Force field-based
calculations of all-silica zeolites show the
role of fluoride as a structure-directing agent (SDA). Calculations
of fully packed organic and fluoride SDAs were possible with an updated
version of zeoTsda software, based on Monte Carlo+Lattice Energy Minimization.
Comparison between fluoride-containing and fluoride-free calculations
using the same organic SDA allowed us to predict which zeolite phase
is obtained in fluoride and hydroxide media and also, through a new
energy decomposition scheme, identify energetic contributions driving
the synthesis outcome.
Zeolites are a large
family of crystalline microporous materials
with, mainly, silicate and aluminosilicate composition. Each topology
can only be synthesized in a specific range of aluminum content, Al/(Si
+ Al), within the interval [0–0.5], and this interval cannot,
in general, be predicted for each structure. Aluminum and organic
structure directing agents (OSDAs), among others, act together in
obtaining the zeolite phase under each specific synthesis condition.
The present study is an attempt to rationalize the role of aluminum
as a structure directing agent in the synthesis of zeolites using
computational chemistry and based on the criteria of energetic stability
using both force field and periodic DFT methods. A proper selection
of cases in which, using the same OSDA, different zeolite phases are
obtained in the presence and absence of aluminum helps to rationalize
how aluminum contributes to the relative stability of the different
competing zeolite phases considered.
Shape' was the first criteria claimed to explain specificity between organic structure directing agents (OSDAs) and zeolite micropores. With the advent of computational chemistry methods applied to study the effectiveness of SDA-zeolite combinations, 'energy' (mainly van der Waals) became the most commonly invoked concept to explain zeolite phase selectivity. The lower the energy the better the SDA. In this study we rescue the concept of 'shape' and we combine it with the concept of 'energy' within the frame of a SDA screening approach to identify new SDAs for the synthesis of cage-based ITE zeolite. Once we identify an appropriate 'shape' fingerprint, filtering through the SDA database can be made fast and accurately. With the 'shape' selection, an automated Monte Carlo software allows to assess suitability using the force-field calculated zeo-SDA 'energy'.The computational approach can be promptly applied to other cage-based zeolites.
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