Crystal structure of pure-silica ZSM-12 with tetraethylammonium cations from X-ray powder diffraction data

Abstract

Pure-silica ZSM-12, containing tetraethylammonium cations (TEA) was synthesized using a ‘two-silica’ source strategy and its structure was determined from laboratory X-ray powder diffraction data. This is the first known zeolite structure in which the positions of the TEA cations have been established. The ordered arrangement of TEA cations in the straight nonintersecting channels of the host material, which is reflected in several weak superstructural diffraction peaks, was determined by using the combination of simulated annealing and Rietveld refinement. In accordance with Raman spectroscopy, the calculations confirmed the presence of a tt. tt form of the TEA cations and the absence of the other stable form ( tg. tg) which was also consistent with the dimensions and elliptical cross-section of the channels. Therefore in ZSM-12 structure pore (channels) geometry is directly connected with the conformation of TEA cations. The compound with a moiety formula C 16H 40N 2O 84Si 42 ( M r = 2784.28, Z = 4) crystallizes in the monoclinic Cc space group with unit cell parameters a = 25.2177(9), b = 15.1903(5), c = 24.4754(7) Å, β = 108.516(2)°. The final structure, which resulted in a good agreement between calculated and observed powder diffraction pattern ( R wp = 10.17%), was additionally confirmed by results from thermogravimetric measurements and FTIR spectroscopy.