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

Abstract

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.

Transport Phenomena in Nanoporous Materials

“…MFI "crystals" are, in reality, intergrowths, so that the crystallographic directions are known to vary within a given particle, following quite different patterns. Since observation in X direction (crystal orientation in the bottom of Figure 10 b) gives rise to profiles essentially identical to those shown in Figure 10 c, [114] it follows that the crystallographic x and y directions must be oriented along the particle X and Y directions with equal probabilities. Since the uptake patterns are found to be independent of the distance from the upper and lower ends of the crystal (in the Z direction), mass transfer in the Z direction is seen to be notably reduced so that the particle Z direction can be assumed to coincide closely with the crystallographic z direction.…”

“…Absolute concentrations become accessible, however, by comparison with the adsorption isotherm for the given system. [113,114] Figure 8, which shows a variety of differently shaped profiles recorded in micro-imaging studies of the methanol-ferrierite system, provides an impression of the wealth of information that can be derived concerning the evolution of concentration profiles during uptake and release. This is also true in general for observation parallel to one (or several) crystal faces as long as the fluxes stemming from the different faces do not interfere with each other.…”

“…Most importantly, this option is not connected with the requirement of strong (c) one-dimensional concentration profiles during 2-methyl-butane uptake at room temperature by a pressure step from 0 to 1 mbar pressure in X direction at three different positions Z as indicated by blue lines in (b). From GueudrØ et al, [114] with permission. 27), open symbols], mid-time regime analyzed by center-line approach (full stars) and the best fit to the complete set of profiles applying the diffusivity-concentration-dependence given by Fujita's model (solid line).…”

“…However, in some exceptional cases, researchers have managed to achieve this by repeating uptake and release experiments with one and the same crystal in different orientations. [113,114] Figure 8, which shows a variety of differently shaped profiles recorded in micro-imaging studies of the methanol-ferrierite system, provides an impression of the wealth of information that can be derived concerning the evolution of concentration profiles during uptake and release. Hitherto such profiles have been derived theoretically (from the solution of the transient diffusion equation) but experimental profiles have been measured for only a few special cases.…”

“…It shows a "rounded-boat"-shaped zeolite crystallite [124] of type MFI (Figure 10 a), together with a schematic of the three different crystal orientations (Figure 10 b) under which uptake of 2methyl-butane was observed. [114] The particle coordinate system is deliberately represented by capital letters (X, Y, Z) and Figure 8. Uptake and release of methanol by ferrierite along the eight-ring channels: Comparison of simulated and experimental profiles for pressure steps 0 to 5 mbar (a), 5 to 0 mbar (b), 0 to 10 mbar (c), 10 to 0 mbar (d), 0 to 40 mbar (e), 40 to 0 mbar (f), 0 to 80 mbar (g), and 80 to 0 mbar (h).…”