NMR study of mass transfer in granulated molecular sieves

Abstract

Nuclear magnetic resonance (NMR) tracer desorption technique in combination with the traditional pulsed field gradient technique is applied to measure directly intracrystalline and long-range molecular transport, as well as molecular exchange rates, between the individual crystallites in granulated molecular sieves. It is found that the granulation process leads to the formation of transport resistances on the external surface of the crystallites, which may be dramatically enhanced during their technical application. As an example, for granulated zeolite NaCaA the enhancement of such surface barriers under hydrothermal conditions and under the influence of a hydrocarbon atmosphere is studied. Since both intra-and intercrystalline transport are found to remain practically unaffected by this treatment, changes in the overall transport behavior are caused by the enhanced transport resistance on the external surface of the zeolite crystallites.

Benefit of Microscopic Diffusion Measurement for the Characterization of Nanoporous Materials

“…Hydrothermal treatment of NaCaA-type zeolites [104,105], and/or contact with hydrocarbons at elevated temperatures [106], are well known to give rise to the formation of transport resistances, notably exceeding the influence of intracrystalline diffusion on the overall uptake or release process of hydrocarbons. In fact, by the application of the NMR tracer desorption technique [66], long-term contact with water vapor or with hydrocarbons at elevated temperatures was found to notably reduce the exchange rate of probe molecules with the surroundings, while their intracrystalline diffusivities remained essentially unaffected.…”

Influence of molecular shape on probing mass transfer resistances on zeolites

“…In this way, it is possible to determine both the intracrystalline self-diffusivity D of the adsorbed molecule and their intracrystalline mean life time T~~,~~ (Karger et al, 1988). Combining these two quantities yields direct information about the existence of surface barrier, since additional surface resistances in molecular mass transfer will affect only T~~,~~, not D (Karger, 1982).…”

“…By probing the properties of the hydrothermally deteriorated zeolite crystallites by N M R self-diffusion measurements with methane as a probe molecule, it has been found that lattice distortions are predominantly in the vicinity of the crystallite surface, since the desorption rates (i.e., 1 / T , , ,~, ) , not the intracrystalline diffusivities, were dramatically reduced (Karger et al, 1988). In this paper, we concentrated on the application of the N M R tracer desorption measurements for determining the intracrystalline mean life, since the general measurement of the intracrystalline diffusivities was prohibited due to the small sizes of crystallites .…”

“…Applying methane as a probe molecule, the origin of the deterioration of granulated zeolite NaCaA under the conditions of industrial use could be attributed to the formation of such "surface barriers" (Karger et al, 1988). Though it was produced by structural changes in or on the outer surface of the crystallites, the strength of the surface barrier is clearly also a function of the probe molecule under study.…”

Kinetics of hard sphere and chain adsorption into circular and elliptical pores

“…15,16 Several groups have performed experiments demonstrating that adsorption kinetics can be altered by modification of the external surfaces of zeolite crystals by chemical vapor deposition of a surface layer 17 or simply by coking of the surface over time under process conditions. [18][19][20] Surface barrier effects have also been observed in experiments involving adsorption into carbon molecular sieves. 21 It is useful to note, however, that surface barrier effects are not important in all situations.…”