A SYNCHROTRON X-RAY-DIFFRACTION, NEUTRON-DIFFRACTION, SI-29 MAS-NMR, AND COMPUTATIONAL STUDY OF THE SILICEOUS FORM OF ZEOLITE FERRIERITE

The crystal structure of siliceous zeolite ferrierite has been determined from a combined analysis of synchrotron X-ray and neutron powder diffraction data (space group Pmnn, a = 18.7202(1) Angstrom, b = 14.07025(8) Angstrom, c = 7.41971(4) Angstrom, Z = 36, V = 1954.32(2) Angstrom(3), R(wp) = 12.33%, chi(2) = 3.55). The structure is subtly different from several previously determined aluminosilicate analogues, which were refined in Immm, particularly in relation to the absence of a linear Si-O-Si bond angle that is now found to be 170 degrees. The energy minimized structure is in excellent agreement with the experimental results, and the well-resolved Si-29 magic-angle spinning NMR spectrum can be assigned on the basis of a bond angle correlation that has been validated for other well-determined siliceous zeolite structures and a 2D INADEQUATE NMR spectrum. This assignment has provided a basis for interpreting the complex NMR spectra that are obtained from aluminosilicate ferrierite samples and enabled us to estimate Si/Al ratios in this multiple T-site system.