New Perspectives for Evaluating the Mass Transport in Porous Catalysts and Unfolding Macro- and Microkinetics.

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.

Unblocking a rigid purine MOF for kinetic separation of xylenes.

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 -xylene/-xylene separation capability with a diffusion selectivity of 6 and high equilibrium adsorption selectivity as indicated by coadsorption experiments.

Influence of vanillic acid immobilization in Nafion membranes on intramembrane diffusion and structural properties.

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.

Relationship between Ethane and Ethylene Diffusion inside ZIF-11 Crystals Confined in Polymers to Form Mixed-Matrix Membranes.

Self-diffusivities of ethane were measured by multinuclear pulsed field gradient (PFG) NMR inside zeolitic imidazolate framework-11 (ZIF-11) crystals dispersed in several selected polymers to form mixed-matrix membranes (MMMs). These diffusivities were compared with the corresponding intracrystalline self-diffusivities in ZIF-11 crystal beds. It was observed that the confinement of ZIF-11 crystals in ZIF-11 / Torlon MMM can lead to a decrease in the ethane intracrystalline self-diffusivity.

Self-diffusion of pure and mixed gases in mixed-linker zeolitic imidazolate framework-7-8 by high field diffusion NMR.

Self-diffusion of pure gases including carbon dioxide, methane, ethylene, ethane, and xenon as well as selected two-component mixtures was studied in hybrid zeolitic imidazolate framework-7-8 (ZIF-7-8) crystals using pulsed field gradient (PFG) NMR. This material was formed by mixing 2-methylimidazolate (ZIF-8 linker) and bulkier benzimidazolate (ZIF-7 linker) in the same framework. The intracrystalline diffusion data measured in mixed-linker ZIF-7-8 was compared with the corresponding data in the parent ZIF-8 material.

Single-crystal-to-single-crystal guest exchange in columnar assembled brominated triphenylamine bis-urea macrocycles.

Self-assembly of brominated triphenylamine bis-urea macrocycles affords robust porous materials. Urea hydrogen bonds organize these building blocks into 1-dimensional columns, which pack via halogen-aryl interactions. The crystals are stable when emptied, present two distinct absorption sites for Xe with restricted Xe diffusion, and exhibit single-crystal-to-single-crystal guest exchange.

Ethane diffusion in mixed linker zeolitic imidazolate framework-7-8 by pulsed field gradient NMR in combination with single crystal IR microscopy.

Pulsed field gradient (PFG) NMR was used in combination with single crystal IR microscopy (IRM) to study diffusion of ethane inside crystals of a mixed linker zeolitic imidazolate framework (ZIF) of the type ZIF-7-8 under comparable experimental conditions. These crystals contain 2-methylimidazolate (ZIF-8 linker) and benzimidazolate (ZIF-7 linker). It was observed that the PFG NMR attenuation curves measured for ethane in ZIF-7-8 exhibit deviations from the monoexponential behaviour, thereby indicating that the ethane self-diffusivity in different crystals of a crystal bed can be different.

Strongly Bound Sodium Dodecyl Sulfate Surrounding Single-Wall Carbon Nanotubes.

NMR techniques have been widely used to infer molecular structure, including surfactant aggregation. A combination of optical spectroscopy, proton NMR spectroscopy, and pulsed field gradient NMR (PFG NMR) is used to study the adsorption number for sodium dodecyl sulfate (SDS) with single-wall carbon nanotubes (SWCNTs). Distinct transitions in the NMR chemical shift of SDS are observed in the presence of SWCNTs.

Relationship between single-file diffusion of mixed and pure gases in dipeptide nanochannels by high field diffusion NMR.

High field NMR diffusometry reveals single-file diffusion of CO/CH4 mixture in dipeptide nanochannels with a coincident mobility for CO and CH4. In contrast to the relationship commonly observed for normal diffusion, this mixture mobility is only slightly smaller than that of pure CO which diffuses much faster than pure CH4.

Relationship between Diffusion and Chemical Exchange in Mixtures of Carbon Dioxide and an Amine-Functionalized Ionic Liquid by High Field NMR and Kinetic Monte Carlo Simulations.

NMR exchange spectroscopy (EXSY) and NMR diffusion spectroscopy (PFG NMR) were applied in combination with kinetic Monte Carlo (MC) simulations to investigate self-diffusion in a mixture of carbon dioxide and an amine-functionalized ionic liquid under conditions of an exchange of carbon dioxide molecules between the reacted and unreacted states in the mixture. EXSY studies enabled residence times of carbon dioxide molecules to be obtained in the two states, whereas PFG NMR revealed time-dependent effective diffusivities for diffusion times comparable with and larger than the residence times. Analytical treatment of the PFG NMR attenuation curves as well as fitting of the PFG NMR effective diffusivities by KMC simulations enabled determination of diffusivities of carbon dioxide in the reacted and unreacted states.

Single-file nanochannel persistence lengths from NMR.

Crystalline solids composed of one-dimensional channels with cross-sectional dimensions below 1 nm represent an intriguing class of materials with important potential applications. A key characteristic for certain applications is the average open channel persistence length, i.e.

Self-diffusion of carbon dioxide in samaria/alumina aerogel catalyst using high field NMR diffusometry.

Pulsed field gradient (PFG) NMR was used to investigate the self-diffusion of carbon dioxide in alumina stabilized samaria aerogel catalyst, a promising porous catalyst for gas-phase reactions featuring high porosity and high surface area. For diffusion studies, the catalyst was prepared in two sample packing types, macroscopic monoliths (i.e.

Combined application of high-field diffusion NMR and molecular dynamics simulations to study dynamics in a mixture of carbon dioxide and an imidazolium-based ionic liquid.

Self-diffusion and related short-time dynamic and structural properties were investigated for mixtures of carbon dioxide and the ionic liquid 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [bmim](+)[Tf2N](-) for a broad range of carbon dioxide molar fractions and at different temperatures. The studies were performed by a novel multinuclear pulsed field gradient (PFG) NMR technique, which combines the advantages of a high magnetic field (17.6 T) and a high magnetic field gradient (up to 30 T/m), in combination with molecular dynamics simulations.

Diffusion of methane and carbon dioxide in carbon molecular sieve membranes by multinuclear pulsed field gradient NMR.

Carbon molecular sieve (CMS) membranes are promising materials for energy efficient separations of light gases. In this work, we report a detailed microscopic study of carbon dioxide and methane self-diffusion in three CMS membrane derived from 6FDA/BPDA(1:1)-DAM and Matrimid polymers. In addition to diffusion of one-component sorbates, diffusion of a carbon dioxide/methane mixture was investigated.

Sorbate Transport in Carbon Molecular Sieve Membranes and FAU/EMT Intergrowth by Diffusion NMR.

In this paper we present and discuss selected results of our recent studies of sorbate self-diffusion in microporous materials. The main focus is given to transport properties of carbon molecular sieve (CMS) membranes as well as of the intergrowth of FAU-type and EMT-type zeolites. CMS membranes show promise for applications in separations of mixtures of small gas molecules, while FAU/EMT intergrowth can be used as an active and selective cracking catalyst.

Pulsed field gradient nuclear magnetic resonance study of time-dependent diffusion behavior and exchange of lipids in planar-supported lipid bilayers.

This work reports the direct experimental observation of lipid exchange between liquid-ordered domains and their liquid-disordered surroundings in 3-component planar-supported multibilayers (1,2-dioleoyl-sn-glycerol-3-phosphocholine/1,2-dipalmitoyl-sn-glycero-3-phosphocholine/cholesterol). The measurements of lipid lateral diffusion and exchange were carried out using proton pulsed field gradient (PFG) NMR spectroscopy with high field strength (17.6 T) and high gradient amplitudes (up to 30 T/m).

Influence of water on diffusion in imidazolium-based ionic liquids: a pulsed field gradient NMR study.

In this work, we applied a novel pulsed field gradient (PFG) NMR option, which combines advantages of high-field (17.6 T) NMR and high magnetic field gradients (up to 30 T/m), to study diffusion of anions, cations and water in two 1-ethyl-3-methylimidazolium-based ionic liquids. Application of high field allows for an easy recording of an NMR signal from small amounts of water added to the ionic liquids.

Application of pulsed field gradient NMR with high gradient strength for studies of self-diffusion in lipid membranes on the nanoscale.

This work demonstrates the feasibility of noninvasive studies of lipid self-diffusion in model lipid membranes on the nanoscale using proton pulsed field gradient (PFG) NMR spectroscopy with high (up to 35 T/m) gradient amplitudes. Application of high gradients affords for the use of sufficiently small diffusion times under the conditions when the width of the gradient pulses is much smaller than the diffusion time. As a result, PFG NMR studies of partially restricted or anomalous diffusion in lipid bilayers become possible over length scales as small as 100 nm.

Heterogeneity of polyelectrolyte diffusion in polyelectrolyte-protein coacervates: a 1H pulsed field gradient NMR study.

Proton pulsed field gradient (PFG) NMR was used to study the diffusion of poly(diallyldimethylammonium chloride) (PDADMAC) in coacervates formed from this polycation and the protein bovine serum albumin (BSA). Application of high (up to 30 T/m) magnetic field gradients in PFG NMR measurements allowed probing the diffusion of PDADMAC on a length scale of displacements as small as 100 nm in coacervates formed at different pH's and ionic strengths, i.e.

Toward observation of single-file diffusion using the tracer zero-length column method.

The tracer zero-length column (ZLC) method has been employed to study the diffusion of toluene in one-dimensional ZSM-12 and SAPO-5 zeolites. A significant deviation in the shape of the measured tracer exchange curves from monoexponential behavior was observed for toluene diffusion in both adsorbents in the limit of long-time asymptotes. In contrast, water/ZSM-12 and acetylene/SAPO-5 systems exhibit tracer exchange curves that are close to monoexponential behavior.

Probing lateral diffusion in lipid membranes on nanoscale by PFG NMR with high gradient strength.

This work demonstrates the feasibility of noninvasive studies of diffusion on a submicrometer length scale in aligned model lipid membranes using pulsed field gradient nuclear magnetic resonance with ultrahigh (up to 35 T/m) gradient strength. Application of such gradients allows the use of sufficiently small diffusion times under conditions of narrow-pulse approximation. As a result, monitoring anomalous or restricted diffusion in lipid membranes on a length scale in the range of 100 nm becomes possible.

Mesoscopic simulations of the diffusivity of ethane in beds of NaX zeolite crystals: comparison with pulsed field gradient NMR measurements.

Mesoscopic kinetic Monte Carlo simulations and pulsed field gradient nuclear magnetic resonance (PFG NMR) measurements are compared in order to investigate the transport of ethane in a bed of NaX crystals. A novel molecular mechanics particle-based reconstruction method is employed for the digital representation of the bed, enabling for the first time a parallel study of the real system and of a computer model tailored to reproduce the void fraction, particle shape and average size of the real system. Simulation of the long-range diffusion of ethane in the bed over the Knudsen, transient, and molecular diffusion regimes is consistent with the PFG NMR measurements in yielding tortuosity factors which depend upon the regime of diffusion; more specifically, tortuosity factors defined in the conventional way are higher in the Knudsen than in the molecular diffusion regime.

Internal concentration gradients of guest molecules in nanoporous host materials: measurement and microscopic analysis.

Evolution of internal concentration profiles of methanol in 2-D pore structure of ferrierite crystal was measured in the pressure range of 0 to 80 mbar with the help of the recently developed interference microscopy technique. The measured profiles showed that both a surface barrier and internal diffusion controlled the kinetics of adsorption/desorption. Furthermore, they indicated that in the main part of the crystal, the z-directional 10-ring channels were not accessible to methanol and that the transport of methanol mainly occurred via 8-ring y-directional channels.