Cordula B Krause

Publications of Cordula B Krause

• 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.

• 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.

• Intracrystalline Transport Resistances in Nanoporous Zeolite X.

  Authors: Armin Feldhoff, Jürgen Caro, Hervé Jobic, Jacques Ollivier, Cordula B Krause, Petrik Galvosas, Jörg Kärger

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

  By applying pulsed-field gradient nuclear magnetic resonance (PFG NMR) in comparison to quasi-elastic neutron scattering (QENS), we sense by measurement of the diffusion of n-octane on differentBy applying pulsed-field gradient nuclear magnetic resonance (PFG NMR) in comparison to quasi-elastic neutron scattering (QENS), we sense by measurement of the diffusion of n-octane on different length scales, transport resistances in faujasite type X (which is isostructural with type Y and differs by the lower Si/Al ratio only) with mutual distances of less than 300 nm. Direct observation of the real structure of zeolite X by transmission electron microscopy identifies them as stacking faults of mirror-twin type on (111)-type planes of the cubic framework. Thus, direct experimental proof is given that, in general, nanoporous host systems such as zeolite crystals cannot be considered as a mere arrangement of cavities. It is rather the presence of structural defects that dominates their properties as soon as transport phenomena with practical relevance, including chemical conversion by heterogeneous catalysis and chemical separation by molecular sieving and selective adsorption, become relevant.

• (1)H NMR signal broadening in spectra of alkane molecules adsorbed on MFI-type zeolites.

  Authors: Ekaterina E. Romanova, Cordula B Krause, Alexander G Stepanov, Ursula Wilczok, Wolfgang Schmidt, Jasper M van Baten, Rajamani Krishna, André Pampel, Jörg Kärger, Dieter Freude

  Solid state nuclear magnetic resonance. 06/2008; 33(4):65-71.

  The anisotropic behavior of C(1)-C(6) alkane molecules adsorbed in MFI zeolite was studied by (1)H nuclear magnetic resonance (NMR) using single-pulse excitation, Carr-Purcell-Meiboom-Gill (CPMG)The anisotropic behavior of C(1)-C(6) alkane molecules adsorbed in MFI zeolite was studied by (1)H nuclear magnetic resonance (NMR) using single-pulse excitation, Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence, Hahn echo (HE) pulse sequence, and magic-angle spinning. The molecular order parameter was obtained by both static (2)H NMR spectroscopy and molecular simulations. This yields an order parameter in the range of 0.28-0.42 for linear alkanes in MFI zeolite, whereas the parameter equals zero for FAU zeolite with a cubic symmetry. Thus, in the case of a zeolite with a non-cubic symmetry like MFI, the mobility of the molecules in one crystallite cannot fully average the dipolar interaction. As a consequence, transverse nuclear magnetization as revealed in the echo attenuation notably deviates from a mono-exponential decay. This information is of particular relevance for the performance of pulsed field gradient (PFG) NMR diffusion experiments, since the occurrence of non-exponential magnetization attenuation could be taken as an indication of the existence of different molecules or of molecules in different states of mobility.

• PFG NMR and QENS diffusion study of n-alkane homologues in MFI-type zeolites

  Authors: Hervé Jobic, Wolfgang Schmidt, Cordula B. Krause, Jörg Kärger

  Microporous and Mesoporous Materials.

  Recent methodological progress in pulsed field gradient (PFG) NMR and quasi-elastic neutron scattering (QENS) is exploited to enhance the range of diffusion measurement of the series of the n-alkaneRecent methodological progress in pulsed field gradient (PFG) NMR and quasi-elastic neutron scattering (QENS) is exploited to enhance the range of diffusion measurement of the series of the n-alkane homologues in MFI-type zeolites (silicalite-1, ZSM-5) up to n-tetradecane. As observed already for the short-chain-length alkanes, the diffusivities are found to be intermediate between those for NaCaA and NaX, in agreement with the trend in the respective pore diameters. Similarly as in NaX, the diffusivities decrease monotonically with increasing chain length, in contrast to NaCaA, where recently for chain lengths above about six carbon atoms a remarkable deviation from this trend has been observed. Though agreeing in their trends, the diffusivities measured by PFG NMR are by up to one order of magnitude below the QENS data. This difference may be referred to internal diffusion barriers in the MFI-type zeolites: since PFG NMR monitors molecular displacements of typically micrometers the determined diffusivities are affected by these additional transport resistances, while these resistances are of no relevance for QENS, where much shorter displacements are recorded.

• H-1 NMR signal broadening in spectra of alkane molecules adsorbed on MFI-type zeolites

  Authors: Ekaterina E. Romanova, Cordula B Krause, Alexander G Stepanov, Ursula Wilczoke, Wolfgang Schmidt, Jasper M van Baten, Rajamani Krishna, André Pampel, Jörg Kärger, Dieter Freude

  Solid State Nuclear Magnetic Resonance, v.33, 65-71 (2008).