Computational studies of zeolite framework stability

For the purpose of detg. the relative stabilities of topol. different Al-free tetrahedral networks, Hartree-Fock-level ab-initio calcns. were done of the relative stability of 3-, 4-, 5-, and 6-unit SiO(OH)2 rings. Very small differences per T unit are found for the 4-, 5-, and 6-rings; however, the energy per T unit is unfavorable for the 3-ring. Rigid ion lattice minimization calcns. were performed on Al-free as well as high-Al-content zeolite systems. The results are discussed for ZSM-5, mordenite and faujasite structures. Very small energy differences, of the order of .apprx.20 kJ/mol, are again found for the Al-free networks. Open structures have less favorable energy than dense structures due to decreased Madelung energy. Large changes in relative energy are found on variation of the Al/Si ratio. Medium- and small-pore zeolites are much more sensitive to an increase in Al content than the wide-pore material. This should be ascribed to stacking of the cations in the channels of the zeolite. The implications of these results for zeolites synthesis are discusse