The wealth of information from transient guest profiles

The application of interference microscopy (IFM) and infrared microscopy (IRM) to monitor the evolution of the concentration of guest molecules in nanoporous host materials opens a new field of diffusion research in condensed matter. It combines the methodical virtues of the profiling methods of solid-state diffusion studies with the benefit of the mobility enhancement in fluids. We are going to illustrate the rich options of diffusion studies provided by this novel experimental approach

The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application Mass-transfer of binary mixtures in DDR single crystals

For the first time, micro imaging by interference microscopy (IFM) is applied for studying uptake and release of guest molecules in nanoporous host material for investigating the adsorption of gas mixtures and for correlating the thus observed behavior with corresponding single-component sorption experiments. Combining the advantages of high spatial and temporal resolution in the order of 0.45 µm and 15 s, respectively, this promising approach results in time-dependent two-dimensional profiles of the intra-crystalline sorbate concentration of the single crystal under study [1, 2, 3]. However, with the lack of differentiation of different species present inside the microporous framework only certain types of experiments seems feasible, notably when the mixture consists of species with large differences in the rate of mass transport under the confinement of the channels, cages and windows of e.g. zeolite ZSM-58, which is well known from previous IFM studies [4]. Figure 1: IFM mixture experiments are enabled by several orders of magnitude difference in the single-component diffusion rates of e.g. CO2 and ethane leading to apparent instantaneous uptake of CO2 and much a slower sorption process for ethane. When provided as a 1:1 gas mixture of ptotal = 400 mbar, CO2 reaches a quasiequilibrium at about 30 % IFM signal (shaded area) followed by additional and much slower ethane adsorption. Hence, influences of the presence of CO2 on the diffusivity of ethane can be examined

Micro-Imaging by Interference Microscopy: A Case Study of Orientation-Dependent Guest Diffusion in MFI-Type Zeolite Host Crystals

Because of the small particle size, orientation-dependent diffusion measurements in microporous materials remains a challenging task. We highlight here the potential of micro-imaging by interference microscopy in a case study with MFI-type crystals in which, although with different accuracies, transient concentration profiles in all three directions can be observed. The measurements, which were performed with “rounded-boat” shaped crystals, reproduce the evolution patterns of the guest profiles recorded in previous studies with the more common “coffin-shaped” MFI crystals. The uptake and release patterns through the four principal faces (which in the coffin-shaped crystals extend in the longitudinal direction) are essentially coincident and there is no perceptible mass transfer in the direction of the long axis. The surface resistances of the four crystal faces through which mass transfer occurs are relatively small and have only a minor effect on the mass transfer rate. As a result of the pore structure, diffusion in the crystallographic c direction (which corresponds to the direction of the long axis) is expected to be much slower than in the transverse directions. This could explain the very low rate of mass transfer observed in the direction of the long axis, but it is also possible that the small end faces of the crystal may have high surface resistance. It is not possible to distinguish unequivocally between these two possibilities. All guest molecules studied (methyl-butane, benzene and 4-methyl-2-pentyne) show the same orientation dependence of mass transfer. The long 4-methyl-2-pentyne molecules would be expected to propagate at very different rates through the straight and sinusoidal channels. The coinciding patterns for uptake through the mutually perpendicular crystal faces therefore provide clear evidence that both the coffin shaped crystals and the rounded-boat-shaped crystals considered in this study, must be intergrowths rather than pure single crystals

Microimaging of transient guest profiles to monitor mass transfer in nanoporous materials

The intense interactions of guest molecules with the pore walls of nanoporous materials is the subject of continued fundamental research. Stimulated by their thermal energy, the guest molecules in these materials are subject to a continuous, irregular motion, referred to as diffusion. Diffusion, which is omnipresent in nature, influences the efficacy of nanoporous materials in reaction and separation processes. The recently introduced techniques of microimaging by interference and infrared microscopy provide us with a wealth of information on diffusion, hitherto inaccessible from commonly used techniques. Examples include the determination of surface barriers and the sticking coefficient's analogue, namely the probability that, on colliding with the particle surface, a molecule may continue its diffusion path into the interior. Microimaging is further seen to open new vistas in multicomponent guest diffusion (including the detection of a reversal in the preferred diffusion pathways), in guest-induced phase transitions in nanoporous materials and in matching the results of diffusion studies under equilibrium and non-equilibrium conditions

The wealth of information from transient guest profiles

The application of interference microscopy (IFM) and infrared microscopy (IRM) to monitor the evolution of the concentration of guest molecules in nanoporous host materials opens a new field of diffusion research in condensed matter. It combines the methodical virtues of the profiling methods of solid-state diffusion studies with the benefit of the mobility enhancement in fluids. We are going to illustrate the rich options of diffusion studies provided by this novel experimental approach