Zeolite synthesis by the excess fluoride approach in the presence of piperidinium-based structure-directing agents

Here, we investigate the effects of fluoride concentration on the phase selectivity of crystallization in (alumino) silicate synthesis mixtures containing a series of N-alkyl-N-methylpiperidinium ions, where alkyl is methyl to hexyl, as organic structure-directing agents (OSDAs). Considerable diversity in the phase selectivity was observed when the HF/OSDA ratio in synthesis mixtures is 2.0 or higher. Using this excess fluoride approach, we synthesized five different aluminosilicate zeolites (i.e., EMM-10P, EU-1, MCM-22P, ZSM-12, and ZSM-58) that did not crystallize under the normal F- concentration conditions (HF/OSDA = 1.0). Our synthesis results also reveal that the same zeolite phase (i.e., levyne, nonasil, octadecasil, or ZSM-5) can crystallize over a wide range (0.5-2.0) of HF/OSDA ratios. More interestingly, F- concentration in the synthesis mixture was found to influence the crystal size and framework Al distribution of levyne.

Automated summary

The study found that adding excess fluoride in silica synthesis mixtures containing various N-alkyl-N-methylpiperidinium ions affects phase selectivity, and the same zeolite phase can crystallize under different HF/OSDA ratios.