Towards Operando Characterisation by Powder Diffraction Techniques of Molecular Sieves

Abstract

Working molecular sieves imply numerous and various atoms and for their characterisation we need chemical selective probes. Thus they can be studied either by neutron powder diffraction or by anomalous X-ray powder diffraction techniques to extract structural information. We will illustrate the complementarities of these methods in the analysis of two different chemical processes on X-type zeolite. In the first case, a fully exchanged barium X-type zeolite was, firstly, characterised by neutron powder diffraction after an ex situ preparation step. During the preparation step, the sample was saturated with a mixture of heavy water and deuterated para-xylene. The selectivity of neutron diffraction for light elements allows the precise location of both water and xylene molecules. In the second example, an X-type zeolite exchanged by both strontium and rubidium cations was studied during the dehydration process. The in situ structural characterisation was performed by recording, for each state of the zeolite (hydrated, dehydrated), three X-ray powder patterns. Two of them were measured at an energy close to the absorption edge of each compensating cation (Sr2+ and Rb+) and one far from both absorption edges. The chemical selectivity of resonant diffraction allows an accurate determination of the distribution of compensating cations (location, distribution and mobility) during the dehydration process. Finally a comparison of some specificities and limitations of both methods are summarized.