Fluorine-containing organic molecules as structure directing agents in the synthesis of crystalline microporous materials. Part III: Synthesis of all-silica zeolites from fluorine-containing derivatives of 1-benzyl-1-methylpyrrolidinium

Abstract

In order to investigate the effect that a variation of the chemical nature of the structure directing agent may have in the products of synthesis, the cation 1-benzyl-1-methylpyrrolidinium (BMPIRROL) and several fluorinated derivatives of this cation have been employed as structure directing agents in the synthesis of pure silica molecular sieves in fluoride medium. It has been found that the hydrogen-bearing cation strongly directs the formation of pure silica zeolite beta, highly crystalline and stable at long crystallization time. On the contrary, employing the same synthesis conditions and molar compositions, only the 1-orthofluorobenzyl-1-methylpyrrolidinium ( o-FBMPIRROL) directs the crystallization of this phase while the 1-metafluorobenzyl-1-methylpyrrolidinium ( m-FBMPIRROL) and the 1-parafluorobenzyl-1-methylpyrrolidinium ( p-FBMPIRROL) direct the formation of the ZSM-12 zeolite. The fluorinated derivatives are more unstable than their hydrogen-bearing counterpart since the co-crystallization of the small pore octadecasil structure was observed for the three fluorinated derivatives. Nevertheless, the results show that the presence of fluorine in the organic molecule and its position in the aromatic ring has a marked effect in the crystallization pathway.