Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2001-06-07
2003-03-04
Phasge, Arun S. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S537000, C204S541000
Reexamination Certificate
active
06527932
ABSTRACT:
The present invention relates to methods of preparing or purifying onium hydroxides of the elements N, S or P by electrodialysis in an electrodialysis apparatus comprising one or more cell units, each cell unit comprising a bipolar membrane and an anion-selective membrane.
Stable or at least largely stable onium compounds, i.e. the onium compounds of the elements N, S and P, play a major part in many fields of chemical synthesis or analysis. The onium compounds of nitrogen in particular, i.e. the quaternary ammonium compounds, cover a wide spectrum of applications. The quaternary ammonium hydroxides, e.g. tetramethylammonium hydroxide (TMAH) and tetraethylammonium hydroxide (TEAH) are strong organic bases which have been known for many years. Such quaternary ammonium hydroxides have found numerous applications, for example for the titration of acids in organic solvents or as electrolytes in polarography. Aqueous solutions of quaternary ammonium hydroxides, especially of TMAH, are often used as developers for photoresists in printed circuits and in chip manufacture. Often, however, the particular application demands that in addition to the abovementioned ammonium hydroxides, industrially available on a large scale, those carrying larger organic substituents be used, for example when employed as phase transfer catalysts or in the production of zeolites.
Many applications moreover require the ammonium hydroxides to be of high purity, for example to prevent the formation of by-products or any contamination of semiconductor elements. The high purity demanded relates to residual levels of halides, sulfates, carbonates and the like, for example in the manufacture of semiconductor elements. If ammonium hydroxides are used in the preparation of zeolites, the high purity relates particularly to as low an alkali metal ion level as possible.
Various methods of preparing quaternary ammonium hydroxides such as TMAH, TEAH or TPAH are known already. Generally, the quaternary ammonium hydroxides are prepared by electrolysis of a salt of a quaternary ammonium compound in an electrochemical cell which includes one or more membranes capable of exchanging cations. The quaternary ammonium salts customarily employed in such processes comprise halides, carboxylates, carbonates and sulfates.
WO 98/09002 describes the preparation of onium hydroxides in an electrochemical cell. This publication describes an electrodialysis process employing a cell unit which includes four volumes, the cell unit being defined by an anode-side bipolar membrane, a first membrane dividing the cell and a second membrane dividing the cell. Described in detail are cell structures which include various arrangements of bipolar membranes each comprising one anion-selective and one or more cation-selective membranes as cell dividers.
EP 0 824 346 describes a method of purifying hydroxy compounds in an electrochemical cell, where the cell unit comprises a bipolar membrane and a cationic membrane. Also described are cell arrangements which, in addition to the cell based on two membranes, include yet further membranes, anionic membranes as a rule.
WO 98/44169 relates to a method of preparing onium hydroxides from a suitable onium salt and of purifying such onium hydroxides. Described inter alia is an arrangement of electrochemical cells, making use of at least a first electrochemical cell and a second electrochemical cell. The first type of electrochemical cells includes a bipolar membrane and at least one further cell delimiter, possible examples of the at least one further cell delimiter including a nonionic microporous diffusion barrier such as a screen, filter, diaphragm etc. or an ionic cell delimiter such as a cation-selective membrane. The second electrochemical cell can include one or more bipolar membranes and one or more cell dividers, cation-selective membranes and anion-selective membranes being specified as being also within the scope of the invention. According to the patent description, the second cell arrangement serves the purpose of removing undesirable amounts of acid from the solution which was originally fed into the circuit. The use of an electrodialysis unit in which a unit cell includes a bipolar and an anionic membrane, with a bipolar membrane or a cation-selective membrane being located on the anode side between the last anion-selective membrane and the anode, for the purpose of preparing onium hydroxides, is not disclosed in the publication.
J. R. Ochoa, Gomez and M. Tarancon Estrada, in
Journal of Applied Electrochemistry,
21, (1991), Short Communication, describe the synthesis of quaternary ammonium hydroxides in an electrolytic cell which includes an anion exchange membrane. Via the procedure described in the publication, quaternary ammonium hydroxides are obtained which still include a high halide content.
U.S. Pat. No. 4,578,161 describes a method of preparing quaternary ammonium hydroxides via electrolysis, which makes use of an electrolytic cell which includes an anion exchange membrane. The solutions of quaternary ammonium hydroxides obtained via the described method are likewise distinguished by a high residual bromide content.
U.S. Pat. No. 5,286,354 likewise relates to a method of preparing inorganic hydroxides and alkoxides by electrolysis. The method described relates to the use of an electrolytic cell in which the volumes which contain the respective electrode are separated by an anion exchange membrane. The solutions of inorganic hydroxides obtained via the method described have very high residual bromide contents.
As electrodialytic processes as a rule are carried out using more than just one cell unit, a complex design of a cell unit, as often described in the prior art, results in an expensive and fault-prone system, depending on the number of cell units employed in the electrodialysis apparatus. It is therefore desirable to keep the complexity of the cell unit as low as possible, to keep the costs of the electrodialysis apparatus low and its stability at the highest possible level. Another drawback of the systems and electrodialysis apparatus known from the prior art is that the bulky onium ions, for example quaternary ammonium ions, have to pass through a cation-selective membrane during the electrodialysis process. Given the limited pore size of such a membrane, it is not possible, using the prior art method, to prepare onium hydroxides whose cations are bulky. Hitherto, the preparation of such onium hydroxides therefore required a complex cell design which has an adverse impact on the reliability of the electrodialysis apparatus and on the economic viability of such a preparative method.
Moreover, the cell structures known from the prior art have a further drawback in that product-containing material streams are in direct contact with the anode or cathode. Said contact can result in the formation of impurities in the material streams and in a reduced service life of the anode or cathode material.
It is therefore an object of the present invention to provide a method of preparing onium hydroxides and a method of purifying these, which ensures a simple design of a cell unit, permits the preparation of onium hydroxides having bulky cations and moreover increases the service life of the anode and cathode material while at the same time affording material streams comprising a low level of impurities.
This object is achieved according to the invention by a method which involves the conversion, by means of an electrodialysis process in an electrodialysis apparatus, of a salt of an onium compound into the onium hydroxide, a cell unit of the electrodialysis apparatus comprising a bipolar membrane and an anion-selective membrane, with a bipolar membrane or a cation-selective membrane being located on the anode side between the last anion-selective membrane and the anode.
The invention therefore relates to a method of preparing onium hydroxides of the elements N, S or P by electrodialysis in an electrodialysis apparatus which includes an anode, a cathode and one or more cell units each compris
Botzem Jörg
Frede Markus
Kober Reiner
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