Ion transport membranes with dense layer containing a catalyst

Abstract: The present invention relates to surface catalyzed ion transport membranes which demonstrate superior oxygen flux. The membranes comprise a dense multicomponent metallic oxide layer having a first surface and a second surface wherein the first surface is coated with a catalyst such as a metal or an oxide of a metal selected from Groups II, V, VI, VII, VIII, IX, X, XI, XV and the F Block lanthanides of the Periodic Table of the Elements. One or more porous layers formed from a mixed conducting multicomponent metallic oxide or a material which is not mixed conducting under process operating conditions may be formed contiguous to the second surface of the dense layer. The claimed membranes are capable of separating oxygen from oxygen-containing gaseous mixtures.

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to surface catalyzed composite ion transport membranes formed from mixed conducting oxides which exhibit substantially improved oxygen flux compared to prior art oxygen-ionically conductive membranes. The membranes possess a composite structure comprising a dense layer having a first surface and a second surface wherein the first surface of the dense layer is coated with a catalyst. The membranes can be used in a wide variety of processes including processes for separating oxygen from oxygen-containing gaseous mixtures. BACKGROUND OF THE INVENTION Solid Solid state membranes formed from oxygen ionically conductive materials are showing promise for use in commercial processes for separating oxygen from oxygen-containing streams. Envisioned applications range from small scale oxygen pumps for medical use to large scale integrated gasification combined cycle (IGCC) plants. This technology encompasses two distinctly different membrane materials, solid electrolytes and mixed conductors. Membranes formed from mixed conductors are sometimes preferred over solid electrolytes in medium- and large-scale processes for separating oxygen from oxygen-containing gaseous mixtures because mixed conductors conduct both oxygen ions and electrons at elevated temperatures and can be operated without external circuitry such as electrodes, interconnects and power-supplies. In contrast, solid electrolytes conduct only oxygen ions and require such external circuitry to be operative. Membranes formed from solid electrolytes and mixed conducting oxides are oxygen selective and can transport oxygen ions through dynamically formed oxygen anion vacancies in the solid lattice when such membranes are subjected to temperatures typically above about 500°C. Examples of solid electrolytes include yttria-stabilized zirconia (YSZ) and bismuth oxide. Examples of mixed conductors include titania-doped YSZ, praseodymia-modified YSZ, and, more importantly, various mixed metal oxides some of which possess the perovskite structure. Membranes formed from mixed conducting oxides which are operated at elevated temperatures can be used to selectively separate oxygen from an oxygen-containing gaseous mixture when a difference in oxygen partial pressures exists on opposite sides of the membrane. Oxygen transport occurs as molecular oxygen is dissociated into oxygen ions which ions migrate to the low pressure side of the membrane where the ions recombine to form oxygen molecules while electrons migrate in a direction opposite the oxygen ions to conserve charge. The rate at which oxygen permeates through the membrane is mainly controlled by two factors, the diffusion rate within the membrane and the kinetic rate of interfacial oxygen exchange; i.e., the rate at which oxygen molecules in the feed gas are converted to mobile oxygen ions at the surface of the feed side of the membrane and back again to oxygen molecules on the permeate side of the membrane. Membranes formed from mixed conducting oxides offer substantially superior oxygen selectivity than polymeric membranes. However, the value of such improved selectivity must be weighed against the higher costs associated with building and operating plants employing membranes formed from mixed conducting oxides because such plants require heat exchangers, high temperature seals and other costly equipment. Typical prior art membranes formed from mixed conducting oxides do not exhibit sufficient oxygen permeance to justify their use in commercial gas separation applications. Japanese patent application 63156516 discloses an oxygen permeation apparatus which utilizes a membrane formed from a mixed sintered body consisting of Sr (1+x)/2 La (1-x)/2 Co 1-x Me x O 3-d and SrMe'O 3 where Me = Fe, Mn, Cr or Va, 0<=x<=1 and Me' = Ti, Zr and Hf. The examples state that modest improvements in oxygen anion conductivity can be achieved by impregnating the entire surfaces of such membranes by immersing the sintered membrane bodies into solutions of silver-, palladium- or platinum-containing compounds. Solid State Ionics 37, 253-259 (1990) further describes the membranes presented in Japanese patent application 63156516 wherein palladium metal is added to the mixture of metallic oxides prior to sintering the mixture of metallic oxides to form a palladium-containing multicomponent metallic oxide. Sintered samples containing palladium showed a higher "oxygen anion conductivity" than samples which did not contain palladium. U.S. Patent 4,791,079 teaches novel mixed ion- and electron-conducting catalytic ceramic membranes consisting of a first layer of impervious mixed ion- and electron-conducting ceramic material and a second layer which is a porous catalyst-containing ion conducting ceramic material. A preferred composition for the second layer is zirconia stabilized with 8 to 15 mole% calcia, yttria, scandia, magnesia and/or mixtures thereof. The membranes are suitable for use in hydrocarbon oxidation and dehydrogenation processes. EP-A-0 438 902 discloses an ion transport membrane comprising a dense layer, a porous layer and a catalyst. No precise arrangement of the three layers is disclosed. The porous layer is made from a solely oxygen-ionically conductive material, but not a mixed conducing material. Further, US-A-5 276 237 discloses a membrane structure having a membrane which is substantially impervious to non-oxygen gases and permits a selective conveyance of a form of oxygen from the first surface of the membrane towards the second surface thereof. The first surface includes an oxygen dissociation catalyst in order to facilitate dissociation of oxygen. The membrane structure may comprise and additional catalyst material coated on the second surface of the membrane. This second catalyst material must permit the transport of the oxygen and is therefore required to form a relatively thin discontinuous film or may be used in form of sufficiently porous supported form of a catalyst. Researchers are continuing their search for thin, ceramic membranes which exhibit superior oxygen flux and sufficient mechanical strength and properties to enable their use in commercial processes. BRIEF SUMMARY OF THE INVENTION The present invention relates to novel surface catalyzed composite ion transport membranes as defined in claims 1 to 13 which are suitable for use in a wide variety of process applications. According to the most general embodiment, the ion transport membranes comprise a dense mixed conducting multicomponent metallic oxide layer having a first surface and a second surface wherein the first surface is coated with a catalyst selected from the group consisting of metals and oxides of metals selected from platinum, palladium, gold, silver, bismuth, barium, vanadium, molybdenum, cerium, ruthenium, manganese, cobalt, rhodium and praseodymium, wherein the second surface is contiguous to a porous layer or plurality of porous layers comprising a mixed conducting mulitcomponent metallic oxide or mixtures thereof, and wherein the dense mixed conducting multicomponent metallic oxide layer is formed one or a mixture of multicomponent metallic oxides represented by the formula A x A x ',A" x" B Y B' Y ,B'' Y" O 3 - z , where A, A', A" are chosen from the groups comprising Groups 1, 2 and 3 and F block lanthanides and B, B', B" are chosen from the D block transition metals according to the Periodic Table of the Elements wherein 0 < x ≤ 1, 0 ≤ x' ≤ 1, 0 ≤ x'' ≤ 1, 0 < y ≤ 1, 0 ≤ y' ≤ 1, 0 ≤ y'' ≤ 1, x+x'+x"=1, y+y'+y" = 1 and z is a number which renders the compound charge neutral. Throughout the Specification and Claims, the "dense" layer shall be interpreted to possess no connected through porosity. Preferably, the dense layer is formed from a mixed conducting multicomponent metallic oxide selected from the group consisting of La 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 3 - z , Pr 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 3 - z and La 0.2 Ba 0.8 Co 0.6 Cu 0.2 Fe 0.2 O 3 - z and the metal of the catalyst is platinum. In a preferred embodiment the average pore radius of the porous layer is less than 10 micrometers and the dense layer has a thickness ranging from 0.01 micrometer to 500 micrometers the porous layer having a thickness ranging from 1 micrometers to 2 millimeters. Preferably the dense mixed conducting multicomponent metallic oxide layer demonstrates an oxygen ionic conductivity ranging from 0.01 Ω -1 cm -1 to 100 Ω -1 cm -1 and an electronic conductivity ranging from 1 Ω -1 cm -1 to 100 Ω -1 cm -1 . In the above general formula A, A' or A" of the enumerated formula is preferably a Group 2 metal selected from the group consisting of calcium, strontium, barium and magnesium. More preferably the dense layer is formed from a mixed conducting multicomponent metallic oxide represented by the formula La x A 1-x Co Y Fe 1-Y O 3 - z wherein x is between 0 and 1, y is between 0 and 1 and A is selected from barium, strontium or calcium. In another embodiment the porous layer comprises a porous material which does not conduct electrons and oxygen ions at temperatures in excess of 500°C. According to a preferred embodiment the porous layer has an average pore radius which increases as a function of distance away from the dense layer. The surface catalyzed ion transport membranes of the present invention can be incorporated into any process wherein the gaseous reactants or products formed from the same do not unduly impact membrane performance. Suitable processes include oxygen production, esp. oxygen recovery from an oxygen containing gas mixture oxidation of organic compounds including hydrocarbons, decomposition of nitrogen- and sulfur-oxides and conversion of methane into higher hydrocarbons and the like. For example, oxygen can be separated from an oxygen-containing gaseous mixture by introducing an oxygen-containing gaseous mixture into a first gas compartment which is separated from a second gas compartment by one of Applicants' surface catalyzed ion transport membranes and establishing a positive oxygen partial pressure difference between the first and second gas compartments by producing an excess oxygen partial pressure in the first gas compartment and/or by producing a reduced oxygen partial pressure in the second gas compartment. The ion transport membrane is situated such that the oxygen-containing feed gas is contacted with the catalyzed surface of the membrane. The oxygen-containing gaseous mixture is contacted with the membrane at a temperature greater than about 500°C to separate the oxygen-containing gaseous mixture into an oxygen permeate stream and an oxygen-depleted gaseous stream and recovering the oxygen permeate stream. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A presents a prior art surface catalyzed ion transport membrane comprising a single dense mixed conducting (MC) layer having one surface coated with a catalyst. FIG. 1B presents a surface catalyzed ion transport membrane comprising a dense MC layer having a first surface which is contiguous to a mixed conducting porous layer and a second surface which is coated with a catalyst. FIG. 1C presents a surface catalyzed ion transport membrane comprising a dense MC layer having a first surface which is coated with a catalyst and a second surface which is contiguous to a plurality of individually deposited MC porous layers having successively larger pore radii as a function of distance away from the dense MC layer. FIG. 1D presents a surface catalyzed ion transport membrane similar to the embodiment according to FIG. 1C comprising two or more discrete MC porous layers wherein each respective layer has a successively larger average pore radius as a function of distance away from the dense MC layer. FIG. 1E presents a surface catalyzed ion transport membrane comprising a dense MC layer having a first surface which is coated with a catalyst and a second surface which is contiguous to a plurality of porous layers wherein one or more of the porous layers not in contact with the dense mixed conducting oxide layer is formed of a material which is not a mixed conductor. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to novel surface catalyzed ion transport membranes as defined above which membranes are suitable for use in a wide variety of process applications including processes for separating oxygen from oxygen-containing gaseous mixtures. According to an embodiment of prior art, the surface catalyzed ion transport membranes have a composite structure comprising a dense multicomponent metallic oxide layer having a first surface and a second surface wherein the first surface is coated with a catalyst. This prior art embodiment is depicted in FIG. 1A which illustrates a dense mixed conducting layer having a single surface which is coated with a catalyst as shall be described in greater detail in this Specification. The configuration of Applicants' ion transport membranes diverge from the prior art by placing a catalyst on only one side of the dense layer of the membrane instead of both sides resulting in unexpectedly improved oxygen flux and by a porous layer or a plurality of porous layer contiguous to the second surface of the dense membrane and comprising a mixed conducting multicomponent metallic oxide or mixtures thereof. For example, Japanese patent application 63156516 and the article presented in Solid State Ionics 37, 253-259 (1990), recited under the section entitled, Background of the Invention, disclose membranes consisting of a dense multicomponent metallic oxide layer having no porous support layer wherein a catalyst is placed on both sides of the dense layer. The claimed ion transport membranes comprise a dense layer of one or a mixture of multicomponent metallic oxides having no connected through porosity which are capable of conducting electrons and oxygen ions at operating temperatures. The dense layer of the membranes of this invention comprising the multicomponent metallic oxide demonstrates electron conductivity as well as oxygen ion conductivity at temperatures greater than about 500°C. Hence, these materials are commonly referred to as mixed conducting oxides. Suitable mixed conducting oxides are represented by the structure A x A' x' A" x" B y B' y' B" y" O 3-z , where A,A',A" are chosen from the group comprising Groups 1, 2 and 3 and the F block lanthanides; and B,B',B" are chosen from the D block transition metals according to the Periodic Table of the Elements wherein 0<x≤1, 0≤x'≤1, 0≤x"≤1, 0<y≤1, 0≤y'≤1, 0≤y"≤1, x+x'+x"=1, y+y'+y"=1 and z is a number which renders the compound charge neutral. All references made to the Periodic Table of the Elements in the Specification and Claims shall refer to the Table adopted by the International Union of Pure and Applied Chemistry. Preferably, A, A' or A" of the enumerated structure is a Group 2 metal selected from the group consisting of calcium, strontium, barium and magnesium. The dense multicomponent metallic oxide layer typically demonstrates an oxygen ionic conductivity ranging from 0.01 ohm -1 cm -1 to 100 ohm -1 cm -1 and an electronic conductivity ranging from about 1 ohm -1 cm -1 to 100 ohm -1 cm -1 . Preferred mixed conducting oxides are represented by the formula La x A 1-x Co y Fe 1-y O 3-z wherein x is between 0 and 1, y is between 0 and 1 and A is selected from barium, strontium or calcium. Most preferably, the dense layer is formed from a multicomponent metallic oxide selected from the group consisting of La 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 3-z' Pr 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 3-z and La 0.2 Ba 0.8 Co 0.6 Cu 0.2 Fe 0.2 O 3-z . Catalysts to be deposited onto the enumerated surface of the dense layer of the ion transport membrane include metals and oxides of metals selected from Groups II, V, VI, VII, VIII, IX, X, XI, XV and the F Block lanthanides of the Periodic Table of the Elements such as platinum, palladium, gold, silver, bismuth, barium, vanadium, molybdenum, cerium, praseodymium, cobalt, ruthenium, rhodium and manganese. The catalyst can be applied to the dense mixed conducting layer of the ion transport membrane by any conventional method including painting a suspension of metal particles onto the dense layer of the membrane; spraying a solution of metal salt onto the dense layer surface or dispersing a metal salt solution onto the same. Other suitable methods include screen printing, dip coating, plasma spraying and flame spraying, physical vapor deposition such as electron bean evaporation or sputtering, and chemical vapor deposition. Applicants emphasize that the entire surface of the dense mixed conducting layer does not have to be coated with catalyst in order to achieve the unexpected benefits of the present invention. For example, any selected pattern of catalyst may be deposited onto the surface of the dense layer of the composite'membrane by screen printing, masking and other techniques. Such patterns can be designed and applied according to currently used techniques which are well known in the art. Referring now to the painting technique for applying catalyst to the dense layer of the composite membrane, the following general procedure is utilized. A desired catalyst, such as platinum, may be applied by coating the enumerated dense layer of the composite catalyst For example, platinum ink #6926 which is commercially available from Engelhard Inc., consisting of micron-sized platinum particles suspended in terpene can be applied to the dense layer of the composite catalyst using a brush or roller. The ink coating is air dried and the membrane is loaded into an apparatus and slowly heated to a temperature above about 550°C in order to volatilize and burn organic binders and solvents which may be present in the catalyst ink. Catalysts may also be applied to the surface of the dense layer by spraying the surface with a solution of the desired catalyst. For example, platinum can be applied in this manner using a 0.01 molar solution of platinum acetylacetonate Pt(Acac) 2 which was prepared by dissolving an appropriate amount of Pt(Acac) 2 in acetone. This solution is loaded into a chromatography sprayer using nitrogen as a carrier gas. The solution is sprayed onto the surface of the dense layer of the composite membrane to yield a 50 nm thick continuous coating of platinum. The acetone solvent is evaporated thereby providing a composite membrane whose dense layer was coated with Pt(Acac) 2 . The membrane is then loaded into a test apparatus and slowly heated to a temperature greater than about 550°C. The temperature should be sufficiently high to decompose the Pt(Acac) 2 to produce the platinum catalyst on the dense layer of the ion transport membrane and organic vapors. A desired catalyst may be applied to the surface of the dense layer of a composite membrane by depositing a solution of the desired catalyst in an appropriate solvent. For example, a solution of Pt(Acac) 2 in acetone can be transferred or dispersed onto the surface of the dense layer of the composite ion transport membrane. The acetone is evaporated leaving the dense layer of the composite membrane coated with Pt(Acac) 2 . The membrane is heated to decompose the Pt(Acac) 2 to form the desired coating of platinum catalyst on the surface of the dense layer of the ion transport membrane. In an alternate embodiment depicted in FIG. 1B, the surface catalyzed ion transport membranes comprise a dense mixed conducting layer having a first surface contiguous to a porous layer and a second surface which is coated with one or more of the enumerated catalysts. This embodiment includes surface catalyzed ion transport membranes comprising a single porous layer formed from one or a mixture of multicomponent metallic oxides referred to in this Specification. Referring to FIG. 1B, the average pore radius of the single porous layer may be constant throughout its cross-section. In another alternate embodiment, the surface catalyzed ion transport membranes comprise a plurality of porous layers. The average pore radius of each respective porous layer may increase with distance from the interface with the dense mixed conducting multicomponent metallic oxide layer as depicted in FIG. 1C., which presents a composite membrane comprising a mixed conducting porous layer which is actually a plurality of individually deposited layers having successively larger pore radii as a function of distance away from the dense mixed conducting oxide layer. Any number of layers are used such that the porous layer effectively possesses a pore structure which is funnel-shaped with an average pore radius increasing from 0.5 to about 10 micrometers or more moving away from the interface with the dense mixed conducting oxide layer. These layers can be fabricated by starting with a green state formed of a plurality of layers wherein each respective layer comprises successively larger particles. A preferred technique for manufacturing ultrathin solid state membranes is presented in United Stated Patent 5 160 618, filed on January 3, 1992, which is assigned to Air Products and Chemicals, Inc., Allentown, PA. In another alternate embodiment depicted in FIG. 1D, the ion transport membranes comprise a multicomponent metallic oxide dense layer having a first surface which is coated with an enumerated catalyst and a second surface contiguous to a plurality of mixed conducting porous layers, each respective porous layer having a discrete average pore radius wherein the average pore radius of each respective porous layer is larger than the average pore radius of the preceding porous layer as function of distance from the dense layer. Each respective porous layer of the membranes of the embodiment according to FIG. 1D is formed from one or a mixture of mixed conducting multicomponent metallic oxides, an oxygen-ionically conductive material, an electronconducting material or a material which does not conduct electrons or oxygen ions under operating temperatures, wherein at least the porous layer adjacent to the dense mixed conducting multicomponent metallic oxide layer is formed from a mixed conducting multicomponent metallic oxide or mixtures thereof and preferably has an average pore radius of less than about 10 micrometers while the average pore radius of each subsequent porous layer has an average pore radius which progressively increases as a function of distance from the dense layer. Any combination of porous layers can be utilized so long as their coefficients of thermal expansion are compatible and chemical reactions are minimized between the respective layers at membrane operating temperatures. Examples of suitable porous materials which are not mixed conducting under process operating conditions include high temperature oxygen compatible metal alloys, metal oxide-stabilized zirconia such a yttria-stabilized zirconia and calcium-stabilized zirconia, ceria, alumina, magnesia, silica, titania and compounds and mixtures thereof. FIG. 1E illustrates a surface catalyzed ion transport membrane comprising two porous layers, the first which is a mixed conducting porous layer situated contiguous to the dense mixed conducting layer and the second porous layer which is formed of a material which is not mixed conducting under process operation conditions, lying adjacent to the first porous layer. Any combination of multicomponent metallic oxides can be used so long as the dense and porous layers are chemically and mechanically compatible under conditions used in the enumerated applications such as in a process for separating oxygen from an oxygen-containing gaseous mixture. The thickness of the porous layers of the composite membranes referred to in each of the above-mentioned embodiments can be varied to ensure sufficient mechanical strength of the composite membrane. The desired thickness of each porous layer is regulated by the following considerations. First, the porosity and average pore radius of each porous layer should be regulated such that oxygen flux is not impeded while maintaining sufficient mechanical strength. Second, the pores or channels of pores within each porous layer should be wide enough so that oxygen flux is not impeded, but not so wide as to cause pore filling during fabrication or fissure of the dense layer during operation. Third, each porous layer should be compatible with the dense layer in terms of chemical reactivity and adhesion and thermal expansion to reduce problems associated with cracking and delamination. The porous layers of the composite membrane are formed from mixed conducting oxides. Such porous mixed conducting oxide layers act as a compatible mechanical support for the dense mixed conducting oxide layer and provide two interacting diffusion paths for oxygen; through the pores, and through the solid. Surface kinetic rate limitations for gas-solid oxygen exchange are mitigated by the availability of a large "active" surface area in the small pore structure of the support, especially in the vicinity of the dense layer. On the other hand, the effect of the small pores in hindering diffusion is mitigated by fast ionic conduction in the solid. The thickness of the dense mixed conducting layer typically ranges from 0.01 micrometer to about 500 micrometers although preferably, the dense layer is fabricated as thinly as permitted by structural integrity considerations and has a thickness of less than about 100 micrometers. The porous mixed conducting oxide layer in contact with, and contiguous to the dense mixed conducting layer has a thickness ranging from 1 micrometer to about 2 millimeters. Porous layers not in contact with the dense mixed conducting layer, whether formed from a mixed conducting oxide or a material which is not a mixed conducting oxide, can be as thick as desired to ensure mechanical strength to the extent that the ultimate thickness does not impede gas diffusion. Typically, the total thickness of the composite membrane is less than about 5 mm, although membranes having a greater thickness are also contemplated. Preferably, the dense layer has a thickness ranging from 0.01 to about 500 micrometers. Surface catalyzed ion transport membranes of this invention which utilize one or more active porous supports formed from mixed conducting oxides provide particularly high oxygen flux because such active porous layers counteract surface kinetic limitations by increasing the active gas-solid interfacial area per unit volume. Therefore, as the average pore radius is reduced while maintaining a given porosity within each porous layer, surface kinetic limitations which reduce oxygen flux can be correspondingly diminished. A fairly thin porous mixed conducting oxide layer having an average pore radius ranging from 0.1 to about 10 micrometers, situated contiguous to a dense mixed conducting layer provides increased interfacial area to counteract surface kinetic limitations, but does not cause any significant pressure drop or resistance to mass transfer. Thin dense layers of the desired multicomponent metallic oxide of any desired thickness can be deposited onto the enumerated porous layers by known techniques. For example, the membrane composites can be manufactured by first forming a porous body from relatively coarse sized particles of the multicomponent metallic oxide. A slurry of finer particles of the same material or a similar, compatible multicomponent metallic oxide may then be coated onto the porous material and cured to the green state, the two layer system then being fired to form the composite membrane. Alternatively, the composite membranes of the present invention can be prepared by applying a dense layer of a desired mixed conducting oxide onto the desired porous substrate by conventional chemical vapor deposition techniques followed by sintering to obtain the desired dense layer. In order to obtain an optimal dense coating, a smaller average pore radius in the surface of the porous support may be used compared to the average pore radius in the bulk. This may be achieved by using two or more porous layers which differ in properties such as pore radius and porosity. The advantages afforded by Applicants' surface catalyzed ion transport membranes can best be understood by developing a thorough understanding of the mechanism by which oxygen is ionically transported through the dense mixed conducting oxide layer of an ion transport membrane. The oxygen flux observed by a conventional ion transport membrane is controlled by "surface kinetic limitations" and "bulk diffusion limitations". Surface kinetic limitations are constraints to oxygen flux caused by one or more of the many steps involved in converting an oxygen molecule in the gas phase on the feed side of the ion transport membrane into mobile oxygen ions and converting the oxygen ions back to oxygen molecules on the permeate side of the ion transport membrane. Bulk diffusion limitations are constraints on oxygen flux relating to the diffusivity of oxygen ions through the dense mixed conducting oxide layer. Additional diffusion constraints can be associated with molecular oxygen moving through the pores of the porous layers. The present invention provides surface catalyzed composite membranes which overcome kinetic limitations on oxygen flux associated with very thin dense mixed conducting oxide layers while providing membranes which maintain their structural integrity under the demanding high temperature conditions associated with processes using such membranes. The membranes of the present invention can be used to recover oxygen from an oxygen-containing gaseous mixture by delivering the oxygen-containing gaseous mixture into a first gas compartment which is separated from a second gas compartment by the subject membrane, establishing a positive oxygen partial pressure difference between the first and second gas compartments by producing an excess oxygen partial pressure in the first gas compartment and/or by producing a reduced oxygen partial pressure in the second gas compartment; contacting the oxygen-containing gaseous mixture with the catalyzed surface of the enumerated ion transport membranes at a temperature greater than about 500°C to separate the compressed oxygen-containing gaseous mixture into an oxygen permeate stream and an oxygen-depleted gaseous stream and recovering the oxygen permeate stream. Any conventional apparatus can be utilized to house the ion transport membranes of the present invention whereby the membrane forms a partition between the first and second gas compartments. A representative apparatus is disclosed in U.S. Patent 5,035,727, issued to Air Products and Chemicals, Inc., Allentown, PA. The surface catalyzed ion transport membranes are situated in the apparatus such that the oxygen-containing gaseous mixture is contacted with the side of the membrane upon which the catalyst resides. The composite membranes of this invention are capable of separating oxygen from oxygen-containing gaseous mixtures containing one or more components selected from carbon dioxide, water and volatile hydrocarbons. The amount of oxygen present in such gaseous mixtures typically ranges between about 0.01 vol.% to 50 vol.% oxygen. The preferred oxygen-containing gaseous mixture is atmospheric air. A difference in oxygen partial pressure between the first and second compartments provides the driving force for effecting the separation when the process temperature is elevated to a sufficient temperature to cause oxygen in the oxygen-containing gaseous mixture residing in the first compartment to adsorb, dissociate and ionize. Oxygen is transported through the membrane in the ionic form. A pure oxygen product is collected in the second gas compartment wherein oxygen ions are converted into neutral oxygen molecules by the release of electrons and reassociation. The second gas compartment resides at lower oxygen partial pressure than the first gas compartment. A positive oxygen partial pressure difference between the first and second gas compartments can be created by compressing air in the first compartment to a pressure sufficient to recover the oxygen permeate stream at a pressure of greater than or equal to about one atmosphere. Typical pressures range from about 15 psia to about 250 psia and the optimum pressure will vary depending upon the amount of oxygen in the oxygen-containing gaseous mixture. Conventional compressors can be utilized to achieve the compression required to practice the present step of the process. Alternately, a positive oxygen partial pressure difference between the first and second gas compartments can be achieved by evacuating the second gas compartment to a pressure sufficient to recover the oxygen permeate. The final step of the process comprises recovering the oxygen-containing gaseous mixture by storing the substantially pure oxygen in a suitable container or transferring the same to another process. The oxygen permeate typically comprises pure oxygen or high purity oxygen defined as a gas generally containing at least about 90 vol.% O 2 , preferably more than about 95 vol% O 2 and especially more than 99 vol.% O 2 . The surface catalyzed ion transport membranes of the present invention can be incorporated into any process wherein the gaseous reactants or products formed from the same do not unduly impact membrane performance. Suitable processes include oxygen production, oxidation of organic compounds including hydrocarbons, decomposition of nitrogen- and sulfur-oxides, methane conversion and the like.

Claims: An ion transport membrane comprising a dense mixed conducting multicomponent metallic oxide layer having a first surface and a second surface wherein the first surface is coated with a catalyst selected from the group consisting of metals and oxides of metals selected from platinum, palladium, gold, silver, bismuth, barium, vanadium, molybdenum, cerium, ruthenium, manganese, cobalt, rhodium and praseodymium, wherein the second surface is contiguous to a porous layer or plurality of porous layers comprising a mixed conducting mulitcomponent metallic oxide or mixtures thereof, and wherein the dense mixed conducting multicomponent metallic oxide layer is formed one or a mixture of multicomponent metallic oxides represented by the formula A x A' x' A" x" B Y B' Y' B" Y" O 3-z , where A, A', A" are chosen from the groups comprising Groups 1, 2 and 3 and F block lanthanides and B, B', B" are chosen from the D block transition metals according to the Periodic Table of the Elements wherein 0 < x ≤ 1, 0 ≤ x' ≤ 1, 0 ≤ x" ≤ 1, 0 < y ≤ 1, 0 ≤ y' ≤ 1, 0 ≤ y'' ≤ 1, x+x'+x" =1, y+y'+y"=1 and z is a number which renders the compound charge neutral. The ion transport membrane according to claim 1, wherein the dense layer is formed from a mixed conducting multicomponent metallic oxide selected from the group consisting of La 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 3-z , Pr 0.2 Ba 0.8 Co 0.8 Fe 0.2 O 3-z and La 0.2 Ba 0.8 Co 0.6 Cu 0.2 Fe 0.2 O 3-z and the metal of the catalyst is platinum. The ion transport membrane according to claims 1 and 2 wherein the average pore radius of the porous layer is less than 10 micrometers and the dense layer has a thickness ranging from 0.01 micrometer to 500 micrometers and the porous layer having a thickness ranging from 1 micrometers to 2 millimeters. The ion transport membrane according to any one of claims 1 to 3 wherein the dense mixed conducting multicomponent metallic oxide layer demonstrates an oxygen ionic conductivity ranging from 0.01 Ω -1 cm -1 to 100 Ω -1 cm -1 and an electronic conductivity ranging from 1 Ω -1 cm -1 to 100 Ω -1 cm -1 . The ion transport membrane according to any one of claims 1 to 4 wherein A, A' or A" of the enumerated formula is a Group 2 metal selected from the group consisting of calcium, strontium, barium and magnesium. The ion transport membrane according to claim 5 wherein the dense layer is formed from a mixed conducting multicomponent metallic oxide represented by the formula La x A 1-x Co Y Fe 1-Y O 3-z wherein x is between 0 and 1, y is between 0 and 1 and A is selected from barium, strontium or calcium. The ion transport membrane according to any one of claims 1 to 6 wherein the porous layer comprises a porous material which does not conduct electrons and oxygen ions at temperatures in excess of 500°C. The ion transport membrane according to any one of claims 1 to 7 wherein the porous layer has an average pore radius which increases as a function of distance away from the dense layer. A process for recovering oxygen from an oxygen-containing gaseous mixture which utilizes the ion transport membrane according to claim 1. A process for oxidizing an organic compound which utilizes the ion transport membrane according to claim 1. A process for converting nitrogen oxide into gaseous nitrogen and oxygen which utilizes the ion transport membrane according to claim 1. A process for converting sulfur oxides into sulfur and oxygen which utilizes the ion transport membrane according to claim 1. A process for converting methane into higher hydrocarbons which utilizes the ion transport membrane according to claim 1.

Inventors: CAROLAN MICHAEL FRANCIS, DYER PAUL NIGEL, CAROLAN, MICHAEL FRANCIS, DYER, PAUL NIGEL
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1995
Main classification: B01D53
IPC codes: B01D53, C01B17, C01B13, B01J23, H01M8, H01B1, C04B41, B01J37, B01D71, B01J20, C01B21