Properties of organic cations that lead to the structure-direction of high-silica molecular sieves

A variety of new organic structure-directing agents (SDA's) are synthesized and used for the synthesis of high-silica molecular sieves. The hydrophobicity and rigidity of these new and other previously known SDA's are evaluated in terms of their phase transfer behavior from water to chloroform and the number of tertiary and quaternary connectivities, respectively. It is found that the phase transfer behavior of the organic cation SDA's is best measured when they are in their iodide form, in that the greatest ability for discrimination between differences in hydrophobicity is possible in this form of the salt. The phase transfer behavior of numerous SDA's shows the same trend as that of simple tetraalkylammonium iodides with respect to the correlation between C/N+ values and percent transferred. The phase transfer results are correlated to the ability for structure-direction in molecular sieve synthesis. It is difficult to obtain a molecular sieve when using extremely hydrophobic SDA's. For hydrophobic, monocationic SDA's, introduction of a second charge into the molecule decreases the hydrophobicity and allows for structure-direction in molecular sieve synthesis. Thus, SDA's with intermediate hydrophobicity are found to be most useful for high-silica molecular sieve synthesis. In terms of SDA geometry, bulky, rigid molecules with limited conformational variability result in near unique formation of a great variety of new high-silica molecular sieves. The use of relatively flexible molecules with a minimum diameter of approximately 5 Angstrom gives more than one molecular sieve.