Siegfried Stapf

Molecular dynamics in confined monomolecular layers. A field-cycling nuclear magnetic resonance relaxometry study of liquids in porous glass

Liquids filled in porous media such as porous glass do not freeze at the bulk freezing temperatures. Two phases must be distinguished. A one to at most two monolayer thick film adsorbed on the inner surfaces does not freeze at all, whereas free liquid within the pores freezes at reduced temperatures relative to the bulk values as predicted by the Gibbs/Thompson equation. The fraction of non‐freezing liquid can be evaluated from the reduction factor of the low‐frequency spin‐lattice relaxation time upon freezing of the free liquid. A method for the determination of the pore size may be established on this basis. Water and tetradecane, i.e., a polar and a nonpolar adsorbate, filled in porous glass have been studied with the aid of field‐cycling nuclear magnetic resonance (NMR) relaxometry. Above the freezing range the frequency dependences of the spin‐lattice relaxation time T1 of the two liquids strongly deviate from each other owing to the different adsorption properties. On the other hand, with frozen sa...

Deactivation and regeneration of a naphtha reforming catalyst

Abstract A series of naphtha reforming catalysts from different stages of the deactivation (coking) and the regeneration (decoking) processes were investigated by NMR and chemical engineering methods. The dependence of the tortuosity on the coke content was determined for both processes by NMR measurements of the intraparticle self-diffusion coefficients of adsorbed liquid n -heptane. The shrinkage of the accessible pore volume as a function of increasing coke content due to the deactivation process is compared to nitrogen adsorption (BET) measurements which show an equivalent behavior. A crude model was adapted to predict qualitatively the relationship between the tortuosity and the average pore diameter. Longitudinal ( T 1 ) and transverse ( T 2 ) NMR relaxation times measured for protons of adsorbed liquid n -heptane, provide information on the pore morphology changes which can be corroborated by the tortuosity measurements. The chemical composition of the coke layer, which was investigated by 1 H magic angle spinning (MAS) and 13 C cross polarization (CP)/MAS NMR spectroscopy, is shown to change during both deactivation and decoking processes. Moreover, the micro-structure of the fresh catalyst and the fully regenerated catalyst was investigated by scanning electron microscopy (SEM). The experimental results indicate that a full recovery of the activity of the clean catalyst is not achieved by the regeneration process, and that the quality of regeneration depends on the coke content reached during the deactivation/regeneration cycle.

The characterisation of fluid transport in porous solids by means of pulsed magnetic field gradient NMR

The determination, by pulsed field gradient (PFG) nuclear magnetic resonance (NMR), of the probability distributions (propagators) of displacements for fluids undergoing transport by both flow and self-diffusion within porous solids is outlined. The nature of the observed propagator, P delta (Z), for the transport of a single aqueous phase through an outcrop sandstone (Fontainebleau) is described. Recent measurements of the propagators for both aqueous and oil phase flow in the limiting saturation states of irreducible water (Swi) and residual oil (Sor) in the same sample are illustrated through the use of difference propagators. These are shown to emphasise the regions of the propagators most affected by the presence of the second, stationary, phases in these limiting saturation conditions. Measurement of the propagators for both oil and aqueous phases undergoing simultaneous flow are also described for the same sandstone sample and the effect of increasing Swi on the nature of the oil flow is briefly discussed. Finally, a new two-dimensional (2-D) experiment is introduced which measures the propagator P delta (X, Z). This is the joint probability for displacements X and Z in time delta. Some preliminary observations of these two-dimensional propagators are shown for single-phase flow in the Fontainebleau sandstone sample, where Z and X are, respectively, displacements in the axial and radial direction for the cylindrical sample for which the pressure gradient is along Z and where bulk radial flow is constrained to be zero.

Characterization of unsaturated porous media by high‐field and low‐field NMR relaxometry

[1] A comparison study of nuclear magnetic resonance relaxometry at high and low magnetic field (7 and 0.1 T) has been initiated for investigating the influence of the magnetic field strength, variable clay content, and different degrees of saturation on the relaxometric properties of four ideal porous media. The samples consisted of medium sand with increasing fractions of kaolin clay ranging from 0 to 15%. Six different volumetric water contents between saturation and θ = 0.05 were used. Changes in water content of the samples were achieved by slow evaporation. T 2 relaxation curves were monitored by the Carr-Purcell-Meiboom-Gill sequence and were further analyzed by inverse Laplace transformation, yielding T 2 distribution functions. Sand shows a slight continuous shift with decreasing water content of a bimodal distribution function of T 2 to faster relaxation at high and low magnetic field. Sand-clay mixtures show broad, bimodal distribution functions for both magnetic field intensities which shift slightly with decreasing water content. Signal amplitude behavior with variation of saturation degree was also monitored. An expected proportionality of the total signal amplitude with water content was observed for all samples at 0.1 T, whereas at 7 T deviations occurred for samples with a clay content higher than 5%, which are assigned to loss of signal in the first echo periods. The relaxivity in unsaturated clay-based porous media is mostly surface dominated, as the weak and comparable dependence of 1/T 2 on T E at both field strengths shows. Nevertheless, for a reliable determination of water content in mixed systems with varying texture and saturation the employment of multiecho sequences at low magnetic field strength are preferable.

Heterogeneities in gelatin film formation using single-sided NMR.

Gelatin solutions were prepared in D(2)O. The drying process of cast solutions was followed with a single-sided nuclear magnetic resonance (NMR) scanner until complete solidification occurred. Spin-spin relaxation times (T(2)) were measured at different layers with microscopic resolution and were correlated with the drying process during film formation. Additionally, the evaporation of the gelatin solution was observed optically from the reduction of the sample thickness, revealing that at the macroscopic level, the rate of evaporation is not uniform throughout the experiment. A crossover in the spatial evolution of the drying process is observed from the NMR results. At the early stages, the gel appears to be drier in the upper layers near the evaporation front, while this tendency is inverted at the later stages, when drying is faster from the bottom. XRD (X-ray diffraction) data showed that a structural heterogeneity persists in the final film.

Features of polymer chain dynamics as revealed by intermolecular nuclear magnetic dipole-dipole interaction: Model calculations and field-cycling NMR relaxometry

Proton NMR phenomena such as spin-lattice relaxation, free-induction decays, and solid echoes are analyzed with respect to contributions by intermolecular dipole-dipole interactions in polymer melts. The intermolecular dipole-dipole correlation function is calculated by taking into account the correlation hole effect characteristic for polymer melts. It is shown that the ratio between the intra- and intermolecular contributions to NMR measurands depends on the degree of isotropy of chain dynamics anticipated in different models. This, in particular, refers to the tube/reptation model that is intrinsically anisotropic in clear contrast to n-renormalized Rouse models, where no such restriction is implied. Due to anisotropy, the tube/reptation model predicts that the intramolecular contribution to the dipole-dipole correlation function increases with time relative to the intermolecular contribution. Therefore, the intramolecular contribution is expected to dominate NMR measurands by tendency at long times (or low frequencies). On the other hand, the isotropic nature of the n-renormalized Rouse model suggests that the intermolecular contribution tends to prevail on long-time scales (or low frequencies). Actually, theoretical estimations and the analysis of experimental spin-lattice relaxation data indicate that the intermolecular contribution to proton NMR measurands is no longer negligible for times longer than 10(-7) s-10(-6) s corresponding to frequencies below the megahertz regime. Interpretations not taking this fact into account need to be reconsidered. The systematic investigation of intermolecular interactions in long-time/low frequency proton NMR promises the revelation of the dynamic features of segment displacements relative to each other in polymer melts.

In situ observation of diffusion and reaction dynamics in gel microreactors by chemically resolved NMR microscopy

The enzymatically catalyzed esterification reaction of propionic acid+1-butanol ⇆ propionic-acid-butyl-ester+water inside an immobilizing hydrogel environment has been investigated by means of spectroscopically resolved nuclear magnetic resonance (NMR) imaging. The alginate hydrogel was in the shape of a 3–4 mm diameter bead, with both the gel-forming water and the solvent (cyclohexane) being fully deuterated to simplify the identification of small amounts of reactants. In the absence of enzymes, the self-diffusion process of the separate reactants (propionic acid and butanol) proved to be severely slowed down compared to pure Fickian self-diffusion, and the concentration buildup inside the gel bead was shown to depend strongly on the properties of the reactants. Two-dimensional, non-chemically resolved images revealed that the diffusion process is not radially symmetric as expected, thus complicating the modelling of the diffusion and reaction kinetics. The reaction itself has been observed with chemical resolution in a time series of up to 40 h, clearly demonstrating the reduction of 1-butanol and production of water inside the gel bead.

Microstructure of porous media probed by NMR techniques in sub-micrometer length scales

It is shown that field-cycling NMR relaxation spectroscopy in combination with pulsed-gradient spin-echo diffusion studies especially in the supercon fringe field version are suitable techniques for the investigation of length scales of porous media in the range 10 A to 10 microns. Data for water adsorbed in fine particle agglomerates, porous glass and ceramics are reported. An orientational structure factor is introduced permitting the characterization of hydrated surfaces on the basis of reorientations mediated by translational displacements of the adsorbed molecules. Known lengths such as the mean pore or particle size have been reproduced in this way. In length scales beyond these structural elements, the geometry of the internal surfaces can be discussed in terms of wavenumber-space fractals.

Monitoring of fluid motion in a micromixer by dynamic NMR microscopy

The velocity distribution of liquid flowing in a commercial micromixer has been determined directly by using pulsed-field gradient NMR. Velocity maps with a spatial resolution of 29 microm x 43 microm were obtained by combining standard imaging gradient units with a homebuilt rectangular surface coil matching the mixer geometry. The technique provides access to mixers and reactors of arbitrary shape regardless of optical transparency. Local heterogeneities in the signal intensity and the velocity pattern were found and serve to investigate the quality and functionality of a micromixer, revealing clogging and inhomogeneous flow distributions.

Translational mobility in surface induced liquid layers investigated by NMR diffusometry

Abstract The self-diffusion properties of plastic succinonitrile and cyclohexane confined in nanoporous glass were investigated by means of NMR field-gradient diffusometry. The results are compared to diffusion measurements in the bulk as well as to the relaxational behaviour of other confined plastically crystalline substances. For all adsorbates, bed low the freezing point a thin layer near the glass surface is found to retain a high translational mobility as compared to the plastic phase in the center of the pore. The behaviour of the unfrozen layer can be described by liquid-like diffusion inside a network of channels with temperature-dependent width assuming fast molecular exchange with the plastic phase.