Jörg Kärger

Publications of Jörg Kärger

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

  Authors: Florian Hibbe, Jürgen Caro, Christian Chmelik, Aisheng Huang, Tom Kirchner, Douglas M Ruthven, Rustem Valiullin, Jörg Kärger

  Journal of the American Chemical Society. 04/2012;

  Micro-Imaging is employed to monitor the evolution of intracrystalline guest profiles during molecular ad- and desorption in cation-free zeolites AlPO-LTA. The measurements are shown to provideMicro-Imaging is employed to monitor the evolution of intracrystalline guest profiles during molecular ad- and desorption in cation-free zeolites AlPO-LTA. The measurements are shown to provide direct evidence on the rate of intracrystalline diffusion and surface permeation and their interrelation. 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.

• Intracrystalline Diffusion in Mesoporous Zeolites.

  Authors: Dirk Mehlhorn, Rustem Valiullin, Jörg Kärger, Kanghee Cho, Ryong Ryoo

  Chemphyschem : a European journal of chemical physics and physical chemistry. 03/2012;

  Specially synthesized extra-large crystallites of zeolite LTA with intentionally added mesoporosity are used for an in-depth study of guest diffusion in hierarchical nanoporous materials by theSpecially synthesized extra-large crystallites of zeolite LTA with intentionally added mesoporosity are used for an in-depth study of guest diffusion in hierarchical nanoporous materials by the pulsed field gradient NMR technique. Using propane as a guest molecule, intracrystalline mass transfer is demonstrated to be adequately described by a single effective diffusivity resulting from the weighted average of the diffusivities in the two (micro- and meso-) pore spaces. Gas-kinetic order-of-magnitude estimates of the diffusivities are in satisfactory agreement with the experimental data and are thus shown to provide a straightforward means for predicting and quantifying the benefit of hierarchically structured nanoporous materials in comparison with their purely microporous equivalent.

• Single-particle and ensemble diffusivities--test of ergodicity.

  Authors: Florian Feil, Sergej Naumov, Jens Michaelis, Rustem Valiullin, Dirk Enke, Jörg Kärger, Christoph Bräuchle

  Angewandte Chemie (International ed. in English). 01/2012; 51(5):1152-5.

• Single-Particle and Ensemble Diffusivities-Test of Ergodicity.

  Authors: Florian Feil, Sergej Naumov, Jens Michaelis, Rustem Valiullin, Dirk Enke, Jörg Kärger, Christoph Bräuchle

  Angewandte Chemie (International ed. in English). 12/2011;

  Coincidence between the mean squared displacements determined for the same molecule over sequential time intervals and for all molecules over the same time interval follows as a consequence of theCoincidence between the mean squared displacements determined for the same molecule over sequential time intervals and for all molecules over the same time interval follows as a consequence of the ergodic theorem. In their Communication (DOI: 10.1002/anie.201105388), C. Bräuchle et al. confirm experimentally this fundamental principle of thermodynamics by diffusion studies using fluorescence microscopy and NMR ensemble measurements with pulsed field gradients.

• How hydrogen bonds influence the mobility of imidazolium-based ionic liquids. A combined theoretical and experimental study of 1-n-butyl-3-methylimidazolium bromide.

  Authors: Miriam Kohagen, Martin Brehm, Yves Lingscheid, Ralf Giernoth, Joshua Sangoro, Friedrich Kremer, Sergej Naumov, Ciprian Iacob, Jörg Kärger, Rustem Valiullin, Barbara Kirchner

  The journal of physical chemistry. B. 11/2011; 115(51):15280-8.

  The virtual laboratory allows for computer experiments that are not accessible via real experiments. In this work, three previously obtained charge sets were employed to study the influence ofThe virtual laboratory allows for computer experiments that are not accessible via real experiments. In this work, three previously obtained charge sets were employed to study the influence of hydrogen bonding on imidazolium-based ionic liquids in molecular dynamics simulations. One set provides diffusion coefficients in agreement with the experiment and is therefore a good model for real-world systems. Comparison with the other sets indicates hydrogen bonding to influence structure and dynamics differently. Furthermore, in one case the total charge was increased and in another decreased by 0.1 e. Both the most acidic proton as well as the corresponding carbon atom were artificially set to zero, sequentially and simultaneously. In the final setup a negative charge was placed on the proton in order to introduce a barrier for the anion to contact the cation via this most acidic hydrogen atom. The following observations were made: changing the hydrogen bonding ability strongly influences the structure while the dynamic properties, such as diffusion and viscosity, are only weakly changed. However, the introduction of larger alterations (stronger hydrogen bonding and antihydrogen bonding) also strongly influences the diffusion coefficients. The dynamics of the hydrogen bond, ion pairing, and the ion cage are all affected by the level of hydrogen bonding. A change in total charges predominantly influences transport properties rather than structure. For ion cage dynamics with respect to transport porperties, we find a good correlation and a weak or no correlation for the ion pair or the hydrogen bond dynamics, respectively. Nevertheless, the hydrogen bond does influence ion cage dynamics. Therefore, we confirm that ionic liquids rather consist of loosely interacting counterions than of discrete ion pairs. Hydrogen bonding affects the properties only in a secondary or indirect manner.

• How to compare diffusion processes assessed by single-particle tracking and pulsed field gradient nuclear magnetic resonance.

  Authors: Michael Bauer, Rustem Valiullin, Günter Radons, Jörg Kärger

  The Journal of chemical physics. 10/2011; 135(14):144118.

  Heterogeneous diffusion processes occur in many different fields such as transport in living cells or diffusion in porous media. A characterization of the transport parameters of such processes canHeterogeneous diffusion processes occur in many different fields such as transport in living cells or diffusion in porous media. A characterization of the transport parameters of such processes can be achieved by ensemble-based methods, such as pulsed field gradient nuclear magnetic resonance (PFG NMR), or by trajectory-based methods obtained from single-particle tracking (SPT) experiments. In this paper, we study the general relationship between both methods and its application to heterogeneous systems. We derive analytical expressions for the distribution of diffusivities from SPT and further relate it to NMR spin-echo diffusion attenuation functions. To exemplify the applicability of this approach, we employ a well-established two-region exchange model, which has widely been used in the context of PFG NMR studies of multiphase systems subjected to interphase molecular exchange processes. This type of systems, which can also describe a layered liquid with layer-dependent self-diffusion coefficients, has also recently gained attention in SPT experiments. We reformulate the results of the two-region exchange model in terms of SPT-observables and compare its predictions to that obtained using the exact transformation which we derived.

• Paramagnetic relaxation enhancement (PRE) as a tool for probing diffusion in environmentally relevant porous media.

  Authors: Filipe Furtado, Petrik Galvosas, Frank Stallmach, Ulf Roland, Jörg Kärger, Frank-Dieter Kopinke

  Environmental science & technology. 08/2011; 45(20):8866-72.

  The transport diffusivity of the paramagnetic molecule 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was measured by monitoring its influence on the NMR transverse relaxation time (T₂) on surroundingThe transport diffusivity of the paramagnetic molecule 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was measured by monitoring its influence on the NMR transverse relaxation time (T₂) on surrounding water protons - also known as paramagnetic relaxation enhancement (PRE). Due to the nature of the PRE effect, few paramagnetic molecules are able to simultaneously reduce the T₂ of many NMR active nuclei, which represents a significant gain in sensitivity. In an aqueous solution, the minimal detectable TEMPO concentration was around 70 ppm. The value of the diffusivity was estimated by fitting the relaxation data, collected as a function of time, with the appropriate solutions of the second Fick's law in respect to the corresponding sample geometry and dimensions. Considering the experimentally determined TEMPO relaxivity in water ("TEMPO-water relaxivity"; R(TEMPO) = (1.05 ± 0.12) × 10⁻³ ppm⁻¹ s⁻¹), the obtained diffusion coefficients (D) of TEMPO in homogeneous solution and in a water saturated sand column (D(bulk) = (6.7 ± 0.4) × 10⁻¹⁰ m² s⁻¹ and D(sand) = (1.4 ± 0.5) × 10⁻¹⁰ m² s⁻¹, respectively) are in good agreement with the expected values (literature values: D(bulk) = 6.6 × 10⁻¹⁰ m² s⁻¹, 1.3 × 10⁻¹⁰ m² s⁻¹ < D(sand) < 2.3 × 10⁻¹⁰ m² s⁻¹). This new approach enables one to determine the diffusivity of paramagnetic molecules in homogeneous (aqueous solution) and porous media with basic NMR equipment, at low concentrations and in a noninvasive manner.

• Nanoporous glass as a model system for a consistency check of the different techniques of diffusion measurement.

  Authors: Christian Chmelik, Dirk Enke, Petrik Galvosas, Oliver Gobin, Andreas Jentys, Hervé Jobic, Jörg Kärger, Cordula B Krause, Jens Kullmann, Johannes Lercher, Sergej Naumov, Douglas M Ruthven, Tobias Titze

  Chemphyschem : a European journal of chemical physics and physical chemistry. 03/2011; 12(6):1130-4.

  The remarkable differences in the guest diffusivities in nanoporous materials commonly found with the application of different measuring techniques are usually ascribed to the existence of aThe remarkable differences in the guest diffusivities in nanoporous materials commonly found with the application of different measuring techniques are usually ascribed to the existence of a hierarchy of transport resistances in addition to the diffusional resistance of the pore system and their differing influence due to the differing diffusion path lengths covered by the different measuring techniques. We report diffusion measurements with nanoporous glasses where the existence of such resistances could be avoided. Molecular propagation over diffusion path lengths from hundreds of nanometers up to millimeters was thus found to be controlled by a uniform mechanism, appearing in coinciding results of microscopic and macroscopic diffusion measurement.

• Correlating surface permeability with intracrystalline diffusivity in nanoporous solids.

  Authors: Lars Heinke, Jörg Kärger

  Physical review letters. 02/2011; 106(7):074501.

  The rates of uptake and release of guest molecules in nanoporous solids are often strongly influenced or even controlled by transport resistances at the external surface ("surface barriers") ratherThe rates of uptake and release of guest molecules in nanoporous solids are often strongly influenced or even controlled by transport resistances at the external surface ("surface barriers") rather than by intraparticle diffusion, which was assumed to be rate controlling in many of the earlier kinetic studies. By correlating the surface resistance with the intracrystalline diffusivity, we develop here a microkinetic model which closely reproduces the experimentally observed results for short-chain alkanes in Zn(tbip), a member of the novel metal-organic framework family of nanoporous materials. It seems likely that this mechanism, which is shown to provide a rational explanation of the commonly observed discrepancies between "macro" and "micro" measurements of intracrystalline diffusion, may be fairly general.

• Guest diffusion in interpenetrating networks of micro- and mesopores.

  Authors: Filipe Furtado, Petrik Galvosas, Maraisa Gonçalves, Frank-Dieter Kopinke, Sergej Naumov, Francisco Rodríguez-Reinoso, Ulf Roland, Rustem Valiullin, Jörg Kärger

  Journal of the American Chemical Society. 02/2011; 133(8):2437-43.

  Pulsed field gradient NMR is applied for monitoring the diffusion properties of guest molecules in hierarchical pore systems after pressure variation in the external atmosphere. Following previousPulsed field gradient NMR is applied for monitoring the diffusion properties of guest molecules in hierarchical pore systems after pressure variation in the external atmosphere. Following previous studies with purely mesoporous solids, also in the material containing both micro- and mesopores (activated carbon MA2), the diffusivity of the guest molecules (cyclohexane) is found to be most decisively determined by the sample "history": at a given external pressure, diffusivities are always found to be larger if they are measured after pressure decrease (i.e., on the "desorption" branch) rather than after pressure increase (adsorption branch). Simple model consideration reproduces the order of magnitude of the measured diffusivities as well as the tendencies in their relation to each other and their concentration dependence.

• The nature of surface barriers on nanoporous solids explored by microimaging of transient guest distributions.

  Authors: Florian Hibbe, Christian Chmelik, Lars Heinke, Sanhita Pramanik, Jing Li, Douglas M Ruthven, Despina Tzoulaki, Jörg Kärger

  Journal of the American Chemical Society. 02/2011; 133(9):2804-7.

  Nanoporous solids are attractive materials for energetically efficient and environmentally friendly catalytic and adsorption separation processes. Although the performance of such materials isNanoporous 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.

• Tracing pore-space heterogeneities in X-type zeolites by diffusion studies.

  Authors: Ziad Adem, Jürgen Caro, Filipe Furtado, Petrik Galvosas, Cordula B Krause, Jörg Kärger

  Langmuir : the ACS journal of surfaces and colloids. 01/2011; 27(1):416-9.

  Pore-space homogeneity of zeolite NaX was probed by pulsed field gradient (PFG) NMR diffusion studies with n-butane as a guest molecule. At a loading of 0.75 molecules per supercage, a wide spectrumPore-space homogeneity of zeolite NaX was probed by pulsed field gradient (PFG) NMR diffusion studies with n-butane as a guest molecule. At a loading of 0.75 molecules per supercage, a wide spectrum of diffusivities was observed. Guest molecules in the (well-shaped) zeolite crystallites were thus found to experience pore spaces of quite different properties. After loading enhancement to 3 molecules per supercage, however, molecular propagation ideally followed the laws of normal diffusion in homogeneous media. At sufficiently high guest concentrations, sample heterogeneity was thus found to be of no perceptible influence on the guest mobilities anymore.

• Charge transport and diffusion of ionic liquids in nanoporous silica membranes.

  Authors: Ciprian Iacob, Joshua Rume Sangoro, Periklis Papadopoulos, Tilman Schubert, Sergej Naumov, Rustem Valiullin, Jörg Kärger, Friedrich Kremer

  Physical chemistry chemical physics : PCCP. 11/2010; 12(41):13798-803.

  Charge transport in 1-hexyl-3-methylimidazolium hexafluorophosphate ionic liquid in oxidized nanoporous silicon membranes is investigated in a wide frequency and temperature range by a combination ofCharge transport in 1-hexyl-3-methylimidazolium hexafluorophosphate ionic liquid in oxidized nanoporous silicon membranes is investigated in a wide frequency and temperature range by a combination of Broadband Dielectric Spectroscopy (BDS) and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). By applying the Einstein-Smoluchowski relations to the dielectric spectra, diffusion coefficient is obtained in quantitative agreement with independent PFG NMR measurements. More than 10-fold systematic decrease in the effective diffusion coefficient from the bulk value is observed in hydrophilic silica nanopores. A model assuming a reduced mobility at the pore-matrix interface is shown to provide a quantitative explanation for the remarkable decrease of effective transport quantities (such as diffusion coefficient, dc conductivity and consequently, the dielectric loss) of the ionic liquid in non-silanized membranes. This approach is supported by the observation that silanization of porous silica membranes results in a significant increase of the effective diffusion coefficient, which approaches the value for the bulk liquid.

• Entropy-driven enhanced self-diffusion in confined reentrant supernematics.

  Authors: Marco G Mazza, Manuel Greschek, Rustem Valiullin, Jörg Kärger, Martin Schoen

  Physical review letters. 11/2010; 105(22):227802.

  We present a molecular dynamics study of reentrant nematic phases using the Gay-Berne-Kihara model of a liquid crystal in nanoconfinement. At densities above those characteristic of smectic A phases,We present a molecular dynamics study of reentrant nematic phases using the Gay-Berne-Kihara model of a liquid crystal in nanoconfinement. At densities above those characteristic of smectic A phases, reentrant nematic phases form that are characterized by a large value of the nematic order parameter S≃1. Along the nematic director these "supernematic" phases exhibit a remarkably high self-diffusivity, which exceeds that for ordinary, lower-density nematic phases by an order of magnitude. Enhancement of self-diffusivity is attributed to a decrease of rotational configurational entropy in confinement. Recent developments in the pulsed field gradient NMR technique are shown to provide favorable conditions for an experimental confirmation of our simulations.

• In situ study on molecular diffusion phenomena in nanoporous catalytic solids.

  Authors: Christian Chmelik, Jörg Kärger

  Chemical Society reviews. 10/2010; 39(12):4864-84.

  As an omnipresent phenomenon in nature, diffusion is among the rate-determining processes in many technological processes. This is in particular true for catalytic conversion in nanoporous materials.As an omnipresent phenomenon in nature, diffusion is among the rate-determining processes in many technological processes. This is in particular true for catalytic conversion in nanoporous materials. We provide a critical review of the possibilities of exploring diffusion phenomena over microscopic dimensions in such media by direct experimental observation. By monitoring the probability distribution of molecular displacements as a function of time, the pulsed field gradient technique of NMR (PFG NMR) records the rate of molecular re-distribution. By varying the observation time, PFG NMR is thus able to trace even hierarchies of transport resistances as occurring, e.g., in catalyst particles in the form of binder-compacted assemblages of zeolite crystallites. Alternatively, and complementary to this information, interference microscopy (IFM) and IR microscopy (IRM) are able to follow the evolution of intracrystalline concentration profiles during uptake and release. This allows, in particular, an accurate quantification of the transport resistances on the surface of the individual crystallites and of the probability that reactant molecules from the gas phase, upon colliding with the external surface, are able to penetrate through such "surface barriers" into the crystal bulk phase. Being able to distinguish between different molecular species, IRM is able to record the evolution of intracrystalline concentration profiles even during multi-component adsorption and catalytic reactions (169 references).

• Assessing molecular transport properties of nanoporous materials by interference microscopy: remarkable effects of composition and microstructure on diffusion in the silicoaluminophosphate zeotype STA-7.

  Authors: Despina Tzoulaki, Lars Heinke, Maria Castro, Pablo Cubillas, Michael W Anderson, Wuzong Zhou, Paul A Wright, Jörg Kärger

  Journal of the American Chemical Society. 08/2010; 132(33):11665-70.

  The influence of the chemical composition and of the storage and activation protocol on the diffusion of methanol into strongly chemically zoned crystals of the silicoaluminophosphate zeotype STA-7The influence of the chemical composition and of the storage and activation protocol on the diffusion of methanol into strongly chemically zoned crystals of the silicoaluminophosphate zeotype STA-7 has been investigated by interference microscopy. Analysis of the evolution of transient intracrystalline concentration profiles reveals that just-calcined SAPO STA-7 crystals with lower Si content (Si/(Si + P) = 0.18) exhibit higher surface permeability and bulk diffusivity than those with higher Si content (S/(Si + P) = 0.37). Remarkably, crystals with the higher Si content which were stored in the calcined form crack during activation along planes of weakness already present in the as-prepared crystals, creating fresh surfaces through regions of lower Si that are much more easily penetrated by the adsorbing methanol than are the original surfaces.

• Comment on "Single-file diffusion of confined water inside SWNTs: an NMR study".

  Authors: Rustem Valiullin, Jörg Kärger

  ACS nano. 07/2010; 4(7):3537; author reply 3537-8.

• Self-assembly and diffusion of block copolymer templates in SBA-15 nanochannels.

  Authors: Konstantin Ulrich, Petrik Galvosas, Jörg Kärger, Farida Grinberg, Jarian Vernimmen, Vera Meynen, Pegie Cool

  The journal of physical chemistry. B. 03/2010; 114(12):4223-9.

  Molecular dynamics of triblock copolymers under confinement by the nanochannels of SBA-15 was investigated using pulsed field gradient (PFG) NMR spectroscopy with high-intensity field gradientMolecular dynamics of triblock copolymers under confinement by the nanochannels of SBA-15 was investigated using pulsed field gradient (PFG) NMR spectroscopy with high-intensity field gradient pulses. The mesoporous material SBA-15 was synthesized using the surfactant Pluronic P123 (EO(20)-PO(70)-EO(20)). The diffusion of P123 in mixtures with water was studied both in bulk and under the condition of confinement by the mesoporous channels of SBA-15. As a result, at room temperature the diffusion of P123 in SBA-15 is more than a factor three smaller than that of the same polymer in the bulk mixture with water. A pronounced atypical temperature dependence of the measured diffusivities was observed both in the confined and in the bulk systems. This atypical temperature dependence was attributed to the transition from the aggregated state to the molecular solution and gave an evidence for a qualitative similarity of the supermolecular organization of the Pluronic/water mixtures under confinements and in the bulk. Confinements were shown to produce significant effects on diffusion properties of Pluronic molecules. The diffusivity of self-associates forming up at room temperature was considerably diminished in comparison to the bulk systems. In contrast, at low temperatures diffusion of the individually dissolved molecules was comparably fast, but subjected to anisotropy induced by channels.

• Mass transfer in a nanoscale material enhanced by an opposing flux.

  Authors: Christian Chmelik, Helge Bux, Jürgen Caro, Lars Heinke, Florian Hibbe, Tobias Titze, Jörg Kärger

  Physical review letters. 02/2010; 104(8):085902.

  Diffusion is known to be quantified by measuring the rate of molecular fluxes in the direction of falling concentration. In contrast with intuition, considering methanol diffusion in a novel type ofDiffusion is known to be quantified by measuring the rate of molecular fluxes in the direction of falling concentration. In contrast with intuition, considering methanol diffusion in a novel type of nanoporous material (MOF ZIF-8), this rate has now been found to be enhanced rather than slowed down by an opposing flux of labeled molecules. In terms of the key quantities of random particle movement, this result means that the self-diffusivity exceeds the transport diffusivity. It is rationalized by considering the strong intermolecular interaction and the dominating role of intercage hopping in mass transfer in the systems under study.

• New Option for Characterizing the Mobility of Organic Compounds in Humic Acids.

  Authors: Khanneh Wadinga Fomba, Petrik Galvosas, Ulf Roland, Jörg Kärger, Frank-Dieter Kopinke

  Environmental science & technology. 11/2009; 43(21):8264-8269.

  A new NMR option for monitoring the mobility of organic contaminants in SOM in the solid state has been successfully applied for the first time. This recently available noninvasive technique, magicA new NMR option for monitoring the mobility of organic contaminants in SOM in the solid state has been successfully applied for the first time. This recently available noninvasive technique, magic angle spinning pulsed-field gradient (MAS PFG) NMR, combines both NMR spectroscopy and diffusometry to selectively monitor the diffusion of compounds sorbed in porous media or polymer matrices. Using this technique, the diffusion of toluene in humic acid particles has been studied. Measurements were performed under varying temperatures from 25 to 80 degrees C. The obtained diffusion coefficients were found to be in good agreement with those obtained from computer simulations reported elsewhere. Our results show a strong influence of the interaction of toluene with humic acid on its diffusion in the matrix even at elevated temperatures of up to 80 degrees C. The Arrhenius plot of the diffusivities shows a decrease in the activation energy of diffusion above 50 degrees C by a factor of 3. This change of activation energy is attributed to a structural change in the humic acid matrix that influences the mobility of toluene.