Mass transfer in mesoporous materials: the benefit of microscopic diffusion measurement

Abstract

We introduce the various options of experimentally observing mass transfer in mesoporous materials. It shall be demonstrated that the exploration of the underlying mechanisms is excessively complicated by the complexity of the phenomena contributing to molecular transport in such systems and their mutual interdependence. Microscopic diffusion measurement by the pulsed field gradient (PFG) technique of NMR offers the unique option to measure both the relative amount of molecules adsorbed and the probability distribution of their displacements over space scales relevant to fundamental adsorption science just as for technological application. These advantages are shown to have cared for a recent breakthrough in our understanding. The examples presented include the measurement of diffusion in purely mesoporous materials and the rationalization of the complex concentration patterns revealed by such studies on the basis of suitably chosen micro-kinetic models. As an interesting feature, transition into the supercritical state is shown to become directly observable by monitoring a jump in the diffusivities during temperature enhancement, occurring at temperatures notably below the bulk critical temperature. PFG NMR studies with hierarchical materials are shown to permit selective diffusion measurement with each of the involved subspaces, in parallel with the measurement of the overall diffusivity as the key parameter for the technological exploitation of such materials. We refer to the occurrence of diffusion hysteresis as a novel phenomenon, found to accompany phase transitions quite in general. Though further complicating the measuring procedure and the correlation between experimental observation and the underlying mechanisms, diffusion hysteresis is doubtlessly among the new options provided by diffusion studies for gaining deeper insight into the structure and dynamics of complex porous systems.