encyclopedia of membrane science and technology || inorganic membranes

INORGANIC MEMBRANES

Shaomin LiuCurtin University, Perth, Australia

Xiaoyao TanTianjin Polytechnic University, Tianjin, China

Kang LiImperial College London, London, UK

1 BACKGROUND

The inorganic membrane can be defined as a permselective barrier or a fine sieve madefrom inorganic materials, which may include ceramics, oxides, metals, carbon, or others.A common feature of these membranes in asymmetric structure prepared by the sol–gelmethod is that they are made in supported form. The cost of support elements and thelengthy steps involved in the synthesis of the separating layers make the price of theseinorganic membranes very high compared to the well-established polymeric membranes.Thus, the usage or industrial application has to be justified for separations at extremeconditions where they offer some unique advantages in terms of selectivity, high chem-ical tolerance, high thermal stability, high fouling resistance, or good sterilization, etc.These extreme conditions include a high temperature operation and a harsh environmentcontaining highly reactive chemicals such as acidic or alkaline substances or corrosivesolvents, all of which preclude the use of the existing polymeric membranes owing totheir low material stability to resist these conditions.

The historical development of inorganic membranes was started in the 1940s for ura-nium isotope separation for military nuclear weapons by the Manhattan project. Thenaturally existing uranium is composed of 99.28% of 238U and 0.72% 235U isotope.In order to enrich the 235U concentration up to the 90% required by weapons grade,advanced separation technologies such as centrifuge, laser, or plasma were not avail-able at that time, and the only practical method was gaseous diffusion by forcing theUF6 (the major precursor of uranium) through semipermeable membranes. Owing to thehigh operation temperature and the highly corrosive feature of UF6, the only survivingmembrane materials were inorganic materials from metals, Al2O3, TiO2, ZrO2, etc.; and

Encyclopedia of Membrane Science and Technology. Edited by Eric M.V. Hoek and Volodymyr V. Tarabara.Copyright © 2013 John Wiley & Sons, Inc.

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2 INORGANIC MEMBRANES

the tubular membrane configuration in asymmetric structure was adopted. Later, in the1950s, some companies in France such as CEA or CGEC also joined the developmentof porous ceramic membranes for uranium enrichment for military purposes. Follow-ing the oil crisis in the 1970s, there has been considerable worldwide research to findnew energy sources to replace the fossil fuels, pushing nuclear energy to the developmentfront. In this context, ceramic membrane development reached its historical golden periodbecause of the 235U enrichment from the natural 0.72% to 3–4% required by the nuclearenergy reactor grade in many countries. With the passage of time, in the 1980s, newcost-effective technologies for uranium enrichment appeared, including centrifugal sepa-ration, laser, plasma, etc., so that the energy-intensive ceramic membrane technology hascompleted its historical task and has been squeezed out from the nuclear market. How-ever, traditionally large companies such as CGEC, SFEC, Rhone-Poulenc, Ceraver, etc.,with their established huge capital investment, started to search out new markets for theirceramic membrane products. In some sense, this opened the era of applying the ceramicmembranes for separations in liquid media for civil applications, with the main operat-ing modes using ultrafiltration, and microfiltration. Owing to easy sterilization by hightemperature treatment and efficient removal of various bacteria, ceramic membranes aresuccessfully applied in industries such as dairy, food, beverage (beer, wine, fruit juices,etc.), pharmaceutical, biological, or water (1). To overcome the brittleness of one singletubular ceramic membrane and reduce the production cost, the monolithic multichannelmembrane element (schematically shown in Figure 46, Membrane Materials and ModuleDevelopment, Historical Perspective) was developed. During this period of develop-ment, many new ceramic membrane companies were formed [i.e., Alcoa and Ceramem(US), Ceram-Filtre (Europe), NGK (Japan), and Toto (Japan)]. With the developmentcontinuing to the 1990s, the advantages of inorganic membranes became well knownand widely applied in many other fields such as energy and biological, environmental,and chemical engineering. Particularly, the inorganic membrane application has beenexpanded to gas separation and high temperature reactions, mirroring a full developmentstage (2).

The inorganic membrane is a very broad topic. According to materials, it can beclassified into zeolite, carbon, palladium, fluorite oxide, perovskite membranes, etc.According to membrane types, it can also be categorized into porous or dense inor-ganic membranes. According to functions, a classification can be made into membranesfor physical separations and membrane reactors for high temperature reactions. Eachof these topics involves synthesis skills, characterization, transport theories, and scale-up problems. It is difficult to give a comprehensive review addressing all these issuesin one article. Some porous inorganic membranes share the common working princi-ples or features with their counterpart polymeric membranes. For example, driven bypressure, concentration, or electric field across the membranes, inorganic membranescan also be operated with modes of pervaporation, reverse osmosis (RO), NF, ultrafil-tration, microfiltration, and dialysis for liquid separation. These applications are quitedependent on pore size and its size distribution of the prepared membranes, whichhave been widely reviewed and therefore are treated in less attention in this article(1, 3). However, most of the recently developed inorganic membranes for gas separa-tion are very specific, with their own unique properties, which will be highlighted inthe article; thus, the focus of this article is placed on the inorganic membrane for gasseparations.

INORGANIC MEMBRANES 3

2 INORGANIC MEMBRANE MORPHOLOGY, PREPARATION,AND CHARACTERIZATION

Inorganic membranes usually have an asymmetric structure consisting of several layersof one or more different inorganic materials. A typical membrane is depicted in Figure 1.

The membrane consists of a macroporous support (Fig. 1, bottom layer with pore size1–15 μm), one or two mesoporous intermediate layers (with pores 0.05–1 μm), and atop microporous (with pores <1 nm) or a dense top layer. The bottom layer providesmechanical support, while the middle layers bridge the pore size differences between thesupport and the top layers where the actual separation takes place (1). Commonly usedmaterials for the macroporous support layer with pore size ranged from 0.5 to 15 μm areAl2O3, TiO2, ZrO2, SiO2, MgO, RuO2, Si3N4, stainless steel, etc. or a combination ofthem. An example of the pore characteristics of a three-layer alumina silica membraneis given in Figure 2. The pore sizes of macroporous support layer (α-Al2O3 thickness1.4 mm) and intermediate mesoporous layer (γ-Al2O3 thickness 3 μm) are in the rangeof 800 and 10 nm, respectively (4). The real thin separating layer located in the bottomcorner in Figure 2 has a thickness of 0.25 μm, with a pore diameter of 3 A. This poresize is between the molecular diameter of H2 (2.9 A) and CO2 (3.3 A) and therefore

Microporous layer

IntermediateMesoporous layer

Macroporous support

FIGURE 1 Schematic representation of an asymmetric composite membrane. (Please refer tothe online version for the color representation of the figure.)

FIGURE 2 SEM image of the cross section of co-doped silica membrane (5 kV and 19, 000ð).Top left corner (α-Al2O3), middle (γ-Al2O3), and bottom right corner (cobalt–silica layer). Source:Reprinted from Reference 4. Copyright (2009), with permission from Elsevier.


the membrane has a very good selectivity of H2/CO2. The above-mentioned ceramicmembranes can only be prepared through multiple steps. As illustrated in Figure 2,an α-Al2O3 support layer is first prepared to provide sufficient mechanical strength forthe membrane, followed by coating one or more intermediate layers (γ-Al2O3) on thesupport layer before a final separation layer can be fabricated. Depending on the tubularor flat sheet (or disk) configurations, the macroporous support layer can be prepared byestablished techniques of slip casting, tape casting, pressing, or extrusion method, thedetails of which can be found elsewhere (1). The mesoporous and separating layers arenormally prepared by the sol–gel method, which was first applied for development ofceramic ultrafiltration membranes by Leenaars and Burggraaf (5).

The sol–gel method has now become one of the most important techniques in the fabri-cation of ceramic membranes with pore size less than 10 nm. A more detailed descriptionof the sol–gel technology can be found elsewhere (6–10). The advantage of the sol–geltechnique is that the pore size of the membrane can be desirably controlled, especially forsmall pores, regardless of the support shape (flat or tubular). The sol–gel technique canbe differentiated as two main methods: the colloidal route and the polymer route. In thecolloidal route, one metal salt (or more than one) is mixed with water to form a sol thatis coated on a membrane support to form a colloidal gel. The colloidal solutions containdense oxide particles such as Al2O3, SiO2, TiO2, or ZrO2, many of which are now com-mercially available such as Locron (Clariant, Germany). On the other hand, in anotherroute, the metal–organic precursors, instead of metal salts, are mixed with organic sol-vent to form a sol, followed by performing a coating on a membrane support where afinal polymer gel will be formed. The following example is one typical experimentalprocedure to use the sol–gel method to prepare inorganic molecular sieving membranesfor hydrogen separation (11).

2.1 Preparation of Cobalt-Doped Silica Molecular Membranevia the Sol–Gel Method

Commercial alumina platelet substrates having a porosity of 30% and an average poresize of 0.5–1 μm were dip coated with Locron solution (Clariant, Germany) for the depo-sition of the γ-alumina layer. Sintering was carried out at 600 ŽC for 2 h at a heating andcooling rate of 1 ŽC/min. A total of four Locron layers were coated to ensure the interme-diate mesoporous layer with a pore size of 4 nm. Cobalt-doped sol–gel was prepared bymixing 42.26 g tetraethoxysilane (TEOS) in 600 g ethanol to form a solution. A secondsolution was then synthesized by the dissolution of 14.84 g cobalt nitrate hexahydrate(Co(NO3)26H2O) in 51.77 g of 30% aqueous H2O2. Subsequently, both solutions werevigorously mixed together by stirring for 3 h in an ice-cooled bath. The platelets werecoated with the stable Si–Co–O solution using a controlled immersion time of 1 min andwithdrawal speed of 2 cm/min. Sintering was carried out at 600 ŽC for 4 h at a heatingand cooling rate of 0.7 ŽC/min. A total of six metal-doped silica layers were coated onthe platelets to ensure the production of defect-free membranes with an expected poresize in the region of (0.35–0.45 nm) for silica-derived materials. Subsequently, a sinter-ing hydrogen-rich atmosphere at 500 ŽC was employed for 15 h to reduce cobalt oxidesembedded in the silica matrix (11).

As can be seen from this description, the major problem of the sol–gel method isthat the preparation is time consuming as the dip-coating procedure using the colloidalor polymer solution needs to be tediously repeated many times to guarantee a qualified


layer, and every repetition requires the careful controlling of the heat treatment program.Obviously, combining the multiple steps into a single one is desirable by cutting pro-duction time and costs, and thereby membrane price. Li et al. (12) demonstrated thatthe multiple-step fabrication process could be combined into a single step using a phaseinversion process to prepare the hollow fiber ceramic membrane for oxygen separation.Figure 3 shows a typical scanning electron microscopic (SEM) image of such an asym-metric membrane (12). As can be seen, a dense and thin skin layer is integrated on theporous support of the same ceramic material. Such a layered ceramic membrane canbe prepared in one extrusion and sintering step. Given here is an example showing theexperimental procedure to prepare such asymmetric membranes.

2.2 Preparation of Asymmetric Ceramic Membranes via the CombinedPhase Inversion and Sintering Method

2.2.1 Materials. Commercially available lanthanum strontium cobalt ferrite,La0.6Sr0.4Co0.2Fe0.8O3�δ (LSCF) powder with particle diameter of 1 μm was used asthe membrane material. PESf (Radel A-300) and N -methyl-2-pyrrolidone (NMP) wereused to prepare the polymer solution as the binder. Polyvinylpyrrolidone (PVP, K90,MW D 630, 000) was used as an additive to adjust the viscosity. Tap water was used asboth internal and external coagulants.

2.2.2 Preparation of Spinning Suspension. The required quantity of NMP was takenin a 1-l wide-neck reaction flask and the PESf was slowly added over a period of 30 minto form the polymer solution. After the polymer solution was formed, a given amountof LSCF was added into the polymer solution slowly, while the stirrer was used at a

Outer “skin”

Honeycombed sub surface

FIGURE 3 SEM image of a layered ceramic membrane. Source: Reprinted from Reference 12.Copyright (2006), with permission from Elsevier. (Please refer to the online version for the colorrepresentation of the figure.)


speed of 300 rpm to ensure that all the LSCF powder was dispersed uniformly in thepolymer solution. PVP as an additive was also introduced into the solution to modulate itsviscosity. Finally, the mixture (spinning suspension) was degassed at room temperatureto remove the air trapped inside the suspension.

2.2.3 Fabrication of Inorganic Hollow Fiber Membranes. The degassed suspensioncontaining the dispersed LSCF was transferred to a stainless steel reservoir and pressur-ized to 20–30 psig using nitrogen. A tube-in-orifice spinneret with orifice diameter/innerdiameter of the tube of 2.0/0.72 (mm) was used to obtain hollow fiber precursors. Theair gap was kept at 2 cm for all spinning runs. Finally, the forming hollow fiber precur-sor was passed through a water bath and immersed inside the bath for at least 12 h tocomplete the solidification process. After being thoroughly washed in water, the hollowfiber precursors were dried at room temperature. They were first heated in the carbolitefurnace at about 500 ŽC for 2 h to remove the organic polymer binder, and then werecalcined at a high temperature between 1200 and 1600 ŽC for about 5 h to allow theparticle fusion and bonding to occur. On the basis of this individual example, a generalprocedure in preparation of ceramic hollow fiber membranes employing a phase inver-sion method can be drawn. This method has three major steps including (i) preparationof a spinning suspension, (ii) spinning of ceramic hollow fiber precursors, and (iii) finalsintering. Preparation of ceramic membranes using the phase inversion method is quitea complex process and in every step there are many factors that may affect the finalmembrane performance. Tan et al. (13, 14) and Liu et al. (15) have provided detaileddiscussions on these factors when they used the method to produce Al2O3, ZrO2 orperovskites hollow fiber ceramic membranes with different asymmetric structures.

2.3 Membrane Characterization

An inorganic membrane is a typical example that performance is determined by its mor-phology and material phase structure. Information about the pore size, shape, density,distribution, and membrane surface properties for porous ceramic membranes and gastightness, crystal structures, and permeation characteristics of specific gases for denseceramic membranes are therefore of importance to membrane researchers, so as to allowa meaningful prediction of separation performances of the membranes. For porous inor-ganic membranes, separation properties are directly determined by the porous morphologyproperties in terms of pore size, pore shape, connectivity, particle packing, surface area,thickness of different layers in the asymmetric structure, etc. Therefore, proper charac-terization of the porous structures is crucial for the development of commercial ceramicmembranes. In general, the characterization of the supported composite membranes ismore demanding than that of most of the symmetric porous membranes because theseparation layer is thin, often with an anisotropic and microporous structure. Therefore,specialized characterization techniques are required. For dense inorganic membranes, thegas tightness, crystal structures, and microstructure, as well as the catalytic activity ofthe membrane surfaces and conductivities of ions and electrons of specific gases andhence the permeation characteristics, are important parameters. General characterizationof ceramic membranes to identify the morphology-/structure-related parameters can usetechniques of SEM, field emission scanning electron microscopy (FESEM), transmis-sion electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction(XRD), etc. On the other hand, permeation-related parameters can be derived via charac-terization techniques such as mercury porosimetry, permeation, bubble point technique,


permporometry, thermoporometry, etc. In addition, to determine the mechanical strengthof ceramic tubular or hollow fiber membranes, the bending strength test can be performed.Experimental procedure and analysis guidelines about all these characterization methodscan be referred in many papers or books (1) and therefore are treated in less attention here.

3 MEMBRANE PROCESSES FOR LIQUID PHASE OR PARTICLESEPARATION

Inorganic membranes can be generally divided into porous or nonporous types, with theformer well developed and widely applied for solid or liquid phase separations but thelatter mainly for gas separation, which will be discussed in the next section. In this subsec-tion, the emphasis is placed on porous inorganic membranes for liquid phase separation,where the application variations are largely dependent on the properties of membranestructure. The membrane pore size, one of the most important characteristics in determin-ing the separation performance, normally refers to the median or mean size of the poresof the separating layer on the membrane. Noteworthy is that although most of the mem-brane pores are not uniform, they tend to be assumed to be uniform and in cylindricalshape to derive some mathematical modeling equations or other theoretical interpretationto predict the membrane behavior in certain situations. According to the pore size rangefrom small to large, the porous membranes can be divided into five types with differentapplications: RO (0.1–1 nm), NFs (1–100 nm), ultrafiltration (0.01–0.1 μm), microfiltra-tion (0.1–5 μm), and filtration (1–200 μm). Albeit widely reviewed by many researchers,the definitions and applications of these membranes are still briefly introduced to makethis article more comprehensive and informative.

3.1 Reverse Osmosis/Nanofiltration

RO processes allow selective passage of a particular species (solvent), while other species,that is, solutes, are retained partially or completely. NF is also driven by pressure and issometimes called loose RO . The main difference between RO and NF membranes is thatRO rejects all the solutes, including monovalent ions, while the NF membrane can onlyreject multivalent ions with no selectivity toward monovalent ions (1). Osmosis is a nat-ural phenomenon where water will pass through a membrane from one side with lowersolute concentration to a higher solute concentration until the osmotic equilibrium isestablished. To reverse the water flow, additional mechanical pressure has to be applied,providing a pressure difference greater than the osmotic pressure difference; as a result,separation of water from a solution becomes possible. This phenomenon is referred to asRO . Applications of RO processes include seawater desalination, wastewater treatment,ultrapure water production for electronic industries, and are also applied in the food sectorto concentrate fruit juice, sugar, coffee, and milk for cheese production. RO is a well-established membrane technology for the treatment of water in a variety of applications.Currently, only polymeric RO/NF membranes are commercially available (1). The majorproblems associated with polymeric RO/NF membranes are fouling and low resistance tochlorine and other oxidants. Ceramic membranes, in this context, are apparently advan-tageous over commercially available polymeric ones in terms of the excellent resistanceto chlorine, oxidants, radiation, and solvents; the high thermal and chemical stability andthe long reliable life of ceramic membranes. The technology to prepare RO/NF ceramicmembrane with pore size larger than 10 nm is very established now. For instance, alumina


ceramic hollow membranes with different pore sizes from 20 to 800 nm are commerciallyavailable from High Flux (Singapore) while tubular ceramic membrane with pore sizeof 10 nm can be purchased from Noritaki (Japan). However, the high cost, low pack-ing density, and/or poor selectivity renders commercially available ceramic membranetechnology economically untenable for RO/NF applications (1). Research efforts for thedevelopment of ceramic membranes with pore size of a few nanometer are still going onwith different oxide phases such as titania (16); (17), zirconia (18), silica–zirconia (19),hafnia (20), and γ-alumina (17). Most of these NF membranes are used to separate thenonaqueous and much value-added solvents. The preparation is by the sol–gel process,where a mesoporous ceramic support is coated with a top oxide layer having the desiredfinal pore size. This provides a great advantage in controlling the pore diameter throughthe proper choice of colloidal solutions at the final coating stage.

3.2 Ultrafiltration and Microfiltration

For ultrafiltration membrane, the mechanism for separation of the solvent from thesolute/colloidal particle is similar to that of RO/NF. The solute rejection efficiency islargely determined by the joint effects of the porous structure of the membranes and theinteractions between the membrane surface and solvent/solutes. The overall solvent trans-fer is often dominated by mass transfer resistances in the membrane as well as at thesolution boundaries. Microfiltration closely resembles conventional filtration processesand separates discrete particles from solution. There is no clear dividing line between thetwo processes of UF and MF. To be consistent with others, the upper pore size limit forMF is less than 10 μm and the lower size limit is set at 0.1 μm. Noteworthy is that MFis the most widely applied process of inorganic membranes to separate small insolubleparticles, bacteria, and yeast cells from broths and aqueous streams (1). Ultrafiltrationand microfiltration membranes have been prepared from a wide range of polymers, andceramic membranes have also been developed for the same applications. The develop-ment of ceramic membranes is mainly driven by the need to produce membranes withgreater chemical and thermal tolerance because the upper temperature limit of polymericmembranes is mostly below 200 ŽC. In addition, most of the above-mentioned poly-mers cannot survive in solvents such as benzene and toluene. Ceramic ultrafiltration andmicrofiltration membranes are prepared from materials such as aluminum oxide, titaniumoxide, and zirconium oxide, as they can withstand high temperatures and harsh chemicalenvironments. Typical applications of ultrafiltration and microfiltration processes usingceramic membranes in the liquid media can be found in the dairy, food, pharmaceutical,biological, paint, paper, and water industries, a detailed review of which can be foundelsewhere (3).

3.3 High Temperature Dust Removal for Clean Energy Delivery

Another promising application of inorganic membranes for microfiltration/filtration isthe high temperature dust removal for clean energy. Some advanced fossil energy powerconcepts such as pressurized fluidized bed combustion (PFBC) and integrated gasifi-cation combined cycle (IGCC) technology require high temperature barrier filters forhigh efficient operation. In these new combustion chambers with high temperature andpressure, ash must be removed from the combustion flue gas stream before entering thegas turbine. Ash particles will destroy the turbine blades. Even small concentrations of


particles can be damaging and cannot be tolerated in the long run. In addition, the par-ticle separation must be performed at very high temperatures for the power generationto reach its optimum efficiency. In gasification systems, the ash and char suspendingin the syngas stream must be removed before the introduction to other downstream airpollution control processes such as hydrogen sulfide removal. The char, once captured,can be recycled into the gasification reactors to increase the overall system efficiency. Ifthe ash and char are not captured before the other downstream equipment, these solidscan foul process piping or columns and gradually fail the entire system (21). Ceramicmicrofiltration/filtration membranes address the critical challenges of particulate removalunder the high temperature oxidizing conditions. Some companies such as CeraMem andUS Filter have pioneered the R&D in this area.

4 INORGANIC MEMBRANES FOR GAS SEPARATION

Compared to the relatively established liquid phase separation or particle removal, theindividual gas separation from gas mixtures using ceramic membranes is a relatively new,but very dynamic and rapidly growing field. Intense research efforts have been madein developing novel ceramic membranes for use in separations/reactions where harshfeed conditions or high temperature operations preclude the use of existing polymericmembranes. There are three types of inorganic membranes suitable for gas separations:microporous, dense ceramic, and metallic membranes; more details are covered in thefollowing subsections. Before the detailed description of these membranes, the conceptsof performance indicators of gas separation membranes are briefly introduced in termsof flux, permeance, permeability, selectivity, and separation factors. For single gas (i )measurements, flux is defined as the amount (molar or volume) of gas passing throughthe unit membrane area per unit time; permeance is the flux divided by the pressuredifference

PiŁ D Ni

(p1� p2)(1)

and permeability is the permeance divided by the membrane thickness. The ideal selec-tivity between components i and j can be written as

Sij DPiŁ

PjŁ (2)

where Ni is the molar (or volume) flux of i and p1, p2 are the pressures at the feed sideand permeate side. In the case of gas mixtures, the permeance is defined by

PiŁ D Ni

(pi1 � pi2)(3)

and the mixture selectivity or separation factor

αij D(yi2/yj 2)

(yi1/yj 1)(4)

where the subscripts 1 and 2 refer to the feed side and permeate side, respectively,and the pressures are partial pressures. It can be seen from Equation 4 that the higher


the value of αij , the greater the degree of separation offered by the membrane. Whenαij becomes infinitely larger, the membrane tends toward being superselective withrespect to the component i . When looking at membranes to enhance performance, thereis often a trade-off between the separation factor and flux or permeance, performinga modification that increases one leads to a decrease in the other. The particularseparation requirements for an individual process need to be assessed comprehensivelyand just selecting the membrane with the highest separation factors may not alwaysbe the optimum choice. Gas transport in larger macropores (>20 nm) is via a viscousflow and gas transport in mesopores (20–2 nm) is typically occurring by a Knudsenmechanism if the mean free path length of molecules is larger than the pore size (1).When the pore is less than 2 nm in the micropore range, the transport is characterizedby molecular sieving effects together with the preferred individual molecular absorptionbehavior. Dense membranes are ideally selective for only one molecule such as H2transported as H atoms in Pd membranes and protons in mixed conducting ceramics.Different transport mechanisms display different behaviors. In this article, we focus ongas transport through the microporous and dense membranes as the membranes withpores larger than 2 nm have little interest for gas separation.

4.1 Microporous Membranes

According to the IUPAC definition, microporous inorganic membranes are referred to asporous membranes with pore diameters smaller than 2 nm (22). The kinetic diameter ofmost gases is less than 1 nm, with examples of small molecules such as H2, CO2, O2,N2, CO, CH4 n-C4 (n-butane), and i -C4 (isobutene) having diameters 0.289, 0.33, 0.346,0.364, 0.376, 0.38, 0.43, and 0.50 nm. Thus, the microporous membranes of practicalseparation interest have pore size in the subnanometer range. For instance, some silicamembranes having pore size of 0.30 nm can effectively separate H2 from CO2. Note-worthy is that the gas selectivity through the membrane is not only determined by thesize sieving effect but also by the individual molecular adsorption on these membranematerials. Microporous inorganic membranes include amorphous (silica or carbon) andcrystalline (zeolite) membranes.

4.1.1 Amorphous Microporous Membranes

4.1.1.1 Silica Membranes Among the amorphous microporous membranes, silica(SiO2) membranes are the most important examples because they can be easilyprepared as thin layers for molecular sieving applications in comparison with othermetal oxides. In the last decade, there was a worldwide effort to develop molecularsieve membranes for hydrogen separation or production with several leading groupsin USA, Holland, Germany, Australia, and Japan. According to the silica precursorstate in liquid or gaseous medium, the preparation can be classified into sol–gel orchemical vapor deposition (CVD) method and the silica precursors can be chosenfrom SiH, SiC4, tetraethylorthosilicate (TEOS), tetramethylorthosilicate (TMOS),phenyltrimethoxysilane (PTMS), dimethoxydiphenylsilane (DMDPS), diethylsilane(DES), etc. The procedures of sol–gel technique can be generally described by fivesteps in sequence: the preparation of a uniform solution containing one of these silicaprecursors, film formation by dip coating on the porous support using the preparedsolution, film drying or gelation, and final heat treatment to the desired microporousfilm. For the colloidal suspension route, the silica particles with different particle size


are firstly prepared by the hydrolysis of silicon alkoxide (i.e., TEOS) under catalyticconditions with the presence of organic solvent and an excess of water; then the poroussubstrates are dip-coated with the colloidal silica sols to form a thin separating layerwith densely compacted silica particles. Normally, this colloidal method produces themembrane in mesoporous structure with pore size larger than 1 nm, which is of littleinterest for small molecular gas separation (23, 24). However, this method is often usedto prepare the intermediate layer to modify the coarse surface of the porous support withlarge pores for the subsequent coating of top thin microporous layer as discussed in oneof the early examples. The first successful silica membranes with good properties for gaspermeation/separation were reported in 1989 by Burggraaf’s group (25). They used asol–gel method to prepare the supported silica membrane. SiO2 polymer sols were firstprepared by the catalyzed hydrolysis process using tetraethoxysilane (TEOS) precursormixed with ethanol, an acid catalyst (HCl or HNO3) and distilled water. The acid catalystreduces hydrolysis but enhances polycondensation rates during the sol preparationprocess. Then the porous support, often in a tubular configuration, was dipped coatedwith the SiO2 polymer sols, followed by aging, drying and heat treatment of the gels.Processes of dip-coating and heating must be repeated to ensure the membrane thicknessand quality. Buckley and Greenblatt investigated the pore characteristics of xerogelsprepared with different contents of TEOS, ethanol, and water (26). They found that byincreasing the alkyl chain length of the alcohol solvent, the xerogel structure changedfrom microporous to mesoporous. In particular, the degree of branching of the polymericsol significantly influences the pore size. In general, lowly branched inorganic polymerslead to gels that can result in membranes with a smaller mean pore size (27, 28). Bywell-controlled processing of every membrane fabrication step, SiO2 membranes with aH2 permeance at 200 ŽC of 2ð 1.0�6 mol/(m2 Ð s Pa) and permselectivities of H2/othercomponents of >500 can be obtained (29). Other researchers used a two-step catalyzedhydrolysis sol–gel process to provide smaller pore size allowing superior gas separationperformance over the conventional single-step process. Diniz da Costa et al. observedthat the silica film derived by two-step sol–gel process has pore size in the region of0.3–0.4 nm, which is much smaller than that prepared by the conventional one-stepprocess due to the presence of different functional groups in the network (30). The poresizes of the silica membrane can also be adjusted by a molecular template method, atechnique inspired from the preparation of zeolite membranes. Pores with interestingsize can be created by molecular templating, for example, incorporation of organicmolecules in the silica network which, after calcination, create pores that mimic thesize and shape of the original template (31). To introduce these template molecules,one way is by co-reacting TEOS with an organic-substituted alkoxysilane (32, 33). Asa result, the membrane pore size and porosity can be tailored by proper choice of thetype and the amount of organic template. For example, Xomeritakis et al. observed thatthe pore size of their membrane without organic template is in the range of 3.5 A;with the tetra-alkylammonium bromides (TEABr) as the template, the silica membranecontains pores in the range 4.0–5.5 A, which is highly permeable to CO2, N2 and CH4but is fairly impermeable to SF6 with larger molecular diameter (32). A major problemfor microporous silica membranes is that these membranes are not stable at hightemperatures, especially in humid atmospheres, leading to loss of permeability. This isattributed to the closure of membrane pore channels by densification, which is catalyzedby moisture, particularly at higher temperatures. In addition, densification can alsocause embitterment of silica film and damage the integrity of the film and the support,


deteriorating the separation properties of the membrane. Several methods have beenapplied to improve the stability of sol–gel-derived silica membranes. One approach is todope a small amount of inorganic oxides such as TiO2, ZrO2, Fe2O3, Al2O3, NiO, etc.(34, 35). Of these metal-doped membranes, cobalt-doped silica membranes showed veryhigh He/N2 selectivity up to 1100 after water vapor treatment at 180 ŽC (11). However,the pore sizes of silica microporous membranes have been enlarged compared to thepure silica membranes by using dopants such as zirconia (36). Another approach is tomake the silica membrane hydrophobic by replacing–OH groups on the pore surfacewith other groups using a hydrophobic methyl template covalently bonded to silica inthe sol–gel process (37). More recently, Duke et al. applied a novel carbonized templateinto the silica micropores that provided hydrostable operation at 200 ŽC with no loss toselectivity observed after almost 200 h of continuous operation (38). In their material,the micropore collapse, which normally occurs in pure silica, was suppressed by thecarbon residue inside the pores, which was introduced by the carbonization (instead ofthe complete removal) of the organic templates. It is expected that the condensationreaction rate at high temperatures can be reduced thus improving the membrane’sthermal stability. However, such hydrophobic microporous silica membranes are limitedto the applications of low temperature and nonoxidative environments. In addition tothe sol–gel method, CVD is a popular method used in the preparation of microporoussilica membranes. It was first reported by Gavalas et al. (39) to deposit silica insidethe pore wall of a Vycor glass tube. In a typical CVD process, the porous substratesor supports are exposed to one or more volatile precursors that is, silicon hydride,tetrachlorosilane, tetramethoxysilane, phenyltrimethoxysilane, which react with H2O orO2 on the support surface to produce the desired deposited layer. An important propertyof the CVD-derived silica membranes is that they have pores with diameters less than1 nm and indicate molecular sieving performance. Therefore, the CVD membranes showenhanced selectivity and good reproducibility, but low permeability. In contrast, thesol–gel-derived membranes generally possess higher permeability, but lower selectivityand a lack of reproducibility. However, the CVD method usually requires substantialcapital investment and controlled conditions of deposition. Thus most of the beginnerscan start from the sol–gel method to take up the research in the field as it is relativelyeasy to perform.

The potential application of silica membrane in H2 separation from CO2 or N2 gasmixtures has spurred much research interest since their discovery in the late 1980s. Thefirst membranes had a permeance in the range of 10�9 mol/(m2 s Pa) and a selectivityof H2/N2 in the order of 10. Now, after 20 years’ development, the permeance has beenenlarged to 10�6 mol/(m2 s Pa) with selectivity of H2 over other gases up to 1000s.However, most of the reported results are still in the research stage and large-scaleapplications have not been reported because of the problems related to the low materialstability in the presence of water vapor, poor reproducibility, high price, and difficultyto produce modules with high surface area to volume ratio. Hopefully, we can see abreakthrough for silica membranes to go to industrials in the next 10 years as currentresearch strategies are intensively addressing these concerns.

4.1.1.2 Carbon Membranes Carbon membrane is another typical example of amor-phous microporous membrane attracting particular interest in the literature. It is relativelyinexpensive and can be prepared basically by carbonizing thermosetting polymers suchas poly(vinylidene chloride) (PVDC), poly(furfuryl alcohol) (PFA), cellulose, cellulose


triacetate, saran copolymer, polyacrylonitrile (PAN), phenol formaldehyde, etc. and vari-ous coals such as coconut shell at high temperatures under a controlled temperature pro-gram and atmosphere (40). Once the carbon precursor is chosen, the synthesis of carbonmembrane normally involves three important steps: pretreatment, pyrolysis/carbonization,and posttreatment, which must be controlled and optimized to ensure good membraneperformance. The pretreatment step involving some physical or chemical methods is toensure the stability of the polymeric precursor and the preservation of its structure duringsubsequent pyrolysis. Pyrolysis or carbonization is a process in which the chosen andpretreated carbon precursor is heated in a controlled atmosphere (vacuum or inert) tothe pyrolysis temperature at a specific heating rate for a sufficiently long thermal soaktime. Pyrolysis is the heart of the carbon membrane synthesis, during which microporesare produced as the result of small gaseous molecules channeling their way out of thesolid matrix of the polymer. The posttreatment is to repair the defects and further adjustthe pore dimensions and distribution of the resultant carbon membrane to meet differentseparation requirements with methods including the postoxidation, CVD, post-pyrolysis,coating, etc. Depending on the different polymer precursors and processing conditions, thepore size of the carbon membranes can be tailored from 4 to 7 A, making the separationmechanism not only on molecular size selective but also on the preferential adsorptiondifference between these gases. For example, some carbon membranes are effective inseparating nonadsorbable or weakly adsorbable gases (i.e., He, H2, air, O2, N2, and CH4)from adsorbable gases, such as hydrocarbons (C2C), NH3, SO2, H2S, and CFCs (41).

It is expected that carbonized materials are stable at high temperatures and resistchemical attack. The challenge for carbon membranes is how to increase the gas per-meation rate for small molecules (1). One approach is to make the membranes in anasymmetric structure with thin carbon layer supported on a robust mesoporous sub-strates. For example, carbon membranes were prepared by ultrasonic deposition of PFAon a porous stainless steel tube, followed by pyrolysis at 723 K to convert the polymerlayer to nanoporous carbon film with a thickness of 21 μm (42). The resulting carbonmembranes give an oxygen, helium, or hydrogen to nitrogen separation factor of 30, 178,or 331, respectively, the largest reported for any membranes working at room temper-atures (1). The difference in the kinetic diameters of these gases and the high qualitycarbon membrane may contribute to such a large separation factor. This requires devel-opment of a delicate technique for the formation of a continuous, defect-/pinhole-free,thin nanoporous carbon film on a porous support. Another approach is using asymmetrichollow fiber membrane precursors from these thermosetting polymers (43–46). Fromthe standpoint of practical large-scale application, the hollow fiber geometry is prefer-able due to their high packing density (membrane area per unit volume of vessel) andease of module construction. An additional advantage is that the carbonization of orig-inal polymeric fibers of several hundred micrometers in diameter can be carried out ina continuous process instead of batch operation (47). Carbon hollow fiber membranes,having an asymmetric structure, showed an improved performance in terms of perme-ability and selectivity for some gases and light hydrocarbons. Carbon hollow fibers arecommercially available from Israel’s Carbon Membranes Ltd. The only problem is thattheir fragility/brittleness may restrict their large application.

4.1.2 Crystalline Microporous Membranes Zeolites are crystalline aluminosilicatemembers of the family of microporous solids with a regular three-dimensional porestructure, which is relatively stable at high temperatures. The atomic structures of


zeolites are based on three-dimensional frameworks of silica and alumina tetrahedra,where the silicon or aluminum ions are surrounded by four oxygen ions in a tetrahedralconfiguration. Clusters of tetrahedra form different boxlike polyhedral units that arefurther linked to build up various frameworks with different channel sizes. Figure 4 is aschematic of the structure of zeolites A, Y, and MFI (48).

Although more than 200 distinct framework zeolite structures are known, only lessthan 10% has been prepared as membranes with significant separation interest. Table 1lists a few potential applications to separate gas and liquid mixtures (49). The separationof liquid mixtures by pervaporation is primarily based on adsorption differences, asthis type of separation is relatively tolerant of membrane defects. Separations basedprimarily on mobility differences are more demanding in terms of membrane quality.For instance, zeolite A membranes have good separation properties for ethanol–waterand other organic–water mixtures but do not have useful selectivity for separation ofsmall molecule gaseous mixtures such as O2 –N2 and H2 –CO2.

As shown in Table 1, all the listed zeolite membranes pores are much larger than thekinetic diameters of the above-mentioned small molecules such as He, H2, CO2, O2, N2,etc.; thus, there is no strong mobility difference or molecular sieving (49). Also, theiradsorption differences are not dramatic and, as a result, these zeolite membranes cannot

A: Zeolite A

B: Zeolite Y C: Zeolite MFI

(a)

(b) (c)

FIGURE 4 Examples of zeolite frame structures. Source: Reprinted from Reference 48 with kindpermission from Springer Science+Business Media. (Please refer to the online version for the colorrepresentation of the figure.)


TABLE 1 Examples of Potential Applications for Zeolite Membranes

Zeolite Membranes (PoreSize, A)

Separation Advantages and Limitations

MFI (5.5 A) zeolite A(4.1 A)

Organic-water mixtures bypervaporation

Highly selective separation withwater (zeolite A) or the organic(MFI) as the selective penetrant

MFI, zeolite A, or others Separation of miscellaneousorganic compounds that haveclose boiling points or areheat sensitive

Potentially useful for specialtychemicals and natural products

Na-Y(7.4 A) CO2 –CH4 (natural gasupgrading)

Resistance to plasticization andfouling by higher hydrocarbons

MFI Normal alkanes from branchedalkanes, aromatics, orcycloalkanes (petroleumrefining)

Higher alkanes are the selectivepenetrants; has to be comparedwith separation by liquefaction

MFI H2-hydrocarbon mixtures(petroleum refining)

Very high selectivity unattainablewith polymeric membranes;separation by distillation difficult

Various zeolites: MFI,Zeolite A, FAU (7.4 A),GIS (4.5 A), etc.

Membrane reactors forequilibrium-limited reactions

Alkanes are the selective penetrantsbut much of methane and someethane will remain in theretentate

provide high separation selectivity for these gases. Another challenge of preparing zeolitemembranes with high selectivity is mainly due to the existence of defects and the difficultyin minimizing them. Efforts in preparation of polycrystalline zeolite membranes started inthe late 1980s, but not until the early 1990s were MFI-type zeolite membranes with verygood permeation and separation properties successfully prepared (22). Since then, zeolitemembrane synthesis has become a very active research field; and a considerable numberof research articles have been published, with over 50 laboratories worldwide conductingresearch on the subject. LTA zeolite membrane for alcohol dehydration or separationfrom other organic–water mixtures by pervaporation is already commercialized in MitsuiEngineering & Shipbuilding Co. in Japan Co., with cost savings up to 49% compared toconventional distillation technology (50).

The application in similar liquid separations has been expanded to other countries,including China, Brazil, Singapore, Lithuania, Finland, Ukraine, Germany, Netherlands,and Switzerland (51, 52). In the gas separation area, Falconer’s group at the University ofColorado Boulder successfully scaled up the tubular SAPO-34 zeolite membrane area by7.5-fold for CO2/CH4 separation with similar performance in selectivity and permeabil-ity (53). Compared to other microporous silica or carbon membranes, zeolite membranesare relatively more thermally or chemically stable and thus have constantly attracted themost attention. Owing to their ‘molecular sieve’ function, zeolite membranes can dis-criminate the components of gaseous or liquid mixtures depending on their molecularsize. The methods to prepare zeolite membranes can be classified into two categories:in situ crystalization and secondary growth (or seeding) methods. Similar to amorphoussilica membranes, all zeolite membranes are prepared in supported asymmetric struc-ture sharing the same requirements for the porous supports to be chosen. Noteworthy


is that the surface of the macroporous support (e.g., alumina) with average size largerthan 0.5 μm usually does not meet the criteria for growing thin zeolite or silica mem-branes. During the synthesis, these zeolite or silica layers will deeply penetrate insidethe porous supports, thus making the membranes too thick to provide a desirable flux.Consequently, the coarse surface of the macroporous supports are often modified by thesol–gel method to obtain a surface layer with pore size around 2–10 nm. Independentof the method, zeolite crystallization can always be started from a synthesis mixture (aclear solution or gel) followed by the hydrothermal treatment. The synthesis mixturecontains four or five components SiO2, Al2O3, Na2O (or K2O), H2O, sometime com-bined with organic template of tetrapropylammonium cation (TPA+) either as TPABror TPAOH (49). The synthesis mixture follows strict mole ratios and each componentmust be added in a specified sequence. SiO2 (zeolite main component) is added in theform of TEOS, colloidal solution, fumed silica, or sodium silicate, while aluminium canbe sourced from aluminium sulphate, aluminium hydroxide, sodium aluminate and lesscommonly aluminium foil. The addition of Na+ is to enhance the rate of crystal growth,but only in a certain range of compositions. Na2O can be added in the form of NaOH,NaCl, or other sodium salt without OH– . The template TPA in the solution helps to buildup the clusters to direct zeolite crystal nucleation or aggregation. For the in situ crystal-lization method, the support surface directly contacts with the synthesis solution or gelat controlled pH. The process should be controlled such that zeolite nuclei are formedon the support surface, rather than in the synthesis solution, which requires creation oflocal supersaturation on the support surface by various physical means (52). Zeolite cannucleate and grow at any points on the support pore surface. Efforts should be made tolimit the unwanted internal growth (penetration) by introducing a barrier substance priorto the membrane growth and removing it after growth. Once the nuclei are formed on thesupport surface, crystals will grow by an interlocking manner with random orientationuntil the formation of a continuous zeolite film. In the secondary growth (or seeding)method, a zeolite seed layer is coated from a sol containing zeolite crystals via the slip-casting or dip-coating method followed by drying, and calcination to increase the particleadherence to the support. Zeolite seeds with a size from nanometer to submicron can beprepared from diminution of larger crystals or growing from zeolite synthesis mixtures atcertain conditions. The seeded support is then placed inside the zeolite synthesis solutionto grow into a continuous zeolite layer. As can be seen, zeolite seeds bypass the nucle-ation process and go directly to the crystal growth (49). The channel orientation of thezeolite crystals and seeded growth of an oriented particle monolayer show a profoundinfluence on the separation performance of the membrane. By seeding of the support,MFI zeolite membrane layers of different crystallographic orientations can be obtained.Recently, Lai et al. have prepared a b-oriented MFI silicalite I membrane (54), whichexhibits much higher flux and selectivity compared to the normal c-oriented membrane.

In the case of template-assisted growth, it is necessary to remove the template bycalcination at temperatures above 400 ŽC at a very slow heating or cooling rate to freethe pores for transport and avoid the zeolite cracking. In order to perform the molecularsieving function, the membranes must have negligible amounts of defects and pinholes oflarger than 2 nm to reduce the gas flux from these large pores. By careful controlling ofthe membrane synthesis conditions, these defects or pinholes can be avoided in the caseof achieving high quality membrane. However, to avoid these intercrystalline (nonzeo-lite) pores sized between 1 and 2 nm is very tricky as their existence is inherited fromone of the synthesis steps. Template removal at high temperatures is usually leading to


the shrinkage of the zeolite crystallites, which can create or enlarge the intercrystallinepores (55). Consequently, the good quality, defect-free zeolite membranes, in principle,also contain nonzeolitic pores in addition to the well-defined zeolitic pores. To elimi-nate nonzeolitic pores, many efforts have been made, for example, by a template-freesecondary growth (56), the posttreatment of filling by wet-impregnation (57), counter-diffusion chemical vapor diffusion, or hydrolysis of organics or silica precursors (58,59). Some of these methods are effective to prevent or to fix these nonzeolitic pores, butthese extra steps definitely confer to the zeolite membrane high cost. This somehow alsoexplains the aforementioned question why zeolite membrane for industrial application inthe area of gas separation has not been reported. The cost of the support elements andthe lengthy steps involved in film synthesis make the zeolite membranes very expensive.

4.2 Dense Inorganic Membranes

The dense inorganic membranes are made from crystalline ceramic materials such as per-ovskites, fluorites, carbonates, or metals, which allow the permeation of small moleculessuch as oxygen, hydrogen, or CO2 through the completely densified membranes.Different from microporous membranes where the separation is based on pure physicalprocesses—adsorption and molecular sieving—the separation mechanism via denseinorganic membranes is more complex and involves not only physical processes but alsochemical surface reactions except for the dense metal membranes, which are still based onthe solution-diffusion model. These membranes are mostly impermeable to all other gases,giving extremely high selectivity toward oxygen, hydrogen, or CO2. In this subsection,the detailed description is on three areas: dense ceramic membranes for O2/H2 separation,dual-phase membrane for CO2 separation, and dense Pd membrane for H2 separation.

4.2.1 Dense Ceramic Membranes for O2/H2 Separation. Dense ceramic (also calledsolid oxide electrolytes) membranes for oxygen and hydrogen permeation are rapidlygrowing research fields attracting increased attention, particularly for oxygen-selectiveceramic membranes due to the larger flux and good material stability with possible appli-cation in clean energy delivery. The more common ceramics possessing O2� conductionare stabilized ZrO2, ThO2, and CeO2, and so forth, among which the most frequently usedis yttria-stabilized zirconia (YSZ, 8% Y2O3 –ZrO2) or Sm-doped ceria (SDC), with muchhigher conductivities than others. Their stability at various temperatures and pressures,effective conductivity, good chemical resistance, mechanical strength, and relatively lowcost make them attractive for many high temperature applications. In addition to thehigh operating temperature, a driving force has to be provided for oxygen to permeatethrough the membrane. This driving force can be either an electrical potential gradientor a chemical potential gradient (i.e., gas component partial pressure) (60). As oxygen istransported in the ionic form, and to fulfill the electric neutrality criteria, there must be asimultaneous flux of electrons in the opposite direction to charge compensate the oxygenflux. According to the different modes of providing the electron fluxes, three differenttypes of membranes can be classified as shown in Figure 5: (i) pure ion conductors viaan external power source and electrodes to provide the electron pathway, which is alsocalled electrical oxygen pump (EOP); (ii) mixed ion–electron conduction (MIEC) viaone single phase; (iii) dual or multiple phases where the oxygen ion conduction andelectron conduction are provided by different material phases.

Good reviews of the research works in these areas have been given by Sunarso et al.(60) and Zhang et al. (61). Driven by the electrical potential gradient, the oxygen flux


(a)

(b) (c)

e

e

O2−

O2−

P1O2

P1O2

P2O2

P2O2

O2 + 4e 2O2− O2 + 4e2O2−

O2−

P1O2

P2O2

>

A

e

FIGURE 5 Schematic of dense ion-conducting ceramic membranes for oxygen separation: (a)pure ion conductor via electrical oxygen pump, (b) mixed conductor via single-phase material,and (c) dual-phase membranes with one for ion conduction and another for electronic conduction.Source: Reprinted from Reference 60. Copyright (2008), with permission from Elsevier.

transport via EOI can be precisely controlled in quantity by applying the electric currentas below (62):

JO2[flux: mol/(s m2)]D I (electrical current)/4F (Faraday constant)/A (membrane area)

(5)

The main advantage of membranes via EOI is that the oxygen can be pumped in eitherdirection regardless of the oxygen partial pressure gradient. Because of the structuralcomplexity with external electric loadings, pure oxygen-ion-conducting membranes areseldom considered for oxygen production in large scale; in contrast, they are broadlyapplied in the fields of sensors or solid oxide fuel cells for energy generation. Despite theexpensive price, the oxygen production via EOP can find their applications in hospitals,submarines, and manned space missions.

The MIEC membranes, driven by oxygen pressure gradients without the require-ment of electrodes and external power source to operate, consist of one single phasecapable of both ionic and electronic conduction. Because the external circuit is unneces-sary, fabrication and operation of the mixed conducting membranes is much simplified.Thus, significant cost reduction can be expected and so mixed conducting materials haveattracted considerable research interest in recent years. Efforts in efficient utilization ofenergy and reduction of emissions have indirectly stimulated research in mixed conduct-ing membranes. One of the most important applications of mixed conducting membranesis high purity oxygen production from air separation, which is of great importance inboth environmental and industrial processes as most large-scale clean energy technologiesrequire oxygen as feed gas (63, 64). Currently, the conventional cryogenic air separation


unit is a major economic impediment to the deployment of these clean energy tech-nologies with carbon capture (i.e., oxy-fuel combustion). Dense MIEC membranes areenvisaged to replace the cryogenics and reduce O2 production costs by 35% or more;which can significantly cut the energy penalty by 50% when integrated in an oxy-fuelpower plant for CO2 capture (65–69). The MIEC membrane is currently attracting signifi-cant research interests not only from research institutes and industries (i.e., Air Products)but also from government. For example, the DOE in the United States has invested$148 million from 1999 to scale up the technology. Since Teraoka et al.’s first report(70) on favorable oxygen permeation through dense MIEC ceramic membranes basedon single-phase perovskite oxides of La1�x Srx Co1�y Fey O3�δ composition in the 1980s,many related studies on new compositions have been carried out. Numerous structuresthat exhibit this interesting MIEC properties have been reported, such as perovskite,brownmillerite, orthoferrite, K2NiF4-type phase, and the Ruddlesden–Popper series (60).Among these structures, the perovskite-structured oxides have been studied intensivelybecause of their high flux values and diverse and controllable properties. The generalformula of the perovskite is ABO3, with the structure shown in Figure 6.

The A-site cations are mainly composed of alkaline earth, alkaline, and lanthanideions, while B-site cations are mainly composed of transition metal ions. When the orig-inal A-site cation of perovskite is partially substituted by another cation with loweroxidation state than the original cations, the electrical neutrality is normally sustainedby the formation of oxygen vacancies or increased oxidation state of the B-site cations.Accordingly, partial substitution of original cations with another cation having higheroxidation state tends to inhibit the formation of oxygen vacancies or increased oxidationstate of the B-site cations. The presence of oxygen vacancies here facilitates the oxygenion movement, which closely affects the oxygen ionic conductivity of the oxide, whilethe electron hopping between the valence-variable metal ions at the B-site makes elec-tronic conduction possible. The resulting MIEC properties, in turn, have granted thesematerials a unique separation mechanism, for example, oxygen permeating through themembrane by surface reactions and ionic/electronic transport through the bulk insteadof by way of the conventional molecular diffusion through the microporous structure. Incase of air separation, oxygen permeation at high temperatures through a MIEC ceramicmembrane from the air side to the low oxygen partial pressure side can be assumed toinclude the following steps in series: (i) mass transfer of gaseous oxygen from the airto the membrane surface (air side); (ii) surface reaction between the molecular oxygenand oxygen vacancies at the membrane surface (air side) to form the oxygen ions by

A

B

O

FIGURE 6 Perovskite structure.


reduction; (iii) oxygen ion (or vacancy) bulk diffusion across the membrane; (iv) sur-face reaction between lattice oxygen and electron–hole at the membrane surface to yieldmolecular oxygen (permeate side) via oxidation; and (v) mass transfer of oxygen fromthe membrane surface to the gas stream (permeate side). Generally, the gas phase resis-tance may be negligible compared to that of bulk diffusion and exchange reactions. As aresult, the oxygen permeation flux through a membrane can be given by Equation 6 andthe detailed derivation can be found elsewhere (71)

JO2D

krDv

[(p 0O2

)0.5 �(

p 00O2

)0.5]

2Lkf

(p 0O2

p 00O2

)0.5C Dv

[(p 0O2

)0.5 C(

p 00O2

)0.5] (6)

where JO2is the oxygen flux through the membrane, L the membrane thickness, p 0O2

andp 00O2

are the oxygen partial pressures in the air and the permeate side, respectively, Dvthe diffusion coefficient of oxygen vacancy; kf and kr are, respectively, the forward andthe reverse reaction rate constants for the surface exchange reaction:

12 O2 C V

žžO

kf/kr !OxO C 2h

ž(7)

where OxO stands for lattice oxygen, V žž

O for oxygen vacancy, and hž

for electron–hole.For a certain material, the oxygen flux is also related to membrane surface morphology,membrane thickness, and operating parameters such as temperature and gas atmosphereconditions in the air side and permeate side. All the strategies such as membrane thick-ness reduction, membrane surfaces in a more porous structure, catalyst deposition, andoperating temperature increase will improve the flux values. However, all these engi-neering considerations become reasonable only on the basis of the prerequisite that themembrane material is sufficiently stable at the real separation or application conditions.There has been a substantial number of articles published on the title subject in the pastdecade, highlighting the remarkable potential and increasing interest in the field (72–74).Several reviews on mixed conducting membranes for oxygen separation are available,and they provide the main understanding on the material composition, structure, prepara-tion as well as the transport mechanism of oxygen permeable membranes (75–78). Forexample, (60) reviewed the development of a mixed conducting, dense ceramic mem-brane during the past three decades. Yang et al. (77) summarized the development andchallenges of perovskite-type materials for oxygen transport membrane reactor applica-tions. Liu et al. (78) wrote a review focused on the transport theory of several classes ofperovskite compounds, including their potential application in different areas and oxygenpermeability improvement efforts by doping (e.g., partial substituting of original cations)with various metal oxide elements. After more than 20 years’ development, super-highoxygen flux values up to 14 ml/(cm2Ðmin) can now be achieved under oxygen gradientcreated by air and inert sweep gas (79). From the viewpoint of flux value, these per-ovskite membranes already reach the commercial target. However, the real application ofthese perovskite membranes is still limited by their intrinsic drawback of low structuralphase instability under practical conditions with the presence of gases such as CO2, SO2,H2O, and so on. Membrane performance degradation was normally observed in long-term operations as the reaction between the membrane materials and these gases wouldfinally fail the membrane function. Although the perovskite stability can be improvedby compositional tailoring with robust elements, the problem of low material stability isstill there and cannot be eradicated completely from the root.


By contrast, the dual-phase membrane is a promising alternative, as these fluoriteoxides possess inherently high chemical stability against these acidic or reducing gases.Previously, the dual-phase membranes were prepared by combining one of the metalphases, usually chosen from Ag, Pd, Au, or Pt, and the other from the ion-conductingphases, with examples such as YSZ, SDC, or Gd-doped ceria (GDC) (80–82). Eachof the conducting properties requires a continuous material pathway, which poses chal-lenges on the preparation protocol regarding the mixture portions, mixing method, theparticle size of the chosen membrane materials, and the high material costs. These highmaterial costs cannot be avoided because of the large amount of precious metals usedto form a continuous electronic-conducting phase. The mismatch of these factors usuallyleads to the very low oxygen permeation fluxes, sometimes several orders of magnitudelower than the single-phase perovskite membranes. Reviewing all the results of currentlydeveloped ionic transport ceramic membranes for O2 separation, we note a good ceramicmembrane addressing all the application criteria is rarely encountered. There is alwaysa balance between the chemical stability and oxygen flux with improvements in oneproperty to the detriment of the other property. In order to get out of this dilemma,very recently, Zhang et al. (83) put forward a new membrane concept consisting ofrobust oxygen-ion-conducting ceramic membrane (i.e., fluorite-based SDC) with twosurface-platinum-coating layers together with an external short circuit, as schematicallyillustrated in Figure 7. In this concept, the mixed conducting function can be realizedvia the oxygen ion diffusion inside the fluorite bulk and electronic conduction along theexternal metal wire, avoiding the mutual obstruction often resulted from the conventionaldual-phase membrane synthesized by powder mixing. When silver paste is used to sealthe ceramic membranes between the two different gas chambers, the external wire is nolonger required as the electronic conduction can be completed via the silver sealing aslong as the silver sealing connects with the coated porous metal layer. This new mem-brane concept has been verified by the direct detection and measurement of electricalcurrents, which agreed well with the theoretical calculation based on the equation cor-relating Faraday constant and oxygen flux. Given the high O2 fluxes, the intermediateoperating temperature, and the well-known high stability of the fluorite membrane struc-ture, this novel membrane is possibly applied without the material stability problem inthe field of oxygen production for clean energy delivery (83).

Similar to MIEC membranes, the mixed proton–hole-conducting solid oxide mem-branes are also known, mostly based on the doped SrCeO3, BaCeO3, SrZrO3, andBaZrO3 perovskite oxides (78). The H2 transport mechanism through these proton–hole-conducting ceramic membranes is very similar to O2 transport through the MIEC mem-branes and therefore not repeated here. In principle, these mixed proton–hole-conductingperovskite membranes also have potential applications, particularly in H2 separation andmembrane reactors for high temperature dehydrogenation reactions. However, they havereceived less attention because of the lower stability of these perovskite in H2-containinggas mixtures such as CO2 and CH4. Another possible reason is their much lower H2flux, which is normally 2 orders of magnitude lower than the O2 flux through the per-ovskite membranes (84, 85). For example, a 1-mm-thick BaCe0.95Nd0.05O3�δ membraneonly displayed 0.02 ml/(cm2Ðmin) at 900 ŽC under H2/He gradients; at relatively simi-lar conditions, however, Ba0.5Sr0.5Co0.8Fe0.2O3�δ exhibited an oxygen flux value up to3.06 ml/(cm2Ðmin) at air/He gradient. Future work in this area should focus on thin-filmtechnology and new compositions not only with high proton and electronic conductivitiesbut also with inherent strong material stability against these acid gases.


Pure ionic conductor Current collector

Conducting wire

Porous electronicconducting layer

Air Air

e

e

O2O2

O− O−e

ee

e

e

e

(a) (b)

Pure electronic conductor

FIGURE 7 Diagrams of novel ion-conducting ceramic membranes with a metal coating layer andmetal wire (a) and novel ion-conducting ceramic membranes with a metal coating layer and silverpaste (b). Source: Reprinted from Ref. 83 with permission of The Royal Society of Chemistry.(Please refer to the online version for the color representation of the figure.)

4.2.2 Dual-Phase Membranes for CO2 Separation at High Temperatures. Manyefforts have been reported on the development of membranes for CO2 separationat high temperatures, with potential application for postcombustion carbon captureby the removal of CO2 from hot flue gases without cooling down to improve theenergy efficiency for clean energy. Some of the polymeric membranes show goodpermselectivity for CO2 separation. However, their permeance is too low to be ofcommercial interest and most of them are not stable at high temperatures or inan acid environment (86, 87). Significant research was conducted in developinginorganic membranes for high temperature CO2 separation. Zeolite membranes offergood permselectivity and high permeance for CO2 over nitrogen in low temperatures(<100 ŽC) (88). The sol–gel-derived microporous zirconia membranes may be used inthe intermediate temperature range from 200 to 350 ŽC for CO2 separation. Microporoussilica membranes exhibit good separation factors at temperatures even less than 200 ŽC.These membranes also have a high CO2 permeance up to 3ð 10�7 mol/(m2 s Pa) (31,89). However, all these inorganic membranes cannot be used for separation of CO2from the flue gas at temperatures larger than 350 ŽC.

Recently, research has been undertaken on the dense dual-phase inorganic membranesfor CO2 separation. Lithium silicate (Li4SiO4) membranes on porous alumina supportscoated with a molten carbonate mixture (20% K2CO3 C 80% Li2CO3) were shown tohave a CO2/N2 selectivity of 4–6 in the temperature range of 525–625 ŽC (90). Thereported permeance for the Li4SiO4 membrane at 525 ŽC was 1.0ð 10�8 mol/(m2ÐsÐPa).Both the selectivity and permeance are too low for practical use. Yamaguchi and cowork-ers (90) hypothesized that a membrane composed of lithium zirconate (LiZrO3) couldseparate CO2 at high temperatures via the formation of two electrolytic species, Li2CO3and ZrO2 by decomposition of LiZrO3. However, their membrane only displayed a sep-aration factor of 5 at 600 ŽC, indicating the presence of unavoidable defects of this


hypothesized membranes. A new class of dual-phase metal–carbonate membrane perms-elective for CO2 has been reported by Lin and coworkers (91).

As shown in Figure 8, this membrane consists of a porous metal phase and a moltencarbonate phase. The metal phase not only serves as a support but also transports elec-trons. CO2 separation can be accomplished, driven by the CO2 partial pressure gradientwith working principles briefly as shown. On the upper stream membrane surface, CO2combines with electrons and oxygen to form CO3

2�, which is transported through themolten carbonate phase. On the downstream membrane surface, the CO3

2� transfersto CO2 and O2 via the release of electrons. The electron transports back through themetal phase toward the upstream membrane surface. No external electrodes and con-nectors are required in this dual-phase membrane. Such dual-phase membranes couldgive selective permeation of CO2 and O2 at high temperatures. Results from high tem-perature permeation experiments showed that the stainless steel dual-phase membranecould separate CO2 between 450 and 650 ŽC. At 650 ŽC, the membrane exhibited aCO2 permeance of 2.6ð 10�8 mol/(m2 s Pa) and a CO2/N2 permselectivity of 16.These dual-phase membranes have two major problems. The metal support is eas-ily oxidized under an O2-containing atmosphere at high temperatures and thereforethe dual-phase membrane will gradually lose the function of CO2 separation. Anotherproblem is the requirement of O2 presence in the feed gas and the simultaneous per-meation of O2 lowering the CO2 selectivity. In spite of these problems, this is a pio-neering dual-phase membrane concept for the development of high temperature CO2separation from flue gas. Recently, Lin and coworkers made further progress by theuse of more stable mixed electronic-ionic-conducting ceramic material of perovskiteLa0.6Sr0.4Co0.2Fe0.8O3�δ (LSCF) to replace the metal phase (92). Compared to themetal–carbonate dual membrane, this ceramic–carbonate dual-phase membrane has animproved material stability and a higher CO2/N2 selectivity, and the requirement of O2presence in the feed gas is not necessary as CO2 can be transferred to CO3

2� via O2�in the solid phase (lattice oxygen) of LSCF. LSCF is a well-known O2 selective mem-brane material because of its mixed conducting properties. When applied as a membraneto separate CO2 from flue gas at high temperature, the presence of O2 in the flue gas

Metal phase

Carbonate phase

CO2,O2containing

gasP ′O2

P′CO2

CO2,O2concentratedgas

P″O2

P″CO2

CO2,O2removed gas

e−

CO3=

FIGURE 8 Schematic illustration of the new dual-phase (metal–carbonate) membrane for carbondioxide separation at high temperatures. (Please refer to the online version for the color represen-tation of the figure.)


will expand the gas permeation not only of CO2 but also of O2, lowering the CO2selectivity. In the long term, the perovskite material cannot tolerate the CO2 attack athigh temperatures, which will gradually deteriorate the membrane unless a robust porousion-conducting ceramic like YSZ or SDC is used.

4.2.3 Pd and Non-Pd-Based Metal Membranes for H2 Production. Metal membranesmade of nickel, palladium, and platinum and other metallic elements in groups 3–5 ofthe periodic table have the ability to dissociate and dissolve hydrogen. In particular,palladium membranes show outstanding capability to transport hydrogen through themetal owing to a much higher solubility of hydrogen in its bulk over a wide temperaturerange (93). This property inspires many studies of using palladium layers as membranesfor the separation and purification of hydrogen. Owing to many advantages such ashigh temperature thermal stability, chemical resistance to corrosive environments, goodmechanical strength, and high permselectivities to H2, Pd-based membranes and reactorshave been extensively studied for production of high purity H2 for semiconductor indus-trials, separation H2 from coal-derived syngas, and other dehydrogenation/hydrogenationreactions in the petroleum and petrochemical industries (94, 95).

The permeation of hydrogen through Pd-based metals is a complex multistep processthat involves three steps: (i) H2 molecules chemisorb dissociatively on one side of themembrane; (ii) atoms dissolve in the metal matrix; (iii) atomic hydrogen diffuses towardthe opposite side. Bulk diffusion is usually the rate-limiting step and based on the clas-sical solution–diffusion model, the permeation flux can be described by the followingexpression:

J D Per

(pH2

)0.51� (

pH2

)0.52

Lthickness(8)

When the hydrogen partial pressure (pH2) of permeate side is close to zero, for

example, when the transferred H2 is removed by the quick reaction, that is, H2 C 12 O2 !

H2O, the permeation flux becomes

J D Per

(pH2

)0.51

Lthickness(9)

The permeability Per, which generally depends on temperature through an Arrheniustype of relationship, can be expressed as Per D Dk(hydrogen diffusivity)ð Ks (Sievertconstant). Extensive analysis of transport mechanisms in Pd–alloy membranes has beenprovided by Barrer (96), Shu et al. (97), and Ward and Dao (98). It should also be men-tioned that although Equation 17 represents the classical transport model, more recentwork indicates that the pressure exponent is frequently 0.7 or 0.8, suggesting that adsorp-tion may be partially controlling.

Although the industrial opportunities for the application of the Pd-based membraneare promising, their potential in the industrial scale, so far, is still doubtful. There are,at least, three hurdles to be solved. First, membrane embrittlement induced by phasetransformation has to be overcome. It is well known that hydrogen–palladium–alloyinteractions can induce the transformation between the hydrides of α phase (stable at lowtemperatures or low hydrogen content) and the β phase (stable at high temperatures orhigh hydrogen content). The repeated phase transformation resulting from the temperaturecycling (reactor start-up and shutdown) or H2 content changes alters the atomic spacingin the metal lattice and causes dimensional changes to distort the membrane (99). These


phase changes are pressure and temperature dependent. The challenge is, therefore, todevelop a Pd alloy that forms the hydride phase only at temperatures substantially differ-ent from the operating conditions. Current Pd-based membranes can only be operated inthe temperature range lower than 600 ŽC, within which there is good permeability and sat-isfactory durability. It should also be emphasized that internal stresses and the consequentcracking of membranes are often induced by reasons other than the β-phase formation.Most palladium alloys seem unsuitable for use as membrane material for large-scaletechnologies, as they have insufficient resistance to specific start-up/shutdown stresses.For example, the widely employed 25% silver–75% palladium alloy has low cyclicresistance and often cracks even if suitable start-up/shutdown procedures are employed.Uemiya and coworkers (100, 101) manufactured some 20-μm-thick Pd–Ag membranesdeposited either on alumina or on Vycor glass supports. However, again, the stability ofthese membranes was poor. The α–β transition caused the formation of cracks, pinholes,and distortions, limiting their lifetime. Chemical and mechanical interactions betweenmetal membrane and ceramic support may also play a role in this context. Collins andWay (102) claimed that their ultrathin Pd-coated membranes (1–15 μm) could withstanda temperature cycle from room temperature to 600 ŽC without delamination. On the otherhand, recent work by Li et al. (103) and Mardilovich et al. (104) has demonstrated thateffective Pd-based membranes with hydrogen permeabilities of the order of 10�6 mol/(m2

s Pa) and H2/N2 selectivities of 1000 or better can be prepared on both α-alumina andporous stainless steel supports. The main factors seem to be the key of the palladiumor Pd/Ag into a wider pores substrate (e.g., α-alumina) with increased stability. Anothercritical problem is represented by the palladium surface contamination of Hg vapor,hydrogen sulphide, CO, SO2, water vapor, thiophene, arsenic, unsaturated hydrocarbons,chlorine carbon from organic materials and coking, etc. Before the large-scale application,strategies to handle these impurities have to be addressed. The last fundamental problemhampering the Pd membrane application is the cost. Pure hydrogen production usingpalladium–alloy membranes to separate hydrogen from hydrogen-rich gas mixtures havebeen employed for many years. For example, (105) reported that units capable of sepa-rating up to 50 N m3/h of hydrogen from methanol–water cracking gas have been usedas constituents of self-contained hydrogen generators. However, it is difficult to make ajustification for sufficient return in capital investment to scale up palladium–alloy mem-branes for hydrogen production owing to the high cost of the noble Pd metal involved. Pdis a precious commodity metal whose price is subject to unpredictable market forces. Toaddress this cost issue, research is being driven in two directions: first to the developmentof supported Pd–alloy membranes with reduced thickness; second, to the development ofalloy membranes containing little or no palladium. To prepare a high quality supportedmembrane, the support would be likely a mesoporous ceramic (20–500 A pores). Thus,progress in the field of ceramic membranes is of major interest for use as supports oras separators and a number of successful studies have been made on the use of bothporous ceramic and stainless supports for Pd membranes (103, 106, 107). Non-Pd–alloymembranes (crystalline or amorphous) are presently being developed in the past twodecades to tackle the problem of high Pd cost (108). However, much development is stillrequired to achieve the performance of existing Pd-based–alloy membranes. Yamaura etal. (109) claimed the (Ni0.6Nb0.4)50Zr50 membrane to have a permeability [1.59ð 10�8

mol/(mÐsÐPa0.5)] exceeding that of pure palladium [0.95ð 10�8 mol/(mÐsÐPa0.5)], whileZr60Ni8Al15Cu15Co2 (110) and Zr60Al15Ni7.5Cu15Co2.5 (111) have permeabilities com-parable to pure palladium. However, the quoted permeance values of the membranes


were all achieved in need of Pd coating to catalyze hydrogen dissociation. Slow H2permeability and the susceptibility to hydrogen embrittlement are important issues forNon-Pd–alloy membranes to battle. Recently, thin and defect-free supported palladiummembranes have been prepared on macroporous α-alumina ceramic tubes using zeoliteas the intermediate layer and displayed much improved performance in permeability,selectivity, and stability due to the excellent Pd anchorage to the zeolite layer (112).

5 CONCLUSIONS

Inorganic membranes have been developed for more than 70 years with historicalcontribution toward the nuclear weapons and nuclear energy. Generally speaking,the application of inorganic membranes in liquid separation or particle removal hasbeen established with a relatively simpler separation mechanism. The application ofinorganic membranes in gas separation or membrane reactors combining the separationand reaction in one unit is a new area, with many more research and developmentopportunities, in particular, in the twenty-first century seeking strategies for clean energyand sustainable development. In this brief review, emphasis is placed only on the gasseparations with different kinds of inorganic membranes. This article gives the readersin the engineering fields a general overview about the inorganic membranes and theirapplications for liquid and gas separations.

ACKNOWLEDGMENT

Shaomin Liu acknowledges the research funding provided by the Australian ResearchCouncil (FT120100178).

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