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

Abstract

[EN] Zeolites are a diverse class of crystalline microporousmaterialsof, mainly, aluminosilicate chemical composition. Organic structure-directingagents (OSDAs) are generally utilized in zeolite synthesis to drivethe outcome to a specific zeolite phase. In addition to OSDA, thepresence and content of aluminum in the gel play a role in drivingthe synthesis under specific conditions. The structure-directing roleof aluminum as well as fluoride in zeolite synthesis was exploredthrough the analysis of three recently synthesized aluminosilicatezeolites, PST-21 (PWO), PST-22 (PWW), and ERS-7 (ESV), using a forcefield simulation approach. An updated and recently proposed methodbased on the calculation of "synthesis energy" is usedto predict the stability of zeolites at pure-silica and aluminosilicategel compositions, also able to include fluoride anions as well asOSDAs, and hence largely general. The results are not only demonstratingthat the calculated structures with lowest "synthesis energy"correspond to those experimentally obtained under "standard"(meaning HF/SDA = 1) synthesis conditions but also that new structuresobtained 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.