Silicalite characterization. Part I: Structure, Adsorptive Capacity and IR Spectroscopy of the Framework and Hydroxyl Modes

The physical properties (microcrystal morphology, crystallinity, internal perfection, adsorptive capacities toward CO and N₂, and IR manifestations of the skeletal modes and of the hydroxyl groups) of a Na- and Al-free silicalite (S) prepared following a specifically designed method are investigated. Comparison is made with the silicalite containing Na and Al impurities prepared following a more conventional (classical) path (SNa). The experimental techniques are X-ray diffraction (XRD), high-resolution electron microscopy (HRTEM), infrared spectroscopy in reflectance mode (FTIR), and volumetric isotherms in a BET apparatus. The most relevant results are as follows: (i) S microcrystals have very regular octagonal prismatic habit, (ii) nanodefects and microcavites are more aboundant in S than in SNa, (iii) hydroxyl groups are present at the internal defects sites, and (iv) N₂ and CO volumetric isotherms at 77 K differ from S and SNa.