The Directing Role of Aluminum in the Synthesis of PST-21 (PWO), PST-22 (PWW), and ERS‑7 (ESV) Zeolites

Zeolites are a diverse class of crystalline microporous
materials
of, mainly, aluminosilicate chemical composition. Organic structure-directing
agents (OSDAs) are generally utilized in zeolite synthesis to drive
the outcome to a specific zeolite phase. In addition to OSDA, the
presence and content of aluminum in the gel play a role in driving
the synthesis under specific conditions. The structure-directing role
of aluminum as well as fluoride in zeolite synthesis was explored
through the analysis of three recently synthesized aluminosilicate
zeolites, PST-21 (PWO), PST-22 (PWW), and ERS-7 (ESV), using a force
field simulation approach. An updated and recently proposed method
based on the calculation of “synthesis energy” is used
to predict the stability of zeolites at pure-silica and aluminosilicate
gel compositions, also able to include fluoride anions as well as
OSDAs, and hence largely general. The results are not only demonstrating
that the calculated structures with lowest “synthesis energy”
correspond to those experimentally obtained under “standard”
(meaning HF/SDA = 1) synthesis conditions but also that new structures
obtained under the recently introduced “excess fluoride approach”
are those which follow with energy slightly larger than the lowest,
as calculated from the list of competing zeolites. With this method,
we were able to rationalize the structure-directing effect of aluminum,
in the presence of fluoride and OSDAs, in the synthesis of zeolites.