Synthesis of silica-sodalite from non-aqueous systems

The aluminosilicate sodalite has been known since antiquity1. It occurs naturally as the polysulphide-containing mineral lapis lazuli1 and as a salt-bearing feldspathoid2 and has also been synthesized as the zeolite basic sodalite with the formula Na6(Al6Si6O24) · xNaOH · (8−2x)H2o (ref. 2). The structure is an open framework of relatively large cages which are accessible, to water molecules and some ions, through relatively small windows of ∼2.1 A free diameter. Large ions or molecules may be trapped in the cages during synthesis. In aluminosilicate sodalite, the cages contain the number of cations required to compensate for the lattice charge, together with varying amounts of H2o and salts such as NaOH and NaCl. Aluminosilicate sodalites usually have an Al/Si ratio of about unity although alumina analogues of sodalite have been synthesized3 and a silica-rich sodalite (TMA sodalite) has been prepared from aqueous hydrothermal systems4,5 in the presence of tetramethylammonium ions. We report here a new method for the synthesis of sodalite, either in a silica-rich aluminosilicate form or in a novel pure-silica form. Our method of synthesis of sodalite is unusual in that the solvent system is essentially non-aqueous. In the absence of alumina, a new material is produced, namely a pure-silica form of sodalite which, following previously published suggestions6, we call silica-sodalite. If the ‘solvent’ system is ethylene glycol then the unit cell composition approximates to Si12O24 · 2C2H4(OH)2.