Electrosynthesis of quaternary ammonium hydroxides

Abstract

Quaternary ammonium hydroxides (QAH) are very useful phase transfer catalysts [I-3] as well as intermediates for synthesizing quaternary ammonium salts (QAS) by neutralization with the appropriate acid. QAS have found great use in phase transfer catalysis and as supporting electrolytes in electroorganic synthesis [4] and electroanalyses. From an industrial point of view, QAH and QAS are not easily available and their cost is high. Thus, it is necessary to develop processes to synthesize these compounds starting from relatively cheap and easily available raw materials such as quaternary ammonium chlorides and bromides. Three major methods have been reported in the literature for the preparation of QAH: (a) reaction of quaternary ammonium chloride or bromide with silver oxide. This method is limited by the use of relatively expensive reagents; (b) anion exchange between potassium hydroxide and quaternary ammonium chloride or bromide in alcoholic solution. This method is generally limited because the yields are not quantitative and therefore QAH becomes contaminated with both starting materials and by product potassium halide, which does not precipitate completely; and (c) electrolysis using a divided cell in which the anolyte is an aqueous solution of quaternary ammonium chloride and is separated from the catholyte by means of a cation exchange membrane [5, 6]. As a result, a pure aqueous solution of QAH is obtained in the catholyte. The method works well with quaternary ammonium cations of low molecular weight such as tetramethyl