Lattice energy-minimization calculation in the further investigation of XRD and NMR studies of zeolite frameworks

A knowledge of the local structure of zeolite frameworks is an important prerequisite for a detailed and quantitative understanding of e.g, catalytic processes. In the present work the sensitivity of 1D and 2D MAS NMR experiments to the local order of framework T atoms and energy-minimization calculations for the geometrical analysis of the complete zeolite framework structure are combined to provide a most detailed view of their structural properties. Correlations of the chemical shifts, delta(Si-29), with the mean distance between Si and its neighboring Si T atoms, (d) over bar(Si-Si), are presented for the three materials MFI, MEL, and MTT. Excellent agreement between experimental and computational results was obtained. In the case of MFI, this most complicated structure was used to demonstrate the viability of the method. The MTT and MEL systems showed that it is possible to deduce the unique assignment of the NMR spectra from energy-minimized structures when it is not possible using the available structural data from X-ray structure analysis. Taken together, all three investigations also demonstrate that improved structural data are obtained from lattice energy minimizations compared to those from Rietveld analysis as long as the topology of the framework is known.