# Patent

## Title
Metal-organic frameworks for adsorption and separation of noble gases

## Abstract

A method including exposing a gas mixture comprising a noble gas to a metal organic framework (MOF), including an organic electron donor and an adsorbent bed operable to adsorb a noble gas from a mixture of gases, the adsorbent bed including a metal organic framework (MOF) including an organic electron donor.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of the earlier filing date of U.S. Provisional Patent Application No. 61/955,921, filed Mar. 20, 2014 entitled “METAL-ORGANIC FRAMEWORKS FOR ADSORPTION AND SEPARATION OF NOBLE GASES.” The aforementioned application is hereby incorporated by reference, in its entirety, for all purposes.


STATEMENT OF GOVERNMENT RIGHTS
This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention.


FIELD
Gas separation.
BACKGROUND
Noble gases (also known as rare gases) are an important class of industrial chemicals essential to numerous applications, including lighting, insulation, cryogenic refrigerants, carrier gases, anesthetics, and excimer lasers. Most high-purity, industrial quantities of noble gases are obtained by separating the gas of interest from air.
Currently, most noble gases are obtained via cryogenic distillation—an extremely energy intensive and therefore costly process. Consequently, large-scale isolation of noble gases via selective adsorption near room temperature is highly desirable. Sorption-based processes using porous materials are generally difficult due to the low reactivity of these gases resulting from their filled valence shell. Thus, sorption-based separation processes must rely primarily on differences in atomic size and weak interactions (van der Waals forces) with surfaces. The processes of pressure-swing adsorption (PSA) and vacuum-swing adsorption (VSA) offer attractive alternatives to cryogenic distillation, in that they can be much more energy efficient and therefore cost-effective. However, PSA and VSA require efficient microporous or mesoporous adsorbents that are both highly selective and possess a high capacity. Traditionally, commercial sorbents have been limited to one of four categories: activated carbons, natural and synthetic zeolites, silica gel, and activated alumina. These materials typically have high surface areas and large pore volumes, enabling physisorption of weakly interacting adsorptives such as noble gases. Unfortunately, they also lack synthetic diversity; while currently many examples of commercial sorbents exist, in general, systematic alteration of their structure and chemistry is difficult. Thus, there is a great need for new microporous sorbents with adsorptive properties designed for specific adsorbates.
SUMMARY
A method including exposing a gas mixture including a noble gas to a metal organic framework (MOF) is described wherein the MOF has an organic electron donor having the formula of:




wherein R1 and R2 are independently selected from a branched or straight chain alkyl moiety that may be substituted (e.g., a perfluoroalkyl), or




wherein Ra, Rb, Rc, Rd, Re, Rf, Rg, and Rh are independently selected from a hydrogen moiety, a straight or branched chain alkyl moiety having one to three carbons, an amine moiety, an ether moiety or a halide moiety and Ri and Rj are independently selected from a trifluoromethyl moiety, a straight or branched chain alkyl moiety having one to three carbons, or a phenyl moiety.
In another embodiment, an adsorbent bed is described that is operable to adsorb a noble gas from a mixture of gases. The adsorbent bed includes:
a metal organic framework (MOF) including an organic electron donor having the formula of:




wherein R1 and R2 are independently selected from a branched or straight chain alkyl moiety that may be substituted (e.g., a perfluoroalkyl, or




wherein Ra, Rb, Rc, Rd, Re, Rf, Rg and Rh are independently selected from a hydrogen moiety, a straight or branched chain alkyl moiety having one to three carbons, an amine moiety or a halide moiety and Ri and Rj are independently selected from a trifluoromethyl, a straight or branched chain alkyl moiety having one to three carbons, or a phenyl moiety.
In another embodiment, a MOF of a cobalt formate is operable for selectively separating a noble gas from a gas mixture.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:
FIG. 1 illustrates a MOF including an electron donor having the formula of General Formula I where R1 and R2 are each methyl groups;
FIG. 2 illustrates a MOF including an electron donor having the formula of General Formula II where Ra-Rh are each hydrogen and Ri and Rj are each trifluoromethyl bridging zinc cations;
FIG. 3 presents representative electron donors for a MOF, the representative electron donors having the formula of General Formula II;
FIG. 4 illustrates a cobalt formate MOF; and
FIG. 5 illustrates a pressure-swing adsorption process.



DETAILED DESCRIPTION
Metal-organic frameworks (MOFs) are crystalline materials including open frameworks comprised of metal cations bridged by organic electron donors. One particularly appealing aspect of MOFs is that they are often shown to be permanently microporous and can exhibit extremely high surface areas, in some cases the highest ever observed. Paired with these properties is a level of synthetic flexibility unprecedented in other classes of microporous materials such as porous carbons and zeolites.
In one embodiment, a class of MOFs is described that have the ability to adsorb (e.g., trap) noble gases and selectively separate such gases from one another and nitrogen. A method including exposing a gas mixture including a noble gas to a MOF is described wherein the MOF includes an organic electron donor having the formula:




wherein R1 and R2 are independently selected from a branched or straight chain alkyl moiety, or




wherein Ra, Rb, Rc, Rd, Re, Rf, Rg, and Rh are independently selected from a hydrogen moiety, a straight or branched chain alkyl moiety having one to three carbons, an amine moiety or a halide moiety and Ri and Rj are independently selected from a trifluoromethyl, a straight or branched chain alkyl moiety having one to three carbons, or a phenyl moiety. Noble gas in the gas mixture exposed to the MOF is adsorbed (e.g., trapped) by the MOF and can then be separated using, for example, pressure (e.g., pressure swing adsorption) or temperature (e.g., temperature swing adsorption).
MOFs including an organic electron donor having the formula of General Formula I may be referred to as Pyra MOFs. The organic electron donor of a Pyra MOF generally has a pyrazole backbone with adjacent nitrogens at positions 3 and 4 of the five membered ring and a carboxylic acid at position 1. The carboxylic acid is deprotonated in the MOF (as illustrated in General Formula I). In one embodiment, a MOF including an organic electron donor having the formula of General Formula I has an R1 and R2 independently selected of an alkyl moiety having one to three carbon atoms. Examples include:
R1, R2 are each methyl or each ethyl;
R1 is methyl;
R2 is ethyl;
R1 and R2 are each n-propyl;
R1 is methyl, R2 is isopropyl; and
R1 is methyl, R2 is n-propyl.
Table 1 shows ideal selectivities (ratio of Henry's constants) for certain Pyra MOFs having an organic electron donor having the formula of General Formula I at 273 K and 292 K.












TABLE 1







Pyra MOF
Kr/N2
Kr/Ar
Xe/N2
Xe/Ar
Xe/Kr


















R1
R2
273
292
273
292
273
292
273
292
273
292





















methyl
methyl
3.95
3.47
4.46
3.88
37.2
25.3
41.9
28.3
9.40
7.29


methyl
ethyl
4.80
4.04
4.91
4.17
49.7
32.2
50.8
33.2
10.3
7.96


ethyl
ethyl
6.09
4.82
5.31
4.35
64.2
37.2
56.0
33.6
10.5
7.73


methyl
isopropyl
4.85
4.46
4.66
3.98
48.9
36.1
47.0
32.2
10.1
8.09


methyl
n-propyl
4.85
4.60
4.82
4.08
48.4
38.1
48.1
33.7
9.99
8.27









FIG. 1 illustrates a Pyra MOF 100 including an organic electron donor having the formula of General Formula I where R1 and R2 are each methyl and bridging copper cations (Cu2+). The MOF may be represented by the formula: Zn4O(C6H6N2O2)3. In one embodiment, a process to form a Pyra MOF including an organic electron donor having the formula of General Formula I where R1 and R2 are each ethyl includes adding a solution of the electron donor 0.25 g in 10 ml of 1:24 EtOH:H2O (V:V) with 0.42 ml Et3N dropwise to a stirred solution of Zn(NO3)2. 6H2O (0.45 g) in 10 ml of 1:1 EtOH:H2O, affording a white suspension. This suspension was refluxed for four hours. The white precipitate was collected by vacuum filtration, and washed with H2O, EtOH, and Et2O. The product was soaked in Et2O for three or more days with three solvent exchanges. A sample of this was degassed on the MasterPrep at 30° C. for one hour, and 120° C. for 48 hours, to obtain activated material for porosimetry.
A MOF including an organic electron donor having the formula of General Formula II where Ra-Rh are each hydrogen and Ri and Rj are each trifluoromethyl is a YAPXIX MOF. The MOF may be represented by the formula: Zn(C17H8F6O4). The deprotonated carboxylic acids bridge a metal cation. FIG. 2 illustrates a YAPXIX MOF with the deprotonated carboxylic acids bridging zinc cations. MOF 200 may be formed by combining an amount of an organic electron donor having the formula of General Formula II with zinc acetate (Zn(OAc)2) in an ethanol and water mixture at 170° C. for 72 hours. In one embodiment, MOF 200 is made by a process described at “One Teflon®-like channeled nanoporous polymer with a chiral and new uninodal 4-connected net: sorption and catalytic properties,” Monge, et al., The Royal Society of Chemistry, Chem. Commun., 2005, 1291-93.
Table 2 shows krypton/nitrogen and krypton/argon gas selectivities for the YAPXIX MOF by comparison of Henry's constants (kH/kH) and volume adsorbed (V/V) from zero atmosphere to one atmosphere and 292 K.













TABLE 2






kH/kH
V/V
V/V
V/V
V/V
V/V


Gases
(0-0.1 atm)
0.2 atm
0.4 atm
0.6 atm
0.8 atm
1 atm







Kr/N2
6.5
5.9
5.0
4.4
4.0
3.7


Kr/Ar
6.3
5.9
4.9
4.3
3.8
3.7









Other examples of YAPXIX-type including an organic electron donor having the formula of General Formula II and shown in FIG. 3 include:
Ra, Rb, Rd, Re, Rg and Rh are each hydrogen; Rc and Rf are each a primary amine group; Ri and Rj are each trifluoromethyl (donor 310);
Ra, Rd, Re and Rh are each hydrogen; Rb, Rc, Rf and Rg are each methyl; and Ri and Rj are each trifluoromethyl (donor 320);
Ra, Rd, Re and Rh are each hydrogen; Rb, Rc, Rf and Rg are each chlorine; and Ri and Rj are each trifluoromethyl (donor 330);
Ra, Rb, Rc, Rd, Re, Rf, Rg and Rh are each hydrogen; and Ri and Rj are each ethyl (donor 340); and
Ra, Rb, Rc, Rd, Re, Rf, Rg and Rh are each hydrogen; and Ri and Rj are each phenyl (donor 350).
In another embodiment, a MOF of cobalt formate is operable for adsorption and separation of noble gases from a mixture of gases such as separation of one noble gas relative to another or separation of a noble gas from a mixture of noble gas and nitrogen. An embodiment of an MOF of cobalt formate has the formula: Co3(HCOO)6DMF.
A MOF of cobalt formate was prepared in a similar manner described by Li et. al., Adv. Func. Mater. 2008, 18, 2205-2214. Cobalt nitrate hexahydate (1 equivalent) and formic acid (6.6 equivalents) were combined in DMF and heated to 100° C. for 20 hours. After cooling to room temperature over a period of 2.5 hours, the solid cobalt formate-DMF complex was isolated by filtration. The cobalt formate-DMF solid was washed with DMF and diethyl ether and then placed under vacuum at room temperature for 24 hours to remove unbound solvent. The DMF in the cobalt formate-DMF complex was removed by heating between 110° C. and 150° C. under vacuum to give cobalt formate.
FIG. 4 illustrates a cobalt formate MOF 400 synthesized as described. The cobalt formate MOF synthesized as described obtained a Kr/N2 ideal selectivity (ratio of Henry's constants) of 5.4 at 292 K, with an uptake of 20 cubic centimeters per gram (cm3/g) at standard temperature and pressure. The described cobalt formate MOF 400 has the ability to adsorb (e.g., trap) noble gases and selectively separate such gases from one another and/or from nitrogen.
In another embodiment, an adsorbent bed is described that is operable to adsorb a noble gas from a mixture of gases. The adsorbent bed includes a MOF such as described above including an organic electron donor having the formula of General Formula I or General Formula II or the synthesized cobalt formate MOF. Such adsorbent bed is operable for use in a pressure-swing adsorption process or a temperature-swing adsorption process. FIG. 5 illustrates a pressure-swing adsorption process. In a pressure swing adsorption process to separate a gas from a mixture of gases, the mixture of gases is passed through an adsorbent bed at a high pressure. Representatively, an adsorbent bed includes pelletized MOFs such as MOFs combined with a binder such as graphite, bentonite, alumina and/or silica or other binder and pressed or extruded under pressure to generate a defined shape (e.g., cylinder, sphere, granule, etc.). U.S. Publication No. 2009/0155588 provides a pellet description. FIG. 5 shows system 500 including vessel 510 containing adsorbent bed 520 of a MOF as described herein. Gas mixture, GM, including gases GA, GB and a noble gas (GN) is passed through bed 520 at pressure P1 (a relatively high pressure such as one to 20 atmospheres). Under these conditions, the noble gas (GN) is selectively adsorbed (e.g., trapped) in adsorbent bed 520 relative to the other gases in the gas mixture (relative to GA and GB). After time (Δt), a pressure P2 that is less than P1 is passed through adsorbent bed 520 and the noble gas (GN) is released.
A temperature-swing adsorption process utilizes temperature to assist in the adsorption and removal of a noble gas from a mixture of gases. Representatively, a mixture of gases is passed through an adsorbent bed, such as adsorbent bed 520 of a MOF such as described herein, at a first temperature (a relatively low temperature). The noble gas is selectively adsorbed (e.g., trapped) in the adsorbent bed relative to the other gases in the mixture. After a time, the temperature of the adsorbent bed is increased to a second temperature and the noble gas is released from the adsorbent bed.
In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments. It will be apparent however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. The particular embodiments described are not provided to limit the invention but to illustrate it. The scope of the invention is not to be determined by the specific examples provided above but only by the claims below. In other instances, well-known structures, devices, and operations have been shown in block diagram form or without detail in order to avoid obscuring the understanding of the description. Where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated in the figure to indicate corresponding or analogous elements, which may optionally have similar characteristics.
It should also be appreciated that reference throughout this specification to “one embodiment”, “an embodiment”, “one or more embodiments”, or “different embodiments”, for example, means that a particular feature may be included in the practice of the invention. Similarly, it should be appreciated that in the description, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects may lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of the invention.

## Claims

1. A method comprising:
exposing a gas mixture comprising a noble gas to a metal organic framework (MOF), including an organic electron donor having the formula of:




wherein R1 and R2 are independently selected from a branched or straight chain alkyl moiety that may be substituted, or




wherein Ra, Rb, Rc, Rd, Re, Rf, Rg, and Rh are independently selected from a hydrogen moiety, a straight or branched chain alkyl moiety having one to three carbons, an amine moiety, an ether moiety or a halide moiety and Ri and Rj are independently selected from a trifluoromethyl, a straight or branched chain alkyl moiety having one to three carbons, or a phenyl moiety; with the proviso that Ra, Rb, Rc, Rd, Re, Rf, Rg, and Rh are not each selected from a hydrogen moiety when Ri and Rj are each a trifluoromethyl moiety.

2. The method of claim 1, wherein the electron donor is the electron donor of General Formula I and R1 and R2 are independently an alkyl moiety having one to three carbon atoms.

3. The method of claim 2, wherein R1 and R2 are each an ethyl moiety.

4. The method of claim 2, wherein R1 and R2 are each a methyl moiety.

5. The method of claim 2, wherein R1 and R2 area methyl moiety and an ethyl moiety, respectively.

6. The method of claim 1, wherein the electron donor is the electron donor of General Formula II and Ri and Rj are each trifluoromethyl.

7. The method of claim 6, wherein Rc and Rf are each an unsubstituted amine moiety.

8. The method of claim 6, wherein Rc and Rf are each a methyl moiety.

9. The method of claim 6, wherein Rb, Rc, Rf and Rg are each a chloride moiety.

10. The method of claim 1, wherein the electron donor is the electron donor of General Formula II and Ri and Rj are each ethyl.

11. The method of claim 1, wherein the electron donor is the electron donor of General Formula II and Ri and Rj are each phenyl.

12. The method of claim 1, wherein exposing the gas mixture to the MOF comprises exposing at a first pressure for a first time period and, after the first time period, the method comprises exposing any noble gas adsorbed to the MOF to a different second pressure.

13. The method of claim 1, wherein exposing the gas mixture to the MOF comprises exposing at a first temperature for a first time period and, after the first time period, the method comprises exposing any noble gas adsorbed to the MOF to a different second temperature.

14. An adsorbent bed operable to adsorb a noble gas from a mixture of gases, the adsorbent bed comprising:
a metal organic framework (MOF) including an organic electron donor having the formula of:




wherein R1 and R2 are independently selected from a branched or straight chain alkyl moiety that may be substituted, or




wherein Ra, Rb, Rc, Rd, Re, Rf, Rg and Rh are independently selected from a hydrogen moiety, a straight or branched chain alkyl moiety having one to three carbons, an amine moiety, an ether moiety or a halide moiety and Ri and Rj are independently selected from a trifluoromethyl, a straight or branched chain alkyl moiety having one to three carbons, or a phenyl moiety; with the proviso that Ra, Rb, Rc, Rd, Re, Rf, Rg, and Rh are not each selected from a hydrogen moiety when Ri and Rj are each a trifluoromethyl moiety.

15. The bed of claim 14, wherein the electron donor is the electron donor of General Formula I and R1 and R2 are independently an alkyl moiety having one to three carbon atoms.

16. The bed of claim 15, wherein R1 is an ethyl moiety.

17. The bed of claim 14, wherein the electron donor is the electron donor of General Formula II and Ri and Rj are each trifluoromethyl.

18. A method comprising:
exposing a gas mixture comprising a noble gas to a metal organic framework (MOF) comprising cobalt formate; and
separating the noble gas from the gas mixture.