Micro-Imaging for the Investigation of Diffusion and Reaction in Nanoporous Materials

• Applicant: Dr. Christian Chmelik
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2015 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 277220792

In the last few years novel synthesis strategies and fabrication technologies led to an unprecedented variety of new nanoporous materials and new options of their technological application. Their advent was accompanied by a wealth of issues related to mass transport, being essential for optimizing the technological use of these materials. Answering the related questions may at the same time lead to fundamentally new knowledge in the field of molecular dynamics under spatial confinement. The successful application of interference and IR microscopy for the measurement of the spatial-temporal dependence of guest concentrations in nanoporous materials provided us with an exceptionally powerful tool for the elucidation of molecular mass transfer. With mainly instrument-based, methodical work, this new access to the measurement of molecular dynamics in host-guest systems (referred to as Micro-Imaging) was established in the course of a previous project. While in this previous work the novel options provided by Micro-Imaging were demonstrated with selected examples they shall now be exploited with their full potential. Besides further instrument-based and methodical work for extending the range of measurement and the accuracy being required by the new tasks, the work will be centered on the following three topics: (i) The first topic is devoted to the influence of the real structure of nanoporous materials on the diffusion of single components and mixtures. This includes the quantification and localization of transport resistances inside the crystals and on their outer surface which act in addition to the diffusional resistance of the genuine pore system. Here, also the interplay of guest-induced structural changes of the host system (often referred to as breathing) and the related changes in the diffusion characteristics shall be studied systematically. With the investigation of molecular diffusion in porous polymers, as aimed for in this topic, new grounds will be explored also on the materials side. (ii) The second topic concerns the measurement of the evolution of intracrystalline concentrations in the interior of nanoporous catalyst crystals, i.e. at the very location where the reaction is taking place. The thus enabled in-situ monitoring of concentration profiles of the involved components has all potentials for establishing a new field of catalysis research. (iii) The third topic is focused on the investigation of the dynamics of phase changes in mesoporous materials. In the hysteresis region of such systems NMR diffusion measurements revealed diverging sorption time constants, despite of unabatedly fast guest diffusivities. Hence, the slowing-down in the equilibration rate has to be attributed to rearrangement processes of large molecular ensembles and their diverging time constants. Exactly these rearrangements shall now be examined directly by Micro-Imaging.

• DFG Programme: Research Grants