NMR Gas Phase Measurements in Adsorbate-Adsorbent Systems

Ernst, E; Kärger, Jörg

doi:10.1515/zpch-1987-26843

Cited by 7 publications

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“…Despite its wide use for the analysis of liquid-phase mixtures, NMR spectroscopy has been used to analyze gas-phase mixtures in only a few cases. − Of those, only one study tested the accuracy of the method with a well-defined gas mixture, and that study was also the only one that reported measurement uncertainties for the NMR analysis. In that work, a gravimetrically prepared, 10-component natural gas surrogate (with light hydrocarbons ranging from methane to hexane, as well as CO 2 ) was analyzed.…”

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Composition Determination of Low-Pressure Gas-Phase Mixtures by ¹H NMR Spectroscopy

et al. 2019

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1 H NMR spectroscopy was used to analyze gas-phase mixtures of methane and propane at pressures near 0.1 MPa. The mixtures were prepared gravimetrically and had low uncertainty in their composition. The primary mixture used for this work had a methane mole fraction of x methane,grav = (0.506875 ± 0.00019) and a propane mole fraction of x propane,grav = (0.493125 ± 0.00019). NMR samples were prepared in two types of commercially available sample tubes that seal with a PTFE piston. Sample pressures ranged from 0.02 to 0.5 MPa. An analysis of measurement uncertainty for the NMR method resulted in combined standard uncertainties that decreased from 0.0082x to 0.0010x, as the pressure increased from 0.02 to 0.5 MPa. The larger uncertainties at lower pressures were primarily caused by uncertainties associated with phasing and baseline correction. A key difficulty in working with gas-phase samples, especially at lower pressures, is that the spectral peaks are inherently broad. Consequently, peak overlap was problematic, and it was not always possible to integrate a high percentage of a peak's intensity. However, with corrections to the integrated areas, based on the assumption of ideal Lorentzian peak shapes, excellent agreement between the NMR analyses and the gravimetric composition was observed across the entire pressure range. These experiments demonstrate the potential of 1 H NMR for quantitative composition determinations of low-pressure gas-phase mixtures.

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“…The use of NMR for vapor–liquid equilibrium (VLE) measurements has been suggested, and in fact, we are currently using 1 H NMR spectroscopy to collect VLE data for binary refrigerant mixtures. NMR spectroscopy can also be used to monitor competitive adsorption, , permeation, or reaction ,,,,,,, of gas-phase mixtures in real time.…”

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Composition Determination of Low-Pressure Gas-Phase Mixtures by ¹H NMR Spectroscopy

et al. 2019

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“…D long-range can also be approximated by the relationship where P inter is the fraction of molecules within the intercrystalline space and D inter is their diffusivity. From experimental 15 N 2 D long-range values, D inter can be calculated using eq 4 if P inter is known . For P inter , values for c gas and c ads are determined from the ideal gas law and the adsorption isotherm measured at 23 °C, respectively, while the bulk and crystalline densities are measured directly on the activated powder.…”

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“…Translational diffusion of adsorbed species in a powdered microporous material is comprised of at least three potentially different diffusion regimes (Figure ). These have been described in detail elsewhere 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.…”

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Direct Observation of N₂Self-Diffusion in Zeolitic Adsorbents Using¹⁵N PFG NMR

et al. 1996

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Standard methodologies used for studying hydrocarbon diffusion (using 1 H PFG NMR) were applied for the first time to the diffusion of 15 N 2 in beds of finely powdered commercial zeolites 5A and 13X. Unlike 1 H PFG NMR on hydrocarbon adsorbents, crystallite size, gradient strength, observation time, and gyromagnetic ratio are more critical in the measurement of N 2 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 m 2 /s over the temperature range studied (289-167 K). We also found the intercrystalline diffusion (D inter ) from the measured long-range diffusivities (D long-range ) and adsorption isotherms. We found them to be similar to bulk diffusion of N 2 at the conditions studied. Finally, the experimentally determined D long-range results were shown to compare favorably to those calculated from simple gas kinetic theory.

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“…Our preliminary results demonstrate the utility of gas phase NMR as a new technique for heterogeneous catalysis. It appears to be particularly suited to follow the relative kinetics of volatile reaction components over a catalyst (when the reaction time scale and the diffusion limitations are compatible with the technique) and to study the competitive adsorption equilibria of volatile compounds over a variety of adsorbents . The advantage of this technique over, for example, gas chromatography, is that NMR resonances not only carry information on the concentrations of reactants and products at any instant without sample extraction but also contain precise and easily decipherable structural information.…”

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Hydrogenation of Fluoroolefins Studied by Gas Phase NMR: A New Technique for Heterogeneous Catalysis

et al. 1996

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NMR Gas Phase Measurements in Adsorbate-Adsorbent Systems

Cited by 7 publications

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Composition Determination of Low-Pressure Gas-Phase Mixtures by ¹H NMR Spectroscopy

Composition Determination of Low-Pressure Gas-Phase Mixtures by ¹H NMR Spectroscopy

Direct Observation of N₂Self-Diffusion in Zeolitic Adsorbents Using¹⁵N PFG NMR

Hydrogenation of Fluoroolefins Studied by Gas Phase NMR: A New Technique for Heterogeneous Catalysis

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NMR Gas Phase Measurements in Adsorbate-Adsorbent Systems

Cited by 7 publications

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Composition Determination of Low-Pressure Gas-Phase Mixtures by 1H NMR Spectroscopy

Composition Determination of Low-Pressure Gas-Phase Mixtures by 1H NMR Spectroscopy

Direct Observation of N2Self-Diffusion in Zeolitic Adsorbents Using15N PFG NMR

Hydrogenation of Fluoroolefins Studied by Gas Phase NMR: A New Technique for Heterogeneous Catalysis

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Composition Determination of Low-Pressure Gas-Phase Mixtures by ¹H NMR Spectroscopy

Composition Determination of Low-Pressure Gas-Phase Mixtures by ¹H NMR Spectroscopy

Direct Observation of N₂Self-Diffusion in Zeolitic Adsorbents Using¹⁵N PFG NMR