Exploring guest dynamics in nanoporous host materials

Diffusion is an omnipresent phenomenon in nature. In the world of molecules, it describes their irregular thermal motion. The interplay of diffusion and interaction of molecules with pore walls of nanoporous materials constitutes the benefit of using such materials in applications of separation and catalysis. The need for understanding the rate-limiting mechanisms, further optimization and development of new processes makes this topic subject of continued fundamental research [1]

Diffusion in Nanoporous Materials: Challenges, Surprises and Tasks of the Day

Diffusion is an omnipresent, most fundamental phenomenon in nature and thus critical for the performance of numerous technologies. This is in particular true for nanoporous materials with manifold applications for matter upgrading by separation, purification and conversion. The path lengths of molecular transportation within the industrial plants range from the elementary steps of diffusion within the micropores of the individual particles up to the matter flow over macroscopic distances. Each of them might be decisive in determining overall performance so that detailed knowledge of all modes of mass transfer is crucial for a knowledge-based optimization of the devices with reference to their transport properties. The rate of mass transfer is particularly complicated to be assessed within the individual (adsorbent) particles/crystallites with pore sizes of the order of molecular dimensions. We are going to present two powerful techniques exactly for this application, operating under both equilibrium (Pulsed Field Gradient (PFG) NMR) and non-equilibrium (Microimaging by interference microscopy and IR microscopy) conditions. The potentials of these techniques are demonstrated in a few showcases, notably including the options of transport enhancement in pore hierarchies. The contribution concludes with a survey on present activities within an IUPAC initiative aiming at the elaboration of “guidelines for measurements and reporting of diffusion properties of chemical compounds in nanoporous materials”

Analysis of Thermal Effects in Infrared and Interference Microscopy: n-Butane-5A and Methanol-Ferrierite Systems

Recently, infrared and interference microscopy methods have been increasingly applied to measure internal concentration gradients and hence the uptake of different adsorbates in zeolite crystals. In contrast to conventional macroscopic batch uptake techniques, these microscopic/mesoscopic methods measure changes associated with single zeolite crystals. The analysis of data from these measurements to determine micropore diffusivities has been performed on the basis of the assumption that isothermal conditions prevail during both the adsorption and desorption experiments. This assumption is critically examined in this paper for the case of methanol diffusion in ferrierite crystals during adsorption and desorption to vacuum. It is shown by both detailed simulation as well as an order of magnitude analysis of time constants for heat transfer and diffusion that the temperature changes in the system are negligible during adsorption due to the high conductive heat transfer rate. However, during desorption to a vacuum, heat conduction is minimal so that heat transfer occurs only by radiation. Temperature changes as large as 5–7K are therefore to be expected at the beginning of the desorption process. However, since the time constant for the desorption process is of the order of hundreds of seconds, this temperature transient dissipates rapidly and has no significant impact on the overall desorption process. Even for the worst case scenario considered here, for both interference and infrared microscopy methods, the systems can be considered as essentially isothermal