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2.
Nat Mater ; 23(2): 262-270, 2024 Feb.
Article in English | MEDLINE | ID: mdl-38123813

ABSTRACT

Porous metal-organic frameworks have emerged to resolve important challenges of our modern society, such as CO2 sequestration. Zeolitic imidazolate frameworks (ZIFs) can undergo a glass transition to form ZIF glasses; they combine the liquid handling of classical glasses with the tremendous potential for gas separation applications of ZIFs. Using millimetre-sized ZIF-62 single crystals and centimetre-sized ZIF-62 glass, we demonstrate the scalability and processability of our materials. Further, following the evolution of gas penetration into ZIF crystals and ZIF glasses by infrared microimaging techniques, we determine the diffusion coefficients and changes to the pore architecture on the ångström scale. The evolution of the material on melting and processing is observed in situ on different length scales by using a microscope-coupled heating stage and analysed microstructurally by transmission electron microscopy. Pore collapse during glass processing is further tracked by changes in the volume and density of the glasses. Mass spectrometry was utilized to investigate the crystal-to-glass transition and thermal-processing ability. The controllable tuning of the pore diameter in ZIF glass may enable liquid-processable ZIF glass membranes for challenging gas separations.

3.
Nat Mater ; 19(4): 374-375, 2020 04.
Article in English | MEDLINE | ID: mdl-32094493
4.
Magn Reson Imaging ; 56: 3-13, 2019 02.
Article in English | MEDLINE | ID: mdl-30322668

ABSTRACT

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.


Subject(s)
Diffusion , Image Processing, Computer-Assisted/methods , Magnetic Resonance Spectroscopy/methods , Nanopores , Porosity
5.
Phys Chem Chem Phys ; 20(37): 23967-23975, 2018 Oct 07.
Article in English | MEDLINE | ID: mdl-30211405

ABSTRACT

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. Measurements of the ethane uptake curves performed by IRM under the same conditions in different ZIF-7-8 crystals of the bed yield different transport diffusivities thus confirming that the rate of ethane diffusion is different in different ZIF-7-8 crystals. The IRM observation that the fractions of ZIF-8 and ZIF-7 linkers are different in different ZIF-7-8 crystals allowed attributing the observed heterogeneity in diffusivities to the heterogeneity in the linker fraction. The quantitative comparison of the average ethane self-diffusivities measured by PFG NMR in ZIF-7-8 with the corresponding data on corrected diffusivities from IRM measurements revealed a good agreement between the results obtained by the two techniques. In agreement with the expectation of smaller aperture sizes in ZIF-7-8 than in ZIF-8, the average ethane self-diffusivities in ZIF-7-8 were found to be significantly lower than the corresponding self-diffusivities in ZIF-8.

6.
Molecules ; 23(3)2018 Mar 15.
Article in English | MEDLINE | ID: mdl-29543777

ABSTRACT

Molecular diffusion is commonly found to slow down with increasing molecular size. Deviations from this pattern occur in some host materials with pore sizes approaching the diameters of the guest molecules. A variety of theoretical models have been suggested to explain deviations from this pattern, but robust experimental data are scarcely available. Here, we present such data, obtained by monitoring the chain length dependence of the uptake of n-alkanes in the zeolitic imidazolate framework ZIF-4. A monotonic decrease in diffusivity from ethane to n-butane was observed, followed by an increase for n-pentane, and another decrease for n-hexane. This observation was confirmed by uptake measurements with n-butane/n-pentane mixtures, which yield faster uptake of n-pentane. Further evidence is provided by the observation of overshooting effects, i.e., by transient n-pentane concentrations exceeding the (eventually attained) equilibrium value. Accompanying grand canonical Monte Carlo simulations reveal, for the larger n-alkanes, significant differences between the adsorbed and gas phase molecular configurations, indicating strong confinement effects within ZIF-4, which, with increasing chain length, may be expected to give rise to configurational shifts facilitating molecular propagation at particular chain lengths.


Subject(s)
Alkanes/chemistry , Metal-Organic Frameworks/chemistry , Zeolites/chemistry , Molecular Structure , Monte Carlo Method
7.
Angew Chem Int Ed Engl ; 57(18): 5156-5160, 2018 04 23.
Article in English | MEDLINE | ID: mdl-29465815

ABSTRACT

Through IR microimaging the spatially and temporally resolved development of the CO2 concentration in a ZIF-8@6FDA-DAM mixed matrix membrane (MMM) was visualized during transient adsorption. By recording the evolution of the CO2 concentration, it is observed that the CO2 molecules propagate from the ZIF-8 filler, which acts as a transport "highway", towards the surrounding polymer. A high-CO2 -concentration layer is formed at the MOF/polymer interface, which becomes more pronounced at higher CO2 gas pressures. A microscopic explanation of the origins of this phenomenon is suggested by means of molecular modeling. By applying a computational methodology combining quantum and force-field based calculations, the formation of microvoids at the MOF/polymer interface is predicted. Grand canonical Monte Carlo simulations further demonstrate that CO2 tends to preferentially reside in these microvoids, which is expected to facilitate CO2 accumulation at the interface.

8.
Chemistry ; 23(53): 13000-13005, 2017 Sep 21.
Article in English | MEDLINE | ID: mdl-28722297

ABSTRACT

Investigation of guest diffusion in porous metal-organic frameworks (MOFs) is of major importance, because many porosity-related properties of MOFs are influenced by diffusion effects. The diffusion of dimethyl sulfoxide (DMSO) in the MOF MIL-53-NH2 (Al) was investigated through pulsed-field-gradient (PFG) NMR spectroscopy. The microporous material was synthesized in small crystallites (under 500 nm), which agglomerated in a large range of particle sizes (from hundreds of nanometers to tens of micrometers), giving a morphologically very heterogeneous sample. No special agglomeration pattern could be observed, which makes a PFG NMR investigation very challenging, yet it represents a realistic situation for the diffusion of guest molecules in porous materials. We were able to distinguish between two diffusion regimes existing in parallel with each other over the total range from 15 to 200 ms of observation times as accessible in the experiments: In the large crystal agglomerates (diameters above 20 µm), guest movement was found to be subdiffusive, with a time exponent κ =0.8 (rather than one as for normal diffusion). Guest diffusion in the remaining, smaller host particles followed the pattern of normal diffusion within a bed of spheres of impenetrable external surfaces, with a size distribution in good agreement with that of the material under study. Diffusion in a rather complex system could thus be referred to a two-region model with new potentials for application to systems of intricate topology.

9.
Chemphyschem ; 18(15): 2094-2102, 2017 Aug 05.
Article in English | MEDLINE | ID: mdl-28524266

ABSTRACT

We investigated single-molecule and ensemble diffusivities in a silica nanopore with a chemically modified surface by molecular dynamics simulations. Solutes with graded polarity (nonpolar ethylbenzene and moderately polar benzyl alcohol) were equilibrated with a 40:60 v/v water/acetonitrile solvent in a 10 nm pore, the surface of which was rendered hydrophobic by modification with alkyl chains. Simulations enable detailed sampling of spatially dependent solvent and solute mobilities, which originate from microheterogeneity induced by the surface modification. Acetonitrile is enriched near the ends of the alkyl chains and forms a high-mobility interface region between the (nonpolar) bonded phase at the surface and the (polar) bulk liquid in the center of the pore. Solvent and solute diffusivities calculated from the time average of a single molecule and from the ensemble average over all molecules, respectively, revealed excellent agreement, which implies validity of ergodicity. The molecular-simulation approach to investigate the time average of a single molecule, on the one hand, and the ensemble average over a larger number of molecules, on the other hand, is general and can be adapted for a variety of surfaces, solvents, and solute molecules by using pores with tailored geometries and surface modifications.

10.
Sci Rep ; 7: 40207, 2017 01 20.
Article in English | MEDLINE | ID: mdl-28106047

ABSTRACT

Nanoporous silicon produced by electrochemical etching of highly B-doped p-type silicon wafers can be prepared with tubular pores imbedded in a silicon matrix. Such materials have found many technological applications and provide a useful model system for studying phase transitions under confinement. This paper reports a joint experimental and simulation study of diffusion in such materials, covering displacements from molecular dimensions up to tens of micrometers with carefully selected probe molecules. In addition to mass transfer through the channels, diffusion (at much smaller rates) is also found to occur in directions perpendicular to the channels, thus providing clear evidence of connectivity. With increasing displacements, propagation in both axial and transversal directions is progressively retarded, suggesting a scale-dependent, hierarchical distribution of transport resistances ("constrictions" in the channels) and of shortcuts (connecting "bridges") between adjacent channels. The experimental evidence from these studies is confirmed by molecular dynamics (MD) simulation in the range of atomistic displacements and rationalized with a simple model of statistically distributed "constrictions" and "bridges" for displacements in the micrometer range via dynamic Monte Carlo (DMC) simulation. Both ranges are demonstrated to be mutually transferrable by DMC simulations based on the pore space topology determined by electron tomography.

11.
Chem Soc Rev ; 45(12): 3439-67, 2016 06 13.
Article in English | MEDLINE | ID: mdl-26759076

ABSTRACT

Adding mesopore networks in microporous materials using the principles of hierarchical structure design is recognized as a promising route for eliminating their transport limitations and, therefore, for improving their value in technological applications. Depending on the routes of physico-chemical procedures or post-synthesis treatments used, very different geometries of the intentionally-added transport mesopores can be obtained. Understanding the structure-dynamics relationships in these complex materials with multiple porosities under different thermodynamical conditions remains a challenging task. In this review, we summarize the results obtained so far on experimental and theoretical studies of diffusion in micro-mesoporous materials. By considering four common classes of bi-porous materials, which are differing by the inter-connectivities of their sup-spaces as one of the most important parameter determining the transport rates, we discuss their generic transport properties and correlate the results delivered by the equilibrium and non-equilibrium techniques of diffusion measurements.

12.
Chemistry ; 22(1): 199-210, 2016 Jan 04.
Article in English | MEDLINE | ID: mdl-26611940

ABSTRACT

Large zeolite crystals of ferrierite have been used to study the deactivation, at the single particle level, of the alkyl isomerisation catalysis of oleic acid and elaidic acid by a combination of visible micro-spectroscopy and fluorescence microscopy (both polarised wide-field and confocal modes). The large crystals did show the desired activity, albeit only traces of the isomerisation product were obtained and low conversions were achieved compared to commercial ferrierite powders. This limited activity is in line with their lower external non-basal surface area, supporting the hypothesis of pore mouth catalysis. Further evidence for the latter comes from visible micro-spectroscopy, which shows that the accumulation of aromatic species is limited to the crystal edges, while fluorescence microscopy strongly suggests the presence of polyenylic carbocations. Light polarisation associated with the spatial resolution of fluorescence microscopy reveals that these carbonaceous deposits are aligned only in the larger 10-MR channels of ferrierite at all crystal edges. The reaction is hence further limited to these specific pore mouths.


Subject(s)
Mouth/chemistry , Oleic Acid/chemistry , Zeolites/chemistry , Catalysis , Isomerism , Microscopy, Fluorescence , Oleic Acid/chemical synthesis
13.
Nat Mater ; 15(4): 401-6, 2016 Apr.
Article in English | MEDLINE | ID: mdl-26689140

ABSTRACT

Nanoporous materials find widespread applications in our society: from drug delivery to environmentally friendly catalysis and separation technologies. The efficient design of these processes depends crucially on understanding the mass transfer mechanism. This is conventionally determined by uptake or release experiments, carried out with assemblages of nanoporous crystals, assuming all crystals to be identical. Using micro-imaging techniques, we now show that even apparently identical crystals (that is, crystals of similar size and shape) from the same batch may exhibit very different uptake rates. The relative contribution of the surface resistance to the overall transport resistance varied with both the crystal and the guest molecule. As a consequence of this crystal diversity, the conventional approach may not distinguish correctly between the different mass transfer mechanisms. Detection of this diversity adds an important new piece of evidence in the search for the origin of the surface barrier phenomenon. Our investigations were carried out with the zeolite SAPO-34, a key material in the methanol-to-olefins (MTO) process, propane-propene separation and adsorptive heat transformation.

14.
Angew Chem Int Ed Engl ; 54(48): 14580-3, 2015 Nov 23.
Article in English | MEDLINE | ID: mdl-26448501

ABSTRACT

Diffusion in nanoporous host-guest systems is often considered to be too complicated to comply with such "simple" relationships as Fick's first and second law of diffusion. However, it is shown herein that the microscopic techniques of diffusion measurement, notably the pulsed field gradient (PFG) technique of NMR spectroscopy and microimaging by interference microscopy (IFM) and IR microscopy (IRM), provide direct experimental evidence of the applicability of Fick's laws to such systems. This remains true in many situations, even when the detailed mechanism is complex. The limitations of the diffusion model are also discussed with reference to the extensive literature on this subject.


Subject(s)
Nanostructures , Porosity , Biological Transport , Magnetic Resonance Spectroscopy , Microscopy/methods , Models, Chemical
15.
Nat Commun ; 6: 8633, 2015 Oct 20.
Article in English | MEDLINE | ID: mdl-26482337

ABSTRACT

Advances in materials synthesis bring about many opportunities for technological applications, but are often accompanied by unprecedented complexity. This is clearly illustrated by the case of hierarchically organized zeolite catalysts, a class of crystalline microporous solids that has been revolutionized by the engineering of multilevel pore architectures, which combine unique chemical functionality with efficient molecular transport. Three key attributes, the crystal, the pore and the active site structure, can be expected to dominate the design process. This review examines the adequacy of the palette of techniques applied to characterize these distinguishing features and their catalytic impact.

16.
Nat Commun ; 6: 7697, 2015 Jul 16.
Article in English | MEDLINE | ID: mdl-26177626

ABSTRACT

Under certain conditions, during binary mixture adsorption in nanoporous hosts, the concentration of one component may temporarily exceed its equilibrium value. This implies that, in contrast to Fick's Law, molecules must diffuse in the direction of increasing rather than decreasing concentration. Although this phenomenon of 'overshooting' has been observed previously, it is only recently, using microimaging techniques, that diffusive fluxes in the interior of nanoporous materials have become accessible to direct observation. Here we report the application of interference microscopy to monitor 'uphill' fluxes, covering the entire period of overshooting from initiation until final equilibration. It is shown that the evolution of the profiles can be adequately predicted from the single-component diffusivities together with the binary adsorption equilibrium data. The guest molecules studied (carbon dioxide, ethane and propene) and the host material (ZSM-58 or DDR) are of practical interest in relation to the development of kinetically selective adsorption separation processes.

17.
Angew Chem Int Ed Engl ; 54(17): 5060-4, 2015 Apr 20.
Article in English | MEDLINE | ID: mdl-25720828

ABSTRACT

Microimaging by IR microscopy is applied to the recording of the evolution of the concentration profiles of reactant and product molecules during catalytic reaction, notably during the hydrogenation of benzene to cyclohexane by nickel dispersed within a nanoporous glass. Being defined as the ratio between the reaction rate in the presence of and without diffusion limitation, the effectiveness factors of catalytic reactions were previously determined by deliberately varying the extent of transport limitation by changing a suitably chosen system parameter, such as the particle size and by comparison of the respective reaction rates. With the novel options of microimaging, effectiveness factors become accessible in a single measurement by simply monitoring the distribution of the reactant molecules over the catalyst particles.

18.
Chemphyschem ; 16(1): 24-51, 2015 Jan 12.
Article in English | MEDLINE | ID: mdl-25123096

ABSTRACT

Diffusion, that is, the irregular movement of atoms and molecules, is a universal phenomenon of mass transfer occurring in all states of matter. It is of equal importance for fundamental research and technological applications. The present review deals with the challenges of the reliable observation of these phenomena in nanoporous materials. Starting with a survey of the different variants of diffusion measurement, it highlights the potentials of "microscopic" techniques, notably the pulsed field gradient (PFG) technique of NMR and the techniques of microimaging by interference microscopy (IFM) and IR microscopy (IRM). Considering ensembles of guest molecules, these techniques are able to directly record mass transfer phenomena over distances of typically micrometers. Their concerted application has given rise to the clarification of long-standing discrepancies, notably between microscopic equilibrium and macroscopic non-equilibrium measurements, and to a wealth of new information about molecular transport under confinement, hitherto often inaccessible and sometimes even unimaginable.

19.
Chemistry ; 20(29): 8862-6, 2014 Jul 14.
Article in English | MEDLINE | ID: mdl-24939723

ABSTRACT

The development of water-mediated proton-conducting materials operating above 100 °C remains challenging because the extended structures of existing materials usually deteriorate at high temperatures. A new triazolyl phosphonate metal-organic framework (MOF) [La3L4(H2O)6]Cl⋅x H2O (1, L(2-) = 4-(4H-1,2,4-triazol-4-yl)phenyl phosphonate) with highly hydrophilic 1D channels was synthesized hydrothermally. Compound 1 is an example of a phosphonate MOF with large regular pores with 1.9 nm in diameter. It forms a water-stable, porous structure that can be reversibly hydrated and dehydrated. The proton-conducting properties of 1 were investigated by impedance spectroscopy. Magic-angle spinning (MAS) and pulse field gradient (PFG) NMR spectroscopies confirm the dynamic nature of the incorporated water molecules. The diffusivities, determined by PFG NMR and IR microscopy, were found to be close to that of liquid water. This porous framework accomplishes the challenges of water stability and proton conduction even at 110 °C. The conductivity in 1 is proposed to occur by the vehicle mechanism.

20.
Nat Mater ; 13(4): 333-43, 2014 Apr.
Article in English | MEDLINE | ID: mdl-24651427

ABSTRACT

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.

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