After outlining the chemical features and properties which make zeolites such an important group of catalysts and sorbents, the article explains how high-resolution solid-state NMR with magic-angle spinning reveals numerous new insights into their structure. Z9Si-MAS-NMR readily and quantitatively identifies five distinct Si(OAl)n(OSi)4-n structural groups in zeolitic frameworks (n = 0,1,. . .4), corresponding to the first tetrahedral coordination shell of a silicon atom. Many catalytic and other chemical properties of zeolites are governed by the short-range Si, A1 order, the nature of which is greatly clarified by 29Si-MAS-NMR. It is shown that, as expected from Pauling's electroneutrality principle and Loewenstein's rule, both in zeolite X and in zeolite A (with Si/Al= 1.00) there are no -Al-0-Allinkages. In zeolite A and zeolite X with Si/Al= 1.00 there is strict alternation of Si and A1 on the tetrahedral sites. Ordering models for Si/AI ratios up to 5.00 (in zeolite y) may also be evaluated by a combination of MAS-NMR experiments and computational procedures. 29Si-MAS-NMR spectra reveal the presence of numerous crystallographically distinct Si(OSi), sites in silicalite/ZSM-5, suggesting that the correct space group for these related porosilicates is not Pnma. 27Al-MAS-NMR clearly distinguishes tetrahedrally and octahedrally coordinated aluminum, proving that, contrary to earlier claims, A1 in silicalite is tetrahedrally substituted within the framework. In combination, "Si-and 27AI-MAS-NMR is a powerful tool for monitoring the course of solid-state processes (such as ultrastabilization of synthetic faujasites) and of gas-solid reactions (dealumination of zeolites with silicon tetrachloride vapor at elevated temperatures). They also permit the quantitative determination offramework %/A1 ratios in the region l.OO

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The products of hydrothermal treatment of zeolite NH,-Na-Y and of treatment of zeolite Na-Y with silicon tetrachloride vapour under different conditions were examined by 29Si and "A1 high-resolution solid-state n.m.r. with magic-angle spinning (m.a.s.n.m.r.) at high magnetic m.a.s.n.m.r. in combination with chemical analysis allowed the amount of framework and non-framework aluminium to be readily and quantitatively determined. However, not all non-framework aluminium is observed by 27Al m.a.s.n.m.r. in most samples. Hydrothermal dealumination yielded materials with Si/A1 ratios of up to 6.92, while the zeolite dealuminated with Si/CI, had framework Si/Al ratios of up to 25 and contained little interstitial aluminium. M.a.s.n.m.r. is superior to X-ray diffraction as a method of determining zeolite composition.Aluminium-deficient zeolites (with Si/A1 ratios > ca. 5 ) are valuable as catalysts and catalytic supports in a variety of commercial processes involving hydrocarbon synthesis, cracking and isomerisation.l$ They combine appropriate pore size and selectivity with high thermal stability induced by the high silicon content. Of particular interest in this respect are aluminium-deficient faujasites, prepared from the synthetic zeolites X and Y (which are synthesised, under mild laboratory conditions, with Si/Al ratios of between 1 and ca. 2.75).In general, the %/A1 ratio of zeolites can be increased using two kinds of treatment : ( i ) Chemical extraction. Zeolites, particularly those with high aluminium content, are unstable in acid media. Mineral acids attack the Si-0-A1 linkages, removing framework aluminium into the interstitial space; such extra-framework A1 can be subsequently leached from the zeolite. A vacancy, most probably comprising a nest of four hydroxyl groups, is left behind. Provided the Si/Al ratio is sufficiently high to prevent the collapse of the structure, materials with very low A1 contents can be prepared in this way. The method is not suitable for 'aluminous' zeolites, such as zeolite A, but works well with materials such as zeolite Y, mordenite, clinoptilolite and erionite. Aluminium can also be extracted using chelating agents such as ethylenediaminetetracetic acid (EDTA), solutions of CrCl,, (NH,),SiF,, gaseous fluorine, COCl, (phosgene) and other reactants.We were particularly interested in the single-step method of preparing highly siliceous zeolites by passing inert gas saturated with SiCl, vapour through a bed of heated dehydrated ~eolite.~-* By prolonged treatment a highly crystalline, but virtually aluminium-free, material is obtained which is formally analogous to microporous 3003 98-2 3004 M.A.S.N.M.R. OF ZEOLITE Y