ANISOTROPIC CHEMICAL SHIELDING, M-SITE ORDERING, AND CHARACTERIZATIONOF EXTRAFRAMEWORK CATIONS IN ETS-10 STUDIED THROUGH MAS MQ-MAS NMR AND MOLECULAR MODELING TECHNIQUES/

The local structural characteristics of Si and Ti sites in ETS-10 as d erived from a combination of high-resolution magic angle spinning (MAS ) NMR spectroscopic and molecular modeling studies are reported. Pure and highly crystalline ETS-10 and aluminum-substituted ETS-10 (ETAS-10 ), devoid of impurity ETS-4 phase, were synthesized and fully studied by MAS and multiple-quantum magic angle spinning (MQ-MAS) NMR. More ac curate assignments of the experimentally observed Si-29 resonances to the crystallographically nonequivalent Si sites are made, and a correl ation with T-site geometry is established. Si-29 slow MAS NMR is shown to he a very general and powerful methodology to unequivocally establ ish heteroatom substitution in the zeolite lattice, and this was used to probe the local symmetry and chemical shielding at different Si sit es in ETS-10 and ETAS-10. Si-29 and Al-27 MAS NMR spectral analysis of ETAS-10 is used to confirm that the aluminum substitution occurs only in Si[4Si,0Ti] silicon sites. This, in turn, was used to generate clu ster models for computer graphics techniques. The electronic structure of such cluster models and the calculated aluminum substitution energ y values pinpoint the topographical location of aluminum in ETAS-10. T he acidity of ETAS-10 is predicted on the basis of the quantum chemica l cluster model calculations. The first application of MQ-MAS NMR to s tudy cation environments in molecular sieves is also reported and is u sed in the present study to investigate the local structural character istics of sodium cations in ETS-10 and ETAS-10.