Hervé Jobic

Publications of Hervé Jobic

• 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.

• 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.

• Molecular Dynamics of n-Octane Inside Zeolite ZSM-5 As Studied by Deuterium Solid-State NMR and Quasi-Elastic Neutron Scattering

  Authors: Alexander G Stepanov, Alexander A Shubin, Mikhail V Luzgin, Hervé Jobic, Alain Tuel

  The Journal of Physical Chemistry B. 05/1998; 102(52):10860--10870.

  The dynamics of a linear alkane, n-octane, adsorbed in zeolite ZSM-5 was studied using deuterium solid-state NMR (2 H NMR) and quasi-elastic neutron scattering (QENS). It has been found that at theThe dynamics of a linear alkane, n-octane, adsorbed in zeolite ZSM-5 was studied using deuterium solid-state NMR (2 H NMR) and quasi-elastic neutron scattering (QENS). It has been found that at the loading of 1.8 molecules per unit cell, adsorbed n-octane molecules are essentially located in the straight channels and diffuse along the direction of the straight channels with a diffusion coefficient D) 12.0 × 10 -11 m 2 /s at 300 K. In the course of translational movement along the straight channels, some coupled rotational motions of all CH n -(n) 2, 3) groups of the hydrocarbon skeleton of the molecule take place. They are reflected in the 2 H NMR spectrum of deuterated n-octane-d 18 , in the temperature range 253-373 K, as fast rotations of the separate methylene and methyl groups simultaneously around two and three C-C bonds of the molecule with a characteristic time τ C ≈ 10 -11 s and an activation energy E R ≈ 10-12 kJ/mol. These internal motions may correspond to fast interconversion between trans and gauche conformations in the adsorbed alkane molecule while the molecule moves along the straight channels. Upon heating at 373 K for 1 h, n-octane molecules, formerly located in the straight channels, become redistributed over straight and zigzag channels. Subsequent translational motion of n-octane consists of two independent modes of motion. One of them represents the movement along the tortuous zigzag channels. The other one represents the movement along the straight channels, disturbed by collisions with the other molecules at the channel intersections. For a loading of 3.5 molecules per unit cell, a liquidlike line shape appears at 253 K in the 2 H NMR spectrum. This line shape corresponds to isotropically reorienting n-octane molecules, changing the direction of their translational motion (from straight to zigzag channels) under collision with the other molecules at the channel intersections. Introduction Study of the dynamic behavior of alkanes sorbed by zeolites is of crucial importance for practical catalysis and of interest from a theoretical point of view. A number of important industrial processes, which involve alkanes, use zeolites either as catalysts or sorbents, e.g., in fluid catalytic cracking 1 or for hydrocarbon separation. 2 Diffusion of hydrocarbon molecules inside zeolite pores plays a critical role in the performances of zeolitic materials as catalysts and adsorbents. Therefore, it is no wonder that much work has been done to measure and characterize the diffusion of hydrocarbons in zeolites. 3,4 From the theoretical point of view, it is of particular interest to have a knowledge of the dynamic properties of long-chain hydro-carbon molecules whose dimensions are commensurable with the sizes of zeolitic pores. The information on the dynamic behavior of large molecules in constrained geometries, whose dynamics are strongly affected by both the zeolitic walls and the regular periodicity of the zeolitic pore system, should aid in elucidating the origin of shape selectivity 5 in catalytic reactions and may be the basis for the development of models for large molecule diffusion and for clarifying the mechanism of diffusion. The mobilities of hydrocarbons sorbed within zeolites are usually probed with pulsed-field gradient NMR (PFG NMR) or quasi-elastic neutron scattering (QENS) techniques. 6,7 These techniques yield information on the intracrystalline diffusion coefficients. By using neutron spectrometers with different time scales, the QENS method can be used to characterize rotational motions. 8,9 This information can also be derived from the analysis of deuterium solid-state NMR (2 H NMR) spectra of adsorbed deuterated molecules. 10-16 The line shape for 2 H NMR is defined completely by intramolecular quadrupole interaction. 17-19 An important point is that the line shape of the 2 H NMR spectrum is especially sensitive to the mode of molecular motion and its rate. 18,19 Therefore, by analyzing 2 H NMR line shapes as well as relaxation rates, one can evaluate the nature of the molecular motion and its rate. Up to now, the dynamics of long-chain linear alkanes (C 8 -C 20) in silicalite, the Al-free analogue of ZSM-5, has been probed only by theoretical methods. 20-22 However, molecular dynamics (MD) simulations of alkanes in silicalite give some idealized view of alkane motion inside the zeolite channels, which may be different from the real dynamic behavior. Therefore, for clarifying the dynamics of linear long-chain alkanes in the catalytically active ZSM-5 zeolite, experimental techniques such as 2 H NMR and QENS studies are expected to be useful. In this paper, we report the results of 2 H NMR and QENS studies of the dynamic behavior of linear n-octane inside ZSM-5 zeolite. ZSM-5 was chosen for this work because of the

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• Diffusion of ammonia in silicalite studied by QENS and PFG NMR

  Authors: Hervé Jobic, Horst Ernst, Wilfried Heink, Jörg Kärger, Alain Tuel, Marc Bée

  Microporous and Mesoporous Materials.

  Ammonia diffusivities in silicalite have been studied by quasi-elastic neutron scattering (QENS) and pulsed-field gradient (PFG) NMR for temperatures between 200 and 500 K and loadings from 1.5 up toAmmonia diffusivities in silicalite have been studied by quasi-elastic neutron scattering (QENS) and pulsed-field gradient (PFG) NMR for temperatures between 200 and 500 K and loadings from 1.5 up to 4.3 molecules per unit cell. The diffusion coefficients obtained by both techniques increase with increasing ammonia concentration. The QENS diffusivities refer to only a certain fraction of molecules, because during the time scale of the measurement the other part of molecules is essentially immobile, in interaction with silanol groups. During the much larger time scale of the PFG NMR experiment, ammonia molecules assume both states of mobility, leading to an average diffusivity which is smaller than the diffusivity of the mobile molecules recorded by QENS. The difference between the diffusivities derived from both techniques decreases when the proportion of immobile molecules is taken into account. The residence time of ammonia in interaction with silanol groups is about two orders of magnitude longer than with oxygen atoms.

• 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.