Inorganic Ion Exchangers with Layered Structures

Abstract

Ion exchange materials are widely distributed in nature. They include clays, zeolites, humic acids, certain coals, and many mineral species. It is not surprising then, that many classes of synthetic inorganic compounds also exhibit ion exchange behavior. Among the most important we may mention synthetic zeolites and aluminosilicates, hydrous oxides, acid salts of polyvalent cations, apatites, heteropolyacids and salts, insoluble ferro­ 'cyanides, condensed phosphates, certain titanates, f3and f31/-alumina, and many glass compositions. The recent literature on several of these classes of compounds has been reviewed (1, 2). In this paper we confine ourselves to a discussion of the layered group IV acid salts. Even within this more restricted area there is a vast array of compounds including, most importantly, the phosphates, but also ar­ senates, molybdates, tungstates, antimonates, pyrophosphates, silicates, silicophosphates, polyphosphates, etc. The compounds of greatest interest are the zirconium phosphates because more is known about their behavior and structure than the others and because of their relationship to ionic conductors. However, brief descriptions of many of the other compounds are included. Renewed interest in inorganic ion exchangers developed with the growth of atomic energy programs worldwide. It was felt that inorganic ion exchangers would be more stable to high temperatures and high radiation fields than the available organic exchangers. It was soon discovered that the group IV phosphates, molybdates, tungstates, etc possessed interesting cation exchange properties (3). The early preparations were amorphous gels of variable composition (4, 5). While these materials were difficult to