Magic‐Angle‐Spinning NMR (MAS‐NMR) Spectroscopy and the Structure of Zeolites

Abstract

AbstractAfter 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.29Si‐MAS‐NMR readily and quantitatively identifies five distinct Si(OAl)n(OSi)4‐nstructural 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, Al order, the nature of which is greatly clarified by29Si‐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OAl linkages. In zeolite A and zeolite X with Si/Al = 1.00 there is strict alternation of Si and Al on the tetrahedral sites. Ordering models for Si/Al 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)4sites 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, Al in silicalite is tetrahedrally substituted within the framework. In combination,29Si‐ and27Al‐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 offrameworkSi/Al ratios in the region 1.00 < Si/Al < 10 000. Since most elements in the periodic table may be accommodated within zeolite structures, either as part of the exchangeable cations or as building units of the anionic framework, there is immense scope for investigation by MAS‐NMR and its variants (cross‐polarization, multiple pulse and variable‐angle spinning) of bulk, surface and chemical properties. Some of the directions in which future research in zeolite science may proceed are adumbrated.