News Column

Patent Issued for Metal-Organic Frameworks for Xe/Kr Separation

August 5, 2014

By a News Reporter-Staff News Editor at Life Science Weekly -- Northwestern University (Evanstone, IL) has been issued patent number 8784536, according to news reporting originating out of Alexandria, Virginia, by NewsRx editors (see also Northwestern University).

The patent's inventors are Ryan, Patrick J. (Vernon Hills, IL); Farha, Omar K. (Morton Grove, IL); Broadbelt, Linda J. (Glenview, IL); Snurr, Randall Q. (Evanston, IL); Bae, Youn-Sang (Evanston, IL).

This patent was filed on August 23, 2013 and was published online on July 22, 2014.

From the background information supplied by the inventors, news correspondents obtained the following quote: "Separating xenon from krypton is an industrially important problem. Xenon (Xe) and krypton (Kr) are used in fluorescent light bulbs, and current technology produces these gases from the cryogenic distillation of air, in which these noble gases are present in small concentrations (1.14 ppmv Kr, 0.086 ppmv Xe). Both xenon and krypton separate into the oxygen-rich stream after distillation, and these gases are concentrated and purified to produce an 80/20 molar mixture of krypton to xenon..sup.1 This final mixture typically undergoes further cryogenic distillation to produce pure krypton and pure xenon. Distillation is an energy-intensive process, and separation of these gases by selective adsorption near room temperature would be much more energy efficient. Additionally, separating krypton from xenon is an important step in removing radioactive krypton-85 during treatment of spent nuclear fuel..sup.2 However, even after cryogenic distillation, trace levels of radioactive krypton in the xenon-rich phase are too high to permit further use..sup.2 If adsorbents could reduce krypton-85 concentrations in the xenon-rich phase to permissible levels, there could be an entirely new supply source of xenon for industrial use. Thus, there is a strong need to develop adsorbent materials for this separation to reduce energy consumption and to reuse byproducts of consumed nuclear fuel.

"There are several examples in the literature where zeolites have been tested for Xe/Kr separation. Previous research has shown NaX zeolite to be a selective adsorbent for xenon over krypton with a selectivity of about 6 with krypton concentrations ranging from 1 to 10,000 ppm..sup.2 Jameson et al..sup.3 showed that NaA zeolite had a selectivity of approximately 4 for binary mixtures of xenon and krypton at 300 K between 1 and 10 bar. They also used molecular simulations to show that ideal adsorbed solution theory (IAST) could accurately predict the selectivities and mixture behavior from the single-component isotherms.

"Metal-organic frameworks,.sup.4-6 or MOFs, are a new class of nanoporous materials. Composed of organic linkers and metal corners, these materials self-assemble in solution to form stable, crystalline frameworks. Coordination bonds between oxygen and nitrogen atoms with metal centers allow for a variety of topologies, and choice of the organic linker allows one to tailor pore sizes and environments for particular applications. As a result, these materials have garnered much attention for hydrogen storage,.sup.7-9 separations,.sup.10,11 and catalysis..sup.12-14

"A number of groups have investigated MOFs for separation of other gases. For example, Bae et al..sup.15 used both experiments and simulation to show a mixed-ligand MOF effectively separates carbon dioxide from methane. Bae et al..sup.16 also showed that exchanging fluorinated-methylpyridine into a MOF could substantially increase the selectivity of carbon dioxide over nitrogen due to the increased polar environment. Pan et al..sup.17 synthesized a microporous MOF with 1D hydrophobic microchannels and demonstrated its ability to separate n-butane from other n-alkanes and olefins. Hartmann et al..sup.18 showed that isobutene can be separated from isobutane using HKUST-1 in a breakthrough system. Yang et al..sup.19, 20 used molecular simulations to predict that HKUST-1 is a promising candidate for separation of carbon dioxide from both air and methane/hydrogen mixtures.

"To date, there are a few publications that report the investigation of Xe/Kr separation using MOFs. Mueller et al..sup.21 measured noble gas adsorption in IRMOF-1 and noticed significantly higher adsorption for the heavier gases, namely xenon and krypton, in MOF-filled containers relative to containers without MOF material. Building on these results, they built a breakthrough system filled with HKUST-1 and showed that a 94/6 molar mixture of krypton/xenon could be purified to over 99% krypton and less than 50 ppm xenon. Greathouse et al..sup.22 recently simulated noble gas adsorption in IRMOF-1. They predicted that IRMOF-1 has a selectivity of about 2.5-3 for Xe over Kr at 298 K and pressures of both 1 and 10 bar."

Supplementing the background information on this patent, NewsRx reporters also obtained the inventors' summary information for this patent: "The present invention envisions a method of separating a particular noble gas in a gas mixture by contacting the gas mixture with an adsorbent material comprising a metal-organic framework (MOF) material having framework pores that are sized to receive no more than one atom of the particular noble gas for selectively adsorbing the particular noble gas from the gas mixture. MOF materials having a relatively high percentage of pores (percentage of total pore volume) that are capable of accommodating the noble gas atom but that have a small enough pore size to receive no more than one such atom are desired to preferentially adsorb the particular noble gas over one or more other noble gases in a multi-component mixture adsorption method embodiment. However, the pore size cannot be so small that it significantly limits overall gas uptake (capacity), which is undesirable. The present invention thus envisions MOF adsorbent materials for separation of a particular noble gas and one or more other noble gases in a gas mixture.

"In an illustrative embodiment of the invention, the present invention provides metal-organic framework (MOF) materials that are selectively adsorbent to xenon (Xe) over another noble gas, such as for example krypton (Kr) and/or argon (Ar), as a result of having a framework voids (pores) sized to this end. MOF materials having 20% or more, preferably 40% or more, of the total pore volume, capable of accommodating a Xe atom but having a small enough pore size to receive no more than one Xe atom are desired to preferentially adsorb Xe over Kr in a multi-component (Xe--Kr gas mixture) adsorption method. The present invention thus envisions MOF adsorbent materials for separation of xenon (Xe) and one or more other noble gases.

"An illustrative Xe-selective MOF material includes characteristic multiple pore size categories within its particular framework wherein 20% or more, preferably 40% or more, of the total pore volume has a size in the range of 0.45-0.75 nm, which compares to the Lennard-Jones diameters of 0.4100 nm and 0.3636 nm for Xe and Kr, respectively. Such MOF materials can selectively adsorb Xe over another noble gas, such as Kr, in a multi-component gas mixture over a pressure range of 0.01 to 1.0 MPa.

"In a particular illustrative embodiment of the present invention, a Xe-selective adsorbent material having a chemical formula unit represented by Cu.sub.2(3,3',5,5'-biphenyltetracarboxylate) and having the NbO topology has been identified, tested and determined to exhibit increased xenon selectivity over a wider pressure range of 0.01 to 1.0 MPa compared to other MOFs materials. The material was found to exhibit a selectively of about 9 to about 11 in the pressure range of 0.1 to 1.0 MPa.

"Advantages and detailed features of the present invention will become more apparent from the following detailed description taken with the following drawings."

For the URL and additional information on this patent, see: Ryan, Patrick J.; Farha, Omar K.; Broadbelt, Linda J.; Snurr, Randall Q.; Bae, Youn-Sang. Metal-Organic Frameworks for Xe/Kr Separation. U.S. Patent Number 8784536, filed August 23, 2013, and published online on July 22, 2014. Patent URL:

Keywords for this news article include: Chemicals, Chemistry, Carbon Dioxide, Northwestern University.

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Source: Life Science Weekly

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