Single-crystal structure analysis and energy minimizations of a MFI-type zeolite at low p-dichlorobenzene sorbate loading

Abstract

The crystal structure of a low-loaded adsorption complex of H-ZSM-5 with p-dichlorobenzene has been studied by single-crystal X-ray diffraction. The controversy in the literature, concerning the preferred location of the p-dichlorobenzene molecule, is explained by different interpretations of the difference electron-density map representing the electron density of the absorbed molecule. The crystal studied contains 2.56 (2) p-dichlorobenzene molecules per unit cell. The absorbed molecules prefer the position at the intersection of channels. Energy calculations, using the Biosym Catalysis and Sorption Software, strongly support this interpretation. In nearly all calculations the orientation of the relaxed molecule is close to the orientation found from the X-ray analysis. A second rotational position at the intersection, suggested from calculations in a fully relaxed structure, resembles the location of the p-xylene molecule at the intersection of channels in the high-loaded H-ZSM-5/8p-xylene complex. The unit cell of Si11.96Al0.04O24.0.32C6H4Cl2(+ 0.04H+), Mr = 767.42, is orthorhombic, Pnma, with a = 20.009 (3), b = 19.909 (4), c = 13.366 (2) Å, V = 5324 (2) Å3, Z = 8, Dx = 1.922 Mg m−3 and μ(Mo Kα) = 0.734 mm−1. The final R(wR) is 0.044 (0.048), w = 1/σ 2(F), for 5306 observed reflections with I> 2.0σ(I) measured at 293 K.