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 silicalite1,5. 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)