THE STRUCTURE OF THE LOW-TEMPERATURE (20 K) FORM OF ZEOLITE ZSM-11 DERIVED FROM SI-29 MAS NMR-SPECTROSCOPY, LATTICE ENERGY MINIMIZATION ANDRIETVELD REFINEMENT

The crystal structure of the low temperature form of ZSM-11 (structure code MEL) was refined from high resolution synchrotron X-ray powder d iffraction data, recorded at 20 K. Based on the results of 2D Si-29 NM R investigations a model was obtained by energy minimization the Rietv eld refinement of which converged in space group <I(4)over bar> (No. 8 2) [a = 20.019(2) Angstrom; c = 13.380(1) Angstrom] to R(I) = 0.143 an d R(wp) = 0.126. Comparison of the theoretically predicted and experim entally determined structures against results from NMR experiments usi ng the average T-T distance relationship between the structural parame ters and the isotropic Si-29 chemical shifts did not show any correlat ion with the results of the structure refinement. However, with the va lues obtained from the energy-minimized structure model a good correla tion between the structural parameters and the chemicals shifts of Si was obtained for one of the two possible assignments of the NMR chemic al shifts to Si-sites. The fit of the energy minimized model to the ex perimental data set gave R(I) = 0.179 and R(wp) = 0.157. Since the NMR experiment gives direct information about the local ordering and stru ctural state of the material this demonstrates the potential of the mo deling experiment and also reveals the limits in accuracy and precisio n of the Rietveld structure refinement.