Determination of the location of coke in catalysts by a novel NMR-based, liquid-porosimetry approach.
In this work, a new technique, suitable for chemically-heterogeneous materials, has been used to characterise the structural properties of porous heterogeneous catalysts. A liquid-liquid exchange (LLE) process within nanoporous catalysts has been followed using NMR relaxometry and NMR diffusometry. In order to validate the new technique, two model materials were used. First, a chemically-pure, sol-gel silica, with a simple, mono-disperse pore-space, was studied. The second model material was a bidisperse, eggshell Pt-alumina catalyst. The Pt-alumina catalyst was studied both fresh, and coked following chemical reaction. The degree of structural and chemical complexity added by coking was restricted by the localisation of the coke deposition to the Pt-eggshell layer. Under so-called 'metered' supply conditions, when a high affinity liquid (water) displaced a low affinity liquid (cyclohexane) from the sol-gel silica, entrapment of the low affinity liquid was observed which was similar to that observed in mercury porosimetry. In a similar experiment, comparing LLE in fresh and coked samples of the Pt-alumina catalyst pellets, it was found, for the fresh sample, that water initially displaced cyclohexane from a sub-set of the most accessible, smallest pores, as might expected under metered conditions, but this did not occur for coked catalysts. This finding suggested coking had removed some smaller pores located close to the surface of the pellet, in agreement with where the Pt-metal was preferentially located and coking was known to have occurred.
Probing the structure of porous pellets: an NMR study of drying.
Partially saturated porous catalyst support pellets have been studied using NMR relaxation, PGSE, and imaging techniques. Spatially nonresolved PGSE data were acquired as a function of the fractional saturation of the pellets during drying. Combination of these data with an estimate of the width of the pore-size distribution, as determined by mercury porosimetry, and the results of numerical simulations of diffusion in a random pore network enabled predictions of the connectivity of the pore structure to be determined. NMR imaging was shown to be effective in discriminating between different models of drying; even when the bulk drying curves are similar, significantly different moisture redistribution phenomena are seen to occur.
Nanofiltration membrane behavior in a supercritical medium
Abstract The permeability values of “TN” organomineral nanofiltration membranes to water, L , and to supercritical CO 2 , G , were compared. The resulting values for G were an order of magnitude higher than for L . The difference may be directly related to the viscosity difference between the two fluids. Temperature- and pressure-related variations in G were also analyzed; for this purpose, Poiseuille's model satisfactorily accounts for experimental behavior, while Knudsen's model is unsuitable. A hysteresis effect was observed on the isotherms corresponding to variations in G versus pressure, suggesting partially irreversible CO 2 adsorption on the micropore walls, that would diminish the radius. This phenomenon could be enhanced by an increased fluid density and viscosity.