PROCESSES USING IMPROVED RHO ADSORBENT COMPOSITIONS

Abstract

Disclosed herein are new processes for adsorbing oxygen using adsorbent compositions comprising RHO zeolites kinetically selective for oxygen. The RHO zeolites can be used in pressure swing adsorption processes for separating oxygen from oxygen containing streams, such as for, but not limited to, purifying a crude argon feed stream or separating oxygen from an air feed stream.

Claims

1. A process of adsorbing oxygen from a feed stream containing oxygen, comprising passing the feed stream through a bed of an adsorbent selective for oxygen so as to adsorb oxygen from the feed stream, thereby producing a product stream depleted in oxygen, wherein the adsorbent comprises a RHO zeolite having a Si/Al ratio of from 3.2 to 4.5 and containing non-proton extra-framework cations, wherein the size, number, and charge of the extra-framework cations that are present in the zeolite are such that 1.8 or fewer non-proton extra-framework cations per unit cell are required to occupy 8-ring sites, and wherein the zeolite has a unit cell axis length of from 14.23 ? to 14.55 ?.

2. The process of claim 1, wherein the feed stream comprises oxygen and one or both of nitrogen and argon, and the product stream is depleted in oxygen and enriched in one or both of nitrogen and argon.

3. The process of claim 1, wherein the size, number and charge of the extra-framework cations that are present in the zeolite are such that 1.6 or fewer non-proton extra-framework cations per unit cell are required to occupy 8-ring sites.

4. The process of claim 1, wherein the zeolite contains at most 6 protons per unit cell.

5. The process of claim 1, wherein the zeolite contains at most 4 protons per unit cell.

6. The process of claim 1, wherein the zeolite contains at most 3 protons per unit cell.

7. The process of claim 1, wherein the zeolite has a unit cell axis length of from 14.23 ? to 14.50 ?.

8. The process of claim 1, wherein the zeolite has a unit cell axis length of from 14.30 ? to 14.45 ?.

9. The process of claim 1, wherein the zeolite has a Si/Al ratio of from 3.6 to 4.2.

10. The process of claim 1, wherein the non-proton extra-framework cations comprise Li.sup.+, Mg.sup.2+, Mn.sup.2+, Fe.sup.2+, Co.sup.2+, Ni.sup.2+, Cu.sup.2+ and/or Zn.sup.2+ cations.

11. The process of claim 1, wherein the non-proton extra-framework cations comprise Li.sup.+ and/or Zn.sup.2+ cations.

12. The process of claim 11, wherein said Li.sup.+ and/or Zn.sup.2+ cations make up the majority of the non-proton extra-framework cations that are present per unit cell.

13. The process of claim 11, wherein said Li.sup.+ and/or Zn.sup.2+ cations make up at least 70% of the non-proton extra-framework cations that are present per unit cell.

14. The process of claim 11, wherein said Li.sup.+ and/or Zn.sup.2+ cations make up at least 80% of the non-proton extra-framework cations that are present per unit cell.

15. The process of claim 1, wherein the zeolite is selected from H.sub.6Li.sub.5.4RHO(3.2), Li.sub.9.0K.sub.0.8RHO(3.9), Li.sub.9.0Na.sub.0.8RHO(3.9), Li.sub.8.3Cs.sub.1.5RHO(3.9), Li.sub.8.0Zn.sub.1.7RHO(3.2), Zn.sub.4.1Na.sub.1.6RHO(3.9), Li.sub.9.2H.sub.0.6RHO(3.9), Li.sub.9.2RHO(4.2), Li.sub.6.0H.sub.1.8Zn.sub.1.0Na.sub.0.6RHO(3.6), Li.sub.7.8H.sub.2.0RHO(3.9), Li.sub.6.8H.sub.3.0RHO(3.9) and Li.sub.5.8H.sub.4.0RHO(3.9).

16. The process of claim 1, wherein the process is a PSA process comprising an adsorption step performed at elevated pressure in which the feed stream is passed through a bed of the adsorbent comprising the RHO zeolite to adsorb oxygen from the feed stream, and a desorption step performed at reduced pressure in which oxygen from the previous adsorption step is desorbed from the bed to regenerate the bed for the next adsorption step.

17. The process of claim 1, wherein the process is process of adsorbing oxygen and nitrogen from a feed stream comprising oxygen, nitrogen, and argon, comprising passing the feed stream through one or more beds of adsorbent comprising a first adsorbent selective for nitrogen to adsorb nitrogen from the feed stream and a second adsorbent comprising the RHO zeolite to adsorb oxygen from the feed stream, thereby producing a product stream enriched in argon and depleted in oxygen and nitrogen.

18. The process of claim 17, wherein the first adsorbent has a Henry's law constant for nitrogen of from 0.5 to 3.0 mmole/gm/bara at 37.78? C.

19. The process of claim 17, wherein the process is a PSA process comprising an adsorption step performed at elevated pressure in which the feed stream is passed through a bed of adsorbent comprising the first and second adsorbents to adsorb nitrogen and oxygen, respectively, thereby producing a product stream enriched in argon and depleted in oxygen and nitrogen, and a desorption step performed at reduced pressure in which oxygen and nitrogen from the previous adsorption step are desorbed from the bed to regenerate the bed for the next adsorption step.

20. The process of claim 19, wherein the bed of adsorbent comprises a first layer comprising the first adsorbent and a second layer comprising the second adsorbent, the first and second layers being arranged such that during the adsorption step the feed stream passes through the first layer and contacts the first adsorbent for adsorption of nitrogen before passing through the second layer and contacting the second adsorbent for adsorption of oxygen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG. 1 is a graph of both O.sub.2 uptake rate and kinetic selectivity of O.sub.2 vs. N.sub.2 plotted against the level of proton exchange in Li.sub.9.0Na.sub.0.8RHO(3.9). .sup.aDuring full H.sup.+ exchange of RHO(3.9) some framework de-alumination occurs resulting in a Si/Al of 4.2, or H.sub.9.2RHO(4.2)

[0047] FIG. 2 is a graph of both O.sub.2 uptake rate and kinetic selectivity of O.sub.2 vs. Ar plotted against the level of proton exchange in Li.sub.9.0Na.sub.0.8RHO(3.9). .sup.aDuring full H.sup.+ exchange of RHO(3.9) some framework de-alumination occurs resulting in a Si/Al of 4.2, or H.sub.9.2RHO(4.2).

[0048] FIG. 3 is a graph showing the variation in O.sub.2 D/r.sup.2 and O.sub.2/Ar kinetic selectivity with change in RHO unit cell axis for selected RHO compositions.

[0049] FIG. 4a is a graph showing isotherm data measured up to 10 atm for O.sub.2 and Ar on Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) at three temperatures.

[0050] FIG. 4b is a graph showing isotherm data measured up to 1 atm for O.sub.2 and Ar on Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) at three temperatures.

[0051] FIG. 5 is a graph of the O.sub.2 vs. N.sub.2 equilibrium selectivity plotted against the level of proton exchange starting from Li.sub.9.0Na.sub.0.8RHO(3.9). .sup.aDuring full H.sup.+ exchange of RHO(3.9) some framework de-alumination occurs resulting in a Si/Al of 4.2, or H.sub.9.2RHO(4.2).

[0052] FIG. 6 is a graph of kinetic selectivity of O.sub.2 vs. N.sub.2 plotted against O.sub.2 uptake rate for a range of RHO and literature zeolites. .sup.aS. Farooq, Gas Separations and Purification, Vol. 9, No. 3, pp 205-212. .sup.bS. Kuznicki, B. Dunn, E Eyring, and D. Hunter, Separation Science and Technology 2009, 44:7, pp 1604-1620.

[0053] FIG. 7 is a graph of kinetic selectivity of O.sub.2 vs. Ar plotted against O.sub.2 uptake rate for a range of RHO and literature zeolites. .sup.aS. Farooq, Gas Separations and Purification, Vol. 9, No. 3, pp 205-212. .sup.bS. Kuznicki, B. Dunn, E Eyring, and D. Hunter, Separation Science and Technology 2009, 44:7, pp 1604-1620.

[0054] FIG. 8 is a graph showing the isotherms of O.sub.2 and N.sub.2 at 5, 23 and 45? C. on Li.sub.6.8H.sub.3.0RHO(3.9).

[0055] FIG. 9 is a graph showing the isotherms of O.sub.2 and N.sub.2 at 5, 23 and 45? C. on Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9).

[0056] FIG. 10 is a graph showing the isotherms of O.sub.2 and Ar at 5, 23 and 45? C. on Li.sub.9.5Na.sub.1.6Cs.sub.0.3RHO(3.2).

[0057] FIG. 11 is a graph showing the isotherms of N.sub.2 and Ar at 23 and 45? C. on Zn.sub.4.1Na.sub.1.6RHO(3.9).

[0058] FIG. 12 is a schematic showing the operation of a 2-bed multi-step Ar PSA cycle. Here F (F1, F2, and F3): feed, CoD: co-current depressurization, I (I1, I2, and I3): idle, EQD1: equalization depressurization 1, DEQD2: dual equalization depressurization 2, DEQD3: dual equalization depressurization 3, CnD (CnD1 and CnD2): counter-current depressurization, PU (PU1 and PU2): product purge, RP (RP1, RP2, and RP3): product re-pressurization, EQR1: top equalization re-pressurization 1, DEQR2: dual equalization re-pressurization 2, DEQR3: dual equalization re-pressurization 3.

[0059] FIG. 13 depicts the results of a comparison of Ar recovery and productivity on RHO and CMS type of adsorbent. The x-axis represents the concentration of O.sub.2 impurity in the product.

[0060] FIG. 14 depicts the results of a sensitivity study using a layering configuration where N.sub.2 selective equilibrium layer is placed at feed end followed by an O.sub.2 selective kinetic layer at product end.

DETAILED DESCRIPTION

[0061] The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments of the invention, it being understood that various changes may be made in the function and arrangement of elements without departing from the scope of the claimed invention.

[0062] The articles a and an as used herein mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of a and an does not limit the meaning to a single feature unless such a limit is specifically stated. The article the preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used.

[0063] As used herein, first, second, third, etc. are used to distinguish from among a plurality of steps and/or features, and is not indicative of the total number, or relative position in time and/or space unless expressly stated as such.

[0064] As used herein, the term comprising means consisting of or including.

[0065] As used herein, the phrase and/or placed between a first entity and a second entity includes any of the meanings of (1) only the first entity, (2) only the second entity, and (3) the first entity and the second entity. The term and/or placed between the last two entities of a list of 3 or more entities means at least one of the entities in the list including any specific combination of entities in this list. For example, A, B and/or C has the same meaning as A and/or B and/or C and comprises the following combinations of A, B and C: (1) only A, (2) only B, (3) only C, (4) A and B and not C, (5) A and C and not B, (6) B and C and not A, and (7) A and B and C.

[0066] As will be understood by those skilled in the art, zeolite structures are often defined in terms of extended structural frameworks of oxygen-linked tetrahedra (L. Pauling, The Nature of the Chemical Bond, 3.sup.rd Edition, Cornell University Press, Ithaca, 1960. D. W. Breck, Zeolite Molecular Sieves, Robert E. Krieger Publishing Co., 1984). In the extended tetrahedral framework, the so-called framework cations of the structural framework (i.e. silicon, Si.sup.4+, and aluminum, A.sup.3+, cations) are surrounded by oxygen anions, O.sup.2?, at the four corners of a tetrahedron. When the charge of the framework cation is 4+, as is the case when the framework cation is the silicon cation, charge neutrality of the extended tetrahedral framework is maintained by sharing each of the oxygen anions between, on average, two tetrahedra, each tetrahedron being a charge neutral SiO.sub.4. Conversely, where the charge of the framework cation is 3+, as is the case when the framework cation is the aluminum cation, similar sharing of oxygen anions between, on average, two tetrahedra leads to each aluminum-centered tetrahedron being negatively charged, i.e., AlO.sub.4.sup.?. The resulting negative charge in the [(Al.sub.xSi.sub.1-x)O.sub.2].sup.x? polyanionic framework is balanced by so-called extra-framework cations that are located at interstitial or non-framework sites in the extended structure. These charge-balancing extra-framework cations (which are often also called non-framework cations) can often be exchanged with other cations by standard cation exchange methods. Exemplary extra-framework cations include alkali and alkaline earth metal cations, transition metal cations, and protons (i.e. hydrogen cations).

[0067] As will also be understood by those skilled in the art, although the extra-framework cations and framework cations present in a zeolite are referred to (as is conventional in the field) as cations, their interactions with the framework oxygen anions of the zeolite are not in practice fully ionic. The bonding of the framework silicon and aluminum cations to the oxygen anions is sufficiently covalent that these framework cations are not readily exchangeable with other cations by standard cation exchange methods. As regards the extra-framework cations, it is likely that smaller, higher charge density cations such as Li.sup.+ or Ca.sup.2+, that create larger distortions in the RHO framework than larger, lower charge density cations such as K.sup.+ and Cs.sup.+, do so, in part, as a result of greater covalent interaction with framework oxygen ions. Also as regards the extra-framework cations, the hydrogen cation or proton may perform its role in balancing negative framework charge by forming a relatively covalent bond with the framework oxygen anion (and indeed it has been hypothesized that in practice said protons associate with said oxygen anions in the form of structural hydroxyl groups; D. W. Breck, Zeolite Molecular Sieves, Robert E. Krieger Publishing Co., 1984).

[0068] As noted supra, RHO zeolites crystallize with a centrosymmetric body centered cubic (bcc) structure, and in their initially synthesized form contain sodium and cesium cations. However, dehydration and/or cation exchange (i.e. substitution of the initially present sodium and cesium cations with other extra-framework cations) can result in distortion of the cubic unit cell structure. As used herein, and unless otherwise indicated, the term RHO zeolite without any further qualifier encompasses both RHO zeolites in their initially synthesized form and RHO zeolites that have been dehydrated and/or subject to cation exchange.

[0069] As used herein, the term non-proton extra-framework cation refers to any extra-framework cation that is not a proton (hydrogen cation). Unless otherwise indicated, all references to numbers of non-proton extra-framework cations that are present per unit cell indicate total numbers of non-proton extra-framework cations (of any and all types) rounded to one decimal place. Thus, a requirement that there are 1.6 or fewer non-proton extra-framework cations per unit cell that are required to occupy 8-ring sites indicates that the maximum number, rounded to one decimal place, of non-proton extra-framework cations per unit cell that may be required to occupy 8-ring sites is 1.6 non-proton extra-framework cations in total.

[0070] Unless otherwise indicated, all references to the number of protons that are present per unit cell indicate the number protons rounded to one decimal place. Thus, a requirement that the zeolite contains at most 3 protons per unit cell indicates that the zeolite contains at most 3.0 protons per unit cell (rounded to one decimal place), and so encompasses also zeolites containing no protons, but excludes zeolites containing 3.1 protons or more per unit cell.

[0071] The extra-framework cation content of a zeolite, namely the numbers, and types, of extra-framework cations (including any protons) that are present per unit cell, can be determined by standard experimental techniques. For example, the extra-framework cation content of a zeolite can be determined by elemental analysis of the solid by energy dispersive spectroscopy (EDX) or by dissolution of a sample and analysis of the solution using inductively coupled plasma optical emission spectroscopy (ICP-OES), as are also described in the Experimental section, infra. Where analysis of a zeolite by EDX or ICP-OES indicates that the negative charge per unit cell of the zeolite is not fully balanced by the non-proton extra-framework cations identified as being present, it is assumed that the remaining negative charge per unit cell is balanced by protons.

[0072] Unless otherwise indicated, all references herein to silicon to aluminum (Si/Al) ratios indicate Si/Al ratios rounded to one decimal place. The Si/Al ratio of a zeolite can, for example, be determined using solid state .sup.29Si NMR. Further details of suitable solid state .sup.29Si NMR techniques and methods of determining the Si/Al ratio of a zeolite from the resulting NMR data are provided in the Experimental section, infra.

[0073] As used herein, the term majority means more than 50%. Thus, reference herein to one or more cations making up the majority of the non-proton extra-framework cations that are present per unit cell of a RHO zeolite indicates that said cation or cations, in their totality, constitute more than 50% of all the non-proton extra-framework cations that are present per unit cell of the RHO zeolite.

[0074] As used herein, all references to percentages of cations that are or must be present per unit cell indicate atomic percent (at. %) unless otherwise indicated. Thus, reference herein to one or more cations making up at least X % of the non-proton extra-framework cations that present per unit cell of a RHO zeolite indicates that said cation or cations, in their totality, constitute equal to or greater than X at. % of all the non-proton extra-framework cations that are present per unit cell of the RHO zeolite. For example, in the zeolite Li.sub.6.0H.sub.1.8Zn.sub.1.0Na.sub.0.6RHO(3.6) there are 7.6 non-proton extra-framework cations per unit cell, of which 7.0 in total are Li.sup.+ or Zn.sup.2+ cations; thus in this composition Li.sup.+ or Zn.sup.2+ cations make up 92.1 at. % of the non-proton extra-framework cations that are present per unit cell, and so this composition would meet a requirement that at least 90% of the non-proton extra-framework cations that are present are Li.sup.+ or Zn.sup.2+ cations.

[0075] As used herein, all references to a zeolite using the designation RHO(X.X), where X.X is number, indicate a RHO zeolite having a silicon to aluminum ratio that is X.X (rounded to one decimal place). Thus, as noted supra, the known prior art mixed-cation RHO zeolite Li.sub.7.1Na.sub.1.93Cs.sub.0.3Al.sub.11.7Si.sub.36.3O.sub.96, which has a silicon to aluminum (Si/Al) ratio of 36.3/11.7=3.1025641, can also be referred to as Li.sub.7.1Na.sub.1.93Cs.sub.0.3RHO(3.1).

[0076] Unless otherwise indicated, all references to the unit cell axis length of a RHO zeolite indicate the unit cell axis length of the RHO zeolite when dehydrated and as measured and determined using X-ray diffraction (XRD). Further details of suitable XRD techniques and methods of determining the unit cell length axis of a RHO zeolite from the resulting XRD data are provided in the Experimental section, infra. For example, the unit cell axis length of a dehydrated sample of RHO zeolite can be determined by Rietveld refinement against the XRD data. Unless otherwise indicated, all references herein to unit cell axis lengths of a RHO zeolite indicate the unit cell axis length rounded to two decimal places.

[0077] As is well known, pressure swing adsorption (PSA) processes comprise an adsorption step which the feed stream containing the substance or substances to be adsorbed is passed through a bed of adsorbent at an elevated pressure, and a desorption step in which substances adsorbed in the previous adsorption step are desorbed from the bed at reduced pressure. In this context, the terms elevated pressure and reduced pressure refer only to the relative pressures in the bed during the two steps, i.e. the pressure during the adsorption step is elevated relative to the pressure during the desorption step but may otherwise be of any suitable pressure for carrying out the adsorption step, and the pressure during the desorption step is reduced relative to the pressure during the adsorption step but may otherwise be of any suitable pressure for carrying out the desorption steps. Suitable operating pressures and cycles for carrying out PSA are well known to those of ordinary skill in the art.

[0078] Disclosed herein are processes, and in particular PSA processes, of adsorbing oxygen from a feed stream containing oxygen, comprising passing the feed stream through a bed of an adsorbent selective for oxygen so as to adsorb oxygen from the feed stream, thereby producing a product stream depleted in oxygen, wherein the adsorbent comprises a RHO zeolite having a Si/Al ratio of from 3.2 to 4.5 and containing non-proton extra-framework cations, wherein the size, number and charge of the extra-framework cations that are present in the zeolite are such that 1.8 or fewer non-proton extra-framework cations per unit cell are required to occupy 8-ring sites, and wherein the zeolite has a unit cell axis length of from 14.23 ? to 14.55 ?.

[0079] Said RHO zeolites used in these processes demonstrate improved productivity, capacity, and/or regenerative properties at ambient (0 to 50? C.) and sub-ambient (<0? C.) temperatures. The present inventors have observed that, surprisingly, said RHO zeolites are suitable for rapid O.sub.2 separation from Ar. The compositions are well suited for use in pressure swing adsorption techniques at ambient (0? C. to 50? C.) temperatures and it is believed that they would be effective at cold temperatures in the process described by U.S. patent application Ser. No. 15/049,704, METHOD FOR ARGON PRODUCTION VIA COLD PRESSURE SWING ADSORPTION (U.S. Pat. No. 9,708,188). In addition to their use in such processes, these and other RHO zeolites described herein may in some cases be suitable for Ar or N.sub.2 PSA applications as well as methane upgrading, i.e., CO.sub.2 removal from methane, applications.

[0080] Without intending to be bound by theory, it is believed that the RHO zeolites described herein achieve their desirable adsorption properties based on the nature of the 8-ring openings of these structures, through which gas molecules must pass to enter the RHO cages. In RHO, these rings are very flexible and can undergo significant distortion from circular to highly elliptical depending on extra framework cation site and type. Optimization of the extra framework cation sites, as well as the generation of elliptical 8-ring openings, are likely important factors in allowing the very rapid uptake of elongated oxygen molecules versus the slow uptake of spherical argon atoms.

[0081] Extra-framework cations in RHO zeolites can occupy at least one of three different sites in the unit cell cage. It is known that small cations, comparable in size to Li.sub.+, i.e. those with a 6-coordinate cation radius as defined by Shannon (R. D. Shannon, Acta Cryst. 1976, A32, 751-767.) of 0.8 ? and below, can reside in 6-ring openings within the unit cell cage. There are 8 of these sites per RHO unit cell in any RHO composition with Si/Al of 5 or less. Smaller cations, 0.8 ? and below, can also occupy one of 12 sites at the center of individual 8-ring openings, and will be required to occupy those sites if all eight of the 6-ring sites are already filled. In addition to Li.sup.+, examples of cations with 6-coordinate Shannon cationic radii smaller than 0.8 ? are Mg.sup.2+ and divalent cations of the first-row transition series and more specifically Mn.sup.2+, Fe.sup.2+, Co.sup.2+, Ni.sup.2+, Cu.sup.2+, and Zn.sup.2+.

[0082] Intermediate size cations, i.e. those with a 6-coordinate Shannon cation radius from 0.8 to approximately 1.3 ?, can reside in one of 12 sites at the center of individual 8-ring openings and, in some cases, at one of 6 sites at the center of two 8-ring windows in the RHO unit cell. They are unable to fit in the 6-ring sites under ambient conditions, i.e. between 0 and 50? C. As an example, in U.S. Pat. No. 5,944,876, Corbin teaches of fully and partially Cd.sup.2+ exchanged RHO zeolites, with Si/Al>3, including RHO compositions with at least 1 Cd.sup.2+ cation per unit cell, with an assortment of other cations. Because of the size of the Cd.sup.2+ cations these compositions require at least one cation, namely the Cd.sup.2+ cation to reside in an 8-ring position.

[0083] The largest cations with a 6-coordinate Shannon cation radius larger than 1.3 ?, including Cs.sup.+, which is required in the RHO crystallization process, occupy one of 6 sites at the center of two 8-ring windows in the RHO unit cell.

[0084] Small, monovalent cations, such as Li.sup.30 are very electropositive and have been shown to cause large elliptical distortions in the 8-ring openings. Larger, divalent cations, such as Ca.sup.2+ and Cd.sup.2+ are also very electropositive and have been shown to cause even larger distortions of the 8-ring openings. In contrast, very small protons, or H.sup.+ cations, cause no distortion of the RHO 8-rings, presumably because they directly bind to one of the zeolite oxygen atoms.

[0085] It has been suggested that while the larger Ca.sup.2+ cations distort and block 8-ring openings, thereby inhibiting gas uptake, the smaller Li.sup.+ cations, while still distorting the 8-rings, could leave enough of the 8-rings open to still permit some gas uptake. This concept was demonstrated, in part, by Corbin in U.S. Pat. No. 7,169,212, who showed that Li.sub.7.1Na.sub.1.93Cs.sub.0.3RHO(3.1) could adsorb O.sub.2 with effective exclusion of N.sub.2. Unfortunately, the O.sub.2 uptake rate of this material is extremely slow and is too slow for PSA applications. While no detailed structural data with cation positions is presented for this material, one can infer that at least 2.23 of the non-proton cations must occupy 8-ring blocking positions. The sodium and cesium cations are too large to fit in 6-rings and fill a total of 2.23 of the 8-ring positions per unit cell. The full cation balance is not reported for this material, and it is possible that additional non-proton cations are forced to reside in the 8-rings. If the cation balance was completed with any monovalent cations other than protons, as many as 3.7 cations would be required to reside in 8-rings. Regardless, the O.sub.2 uptake rate reported is very slow and is consistent with at least 2.2 non-proton cations being forced to reside in the 8-rings of this composition, based on other comparative examples.

[0086] As a comparative example Li.sub.9.5Na.sub.1.6Cs.sub.0.3RHO(3.2) was prepared, which nominally contains the same number of cations/unit cell as the material reported by Corbin, and was targeted to achieve the same number of Na.sup.+ and Cs.sup.+ cations. In this composition, one can infer that at least 3.4 of the cations must occupy 8-ring blocking positions. The sodium and cesium cations are too large to fit in 6-rings and fill a total of 1.9 of the 8-ring positions per unit cell. Of the 9.5 Li.sup.+ cations per unit cell, once all 8 of the 6-ring positions are filled, 1.5 must go in 8-ring positions, making a total of 3.4 cations in 8-ring positions in this unit cell. This material showed nitrogen exclusion behavior like the above RHO composition of Corbin, and the oxygen uptake rate observed was exceedingly slow.

[0087] For a RHO material with Si/Al of 3.2, even when all of the cations are small enough to fit in 6 ring windows, if they are monovalent, such as Li.sup.+, 3.4 of them would be required to reside in 8-ring windows, based on the charge balance required for a RHO(3.2) material. In a comparative example, described infra, fully lithium exchanged Li.sub.11.4RHO(3.2) has been demonstrated to show good kinetic selectivity for oxygen over nitrogen and argon, but the oxygen uptake rate is still much slower than CMS and indeed is comparable to the material described by Corbin.

[0088] In other comparative examples, described infra, it has been shown that fully proton exchanged H.sub.10.7RHO(3.5) and H.sub.9.2RHO(4.2) adsorb O.sub.2, N.sub.2, and Ar very rapidly, but non-selectively, consistent with an absence of distortion in the 8-ring windows.

[0089] For RHO compositions with Si/Al between 3.2 and 4.5, the extent of aluminum substitution, and consequently the formal negative charge which must be balanced, ranges from 11.4 to 8.7. In the RHO zeolites used according to the present invention, the non-proton extra-framework cations are chosen such that 1.8 or fewer non-proton extra-framework cations, and most preferably 1 or fewer non-proton extra-framework cations are forced to reside, by virtue of their size, charge, and/or the total number of extra-framework cations, in 8-ring blocking sites. For example, as previously discussed, Li.sub.11.4RHO(3.2) is forced to have at least 3.4 Li.sup.+ cations in 8-ring blocking positions, once all 6-ring positions are filled, and thus is not suitable for use in the present invention. Conversely, the novel RHO zeolite Zn.sub.5.7RHO(3.2) satisfies the charge balance of RHO(3.2) with potentially no cations in 8-ring windows, i.e., all cations are small enough to fit in 6-rings (the Shannon 6-coordinate cation radius for Zn.sup.2+ is 0.74 ?), and there are enough 6-rings to hold all of the cations. In actuality, some of the Zn.sup.2+ cations are observed to reside in 8-rings in the Zn.sub.5.7RHO(3.2) unit cell but, by virtue of their size and number, there is the potential for them to move between the 6 and 8 rings. The Zn.sub.5.7RHO(3.2) composition shows excellent kinetic selectivity for O.sub.2 vs. N.sub.2 and O.sub.2 vs. Ar, and its O.sub.2 D/r.sup.2 is 900 times faster than that of Li.sub.11.4RHO(3.2), 915 times faster than that of Li.sub.9.5Na.sub.1.6Cs.sub.0.3RHO(3.2), and 10 times faster than an example CMS material. This composition as well as other new RHO zeolite compositions described herein, including Zn.sub.4.1Li.sub.1.1Na.sub.0.5RHO(3.9) and Zn.sub.4.9RHO(3.9), also show excellent equilibrium selectivity for N.sub.2 over argon, making them potentially useful for removal of trace N.sub.2 from argon. The larger unit cell composition Zn.sub.4.1Na.sub.1.6RHO(3.9) also shows excellent equilibrium selectivity for N.sub.2 over argon, but shows lower kinetic selectivity for O.sub.2 vs. Ar than some of the compositions of this invention.

[0090] In addition to choosing small, divalent cations, the number of distorting cations required to be in 8-ring windows can also be decreased by increasing the Si/Al ratio. As the Si/Al ratio of RHO goes from 3.2 to 3.9 to 4.2, LiRHO compositions go from Li.sub.11.4RHO(3.2), to Li.sub.9.8RHO(3.9), to Li.sub.9.2RHO(4.2). In the present examples, the composition Li.sub.9.8RHO(3.9) was found to contain low levels of sodium, and actually has the composition Li.sub.9.0Na.sub.0.8RHO(3.9) by ICP analysis. Required 8-ring occupancy in this series goes from 3.4 to 1.8 to 1.2. O.sub.2 D/r.sup.2 increases from 1 to 67 to 2400 relative to that of Li.sub.11.4RHO(3.2), while O.sub.2 vs. N.sub.2 and Ar kinetic selectivities remain high for all. For oxygen removal, both the Li.sub.9.0Na.sub.0.8RHO(3.9) (nominally Li.sub.9.8RHO(3.9)) composition and the Li.sub.9.2RHO(4.2) composition are potentially useful. Both show very high kinetic selectivity for O.sub.2 vs. Ar, while the first composition has an O.sub.2 D/r.sup.2 uptake rate comparable to that of CMS and the second composition has an O.sub.2 D/r.sup.2 uptake rate 27 times faster than CMS.

[0091] A third way to reduce the number of distorting cations required to occupy blocking 8-ring positions involves the substitution of blocking cations with protons, which presumably do not distort the 8-rings. Corbin in U.S. Pat. No. 7,169,212, mentions the possibility that mixed cation RHO materials with partial exchange of H+(e.g. H,CsRHO) could be prepared which would give at least some of the desired distortion and smaller pore size, but no specific compositions were reported. Recently, Paul A. Wright and co-workers (J. Am. Chem. Soc. 2012, 134, 17628) described the preparation of mixed Li.sub.9.8-xH.sub.xRHO(3.9), but no adsorption data was reported. In the present application, a number of Li.sub.9.8-xH.sub.xRHO(3.9) compositions have been prepared, and are described infra, in which the number of non-proton cations forced to reside in 8-ring blocking sites have been lowered to 1 or fewer. Unless otherwise indicated, all adsorption data, both equilibrium and kinetic, was measured at 30? C. As shown in FIGS. 1 and 2, substitution of Li.sup.+ cations in Li.sub.9.0Na.sub.0.8RHO(3.9) with varying numbers of protons leads to a dramatic increase in O.sub.2 D/r.sup.2 (up to 40 times higher than Li.sub.9.0Na.sub.0.8RHO(3.9)). Surprisingly, good O.sub.2 vs. N.sub.2 and O.sub.2 vs. Ar kinetic selectivity is maintained to up to a composition of Li.sub.5.8H.sub.4RHO(3.9) and H.sub.6Li.sub.5.4RHO(3.2). Results of Ar PSA simulations on Li.sub.6.8H.sub.3RHO(3.9) shown in Table 4 and Example 11, show a large improvement in Ar recovery and productivity over CMS materials, though not quite as high as the novel composition, Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9).

[0092] A comparison of the unit cell axis data in Table 3 and FIG. 3 with the O.sub.2 D/r.sup.2 and O.sub.2/Ar kinetic selectivity data show the optimal RHO materials for rapid separation of O.sub.2 from Ar. These are RHO compositions, having 1.8 or fewer and preferably 1.6 or fewer and most preferably 1.0 or fewer non-proton extra-framework cations per unit cell, which, by virtue of their size, charge and/or number, are required to reside in 8-ring sites; and having a cubic or approximately cubic unit cell defined by a unit cell axis between 14.23 and 14.55 ?, more preferably between 14.23 and 14.50 ?, and most preferably between 14.30 and 14.45 ?. Those with a smaller unit cell axis, including Li.sub.11.4RHO(3.2), show slower O.sub.2 uptake than typical CMS materials, while those with larger unit cell axes, like H.sub.9.2RHO(4.2), show lower kinetic selectivity for O.sub.2 over Ar than CMS materials. Also described herein are new RHO zeolite compositions with larger unit cell axes, from 14.45 ? to 14.85 ?, that additionally or alternatively show potential for the rapid separation of trace N.sub.2 from Ar by an equilibrium separation process. A comparison of the unit cell and uptake data in Table 3 shows that both Zn.sub.4.9RHO(3.9) (with unit cell axis=14.54 ?) and Zn.sub.4.1Li.sub.1.1Na.sub.0.5RHO(3.9) (unit cell axis=14.49 ?) rapidly adsorb N.sub.2 with equilibrium selectivity of N.sub.2 vs. Ar of approximately 3. FIG. 11 shows the isotherm data for Zn.sub.4.1Na.sub.1.6RHO(3.9) confirming high N.sub.2 vs Ar equilibrium selectivity over a range of pressures and temperatures. It is likely that these compositions would also be useful for other rapid separations including that of CO.sub.2 from methane.

[0093] A number of the RHO zeolite compositions described herein provide high kinetic selectivity for O.sub.2 versus Ar and O.sub.2 versus N.sub.2 adsorption at ambient temperatures. As shown in FIG. 4b, an O.sub.2 versus Ar equilibrium selectivity close to 1 is observed on Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) at 23? C. Effective equilibrium selectivity for O.sub.2 versus Ar is observed as the adsorption temperature is dropped below 23? C., which is likely associated with temperature dependent contraction of the RHO 8 ring windows as well as reduced vibration of the rings. Thus, these RHO materials appear to have ideal properties for removal of O.sub.2 from argon containing streams.

[0094] The O.sub.2 vs. N.sub.2 equilibrium selectivity of low-silica zeolites is typically less than 0.5, due to the stronger quadrupole interactions between N.sub.2 and electropositive extra framework cations of the zeolite. The RHO zeolites also show this behavior, but as can be seen in FIG. 5, the O.sub.2 vs. N.sub.2 equilibrium selectivity can be significantly improved by increasing the number of protons exchanged for Li.sup.+ in a Li.sub.9.0Na.sub.0.8RHO(3.9) zeolite. Complete exchange to the fully protonated RHO increases the O.sub.2/N.sub.2 equilibrium selectivity to 0.76. Unfortunately, the lack of 8-ring distortion by these cations leads to negligible kinetic selectivity between O.sub.2 and N.sub.2. By balancing improved O.sub.2 vs. N.sub.2 equilibrium selectivity with high kinetic selectivity and rate, it appears that an optimum may in some instances be achieved on exchange of 3 to 4 protons (FIGS. 1 and 5).

[0095] Also disclosed herein are processes, and in particular PSA processes, of adsorbing oxygen and nitrogen from a feed stream comprising oxygen, nitrogen, and argon, comprising passing the feed stream through one or more beds of adsorbent comprising a first adsorbent selective for nitrogen to adsorb nitrogen from the feed stream and a second adsorbent selective for oxygen to adsorb oxygen from the feed stream, thereby producing a product stream enriched in argon and depleted in oxygen and nitrogen. In said processes, the second adsorbent preferably comprises a RHO zeolite as described above as being preferred for use in processes for adsorbing oxygen from a feed stream containing oxygen. The first adsorbent preferably has a Henry's law constant for nitrogen of from 0.5 to 3.0 mmole/gm/bara at 37.78? C. In certain embodiments, the first adsorbent may also comprise a RHO zeolite of the type described above as being preferred in the context of processes for adsorbing oxygen, except that in the case of the first adsorbent said RHO zeolite has a unit cell axis of from 14.45 ? to 14.85 ? (instead of having a unit cell axis of from 14.23 ? to 14.55 ? as is preferred for the RHO zeolites used for the selective adsorption of oxygen).

[0096] Several of the new RHO zeolite compositions described herein that have unit cell axis from 14.45 ? to 14.85 ? show potential for the rapid removal of trace N.sub.2 from Ar by an equilibrium separation process. Both Zn.sub.4.9RHO(3.9) (with unit cell axis=14.54 ?) and Zn.sub.4.1Li.sub.1.1Na.sub.0.5RHO(3.9) (unit cell axis=14.49 ?) show rapid uptake of N.sub.2 with equilibrium selectivity of N.sub.2 vs. Ar of 3.

[0097] Also described herein are new methods of making RHO zeolite compositions, including the new RHO zeolite compositions described herein.

[0098] In particular, described herein is a convenient method of preparing the RHO zeolite compositions using reduced levels of templating agent. As previously described by Chatelain in (Microporous Materials, 1995, 4, 231), RHO(3.9) can be readily prepared using 18-crown-6 as a templating or structure directing agent. While effective, 18-crown-6 is expensive, and the literature preparation uses this reagent in approximately stoichiometric amounts with CsOH. The present application describes a process whereby seeding the RHO preparation gel composition with approximately 10 wt. % of Na.sub.6.8Cs.sub.3.0RHO(3.9) (relative to the amount of RHO(3.9) product produced) allows the quantity of 18-crown-6 used to be cut by 80%. Use of small amounts of Na.sub.8.4Cs.sub.3.0RHO(3.2) seed material also allows RHO(3.2) to be prepared much more reliably. While as synthesized Na.sub.8.4Cs.sub.3.0RHO(3.2) and Na.sub.6.8Cs.sub.3.0RHO(3.9) are used in seeding the targeted RHO preparations, it is believed that any RHO material with the target Si/Al would be effective.

[0099] The present inventors have also observed that, when preparing mixed cation RHO zeolites, particularly those containing Li.sup.+ and Zn.sup.2+ cations, the order of cation exchange significantly impacts the loading of Zn.sup.2+ achievable in the zeolite. This can be seen from the exchange data in Table 1. When starting with Li.sub.11.4RHO(3.2), a single Zn-exchange with >100:1 molar ratio of 2M Zn(NO.sub.3).sub.2 exchange solution concentration to zeolite concentration leads to the replacement of only 2.4 of the 11.4 Li.sup.+ cations. A second exchange under the same conditions replaces only one additional Li.sup.+ cation. Complete replacement of Li.sup.+ with Zn.sup.2+ is apparently a very slow and difficult process, perhaps because most of the Li.sup.+ cations must be exchanged from very small 6-ring sites. In contrast exchanging Zn.sup.2+ into RHO appears to proceed much more easily starting from the sodium exchanged RHO. Only 3 exchanges, using only a 40:1 molar ratio of 1.5 M Zn(NO.sub.3).sub.2 exchange solution concentration to zeolite concentration, are required to fully load the RHO structure with Zn.sup.2+ cations. Back-exchange of the resulting Zn.sub.5.7RHO(3.2) or Zn.sub.4.9RHO(3.9) composition with Li.sup.+ appears to proceed smoothly.

EXAMPLES

[0100] The compositions described herein in the following examples were characterized in the following manner. In addition to measuring their adsorption properties, novel zeolite compositions were characterized by X-ray diffraction, .sup.29Si NMR, by elemental analysis using ICP-OES, and by scanning electron microscopy (SEM).

[0101] Powder x-ray diffraction (XRD) patterns of hydrated and dehydrated samples were measured in Debye-Scherrer geometry on a Stoe STAD i/p diffractometer with monochromated Cu K?.sub.1 X-rays (?=1.54056 ?). In addition, laboratory powder X-ray diffraction for Rietveld refinement was performed on samples in quartz glass capillaries that had been activated at the glass line at 623 K for 10 hours and sealed using a blow-torch. The unit cell axis length and the number, and position of extra framework cations for Li,M-Rho samples were determined by Rietveld refinement against the laboratory PXRD data, using the GSAS suite of programs.

[0102] Solid state .sup.29Si NMR spectra were obtained at ambient temperature on a Bruker Avance II 300 FT-NMR spectrometer, equipped with a 7 mm MAS probe. The acquisition was carried out using one pulse employing an 8-second recycle delay while the rotor was spun at 5000 Hz at magic angle. Peak deconvolution was performed using GRAMS/32 Al (version 6.00) software. Mixed Gaussian/Lorentzian line shapes were employed. From the relative peak areas, the Si/Al ratio was calculated using Equation 1.

[00001]$\frac{\mathrm{Si}}{\mathrm{Al}}=\frac{\underset{n=0}{\overset{4}{.Math.}}.Math.I_{{\mathrm{Si}}_{\left(\mathrm{nAl}\right)}}}{\underset{n=0}{\overset{4}{.Math.}}.Math.0.25.Math..Math.n.Math..Math.I_{{\mathrm{Si}}_{\left(\mathrm{nAl}\right)}}}$

[0103] where: Si/Al=Silicon to Aluminum ratio, I=Relative area of NMR peak.

[0104] Si.sub.(nAl)=Silicon with n aluminum atoms as nearest neighbor bound through oxygen.

[0105] n=Number of nearest aluminum atoms represented by the NMR peak.

[0106] The cation exchange level was determined by elemental analysis using established methods, which involved either direct analysis on the solid zeolite by energy dispersive spectroscopy (EDX) or dissolution of the solid and subsequent analysis of the solution using inductively coupled plasma optical emission spectroscopy (ICP-OES).

[0107] A Perkin Elmer 3000DV Inductively Coupled Plasma Optical Emission Spectrometer was used for the sample analysis.

[0108] SEM analyses were performed using a Hitachi S-4800 field-emission SEM operated at 2 kV accelerating voltage.

[0109] EDX analysis on all samples was performed in a JEOL JSM 5600 SEM, with an Oxford INCA Energy 200 EDX analyser.

Example 1: Synthesis of Na.SUB.8.4.Cs.SUB.3.0.RHO(3.2)

[0110] The modified RHO synthesis method described by Corbin et al. in U.S. Pat. No. 7,169,212, was used to prepare Na.sub.8.4Cs.sub.3.0Al.sub.11.4Si.sub.36.6O.sub.96, or Na.sub.8.4Cs.sub.3.0RHO(3.2), a RHO zeolite composition with nominal Si/Al=3.2. The purity of the crystalline material was determined by XRD and Si/Al by .sup.29Si NMR. Both matched reported data for the hydrated form of RHO(3.2) zeolites. SEM images show an average particle size of 2.5 m.

Example 2: Synthesis of Na.SUB.6.8.Cs.SUB.3.0.RHO(3.9)

[0111] RHO(3.9) was prepared by the method of Chatelain et al. (Microporous Materials, 1995, 4, 231). Crystallization and isolation followed by calcination in air at 550? C. gives Na.sub.6.8Cs.sub.3.0Al.sub.9.8Si.sub.38.2O.sub.96 or Na.sub.6.8Cs.sub.3.0RHO(3.9). The purity of the crystalline material was determined by XRD and the Si/Al ratio by .sup.29Si NMR. Results from both methods matched reported data for the hydrated form of RHO(3.9) zeolites. SEM images show an average particle size of ?1 ?m.

Example 3: Synthesis of RHO Zeolites with Reduced Levels of Templatinq Agent Na.SUB.8.4.Cs.SUB.3.0.RHO(3.2)

[0112] Aluminum isopropoxide (3.67 g) was dissolved in aqueous NaOH solution (50 weight %, 4.5 g) at 100? C. and left to cool to 25? C. Aqueous CsOH solution (50 weight %, 2.7 g) was then added with stirring, followed by 18-crown-6 (0.27 g). Next, Na.sub.8.4Cs.sub.3.0RHO(3.2) was added as a seed material (0.6 g), followed by colloidal silica (Ludox 30, 20 g), and then, distilled water (0.35 g). Once homogeneous, the gel was aged at 25? C. for 4 days before crystallizing in a polypropylene bottle at 90? C. for 5 days under static conditions. The resulting white solid was filtered and washed before being dried at 90? C. overnight. The product was placed in a ceramic dish and slowly heated in a Fisher Scientific muffle furnace under 10 L/min ambient air purge to 300? C. at 0.4? C./min, and then heated to 550? C. at 1? C./min. Calcination of the product continued under 10 L/min ambient air purge at 550? C. for 24 hours. Approximately 6 g of pure RHO(3.2) product was obtained. Na.sub.6.8Cs.sub.3.0RHO(3.9):

[0113] NaOH (0.59 g) and 18-crown-6 (0.27 g) were dissolved in CsOH solution (50 weight %, 1.8 g) and distilled water (0.78 g) before adding sodium aluminate (1.82 g) and stirring until homogenous. Na.sub.6.8Cs.sub.3.0RHO(3.9) was added as seed material (0.6 g), followed by colloidal silica (Ludox 40, 15 g), and the mixture was stirred until homogenous. The gel was aged at 25? C. for 4 days before crystallizing in a polypropylene bottle at 90? C. for 5 days under static conditions. The resulting white solid was filtered and washed before being dried at 90? C. overnight. The product was placed in a ceramic dish and slowly heated in a Fisher Scientific muffle furnace under 10 L/min ambient air purge to 300? C. at 0.4? C./min, and then heated to 550? C. at 1? C./min. Calcination of the product continued under 10 L/min ambient air purge at 550? C. for 24 hours. Approximately 6 g of pure RHO(3.9) was obtained.

Example 4: Cation Exchange of RHO Zeolites

[0114] A variety of exchanged RHO(3.2 to 4.2) materials were prepared through ion exchange of the starting Na.sub.5.4Cs.sub.3.0RHO(3.2) and Na.sub.6.8Cs.sub.3.0RHO(3.9) from Examples 1 through 3, as well as H.sub.9.2RHO(4.2) from Example 6. Ammonium exchanged RHO samples were prepared by repeated (8 times) exchange with a 40-fold excess (mole % basis) of 1M ammonium chloride solution at 90? C. for at least 4 hrs. Sodium exchanged RHO materials were prepared from ammonium RHO zeolites through repeated (8 times) exchange with a 40-fold excess (mole % basis) of 1M sodium chloride solution at 90? C. for at least 4 hrs. The resulting sodium RHO could be readily exchanged with Zn.sup.2+ or Cu.sup.2+ using excess 1.5 M Zn(NO.sub.3).sub.2 or 1.5M Cu(NO.sub.3).sub.2 solutions at 90? C. Usually, two exchange steps were carried out to ensure complete exchange. Exchange of NaRHO to the LiRHO forms was more difficult and required at least 6 exchanges with 1M lithium chloride at 90? C. for at least 4 hrs. Exchange of LiRHO materials with Zn.sup.2+ using Zn(NO.sub.3).sub.2 solutions was very difficult to drive to completion and typically only exchanged 1 to 1.5 Zn.sup.2+ cations per unit cell into the LiRHO composition. Final exchange compositions were determined by ICP-OES or EDX. Where the analysis gives a cation charge balance which is lower than that needed for the number of aluminum atoms/unit cell in a given RHO composition, the difference is assumed to be made up with protons, e.g. Li.sub.5.2Zn.sub.1.8Na.sub.0.5RHO(3.9) is adjusted to Li.sub.5.2Zn.sub.1.5H.sub.0.5Na.sub.0.5RHO(3.9) to fully balance the charge of the 9.8 alumina centers/unit cell. A number of compositions and exchange conditions are shown in Table 1.

TABLE-US-00001 TABLE 1 Exchange Conditions for Pure and Mixed Cation RHO Samples Salt/zeolite Contact Exchange Exchange Starting RHO Salt Solution (molar ratio) Time (hrs) Temperature ? C. Repeats Product Composition Na.sub.8.4Cs.sub.3.0RHO(3.2) 1M NH.sub.4Cl 40 >4 90 8 (NH.sub.4).sub.11.4RHO(3.2) (NH.sub.4).sub.11.4RHO(3.2) 1M NaCl 40 >6 90 8 Na.sub.11.4RHO(3.2) Na.sub.11.4RHO(3.2) 2M Zn(NO.sub.3).sub.2 40 >4 90 4 Zn.sub.5.7RHO(3.2) Na.sub.11.4RHO(3.2) 1.5M Cu(NO.sub.3).sub.2 30 >4 90 4 Cu.sub.5.7RHO(3.2) Na.sub.11.4RHO(3.2) 1M LiCl 40 >4 90 8 Li.sub.11.4RHO(3.2) Na.sub.6.8Cs.sub.3.0RHO(3.9) 1M NH.sub.4Cl 40 >4 90 8 (NH.sub.4).sub.9.8RHO(3.9) (NH.sub.4).sub.9.8RHO(3.9) 1M NaCl 40 >6 90 8 Na.sub.9.8RHO(3.9) (NH.sub.4).sub.9.8RHO(3.9) 2M Zn(NO.sub.3).sub.1 >100 >4 60 4 Zn.sub.4.9RHO(3.9) Na.sub.9.8RHO(3.9) 2M Zn(NO.sub.3).sub.2 40 >4 90 3 Zn.sub.4.1Na.sub.1.6RHO(3.9) Na.sub.9.8RHO(3.9) 1.5M Cu(NO.sub.3).sub.2 30 >4 90 4 Cu.sub.4.9RHO(3.9) Na.sub.9.8RHO(3.9) 1M LiCl 40 >4 90 8 Li.sub.9.0Na.sub.0.8RHO(3.9) Li.sub.9.0Na.sub.0.8RHO(3.9) 0.07M NH.sub.4Cl 2 >4 90 1 Li.sub.7.8(NH.sub.4).sub.2RHO(3.9) Li.sub.9.0Na.sub.0.8RHO(3.9) 0.1M NH.sub.4Cl 3 12 90 1 Li.sub.6.8(NH.sub.4).sub.3RHO(3.9) Li.sub.9.0Na.sub.0.8RHO(3.9) 0.1M NH.sub.4Cl 4 12 90 1 Li.sub.5.8(NH.sub.4).sub.4RHO(3.9) Li.sub.11.4RHO(3.2) 2M Zn(NO.sub.3).sub.2 >100 >4 60 1 Li.sub.9.0Zn.sub.1.2RHO(3.2) Li.sub.9.0Zn.sub.1.2RHO(3.2) 2M Zn(NO.sub.3).sub.2 >100 >4 60 1 Li.sub.8.0Zn.sub.1.7RHO(3.2) Li.sub.9.2H.sub.0.6RHO(3.9) 2M Zn(NO.sub.3).sub.2 >100 0.5 60 1 Li.sub.7.4Zn.sub.1.2RHO(3.9) Zn.sub.4.1Na.sub.1.6RHO(3.9) 1M LiCl 30 12 90 1 Zn.sub.4.1Li.sub.1.1Na.sub.0.5RHO(3.9) Zn.sub.4.1Li.sub.1.1Na.sub.0.5RHO(3.9) 1M LiCl 30 12 90 1 Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9)

Example 5: Synthesis of H.SUB.10.7.RHO(3.5)

[0115] Na.sub.8.4Cs.sub.3.0RHO(3.2), from Example 1 was mixed with a 10-fold excess (mole % basis) of 1M ammonium chloride solution at 90? C. for at least 4 hrs. After mixing, the material was filtered. The ammonium chloride mixing (exchange) was repeated 8 times to fully convert the material to the ammonium exchanged RHO(3.2). After filtering, the material was rinsed 3 times with a 3-fold excess (weight % basis) of DI water and was dried overnight at 90? C. Typically, 75 g of the ammonium exchanged RHO(3.2) was placed in a ceramic dish and calcined in a purged Fisher Scientific muffle furnace. While the oven was purged with ambient air at 5 L/min, the material was heated at a rate at 0.8? C./min in air to 550? C. and calcined at this temperature for 24 hrs to prepare the proton exchanged RHO composition. Solid State .sup.29Si NMR demonstrated that some de-alumination had occurred during calcination, leading to a Si/Al of 3.5. Calcination conditions and products from ammonium substituted RHO materials are shown in Table 2.

TABLE-US-00002 TABLE 2 Calcination of NH.sub.4-substituted RHO materials Calcination Calcination Temperature Product Starting RHO time (hrs) ? C. Composition (NH.sub.4).sub.11.4RHO(3.2) 24 550 H.sub.10.7RHO(3.5) (NH.sub.4).sub.9.8RHO(3.9) 24 550 H.sub.9.2RHO(4.2) Li.sub.7.8(NH.sub.4).sub.2RHO(3.9) 24 550 Li.sub.7.8H.sub.2RHO(3.9) Li.sub.6.8(NH.sub.4).sub.3RHO(3.9) 24 550 Li.sub.6.8H.sub.3RHO(3.9) Li.sub.5.8(NH.sub.4).sub.4RHO(3.9) 24 550 Li.sub.5.8H.sub.4RHO(3.9)

Example 6: Synthesis of H.SUB.9.2.RHO(4.2)

[0116] RHO(3.9), Na.sub.6.8Cs.sub.3.0RHO(3.9), from Example 2 was mixed with a 10-fold excess (mole % basis) of 1M ammonium chloride solution at 90? C. for at least 4 hrs. After mixing, the material was filtered. The ammonium chloride mixing (exchange) was repeated 8 times to fully convert the material to the ammonium exchanged RHO(3.9). After filtering, the material was rinsed 3 times with a 3-fold excess (wt. % basis) of DI water and was dried overnight at 90? C. Typically, 75 g of the ammonium exchanged RHO(3.9) was placed in a ceramic dish and calcined in a purged Fisher Scientific muffle furnace. While the oven was purged with ambient air at 5 L/min, the material was heated at a rate of 0.8? C./min in air to 550? C. and calcined at this temperature for 24 hrs to prepare the proton exchanged RHO composition, as shown in Table 2. Solid State .sup.29Si NMR demonstrated that some de-alumination had occurred during calcination, leading to a Si/Al of 4.2.

Example 7: Synthesis of Mixed LiHRHO(3.9) and Li,HRHO(3.2) Zeolites

[0117] Li.sub.9.0Na.sub.0.8RHO(3.9), from Example 4, was mixed with 2:1, 3:1, and 4:1 stoichiometric ratios of 1M ammonium chloride solution at 90? C. for at least 4 hrs. After mixing, the material was filtered. After filtering, the material was rinsed 3 times with a 3-fold excess (wt. % basis) of DI water and was dried overnight at 90? C. Typically, 5-10 g of the partially ammonium exchanged RHO(3.9) samples were placed in a ceramic dish and calcined in a purged Fisher Scientific muffle furnace. While the oven was purged with ambient air at 5 L/min, the material was heated at a rate of 0.8? C./min in air to 550? C. and calcined at this temperature for 24 hrs to prepare the mixed Li.sub.7.8H.sub.2RHO(3.9), Li.sub.6.8H.sub.3RHO(3.9), and Li.sub.5.8H.sub.4RHO(3.9) compositions, as shown in Table 2. The extent of ion exchange was confirmed by inductively coupled plasma optical emission spectroscopy (ICP-OES). An analogous process was carried out to prepare mixed Li,HRHO(3.2) zeolites, including H.sub.6Li.sub.5.4RHO(3.2), after exchanging Li.sub.11.4RHO(3.2) from example 4 with a 6:1 stoichiometric ratio of 1M ammonium chloride solution at 90? C. for 4 hrs. While the resulting LiHRHO(3.2) composition is referred to in this application as H.sub.6Li.sub.5.4RHO(3.2), based on the starting partially ammonium exchanged RHO(3.2); it is likely that some de-alumination to a slightly higher framework Si/Al ratio has occurred. ICP-OES results suggest the actual Si/Al may be closer to 3.5.

Example 8: Adsorption Rate Uptake Measurements

[0118] The mass transfer properties of the adsorbents were evaluated using a standard volumetric adsorption apparatus. The experiment consisted of exposing an adsorbent sample, which is initially at vacuum and 30? C., to a measured amount of O.sub.2, N.sub.2, or Ar at 760 Torr (101 kPa). The change in pressure was then followed as a function of time. The pressure time data is then subtracted from a similar pressure history using the same weight of quartz beads in the place of the adsorbent sample to obtain a plot of the amount of gas adsorbed as a function of time, also known as an uptake curve. From the initial slope of the uptake curve, a diffusion parameter for the test gas in units of inverse time (sec.sup.?1) can be obtained. It is understood that the heat dissipation from adsorbent due to this step change in adsorbate loading during the kinetic measurement can affect the diffusion parameter when isothermal model is used in the calculation of the parameters. It is important to note that the heat of adsorptions for oxygen, nitrogen and argon on the RHO adsorbents considered here are significantly lower than most of the known zeolite adsorbents. Therefore, the diffusional parameter calculated under the assumption of isothermal behavior should be a reasonable estimate of the diffusion parameter.

[0119] A pseudo-equilibrium capacity can be defined for a given adsorbent sample over the timeframe of the experiment as follows. The pressure reduction of a gas over an adsorbent sample weighing 2.2 g is measured starting at 760 Torr (101 kPa) until the rate of pressure reduction is <1 Torr/min. A term Pmax is defined as the total pressure reduction or gas uptake over a 2.2 g sample of adsorbent after subtraction from the pressure reduction of the 2.2 g glass bead blank. The Pmax together with the system volume, thus defines a pseudo-equilibrium capacity. These Pmax values are given for various adsorbents in Table 3.

TABLE-US-00003 TABLE 3 Structural and Adsorption Data for Adsorbents of the Invention and Prior Art Materials. Required minimum 8-ring occupancy RHO of non- Particle Kinetic Kinetic O.sub.2 N.sub.2 Ar Unit Cell proton O.sub.2 size Selectivity Selectivity Pmax Pmax Pmax O.sub.2 Pmax*/ O.sub.2 Pmax*/ Adsorbent axis (?) cations D/r.sup.2 (?m) O.sub.2/N.sub.2 O.sub.2/Ar (torr) (torr) (torr) N.sub.2 Pmax* Ar Pmax* Li.sub.9.5Na.sub.1.6Cs.sub.0.3RHO(3.2).sup.d 14.219 3.4 9.84E?05 2.5 ND 100 36 ND ND >10 >10 Li.sub.11.4RHO(3.2) 14.167 3.4 1.00E?04 2.5 ND ND ND ND ND ND ND H.sub.6Li.sub.5.4RHO(3.2).sup.c ND 0 1.01E?01 2.5 3.70E+01 >50 50 75 42 0.67 1.2 Li.sub.9.4Ca.sub.1.0RHO(3.2) 14.155 2.4 ND 2.5 ND ND ND ND ND ND ND Li.sub.9.0Zn.sub.1.2RHO(3.2).sup.c 14.219 2.2 1.97E?03 2.5 17.2 17.8 43 37 5 1.15 8.2 Li.sub.9.0K.sub.0.8RHO(3.9).sup.c 14.255 1.8 6.70E?03 1.0 37 41 47 53 14 0.89 3.3 Li.sub.9.0Na.sub.0.8RHO(3.9).sup.c ND 1.8 6.70E?03 1.0 30 100 53 65 5 0.82 11.6 Li.sub.8.3Cs.sub.1.5RHO(3.9) 14.362 1.8 5.49E?02 1.0 39 301 46 81 38 0.57 1.2 Li.sub.8.0Zn.sub.1.7RHO(3.2).sup.c ND 1.7 3.14E?03 1.0 22.4 23 43 43 3 1 12.3 Zn.sub.4.1Na.sub.1.6RHO(3.9) ND 1.6 3.10E?01 1.0 13.5 35 56 160 56 0.35 1.0 Li.sub.9.2H.sub.0.6RHO(3.9) ND 1.2 1.77E?02 1.0 65 117 46 79 14 0.58 3.2 Li.sub.9.2RHO(4.2) ND 1.2 2.40E?01 1.0 51 323 52 0.11 0.18 0.08 1.4 Li.sub.7.8Ca.sub.1.0RHO(3.9) 14.211 0.8 ND 2.5 ND ND ND ND ND ND ND Li.sub.7.4Zn.sub.1.2RHO(3.9) 14.300 0.6 3.52E?02 1.0 40.3 207 48 80 21 0.6 2.3 Li.sub.6.0H.sub.1.8Zn.sub.1.0Na.sub.0.6RHO(3.6) ND 0.6 2.04E?01 1.0 58 485 51 0.11 0.21 0.10 1.1 Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) 14.330 0.5 3.70E?01 1.0 53 525 52 118 47 0.44 1.1 Zn.sub.4.1Li.sub.1.1Na.sub.0.5RHO(3.9) 14.489 0.5 9.73E?01 1.0 23 45 52 144 52 0.36 1.0 Cu.sub.3.4Li.sub.2.8Na.sub.0.2RHO(3.9) 14.867 0.2 7.60E?01 1.0 2.1 0.83 60 92 62 0.65 1.0 Li.sub.7.8H.sub.2.0RHO(3.9) ND 0 3.60E?02 1.0 51 171 53 85 29 0.62 1.8 Zn.sub.5.7RHO(3.2) 14.437 0 9.00E?02 2.5 43 282 39 91 30 0.43 1.3 Li.sub.6.8H.sub.3.0RHO(3.9) ND 0 1.10E?01 1.0 43 224 51 81 39 0.63 1.3 Zn.sub.4.9RHO(3.9) 14.541 0 3.70E?01 1.0 10 45 50 141 48 0.35 1.0 Li.sub.5.8H.sub.4.0RHO(3.9) ND 0 3.73E?01 1.0 31 120 52 80 47 0.65 1.1 H.sub.10.7RHO(3.5) ND 0 5.80E?01 1.0 4.1 3.7 39 50 39 0.78 1.0 Cu.sub.5.7RHO(3.2) 14.915 0 6.10E?01 2.5 1.4 0.7 52 72 53 0.72 1.0 H.sub.9.2RHO(4.2) 15.035 0 7.10E?01 1.0 1.3 0.8 69 91 76 0.76 0.9 Cu.sub.4.9RHO(3.9) 14.938 0 7.20E?01 1.0 1.6 0.94 66 92 68 0.72 1.0 4A NA NA 4.80E?01 5 25 16 41.8 113 39 0.37 1.1 CMS NA NA 8.83E?03 4.5 36.5 64.7 107.3 107 107 1.0 1.0 RS10.sup.a NA NA 9.94E?03 ND 35.0 35 ND ND ND ND ND Ba-RPZ-3.sup.b NA NA 2.43E?03 ND 1.0 6 ND ND ND ND ND .sup.aS. Farooq, Gas Separations and Purification, Vol. 9, No. 3, pp 205-212. .sup.bS. Kuznicki, B. Dunn, E Eyring and D. Hunter, Separation Science and Technology, 2009, 44: 7, pp 1604-1620. .sup.cN.sub.2 and Ar not fully equilibrated and their rates are overestimated. The Pmax reported reflects pressure drop over 30 minutes of measurement. .sup.dO.sub.2 not fully equilibrated and its rate is overestimated. The Pmax reported reflects pressure drop over 30 minutes of measurement. NA = Not applicable ND = Not Determined

[0120] The theory behind the derivation of the diffusion parameter is given by the Fickian Diffusion model in which a special case of a more rigorous chemical potential driving force model for adsorbate transport within the adsorbent particle is used. The effect of finite system volume on overall kinetics is also considered. The analytical form of the model is given by Equation 2:

[00002]$f?\left(t\right)=1-6.Math.\underset{n=1}{\overset{?}{.Math.}}.Math.\frac{\exp ?\left(-\frac{D.Math..Math.P_n^2.Math.t}{r^2}\right)}{\frac{9.Math..Math.?}{1-?}+\left(1-?\right).Math.P_n^2}$

[0121] Where f(t) is the fractional uptake, D is the intra-crystalline diffusivity, r is crystal radius (diffusional length scale), t is the time, ? is the fraction of adsorbate ultimately adsorbed by the adsorbent and P.sub.n are the non-zero roots of Equation 3:

[00003]$\tan .Math..Math.P_n=\frac{3.Math..Math.P_n}{3+\left(\frac{1}{?}-1\right).Math.P_n^2}$

as set forth in chapter 6 of Ruthven, D. M. Principles of Adsorption and Adsorption Processes, John Wiley and Sons, New York, 1984.

[0122] Kinetic selectivity parameters were measured for the RHO compositions of this invention and compared with other zeolite and carbon molecular sieve (CMS) materials tested internally and from the literature. All of the RHO samples described herein were activated under vacuum (<10 mPa) at 400? C. for at least 8 hours to remove water and CO.sub.2 prior to adsorption measurements. The results are compiled in Table 3.

[0123] The ambient temperature data in Table 3 and FIGS. 6 and 7 represent a wide range of RHO phases with Si/Al between 3.2 and 4.2, which show significantly better kinetic selectivity for O.sub.2 vs. both Ar and N.sub.2 than CMS or known commercial zeolites, while maintaining O.sub.2 uptake rates that are 10 to 50? faster than CMS. Surprisingly, the rates are some 1000 to 5000? faster than the Li-rich RHO materials reported by Corbin in U.S. Pat. No. 7,169,212.

[0124] CMS, RS10, 4 A, and the MOF Ba-RPZ-3 show poorer selectivities at slower rates, apart from 4 A zeolite. While 4 A zeolite shows very fast O.sub.2 uptake, its selectivity for O.sub.2 vs. Ar adsorption is much lower than most of the RHO materials.

[0125] The O.sub.2 vs. N.sub.2 equilibrium selectivity of low-silica zeolites is typically less than 0.5, owing to the stronger quadrupole interactions between N.sub.2 and electropositive extra framework cations of the zeolite. The equilibrium data in Table 3 demonstrate that a number of the RHO zeolites also show this behavior, but as can be seen in FIG. 5, the O.sub.2 vs. N.sub.2 equilibrium selectivity can be significantly improved by increasing the number of protons exchanged for Li.sup.+ in a Li.sub.9.0Na.sub.0.8RHO(3.9) zeolite starting zeolite. Complete exchange to the fully protonated RHO results in slight de-alumination, but also increases the O.sub.2/N.sub.2 equilibrium selectivity to 0.76. Unfortunately, the lack of 8-ring distortion by these cations leads to negligible kinetic selectivity between O.sub.2 and N.sub.2. By balancing improved O.sub.2 vs. N.sub.2 equilibrium selectivity with high kinetic selectivity and rate, it appears that an optimum is achieved on exchange of 3-4 protons (FIGS. 1 and 5).

Example 9: Isotherm Measurements

[0126] Isotherms at various temperatures were measured on Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO (3.9), Li.sub.6.8H.sub.3.0RHO(3.9), the Li.sub.9.5Na.sub.1.6Cs.sub.0.3RHO(3.2) analog to the material of Corbin, and Zn.sub.4.1Na.sub.1.6RHO(3.9) using a 3FLEX Surface Characterization Unit from Micromeritics for pressures measured up to 1 atm absolute or on a VTI HPA 300 Adsorption Unit for pressures measured up to 10 atm absolute. Isotherms were collected for O.sub.2, N.sub.2 and Ar at 5, 23, and 45? C. for the first three samples and on N.sub.2 and Ar at 23 and 45? C. for Zn.sub.4.1Na.sub.1.6RHO(3.9). Isotherm plots comparing O.sub.2 and Ar capacities measured up to 10 atm are shown in FIGS. 4a for Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO (3.9).

[0127] The isotherms measured up to 1 atm for Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) are shown in FIG. 4b, and an O.sub.2 vs. Ar equilibrium selectivity of close to 1 is observed at 23? C. Effective equilibrium selectivity for O.sub.2 vs. Ar is observed as the adsorption temperature is dropped below 23? C., which is likely associated with temperature dependent contraction and reduced vibration of the RHO 8 rings. The O.sub.2 and N.sub.2 isotherms for Li.sub.6.8H.sub.3.0RHO(3.9) in FIG. 8 and Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) in FIG. 9 show improved O.sub.2/N.sub.2 equilibrium selectivity for Li.sub.6.8H.sub.3.0RHO(3.9) vs Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9). The isotherms for the Li.sub.9.5Na.sub.1.6Cs.sub.0.3RHO(3.2) are shown in FIG. 10. A curious feature of the isotherm data for this composition is that the O.sub.2 capacities at 5 and 23? C. are nearly identical. It is unclear if this is a consequence of the extreme slowness of the O.sub.2 uptake at 5? C., or if the slight contraction of the 8 ring windows going from 23 to 5? C. leads to fewer accessible sites for the oxygen. The N.sub.2 and Ar isotherms for Zn.sub.4.1Na.sub.1.6RHO(3.9) are shown in FIG. 11.

Example 10: Argon Production Via Ambient Temperature PSA with Li.SUB.5.2.Zn.SUB.1.8.H.SUB.0.5.Na.SUB.0.5.RHO(3.9)

[0128] A 2-bed multi-step pressure swing adsorption (PSA) process cycle is used to evaluate process performance indicators in the form of primary product (Ar) recovery and productivity using Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) adsorbent. The adsorbent characteristics and adsorbent bed characteristics used in the simulation are shown in Table 4.

TABLE-US-00004 TABLE 4 Characteristics of the bed and adsorbent as well as the operating conditions used to evaluate process performance indicators. Adsorbent Characteristics Adsorbent type Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO (3.9) Li.sub.6.8H.sub.3.0RHO Zn.sub.4.1Li.sub.1.1Na.sub.0.5RHO (3.9) (3.9) Adsorbent diameter (m) 0.002 0.002 0.002 Total void fraction 0.65 0.65 0.65 Interstitial void fraction 0.40 0.40 0.40 Bulk density (kg/m.sup.3) 800.92 800.92 800.92 Rate constant (D/r.sup.2) for Oxygen (1/s) 0.3700 0.1100 0.9730 Nitrogen (1/s) 0.00698 0.00256 0.0423 Argon (1/s) 0.00070 0.00049 0.0216 Bed Characteristics Number of beds 2 2 2 Bed length (m) 2.40 2.40 2.40 Bed inside diameter (m) 2.0 2.0 2.0 Middle port from bottom 1.35 1.35 1.35 (m) Bed wall thickness (m) 0.016 0.016 0.016 Feed end head space 0.096 0.096 0.096 (m.sup.3) Exit end head space (m.sup.3) 0.113 0.113 0.113 Operating Conditions Temperature (? C.) 37.78 37.78 37.78 .sup.1Feed pressure (bara) 7.90 7.90 7.90 .sup.2Purge pressure (bara) 1.09 1.09 1.08 .sup.3Purge to feed ratio 0.14 0.14 0.36 Feed mole fraction for Oxygen 0.20 0.20 0.20 Nitrogen 0.0005 0.0005 0.001 Argon 0.7995 0.7995 0.799 Cycle time (s) 60 64 300 Process Performance Primary impurity O.sub.2 O.sub.2 N.sub.2 removed Primary product impurity 2.0 2.0 1.0 (ppm) Argon recovery (%) 61.44 56.01 38.67 Productivity (nm.sup.3/h/m.sup.3) 283.12 215.51 87.35 .sup.1Pressure is at the middle of the bed and at the end of feed step, .sup.2Pressure is at the middle of the bed and at the end of purge step, .sup.3Ratio is based on average purge and average feed flows in lb-moles/h.

[0129] The PSA cycle is operated by following the sequence shown in FIG. 12 at a feed pressure of 7.90 bara and a temperature of 37.78? C. At the start of the cycle (F1/RP1), the bed is pressurized to the highest-pressure level of the cycle with the addition of primary product and feed gas from top and bottom end of the bed, respectively. No product is withdrawn during this phase of process cycle. The feed step (F2 and F3) is continued until the mass transfer zone of preferentially adsorbed component (O.sub.2) reaches the exit end of the bed without substantially breaking through it, and the un-adsorbed gas (first purified Ar) is discharged from the outlet end of bed. The feed gas consists of 79.95 mole % Ar, 20.00 mole % O.sub.2 and balance N.sub.2. After the feed step, the bed pressure is reduced to 7.79 bara through stopping feed flow and extracting product from top end of the bed (CoD). At the termination of CoD step, there is an idle step (11). Then, the bed (EQD1) is connected with the second bed undergoing equalization re-pressurization step (EQR1) and a portion of the void as well as desorbed gas is transferred from the product end of first bed to the product end of second bed, thus lowering the first bed pressure to approximately 6.67 bara at the end of this step (EQD1). Following this step, a dual end equalization de-pressurization step (DEQD2) is introduced to transfer more co-adsorbed as well as void gases from the first bed to the second bed from top as well as middle of the first bed to the bottom of the second bed until the pressure of the first bed goes down to approximately 4.54 bara. The dual end depressurization step is then coupled with counter-current depressurization step (DEQD3/CnD1) which further reduces the pressure of the bed to approximately 3.03 bara. The column is then counter-currently de-pressurized (CnD2) and purged (PU1 and PU2) at 1.09 bara, and thereafter, re-pressurized (RP2 and RP3) counter-currently with primary product gas at 1.51 bara. Following the re-pressurization step, the column is subsequently pressurized through pressure equalization re-pressurization steps (EQR1, DEQR2, and DEQR3) to bring back the pressure level for initiation and repetition of the cycle. Note that three idle steps (11, 12, and 13) are incorporated into the cycle schedule and during this step the bed is isolated and all valves leading to it are closed.

[0130] With all the steps, the full cycle completes in 60 seconds. The net O.sub.2-free (2 ppm O.sub.2 in primary product) Ar recovery from the feed gas is 61.44% and the productivity is 283.12 Nm.sup.3/h/m.sup.3 bed. The Ar recovery and productivity benefits of RHO type adsorbent are compared with carbon molecular sieve (CMS) based PSA process in FIG. 13 for 1, 2, and 5 ppm O.sub.2 in primary product. Note that the process conditions and the cycle sequence are kept same as mentioned above for the comparison. These performance benefits result in the following commercial advantages:

[0131] Argon recovery is improved from about 40 to 45% in CMS to 61% in RHO adsorbents for product O.sub.2 purity range from 1 to 5 ppm. In the case of process integration, this higher recovery reduces recycle back to the distillation column.

[0132] Argon productivity is improved about 5 times versus a CMS based PSA process, reducing bed size and cost and enabling crude argon purification of larger feed flows.

[0133] Use of RHO zeolites in place of CMS eliminates the safety concern of combustible carbon particles in an O.sub.2-rich environment if the PSA waste stream is recycled back to the distillation column is used. Thus, filters present in CMS Argon PSA are eliminated, resulting in reduced equipment and capital expenditure.

Example 11: Argon Production Via Ambient Temperature PSA with Li.SUB.6.8.H.SUB.3.0.RHO(3.9)

[0134] This example compares the process performances in terms of argon recovery and productivity of the RHO composition Li.sub.6.8H.sub.3.0RHO(3.9) with the RHO composition Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9). The 2-bed PSA process cycle described above is used to evaluate the process performances. The adsorbent and bed characteristics as well as operating conditions are summarized in Table 4. For both cases, the PSA process is independently optimized for final evaluation. It is worth noting that like Example 10, this example deals with primarily oxygen removal by the PSA process. The process performances are summarized in Table 4. Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) is capable of enhancing recovery and productivity by about 10% and about 31% respectively due to its higher oxygen rate and higher oxygen over argon selectivity. It is worth noting that the simulation results are obtained for the purpose of demonstration. A different combination of argon recovery and productivity can be obtained by changing different parameters. For example, a productivity gain can be achieved by reducing cycle time which would affect the argon recovery to some extent.

Example 12: Simulation of Removal of Trace N.SUB.2 .from Ar with Zn.SUB.4.1 .Li.SUB.1.1 .Na.SUB.0.5.RHO(3.9)

[0135] This example deals with the removal of predominantly trace nitrogen from crude argon stream using a nitrogen selective RHO adsorbent, Zn.sub.4.1Li.sub.1.1 Na.sub.0.5RHO(3.9). The 2-bed PSA cycle mentioned with Example 10 is used to evaluate the efficiency of this adsorbent for removing trance nitrogen from argon stream. The adsorbent and bed characteristics as well as the operating conditions are summarized in Table 4. It is worth mentioning that the adsorbent is also kinetically selective towards nitrogen, therefore, effective selectivity (which is a function of equilibrium and kinetic selectivity) is essentially higher. Another important characteristic of the adsorbent is that the isotherms of nitrogen, oxygen, and argon are less steep than commonly known thermodynamically selective zeolites. This means that a smaller purge to feed ratio is sufficient to maintain a reasonable performance.

[0136] The results from the simulation is summarized in Table 4. With 1000 ppm nitrogen in the feed and 1.0 ppm nitrogen in the product stream, an argon recovery and productivity of 38.67% and 87.35 nm.sup.3/h/m.sup.3 can be achieved at 37.78? C. and 7.90 bara.

Example 13: Simulation of Layered Bed for Ar PSA with Trace N.SUB.2 .and O.SUB.2 .Removal from Ar Stream Using Adsorbents from Present Invention in Both Layers

[0137] This example is presented to illustrate the performance of a layered bed comprising of an equilibrium selective layer and a kinetically selective layer to simultaneously remove oxygen and nitrogen to produce very high purity argon stream using a pressure swing adsorption process at ambient temperature. The novelty can be understood through arrangement of the two layers in the same column in a pressure swing adsorption process at ambient temperature.

[0138] Two cases (Case 1 and Case 2) are created to better understand the invention. In Case 1, the adsorbent materials inside the columns are arranged in two separate layers: first layer of adsorbent with kinetic selectivity to one of the contaminant gases (in this case, oxygen), preferentially where the product gas (in this case argon) has very slow diffusion kinetics. The second layer is comprised of an adsorbent material where the separation is enabled by differences in equilibrium capacities, where the contaminant gas (mainly nitrogen) is more adsorbed than the product gas. The Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) is used in the first layer and the Zn.sub.4.1Li.sub.1.1 Na.sub.0.5RHO(3.9) is used in the second layer. In Case 2, a reverse scenario is created, i.e., equilibrium adsorbent as the first layer and kinetic adsorbent in the second layer. The adsorbent and bed characteristics, layering information, and operating conditions can be found in Tables 4 and 5. Note that, the total bed length is maintained constant for both cases.

TABLE-US-00005 TABLE 5 Process summary for simultaneous oxygen and nitrogen removal using a layered bed conventional 2-bed PSA process. Case 1 Case 2 Bed Characteristics Layering configuration Feed end O.sub.2 removal N.sub.2 removal (kinetic) layer (equilibrium) Product end N.sub.2 removal O.sub.2 removal (equilibrium) layer (kinetic) layer Adsorbent for O.sub.2 removal Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5 RHO (3.9) Adsorbent for N.sub.2 removal Zn.sub.4.1Li.sub.1.1Na.sub.0.5RHO Zn.sub.4.1Li.sub.1.1Na.sub.0.5RHO (3.9) (3.9) Number of beds 2 2 Total bed length (m) 3.9 3.9 O.sub.2 removal layer length 1.5 2.4 (m) Bed inside diameter (m) 2.0 2.0 Middle port from bottom 0.90 1.5 (m) Operating Conditions Temperature (? C.) 37.78 37.78 .sup.1Feed pressure (bara) 7.90 7.90 .sup.2Purge pressure (bara) 1.07 1.09 .sup.3Purge to feed ratio 0.12 0.15 Feed mole fraction for Oxygen 0.20 0.20 Nitrogen 0.001 0.001 Argon 0.799 0.799 Cycle time (s) 170 120 Process Performance O.sub.2 in primary product 2.0 2.0 (ppm) N.sub.2 in primary product 1.0 1.0 (ppm) Argon recovery (%) 28.63 33.32 Productivity (nm.sup.3/h/m.sup.3) 42.88 61.42 .sup.1Pressure is at the middle of the bed and at the end of feed step, .sup.2Pressure is at the middle of the bed and at the end of purge step, .sup.3Ratio is based on average purge and average feed flows in lb-moles/h.

[0139] The aforementioned processes (Cases 1 and 2) are applied to produce a product argon stream containing nitrogen and oxygen of 1.0 ppm and 2.0 ppm, respectively at 37.78? C. and 7.90 bara. As can be seen from Table 5, the feed gas consists of 0.1 mole % nitrogen, 20 mole % oxygen and balance argon. The 2-bed PSA cycle mentioned with previous examples is used for performance evaluation. From Table 5, it is clear that the second case performs better than Case 1 (about 16% better argon recovery and about 43% better productivity).

[0140] The purge is more efficient when an equilibrium layer is added in the feed end followed by a kinetic layer in the product end. The RHO adsorbents mentioned here are both capable of removing both oxygen and nitrogen to some extent from the argon stream. Thus, the nitrogen mass transfer zone from equilibrium layer can be pushed further towards the kinetic layer. This implies that the bed utilization is higher in conjunction with effective purge.

[0141] An interesting feature with the layering configuration is that the middle bed pressure transfer used in the PSA cycle plays an important role in providing an incremental benefit when equilibrium layer is placed in the feed end. The optimum location for the middle bed port is at the interface between equilibrium layer and kinetic layer as can be seen from Table 5. No significant benefit is observed with middle bed port location when kinetic layer (O.sub.2 removal layer) is placed in the feed end.

[0142] With the preferred layering sequence discussed in Table 5, Case 2, it is observed from examples in FIG. 14 that increasing the proportion of O.sub.2 selective kinetic layer can enhance argon recovery at the expense of reduced productivity. The PSA cycle configuration, adsorbent and bed characteristics, operating conditions, and product O.sub.2 and N.sub.2 levels are kept same as Case 2 mentioned above. The total bed length is also kept constant at 3.9 m.

TABLE-US-00006 TABLE 6 Process summary for simultaneous oxygen and nitrogen removal using a preferred layering configuration as defined in Table 5, Case 2 utilizing known adsorbents. Layering configuration Feed end layer N.sub.2 removal N.sub.2 removal N.sub.2 removal N.sub.2 removal (equilibrium) Product end layer O.sub.2 removal O.sub.2 removal O.sub.2 removal O.sub.2 removal (kinetic) Adsorbent for O.sub.2 Li.sub.6.8H.sub.3.0 RHO(3.9) Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) removal layer Adsorbent for N.sub.2 Zn.sub.4.1Li.sub.1.1Na.sub.0.5RHO(3.9) AgLiLSX CaX NaX Number of beds 2 2 2 2 Total bed length (m) 3.9 3.9 3.9 3.9 O.sub.2 removal layer length 2.4 2.4 2.4 2.4 Bed inside diameter 2.0 2.0 2.0 2.0 Middle port from 1.5 1.5 1.5 1.5 For N.sub.2 layer .sup.1K.sub.H, N2 (mmole/gm/bara) 0.71 2.38 3.69 0.37 Effective N.sub.2/Ar 6.55 18.77 17.33 3.32 Temperature (? C.) 37.78 37.78 37.78 37.78 .sup.2Feed pressure (bara) 7.90 7.90 7.90 7.90 .sup.3Purge pressure (bara) 1.07 1.10 1.12 1.08 .sup.4Purge to feed ratio 0.11 0.21 0.21 0.12 Feed mole fraction for Oxygen 0.20 0.20 0.20 0.20 Nitrogen 0.001 0.001 0.001 0.001 Argon 0.799 0.799 0.799 0.799 Cycle time (s) 120 100 120 260 O.sub.2 in primary product 2.0 2.0 2.0 2.0 N.sub.2 in primary product 1.0 1.0 1.0 1.23 Argon recovery (%) 32.18 33.46 9.90 6.72 Productivity (nm.sup.3/h/m.sup.3) 58.03 78.05 15.89 4.79 .sup.1K.sub.H, N2 is the Henry's law constant for N.sub.2 at 37.78? C.. .sup.2Pressure is at the middle of the bed and at the end of feed step, .sup.3Pressure is at the middle of the bed and at the end of purge step, .sup.4Ratio is based on average purge and average feed flows in lb-moles/h.

Example 14: Simulation of a Layered Bed for Ar PSA with Trace N.SUB.2 .and O.SUB.2 .Removal from Ar Stream Using Li.SUB.6.8.H.SUB.3.0 .RHO(3.9) as O.SUB.2 .Removal Layer and Zn.SUB.4.1.Li.SUB.1.1 .Na.SUB.0.5.RHO (3.9) as N.SUB.2 .Removal Layer

[0143] This example illustrates the performance of a layered bed pressure swing adsorption (PSA) process for trace N.sub.2 removal and for O.sub.2 removal in which the Zn.sub.4.1Li.sub.1.1 Na.sub.0.5RHO (3.9) adsorbent is used as N.sub.2 removal layer and the Li.sub.6.8H.sub.3.0RHO(3.9) adsorbent is used as O.sub.2 removal layer. The adsorbents inside the PSA bed are arranged in the same manner as explained with Case 2 in Example 13: N.sub.2 removal layer is used in the feed end (as first layer) and O.sub.2 removal layer is in the product end (as second layer).

[0144] An adsorption process simulator is used to evaluate the layered bed PSA performance using the above layering configuration for a feed gas composition of 0.1 mole % nitrogen, 20.0 mole % oxygen and balance argon. The feed pressure is 7.90 bara, purge pressure is 1.07 bara and temperature is 37.78? C. Note that the 38.46% of the total bed length is filled with adsorbent for N.sub.2 removal layer and 61.54% of the length is filled with O.sub.2 removal layer. The adsorbent and bed characteristics, layering information, and operating conditions can be found in Table 6. The 2-bed PSA cycle mentioned with previous examples is used for performance evaluation.

[0145] The process is simulated to produce a product argon stream containing nitrogen and oxygen of 1.0 ppm and 2.0 ppm, respectively. The process performance is summarized in Table 6. It is clear from the table that the adsorbent Li.sub.6.8H.sub.3.0RHO(3.9) is not as efficient as the adsorbent Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9). With Li.sub.6.8H.sub.3.0RHO(3.9) the argon recovery is 3.4% worse and the productivity is 5.5% worse than with Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9).

Example 15: Simulation of a Layered Bed for Ar PSA with Trace N.SUB.2 .and O.SUB.2 .Removal from Ar Stream Using the Inventive RHO, Li.SUB.5.2.Zn.SUB.1.8.H.SUB.0.5.Na.SUB.0.5.RHO (3.9) as O.SUB.2 .Removal Layer and Known Adsorbents for N.SUB.2 .Removal Layer

[0146] This example is presented to illustrate the performances of layered bed comprising of known adsorbents for trace nitrogen removal (selected from the group formed by mordenite, ferrierite, clinoptilolite and the type A, X, Y, or mixture therefrom) and the inventive RHO, Li.sub.5.2Zn.sub.1.8H.sub.0.5Na.sub.0.5RHO(3.9) for oxygen removal layer to simultaneously remove oxygen and nitrogen to produce very high purity argon stream using a pressure swing adsorption (PSA) process at ambient temperature. The adsorbents inside the PSA bed are arranged in the same manner as explained with Case 2 in Example 13: nitrogen removal layer is in the feed end (as first layer) and oxygen removal layer is in the product end (as second layer).

[0147] The inventors have identified that employing a suitable conventional adsorbent for nitrogen removal layer (selected from the group formed by mordenite, ferrierite, clinoptilolite, chabazite and the type A, X, Y, or mixture therefrom) can enhance the overall performance of the layered bed process. The criterion used for selecting a suitable adsorbent for nitrogen removal is based on the Henry's Law constant for nitrogen (K.sub.H,N2). The Henry's Law constant for an adsorption isotherm is defined as the initial isotherm slope. See, for example, Physical Adsorption of Gases, Young, D. M. and Crowell, A. D., p. 104 (Butterworths, London 1962). The unit of the constant is in amount of gas adsorbed per unit weight of adsorbent per unit of pressure (e.g., mmole of gas adsorbed/gm of adsorbent/bar absolute pressure).

[0148] An adsorption process simulator is used to evaluate the layer bed PSA performance using the above layering configuration with different adsorbents for nitrogen removal for a feed gas composition of 0.1 mole % nitrogen, 20.0 mole % oxygen and balance argon. The feed pressure is 7.90 bara and temperature is 37.78? C. Note that the 38.46% of the total bed length is filled with adsorbent for nitrogen removal layer and 61.54% of the length is filled with oxygen removal layer. The adsorbent and bed characteristics, layering information, operating conditions and process performances are summarized in Table 6. The 2-bed PSA cycle mentioned with previous examples is used for performance evaluation.

[0149] It is clear from Table 5, Case 2 and Table 6 that there is a preferred range of 0.5 to 3.0 mmole/gm/bara at 37.78? C. for Henry's law constant for nitrogen which significantly improves argon recovery and productivity under layering configuration. For example, when the product end of the bed (second layer) consists essentially of the RHO adsorbent from the invention (predominantly for oxygen removal) and the feed end (first layer) consists of AgLiLSX (predominantly for nitrogen removal), the performances are significantly better than other adsorbents considered in Table 6. Note that the K.sub.H,N2 for AgLiLSX is 2.38 mmole/gm/bara and the effective selectivity for nitrogen over argon (function of Henry's law constant and kinetics as set forth in Pressure Swing Adsorption Ruthven, D. M.; Farooq, S. and Knaebel, K. S., p. 52 (VCH, New York, 1994) is 17.33 at 37.78? C. Above the suitable range of the Henry's constant for nitrogen, the adsorbate (nitrogen) is more strongly adsorbed and it is impractical to use a PSA process for gas removal. For example, the CaX adsorbent, which has a K.sub.H,N2 of 3.69 mmole/gm/bara at 37.78? C., shows a significant performance drop, even though it has an effective nitrogen over argon selectivity similar to AgLiLSX, as can be seen in Table 6. Alternatively, too low of Henry's constant for nitrogen having poor effective nitrogen over argon selectivity would result in very poor performances as can be seen from Table 6 with NaX adsorbent. Note that the performances from the inventive Zn.sub.4.1Li.sub.1.1 Na.sub.0.5RHO(3.9) for nitrogen removal having reasonable Henry's constant performs quite well under layering model for simultaneous oxygen and nitrogen removal (Table 5, Case 2) even though its effective nitrogen over argon selectivity is relatively low at 6.55.