Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
2002-07-19
2004-03-30
Phasge, Arun S. (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S468000, C204S263000, C204S265000
Reexamination Certificate
active
06712949
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to methods for the synthesis of stable, aqueous, mainly acidic solutions of hydrogen peroxide and improved electrocatalytic electrodes, cells and systems for more efficient production of high concentration peroxide solutions.
BACKGROUND OF THE INVENTION
Hydrogen peroxide is a strong oxidant, which is also an environmentally favored chemical for various applications, such as in chemical synthesis, water treatment, pulp and paper bleaching and waste treatment. Hydrogen peroxide is also widely used as an alternative for chlorine in view of its more benign affects on the environment. Use of hydrogen peroxide by various industries is such that the demand has been growing at a steady rate (>7% annually). Consequently, new and improved processes for commercial and on-site production of hydrogen peroxide would be of substantial interest to many industries.
Hydrogen peroxide has been produced by chemical methods, as well as electrochemical methods. Representative catalytic chemical methods include such processes as oxidation of secondary alcohols, e.g. isopropanol, to ketone and peroxide; anthraquinone auto-oxidation by cyclic reduction/oxidation; oxidation of alkali metals to peroxides; metal to peroxide followed by hydrolysis; synthesis by the direct combination of hydrogen and oxygen over noble metal catalyst, and so on.
The above mentioned cyclic anthraquinone process is an auto-oxidation process, often chosen for large scale manufacturing of hydrogen peroxide. This chemical process relies on mixed organic solvents to maximize the solubility of anthraquinone to achieve high yields of peroxide. In this process, an alkylanthraquinone, a quinoid, is chemically reduced with hydrogen in the presence of a catalyst to the corresponding 2-alkyl 9,10-dihydroanthraquinone. The 2-alkyl 9,10-dihydroanthraquinone is then oxidized in the presence of oxygen back to the corresponding quinoid, plus hydrogen peroxide. Desirably, the alkylanthraquinone is then available for separation and recovery as recycle in the further synthesis of hydrogen peroxide.
Some of the more significant problems associated with the cyclic anthraquinone process relate to peroxide contamination from the organic solvent system and the requirement of catalyst removal prior to the oxidation step as a safety precaution to avoid potentially explosive reactions. Consequently, several separation steps are required in the process. While a number of improvements have been made in the cyclic anthraquinone process, a purely aqueous process would be highly desirable for ease of operation.
Other significant problems associated with catalytic chemical methods have included potentially explosive reactions occurring, i.e., safety concerns with hydrogen and oxygen reacting; the generation of undesirable by-products, like acetone, and so on.
Generally, electrochemical methods for the synthesis of hydrogen peroxide offer some important advantages over chemical methods, including higher purity, fewer separation steps, fewer unwanted by-products, greater safety and fewer environmental concerns.
Representative methods for the electrochemical production of hydrogen peroxide include, for example, the electrolysis of ammonium sulfate followed by the hydrolysis of persulfate to peroxide; cathodic reduction of oxygen to alkaline peroxide solution; redox polymer with quinoid groups in the reduced form to effectuate oxygen reduction to peroxide; electrochemical reduction of a quinone to continuously regenerate hydroquinone in aqueous medium, etc.
Notwithstanding the number of substantial advantages associated with electrochemical synthesis methods for the production of hydrogen peroxide, some electrolytic processes have experienced shortcomings. For example, in the electrolytic method wherein sulfuric acid or sulfate salts of potassium, sodium or ammonium are oxidized to persulfate, capital equipment costs can be high due to costly platinum anodes corroding resulting in losses in valuable platinum metal.
The so called Dow Process for on-site electrolytic production of hydrogen peroxide employs cathodic reduction of oxygen in a trickle bed cell. The hydrogen peroxide solution is used directly for pulp bleaching, de-inking recycled paper, etc. However, the hydrogen peroxide produced according to this method has an alkaline pH, rendering it less stable. Consequently, alkaline pH peroxides produced by such methods are not entirely suitable for transportation, long-term storage, or for use in other applications, like mining, chemical synthesis, and certain environmental applications.
Formation of hydrogen peroxide through electrochemical oxygen reduction is described in several patents and technical publications, including U.S. Pat. No. 5,112,702 to Berzins et al; Canadian Pat. 2,103,387 to Drackett; German DE 4 311 665 to Hilricha, et al; Japan. Kokai JP 06,600,389 (1994) to Otsuka, et al;
Electrochem acta,
35(2), 1990, 319-22. Kalu, et al,
J. Applied Electrochemistry,
20, p. 932-940, 1990, describe a method for simultaneous production of sodium chlorate and hydrogen peroxide using a cathodic oxygen reduction cell. Foller et al,
J. Applied Electrochem.,
25, p. 613-27, 1995, reported on the use of gas diffusion electrodes for the preparation of hydrogen peroxide. However, the peroxides had an alkaline pH, and were of generally poor stability.
While the chemical and electrochemical processes for the production of hydrogen peroxide each offer several important benefits, it would be highly desirable to have a modified process which offers the advantages of an aqueous medium for safety and environmental concerns, along with higher purity and minimal by-products of an electrochemical process, but which mimics certain features of established catalytic chemical process technology to enable the production of more stable peroxides at acid pH ranges, and at higher yields.
The literature describes methods relating to the preparation and use of redox polymers for the chemical and electrochemical synthesis of hydrogen peroxide. For example, Manecke,
Angew. Chem.,
68, 582, 1956, used a redox polymer formed by the condensation polymerization of hydroquinone and formaldehyde. In a column reactor, workers were able to produce 2N hydrogen peroxide (3% by wt.) by passing oxygen-saturated water and recycling through the bed. Manecke, et al,
Electrochem.,
62, 311, 1958; Izoret, G.,
Ann. Chim.
, (Paris), 7, 151, 1962 describe passing a solution of sodium dithionite or other reducing agent for regenerating a resin. U.S. Pat. No. 2,992,899 (Manecke) disclose a process for peroxide synthesis using an insoluble oxidation/reduction resin by passing oxygen saturated water through a bed of the resin.
U.S. Pat. No. 4,647,359 to Lindstrom discloses the preparation of electrocatalytic gas diffusion electrodes employing noble metal catalyzed carbon cloth.
U.S. Pat. No. 6,274,114 to Ledon et al disclose a two step electrochemical process for the generation of hydrogen peroxide wherein cobalt is oxidized in an electrochemical cell to form a Co
+3
complex, which is then reacted with oxygen to form peroxide and reduced cobalt (Co
+2
).
In a 1969 patent, U.S. Pat. No. 3,454,477 to Grangaard, there is disclosed an electrochemical method for producing hydrogen peroxide using a quinone redox polymer deposited on a porous graphite cathode, and operated in an alkaline electrolyte. However, the performance of this system indicates poor stability of the electrode, as well as the production of peroxides at low concentrations generated at current efficiencies of less than 25%.
Accordingly, there is a need for more reliable and efficient semi-electrochemical/chemical or hybrid methods, apparatus and systems for the synthesis of hydrogen peroxide in stable aqueous medium at high concentrations, and which are suitable for scaling-up for large manufacturing installations, as well as for smaller on-site peroxide generation.
SUMMARY OF THE INVENTION
It is therefore one principal object of this invention to provide for improved methods
Ellis Howard M.
Phasge Arun S,.
The Electrosynthesis Company, Inc.
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