Silicon and aluminum ordering in frameworks of FAU and EMT aluminosilicate zeolites crystallized in the presence of crown ethers

The siting pattern of Si and Al atoms in the frameworks of a FAU zeolite, an EMT zeolite, and a FAU/EMT structural intergrowth crystallized in presence of 15-crown-5 ethers and 18-crown-6 ethers and with an Al/ (Si+Al) ratio of 0.217 is determined using a combination of Si-29 MAS NMR and model generation by computer algorithm. In the three zeolite samples, the Si(nAl) distributions determined with Si-29 MAS NMR are very similar. They are compared with hypothetical distributions in computer-generated models of framework fragments comprising 1440 T-atoms. The model building starts from a pure silicon dioxide framework. The appropriate number of Al atoms are substituted for Si, one atom at a time, according to selection rules with a physical basis. A model of random Al siting obeying the Lowenstein rule is in disagreement with the experimental Si(nAl) distribution. A framework model that allows one to fit the experimentally obtained Si-29 MAS NMR spectra takes into account the position of the crown ether molecules in the zeolite cavities and allows Al-enrichment in the six-membered rings perpendicular to 111(CUB) Or 001(HEX) crystallographic directions and Si-enrichment in the remaining T-sites, which are in positions near the hydrophobic ethane bridges of the crown ether molecules. This model classifies different sets of the 12-membered rings as siliceous or aluminum-rich and consequently explains the experimentally observed anomalous population of sodium cations at the different cation sites in sodium-exchanged samples.