Metal organic framework absorbent platforms for removal of CO2 and H2S from natural gas

Provided herein are metal organic frameworks comprising metal nodes and N-donor organic ligands which have high selectivity and stability in the present of gases and vapors including H2S, H2O, and CO2. Methods include capturing one or more of H2S, H2O, and CO2 from fluid compositions, such as natural gas.

BACKGROUND

Today there is an increasing global desire to reduce greenhouse gas emissions and develop clean alternative vehicle fuels. Methane (CH4), the primary component of natural gas (NG), is of particular interest as it is abundant and has lower carbon dioxide (CO2) emission and more efficient combustion than other hydrocarbons due its high H/C ratio. Biogases, including landfill gas, are also seen as promising renewable energy resources, but, like NG, they contain significant amounts of water, CO2, and hydrogen sulfide (H2S) which must be removed before being transported, stored, and burned as a fuel. For example, NG must contain less than 1-2% CO2and 4 ppm H2S to meet fuel gas specifications for pipeline transportation. Within many industries, gas dehydration and removal of CO2and H2S remain some of the most intensive and challenging separations, in part due to the intolerance of many technologies to water.

Available technologies for refining NG and other biogases are often costly, multi-stage processes. Amine scrubbing is a common liquid phase system used to remove acid gases such as CO2and H2S from NG. However, stagnant historical operating efficiencies, and the excessive oxidative degradation, evaporation, and the corrosive nature of the alkanolamine aqueous solutions create a myriad of performance, safety, and environmental concerns. Solid, porous material systems, such as zeolite and metal organic frameworks (MOFs), offer more environmentally friendly alternatives for CO2capture, but require cumbersome, multi-stage processes. For example, zeolite has single-species selectivity for CO2and cyclic adsorption performance in the presence of moisture that require prior dehydration and H2S removal stages. MOFs, similarly, can be designed for CO2capture but exhibit prohibitively low selectivity for acid gases such as H2S. Further, MOFs exhibit low selectivity and capture at low CO2partial pressures.

MOFs generally include porous crystals which are assembled from modular molecular building blocks, and provide a wide array of advantageous material properties, including high surface area, porosity, stability, and sorption potential. While the available building block options, and combinations thereof, are virtually limitless, such potential highlights the statistical difficulty in identifying and assembling MOFs with desired and particularized material properties and multi-faceted functionality. For example, many MOFs exhibit high selectivity towards a particular molecular species, but are highly intolerant to water and H2S.

SUMMARY

In general, this disclosure describes porous metal organic frameworks (MOFs). In particular, this disclosure describes MOFs suitable for the capture and removal of gases and/or vapors from fluids. It should be noted that although the embodiments of this disclosure are described with respect to examples for gas capture, the embodiments described herein are generally applicable to many fields including gas molecule separation, gas storage, catalysis, sensors, drug delivery, rare gas separation, and proton conductivity.

In one embodiment, a method of capturing chemical species from a fluid composition comprises contacting a first metal organic framework characterized by the formula [MaMbF6-n(O/H2O)w(Ligand)x(solvent)y]zwith a fluid composition comprising one or more of carbon dioxide, water, and hydrogen sulfide and capturing one or more of carbon dioxide, water, and hydrogen sulfide from the fluid composition.

DETAILED DESCRIPTION

Provided herein are a series of highly stable and highly tunable MOFs with high affinity and stability to water and H2S. Such qualities allow for efficient and cost effective methods for dehydrating gases, vapors, and solvents capable of replacing many cumbersome and expensive industrial processes. Further, this novel series of MOFs can be designed with a variety of pore sizes and assembled with and without open-metal sites, affording tunable properties for a variety of separation applications. For example, the MOFs provided herein can capture chemical species from fluid compositions under conditions and in the presence of chemical species which render known gas capture technologies inefficient, impracticable, or inoperable. In particular, the MOFs provided herein can capture one or more of H2O, CO2and H2S from a fluid composition, such as natural gas, while maintaining high structural integrity during both adsorption and desorption.

As used herein, “fluids” can refer to a gas, liquid, or combination thereof. A gas or liquid can include one or more components. For example, a fluid can include a gas stream comprising CO2, H2S and water vapor.

As used herein, “refining” refers to removing one or more unwanted components or separating one or more components from remaining components of a composition, such as a fluid. For example, refining can include removing a fraction of H2S from a fluid, such as natural gas.

As used herein, “poly-functional” refers to the characteristic of having more than one reactive or binding sites. For example, a poly-functional ligand can attach to a metal ion in multiple ways, bridge multiple metal ions, or combinations thereof. Specifically, pyrazine is a poly-functional ligand.

Gas storage and separation using porous materials has experienced significant development in recent years in various industrial applications related to energy, environment, and medicine. Among porous materials, metal organic frameworks (MOFs) are a versatile and promising class of crystalline solid state materials which allow porosity and functionality to be tailored towards various applications. MOF crystal chemistry uses a molecular building block (MBB) approach that offers potential to construct MOFs where desired structural and geometrical information are incorporated into the building blocks prior to the assembly process.

Generally, MOFs comprise a network of nodes and ligands, wherein a node has a connectivity capability at three or more functional sites, and a ligand has a connectivity capability at two functional sites each of which connect to a node. Nodes are typically metal ions or metal containing clusters, and, in some instances, ligands with node connectivity capability at three or more functional sites can also be characterized as nodes. In some instances, ligands can include two functional sites capable of each connecting to a node, and one or more additional functional sites which do not connect to nodes within a particular framework. A MBB can comprise a metal-based node and an organic ligand which extrapolate to form a coordination network. Such coordination networks have advantageous crystalline and porous characteristics affecting structural integrity and interaction with foreign species (e.g., gases). The particular combination of nodes and ligands within a framework will dictate the framework topology and functionality. While essentially limitless combinations of nodes and ligands exist, to date, very few MOF materials are H2S stable which consequently preclude their use in gas separation.

As disclosed in co-owned U.S. Application No. 62/044,928, a series of isoreticular MOFs with periodically arrayed hexafluorosilicate (SiF6) pillars, called SIFSIX-2-Cu-i and SIFSIX-3-Zn, SIFSIX-3-Cu and SIFSIX-3-Ni showed particularly high CO2selectivity and capture. These properties in SIFSIX-3-M materials suggest broad applications from ppm level CO2removal to bulk CO2separation. However, with the exception of SIFSIX-3-Ni, the SIFSIX-3-M materials were not tolerant to H2S. And although these materials exhibit high structural structurally in the presence of CO2, extensive exposure of all SIFSIX-3-M materials to moisture detrimentally induces a phase change and the formation of new 2D stable materials. These 2D materials exhibit relatively unaltered selectivity but diminished CO2uptake. This indicates that the SIFSIX-3-M materials series is not sufficiently robust to handle CO2and H2S capture in most critical applications throughout the oil and gas and renewable fuels industries, especially in applications which bring the materials into contact with moisture.

Provided herein are novel functionalized MOFs suitable for the removal of acid gases, particularly CO2and H2S, which additionally exhibit high water vapor tolerance and stability over thousands of cycles. These MOFs eliminate the safety, efficiency, and environmental concerns associated with amine scrubbing techniques while providing high selectivity toward acid gases and negligible uptake of other components, including CH4. The proposed class of materials will permit NG and biofuels refining approaches based on simultaneous removal of CO2, H2S and water vapor. The benefits of this innovative approach include the following: (i) no environmental and safety hazards germane to amine scrubbing (ii) no preliminary separate desulfurization is necessary (iii) no separate gas drying is needed, and (iv) no compression-decompression/cooling of NG is required. Further, the MOFs provided herein offer exceptional thermal and mechanical stability, particularly during adsorption/desorption.

MOFs as provided herein comprise one or more MBBs. Generally, a MBB, or a network of MBBs, can be represented by the formula [(node)a(ligand)b(solvent)c]n, wherein n represents the number of molecular building blocks. Solvent represents a guest molecule occupying pores within the MOF, for example as a result of MOF synthesis, and can be evacuated after synthesis to provide a MOF with unoccupied pores. In one example, an evacuated MOF can be subsequently enriched with a guest molecule compatible with the MOF framework and/or pores for a particular purpose (e.g., to outfit the MOF for use as a sensor). In other embodiments, guest molecules can include adsorbed gases, such as H2S. While guest molecules can impart functionality onto a MOF, such are not a permanent fixture of the MOF. Accordingly, the value of c can vary down to zero, without changing the definitional framework of the MOF. Therefore in many instances, MOFs as provided herein will be defined as [(node)a(ligand)b]n, without reference to a solvent or guest molecule component.

In some embodiments, a ligand can comprise a polydentate, or poly-functional ligand, such as a bi-functional ligand, a tri-functional ligand, or ligands with four or more functional sites. In some embodiments, a ligand can comprise an N-donor linker. In some embodiments a ligand can comprise a poly-functional ligand. In some embodiments, a ligand can comprise a plurality of N-donor functional groups. In some embodiments, a ligand can comprise a monocyclic or polycyclic group structure, wherein the cyclic groups can be aromatic or non-aromatic. In some embodiments, a ligand can comprise a nitrogen-containing monocyclic or polycyclic group structure. In some embodiments, a ligand can comprise a nitrogen-containing heterocyclic ligand, including pyridine, pyrazine, pyrimidine, pyridazine, triazine, thiazole, oxazole, pyrrole, imidazole, pyrazole, triazole, oxadiazole, thiadiazole, quinoline, benzoxazole, benzimidazole, and tautomers thereof.

The utility of MOFs such as those provided herein are highly dependent upon the framework's structural features such as structural strength, density, functionality, pore aperture dimensions, pore dimensions, the ratio of pore aperture dimensions to pore dimensions, pore accessibility, and the presence of a plurality of pore dimensions and/or pore aperture dimensions (e.g., a poly-porous MOF). The originality of this new class of crystalline porous materials is based, in part, on the fact that the shape of cavities, (i.e. square or rectangle based channels), is controlled from a structural point of view using appropriate cations and organic linkers. The novel MOF architectures disclosed herein offer a novel improvement on some MOF architectures by replacing silicon components with other metals, such as Al3+, Fe2+, Fe3+, V3+, V4+, V5+, Nb5+, to afford highly stable materials with or without open metals sites. In some embodiments, the use of specific cations, such as Al+3, Fe+2, Fe+3, Cr2+, Cr3+, Ti3+, V3+, V5−, Sc3+, In3+, Y3−, in Mbsite positions can introduce open-metal sites within the channels that enhance properties of gas capture.

These, and other features, collaborate to achieve MOFs with high affinity and stability to water and H2S. Additionally, the novel series of MOFs structures disclosed herein can be designed with a variety of pore sizes and/or open-metal sites which afford tunable properties for a variety of gas/vapor/solvent separation applications. Tuning, in some embodiments, can include modification of the organic and/or inorganic components of the MOF. For example, lighter metal-based clusters can be used to lower the framework density and increase the relative wt. % of captured CO2and/or H2S. Further, the MOF platforms as provided herein allow for an unprecedented high degree of tuning control at the molecular level, allowing the size and shape of channels within a MOF architecture to be rigorously controlled and adapted to specific separation of numerous gases, beyond CO2and H2S.

In some embodiments, a representative MaMbFwOx(H2O)y(Ligand)zMOF structure can include a Ni Maconstituent, an Mbconstituent group selected from one of Al, Fe, V, or Nb, and a Ligand comprising a pyrazine constituent group, wherein x can vary from 0 to 10. All such embodiments offer high affinity and stability to water vapor and H2S, unlike the Cu and Zn-based analogues of SIFSIX-3-M materials made with Si. In some embodiments a MOF characterized by the formula [MaMbF6-n(O/H2O)n(Ligand)2(solvent)x]nwherein Maequals Ni, Mbequals Al, Fe, V or Nb, and ligand equals pyrazine, the pore size (channel size) of the resulting MOF can be about 3.3 Å to about 3.8 Å. In some embodiments, the channels are square/rectangular. In the same or in an alternative embodiment, a MOF can have a specific surface area of about 250 m2/g to about 500 m2/g. In either of the same MOFs or in an alternative embodiment, a MOF can have a pore volume of about 0.1 cm3/g to about 0.25 cm3/g. In a different embodiment, a more elongated ligand can provide an analogous MOF with much higher porosity.

FIG. 1Aillustrates a method100for capturing120one or more chemical species from a fluid composition110via a first MOF105. A method100for capturing120one or more chemical species from a fluid composition110can comprise contacting115a metal organic framework105characterized by the formula [MaMbF6-n(O/H2O)w(Ligand)x(solvent)y]zwith a fluid composition110. Fluid composition110can comprise two or more chemical species. Fluid composition110can comprise natural gas. In one embodiment, fluid composition110comprises one or more of carbon dioxide, water, and hydrogen sulfide. In such an embodiment, fluid composition110can further comprise methane.

Method100can further comprise capturing120one or more chemical species from the fluid composition110. Capturing120can comprise capturing one or more of carbon dioxide, water, and hydrogen sulfide. Capturing120can comprise capturing two or more of carbon dioxide, water, and hydrogen sulfide. In some embodiments, capturing120comprises physical adsorption of the one or more captured chemical species by the first MOF105. In some embodiments, capturing120comprises chemisorption of the one or more captured chemical species by the first MOF105. Chemisorption can occur by one or more captured chemical species chemically interacting with one or more open metal sites of the first MOF105. In other embodiments, capturing120comprises physical adsorption and chemisorption of the one or more captured chemical species by the metal organic framework. Capturing120can comprise wholly or partially containing a chemical species within a pore of a MOF. In some embodiments, capturing120consists of chemisorption. In some embodiments, capturing120consists of physical adsorption. Capturing120can occur in a capturing environment. A capturing environment can comprise one or more of ambient temperature or pressure. A capturing environment can comprise a pressurize-controlled vessel. A capturing environment can comprise a temperature-controlled vessel. A capturing environment can comprise a pressure and temperature-controlled vessel. A capturing environment can comprise the first MOF105in the form of a fixed bed, a packed column, or combinations thereof.

Capturing120can further comprise changing the temperature of the capture environment to alter the affinity of one or more chemical species for the MOF. Similarly, capturing120can further comprise changing the pressure of the capture environment to alter the affinity of one or more chemical species for the MOF. Additionally, capturing120can further comprise changing the temperature and pressure of the capture environment to alter the affinity of one or more chemical species for the MOF. In such embodiments, the chemical species can particularly include carbon dioxide, water, and hydrogen sulfide.

Method100can further comprise desorbing one or more of carbon dioxide, water, and hydrogen sulfide from the MOF. Method100can further comprise desorbing two or more of carbon dioxide, water, and hydrogen sulfide from the MOF. Desorbing can occur in a desorbing environment. A desorbing environment can comprise one or more of ambient temperature or pressure. A desorbing environment can comprise a pressurize-controlled vessel. A desorbing environment can comprise a temperature-controlled vessel. A desorbing environment can comprise a pressure and temperature-controlled vessel. While a MOF can capably capture a number of gaseous or vapor phase species, a relative affinity hierarchy will, in principle, exist among those species. In embodiments where method100comprises desorbing two or more of carbon dioxide, water, and hydrogen sulfide from the MOF, carbon dioxide, water, and hydrogen sulfide can be sequentially desorbed in descending order of affinity. In a particular embodiment, H2S is desorbed first, CO2is desorbed second, and H2O is desorbed last.

In some embodiments, the relative affinity hierarchy of chemical species can be manipulated. Accordingly, desorbing can further comprise changing the temperature of the desorbing environment to alter the affinity of one or more chemical species for the MOF. Similarly, desorbing can further comprise changing the pressure of the desorbing environment to alter the affinity of one or more chemical species for the MOF. Additionally, desorbing can further comprise changing the temperature and pressure of the desorbing environment to alter the affinity of one or more chemical species for the MOF. In such embodiments, the chemical species can particularly include carbon dioxide, water, and hydrogen sulfide. In some embodiments, one or more of capturing and desorbing is effected by multicolumn pressure-temperature swing adsorption. Multicolumn pressure-temperature swing adsorption is known in the art.

Method100can further comprise contacting115the fluid composition110with a second MOF, characterized by the formula [McMdF6-n(O/H2O)w(Ligand)x(solvent)y]z. In some embodiments, the second MOF is different from the first MOF105. In some embodiments, contacting115the fluid composition with the first MOF105and the second MOF occurs separately. In some embodiments, contacting115the fluid composition with the first MOF105and the second MOF occurs sequentially. In some embodiments, contacting115the fluid composition with the first MOF105and the second MOF occurs simultaneously. Method100can further comprise contacting115the fluid composition110with three or more MOFs.

In some embodiments, the first MOF comprises one or more MOFs as provided herein. For example, the first MOF can comprise an MOF characterized by the formula [NiNbF6-n(O/H2O)w(Ligand)x(solvent)y]z. In some embodiments the second MOF comprises one or more MOFs as provided herein. For example, the second MOF can comprise an MOF characterized by the formula [NiMdF6-n(O/H2O)w(Ligand)x(solvent)y]zwherein Mdcomprises Al+3, Fe+2, or Fe+3In some embodiments, the MOF is characterized by the formula [NiNbF6-n(O/H2O)w(Ligand)x(solvent)y]zand the second MOF is characterized by the formula [NiMdF6-n(O/H2O)w(Ligand)x(solvent)y]zwherein Mdcomprises Al+3, Fe+2, or Fe+3.

FIG. 1Billustrates a method101for removing one or more of carbon dioxide, water, and hydrogen sulfide from natural gas via a MOF. Removing can one or more of carbon dioxide, water, and hydrogen sulfide from natural gas can occur simultaneously. Removing can one or more of carbon dioxide, water, and hydrogen sulfide from natural gas can occur separately. Removing can occur in a multicolumn pressure-temperature swing adsorption (PTSA) system. In one embodiment, the MOF comprises one of Ni(Al/Fe/V/Nb)F5(O/H2O)x(pyrazine)2(solvent)x. In another embodiment, the MOF comprises two or more of Ni(Al/Fe/V/Nb)F5(O/H2O)x(pyrazine)2(solvent)x.

MOFs as provided herein can be fabricated using a solvo(hydro)thermal synthetic procedure. As shown inFIG. 2, a method for fabricating200a MOF230can include combining205reactants. Reactants can include one or more of a fluorhydric acid solution206with a Ni2+source207, a second metal source208, and a solvent209to form a mixture210. A Ni2+source207can include one or more of nickel nitrate, hydrated nickel nitrate, nickel chloride, hydrated nickel chloride, nickel fluoride, hydrated nickel fluoride, nickel oxide, or hydrated nickel oxide. The second metal source208can include an Al+3source, an Fe−2source, an Fe+3source, a Cr2+source, a Cr3+source, a Ti3+source, a V3+source, a V5+source, a Sc3+source, an In3+source, a Nb5+source, or a Y3+source, for example. These, metals can be in the form of nitrates, hydrated nitrates, chlorides, hydrated chlorides, fluorides, hydrated fluorides, oxides, hydrated oxides, and combinations thereof. The solvent209can include one or more of H2O, dimethylformamide (DMF), and diethylformamide (DEF).

The method for fabricating200can further comprise to reacting215the mixture210, sufficient to form a reacted mixture220. Reacting215can include contacting the fluorhydric acid solution206, the Ni2+source207, the second metal source208, and the solvent209. Reacting215can further comprise stirring or agitating the mixture210, or heating the mixture210. Heating the mixture210can comprise heating to a temperature between about 80° C. to about 200° C. The reacted mixture220can be further processed225to provide a fabricated MOF230. Processing220can include one or more of filtering the reacted mixture220, rinsing the reacted mixture220with water, removing excess reactants from the reacted mixture220. In some embodiments, guest molecules are optionally evacuated from a fabricated MOF230. Guest molecules can include solvent guest molecules, or derivatives thereof.

One MOF synthesis strategy provided herein comprises linking inorganic chains using appropriate N-donor based linkers to deliberately generate channels along one crystallographic direction. The inorganic chains are built up from the trans-connection between MaN4F2and MbF4(H2O)2octahedra or between MaN4F2and MbF5(H2O) octahedra or between MaN4F2octahedra and MbF5(O) octahedra.FIG. 3Aillustrates an example of an inorganic chain, built up from MaN4F2and MbF5(H2O) octahedra. The resulted inorganic chains are linked to each other using bi-functional N-donor organic ligands, thereby generating channels with different sizes and shapes depending on the nature of the organic linker.FIG. 3Billustrates a schematic view of one embodiment of a MOF comprising a NiNbF5O(pyrazine)2structure, viewed along the c-axis.

In one embodiment, a representative MaMbFxOy(Ligand)2MOF structure can include a Ni Maconstituent, a Nb Mbconstituent group, and a Ligand comprising a pyrazine constituent group.FIG. 4Aillustrates powder X-ray diffraction data of this MOF, characterized by the formula NiNbF5O(pyrazine)2(solvent)x, confirming the high stability of the MOF in the presence of water.FIG. 4Billustrates powder X-ray diffraction data of this MOF, confirming the high stability of the MOF in the presence of H2S.

FIGS. 5A-Cillustrates the broad potential for MOFs characterized by the formula Ni(Al/Fe/V/Nb)F5(O/H2O)x(pyrazine)2(solvent)x, and variants thereof.FIG. 5Aillustrates CO2isotherms at 298 K for each of these four MOFs, which show extremely high sorption of CO2across all test pressures (0-1 bar),FIG. 5Billustrates H2S isotherms at 298K for three MOFs characterized by the formula Ni(Al/Fe/V)F5(O/H2O)x(pyrazine)2(solvent)z, which shows across all test pressures (0-1, wherein 1 corresponds to a 0.1 H2S partial pressure).

FIG. 5CIllustrates H2O sorption isotherms at 298K for Ni(Al)F5(O/H2O)x(pyrazine)2(solvent)z, indicating the high suitability of this MOF for gas dehydration applications, among others. These results demonstrate the value of the MOF embodiment characterized by the formula Ni(Al/Fe/V/Nb)F5(O/H2O)x(pyrazine)2(solvent)xas a platform for refining a number of valuable hydrocarbon gases and fluids, including methane, natural gas, and biogas. The exemplary performance and properties of the Nb5+based MOFs disclosed herein are notably achieved in spite of having no open metal sites. These and other results can be expected in similar other embodiments, with or without open metal sites, such as MOF structure characterized by the formula NiMbF5O(pyrazine)2, wherein Mbcan be one of the following Al+3, Fe−2, Fe+3, Cr2+, Cr3+, Ti3+, V3+, V5+, SC3+, In3+, Nb5+, Y3+. These and other results can be expected in similar other embodiments, with or without open metal sites, such as MOF structure characterized by the formula MaNbF5O(pyrazine)2, wherein Macan be one of the following cations: Cu2+, Zn2+, Co2+, Ni2+, Mn2+, Zr2+, Fe2+, Ca2+, Ba2+, Pb2+, Pt2+, Pd2+, Ru2+, Rh2+, Cd2+, Mg+2, Al+3, Fe+2, Fe+3, Cr2+, Cr3+, Ru2+, Ru3+. These and other results can also be expected in similar other embodiments, with or without open metal sites, such as MOF structure characterized by the formula Ma(Al/Fe/V)F5O(pyrazine)2, wherein Macan be one of the following cations: Cu2+, Zn2+, Co2+, Ni2+, Mn2+, Zr2+, Fe2+, Ca2+, Ba2+, Pb2+, Pt2+, Pd2+, Ru2+, Rh2+, Cd2+, Mg+2, Al+3, Fe+2, Fe+3, Cr2+, Cr3+, Ru2−, Ru3+.

In order to study the performance of NiAlF5O(pyrazine)2for adsorption of CO2and H2S, a series of cyclables testing were carried out in a flow mode system using the sequence of tests described inFIG. 6.

NiAlF5O(pyrazine)2, a material from a related SiF6platform which is known to be very selective to CO2was tested. Interestingly, the gas adsorption testings using CO2/H2S/CH4:5/5/90 at 25° C. in TSR mode showed a retention time in the column of 27 min/g for H2S and CO2, respectively (FIG. 7), indicative of similar affinity of the NiAlF5O(pyrazine)2framework to both CO2and H2S. These results showed that H2S and CO2could be removed simultaneously with the same affinity.

Most importantly, heating the adsorption column in a second TSR (temperature swing regeneration mode) cycle to 50° C. for NiAlF5O(pyrazine)2did not significantly affect the retention time (25 vs 27 min/g) for both H2S and CO2respectively (FIG. 8).

FIG. 9shows that TSR mode could be practiced for NiAlF5O(pyrazine)2using reactivation at 50° C. and even VSR mode at 25° C. with a minimum loss in the H2S retention time while keeping the same affinity for H2S and CO2.FIG. 9shows that for NiAlF5O(pyrazine)2, the H2S and CO2uptakes were reduced by 20% between TSR and VSR (vacuum swing regeneration) mode when adsorption is carried at 50° C.

Finally, the reproducibility test (6th cycle) after 5 adsorption-desorption tests showed that NiAlF5O(pyrazine)2exhibit a reproducible H2S and CO2adsorption performance. In this case, NiAlF5O(pyrazine)2can be utilized as a potential material for the simultaneous removal of H2S and CO2.

With the exception of NiAlF5O(pyrazine)2, it was shown that other MOF materials studied exhibit higher selectivity and higher retention time for H2S than CO2at 25° C. (FIG. 10). Y-1,4-NDC-fcu-MOF is the best material from the studied materials for H2S removal with retention time of 70 min/gm (5% H2S, 10 cm3/min flowrate). Retention time for H2S decreases for almost 40% when temperature is increased to 50° C. (FIGS. 11, 12), with highest retention time of 41 min/g (5% H2S, 10 cc/min flowrate) for Y-1,4-NDC-fcu-MOF. Retention time of H2S and CO2for Y-1,4-NDC-fcu-MOF and Y-fum-fcu-MOF does not change during VSR at 25° C. (FIG. 13), while retention time for all the samples does not change during VSR at 50° C. (FIG. 14).

Unlike all other studied materials here, NiAlF5O(pyrazine)2has the same retention time of 28 min/gm (5% H2S, 5% CO210 cm3/min flowrate) for both H2S and CO2at 25° C. (FIG. 10). By increasing temperature to 50° C., the retention time for CO2and H2S only decrease slightly to 26 min/gm (5% H2S, 5% CO210 cc/min flowrate) unlike other materials (FIGS. 11, 12). During VSR at 25° C. for NiAlF5O(pyrazine)2, the retention time for H2S decreases slightly (FIG. 13), however the retention time for H2S and CO2remains unchanged during VSR at 50° C. (FIG. 14). NiAlF5O(pyrazine)2is a good candidate for simultaneous removal of H2S and CO2over the wide temperature range, while other studied materials can be good candidates for removal of H2S first and then CO2in the presence of CH4containing gas streams at 25° C.

As shown inFIG. 15, uptake for all the materials remains practically unchanged between 1stcycle and 6thcycle under the same condition showing that all these materials selected for mixed gas testing are very stable.

The CO2and H2S uptakes for all the MOFs studied in the course of this project, using TSR and VSR mode, are reported in Tables 1 and 2.

TABLE 1Summary of H2S and CO2adsorption uptakes of the best MOFs inTSR mode using CO2/H2S/CH4(5/5/90) Gas mixture (1 bar).CO2uptakeH2S uptakewt %wt %TSR modeTSR modeMaterials298K323K298K323KGa-soc-MOF0.580.393.111.74Y-FTZB-fcu-MOF0.980.783.302.04Y-1,4-NDC-fcu-MOF1.180.745.313.14Y-fum-fcu-MOF0.780.493.942.58NiAlF5(H2O)(pyrazine)2(solvent)2.752.552.121.97NiSiF6 DPA-i1.571.323.942.27* CH4uptake not detected by the flow mode system and CO2/CH4, H2S/CH4selectivity is calculated assuming 0.1 mg as the lowest detectable uptake

Because if the low uptake of CH4vs H2S and CO2during the mixed gas tests, it was impossible to detect and quantify the CH4uptake for all the studied materials. Accordingly, in order to determine the CO2/CH4and CO2/H2S selectivity (Tables 3 and 4), it was assumed that the non-detectable uptake of CH4is equal the accuracy of the set-up (0.1 mg).

TABLE 3Summary of CO2/CH4and H2S/CH4adsorption selectivities determinedfor the best MOFs in TSR mode using CO2/H2S/CH4(5/5/90) at 1 barGas mixture (1 bar).CO2/CH4H2S/CH4Selectivity*Selectivity*TSR modeTSR modeMaterials298K323K298K323KGa-soc-MOF38525726351478Y-FTZB-fcu-MOF64251427961735Y-1,4-NDC-fcu-MOF77148245002667Y-fum-fcu-MOF51432133422185NiAlF5(H2O)(pyrazine)2(solvent)1800167118001671NiSiF6 DPA-i102886733421928*CH4uptake not detected by the flow mode system and CO2/CH4, H2S/CH4selectivities are calculated assuming the uptake of CH4is equal to 0.1 mg, which is the lowest detectable uptake by the set-up.

TABLE 4Summary of CO2/CH4and H2S/CH4adsorption selectivitiesdetermined for the best MOFs in VSR mode using CO2/H2S/CH4(5/5/90) at 1 bar Gas mixture (1 bar).CO2/CH4H2S/CH4Selectivity*Selectivity*VSR modeVSR modeMaterials298K323K298K323KGa-soc-MOF32122519921382Y-FTZB-fcu-MOF57851425071735Y-1,4-NDC-fcu-MOF77151443712700Y-fum-fcu-MOF51438532782185NiAlF5(H2O)(pyrazine)2(solvent)1607167115101671NiSiF6 DPA-i102886733421928*CH4uptake not detected by the flow mode system and CO2/CH4, H2S/CH4selectivities are calculated assuming the uptake of CH4is equal to 0.1 mg, which is the lowest detectable uptake by the set-up.

Overall, all the materials selected exhibit very high selectivity for H2S vs CH4. As expected, the NiAlF5O(pyrazine)2exhibited the highest selectivity for CO2. It is noted that adsorption selectivity values higher than 700-1000 are often attached with elevated degree of inaccuracy, hence, this values should be considered as infinite. Accordingly, the high values for CO2/CH4and H2S/CH4selectivities reported in tables 3 and 4 are highly qualitative.

In summary, highly stable recyclable MOF materials for H2S and CO2removal from natural gas with a wide range of H2S/CO2selectivity are described that can be used for TSR and VSR at room and higher temperatures. Highly selective materials are shown for CO2having open metals sites (such as NiAlF5(H2O)(pyrazine)2(solvent)) that have the potential to remove CO2and H2Ssimultaneously with high selectivity.