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RESEARCH NOTES Pure- and Mixed-Gas Sorption Measurements on Zeolitic Adsorbents via Gas-Phase Nuclear Magnetic Resonance Frank Rittig,† David J. Aurentz, Charles G. Coe, Ronald J. Kitzhoffer, and John M. Zielinski* Air Products & Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195-1501
Gas-phase NMR spectroscopy has been applied successfully in measuring pure- and mixed-gas equilibria on a 5A adsorbent for carbon monoxide, methane, and nitrogen. The reliability of the technique is established by comparing data collected here to pure-gas isotherms measured at 23 °C by standard volumetric expansion techniques in the pressure range of 0.5-5 atm. Mixedgas equilibria data for binary gas mixtures as well as the ternary gas mixture are compared with predictions based on heterogeneous ideal solution adsorption theory analysis. Introduction Reliable descriptions of mixed-gas equilibria in zeolitic adsorbents are essential for the design, optimization, and accurate simulation of commercial gas separation processes. Our recent success with measuring polymer/solvent vapor-liquid equilibrium by vaporphase Fourier transform infrared (FTIR) spectroscopy1 has prompted us to apply gas-phase nuclear magnetic resonance (NMR) in an analogous manner to measure pure- and mixed-gas isotherms in adsorbent/adsorbate systems. Although pure-component isotherm data are readily available and easily accessible, multicomponent equilibria are generally not measured even though predictions of multicomponent equilibria based exclusively on pure-component data are often inaccurate. Because different molecular species in the gas phase can be monitored separately,2 high-resolution NMR provides a simple means by which to perform multicomponent adsorption measurements. Other experimental techniques that have been recently developed, e.g., the isotope exchange technique (IET), are also able to measure mixed-gas isotherms insofar as the gas molecules can be distinguished by mass spectroscopy.3 The motivation to develop this experimental technique arose from our need to accurately quantify gasphase partial pressures and Gibbsian surface excesses (GSE) within sealed samples prepared for pulsed field gradient (PFG)-NMR diffusion measurements.4,5 Traditional experimental investigations employ either gravimetric6 or volumetric methods to acquire GSE values. These techniques are, however, mostly restricted to pure adsorbates. The experimental measurement of multicomponent adsorption isotherms is more challenging than that for pure components because of the large number of system variables involved. Consequently, the * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (610) 481-7975. Fax: (610) 4816517. † Current address: BASF AG, GKP/P-G200, D-67056 Ludwigshafen, Germany.
problem of accurately predicting multicomponent gasphase equilibria from single-component adsorption data must be addressed to expedite further development of gas separation processes. Although the sorbate loading on our NMR samples composed of a single gas could be accurately estimated from pure-component isotherm data, the loading of mixed-gas samples could not be predicted with confidence because multicomponent isotherms for the systems of interest were not available and predictions obtained from heterogeneous and/or ideal adsorption solution theory (HIAST and IAST)7 analyses of pure-component data, although accurate in some instances,8 are poor in others.9,10 For our multicomponent PFG-NMR diffusion studies,4,5 direct measurements of gas partial pressure and loading constituted the most desirable alternative. Previous studies11 have demonstrated that NMR spectroscopy can significantly contribute to the understanding of dynamic properties in adsorbate/adsorbent systems. Throughout the last several decades, this technique has proven useful in studying in situ reactions such as the catalytic conversion of methanol to hydrocarbons (the MTG process12). While gas-phase 19F NMR experiments have been described by Roe et al.13,14 to measure slow kinetics, these experiments require an internal standard, which is impractical for adsorbent/ adsorbate systems. In 1987 Ka¨rger and Ernst15 presented preliminary evidence that gas-phase 1H NMR could be successfully applied to measure pure and multicomponent equilibria for adsorbent/adsorbate systems without the use of an internal standard. The objective of the work presented here is to more fully evaluate gas-phase NMR, as a technique for measuring pure and multicomponent isotherms for systems containing various NMR active nuclei (1H, 13C, and 15N). The accuracy of our results is determined by comparing the measured GSE values of pure carbon monoxide, methane, and nitrogen with data from conventional volumetric expansion analysis. Another aspect of this study was to determine the advantages and limitations of gas-phase NMR, particu-
10.1021/ie020133o CCC: $22.00 © 2002 American Chemical Society Published on Web 07/24/2002
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Figure 1. Schematic of the volumetric expansion apparatus used to prepare samples for the gas-phase NMR and PFG-NMR studies.
larly for 15N NMR, which exhibits low signal intensities because of its unfavorably low gyromagnetic ratio. Opposing this drawback is the benefit gained because of the short spin-lattice relaxation times, T1, inherent of all gas-phase NMR experiments. Short T1 values are due to the relaxation mechanism being dominated by spin rotation.13 For the pressures considered here, the spin-lattice relaxation is remarkably rapid for all gases in our study (≈10 ms) and therefore leads to short data acquisition times and consequently to fast signal accumulations. Experimental Section Pelletized 5A zeolites were obtained from UOP (Des Plaines, IL). Initially, the samples were activated (to remove adsorbed water) under dynamic vacuum (
