Journal of physical chemistry vol:100 issue:12 pages:4970-4975
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.