Abstract: A series of high silica mordenite samples synthesized from amine-free system using fluoride as structure-directing agent were characterized by XRD, SEM, FT-IR, and MAS NMR. The results confirmed that the samples were truly mordenite with high SiO2/Al2O3 molar ratio. The unit cell constants a, b, c and the unit cell volume (a×b×c) of M-36 sample were smaller than those of mordenite with silica alumina molar ratio(SAR)=10 reported in the literature, which indicated the unit cell contraction caused by the replacement of Al atoms by smaller Si atoms. However, compared with the sample with SAR=31.5 synthesized from free-fluoride system, the lattice parameters of M-36 were somewhat increased in the a and c directions, but elongated obviously in b direction, which indicated some influences on 8-membered ring parallel to the b axis of mordenite structure should be brought by using fluoride as structure-directing agent. As the SAR of mordenite samples increased, a variety of crystal morphologies (cuboids, hexagonal prismatic crystals, rectangular parallelepiped crystals) was observed and a wide variation in the crystallite size was presented. As the SAR changed from 14 to 55, in the infrared spectra of the samples, the band attributed to T—O bending vibration shifted from 434 cm-1 to 450 cm-1, and its absorption intensity also increased. The band at 720 cm-1 which corresponded to the internal symmetric stretching vibration of the AlO4 tetrahedral in the mordenite from a weak band developed into a broad band and nearly disappeared at last, indicating that the number of SiO4 in the framework was increased. In 27Al MAS NMR spectra, almost all high silica mordenite samples presented a peak at δ(55.6) and no non-framework Al species peak at about δ(0), which was different from steamed or acid-leached siliceous mordenite.

Key words: Fluoride anion, High silica mordenite, Characterization