| Abstract
| - The silicalite/o-xylene and silicalite/m-xylene complexes have been studied by room-temperature powderX-ray diffraction and flexible-lattice energy-minimization simulations. The locations and occupancies of theadsorbed xylene molecules are obtained by Rietveld refinement in the Pnma space group (silicalite/o-xylene,a = 19.9966(13) Å, b = 19.9402(12) Å, c = 13.3738(8) Å; silicalite/m-xylene, a = 20.0509 (7) Å, b =19.9268(7) Å, c = 13.3875(5) Å; α = β = γ = 90°). The adsorption sites for o- and m-xylene are locatednear the channel intersections of silicalite, with the methyl groups oriented approximately into the straightand sinusoidal channels. At room temperature, o-xylene occupies all the channel intersection sites to achievea loading of 4 molecules per unit cell, whereas m-xylene adsorbs to a loading of about 3 molecules per unitcell. There are two possible adsorption sites for the xylene molecules, which are related to each other by amirror plane (perpendicular to the b axis). Unlike the case of p-xylene in silicalite, the centroid of the aromaticring does not lie in the mirror plane. Adsorption of o-xylene or m-xylene causes considerable distortion ofthe straight channels, which become highly elliptical in cross section. The locations of the adsorbed moleculesand the distortion of the zeolite framework are confirmed by energy-minimization calculations. The importanceof framework flexibility in the adsorption thermodynamics of xylene/silicalite systems is discussed.
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