SINGLE-CRYSTAL STRUCTURE-ANALYSIS AND ENERGY MINIMIZATIONS OF A MFI-TYPE ZEOLITE AT LOW P-DICHLOROBENZENE SORBATE LOADING

The crystal structure of a low-loaded adsorption complex of H-ZSM-5 wi th p-dichlorobenzene has been studied by single-crystal X-ray diffract ion. The controversy in the literature, concerning the preferred locat ion of the p-dichlorobenzene molecule, is explained by different inter pretations of the difference electron-density map representing the ele ctron density of the adsorbed molecule. The crystal studied contains 2 .56 (2) p-dichlorobenzene molecules per unit cell. The adsorbed molecu les prefer the position at the intersection of channels. Energy calcul ations, using the Biosym Catalysis and Sorption Software, strongly sup port this interpretation. In nearly all calculations the orientation o f the relaxed molecule is close to the orientation found from the X-ra y analysis. A second rotational position at the intersection, suggeste d from calculations in a fully relaxed structure, resembles the locati on of the p-xylene molecule at the intersection of channels in the hig h-loaded H-ZSM-5/8p-xylene complex. The unit cell of Si11.96Al0.04O24O 24.0.32C(6)H(4)Cl(2)(+0.04H(+)), M(r) = 767.42, is orthorhombic, Pnma, with a = 20.009 (3), b = 19.909 (4), c = 13.366 (2) Angstrom, V = 532 4 (2) Angstrom(3), Z = 8, D-x = 1.922 Mg m(-3) and mu(MoK alpha) = 0.7 34 mm(-1). The final R(wR) is 0.044 (0.048), w = 1/sigma(2)(F), for 53 06 observed reflections with I > 2.0 sigma(I) measured at 293 K.