Characterization of zeolites by high-resolution solid-state nuclear magnetic resonance.
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The unique catalytic and adsorbent properties, crystal structures, ion-exchange and other characteristics of zeolites have been extensively investigated in recent years. The high-resolution solid-state <SUP>29</SUP>Si-and <SUP>27</SUP>Al-nuclear magnetic resonance (NMR) with magic angle spinning (MAS) have proved to be an useful tool for the chemical structural characterization of zeolites.<BR>The article reviews how high-resolution solid-state NMR with MAS reveals unique insights into zeolite structures from recent literatures. Since five distinct Si-Al configurations, Si (OAl) <SUB>n</SUB> (OSi) <SUB>4-n</SUB> (n=0-4), in zeolitic frameworks can be identified by <SUP>29</SUP>Si-MAS NMR, several interesting problems have been enthusiastically discussed on <SUP>29</SUP>Si-NMR analysis. In the present paper, the following topics are introduced and discussed:(1) Assignments of <SUP>29</SUP>Si-chemical shifts and Si-Al configurations for aluminosilicates.(2) Discussion of Loewenstein's rule, the presence of Al-O-Al ordering, in the zeolite A.(3) Characteristic changes of Si/Al ratio in faujasites (zeolites X and Y) and mordenite studied by <SUP>29</SUP>Si-NMR spectra.(4) <SUP>29</SUP>Si-NMR spectral studies on the framework structures of pentasil zeolites such as ZSM-5, ZSM-11 and silicalite.(5) Stuidies of Ga<SUP>3+</SUP> substitution for Al<SUP>3+</SUP> in the zeolitic frameworks of faujasites studied by <SUP>29</SUP>Si-NMR spectra.(6) Other <SUP>29</SUP>Si-NMR spectral studies on zeolitic framework. On the other hand, <SUP>27</SUP>Al-NMR has been used to estimate the compositions on tetrahedrally and octahedrally coordinated aluminum states. For instance, the dealumination mechanisms have been investigated by the <SUP>27</SUP>Al-NMR spectral behaviors.<BR>Furthermore the application of high-resolution solid-state <SUP>13</SUP>C-NMR to the characteristic changes of organic compounds, in the zeolitic frameworks has been discussed.
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