INFLUENCE OF THE SOURCE OF SILICON AND ALUMINUM IN THE HYDROTHERMAL SYNTHESIS OF SODALITE

The synthesis of sodalite from aluminium hydroxide, aluminium isopropoxide and pseudo-boehmite (Catapal alumina) as the source of Al, and fumed silica and sodium silicoglycolate (Na-Si(v)) as the source of Si has been monitored by solid-state NMR, X-ray diffraction (XRD) and IR spectroscopy. The use of Na-Si(v) does not affect the crystallization time, but considerably shortens the preparation of the synthesis gel. Aluminium is not significantly incorporated into the sodalite structure when Al(OH)3 is used. Pseudo-boehmite gives multi-phasic products in which purely siliceous sodalite is always accompanied by (Si,Al)-sodalites. Some of the Al occupies non-framework positions, and parent gels with Si:Al = 5.0, 2.0 and 1.0 gave solids with Al(iv):Al(vi) = 0.73, 1.3 and 3.0, respectively. Only aluminum isopropoxide leads to phase-pure products in which all the Al is four-coordinate and Si:Al = 3.49 and 3.13 with fumed silica and Na-Si(v), respectively. The formula N(Al) = 51.855 - 0.0465nu(as)(Si-O-T) allows the Si:Al ratio to be calculated directly from the FTIR spectrum. With the exception of the material with Si:Al = 1.0 and a(o) = 8.94 +/- 0.1 angstrom, the unit cell parameter of sodalites is 8.83 +/- 0.1 angstrom irrespective of the Al content.