Crystal structure of tetrapropylammonium fluoride-silicalite

Abstract

The hydrophobic and organophilic nature¹ of silicalite may prove commercially important for the removal of organic compounds from waste water. The framework linkage of silicalite¹ is topologically the same as that of synthetic zeolite ZSM-5 (ref. 2), a shape-selective catalyst³ capable of converting methanol into water and hydrocarbons useful in automobile engines⁴. The tetrapropylammonium (TPA) fluoride (F)-containing precursor to fluoride-silicate⁵ crystallizes from a hydrothermal system containing silica, TPA⁺ and F⁻ ions. Destruction of the organic cation during heating in air gives fluoride-silicalite, a polymorph of silica with some properties similar to those of silicalite^1,5. We show here how a TPAF complex in the precursor lies at the tetrahedral intersection of the 10-ring channels of a silica framework in a position consistent with a template scheme for crystallization⁶. There is not enough space to permit replacement of TPA by the tetra-n-butyl ammonium complex used in the synthesis of ZSM-11 (ref. 7) and its silica counterpart, silicalite-2 (ref. 8)