Cooperative Structure-Directing Effect of Fluorine-Containing Organic Molecules and Fluoride Anions in the Synthesis of Zeolites

Abstract

Dibenzyldimethylammonium (DBDM) cations, singly fluorinated at the ortho, meta, and para positions of the aromatic ring, have been investigated as structure-directing agents (SDAs) for the synthesis of all-silica zeolites in fluoride media and compared with the nonfluorinated molecule. The nonfluorinated DBDM cation can direct the synthesis either to an all-silica zeolite beta or to an all-silica zeolite ZSM-50 with the EUO framework topology. Under the same conditions that DBDM gives zeolite beta, benzyl-orthofluorobenzyldimethylammonium (o-FDBDM) readily directs the crystallization of pure all-silica ZSM-50 [Cmma, a = 13.726(3) Å, b = 22.171(5) Å, c = 20.254(4) Å]. Replacing o-FDBDM by the para fluoro derivative (p-FDBDM) results in a much slower crystallization of ZSM-50 whereas no crystalline product results if the meta fluoro derivative (m-FDBDM) is added. Single-crystal diffraction studies of selected ZSM-50 crystals prepared using DBDM and o-FDBDM determine the location of the template and of charge-balancing fluoride atoms bound to framework silicon atoms and included within [415462] cages in the framework. Furthermore, the o-FDBDM is found to order within the structure, with the fluorine-containing aromatic rings occupying channel, rather than cavity, locations. Computer simulations indicate how the position of fluorine in the SDA influences the template ordering.