The location of o- and m-xylene in silicalite by powder X-ray diffraction

The silicalite/o-xylene and silicalite/m-xylene complexes have been studied by room-temperature powder X-ray diffraction and flexible-lattice energy-minimization simulations. The locations and occupancies of the adsorbed xylene molecules are obtained by Rietveld refinement in the Pnma space group (silicalite/o-xylene, a = 19.9966(13) Angstrom, b = 19.9402(12) Angstrom, c = 13.3738(8) Angstrom; silicalite/m-xylene, a = 20.0509 (7) Angstrom, b = 19.9268(7) Angstrom, c = 13.3875(5) Angstrom; alpha = beta = gamma = 90 degrees). The adsorption sites for o- and m-xylene are located near the channel intersections of silicalite, with the methyl groups oriented approximately into the straight and sinusoidal channels. At room temperature, o-xylene occupies all the channel intersection sites to achieve a loading of 4 molecules per unit cell, whereas m-xylene adsorbs to a loading of about 3 molecules per unit cell. There are two possible adsorption sites for the xylene molecules, which are related to each other by a mirror plane (perpendicular to the b axis). Unlike the case of p-xylene in silicalite, the centroid of the aromatic ring does not he in the mirror plane. Adsorption of o-xylene or m-xylene causes considerable distortion of the straight channels, which become highly elliptical in cross section. The locations of the adsorbed molecules and the distortion of the zeolite framework are confirmed by energy-minimization calculations. The importance of framework flexibility in the adsorption thermodynamics of xylene/silicalite systems is discussed.