J. Phys. Chem. 1996, 100, 16263-16267
16263
Direct Observation of N2 Self-Diffusion in Zeolitic Adsorbents Using
15N
PFG NMR
P. L. McDaniel,† C. G. Coe,*,† J. Ka1 rger,‡ and J. D. Moyer† Air Products and Chemicals, Inc., 7201 Hamilton BlVd., Allentown, PennsylVania 18195-1501, and Fakulta¨ t fu¨ r Physik und Geowissenschaften der UniVersita¨ t Leipzig, Linne´ strasse 5, D-04103 Leipzig, Germany ReceiVed: April 9, 1996X
Standard methodologies used for studying hydrocarbon diffusion (using 1H PFG NMR) were applied for the first time to the diffusion of 15N2 in beds of finely powdered commercial zeolites 5A and 13X. Unlike 1H PFG NMR on hydrocarbon adsorbents, crystallite size, gradient strength, observation time, and gyromagnetic ratio are more critical in the measurement of N2 self-diffusion in commercial-grade crystals. In fact, with the available hardware configuration and crystallite diameter constraints, we were restricted to the measurement of long-range diffusivities. These diffusivities were found to be similar for both adsorbents, varying from 3.2 × 10-6 to 1.3 × 10-8 m2/s over the temperature range studied (289-167 K). We also found the intercrystalline diffusion (Dinter) from the measured long-range diffusivities (Dlong-range) and adsorption isotherms. We found them to be similar to bulk diffusion of N2 at the conditions studied. Finally, the experimentally determined Dlong-range results were shown to compare favorably to those calculated from simple gas kinetic theory.
Introduction Pulsed field gradient nuclear magnetic resonance (PFG NMR) techniques have successfully been used to probe translational diffusion processes of adsorbed species in zeolitic materials.1-3 The focus has been on using 1H PFG NMR to measure hydrocarbon self-diffusion within individual zeolite crystallites to explore the influence of the molecular-sized environment on diffusion.3-5 The variation in self-diffusion coefficient can then be related to topological characteristics such as pore size, channel structure, or coadsorbent effects in multicomponent systems.6-9 For over 20 years, N2 selective zeolites have been used to separate O2 from air using pressure swing adsorption. It is wellknown that the air separation properties of zeolites are strongly influenced by both equilibrium and kinetic effects. To develop structure/property relationships for these systems, it is necessary to separate the fundamental influences of topology, extraframework cations, and nonframework species on adsorptive properties. To achieve a fundamental understanding of the relationship among these effects, it is desirable to find a method whereby diffusing species can be directly observed and the influence of framework characteristics determined. Zeolites 5A and 13X are both commonly used commercially to produce oxygen by pressure swing adsorption and are examples of small and large pore zeolites. The extent of calcium exchange and dehydration conditions can significantly influence the adsorption properties for weakly interacting gases such as N2. We chose a CaNaA (5A) sample which when fully dehydrated had similar N2 loadings and heats of adsorption to that of 13X (NaX) powder. Using these adsorbents, which have similar dependencies of N2 loading on temperature, we could directly examine the influence of structural properties on translational diffusion. Translational diffusion of adsorbed species in a powdered microporous material is comprised of at least three potentially different diffusion regimes (Figure 1). These have been * To whom correspondence should be addressed. † Air Products and Chemicals, Inc. ‡ Universita ¨ t Leipzig. X Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01064-7 CCC: $12.00
Figure 1. Translational diffusion or displacement regimes in a powdered microporous material [Rintra (solid), Rinter (dashed), Rlong-range (bold)].
described in detail elsewhere10 and will only be summarized here. Diffusion within the pore system of an individual zeolite crystallite can be influenced by topology, cation type, or extraframework species. This diffusion regime is described by intracrystalline diffusion (Dintra). During the experimental observation time, displacements (Rintra) of less than the crystallite diameter occur. As the adsorbed gas moves from the micropore system of one crystallite through the intercrystalline void and into the micropore system of another crystallite, different diffusion and displacement regimes are encountered. Diffusion through the intercrystalline voids (Dinter) is further characterized by another displacement distance (Rinter). Further, if during the observation time, the diffusing molecule moves through multiple intra- and intercrystalline domains, the displacement becomes large relative to the diameter of an individual crystallite. This movement is denoted as Dlong-range and is characterized by the displacement Rlong-range. The displacements are related as shown by eq 1
Rlong-range ) ∑(Rintra)i + ∑(Rinter)j i
(1)
i
From the diffusion experiment, determination of the specific regime (Dintra or Dlong-range) that is being probed is made with knowledge of the magnitude of the molecular displacements relative to the crystallite diameter. The mean-square displace© 1996 American Chemical Society
16264 J. Phys. Chem., Vol. 100, No. 40, 1996
McDaniel et al.
ment (〈(r)2〉) for an unrestricted diffusion process is given by eq 21
〈(r)2〉 ) 6D∆
(2)
where ∆ is the experimental observation time and D is the diffusion coefficient. Several instrumental and intrinsic nuclear considerations limit the feasibility of the direct determination of each diffusion coefficient, especially when low gyromagnetic ratio nuclei such as 15N are of interest. The success of 1H PFG NMR in characterizing diffusion phenomena led us to extrapolate the experimental methodologies to the direct observation of N2 in zeolitic systems using 15N PFG NMR. In this study our goal was to understand the limitations and boundaries of 15N PFG NMR specifically as they apply to measuring N2 self-diffusion in zeolitic materials. Toward this end, we examined the effects of temperature, N2 loading, and pore size on N2 self-diffusion in commercially available CaNaA and NaX zeolite powders. Additionally, the fundamental limitations of the technique, especially as they apply to N2 diffusion in a typical commercial powdered adsorbent, were determined along with the implications for probing the different diffusion regimes in a powdered zeolite. Experimental Section Sample Preparation. Powdered crystalline zeolites averaging 1.5 µm in diameter of 70Ca30NaA (5A) and NaX (13X) were obtained from Linde Division of Union Carbide Corp. Initially, the samples were activated to remove adsorbed water under dynamic vacuum (
