Christian Chmelik

Publications of Christian Chmelik

• 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.

• 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.

• 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.

• Novel MOF-membrane for molecular sieving predicted by IR-diffusion studies and molecular modeling.

  Authors: Helge Bux, Christian Chmelik, Jasper M van Baten, Rajamani Krishna, Jürgen Caro

  Advanced materials (Deerfield Beach, Fla.). 11/2010; 22(42):4741-3.

• 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).

• 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.

• Direct assessment of molecular transport in mordenite: dominance of surface resistances.

  Authors: Lei Zhang, Christian Chmelik, Adri N C van Laak, Jörg Kärger, Petra E de Jongh, Krijn P de Jong

  Chemical communications (Cambridge, England). 11/2009;

  For the first time, macroscopic and microscopic diffusion measurements were combined for a single system: n-hexane in mordenite. Using tapered element oscillating microbalance and IR microimagingFor the first time, macroscopic and microscopic diffusion measurements were combined for a single system: n-hexane in mordenite. Using tapered element oscillating microbalance and IR microimaging results, we unequivocally proved the dominant role of surface barriers for molecular transport.

• Inside Cover: Ensemble Measurement of Diffusion: Novel Beauty and Evidence (ChemPhysChem 15/2009).

  Authors: Christian Chmelik, Lars Heinke, Pavel Kortunov, Jing Li, David Olson, Despina Tzoulaki, Jens Weitkamp, Jörg Kärger

  Chemphyschem : a European journal of chemical physics and physical chemistry. 10/2009; 10(15):2550.

• Ensemble Measurement of Diffusion: Novel Beauty and Evidence.

  Authors: Christian Chmelik, Lars Heinke, Pavel Kortunov, Jing Li, David Olson, Despina Tzoulaki, Jens Weitkamp, Jörg Kärger

  Chemphyschem : a European journal of chemical physics and physical chemistry. 09/2009;

  Recording the evolution of concentration profiles in nanoporous materials opens a new field of diffusion research with particle ensembles. The technique is based on the complementary application ofRecording the evolution of concentration profiles in nanoporous materials opens a new field of diffusion research with particle ensembles. The technique is based on the complementary application of interference microscopy and IR micro-imaging. Combining the virtues of diffusion measurements with solids and fluids, it provides information of unprecedented wealth and visual power on transport phenomena in molecular ensembles. These phenomena include the diverging uptake and release patterns for concentration-dependent diffusivities, the mechanisms of mass transfer at the fluid-solid interface and opposing tendencies in local and global concentration evolution.

• Assessing Surface Permeabilities from Transient Guest Profiles in Nanoporous Host Materials.

  Authors: Despina Tzoulaki, Lars Heinke, Hyuna Lim, Jing Li, David Olson, Jürgen Caro, Rajamani Krishna, Christian Chmelik, Jörg Kärger

  Angewandte Chemie (International ed. in English). 05/2009;

• Assessing guest diffusivities in porous hosts from transient concentration profiles.

  Authors: Lars Heinke, Despina Tzoulaki, Christian Chmelik, Florian Hibbe, Jasper M van Baten, Hyuna Lim, Jing Li, Rajamani Krishna, Jörg Kärger

  Physical review letters. 03/2009; 102(6):065901.

  Using the short-chain-length alkanes from ethane to n-butane as guest molecules, transient concentration profiles during uptake or release (via interference microscopy) and tracer exchange (via IRUsing the short-chain-length alkanes from ethane to n-butane as guest molecules, transient concentration profiles during uptake or release (via interference microscopy) and tracer exchange (via IR microimaging) in Zn(tbip), a particularly stable representative of a novel family of nanoporous materials (the metal organic frameworks), were recorded. Analyzing the spatiotemporal dependence of the profiles provides immediate access to the transport diffusivities and self-diffusivities, yielding a data basis of unprecedented reliability for mass transfer in nanoporous materials. As a particular feature of the system, self- and transport diffusivities may be combined to estimate the rate of mutual passages of the guest molecules in the chains of pore segments, thus quantifying departure from a genuine single-file system.

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

  Authors: Pavel Kortunov, Lars Heinke, Sergey Vasenkov, Christian Chmelik, Dhananjai B Shah, Jörg Kärger, Rainer A Rakoczy, Yvonne Traa, Jens Weitkamp

  The journal of physical chemistry. B. 12/2006; 110(47):23821-8.

  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 developedEvolution 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. The roof-like part of the crystal was almost instantaneously filled/emptied during adsorption/desorption, indicating accessible 10-ring channels in this section. The measured profiles were analyzed microscopically with the direct application of Fick's second law, and the transport diffusivity of methanol in ferrierite was determined as a function of adsorbed phase concentration. The transport diffusivity varied by more than 2 orders of magnitude over the investigated pressure range. Transport diffusivities, calculated from measured profiles from small and large pressure step changes, were all found to be consistent. Simulated concentration profiles obtained from the solution of Fick's second law with the calculated functional dependence of diffusivities on concentration compared very well with the measured concentration profiles, indicating validity and consistency of the measured data and the calculated diffusivities. The results indicate the importance of measuring the evolution of concentration profiles as this information is vital in determining (1) the direction of internal transport, (2) the presence of internal structural defects, and (3) surface/internal transport barriers. Such detailed information is available neither from common macroscopic methods since, they measure changes in macroscopic properties and use model assumptions to predict the concentration profiles inside, nor from microscopic methods, since they only provide information on average displacement of diffusing molecules.

• Regular intergrowth in the AFI-type crystals: influence on the intracrystalline adsorbate distribution as observed by interference and FTIR-microscopy.

  Authors: Enrico Lehmann, Christian Chmelik, Holger Scheidt, Sergey Vasenkov, Brigitte Staudte, Jörg Kärger, Friedrich Kremer, Gabriela Zadrozna, Jan Kornatowski

  Journal of the American Chemical Society. 08/2002; 124(29):8690-2.

  Interference microscopy and FTIR microscopy are applied to study intracrystalline concentration profiles of methanol in CrAPO-5 zeolite crystals. By using both techniques, the high spatial resolutionInterference microscopy and FTIR microscopy are applied to study intracrystalline concentration profiles of methanol in CrAPO-5 zeolite crystals. By using both techniques, the high spatial resolution of interference microscopy is complemented by the ability of FTIR spectroscopy to pinpoint adsorbates by their characteristic IR bands. For the first time two-dimensional concentration profiles of an unprecedented quality are reported which show a nonhomogeneous distribution of adsorbate in zeolite crystal under equilibrium with the adsorbate vapor. These nonhomogeneous profiles are attributed to regular intergrowth effects in CrAPO-5. A possible internal structure of CrAPO-5 crystals is suggested.

• Exploring the nature of surface barriers on MOF Zn(tbip) by applying IR microscopy in high temporal and spatial resolution

  Authors: Christian Chmelik, Florian Hibbe, Despina Tzoulaki, Lars Heinke, Jürgen Caro, Jing Li, Jörg Kärger

  Microporous and Mesoporous Materials.

  The options of IR microscopy for monitoring the evolution of guest (propane) profiles in nanoporous materials (MOF Zn(tbip)) during transient sorption experiments are exploited in a phenomenologicalThe options of IR microscopy for monitoring the evolution of guest (propane) profiles in nanoporous materials (MOF Zn(tbip)) during transient sorption experiments are exploited in a phenomenological study of the formation and variation of transport resistances on the surface of these materials (“surface barriers”). Novel crystal faces, produced by a simple breaking of the crystals, are found to exhibit notably reduced surface resistances while, with increasing time of storage under ambient atmosphere, the surface resistances are found to be enhanced. Differences in the permeabilities on opposite crystal faces appear in a notable asymmetry in the transient concentration profiles. Symmetry is re-established in crystals with two fresh fracture faces.

• Direct assessment of molecular transport in mordenite: dominance of surface resistances

  Authors: L. Zhang, Christian Chmelik, A.N.C. van Laak, Jörg Kärger, P.E. de Jongh, K.P. de Jong

  For the first time, macroscopic and microscopic diffusion measurements were combined for a single system: n-hexane in mordenite. Using tapered element oscillating microbalance and IR microimagingFor the first time, macroscopic and microscopic diffusion measurements were combined for a single system: n-hexane in mordenite. Using tapered element oscillating microbalance and IR microimaging results, we unequivocally proved the dominant role of surface barriers for molecular transport.