INTERPRETATION OF SI-29 MAS NMR-SPECTRA BY X-RAY DATA - APPLICATION TO THE MONOCLINIC AND ORTHORHOMBIC FRAMEWORK OF ZEOLITE ZSM-5

The previously established relation between isotropic 29Si chemical shifts δ and mean SiOSi bond angles α of the form δ = A cosα (cosα - 1) + B is applied to calculate "theoretical" 29Si n.m.r. spectra of monoclinic ( P21 n) and orthorhombic (Pnma) frameworks of zeolite ZSM-5. Mean SiOSi bond angles from recently published single crystal X-ray refinements of improved accuracy are used in the calculations. Good agreement between theoretical and experimental spectra as well as between the bond angles from XRD and those calculated from 29Si chemical shifts has been found for the monoclinic form of high-silica ZSM-5. The theoretical spectrum of orthorhombic as-synthesized TPA-ZSM-5 cannot be directly compared with the experimental spectrum because of insufficient resolution of the latter. However, the spectral patterns observed for several sorbate loaded ZSM-5 frameworks of orthorhombic symmetry are very similar to the calculated spectrum. The characteristic changes of the 29Si n.m.r. spectra induced by the transition from monoclinic to orthorhombic symmetry of the ZSM-5 framework are discussed and related to the changes in the local structure of distinct Si sites. © 1990.