Uncommon Synergy between Adsorption and Diffusion of Hexane Isomer Mixtures in MFI Zeolite Induced by Configurational Entropy Effects

While <i>n</i>-hexane (nC6) can adsorb at any location within the MFI zeolite pore network, configurational considerations cause the branched isomer 2-methylpentane (2MP) to locate preferentially at the channel intersections. For adsorption of nC6/2MP mixtures, infrared microscopy (IRM) measurements show that the adsorption selectivity favors the linear isomer by about one order of magnitude when the total mixture loading, Θ_t, exceeds four molecules per unit cell at which all intersection sites are fully occupied. The IRM data are in quantitative agreement with configurational-bias Monte Carlo (CBMC) simulations. IRM monitoring of the transient uptake of nC6/2MP mixtures within crystals of MFI exposed to step increases in the pressures shows that the configurational entropy effect also leaves its imprint on the uptake characteristics. For operating conditions in which Θ_t > 4, increase in the 2MP partial pressure in the bulk gas phase leads to a reduction in the 2MP uptake; this reduction leads to a concomitant and synergistic increase in the diffusivities of both isomers due to reduced extent of intersection blocking

Microimaging of Transient Intracrystalline Concentration Profiles during Two-Component Uptake of Light Hydrocarbon–Carbon Dioxide Mixtures by DDR-Type Zeolites

We report here the application of interference microscopy to study intracrystalline diffusion in a binary adsorbed phase. Hitherto the application of interference microscopy to the study of intracrystalline diffusion has been limited to single-component systems since only for a single-component system is the measured phase shift directly related to the concentration. However, when the difference in diffusivity between the components is sufficiently large, the faster component reaches equilibrium on a time scale that is short compared with the diffusion time for the slower component. Under these conditions, which are fulfilled for the system studied (CO₂–C₂H₆ in a large crystal of zeolite DDR), it is possible to determine the transient concentration profiles for both components and hence to determine the influence of the fast component on diffusion of the slower species. It is shown that measurements of this type offer a possible approach for experimental determination of the diffusion matrix. For the system studied, diffusion of ethane was found to be essentially unaffected by the presence of CO₂, in conformity with the results of an earlier study carried out by the zero length column technique

On the Impact of Sorbent Mobility on the Sorbed Phase Equilibria and Dynamics: A Study of Methane and Carbon Dioxide within the Zeolite Imidazolate Framework-8

The presented work aims at exploring the influence of the mobility of the sorbent framework on both the equilibrium and the kinetic properties of the sorbed phase by means of molecular dynamics computer experiments under isochoric–isothermal and isobaric–isothermal statistical ensembles for several host model options, combined by Widom averaging along the entire trajectory of the host–guest system toward rigorously obtained sorbate isotherms within a fully flexible lattice. The methodology is adapted to the study of the self-diffusivity and the collective (Maxwell–Stefan and transport) diffusivities of carbon dioxide (CO₂) and methane (CH₄) within the zeolite imidazolate framework-8 (ZIF-8). The simulation predictions are compared with measurements from pulsed-field gradient nuclear magnetic resonance (PFG NMR), as well as with recently conducted infrared microscopy (IRM) experiments elaborated on the basis of the current modeling in the flexible ZIF-8. The modeling results reveal a significant influence on sorbate transport exerted by the 2-methilimidazolate ligands surrounding the cage-to-cage entrances, whose apertures are commensurate with the guest molecular dimensions. Moreover, calculations of the singlet probability density distribution of the sorbate molecules at selected regions within the imidazolate framework provide a plausible explanation of the transport diffusivity as a function of sorbate occupancy, measured via IRM

The Nature of Surface Barriers on Nanoporous Solids Explored by Microimaging of Transient Guest Distributions

Nanoporous solids are attractive materials for energetically efficient and environmentally friendly catalytic and adsorption separation processes. Although the performance of such materials is largely dependent on their molecular transport properties, our fundamental understanding of these phenomena is far from complete. This is particularly true for the mechanisms that control the penetration rate through the outer surface of these materials (commonly referred to as surface barriers). Recent detailed sorption rate measurements with Zn(tbip) crystals have greatly enhanced our basic understanding of such processes. Surface resistance in this material has been shown to arise from the complete blockage of most of the pore entrances on the outer surface, while the transport resistance of the remaining open pores is negligibly small. More generally, the revealed correlation between intracrystalline diffusion and surface permeation provides a new view of the nature of transport resistances in nanoporous materials acting in addition to the diffusion resistance of the regular pore network, leading to a rational explanation of the discrepancy which is often observed between microscopic and macroscopic diffusion measurements

Monitoring Molecular Mass Transfer in Cation-Free Nanoporous Host Crystals of Type AlPO-LTA

Micro-imaging is employed to monitor the evolution of intra-crystalline guest profiles during molecular adsorption and desorption in cation-free zeolites AlPO-LTA. The measurements are shown to provide direct evidence on the rate of intra-crystalline diffusion and surface permeation and their inter-relation. Complemented by PFG NMR and integral IR measurements, a comprehensive overview of the diffusivities of light hydrocarbons in this important type of host materials is provided

Anomalous Relationship between Molecular Size and Diffusivity of Ethane and Ethylene inside Crystals of Zeolitic Imidazolate Framework-11

Multinuclear pulsed field gradient (PFG) NMR was used to study the self-diffusion of ethane and ethylene inside loosely packed beds of zeolitic imidazolate framework-11 (ZIF-11) crystals. Diffusion measurements were performed at different temperatures under conditions where the length scales of displacements were smaller than or comparable with the mean size of the ZIF-11 crystals. For the crystal beds loaded with a single sorbate, these studies showed a larger intracrystalline diffusivity for ethane than that for ethylene under the same or comparable experimental conditions, an unexpected result due to the larger size of ethane compared to that of ethylene. PFG NMR diffusion studies of ZIF-11 beds loaded with ethane/ethylene mixtures revealed that substituting a fraction of ethane molecules by ethylene molecules decreases the intracrystalline diffusivity of ethane. These results in combination with an observation of a higher-for-ethylene-than-for-ethane activation energy of intracrystalline self-diffusion in single-sorbate systems suggest a hindering effect of ethylene molecules on the intra-ZIF diffusion. To further confirm and investigate the anomalous relationship between ethane and ethylene intracrystalline diffusivities in ZIF-11, uptake curves for ethane and ethylene were measured in single crystals using IR microscopy and in a thin crystal bed using a volumetric technique. The diffusion data obtained from these complementary uptake studies under the same or comparable conditions were found to be consistent with the PFG NMR measurements. The observed anomalous relationship between ethane and ethylene diffusivities in ZIF-11 is discussed in the context of the flexibility of the ZIF-11 framework