the
middle compartment 25 through the cation selective membrane 27 into the
cathode
compartment 24.
Referring to Fig. 3, an electrochemical cell operated according to the
invention
comprises an anode 21 in an anode compartment 23 and a cathode 22 in a cathode
compartment 24. However, in contrast to the cell of Fig. 2 there is only one
ion selective
membrane 27, which preferably is cation selective, and there is no middle
compartment.
In all other aspects the cell is identical to the one of Fig. 2, the
description of which is
referred to.
If the cell in Fig. 3 is provided with a cation selective membrane 27 and is
used
in an embodiment where chloride reacts at the anode 21 to form chlorine that
is
hydrolyzed further to form chlorate, the anolyte is preferably an aqueous
solution
comprising NaCI, NaCIO3, or the corresponding potassium salts KCI or KCIO3,
and a
buffer such as chromate, dichromate or any other suitable salt. NaCI or KCI is
fed to the
anode compartment 23 through inlet stream 33 while cations such as Na+ or H+
are
transferred through the membrane 27 into the cathode compartment 24. In order
to
compensate for loss of Na+ or K+ through the membrane 27 and neutralising H+
formed in
the anodic reactions it may be appropriate to add some NaOH or KOH at any
suitable
position, e.g. to the inlet stream 33 or the recycle stream 37. In all other
aspects, like the
handling of product 34 and recycle streams 37, the operation is equivalent to
the
corresponding embodiment performed in the cell of Fig. 2, the description of
which is
referred to.
Referring to Fig. 4 an electrochemical cell operated according to the
invention
comprises an anode 21 in an anode compartment 23 and a cathode 22 in a cathode
compartment 24. However, in contrast to the cell of Fig. 2, the middle
compartment is
replaced by a bipolar membrane 40 separating the cell compartments 23, 24. The
bipolar
membrane 40 comprises an anion selective membrane 26 and a cation selective
membrane 27 laminated together on each side of a catalyst layer 45. Water from
the
anolyte pass into the catalyst layer where it is split to protons passing into
the cathode
compartment 24 and hydroxide ions passing into the anode compartment 23. In
all other
aspects the cell is identical to those of Figs. 2 and 3, the descriptions of
which are
referred to.
If the cell of Fig. 4 is used in an embodiment where chloride reacts at the
anode
21 to form chlorine that is reacted further to form chlorate, the anolyte is
preferably an
aqueous solution comprising NaCI, NaCIO3, or the corresponding potassium salts
KCI or
KCIO3, and a buffer such as chromate, dichromate or any other suitable salt.
NaCI or KCI
is fed to the anode compartment 23 through inlet stream 33. Inside the bipolar
membrane
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18
water is split into protons and hydroxide ions. The protons move into the
cathode
compartment 24 and facilitates the oxidation of reduced mediator to form
hydrogen
peroxide, while the hydroxide ions move into the anode compartment 23
facilitating the
hydrolysis of chlorine. In all other aspects, like the handling of product 34
and recycle
streams 37, the operation is equivalent to the corresponding embodiment
performed in
the cell of Fig. 2, the description of which is referred to.
In all the embodiments described in Figs. 2-4 the cathodic process may be the
same, i.e. reduction of the mediator at the cathode 22 and transfer of cations
like H+ or
Na+ from the middle compartment 25 or the anode compartment 23 through the
cation
selective membrane 27.
The invention will now be further described through the following Example. If
not
otherwise stated, all parts and percentages refer to parts and percent by
weight.
Example: A solution containing 50 ml of the ionic liquid 1-butyl-3-methyl-
imidazolium hexaflourophosphate [BMIM] [HFP] with 0.8 g 2-ethyl-9,10-
anthraquinone
(EAQ) was poured into the cathode compartment of a small reactor. Nitrogen gas
saturated with water was purged into the solution for 30 minutes to dissolve
gases in the
solution and to saturate the solution with water to an estimated content of
about 3-5 wt%.
On top of the organic phase an aqueous phase of 40 ml 0.5 M H2SO4 was added to
supply protons. A cathode of circular platinum mesh with a diameter of 3 cm
was placed
in the organic phase and a platinum mesh anode was placed in a separate
compartment
containing 10 mM NaOH aqueous solution. The anode and cathode compartments
were
separated with a non-selective ceramic membrane (diaphragm). The catholyte was
stirred
by a magnetic bar located in the organic phase in the cathode compartment. To
keep
track of the cathodic potential a Calomel reference electrode was placed in
the cathode
compartment close to the cathode. The anode compartment contained an aqueous
solution of 150 g/I NaCI and 10 g/I sodium dichromate. At a temperature of 68
C a current
of 0.2A was placed between the anode and cathode. During the experiment a few
droplets of NaOH (1M) was added to the anolyte to keep the pH between 6 and 7.
In the
cathode compartment hydrogen peroxide was generated. In the anode compartment
chloride was oxidized to chlorine which eventually formed chlorate. After 20
minutes the
experiment was terminated and a current efficiency for chlorate formation was
calculated
to 59%.
