Influence of the hydrophobic character of structure-directing agents for the synthesis of pure-silica zeolites

Abstract

Tetrapropylammonium, ethanoltripropylammonium and diethanoldipropylammonium cations were used as structure-directing agents for the synthesis of pure-silica, MFI-type zeolites. These molecules are employed to investigate the influence of the organic hydrophobicity/hydrophilicity on zeolite formation. Ethanoltripropylammonium cation can direct the formation of Si-ZSM-5 but at a much slower rate than tetrapropylammonium cation. Diethanoldipropylammonium alone is unable to influence the formation of Si-ZSM-5 but is able to do so with the addition of either seeds or a small amount of tetrapropylammonium. Thus diethanoldipropylammonium cannot elicit nucleation but can support crystal growth. Solid-state nuclear magnetic resonance results show that ethanolalkylammonium molecules are occluded intact within the zeolite pores; however, tetrapropylammonium (more hydrophobic) is preferentially occluded. The results show that control of the hydrophobicity of the organic structure-directing agent is necessary for the synthesis of pure-silica zeolites.