CHARACTERIZATION OF GUEST MOLECULES ADSORBED ON ZEOLITES OF KNOWN STRUCTURE BY COMBINED X-RAY-POWDER PROFILE REFINEMENTS AND CONVENTIONAL DIFFERENCE-FOURIER TECHNIQUES .1. LOCALIZATION OF THE BENZENE MOLECULE IN A PENTASIL TYPE ZEOLITE

It has been possible to localize the benzene guest molecule in a pentasil-type zeolite by combining full-profile x-ray powder diffraction structure refinements with conventional difference-Fourier synthesis, after extraction of the integrated intensities from the powder pattern. The investigations have been performed on the calcined H//4//. //8//3Al//4//. //8//0B//0//. //0//3Si//9//1//. //1//7O//1//9//2 phase (H-ZSM-5 form) which has been saturated with benzene vapor at low pressure. The same data-collection has been processed in two different ways. A trial-and-error procedure shows that the benzene molecule appears to be present at the channel-intersection with two simultaneous orientations. Direct characterization by the interpretation of a difference-Fourier map reveals the presence of two peaks situated on the mirror plane. Combining the results yielded by both methods shows that the benzene molecule rotates freely about an axis situated on the mirror plane.