FROM HYDROCARBON GAS

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 1 19(e) to U.S. Provisional Patent Application No. 62/525,263, filed June 27, 2017, the contents of which are hereby incorporated by reference.

FIELD

The present invention relates generally to membrane separation of gases. More particularly, the present invention relates to membrane separation of acid gases, including hydrogen sulfide.

BACKGROUND

Hydrogen sulfide (H2S) is a flammable, highly toxic gas that causes nearly instant death when its concentrations are over 1000 parts per million (ppm). Even 5 ppm levels cause eyes, nose, and throat irritation. To address these health concerns and also address corrosion concerns, selective removal of H ₂ S from industrial gas streams in natural gas processing, biogas upgrading, and geothermal energy production is a high priority target. Raw natural gas contains many impurities but H2S and carbon dioxide (CO2) are arguably the two most important to remove after primary dehydration. Over 40% of proven raw natural gas reserves in the United States are termed "sour," which means the gas contains H2S in amounts of about 4 ppm or more measured at standard temperature and pressure. In certain areas in the Middle East (e.g. Qatar, Saudi Arabia), oil and gas reservoirs can contain H2S in amounts of up to 20 mol % or 200,000 ppm. Large reserves of natural gas remain untapped today due to the difficulties involved in processing such low quality gas. Thus, identifying materials and methods for efficiently removing H2S, CO2, other acid gases, and combinations thereof from these feeds is a globally important topic.

Sour gas treatment operations are dominated by absorption-based processes.

But such processes are highly energy intensive, can cause environmental concerns, and require high capital and maintenance costs, especially when acid gas concentrations are high. Membrane separation techniques are preferable to absorption techniques, due to advantages such as higher energy efficiency, smaller footprints, and reduced environmental impact. But currently known membranes for

H2S separation are rubbery polymers that provide efficient H2S separation only at low H2S partial pressures. Known membranes are unsuitable for H2S separations at realistic aggressive well-head pressures, which can exceed 60 bar.

There remains a need for more robust membranes to process gas feeds with combined high CO2 and H2S total acid gas partial pressures. In particular, membranes are required having good intrinsic stability and attractive CO2 and H2S selectivities relative to CH ₄ under aggressive feed conditions of 20 bar or more.

SUMMARY

In some embodiments, a method for separating at least one acid gas from a mixture of gases includes contacting a hydrocarbon-based feedstock comprising hydrogen sulfide and one or more hydrocarbon gases with a membrane comprising a spirobisindane-based polymer of intrinsic microporosity (PIM) or an amidoxime- functionalized PIM (AO-PIM), wherein the membrane separates the mixture into a permeate stream that comprises hydrogen sulfide and a retentate stream that is depleted in hydrogen sulfide as compared to the hydrocarbon-based feedstock. In some examples, the spirobisindane-based PIM includes PIM-1 . In some examples, the AO-PIM includes an amidoxime-functionalized polymer or copolymer derived from triptycene, Troger's base, ethanoanthranene, a phthalocyanine, a spirobisindane, or a benzidioxane. Optionally, the hydrocarbon-based feedstock comprises hydrogen sulfide in an amount of up to 20 mol %. In some examples, the retentate comprises hydrogen sulfide in an amount of less than 50 parts per million. In other examples, the retentate comprises hydrogen sulfide in an amount of up to 0.5 % and other cases up to 1 % on a molar basis when further polishing with additional systems such as amine absorption are to be used.

The hydrocarbon-based feedstock can further include carbon dioxide, and when it does, the permeate stream comprises carbon dioxide, and the retentate stream is depleted in carbon dioxide as compared to the hydrocarbon-based feedstock. In some examples, the retentate stream comprises carbon dioxide in an amount of less than 90 mol %. In further examples, the retentate stream comprises carbon dioxide in an amount of less than 10 mol %. In still further examples, the retentate stream comprises carbon dioxide in an amount of less than 2 mol %.

In any method described herein, the hydrocarbon-based feedstock optionally comprises a pressure of at least 10 bar (e.g., at least 20 bar, or at least 55 bar, or at least 70 bar). For example, the hydrocarbon-based feedstock can include a pressure of from 10 bar to 100 bar (e.g., from 20 bar to 70 bar, or from 55 to 70 bar). In some examples, the hydrocarbon-based feedstock comprises natural gas, shale gas, biogas, or a combination thereof. In some examples, the hydrocarbon-based feedstock further includes one or more of water, nitrogen, oxygen, and mercaptan, and the retenate stream is depleted in the water, nitrogen, oxygen, or mercaptan.

In any method described herein, the AO-PIM can include the following compound.

AO-PIM-1 .

In any method described herein, the membrane can include an integrally- skinned asymmetric structure in flat sheet geometry. Alternatively, in any method described herein, the membrane can include a composite structure in flat sheet geometry. As another alternative, in any method described herein, the membrane can include an integrally-skinned asymmetric structure in hollow fiber geometry. In still another alternative, in any method described herein, the membrane can include a composite structure in hollow fiber geometry.

In some embodiments, a method for separating at least one acid gas from a mixture of gases includes contacting a mixture of gases comprising an acid gas with a membrane, wherein the membrane separates the mixture of gases into a permeate stream that comprises at least some of the acid gas and a retentate stream that is depleted in the acid gas as compared to the mixture of gases, wherein the membrane comprises a hydrogen sulfide permeability of at least 500 Barren Optionally, the membrane comprises a hydrogen sulfide permeability of at least 1500 Barrer at a pressure of 10 bar and a gas feed of 20 mol% H ₂ S, 20 mol % C0 ₂ , 60 mol % CH ₄ . In some examples, the membrane comprises a hydrogen sulfide permeability of at least 3000 Barrer at a pressure of up to 77 bar and a gas feed of 20 mol% H2S, 20 mol % CO2, 60 mol % CH ₄ . In any method described herein, the membrane can include a hydrogen sulfide/methane selectivity of greater than about 20 at a pressure of 10 bar and a gas feed of 20 mol% H ₂ S, 20 mol % C0 ₂ , 60 mol % CH ₄ . For example, the membrane can further include a hydrogen sulfide/methane selectivity of greater than about 40 at a pressure of 30 bar and a gas feed of 20 mol% H ₂ S, 20 mol % C0 ₂ , 60 mol % CH ₄ . In still other examples, the membrane can include a hydrogen sulfide/methane selectivity of greater than about 60 at a pressure of up to 77 and a gas feed of 20 mol% H ₂ S, 20 mol % C0 ₂ , 60 mol % CH ₄ .

In any method described herein, the acid gas may include hydrogen sulfide. In any method described herein, the acid gas optionally includes carbon dioxide. In some examples, the mixture of gases has a pressure of at least 10, or at least 20 bar. In any method described herein, the membrane can comprise a spirobisindane-based PI M or an AO-PIM comprising an amidoxime-functionalized polymer or copolymer derived from triptycene, Troger's base, ethanoanthranene, a phthalocyanine, a spirobisindanes, or a benzidioxane. In any method described herein, the AO-PIM can include the following compound.

AO-PIM-1 .

Disclosed herein is a polymer membrane including a spirobisindane-based PIM or an AO-PIM comprising an amidoxime-functionalized polymer of intrinsic microporosity (AO-PI M) and hydrogen sulfide sorbed on the spirobisindane-based PIM or the AO-PIM. The AO-PI M can include an amidoxime-functionalized polymer or copolymer derived from triptycene, Troger's base, ethanoanthranene, a phthalocyanine, a spirobisindanes, or a benzidioxane. In some examples, the AO-PIM can include the following compound

AO-PIM-1 .

The details of one or more embodiments are set forth in the drawings and description below. Other features, objects, and advantages will be apparent from the drawings, the description, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A clear conception of the advantages and features of one or more embodiments will become more readily apparent by reference to the exemplary, and therefore non-limiting, embodiments illustrated in the drawings:

FIG. 1 A is a graph of C02/CH ₄ selectivity vs. CO2 permeability (Barrer) at 35 °C and compares membranes according to embodiments disclosed herein with various known membranes.

FIG. 1 B is a graph of hbS/ChU selectivity vs. H2S permeability (Barrer) at 35 °C and compares membranes according to embodiments disclosed herein with various known membranes.

FIG. 2A is graph of CO2 pure gas permeation isotherms at 35 °C for polymer membranes according to embodiments disclosed herein.

FIG. 2B is graph of H2S pure gas permeation isotherms at 35 °C for polymer membranes according to embodiments disclosed herein.

FIG. 3A is a graph of CO2 permeability (Barrer) as a function of mixed gas feed pressure (bar) at 35 °C for a feed gas of 20 mol. % H ₂ S, 20 mol. % C0 ₂ , and 60 mol. % CH ₄ for polymer membranes according to embodiments disclosed herein and one known polymer membrane.

FIG. 3B is a graph of H2S permeability (Barrer) as a function of mixed gas feed pressure (bar) at 35 °C for a feed gas of 20 mol. % H ₂ S, 20 mol. % C0 ₂ , and 60 mol. % CH ₄ for polymer membranes according to embodiments disclosed herein and one known polymer membrane. FIG. 4A is a graph of C0 ₂ /CH ₄ selectivity as a function of mixed gas feed pressure (bar) at 35 °C for a feed gas of 20 mol. % H ₂ S, 20 mol. % C0 ₂ , and 60 mol. % CH ₄ for polymer membranes according to embodiments disclosed herein and one known polymer membrane.

FIG. 4B is a graph of H ₂ S/CH ₄ selectivity as a function of mixed gas feed pressure (bar) at 35 °C for a feed gas of 20 mol. % H ₂ S, 20 mol. % C0 ₂ , and 60 mol. % CH ₄ for polymer membranes according to embodiments disclosed herein and one known polymer membrane.

FIG. 5A is a graph of combined C0 ₂ + H ₂ S permeability (Barrer) as a function of mixed gas feed pressure (bar) at 35 °C for a feed gas of 20 mol. % H ₂ S, 20 mol. % C0 ₂ , and 60 mol. % CH ₄ for polymer membranes according to embodiments disclosed herein and one known polymer membrane.

FIG. 5B is a graph of combined acid gas selectivity as a function of mixed gas feed pressure (bar) at 35 °C for a feed gas of 20 mol. % H ₂ S, 20 mol. % C0 ₂ , and 60 mol. % CH ₄ for polymer membranes according to embodiments disclosed herein and one known polymer membrane.

FIG. 6 is a graph of C0 ₂ and H ₂ S permeabililty and C0 ₂ /CH ₄ and H ₂ S/CH ₄ selectivity at 35 °C as a function of operating time for polymer membranes according to embodiments disclosed herein.

FIG. 7A is a graph of combined acid gas selectivity vs. combined C0 ₂ + H ₂ S permeability (Barrer) at 35 °C and compares membranes according to embodiments disclosed herein with various known membranes.

FIG. 7B is a graph of H ₂ S/CH ₄ selectivity as a function of H ₂ S partial pressure at 35 °C and compares membranes according to embodiments disclosed herein with various known membranes.

DETAILED DESCRIPTION OF THE INVENTION

Described herein are materials and methods for purifying sour gas streams. The materials and methods can selectively remove acid gases, including H ₂ S, from a hydrocarbon-based feed gas, thus purifying the hydrocarbon gas. The materials and methods described herein can be used to efficiently purify feed gases having up to 20 mol % or more H ₂ S and that have pressures up to 77 bar. The membranes have high H ₂ S/CH ₄ selectivity, e.g., up to 75, and ultrahigh H ₂ S permeability, e.g., greater than 4000 Barrer at 77 bar, which is two to three orders of magnitude higher than commercially available glassy polymeric membranes. The materials and methods described herein can be used to convert previously untapped reserves of poor-quality, sour natural gas into usable natural gas.

In some aspects, a method for separating at least one acid gas from a mixture of gases includes contacting a hydrocarbon-based feedstock including an acid gas and a hydrocarbon gas with a membrane comprising a polymer of intrinsic microporosity (PIM) or an amidoxime-functionalized polymer of intrinsic microporosity (AO-PI M), wherein the membrane separates the hydrocarbon-based feedstock into a permeate stream that comprises at least some of the acid gas and a retentate stream that is depleted in the acid gas as compared to the hydrocarbon-based feedstock. Optionally, the acid gas includes hydrogen sulfide, carbon dioxide, or a combination thereof.

Microporous organic polymers (MOPs) as a group have well-defined pore structures, strong covalent bonds, and potential for use in membrane-based separation methods. But MOPs demonstrate insufficient selectivity for certain gas separations. A certain group of MOPs known as polymers of intrinsic microporosity (PI Ms) have rigid and contorted backbone structures and interconnected voids. PI Ms have a high Brunauer-Emmett-Teller (BET) surface area and a pore size of less than 2 nm, which allows these polymers to behave like molecular sieves. Examples of PIMs include polymers and copolymers of triptycene, Troger's base, ethanoanthranene, phthalocyanines, spirobisindanes, and benzidioxanes. PIMs are attractive materials for membrane separations because they are solution processable and have structural diversity for gas molecules with different sizes and shapes. They also show high gas permeability, and moderate selectivity based on differences in size of diverse gases where the size difference is at least 0.5 A; however, the H ₂ S/CH ₄ gas pair only differs by about 0.2 A in size. Few studies have specifically considered PIMs for H2S separations from natural gas feeds, and those that have considered PIMs found permeabilities and selectivities for H2S no better than the permeabilities and selectivities of other known membrane materials. For example, the hydroxyl functionalized PIM-6FDA-OH was considered for simultaneous separation of CO2 and H2S from aggressive sour natural gas streams, but its permeability was significantly lower than known rubbery materials., In contrast to other PIMs and PIM derivatives, the spirobisindane-based PIMs and AO-PI Ms described herein form membranes that have H2S permeabilities and selectivities for H2S that greatly exceed those of other known polymeric membranes. Spirobisindane-based PIMs described herein include PIM-1 , which has the 000.

PIM-1

AO-PI Ms described herein include amidoxime-functionalized polymers and/or copolymers derived from triptycene, Troger's base, ethanoanthranene, a phthalocyanine, a spirobisindanes, or a benzidioxane. In one example, an AO-PIM described herein is an amidoxime-functionalized spirobisindane-based PIM, such as AO-PI M-1 . AO-PI M- 10-100,000.

AO-PIM-1

Another example of an AO-PIM is an amidoxime-functionalized triptycene-based PIM (AO-TRIP-PI M). For example, one exemplary AO-Trip-Pi M has the structure shown below.

AO-Trip-PIM HO

Another example of an AO-PI M is an amidoxime-functionalized Troger's base-based PIM (AO-TB-PIM). For example, one exemplary AO-TB-PIM has the structure shown below.

Another example of an AO-PI M is an amidoxime-functionalized ethanoanthranene - based PIM (AO-EA-PIM). For example, one exemplary AO-EA-PIM has the structure shown below.

AO-EA-PIM

In one example, an AO-PIM described herein can be formed by synthesizing a parent polymer of intrinsic microporosity and then functionalizing the parent polymer with amidoxime to form the AO-PIM.

In some examples, PIM monomers can be copolymerized with dicyanotetrafluorobenzene, which provides PI Ms including nitrile groups. An AO-PIM can then be formed by reacting the nitrile groups with hydroxyl amine under reflux conditions to form the amidoxime groups. For example AO-PI M-1 can be formed by exemplary Scheme 1 .

Other AO- PI Ms can be prepared in a variety of ways known to one skilled in the art of organic synthesis or variations thereon as appreciated by those skilled in the art. The AO-PI Ms can be prepared from readily available starting materials, and optimum reaction conditions may vary with the particular reactants or solvents used, but such conditions can be determined by one skilled in the art.

AO-PIM's described herein can be used to form membranes for selectively separating H ₂ S gas. For example, membranes can be formed by dissolving an AO- PIM in an appropriate solvent, pouring a thin layer of the solution onto a smooth level surface or dipping a support such as a porous hollow fiber into a solution of the functionalized PIM and evaporating the solvent to form a vitrified film. The concentration of the AO-PI M solution is not critical and in some examples may be from 1 to 50 wt % AO-PI M. Any solvent that can dissolve the AO-PIM may be used. Optionally, the solvent may be dimethylformamide, dimethylacetamide, or N- methylpyrrolidone. The vitrified film can be subjected to further processing, such as soaking in solvent, drying, annealing, and/or low pressure (e.g., vacuum) treatment to remove the solvent. AO-PIM membranes for use methods described herein may have thickness of up to about 10 micron. In some examples, the membrane thickness may be up to 1 micron, or up to 0.1 micron.

The membranes described herein can have symmetric or asymmetric microporous structures. They may have a flat sheet geometry or a hollow fiber geometry. Various typical membrane structures are known to one skilled in the art and are described in common texts, for example, Basic Principles of Membrane Technology, 2 ^nd Ed., Marcel Mulder, Kluwer Academic Publishers, 1996, The Netherlands; and Lonsdale, H. K. , The Growth Of Membrane Technology, Journal of Membrane Science, 10 (1982) 81 -181 . In some examples, a membrane described herein can have an integrally-skinned asymmetric structure in a flat sheet geometry or a composite structure in a flat sheet geometry. Alternatively, the membrane can have an integrally-skinned asymmetric structure in a hollow fiber geometry or a composite structure in a hollow fiber geometry. A membrane having a flat sheet geometry can be formed as described above. A membrane having a hollow fiber geometry can be formed by extruding a polymer dope through a spinneret, as is known in the art. The dope can be a solution of the AO-PIM polymer, similar to that described above for forming a vitrified film.

Surprisingly, the membranes described herein have hydrogen sulfide gas permeabilities and selectivities significantly higher than any other material known for such separations. For example, the membranes have hydrogen sulfide permeabilities of at least 500 Barrer (e.g., at least 1000 Barrer, at least 3000 Barrer, or at least 4000 Barrer). Accordingly, in one aspect a method for separating an acid gas from a mixture of gases includes contacting a mixture of gases including an acid gas with a membrane, wherein the membrane comprises a hydrogen sulfide permeability of at least 500 Barrer, and wherein the membrane separates the mixture of gases into a permeate stream that comprises at least some of the acid gas and a retentate stream that is depleted in the acid gas as compared to the mixture of gases. Optionally, the acid gas includes hydrogen sulfide, carbon dioxide, or a combination thereof. Optionally, the mixture of gases further includes a hydrocarbon gas, nitrogen, oxygen, or a mixture thereof. Permeability and selectivity are intrinsic material properties used to characterize membrane material productivity and separation efficiency, respectively. The permeability (Pi) of penetrant i can be defined as the steady-state flux ( ) normalized by transmembrane pressure differential (Δρ;) and thickness of the membrane (/),

Pi =— (Eq. 1 )

¹ A _Pi

Permeability is usually given in the unit of Barrer, where 1 Barrer = 1 χ 10 ^~10 cm ³ (STP) cm / (cm ² s cmHg). For non-ideal gas feeds, the partial pressure difference in Eq. 1 is simple replaced by the penetrant fugacity difference (Δ i). The fugacity-based permeability (Pf), thereby, is given by Eq. 2,

P* = i (Eq. 2)

¹ Afi

The transport of gas molecules through polymer materials is governed by the sorption-diffusion mechanism. Hence, permeability of polymer membranes can be represented as the product of the diffusion coefficient (D) and sorption coefficient (S) of penetrant /within the membrane,

Pi = DiSi (Eq. 3)

The apparent diffusion coefficient Z? (cm ² /s) can be estimated from the equation D = 1 ² /6Θ, where / is the thickness of the membrane and Θ is the time lag calculated through single gas permeation tests. Thus, the sorption coefficient S (cm ³ (STP)/cm ³ cmHg) can be deduced from the sorption-diffusion model.

For a given gas pair under conditions where the downstream pressure is much less than the upstream pressure, the ideal selectivity (c¾/j) is defined as the ratio of gas permeabilities for the fast gas (/) and slow gas (/),

pi ^D i ^s i

*i/j= ^ ⁼ 7ΓΤ ^{= OC} D ^S (Eq. 4)

Eq. 4 allows the ideal selectivity of a membrane to be decoupled into the product of mobility selectivity (<¾) and the solubility selectivity (as).

For permeation of mixed gas feeds, a separation factor based on gas chromatographic measurement of upstream and downstream compositions indicate the membrane separation efficiency. When the permeate pressure is negligible, the separation factor equals to the ratio of the component mole fractions in the downstream direction, y, and upstream direction, x,

M j>

^« "> ⁼ τ≠> ^(Eq - ⁵⁾

Again, for non-ideal mixtures, like the acid gas mixtures described herein, the separation factor equals the ratio of the mixed gas permeabilities of components / and /, based on the fugacity-based driving force of permeability is preferred,

The permeabilities and selectivities of AO-PI M membranes were measured using fresh AO-PIM-1 membranes, aged AO-PI M-1 membranes (ambient conditions, 6 months), and rejuvenated AO-PIM-1 membranes (aged membranes soaked in methanol then hexane and dried). The aged film has much lower H2S and CO2 permeabilities than the freshly cast membrane, but the rejuvenated sample exhibits H2S and CO2 permeabilities similar to those of freshly cast film. The rejuvenated sample shows slightly lower H2S/CH4 and CO2/CH ₄ ideal permselectivities due to a somewhat higher CH ₄ permeability than in the fresh membrane. In any case, these results show that the decreased productivity in the aged AO-PIM-1 films can be significantly rejuvenated through a simple solvent treatment.

The membranes described herein have significantly higher permeabilities and selectivities for H2S and CO2 than membranes made from other known materials. It is known that a tradeoff exists between permeability and selectivity, where membranes having high selectivity tend to have low permeability, and vice versa. The Robeson limit is an upper bound that represents the general trend in the tradeoff between permeability and selectivity for a given gas over a range of materials. While the Robeson limit is not fixed and has evolved as materials having improved properties have been developed, it is a good measure of the current state of the art.

The permeabilities and selectivities for the membranes described herein are shown in FIGs. 1 A-B, which are pure gas permeability-selectivity trade-off curves that compare the fresh, aged, and rejuvenated AO-PIM-1 membranes with membranes made from typical glassy polymers, including cellulose acetate (C. S. K. Achoundong et ai, Silane modification of cellulose acetate dense films as materials for acid gas removal. Macro molecules 46, 5584-5594 (2013)); 6FDA-based polyimides 6 FDA- DAM :DABA 3:2 (B. Kraftschik, et ai, Dense film polyimide membranes for aggressive sour gas feed separations. J. Membr. Sci. 428, 608-619 (2013)) and 6FDA- mPDA:6FDA-durene (G. O. Yahaya, et ai , Aromatic block co-polyimide membranes for sour gas feed separations. Chem. Eng. J. 304, 1020-1030 (2016)); crosslinkable polyimide PEGMC (B. Kraftschik, W. J. Koros, Cross-linkable polyimide membranes for improved plasticization resistance and permselectivity in sour gas separations. Macro molecules 46, 6908-6921 (2013)); fluorinated polyamide-imide 6F-PAI (J. T. Vaughn, W. J. Koros, Analysis of feed stream acid gas concentration effects on the transport properties and separation performance of polymeric membranes for natural gas sweetening: A comparison between a glassy and rubbery polymer. J. Membr. Sci. 465, 107-1 16 (2014); J. T. Vaughn, et ai, Effect of thermal annealing on a novel polyamide-imide polymer membrane for aggressive acid gas separations. J. Membr. Sci. 401 -402, 163-174 (2012)); and intrinsically microporous polyimides PIM-1 (P. M. Budd et ai, Gas separation membranes from polymers of intrinsic microporosity. J. Membr. Sci. 251 , 263-269 (2005)), PIM-P1 -3 (B. S. Ghanem, et ai , High-performance membranes from polyimides with intrinsic microporosity. Adv. Mater. 20, 2766-2771 (2008)), and PI M-6FDA-OH (S. Yi, et ai, A high-performance hydroxyl-functionalized polymer of intrinsic microporosity for an environmentally attractive membrane-based approach to decontamination of sour natural gas. J. Mater. Chem. A 3, 22794-22806 (2015)). FIG. 1 A also includes the published 1991 and 2008 Robeson upper bound plots for state of the art polymer membranes for CO2 separations. (L. M. Robeson, Correlation of separation factor versus permeability for polymeric membranes. J. Membr. Sci. 62, 165-185 (1991 ); L. M. Robeson, The upper bound revisited. J. Membr. Sci. 320, 390-400 (2008).) The C0 ₂ and CH ₄ permeabilities were tested at feed pressures of 2 bar and 35 °C. No Robeson plot is available for H2S separations, but FIG. 1 B includes a solid line showing the experimental hbS/ChU upper bound between H2S permeability and hbS/ChU selectivity in typical glassy polymers. H2S permeability was measured at 1 bar feed pressure and 35 °C to avoid plasticization effects.

The AO-PI M membranes described herein are significantly more permeable than conventional hydroxyl functionalized polyimides, such as PIM-6FDA-OH, for which data is included in FIGs. 1A-B. The amidoxime-functionalized polymers described herein offer extensive interchain and intrachain hydrogen bonding, which contributes to the tight microstructure of AO-PIMs and a balance between intrachain rigidity and intrachain spacing. This balance in properties produces the exceptional CO2 separation performance of AO-PIM-1 , which surpass the most recent 2008 Robeson upper bound of state-of-the-art polymer membranes for the C02/CH ₄ separations.

Plasticization occurs when a penetrant significantly increases the chain mobility of polymer segments, resulting in increased diffusion coefficients of all penetrants through the membrane, thereby increasing the permeability and lowering separation efficiency. The AO-PIM membranes described herein show excellent C02-induced plasticization resistance. FIG. 2A shows the onset of C02-induced plasticization around 14 bar with a CO2 permeability of about 1 100 Barrer for fresh, aged, and rejuvenated AO-PIM-1 membranes. For comparison, a membrane of PEGMC, which is a PEG crosslinked derivative of the PI M-6 FDA-DAM : DABA, showed negligible plasticization up to 20 bar pure CO2, but the CO2 permeability was significantly lower at only 70-80 Barrer. B. Kraftschik, W. J. Koros, Cross-linkable polyimide membranes for improved plasticization resistance and permselectivity in sour gas separations. Macro molecules 46, 6908-6921 (2013). The rejuvenated AO-PIM-1 membrane demonstrates even better C02-induced plasticization resistance than the freshly cast membrane, but slightly lower CO2 permeability. As shown in FIG. 2B, no obvious H2S- induced plasticization of the fresh and rejuvenated AO-PIM-1 membranes occurs in the pure H2S feed up to 2.0 bar feed pressure at 35 °C, with very slight plasticization for the aged film. Notable plasticization is observed above 4.0 bar of pure H2S. However, the tradeoff between stability and productivity with the AO-PIM membranes would be attractive in many applications.

Below the onset of strong plasticization, the permeability coefficient and permselectivity can be decoupled into diffusivity and solubility contributions to obtain a better understanding of the role of amidoxime functionalization in hbS/ChU and C02/CH ₄ selectivity of the PIMs material. The apparent diffusion coefficient (D) was estimated using the time-lag method, and sorption coefficients (S) were deduced using S = P/D for feed pressures below the onset of plasticization. The CO2 and CH ₄ permeabilities were tested at 2 bar, and H2S was measured at 1 bar. This analysis is summarized for the AO-PIM-1 films with comparison to other glassy polymers in Table 1 . Table 1

After aging under ambient conditions for six months while not in use, the AO- PIM-1 film showed a drop of about 80% in diffusion coefficients for all three gases as compared with the freshly cast sample and a drop of from about 50 % to 70 % in sorption coefficients compared with the freshly cast sample. The AO-PIM-1 aging response is quite different from the aging phenomena of its parent PIM-1 and other PI Ms, where the aging process is dominated by a decrease in diffusion coefficients. The combined drop in both diffusion and sorption coefficients upon aging for six months under ambient conditions leads to a dramatic AO-PIM-1 permeability loss of nearly 90% relative to the freshly cast sample. The rejuvenation process led to recovery and even increased diffusion coefficients while the sorption coefficients remained slightly lower in comparison with the freshly made membrane. Ultimately, the rejuvenated AO-PIM-1 film exhibits slightly higher gas permeabilities and lower C02/CH ₄ and H2S/CH4 selectivities than that in the freshly cast sample. Despite the lower diffusion coefficient in the aged AO-PI M-1 film, the aged sample shows higher C02/CH ₄ diffusion selectivity in comparison with the freshly cast and rejuvenated films and also their parent PIM-1 . Without wishing to be bound by theory, such a trend may reflect generalized tightening of the AO-PIM-1 during aging.

Functionalization of the parent PIM-1 with amidoxime moieties significantly reduces the CO2 and CH ₄ diffusion and sorption coefficients. For example, the diffusion and sorption coefficients of CO2 and CH ₄ for the AO-PIM-1 film are about 50 % to 60 % less than those in the parent PIM-1 . By contrast to the reduced diffusion and sorption coefficients, the freshly-cast AO-PIM-1 film showed a remarkable increase in diffusion selectivity over the PIM-1 film. This increase in diffusion selectivity occurred simultaneously with a slight reduction in the solubility selectivity relative to the parent PIM-1 membrane. Without wishing to be bound by theory, a tightened structure in AO-PIM-1 may contribute to the enhanced C02/CH ₄ diffusion-based discrimination over the parent PIM-1 material.

The diffusion coefficients of H2S are always smaller than that of CH ₄ in all the freshly made, aged, and rejuvenated AO-PIM-1 films. This trend may seem surprising, given the smaller kinetic diameter of H2S (3.6 A) compared to CH ₄ (3.8 A). This trend reflects strong hydrogen bonding of the highly polar H2S with amine and hydroxyl groups in the amidoxime moieties in AO-PIM-1 . Such a trend may lead to a lower diffusion coefficient resulting from the tendency of H2S molecules to "stick" to the sorption sites. This so called "stickiness" of H2S within the polymer matrix can lead to a higher activation energy of diffusion of H2S than expected. Based on these observations, the overall hbS/ChU selectivity of AO-PIM films is mainly contributed by the sorption selectivity, whereas C02/CH ₄ selectivity is controlled by both factors, even though the diffusion selectivity is dominant.

The freshly cast, aged, and rejuvenated AO-PIM-1 membranes were further investigated using ternary mixed gas permeation measurements with a mixed gas feed of H ₂ S, C0 ₂ , and CH ₄ at mole percentages of 20 %, 20 %, and 60 %, respectively, and a feed pressure of up to 77 bar. Membrane productivity and separation efficiency of the AO-PIM-1 in the ternary mixture under these challenging feed conditions are shown in FIGs. 3A-B, 4A-B, and 5A-B. These highly aggressive conditions greatly exceed typical wellhead pressure threshold of about 68 bar in realistic field operations.

FIG. 3A shows CO2 penetrant-induced plasticization did not occur below 42 bar for freshly cast AO-PI M-1 films, but occurred at about 30 bar for the aged and rejuvenated AO-PIM-1 membranes. FIG. 3B shows strong hbS-induced plasticization effects were observed in the AO-PI M-1 membranes, even under the lowest feed pressure tested. H2S permeabilities were much higher than that of CO2 under the same feed pressures, suggesting that H2S in the mixed gas feeds competes more effectively than CO2 in the polymer matrix. More importantly, the hbS/ChU selectivity in all the freshly cast, aged, and rejuvenated AO-PI M-1 films does not decrease due to plasticization effects as does C02/CH ₄ selectivity, as shown by FIGs. 4A-B.

The aforementioned effects lead to an increased hbS/ChU selectivity with increasing CO2 and H2S partial pressure, notwithstanding the fact that hbS-induced plasticization effect was observed even at the lowest feed pressure that was measured. Mixed gas permeation results, shown in FIGs. 5A-B, indicate a plasticization enhanced selectivity for the separation of H2S from sour natural gas feeds using the family of amidoxime-functionalized polymers of intrinsic microporosity (e.g. AO-PIM-1 ). Therefore, penetrant-induced plasticization, which tends to deteriorate the membrane performance of C02/CH ₄ separation using conventional glassy polymers, actually enhances separation efficiency of H2S from sour gas mixtures for the AO-PI M family of polymers.

At the high end of the mixed gas feed pressure range (e.g., about 77 bar), the H2S/CH ₄ selectivity reached nearly 75 in the freshly cast AO-PIM-1 membrane. This remarkable selectivity exceeds that of all glassy polymers and even most rubbery polymers. Moreover, the AO-PIM-1 ultrahigh H2S permeability of about 4300 Barrer under the extremely aggressive feed pressures up to 77 bar makes it a breakthrough material, with combined high selectivity and high permeability. Table 2 summarizes most of the studies of ternary mixed gases containing H2S using polymeric dense films for acid gas separations from natural gas feeds.

Table 2

The unique separation performance at the exceedingly challenging feed conditions makes AO-PI M materials extremely impressive, since most previous literature reports on rubbery polymers, which usually show high H2S/CH4 selectivity, were investigated at low feed pressures. As shown in Table 2, ternary mixed gas permeation with up to 20 % H2S at feed pressures up to 77 bar show the AO-PI M materials to be outstanding candidates for challenging H2S separations. Moreover, FIG. 6 shows this performance is well maintained under 220 hours of long term continuous operation with a 20 mol % H2S, 20 mol % CO2, 60 mol % CH ₄ mixture at 8.6 bar.

Because the intrinsic differences and somewhat opposing mechanisms between glassy and rubbery materials in terms of CO ChU and hbS/ChU separation performance can complicate comparisons of separation efficiency for overall sour natural gas separation, we further investigated the combined acid gas selectivity (CTCAG), which is the ratio of combined acid gas permeability (PH2S+PCO2) and methane permeability (PCH ₄ ). FIG. 7A compares the overall separation performance of the AO- PI M membranes described herein, and the literature data listed in Table 2 via a combined acid gas productivity-efficiency trade-off plot. FIG. 7B compares the H2S separation performance of AO-PI M membranes described herein with other polymers. The typical glassy polymers represented on Table 2 and FIGs. 7A and 7B are PIM- 6FDA-OH, cross-linked DEGMC, cross-linked TEBMC, cellulose acetate, 6 FDA- DAM :DABA (3:2), 6F-PAI-1 ; and 6FDA-mPDA:6FDA-durene. The typical rubbery polymers represented on Table 2 and FIGs. 7A and 7B are Pebax 1074, PU1 , PU2, PU3, and PU4. We have included on FIGs. 7A-B proposed upper bound plots for typical glassy polymers and typical rubbery materials to reveal single H2S and combined acid gas (C02+H2S) productivity-efficiency relationships based on the data in the literature. As shown in FIGs. 7A-B, both H2S separation performance and the overall performance of the AO-PI M membranes is located at the far upper right quadrant and well above an upper bound for typical glassy and rubbery polymers. Thus, the materials and methods for separating acid gases disclosed herein are significantly better than known materials and methods.

Examples

Materials: 1 ,4-dicyanotetrafluorobenzene (DCTB) was purchased from Matrix Scientific, and 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethyl-1 , 1 '-spirobisindane (TTSBI) (98%) was supplied by Alfa Aesar, USA. Ν,Ν-Dimethylacetamide (DMAc) (99%), toluene (anhydrous, 99.8%), potassium carbonate (anhydrous, 99.5%), chloroform (99.5%), n-hexane (anhydrous, >99%), methanol (99.5%), tetrahydrofuran (99.8%), and hydroxylamine (50 wt. % solution in water, 99.999%) were purchased from Sigma- Aldrich, USA. C0 ₂ and CH ₄ with 99.999% purity were obtained from Airgas Inc., USA. H2S (99.6% purity) and a ternary gas mixture with a composition of 20 mol. % H2S, 20 mol. % CO2, and 60 mol. % CH ₄ were supplied by Praxair Inc., USA.

Example 1 : Synthesis of AO-PI M-1

To synthesize the parent polymer PIM-1 , a flask was charged with 5.1 g (15 mmol) TTSBI, 3.0 g (15 mmol) DCTB, 6.2 g (45 mmol) K2CO3, and 25 mL DMAc. The mixture was vigorously stirred for 6 min under a flow of inert atmosphere of argon at 155 °C, and then 20 mL toluene was added into the mixture under stirring. Afterwards, 0.2 mL Dl water and 2 mL toluene was added to the mixture. The resultant polymer was poured into methanol and filtered. The product was then re-precipitated from a chloroform / methanol mixture, and refluxed in boiled Dl water, then filtered and dried at 120 °C under vacuum, to give PIM-1 as a bright yellow powder.

The amidoxime-functionalized PIM-1 (AO-PI M-1 ) was prepared by dissolving 0.6 g PIM-1 powder in 40 mL tetrahydrofuran (THF) and heating to 65 °C under N ₂ . Then 6.0 mL hydroxylamine was added dropwise. The mixture was then refluxed at 69 °C for 20 hrs under stirring. The resultant polymer was then cooled to room temperature and purified by addition of ethanol, filtered, thoroughly washed with ethanol, and dried at 1 10 °C for 3 h to yield AO-PIM-1 as an off-white solid.

Example 2: Membrane Fabrication

Vacuum dried PIM-1 polymer was dissolved in chloroform to form a 2-3 wt% polymer solution and placed on a roller for at least 24 h for mixing. The polymer solution was then filtered and used to prepare dense films by a solution casting method in a glove bag at room temperature to achieve slow evaporation (3-4 days). The vitrified films were then removed and soaked in methanol, air-dried, and then heated at 120 C in a vacuum oven for 24 h to remove any residual solvent.

Vacuum dried AO-PIM-1 polymer was dissolved in DMAc to form a 2-3 % w/v concentration polymer dope. Then the polymer dope was filtered and poured onto a leveled glass plate with a stainless steel ring. The solvent was slowly evaporated at 45 °C for two days to form vitrified films, and the vitrified membranes were soaked in methanol, air-dried, and annealed at 120 °C under high vacuum to remove residual solvent trapped in the micropores. Example 3: Membrane Aging

An AO-PIM-1 membrane of Example 2 was aged under ambient conditions (i.e., about 25-30 °C and 1 atm with a relative humidity between about 50 % and 70 %) for six months to form an aged AO-PIM-1 membrane as might be the case for storage before use in an active feed situation.

Example 4: Membrane Rejuvenation

An aged AO-PI M-1 membrane of Example 3 was dried at 120 °C for 12 h, soaked in pure methanol for 24 h and n-hexane for another 24 h, air-dried, and then post-dried at 120 °C under vacuum for 24 h to remove residual solvents, forming a rejuvenated AO-PIM-1 membrane.

Example 5: Gas Permeation Measurements

Dense film permeation was conducted using a constant volume/variable pressure permeation apparatus described in S. Yi, X. Ma, I. Pinnau, W. J. Koros, A high-performance hydroxyl-functionalized polymer of intrinsic microporosity for an environmentally attractive membrane-based approach to decontamination of sour natural gas. J. Mater. Chem. A 3, 22794-22806 (2015).

Pure and ternary mixture permeation experiments were performed at 35 °C using H2S, CO2, and CH ₄ as well as a mixture of these three components (20 mol. % H2S, 20 mol. % CO2, and 60 mol. % CH ₄ ). For each feed gas, films prepared as described in Examples 2-4 were dried at 120 °C under high vacuum for 24 h and immediately installed in a membrane cell. After installation in the membrane cell and prior to the permeation test, the membranes were degassed in the permeation system on both sides under high vacuum at 35 C for at least 24 h to ensure complete degassing of the membrane and system.

The permeation cell was additionally enclosed in a large ventilated cabinet as a secondary compartment to prevent H2S exposure if a leak was to occur in the system. In addition, two pneumatically actuated valves were used in place of standard hand-operated valves and controlled by a Labview® program for additional safety. In addition, the downstream actuated valve was programmed to shut down when the downstream pressure reached a certain maximum pressure to avoid over- pressurization that may damage the pressure transducers, and also to prevent unintended release of large quantities of H2S and minimize operator risk when handling H2S.

Pure CO2 and CH ₄ gases were introduced to the upstream side at 35 °C and 2 bar, while H2S was introduced to the upstream side at 35 °C and 1 bar to minimize the plasticization effect. For each of the three different membrane samples (freshly made, aged, and rejuvenated), the CO2 and CH ₄ pure gas permeation isotherms were measured at pressures from 2 to 20 bar, and the H2S pure gas permeation isotherm was measured a pressures from 1 to 8 bar. In each case, as the gas molecules diffused through the membrane, the increase in permeate pressure was measured by a downstream pressure transducer (MKS Instruments, Dallas, TX, USA) and recorded using LabVIEW® software (National Instruments, Austin, TX, USA) until the so called "steady-state" was reached (after at least 10 times the diffusion time lag). After apparent steady state was achieved, the permeate receiver was again evacuated and a second pressure increase test was done to verify that the steady state permeation rate was truly achieved. A plot of the permeate pressure vs. time (dp/dt) was created and the slope of the plot was used to calculate the permeability coefficient in Eq. 2. This data was used for Figs. 1 A-B, 2A-B, and 6 and Table 2.

In the case of ternary mixed gas permeation experiments, the retentate flow rate and upstream pressure were well maintained with the stage cut at 1 % or below using 1000 D syringe pumps (Teledyne Isco Inc., Lincoln, NE, USA) and a metering valve. The stage cut represents the ratio of permeate to retentate flow. The downstream composition was measured using a Varian 450-GC (Agilent Technologies, USA). The permeate gas composition is tested at least three times, or until the steady state operation is confirmed. Mixed gas permeation values are based on the average of at least three gas chromatograph (GC) permeate composition measurements at each pressure point up to approximately 77 bar. Since CO2 and H2S are highly condensable gases, all permeation measurements in this work were calculated using the fugacity-based driving force definition of dense film permeability for highly challenging feeds. This data was used to for Figs. 3A-B, 4A-B, 5A-B, and 7A-B.

As demonstrated by the above Examples, post-synthesis functionalization of hyper-rigid polymers with amidoxime groups provides a strategy for achieving ultra- permeable, highly selective membrane materials for practical natural gas sweetening and additional challenging gas pair separations. Numerous characteristics and advantages have been set forth in the foregoing description, together with details of structure and function. While the invention has been disclosed in several forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions, especially in matters of shape, size, and arrangement of parts, can be made therein without departing from the spirit and scope of the invention and its equivalents as set forth in the following claims. Therefore, other modifications or embodiments as may be suggested by the teachings herein are particularly reserved as they fall within the breadth and scope of the claims here appended.