NMR studies of zeolites and related materials
Magic-angle spinning NMR has been used for the first time to investigate two new classes of molecular sieve-type materials. Firstly, various substituted high-silica (ZSM-5-type) molecular sieves have been studied. The substituting elements (B, Fe, Cr, Ti, La.) are (with the exception of boron) unobservable by NMR and so the effects of their substitution are observed indirectly by (^29)Si NMR. Calcination (thermal treatment in air) affects the (^29)Si NMR spectra of several of the samples. Substitution of paramagnetic elements (Fe, Cr) affects the (^29)Si NMR spectra causing substantial line broadening and the appearance of spinning sidebands. The use of other spectroscopic methods of investigation combined with NMR measurements yields a considerable amount of information concerning the environment(s) of the substituting elements. Secondly, several members of a new class of crystalline microporous molecular sieves based on three-dimensional frameworks of phosphorus and aluminium have been studied. Heteroatom substitution (Si, B) into molecular sieves possessing novel and zeolite-related structures has been investigated and the general mode of silicon substitution has been elucidated. Again, calcination produces significant changes in the NMR spectra of these samples. Ordering of the aluminium and phosphorus framework tetrahedra is normally one of direct alternation in which case unique resonances are observed in the (^27)Al and (^31)P NMR spectra. For several framework structures, crystallographically in equivalent sites exist, and (^31)P NMR (also (^29)Si NMR) is particularly sensitive to this phenomenon. (^31)P CP NMR provides information on the interaction of the organic template with the framework and on the possible sources of proton reservoir necessary for the CP experiment.