THE COMBINATION OF SYNCHROTRON POWDER DIFFRACTION AND HIGH-RESOLUTION SOLID-STATE NMR EXPERIMENTS

Structural investigations on three zeolites, ZSM-11, ZSM-12 and ZSM-5, have been carried out by a combination of high-resolution solid-state Si-28 NMR experiments and Rietveld refinement of synchrotron X-ray powder data. These techniques, being sensitive to short- and long-range order, respectively, gave a most complete picture of the structure of the solid state. Variable temperature NMR experiments of ZSM-11 revealed a displacive phase transition with a change in symmetry at about 300 K. A Rietveld refinement of a data set recorded at 373 K confirmed the proposed structure model. The structure of ZSM-12 was refined from room temperature synchrotron powder data. Space group ambiguities because of pseudo-symmetry of the silica framework of the zeolite were resolved with the aid of Si-29 NMR experiments leading to C2/c as the true space group symmetry. Si-29 NMR and X-ray studies of ZSM-5 showed that the symmetry of the crystal structure depends on the temperature, the concentration of the guest molecule, and on the number of Al atoms in the framework. A full pattern refinement of silica-ZSM-5 loaded with two molecules p-dichlorobenzene per unit cell showed that the site occupied by the sorbed molecule is not at the intersection of the channels but shifted by 1/4 along b0.