Why some zeolites can not be obtained as aluminosilicates?

Zeolites are the largest family of crystalline polymorphs, formed by tetrahedra connected by vertices, giving rise to microporous solids with numerous applications in separation and catalysis. Originally discovered as aluminosilicate minerals in ۱۷۵۶, approximately ۶۰ polymorphs of natural origin are known to date, with composition Mn+aAlxSi۱-xO۲ (x=n·a), M being an inorganic cation (Na+, K+, Mg+۲, Ca+۲) located in the micropore and close to the Al of the crystal lattice. It was not until ۱۸۶۲ when for the first time a zeolite was obtained by laboratory synthesis by the hydrothermal method, from a silica-alumina gel in a basic medium, at a temperature of ۱۴۰-۲۰۰ °C, and autoclaved under autogenous pressure. In ۱۹۶۰ Barrer replaced the inorganic cations by organic cations, typically quaternary nitrogen molecules, and the number of known zeolites rapidly increased to more than ۲۵۵ known today [۱].Not all chemical compositions are possible; ۹۹ zeolites have been obtained as aluminosilicates, of which ۳۵ have also been obtained as pure silica, and on the other hand, ۳۶ zeolites that do not exist as aluminosilicates have been obtained as pure silica. Being the synthesis gel practically common for the synthesis of any alumino-silicate, the major synthesis variable is the organic cation (structure directing agent, SDA) used. Other variables (time, pH, temperature, etc) are also important. There are few studies able to rationalize the most favorable chemical compositions for each structure. Our recent study allow us to propose an explanation [۲].We have used the OSDB database which contains bibliographic information on which organic cations have been used for the synthesis of each zeolite. Using our specific software, zeoTsda, based on interatomic potentials, geometry optimization and Monte Carlo, we obtained the energy and atomic positions of the zeolite+'organic cation' unit cell corresponding to the maximum (optimal) number of organic cations that fit in the micropore, all for the ۱۹×۱۲۱ selected combinations. With the energy values obtained and the recent definition of 'synthesis energy' [۲], we obtain an energy value whose minimum, for each organic cation, tells us which zeolite would be obtained in the synthesis, using silica/alumina gel. The results indicate that with these ۱۲۱ organic cations, there are only a few favorable aluminosilicate zeolites (AEI, CHA, BEA), while the other ۱۶ zeolites are not stable.