The structure of the low temperature (20 K) form of zeolite ZSM-11 derived from ²⁹Si MAS NMR spectroscopy, lattice energy minimization and Rietveld refinement

Abstract

The crystal structure of the low temperature form of ZSM-11 (structure code MEL) was refined from high resolution synchrotron X-ray powder diffraction data, recorded at 20 K. Based on the results of 2D ²⁹Si NMR investigations a model was obtained by energy minimization the Rietveld refinement of which converged in space group I[unk] (No. 82) [a = 20.019(2) Å; c = 13.380(1) Å] to R_I, = 0.143 and R_wp = 0.126.

Comparison of the theoretically predicted and experimentally determined structures against results from NMR experiments using the average T–T distance relationship between the structural parameters and the isotropic ²⁹Si chemical shifts did not show any correlation with the results of the structure refinement. However, with the values obtained from the energy-minimized structure model a good correlation between the structural parameters and the chemicals shifts of Si was obtained for one of the two possible assignments of the NMR chemical shifts to Si-sites. The fit of the energy minimized model to the experimental data set gave R_I, = 0.179 and R_wp = 0.157.

Since the NMR experiment gives direct information about the local ordering and structural state of the material this demonstrates the potential of the modeling experiment and also reveals the limits in accuracy and precision of the Rietveld structure refinement.