New methods for the structural characterization of shape-selective zeolites
Two fundamentally new and powerful methods of studying zeolites have recently become available. High-resolution electron microscopy records structure, of both perfect and imperfect zeolites, directly in real space. This technique can reveal information about the presence or lack of structural integrity in situations where X-ray studies are not helpful; and it has, for example, proved possible to detect embryonic regions of ZSM-5 in an amorphous precursor and to pin-point structural faults of various kinds in otherwise well-ordered systems. To date this technique has not revealed the presence of previously postulated intermediate structural variants between ZSM-11 and ZSM-5 and members. It has also shown that when faujasite is dealuminated by gaseous SiCl4, thereby enhancing the Si/Al ratio by factors of 10–25, the structure remains intact at the atomic level. Magic-angle-spinning nuclear magnetic resonance (MASNMR) results, based on 29Si, afford information pertaining to Si,Al ordering. We show that dealuminated faujasite (Si/Al of ca. 2.45) originally consists of predominantly Si—(OAl)3, Si—(OAl)2 and Si—(OAl) groups, but upon dealumination the environment is almost exclusively Si—(O-Si)4. The selective behaviour of this highly dealuminated faujasite appears promising. Other long-standing problems in Si,Al ordering and catalysis are solvable using MASNMR, in combination with electron and neutron diffraction.