In this work, a family of fluorinated polyimides (PIs) featuring trifluoromethyl-substituted biimidazole rings per repeating unit was synthesized and systematically modified to evaluate the impact of ionic functionalization and ionic liquid (IL) incorporation on membrane structure and gas transport behavior. The base PI, was quaternized to yield the corresponding ionic form and subsequently blended with IL to form a composite membrane. Gas permeability measurements showed a consistent decrease across the series (neutral > ionic > ionic + IL), however, these biimidazole-based materials exhibited significantly enhanced permeability compared to previously reported mono-imidazole analogues. Pulsed field gradient (PFG) NMR studies of CO2 and CH4 self-diffusion revealed that electrostatic interactions play a prominent role in CO2 diffusion inside the ionic membranes, especially when IL is present. The activation energy of self-diffusion was observed to increase across the series (neutral < ionic < ionic + IL) for CO2, while no difference, within uncertainty, was observed for CH4. An increased level of local intramembrane mobility observed for CO2 as a result of the IL addition indicates a preference for CO2 molecules to diffuse in the IL-rich membrane environments.

Carbon molecular sieve (CMS) membranes are emerging as high-performance materials for molecular separations. For CMS membranes the existence of a dense surface layer-termed the "hyperskin"-has been postulated in the literature. In this study, we provide direct, transport-based evidence for hyperskin existence and quantify hyperskin permeance in CMS membranes. This was achieved by comparing microscopic self-diffusion coefficients from pulsed field gradient (PFG) NMR with macroscopic corrected diffusivities from permeation and vapor sorption. The study was performed for methanol, p-xylene, and o-xylene in flat-sheet CMS membranes formed via pyrolysis of crosslinked poly(vinylidene fluoride)(PVDF) films. While PFG NMR was used to measure self-diffusion coefficients in the membrane bulk, isolated from surface effects, the diffusion data from permeation included all contributions including the surface resistance. In all cases, self-diffusivities from PFG NMR exceeded the corresponding corrected diffusivities from permeation by over an order of magnitude, clearly indicating the presence of a surface transport barrier. Quantitative analysis revealed that hyperskin permeance decreases systematically with molecular size-from 8.6 x 10-8 m/s for methanol (3.6 & Aring;) to 1.3 x 10-8 m/s for o-xylene (6.5 & Aring;). Our findings demonstrate that thin surface layers can dominate overall membrane resistance, and must be explicitly considered in modeling and performance optimization. This work introduces a new experimental framework to quantify surface transport resistances in CMS membranes and informs strategies for membrane design and processing.

The random motion (the diffusion) of guest molecules in nanoporous host materials is key to their manifold technological applications and, simultaneously, a ubiquitous phenomenon in nature quite in general. Based on a specification of the different conditions under which molecular diffusion in nanoporous materials may occur and of the thus resulting relevant parameters, a survey of the various ways of the measurement of the determining parameters is given. Starting with a condensed introduction to the respective measuring principles, the survey notably includes a summary of the various parameters accessible by each individual technique, jointly with an overview of their strengths and weaknesses as well as of the respective ranges of observation. The presentation is complemented by basic relations of diffusion theory and molecular modeling in nanoporous materials, illustrating their significance for enhancing the informative value of each measuring technique and the added value attainable by their combination. By providing guidelines for the measurement and reporting of diffusion properties of chemical compounds in nanopores, the document aims to contribute to the clarification and standardization of the presentation, nomenclature, and methodology associated with the documentation of diffusion phenomena in nanoporous materials serving for catalytic, mass separation, and other relevant purposes.

Mixed-matrix membranes (MMMs) have been heavily studied due to their compelling performance in gas separation applications. Direct measurement of gas diffusivities in MMMs is essential for constructing reliable transport models, but this measurement remains challenging. This study investigates CH4 and CO2 diffusion in PIM-COOH and a PIM-COOH/UiO-66-NH2 MMM with 30 vol% MOF loading, employing 13C pulsed field gradient nuclear magnetic resonance (PFG NMR) and various macroscopic techniques like the time-lag method and sorption analysis. PFG NMR revealed uniform transport properties across pure PIM-COOH films and MMMs on micrometer scales, indicating well-integrated structures without significant defects or MOF clustering. Macroscopically-measured diffusivities derived from the time-lag method showed strong pressure dependence due to the limitations of applying a linear sorption model to microporous materials. In contrast, corrected secant diffusivities aligned closely with self-diffusivities from PFG NMR and exhibited minimal pressure dependence. Application of another common analysis approach, the use of transient sorption experiments, yielded diffusivities with large uncertainties. This comparative study presents the range of diffusivities found by applying various techniques to capture transport and self-diffusivities, highlighting the need to carefully validate model assumptions for diffusion measurements in microporous polymer and MOF-based membrane materials. Of note, this study highlights the strength of PFG NMR in directly probing microscopic transport.

In this study, we report the synthesis and investigate the structural and gas transport characteristics of polyimides containing an imidazole ring in the main chain. First, a neutral polyimide (PI-Im-6FDA) was synthesized through polycondensation, followed by quaternation of the imidazole rings, producing an ionic-polyimide (PI-Im(Me)-6FDA). Interestingly, although the neutral form of the polyimide was able to form free-standing films, the ionic-polyimide was not able to form a self-standing film by itself. However, when free ionic liquid (IL) was added, good quality self-standing films were readily achieved (PI-Im(Me)-6FDA + IL). These three materials exhibited significant thermal stability, with a 10% weight loss occurring only when T > 400 degrees C. The solubility profiles of the polymers in various solvents demonstrated that the ionic modification enhances solubility in common organic solvents like acetone. The molecular weights were determined to be 91 kDa for PI-Im-6FDA and 84 kDa for PI-Im(Me)-6FDA, indicating substantial polymeric chain lengths. Additionally, computational simulations indicate an increase in density and a decrease in free volume fraction with ionic content, suggesting a more compact polymer structure, as evidenced by reduced d-spacing values. Further, macroscopic gas transport measurements show that PI-Im-6FDA exhibited superior overall permselectivity performance, while PI-Im(Me)-6FDA + IL enhances the permeability of larger gases like CH4 and N-2, making it a promising material for specific gas separation applications. Microscopic self-diffusion measurements performed by pulsed field gradient nuclear magnetic resonance (PFG NMR) techniques reveal a significant increase in the gas self-diffusivity upon IL addition. This result is in qualitative agreement with the transport diffusivity data from the macroscopic transport measurements. PFG NMR data also show that all materials have uniform transport properties on the length scales exceeding the smallest values of the root mean square displacements (1-2 mu m) used in the measurements.

This work focuses on quantification of microscopic self-diffusion of gas molecules in mixed-matrix membranes (MMMs) formed by dispersing UiO-66-NH2 metal-organic framework (MOF) crystals in 6FDA-Durene polyimide. Self-diffusion measurements were performed by 13C pulsed field gradient nuclear magnetic resonance (PFG NMR) for pure CO2 and CH4 with the spatial resolution in the range of 0.5-24 mu m and for different MOF loadings between 12.5 and 50 weight percent. Diffusion measurements performed for each gas in the MMM with the lowest MOF loading of 12.5 weight percent yielded a single diffusivity for all measured diffusion times corresponding to a diffusion under the condition of a fast exchange between the UiO-66-NH2 crystals and the surrounding polymer phase. However, as the UiO-66-NH2 loading was increased, two molecular ensembles were observed for both CO2 and CH4: 1) an ensemble corresponding to diffusion inside UiO-66-NH2 crystals and through the MOF-polymer interfaces, and 2) an ensemble corresponding to diffusion mainly in the polymer phase of the MMMs. This behavior can be explained by the formation of MOF clusters at higher MOF loadings. Quantification of the intra-cluster diffusivity, average cluster size, and the dependence of these properties on the MOF loading are presented and discussed. The reported measurements can serve as a framework to quantify discrete microscopic diffusion characteristics and sizes of interconnected MOF clusters in MMMs as MOF loading increases to reach the desired outcome of gas percolation over a spanning MOF cluster, viz a cluster of interconnected MOF crystals spanning an entire MMM.

Pulsed field gradient (PFG) NMR at high magnetic field was used to study microscopic diffusion of dimethyl methyl phosphonate (DMMP), a common chemical warfare agent (CWA) simulant, and water in Nafion membranes. PFG NMR measurements were performed for a broad range of molecular displacements. The self-diffusivities were measured as a function of the DMMP concentration for several fixed water concentrations. The measured data suggest that DMMP and water diffuse in different regions of Nafion. While water mostly diffuses in hydrophilic regions of the membrane, viz. water channels, DMMP diffusion is mostly limited to interfacial perfluoroether regions between these water channels and the semi-crystalline matrix.

Mixed-matrix membranes (MMMs) represent a promising membrane type for gas and liquid separations. Such membranes can be formed by dispersing crystals of metal–organic frameworks (MOFs) in polymers. For liquid separations, crosslinked polymers are desirable because crosslinking reduces polymer dilation and plasticization effects by liquid sorbates. However, polymer crosslinking processes can unfavorably change transport and related structural properties of MOF fillers. In this work, 13C pulsed field gradient (PFG) NMR was used to investigate possible changes in intra-MOF self-diffusion of p-xylene and o-xylene after MMM polymer crosslinking. The studied MMMs were formed by dispersing MOF crystals of the type ZIF-71 in Torlon (a poly(amide-imide)) or Matrimid (a polyimide) polymers. The reported PFG NMR data indicate that the polymer crosslinking process used does not influence the intra-ZIF diffusivities when all effects on these diffusivities that are related to diffusing molecules crossing over the crystal boundaries or reflected away from these boundaries are removed. In contrast, these data show that the crosslinking process resulted in a significant decrease of the effective size of ZIF-71 crystals inside the studied MMMs, especially in the ZIF-71/Matrimid MMM. This crystal size decrease is attributed to a partial degradation of the ZIF-71 crystals inside the MMMs due to the crosslinking process. This conclusion was found to be in agreement with the results of electron microscopy analysis. PFG NMR studies and data analysis similar to those presented here can be used for quantifying any types of MOF crystal degradation inside MOF-based MMMs.

Since the first studies reporting on its surprising catalytic properties, nanoporous gold (npAu) has emerged as a novel and ever since intensively investigated type of Au based catalyst. To judge its genuine catalytic potential and to be able to optimize its use in applications, it is mandatory, however, to quantify the influence of mass transport in the porous structure on the observed catalytic rates, i.e., to study the interplay between diffusion and reaction. To this end, we used pulsed field gradient (PFG) NMR for the first time to directly determine the diffusivities of reaction gases in a nanoporous metal - in this case for CO and CO2 as species involved in low temperature CO oxidation efficiently catalyzed by npAu. By comparing the diffusion coefficients within the 20 nm pores of the material with the values in the bulk gas phase, the tortuosity of npAu's pore system was assessable as the central geometrical parameter describing the extent to which diffusive transport in the pore system is slowed down. This knowledge allowed us in the following to disentangle the contributions of mass transport and the kinetics of the sur-face reaction (microkinetics). In particular, we were able to determine the rate constant and turnover fre-quency for low-temperature CO oxidation without previous ambiguities arising from potential transport limitations and to compare the results with other reported values. Based on the results, it was further-more possible to predict optimized dimensions of the catalyst, resulting in minimized or even suppressed diffusion limitations. These predictions could be successfully verified, using np-Au platelets with lateral dimensions in the range of a few hundred microns. In this way, the catalytic conversion could be ramped up by 50 % and an activity level advanced which reflected the microkinetic potential of np-Au.(c) 2022 Published by Elsevier Inc.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Pulsed field gradient (PFG) NMR in combination with quasielastic neutron scattering (QENS) was used to investigate self-diffusion of water and acetone in Nafion membranes with and without immobilized vanillic acid (VA). Complementary characterization of these membranes was performed by small angle X-ray scattering (SAXS) and NMR relaxometry. This study was motivated by the recent data showing that an organic acid, such as VA, in Nafion can preserve its catalytic activity in the presence of water even at high intra-polymer water concentrations corresponding up to 100% ambient relative humidity. However, there is currently no clear understanding of how immobilized organic acid molecules influence the microscopic transport properties and related structural properties of Nafion. Microscopic diffusion data measured by PFG NMR and QENS are compared for Nafion with and without VA. For displacements smaller than the micrometer-sized domains previously reported for Nafion, the VA addition was not observed to lead to any significant changes in the water and/or acetone self-diffusivity measured by each technique inside Nafion. However, the reported PFG NMR data present evidence of a different influence of acetone concentration in the membranes with and without VA on the water permeance of the interfaces between neighboring micrometer-sized domains. The reported diffusion data are correlated with the results of SAXS structural characterization and NMR relaxation data for water and acetone.

The separation of xylene isomers still remains an industrially challenging task. Here, porous purine-based metal-organic frameworks (MOFs) have been synthesized and studied for their potential in xylene separations. In particular, Zn(purine)I showed excellent para-xylene/ortho-xylene separation capability with a diffusion selectivity of 6 and high equilibrium adsorption selectivity as indicated by coadsorption experiments. This high selectivity is attributed to the shape and size of the channel aperture within the rigid framework of Zn(purine)I.

In this article we shed light on newly emerging perspectives to characterize and understand the interplay of diffusive mass transport and surface catalytic processes in pores of gas phase metal catalysts. As a case study, nanoporous gold, as an interesting example exhibiting a well-defined pore structure and a high activity for total and partial oxidation reactions is considered. PFG NMR (pulsed field gradient nuclear magnetic resonance) measurements allowed here for a quantitative evaluation of gas diffusivities within the material. STEM (scanning transmission electron microscopy) tomography furthermore provided additional insight into the structural details of the pore system, helping to judge which of its features are most decisive for slowing down mass transport. Based on the quantitative knowledge about the diffusion coefficients inside a porous catalyst, it becomes possible to disentangle mass transport contributions form the measured reaction kinetics and to determine the kinetic rate constant of the underlying catalytic surface reaction. In addition, predictions can be made for an improved effectiveness of the catalyst, i.e., optimized conversion rates. This approach will be discussed at the example of low-temperature CO oxidation, efficiently catalysed by npAu at 30 °C. The case study shall reveal that novel porous materials exhibiting well-defined micro- and mesoscopic features and sufficient catalytic activity, in combination with modern techniques to evaluate diffusive transport, offer interesting new opportunities for an integral understanding of catalytic processes. Graphical Abstract

13 C pulsed field gradient (PFG) NMR was used to quantify self-diffusion of a p -xylene/ o -xylene mixture inside zeolitic imidazolate framework-71 (ZIF-71) crystals dispersed in a Torlon polymer to form a ZIF-71/Torlon mixed-matrix membrane (MMM). The corresponding reference PFG NMR measurements with a bed of a loosely packed bed of ZIF-71 crystals were also performed. This study is motivated by a demonstrated potential of ZIF-based MMMs for liquid separations. Selective 13 C isotopic labelling of each xylene isomer was used to ensure that the self-diffusivity of each isomer in the mixture is measured independently and correctly. The diffusion measurements were performed at different diffusion times at 296 K. The observed dependencies of intra-ZIF self-diffusivities in ZIF-71/Torlon MMMs and the corresponding ZIF-71 crystal beds on diffusion time were explained by an influence of the external crystal surface on the studied diffusion process. Analysis of the time dependencies of the self-diffusivities measured for each xylene isomer allowed obtaining intra-ZIF self-diffusivities not perturbed by crystal surface effects as well as the corresponding diffusion selectivity for the studied sorbate mixture. The reported selectivity of around 4 demonstrates promising separation behavior of the considered membrane type.

Pulsed field gradient (PFG) NMR at high magnetic field is used to study molecular self-diffusion of ethylene, ethane, and propylene in a 2-ethylimidazole (eIm) linker-doped ZIF-8. These eIm-doped materials were synthesized by incorporating increasing fractions of the non-native eIm linker, which substitutes the native 2-methylimidazole linker in the ZIF-8 framework. Self-diffusion of the three hydrocarbon gases was measured in the limits of a high and low loading pressure in six different eIm-doped ZIFs with the eIm linker fractions in the framework between 0 and 22.2%. It was found that the elm linker substitution shows a tendency of increasing gas intracrystalline diffusivities, particularly for larger elm linker fractions and sorbates at low loading pressures. It was also observed that the ethylene/ethane diffusion selectivity, viz. the ratio of the ethylene and ethane intracrystalline diffusivities, does not show any dependence, within uncertainty, on the extent of the linker substitution. Based on these results it was concluded that the elm linker substitution is likely to introduce small changes in the ZIF-8 microstructure which result in slightly larger pore aperture sizes, but smaller framework flexibility in comparison with the parent ZIF-8 material.

Confinement of crystals of metal–organic frameworks (MOFs) in polymers to form MOF/polymer mixed-matrix membranes (MMMs) can lead to changes in intra-MOF diffusion. Such changes in intra-MOF diffusion were previously demonstrated for light gases, and attributed to a reduction in MOF framework flexibility. However, to our knowledge, no direct measurements of intra-MOF diffusion in MMMs and of the related MOF confinement effect on diffusion have been reported for organic liquids. In this work, 13C pulsed field gradient (PFG) NMR was used to quantify self-diffusion of methanol, ethanol, p-xylene, and o-xylene inside MOF crystals of the type ZIF-71, which were dispersed in a Torlon polymer to form MMMs. The intra-ZIF self-diffusivities in the MMMs were compared with the corresponding self-diffusivities measured in beds of ZIF-71 crystals. The observed self-diffusivity dependencies on diffusion time were explained by crystal boundary effects. The corresponding values of intra-ZIF self-diffusivities not perturbed by such effects were found to be the same, within uncertainty, in the MMMs and ZIF-71 beds. The observed lack of an influence of the ZIF-71 confinement in Torlon on diffusion is explained, and an option to increase diffusion selectivity is discussed.

High magnetic fields (up to 17.6 T) in combination with large magnetic field gradients (up to 25 T/m) were successfully utilized in pulsed field gradient (PFG) NMR studies of gas and liquid diffusion in nanoporous materials. In this mini-review, we present selected examples of such studies demonstrating the ability of high field PFG NMR to gain unique insights and differentiate between various types of diffusion. These examples include identifying and explaining an anomalous relationship between molecular size and self-diffusivity of gases in a zeolitic imidazolate framework (ZIF), as well as revealing and explaining an influence of mixing different linkers in a ZIF on gas self-diffusion. Different types of normal and restricted self-diffusion were quantified in hybrid membranes formed by dispersing ZIF crystals in polymers. High field PFG NMR studies of such membranes allowed observing and explaining an influence of the ZIF crystal confinement in a polymer on intra-ZIF self-diffusion of gases. This technique also allowed measuring and understanding anomalous single-file diffusion (SFD) of mixed sorbates. Furthermore, the presented examples demonstrate a high potential of combining high field PFG NMR with single-crystal infrared microscopy (IRM) for obtaining greater physical insights into the studied diffusion processes.

Pulsed field gradient (PFG) NMR at high field was utilized to directly observe a transition between two different diffusion regimes in a Nafion 117 membrane loaded with water and acetone. Although water self-diffusivity at small water loadings was observed to be diffusion time-independent in the limit of small and large diffusion times, it showed a significant decrease with increasing diffusion time at intermediate times corresponding to root mean square displacements on the order of several microns. Under our experimental conditions, no self-diffusivity dependence on diffusion time was found for water at large water loadings and for acetone at all studied acetone loadings. The diffusion time-dependent self-diffusivity at small water concentration is explained by the existence of finite domains of interconnected water channels with sizes in the range of several microns that form in Nafion in the presence of acetone. The domain sizes and permeance of transport barriers separating adjacent domains are estimated based on the measured PFG NMR data. At large water concentrations, the water channels form a fully interconnected network, resulting in time-independent self-diffusivity. The absence of such a percolation-like transition with increasing molecular concentration for acetone is attributed to a difference in the regions available for water and acetone diffusion in Nafion. The diffusion data are correlated with and supported by structural data obtained using small-angle X-ray and neutron scattering techniques. These techniques reveal distinct water channels with radial dimensions in the nanometer range increasing upon water addition, while acetone appears to be in an interfacial perfluoroether region, reducing the size of the radial channel dimension.

Application of pulsed field gradient (PFG) NMR to studying molecular diffusion in beds of nanoporous materials has given rise to novel insights and paradigm shifts in our understanding, which are reviewed in the present contribution. This gain in information is, in particular, related to the ability of PFG NMR to discriminate between various mechanisms affecting mass transfer in such systems. Examples include, inter alia, the sensitivity of PFG NMR toward transport enhancement in pore hierarchies as well as toward transport resistances acting, in addition to the diffusional resistance of the genuine pore space, either on the crystal surfaces or in their interior.