Porous Inorganic Membranes for CO2 Capture: Present and Prospects

Review pubs.acs.org/CR

Porous Inorganic Membranes for CO2 Capture: Present and Prospects Marc Pera-Titus* Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), Université de Lyon, UMR 5256 CNRSUniversité Lyon 1, 2 Av. A. Einstein, 69626 Villeurbanne Cedex, France Eco-Efficient Products and Processes Laboratory (E2P2L), UMR 3464 CNRSSolvay, 3966 Jin Du Road, Xin Zhuang Industrial Zone, 200012 Shanghai, China S Supporting Information *

3.1.1. Microporous Silica Membranes 3.1.2. Amine-Functionalized Mesoporous Membranes 3.1.3. Ionic Liquid Membranes (RTILMs) Based on Mesoporous Alumina and Silica 3.2. Zeolite Membranes 3.2.1. Effect of the Operation Variables on the CO2 Permeation and Separation Properties 3.2.2. Influence of the Membrane Structure on the CO2 Permeation and Separation Properties 3.3. MOF Membranes 3.3.1. H2 Separation: Molecular Sieving Mechanism 3.3.2. CO2 Separation via Preferential Adsorption 3.4. Taxonomy of Materials Based on Permeation Properties under Ideal/Real Conditions 3.4.1. CO2/N2 Separation 3.4.2. CO2/CH4 Separation 3.4.3. CO2/H2 and H2/CO2 Separation 3.4.4. Insights into in Silico Studies 4. Membrane Synthesis and Microstructure 4.1. Membrane Supports: From Tubes to Hollow Fibers 4.2. Silica Membranes 4.2.1. Sol−Gel Routes 4.2.2. Polymer Route in the Presence of a Surfactant 4.2.3. Chemical Vapor Deposition/Infiltration 4.2.4. Modification of Silica Membranes 4.3. Zeolite Membranes 4.3.1. Direct in Situ Crystallization 4.3.2. Secondary Growth Method 4.4. MOF Membranes 4.4.1. Solvothermal Synthesis 4.4.2. Stepwise Layer-by-Layer Synthesis 4.5. Membrane Microstructure 4.5.1. Macrodefects 4.5.2. Grain Boundaries

CONTENTS 1. Introduction 1.1. Precombustion, Oxy-Combustion, and Postcombustion CO2 Capture: Which Separations Are To Be Addressed? 1.2. Pre- and Postcombustion CO2 Capture Technologies: Adsorption vs Membranes 1.3. Polymer/Mixed Matrix Membranes, Dense Membranes, and PIMs for Pre- and Postcombustion CO2 Capture 1.4. R&D Concepts from Academia and Industry for CO2 Capture Based on PIMs 1.5. PIMs for CO2 Capture: The Triple Challenge 2. Membrane Separation: Adsorption vs Diffusion Selectivities 2.1. Adsorption Mechanisms 2.1.1. Amine-Functionalized Silicas 2.1.2. Zeolites 2.1.3. Metal−Organic Frameworks 2.1.4. Taxonomy of Materials Based on Adsorption Selectivity 2.2. Diffusion Mechanisms: Molecular Sieving and Correlation Effects for CO2 Separation 2.2.1. Diffusion Mechanisms in Mesoporous Solids: Selectivity beyond the Knudsen Threshold? 2.2.2. Diffusion Mechanisms in Zeolites and MOFs 2.2.3. High-Temperature Mechanisms: Application to H2 Separation 2.2.4. Taxonomy of Materials Based on Diffusion Selectivity 3. CO2 Permeation and Separation Properties 3.1. Silica Membranes © XXXX American Chemical Society

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Received: May 2, 2013

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dx.doi.org/10.1021/cr400237k | Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 4.6. Taxonomy of Materials Based on Layer Intergrowth and Microstructure 5. Final Remarks and Outlook Associated Content Supporting Information Author Information Corresponding Author Notes Biography Acknowledgments Dedication Glossary Greek Symbols Subscripts Superscripts Acronyms (for more acronyms see the Supporting Information) References

Review

2012.6,7 New stringent mandatory post-Kyoto targets (postponed until 2015) will be negotiated for the horizon 2015− 2020 after endorsement of Adaptation Funds. In the past 15 years, the European Union (EU) has passed some pioneering legislation to meet Kyoto targets.8−12 The EU-15 committed to cutting GHG emissions by 8% below 1990 levels before 2012, by 20% onward before 2020, and by 80−95% before 2050 by setting an ambitious European Climate Change Program (ECCP).13−15 The proposed strategy relies on the premise that reducing the energy demands by increasing energy efficiency and productivity and promoting the transition to a low-carbon sustainable economy (e.g., by using renewable energies, natural gas cogeneration and integrated gasification combined cycle (IGCC) power plants, and biofuels) is the best way to cut CO2 emissions and boost economic growth. Moreover, despite the serious concerns among the scientific community and the negative public opinion about the future of “green” nuclear energy after the Fukushima disaster (March 11, 2011),16 it appears that the role of nuclear energy will still be manifest in the forthcoming years to promote energy autonomy and to meet the headline CO2 emission targets. In this regard, the midterm implementation of third-generation EPR reactors combined with fuel cells and batteries seems a realistic scenario. Nevertheless, according to the present state-of-the-art, many efforts are still necessary to reduce fuel cell costs (€6000−8000 per kilowatt vs €30−50 per kilowatt for thermal systems17), improve hydrogen storage technologies, and boost the power storage capacity of accumulators. Note that in any case neither fuel cells nor batteries involve per se a net zero-carbon balance, since CO2 is unavoidably generated from steam reforming (hydrogen production) or combustion (power generation) of carbon, natural gas, or biomass. In this context, CO2 capture, transport, and long-term storage or sequestration (CCS) is visualized as a promising strategy for mitigating CO2 emissions at short- and midterms, especially in stationary sources, as put forward in a number of recent technical reports and papers.18−23 Three strategies have been invoked for long-term large-scale CO2 storage: (1) geological storage in depleted oil and gas fields, deep saline aquifers, or unminable coal seams (formation of CO2 pools or solid gas hydrates), (2) deep ocean storage in “CO2 lakes” and marine sediments, and (3) industrial fixation in inorganic carbonates. Injection of CO2 into suitable depleted oil reservoirs has been used for a long time in Texas for enhanced oil recovery (EOR). The maximum available CO2 geological and deep ocean storage capacity has been estimated at 2 and 1 Ttonnes, respectively.24 CO2 transport to the storage site could be carried out either at high pressure by gas duct or by liquefying CO2 at moderate pressure (up to 7 bar) and −20 °C (offshore transport by ship). Among the three steps of the CCS chain, CO2 capture is by far the most expensive one, accounting for 50−90% of the overall chain cost depending on the CO2 emission source.5 Chemical absorption with alkanolamines (or variants) constitutes today the benchmark technology for CO2 capture from flue gas steams in large emission sources.25 In this technology, the sorbed CO2 in the form of stable carbamate and/or bicarbonate species must be further liberated in a separate vessel by raising the temperature and/or lowering the pressure above the solution, the regenerated solvent being recycled to the absorption unit. Despite the significant improvements in terms of liquid stability in the presence of O2 and SO2,26

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1. INTRODUCTION There is growing scientific consensus that the rising atmospheric levels of CO2 as a result of man-made activities (e.g., fossil fuel burning, cement and lime production, ammonia and ethylene oxide synthesis, aluminum and glass industries, fermentation, deforestation, use of fertilizers) is responsible for the warming effect on the climate. Indeed, CO2 emissions account for ca. 70% of the gaseous irradiative force causing the greenhouse effect, followed by CH4 and N2O.1,2 Since preindustrial times, the CO2 concentration in the troposphere has increased from 280 ppmv in 1750 to >380 ppmv at present, with an annual increase of about 1 ppm.1 This increase has been escorted by a rise of about 0.6−0.7 °C in the earth’s surface temperature during the past century, with a prominent increase during the past 20 years from an annual growth rate of 1.1% in the period 1990−1999 to >3% during the period 2000−2004.3 Worldwide CO2 emissions from fuel combustion reached a level of 30 Gtonnes in 2010, 41% of which being related to energy production.4 On current trends, the Intergovernmental Panel on Climate Change (IPCC) projects an average increase of the global temperature by 1.8−4.0 °C during this century, triggering irreversible consequences for mankind and ecosystems (e.g., increase of ocean levels).5 Climate models also suggest that the negative effects of a global temperature increase could even be amplified due to cumulative mechanisms (e.g., massive liberation of CO2 or CH4 stored in carbon hydrates in the oceans, fusion of perpetual ice, or even a change in the pattern of the Gulf Stream), driving the climate to hardly predictable evolution scenarios. Since the beginning of the 1990s, climate change has moved up high in the international political agenda. The Kyoto Protocol, approved by more than 140 nations in 1997 under the auspices of the UN Framework Convention on Climate Change (UNFCCC) and extended during the last climate summit held in Doha (Qatar) in October 2012, constitutes an international milestone to counteract global warming.6,7 The goal is to use market forces to restrict emissions by creating “flexible mechanisms”, such as a trading allowance greenhouse gas (GHG) emission market, a clean development mechanism (CDM), and a joint implementation (JI) strategy. Industrialized countries were entitled to cut emissions of GHGs (including CFCs and HFCs, responsible for stratospheric ozone depletion) by 5.2% from 1990 levels in the period 2008− B


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Figure 1. CO2 capture strategies as a function of the combustion type, implementation in the overall strategy for energy production and CO2 capture/storage, strategic target separations, and relevant FP6 and FP7 European projects recently accomplished.

than polymers, offering potentially longer lifetimes in service and more reduced maintenance costs. Second, PIMs offer higher permeabilities (usually at the expense of lower separation factors), allowing smaller layouts and investment costs than polymer membranes. Finally, unlike adsorbents, the possibility of operating in continuous mode and treating higher flow rates offers PIMs an added value to their industrial implementation. In addition to these general advantages, a new generation of hybrid materials based on functionalized silicas, novel zeolites, and metal−organic frameworks (MOFs) offer a plethora of synthetic possibilities to generate new membranes with suitable functional groups for CO2 capture via selective adsorption and/ or diffusion.30,31 Recent reviews compiling CO2 adsorption data on such materials for pressure-swing and thermal-swing adsorption column design,32−36 and covering thin film synthesis,37−40 have recently appeared, but with only brief insights into membrane design for CO2 and H2 separations. We intend here to fill this gap with a special focus on the assets and limits of such materials in light of current industrial challenges in gas separation technologies for different pre- and postcombustion CO2 capture scenarios (e.g., high- and lowtemperature applications). To this aim, a compilation of the most relevant and updated recent permeation and separation data, structural stability, and optimization perspectives of the above-stated materials for membrane design is presented. The reader will find along the review a multidisciplinary vision of the membrane field, including solid-state chemistry, materials science, separation, and process design (R&D). The lack of exchange between these different disciplines is usually recognized as an important barrier to breakthrough discoveries. This paper intends to provide some keys to the different communities, including industrial researchers, to facilitate reciprocal exchange.

toxicity, and regeneration energy demands, this process is intrinsically noncontinuous (i.e., semicontinuous) and still highly energy intensive (>2.5 GJ/tonne of CO2 avoided27 or €12.5−62.5 per tonne of CO2 avoided for coal power stations24). This review focuses on membranes and more specifically on porous inorganic membranes (PIMs) as alternative candidates to chemical absorption and other mature and prospective technologies for CO2 capture in different pre- and postcombustion scenarios. In general terms, a membrane can be defined as a selective barrier allowing the separation of one or more species from a mixture driven by their preferential affinity for the membrane. Unlike solid adsorbents, the term “affinity” is regarded here in a broad sense, since it comprises not only the favorable interaction of one or more molecules from a mixture with the solid, but also its/their promoted/inhibited diffusion within the active film or top layer. Although some studies point out the advantages of chemical absorption over membrane technologies for CO2 capture,28,29 most of these statements focus on nonoptimized membrane materials (e.g., resistant to water and acids, manufacture of thin layers, reproducibility of synthesis protocols), offering a biased view of the field. Joining the view of Favre,28 three main challenges are addressed for boosting membrane technologies to promote industrial implementation (section 1.3): (1) the selectivity challenge, (2) the energy challenge, and (3) the productivity challenge. These challenges impose not only the optimization of existing membrane materials, but also the development of new materials with balanced adsorption and diffusion properties for a given CO2 capture application. Although PIMs for gas separation are certainly still in an early technological stage, they show potential for pre- and postcombustion CO2 capture for different reasons. First, PIMs show higher thermal, chemical, and mechanical stability C


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Table 1. Typical Flue and Exhaust Gas Compositions for Stationary and Mobile CO2 Emission Sourcesa molecule CO2 (%, v/v) O2 (%, v/v) N2 (%, v/v) CO (%, v/v) H2 (%, v/v) H2O (%, v/v) SO2 (ppmv) H2S (ppmv) NOx (ppmv) HCl (ppmv) dioxine (ppbv)

coal combustion flue gas

coal-fired IGCC flue gas

gas-fired flue gas

waste incineration flue gas

coal gasification fuel gas

methane steam reforming flue gas

∼11

∼7

∼3

6−12

4−13

27−34

∼6 ∼76

∼12 ∼66

∼14 ∼76

7−14 balance 0.001−0.060

balance 40−58

0.3−2.2 0.1 μmol·m−2·s−1·Pa−1), mitigating the capital costs (CAPEX). Increasing the gas permeance is usually considered easier to achieve than boosting the membrane selectivity. Moreover, the membranes have to keep a high permeability under real operation conditions, especially in the presence of high water partial pressures, and in some cases resist harsh environments depending on the combustion/syngas generation conditions, respectively, for post- and precombustion applications (e.g., presence of NOx and SOx, Table 2). Table 4 lists the main textural and structural properties of the different silica, zeolite, and MOF/zeolitic imidazolate framework (ZIF) materials considered in this review offering potential perspectives for membrane design in different preand postcombustion CO2 capture that scenarios. When required, additional materials will be portrayed to provide a systematic view of the field. To properly compare the results reported by different authors, the following classical definitions are hereinafter used for PIMs (see additional definitions for polymer membranes and MMMs in refs 96 and 97): (1) Gas permeance (Πi), flux within the membrane (Ni) divided by the log difference of partial pressure between the retentate and permeate (ΔPi) Πi (mol · m−2· s−1· Pa−1) =

Ni ΔPi

comprising not only the favorable interaction of one or more molecules from a mixture with the solid, but also its/their promoted/inhibited motion within the porous framework. Two key selectivities then emerge (i.e., adsorption and diffusion) which should be maximized at the first step of membrane design. Unfortunately, systematic studies providing concomitant adsorption and diffusion properties of membrane materials for CO2/N2 and CO2/H2 separations are dramatically missing. In silico studies relying on molecular simulations appear possible and feasible to fill this gap,111,112 but only if accurate information about sorbate arrangement (e.g., presence of clustering effects, description of “pockets”) and correlation effects between the diffusing sorbate species, as well as detailed information on the membrane microstructure (by far the most complex aspect!), is well-known and conveniently described. These questions will be tackled in more detail in section 4. Our aim in this section is to address the main adsorption and diffusion mechanisms allowing CO2 discrimination in microand mesoporous frameworks that can be exploited for membrane design with a devoted connection to the framework structure and surface chemistry. 2.1. Adsorption Mechanisms

The adsorption properties of a solid sorbent are governed by the nature and strength of force fields and their distribution along the active surface and pores. These interactions depend on the structure, framework composition, crystal size, and purity of the sorbent. At first glance, one can make use of two main forces for sorbate discrimination: (1) electrostatic forces (e.g., polarization forces, surface field−molecular dipole and surface field gradient−molecular quadrupole interactions), and (2) van der Waals or nonspecific forces, directly correlated with the sorbate molecular polarizability. Electrostatic forces are strongly linked to Lewis acid−base interactions and segregation and clustering effects at the molecular level, while van der Waals forces increase with the degree of sorbate confinement following the classical analysis of Derouane.113−115 The different energetic contributions to adsorption of a given sorbate (in particular CO2) can be discriminated experimentally by calorimetric techniques. This point is addressed below for each family of materials. CO2 and N2 adsorption on polar surfaces is mainly promoted by surface field gradient−molecular quadrupole interactions. Conversely, adsorption of large nonpolar molecules (e.g., hydrocarbons) is essentially ascribed to molecular polarizability and in some cases configuration entropy effects.116 Keeping these ideas in mind, some general trends on the adsorption selectivity can be established that will be exposed at the end of this section. A list of the most relevant CO2, CH4, N2, and H2 adsorption data on amine-functionalized silica, zeolite, and MOF materials, including a broad compilation of Henry’s constants and heats of adsorption, can be found in the Supporting Information (Tables S2−S5). Nonspecific forces play an important role in N2 and H2 adsorption, leading in some cases to compensation effects between the enthalpy and entropy of adsorption.117,118 2.1.1. Amine-Functionalized Silicas. 2.1.1.1. Chemical vs Physical Adsorption. Micro- and mesoporous silica materials show an inherent CO2 adsorption capacity due to the presence of surface silanol groups (usually in the range of 1−5 SiOH groups/nm2) with weak alkaline behavior, providing only modest CO2/N2 and CO2/CH4 adsorption selectiv-

(1)

(2) Gas permeability (Pi), gas permeance multiplied by the top-layer effective thickness Pi (mol · m−1· s−1· Pa−1) = ΠiS

(2)

(3) Permselectivity (Pij), quotient between the pure gas permeances of species i and j

Pij =

Πi Πj

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(4) Separation factor (SFij), feed-to-permeate ratio between the molar compositions of species i and j SFij =

yi /xi yj /xj

(4)

2. MEMBRANE SEPARATION: ADSORPTION VS DIFFUSION SELECTIVITIES The first idea to keep in mind when developing a porous membrane for a target separation is to choose a material with a convenient affinity for the desired species to separate. As stated above, the term “affinity” has to be regarded in a broad sense, K


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ities.116,117 Indeed, the zero-loading isosteric heats of CO2, N2, and CH4 adsorption on TPABr-templated amorphous silicas are as low as 24, 21, and 22 kJ/mol, respectively. The adsorption strength becomes even more moderate in the presence of water vapor, blocking partially the accessibility of CO2 to the silica micropores. The intrinsic CO2 adsorption strength of silicas can be tuned either by impregnating Lewis acids such as imidazolium-based ionic liquids (ILs),119 or by incorporating alkaline surface groups such as amines (e.g., physically adsorbed, covalently grafted, or by formation of SiNH2 surface groups via ammonolysis), taking advantage of the well-known amine chemistry gained from chemical adsorption in wet scrubbers. The final adsorption properties depend not only on the nature of the amine (i.e., primary, secondary, or tertiary), but also on the loading of accessible amine surface groups and their interaction with the silica walls. Jones and co-workers120 have recently published a devoted review on the potentials of amine−oxide hybrid materials for acid gas separation (including CO2). In addition to electrostatic forces, the interaction between CO2 and alkaline amines can be governed by two additional mechanisms involving, respectively, the formation of surface carbamates (C−N bonds) and ammonium (bi)carbonates (monodentate and bidentate). In the former mechanism, amine moieties behave as both Lewis and Brønsted bases, promoting the formation of C−N bonds as acid−base adducts through a mechanism involving zwitterion intermediates with a maximum amine efficiency of 0.5 mol of CO2/mol of N. The amine efficiency increases with its alkaline character and loading, promoting deprotonation of the zwitterion intermediate.121,122 Characteristic IR bands for carbamates have been reported at 1595−1680 cm−1 (ν(N−H), ν(CO)), 1441− 1563 cm−1 (νas(COO−), δ(N−H)), and 1330−1430 cm−1 (νs(COO−)),122−125 while characteristic 13C NMR resonances have been reported at 160 and 164 ppm, respectively, for carbamates and carbamic acid.126 In contrast, under humid conditions, amines behave solely as Brønsted bases, promoting a maximum amine efficiency of 1.0 mol of CO2/mol of N. Water can also stabilize the amine surface groups upon CO2 adsorption/desorption cycles.127 The formation of (bi)carbonate surface species during humid CO2 adsorption yields IR bands centered at 1470−1493 and 1422− 1432 cm−1, respectively, for monodentate and bidenate bicarbonates and at 1337−1363 and 1541−1575 cm−1, respectively, for monodentate and bidentate carbonates.128−130 Note that the presence of (bi)carbonates does not exclude the formation of carbamates at the early stage of adsorption, transforming further into more stable (bi)carbonates. The nature and number of (bi)carbonate species can also change during CO2 adsorption due to partial displacement of CO2 by water, evolving from mono- to bidentate with stronger binding energies. The relative importance of each mechanism depends on the nature of the amine moieties and on the presence of humidity in the gas phase.118,131 In general terms, primary and secondary amines (sterically hindered or not) tend to react directly with CO2, forming carbamates, whereas (bi)carbonates are favored for tertiary amines with a more pronounced alkaline nature. 2.1.1.2. Synthetic Approaches for Amine Loading. Within this general framework, three approaches can be in principle considered for amine loading: (1) incorporation of amine groups on silica walls either by direct co-condensation of

aminoalkosilanes during the synthesis, (2) incorporation of amino groups by impregnation or grafting, and (3) ammonolysis, involving the direct generation of SiNH2 moieties with no need for an intermediate carbon chain. The simplest method for incorporating amines in silicas is by wet impregnation of an amine polymer, most often poly(ethylenimine) (PEI), tetraethylenepentamine (TEPA), or even dendrimers, from a solution and further evaporation of the solvent. The amine polymer is stabilized by the formation of hydrogen bonds with surface silanol groups, which facilitates its distribution throughout the pore volume. This interaction results in a decrease of the maximum decomposition temperature with the polymer loading.132 This approach, generating the so-called “molecular baskets” in the case of mesoporous silicas,133−140 is discouraged for membrane functionalization, since a significant amount of amine groups can be sterically hindered within the pore volume and also form agglomerates between particles, both aspects affecting negatively the CO2 adsorption/diffusion properties and being at the origin of anomalous adsorption patterns with temperature (i.e., increase of CO2 loading with increasing temperature). A similar conclusion can be in principle drawn for recently reported dual impregnated tethered amine−silicas.141 However, CO2 uptakes higher than 1.2 mmol/g can be achieved at low pressures ( m2 and k1 > k2 for most host/guest systems. Equation 12 relates −Ψ/RT with Π in such a way that V


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C

∑ i=1

qi = xiq T

C ∂ ln γi(Φ) xi + RT ∑ ∂Φ qi°(Φ) i=1

Review

(16) (17)

where qT is the total loading, qi°(Φ) is the loading of pure species i at surface potential Φ, and qi is the loading of species i for the given mixture. 2.1.4. Taxonomy of Materials Based on Adsorption Selectivity. Given the CO2 adsorption properties of the different amine-modified silica, zeolite and MOF materials mentioned in the previous sections, a taxonomy of the different materials can be established relying on the relative affinity of the different materials for CO2 adsorption. Krishna and van Baten,99,111 Scholl and Keskin,368−371 and Smit372,373 reported comprehensive series of CBMC simulation studies on the ability of different zeolites, MOFs, and ZIFs to discriminate CO2 from mixtures in view of adsorbent and membrane design. In these studies, the selectivities are computed using an expression analogous to eq 4: qi xj Siads ,j = qj xi (18) A complete and critical summary of the main trends obtained in these series is presented below, which has been extended to amine-functionalized silicas. Figures 20 and 21 compile some simulated trends for the CO2/H2, CO2/N2, and CO2/CH4 selectivities as a function of the total fugacity of the feed stream for a series of zeolites, MOFs and ZIFs. For the sake of comparison, Figure 20 also includes some experimental trends for the CO2/N2 selectivity on amine-functionalized silicas. Among the different zeolite materials considered, these simulations point out the supremacy of NaX zeolites for separating CO2 at moderate pressures. Likewise, in the case of MOF materials, rho-ZMOF shows values higher than 50 for CO2/H2, CO2/N2, and CO2/CH4 selectivities. MgMOF-74, with exposed metal cation sites, exhibits high selectivities for CO2 separation that are comparable to those obtained for rhoZMOF. The adsorption selectivity decreases most often with the CO2 loading in the feed.369 The use of covalent organic frameworks (COFs) might provide adsorption selectivites up to 120 at 5 bar and room temperature for the separation of equimolar CO2/H2 mixtures and up to 4 at 20 bar and room temperature for CO2/CH4 equimolar mixture separation.374 Figure 22 shows the evolution of the mixture CO2/N2 adsorption selectivities in cation-exchanged Y zeolites, reflecting the crucial role of the cation size in the CO2/N2 selectivity by tuning the electrostatic interactions in the FAU windows and cavities (see section 2.1.2.2 for more mechanistic insights). Other cage-type zeolite materials not represented in Figure 22 might even show higher adsorption selectivities due to preferential CO2 adsorption in cage windows and pockets (e.g., all-silica AFX zeolite99). As has been suggested by Krishna, these “pocket” centers are hardly foreseeable to be active in real CO2 capture applications due to their small size, limiting CO2 diffusion in and out the pockets. Likewise, the preferential location of CO2 molecules in window regions explains the high selectivities in all-silica CHA.208,375 In general terms, ZIF materials show lower CO2 separation selectivities than MOF-177, MgMOF-74, and rho-ZMOF (Figure 21). Among the different ZIF materials, those based on the original ZIF-8 framework with imidazolate frameworks

Figure 20. Evolution of the CO2/H2 (top), CO2/N2 (middle), and CO2/CH4 (bottom) adsorption selectivity as a function of the total gas fugacity computed from CBMC simulations for a series of representative zeolite frameworks (all-silica FAU, LTA, DDR, CHA, and MFI and NaY, NaX, NaZSM-5, and NaA). Images adapted from ref 111. More trends in adsorption selectivities can be found in the same reference and its corresponding Supporting Information.

modified with HCO and NO2 groups (denoted as ZIF-90 and ZIF-NO2, respectively) show the most promising CO2/N2 and CO2/CH4 selectivities. ZIF-3 and ZIF-90 appear to be more indicated a priori for CO2/H2 separations, with selectivities >100. However, experimental validation is still required on these materials. For the different separations, the selectivity remains almost constant up to 0.1 MPa for ZIF-2, ZIF-4, ZIF-8 and ZIF-9, and up to 1 MPa for ZIF-5. In contrast, at high pressures, the qualitative behavior of the different selectivities depends specifically on the particular ZIF: the selectivities tend to increase for ZIF-2 and ZIF-8, while they show a decreasing trend for ZIF-9 and ZIF-10. ZIF-3 also shows an increasing trend in the separation of CO2/CH4 equimolar mixtures, W


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Figure 21. Evolution of the CO2/H2 (top), CO2/N2 (middle), and CO2/CH4 (bottom) adsorption selectivity as a function of the total gas fugacity computed from CBMC simulations for different MOFs (left) and ZIFs (right). Images adapted from refs 99, 279, 371, 375, and 376.

whereas the behavior is more complex (presence of a minimum) in the separation of CO2/H2 (15−25% CO2) mixtures. This observation has been attributed to the different cage sizes in the different ZIFs and to the relative interplay between enthalpic and entropic factors. In the case of ZIF-68 and ZIF-70, Sholl and co-workers370 reported a prominent decline of the CO2/CH4 adsorption selectivity until 1 MPa total pressure in the separation of 10:90 CO2/CH4 mixtures. Beyond this pressure, the selectivity remained essentially unchanged (ZIF-68), or showed a slight increase with the pressure (ZIF-70). 2.2. Diffusion Mechanisms: Molecular Sieving and Correlation Effects for CO2 Separation

In addition to adsorption selectivity, the separation capacity of a membrane can be strongly affected by the selective diffusion of one or more species of a mixture. The diffusion selectivity of a membrane depends not only on the relative kinetic diameter of the molecules, but also on the diffusion mechanisms involved,

Figure 22. Adsorption selectivity of ion-exchanged FAU-type zeolites for a pure and an equimolar CO2 and N2 mixture at 308 K and 101 kPa total pressure. Data obtained from ref 377.

X


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Table 6. Summary of the Structural Features of Silicas, Zeolites, and MOFs with Potential for Membrane Design for CO2 Capture

Table 7. Transport Diffusivities of CO2, N2, and CH4 on Zeolite and MOFs at Low Coverage (9 μmol·m−2·s−1· Pa−1) in the separation of CO2/H2 equimolar mixtures at 296 K and 1010 kPa. The CO2/H2 separation factor could even be improved below room temperature (277 K), reaching a value >50. The lack of optimal CO2/H2 separation properties for microporous membranes relies on the opposing separation mechanism between CO2 and H2: while selective adsorption acts as the driving force for CO2 passage, H2 is transferred mainly by selective diffusion due to its small molecular size. In contrast, preferential H2/CO2 separation for precombustion CO2 capture applications has been reported by several authors. Hong et al.533 reported a H2/CO2 separation factor of 5 and a H2 permeance of 0.01 μmol·m−2·s−1·Pa−1 at 295 K on a boronsubstituted MFI membrane (Si/B = 12.5). An ALPO-4 membrane has also shown potential for H2 separation, with a H2/CO2 separation factor as high as 10 and a H2 permeance of 0.2 μmol·m−2·s−1·Pa−1. Other studies have reported preferential H2 separation at high temperature by promoting H2 diffusion pathways and blocking CO2 adsorption. Two topical examples have been reported on MFI501,544 and DDR540,545 membranes. Kanezashi et al. 501 reported H 2 /CO 2 separation factors and H 2 permeances of about 3 and 1.3 μmol·m−2·s−1·Pa−1, respectively, at 673 K from equimolar H2/CO/CO2 mixtures on ZSM-5 membranes blocking nonzeolite pores. Van den Berg et al.540 reported preferential H2 separation from H2/CO2 mixtures on DDR with H2/CO2 separation factors up to 4 and H2 permeances of about 0.04 μmol·m−2·s−1·Pa−1 at 953 K. LTA/ carbon composite membranes have also shown promising perspectives for H2 permeation at high temperature (423 K) despite a modest H2/CO2 separation capacity, most likely due to only partially intergrown layers.546 3.2.1. Effect of the Operation Variables on the CO2 Permeation and Separation Properties. 3.2.1.1. Effect of temperature. Figures 36−40 plot some examples of the influence of temperature on the CO2 permeation and separation properties toward separation of CO2/CH4 and CO2/N2 mixtures on a series of representative zeolite membranes (i.e., MFI, T, SAPO-34, and DDR). The CO2 permeance shows in most cases a decreasing trend with temperature, which is accompanied by a decreasing trend of the CO2/N2 and CO2/CH4 separation factors as the CO2 loading is reduced in both zeolites. A maximum of the CO2 permeance/ separation factor can be obtained, even below room temperature, as in the case of SAPO-34 membranes.

Figure 36. Permeance (red, blue) and separation factor (green) of an equimolar CO2/N2 mixture as a function of temperature for a ZSM-5 membrane subjected to postsynthesis modification with SiO2. Image adapted from Shin et al.498

Figure 37. Gas permeation and separation factor of a CO2/N2 equimolar mixture as a function of temperature for a NaY zeolite membrane under dry (open symbols) and moist (closed symbols) conditions. Image adapted from Gu et al.515

The gas permeation behavior within zeolite membranes can show in certain cases a dramatic increase with the temperature beyond a threshold value (about 400−500 K for light hydrocarbons in silicalite-1 membranes494,547−549). Generally, the membrane permeance does not follow an adsorption-onlydriven mechanism after going through a maximum in keeping with adsorption, increasing again at higher temperatures.548,550 To face the complication of an increasing flux at high temperatures, some authors have introduced a “gas-activated transport” 549,551 on top of a MS mechanism on the guidance of AL


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Figure 38. Permeance and separation factor of CO2/N2 mixtures as a function of temperature for a zeolite T membrane in dead-end mode. Image adapted from Cui et al.177

Figure 40. Permeance (red, blue) and separation factor (green) of equimolar CO2/CH4 (top) and CO2/N2 (bottom) mixtures as a function of temperature at different feed pressures for a DDR membrane. Image adapted from van den Berg et al.181

usually increase with the CO2 partial pressure. Competitive adsorption occurs, the strongest adsorbed component, i.e., CO2, occupying most of the sorption sites. Consequently, CO2/N2 and CO2/CH4 separation factors often show higher values than the corresponding permselectivities, the effect of pore blocking increasing with the CO2 feed composition. CO2 fluxes are only slightly affected by weakly adsorbed species, except at very low CO2 feed concentrations, where N2 can compete with CO2 for the sorption sites. This poses obvious limitations for achieving high selectivities in CO2 separation at low CO2 molar fractions, which is the case often encountered in practical postcombustion CO2 capture applications. Some zeolite membranes, however, still offer high separation factors at relatively low CO2 concentrations. A paradigmatic example is zeolite T membranes. Cui et al.177 showed that these membranes, when duly intergrown, can show CO2/CH4 and CO2/N2 separation factors up to 70 and 250, respectively, at CO2 feed compositions as low as 10% (Figure 42). However, the CO2 mixture permeances remain at relatively low values (3000 h) are needed to conceive realistic applications, focusing specifically on the effect of water and impurities such as acidic or alkaline vapors, promoting in some cases dealumination or desilication under current operation conditions. However, most of the reported permeation and

Figure 46. Permeance and separation factor of a CO2/N2 mixture as a function of the total pressure for a zeolite T membrane. Image adapted from Cui et al.177

constant adsorption and diffusion selectivities with pressure (Figures 20 and 27); (2) the CO2/H2 separation factor shows a maximum with the feed pressure for narrow-pore zeolite membranes and a decreasing trend for NaY and NaX zeolite membranes, which can be well accounted for by the trends of CO2/H2 adsorption selectivities; (3) the CO2/CH4 separation AO


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Figure 50. Gas permeation and separation factor of CO2/N2 equimolar mixtures as a function of the water partial pressure for Ge-MFI (top) and NaY (bottom) zeolite membranes. Images adapted from Nicolas and Pera-Titus88 and Gu et al.515

presence of water due to pore blockage (about 2 orders of magnitude for CO2 at 300 K), the initial values only being partially recovered at temperatures of >425 K. Gu et al.515 also reported a drastic reduction of steady-state CO2 and N2 gas permeances with the water partial pressure at 473 K (more than 1 order of magnitude) for partial pressures >20 kPa. Beyond this level, the CO2/N2 separation factors showed a progressive decrease from a maximum value of about 4.5. Keeping this picture in mind, hydrophobic or low hydrophilic materials such as MFI membranes, relying on selective CO2 adsorption, appear to be more suitable for repelling water and preventing pore blockage during operation. Indeed, Nicolas and Pera-Titus88 reported a net reduction of the gas permeance with water incorporation to the feed, irrespective of the degree of B, Ge or Cu substitution in MFI− alumina hollow fibers. Beyond 2% humidity, the CO2 and N2 permeances showed a steady-state decrease of the CO2 permeance to ca. 40% of the initial value when incorporating water into the feed stream. However, at lower water concentrations, the permeance reduction appeared to be more sudden, probably due to favored water adsorption in micropores (sitting near cations). The CO2 and N2 permeation patterns were accompanied by CO2/N2 separation factors showing a slight increase with the water concentration in the feed stream. Such a positive effect might be attributed to partial water condensation in intercrystalline mesopores, blocking nonselective defective pathways. Moreover, an effect of partial intercrystalline pore shrinkage due to crystal swelling upon water adsorption cannot be excluded (section 4.5.2.2).

Figure 49. Evolution of the CO2/H2 (top), CO2/N2 (middle), and CO2/CH4 (bottom) permeation selectivity (separation factor) as a function of the total gas fugacity computed from CBMC + MD simulations on different zeolite frameworks (all-silica FAU, LTA, DDR, CHA, MFI, ERI, ITQ-29 + NaY, NaX, SAPO-34). Data adapted from ref 111.

separation data have been measured for dry flue gases, omitting the effect of water on pore blockage. In the case of membranes operating via selective CO2 adsorption, care should be taken when dealing with strongly hydrophilic zeolites (e.g., FAU) provided that the relative humidity is kept below a threshold value determined by the form of the water isotherm. Following the study of Gu et al.,515 this threshold humidity can be extended to higher values at higher temperatures driven by a reduction of the water loading. Figure 50 plots the evolution of the CO2 permeance and CO2/N2 separation factor as a function of temperature for an FAU membrane subjected to dry and humid atmospheres (2.64 kPa water pressure). The CO2 and N2 permeances showed a drastic decrease in the AP


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Systematic studies on the permeation properties of highquality SAPO-34 and DDR membranes have been reported, respectively, by Noble and Falconer552 and Himeno et al.180 for the separation of CO2/CH4 mixtures. Figure 51 reproduces

Figure 52. Transient permeation of CO2 and CH4 within SAPO-34 membranes with different initial qualities in the presence of a humid atmosphere. The humidifier was turned off at time 0 h. Image adapted from Poshusta et al.552

permeance might be promoted due to preferential passage within defective domains. The presence of large amounts of water and linear hydrocarbons in the gas stream can promote crystal expansion during the permeation experiments, blocking intercrystalline domains and increasing accordingly the membrane selectivity to CO2.553 Such observation provides additional evidence of the crucial and subtle role of the microstructure on the membrane permeation and separation properties (see section 4.5.2.2 for more details). Hedlund and co-workers506 reported a dramatic increase of the CO2/CH4 and CO2/H2 selectivity for equimolar mixtures in the presence of water vapor for HZSM-5 membranes (Si/Al > 100) modified with methylamine. Figure 53 plots an example of the permeation and separation trends for ternary CO2 (49.6 kPa)/H2 (49.6 kPa)/H2O (2.1 kPa) mixtures during a complete adsorption/desorption cycle. An interesting hysteretic phenomenon was observed, showing higher hydrogen (or methane) permeances when cooling. The authors attributed this phenomenon to partial water desorption when cooling, reducing channel blockage by water. The CO2/H2 separation factor was promoted by water adsorption, showing a maximum with the temperature at a value of 6.5 with a hydrogen permeance as high as 1.3 μmol·m−2·s−1·Pa−1 at about 325 K. This behavior contrasts with the much lower CO2 /H 2 separation factor (about 3) near room temperature for an unmodified MFI membrane. Water vapor can also exert a positive effect on H2 separation from CO2/H2 mixtures in high-temperature applications. Wang and Lin544,561 reported interesting examples on ZSM-5 membranes (Figure 54). The H2 and CO2 permeances both showed a sustained decrease with the water pressure, being more dramatic for the H2 permeance. The H2/CO2 separation factor showed a value of >3 irrespective of the water pressure. Moreover, the introduction of H2 either in the feed or in the permeate streams has been reported to provide a stabilizing effect on the permeation of hydrocarbons and xylenes within ZSM-5 membranes. Such an effect could play an important role in the separation of CO2/H2 or H2/CO2 mixtures.555 Finally, Nicolas and Pera-Titus88 studied the effect of NO and propane addition on the CO2/N2 separation factors of MFI−alumina hollow fibers. The presence of NO in the feed (5000 ppm) did not significantly modify the CO2 /N 2 separation properties of the material. Although NO permeated faster than N2, the CO2/N2 separation factor was not affected

Figure 51. Permeance and selectivity as a function of time of an equimolar CO2/CH4 mixture containing (top) 0.02% water at 297 K for a SAPO-34 membrane and (bottom) 3% water at 298 K for a DDR membrane. In both cases, the experiments were performed in WK mode at a feed and permeate pressure of about 100 kPa. Images adapted from Tomita et al.527and Poshusta et al.552

some representative results obtained by both groups, showing the effect of water introduction in simulated flue gases on the membrane permeation properties. In both cases, the membranes showed gas permeances and separation factors that were fairly stable with time after water introduction. Water only contributed to a significant decrease of CO2 permeance in the case of DDR membranes. As all-silica DDR membranes have an intrinsic hydrophobic nature, the reduction of CO2 permeance in these materials might be ascribed to single-file diffusion effects due to preferential passage of water compared to CO2 and CH4 on the basis of the smaller kinetic diameter of the former (Table 2). Noble and Falconer552 reported an interesting study on the long-term stability of SAPO-34 membranes in the presence of water. A typical permeation pattern observed for low- and intermediate-quality SAPO-34 membranes is reproduced in Figure 52. Such materials might suffer from aging effects after long exposure to humid atmospheres, and this phenomenon might be accelerated in the presence of nonzeolite pores by increasing the accessibility of water to SAPO-34 crystals. As a consequence, CO2 permeation is reduced, while the CH4 AQ


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Figure 54. High-temperature permeance (red, blue) and separation factor (green) of a CO2/H2 equimolar mixture for a MFI membrane as a function of the water partial pressure. Image adapted from Zhu et al.561

of propane in the MFI framework and to correlation effects in the surface diffusive patterns between the sorbate species, slowing the permeation of lower adsorbing species, i.e., NO and N2. The presence of a maximum in the curve plotting the evolution of the CO2 permeance with the temperature reflected a competitive adsorption between CO2 and propane. Such effects were predicted and modeled by Krishna and van Baten.425 The beneficial effect of propane adsorption in the CO2/N2 separation factor was hindered beyond 373 K. 3.2.2. Influence of the Membrane Structure on the CO2 Permeation and Separation Properties. 3.2.2.1. Influence of Exchangeable Cations. Following the comments stated in section 2.1.2.2 on the role of exchangeable cations on the CO2 adsorption properties of high-Al zeolites, the CO2 permeance and separation capacity of zeolite membranes can be affected by the presence of cations. A series of comprehensive studies on this issue were reported by Kusakabe and co-workers377,392,556,557 for FAU zeolite membranes. The CO2 permeance within these membranes decreases in the order Li > K > Na and Ba > Ca > Mg due to a combined effect of variable CO2/N2 sorption capacity and CO2/N2 surface diffusivities. Figure 57 plots some pure and mixture CO2 permeation data on ion-exchanged FAU membranes. In general terms, NaY and KY membranes show the highest CO2/N2 separation factors. Partial impregnation of Rb+ and Cs+ cations in the zeolites might increase the CO2/N2 separation factor to 150 depending on the cation exchange level.556 FAU zeolites, incorporating higher Al amounts, provide improved CO2/N2 and CO2/CH4 separation factors.557 In the case of MFI zeolite membranes, the CO2 permeation performance can only be tuned for low Si/Al ratios. For a Si/Al ratio of 25, Aoki et al.558 reported increasing pure CO2 permeances in the order K+ < Ba2+ ≈ Ca2+ < Cs+ < Na+ ≈ H+, approximately matching a decrease in the cation size. Lindmark and Hedlund505 reported a comprehensive study on the CO2/H2 permeation and separation properties of silicalite-1 and ion-exchanged MFI membranes with Li, Na, and Ba (Figure 58). Ba-MFI membranes show the highest CO2/H2 separation factors, but at the expense of a lower CO 2 permeance due to a reduction of the channel size upon Ba incorporation into the MFI framework. Li et al.559 also showed a variation of the H2 and CO2 permeation properties with the

Figure 53. Ternary CO2/H2/H2O permeances and separation factors as a function of temperature for an H-ZSM-5 membrane in the separation of H2 (49.6 kPa)/CO2 (49.6 kPa)/H2O (2.1 kPa) mixtures at atmospheric pressure under WK conditions. Images adapted from Lindmark and Hedlund.506

by the presence of NO for the entire range of temperature tested. Figures 55 and 56 summarize the results obtained on the separation of a feed mixture with molar composition 10:1:2:0.5:86.5 CO2/propane/O2/NO/N2. Introducing propane to the feed stream led to a significant reduction of the CO2, N2, and NO permeances, the former by a factor of 3, but still maintaining a value of >0.4 μmol·m−2·s−1·Pa−1 at 303 K for the Al-MFI, B-MFI-50, and Cu-MFI samples (>0.7 μmol·m−2· s−1·Pa−1 at 373 K for Al-MFI and Cu-MFI samples). The samples preferentially permeated propane at a level of 1.5 μmol·m−2·s−1·Pa−1 at 303 K. In contrast, the room-temperature CO2/N2 separation factor increased to a value of about 6 in the presence of propane. The authors attributed such an observation to the strong adsorption AR


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Figure 55. Evolution of the CO2 (open circles), N2 (open squares), C3H8 (inverse open triangles), and NO (open tilted squares) permeances and CO2/N2 separation factor (filled triangles) as a function of temperature for the Al-MFI, B-MFI-50 (Si/B = 50), Ge-MFI-10 (Si/Ge = 10), and CuZSM-5 hollow fibers. Experimental conditions: feed composition 10:1:2:0.5:86.5 CO2/C3H8/O2/NO/N2; feed flow rate, 500 cm3 (STP)/min; sweep flow rate, 200 cm3 (STP)/min; feed pressure, 200 kPa; transfiber pressure, 50 kPa. Images adapted from Nicolas and Pera-Titus.88

Figure 56. Comparison of the CO2/N2 permeation and separation performance of Al-MFI, B-MFI-50, Ge-MFI-10, and Cu-MFI hollow fibers without and with the presence of propane in the feed stream. Experimental conditions: feed flow rate, 500 cm3 (STP)/min; sweep flow rate, 80 cm3 (STP)/min; feed composition 10:89:0.5 CO2/N2/NO; feed pressure, 200 kPa; transfiber pressure, 50 kPa. Images adapted from Nicolas and PeraTitus.88

polarization effects at the feed/membrane interface can also exert a positive effect on the CO2 separation in SAPO-34 membranes.554 3.2.2.2. Influence of Silica Deposits. The separation performance of zeolite membranes can be improved by preferential sealing of nonzeolite pores (see section 4.5.1.2). Shin et al.498 used this approach to improve the separation performance of MFI membranes toward the separation of CO2/N2 mixtures. Furthermore, the separation performance of zeolite membranes can be tuned to make them selective for the less adsorbing species at low temperatures. Hong et al.533 found

degree of Ge substitution into the MFI framework (Figure 59). However, no separation properties were reported. In the case of SAPO-34 membranes, the CO 2 /CH 4 separation properties are strongly affected by the crystal size and the Si/Al ratio in the top layer.540,560 An optimal crystal size was found at about 0.8 μm, whereas an optimal Si/Al ratio was found at 0.13, providing the highest selectivities. Higher Si/ Al ratios might enhance the presence of the SAPO-5 phase and nonzeolite pores in the zeolite layers, reducing the CO2/CH4 separation factors to a value of ca. 40 and increasing the CO2 permeance to a value of >2 μmol·m−2·s−1·Pa−1. Concentration AS


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Figure 57. Effect of ion exchange on the gas permeance in the separation of CO2/N2 mixtures for ion-exchanged Y-type membranes. Image adapted from Kusakabe and co-workers.377,392

that the intrinsic CO2 separation capacity of B-ZSM-5 membranes in the separation of CO2/H2 mixtures could be tuned to H2 by silylation. Figure 60 plots the evolution of the H2 permeance and H2/CO2 separation factor as a function of temperature for this membrane after modification. Zhu et al.561 monitored a progressive increase of the H2/CO2 separation factor at 723 K upon CVD of silica on a ZSM-5 membrane. The same group reported an improvement of the H2/CO2 separation factor for a DDR membrane. Contrary to the trends usually observed in zeolite membranes, the H2 permeance and H2/CO2 separation factors can experience a dramatic enhancement at high temperature in these materials. This observation is attributed to a strong promotion of H2 mass transfer by surface diffusion within silica domains at higher temperatures.

Figure 58. Ternary CO2/H2/H2O permeances (top and middle) and separation factors (bottom) as a function of temperature for silicalite-1 and ion-exchanged ZSM-5 membranes (Li, Na, Ba) in the separation of H2 (49.6 kPa)/CO2 (49.6 kPa)/H2O (2.1 kPa) mixtures at atmospheric pressure under WK conditions. Images adapted from Lindmark and Hedlund.505

3.3. MOF Membranes

below. Section 4 compiles the main details on the manufacture, layer intergrowth, and microstructure of the different materials. 3.3.1. H2 Separation: Molecular Sieving Mechanism. 3.3.1.1. MOFs Based on Cu(II) Clusters. The first and chronologically older family of MOF membranes displaying H2 separation is based on materials including Cu(II) clusters. An example of this material is HKUST-1 with a mean pore size of 9 Å. Guo et al.585 prepared HKUST-1 films (60 μm) on partially oxidized Cu supports with a H2/CO2 (equimolar) mixture separation factor of 6.8 at room temperature. The H2 permeance increased while the permselectivity decreased with the temperature in the range of 273−343 K. Guerrero et al.586

A few studies have appeared recently providing CO2 and H2 permeation and separation data on continuous and wellintergrown MOF films. Tables 14 and 15 collect the available data reported in the literature. Most of the successful membrane materials show preferential H2 separation over CO2 at moderate to high temperatures beyond the Knudsen selectivity threshold, promoting in some cases molecular sieving effects. Preferential CO2 separation also appears possible, but on duly functionalized materials providing optimal electrostatic potentials for selective adsorption. The main separation characteristics of the different families of MOF materials with proven separation properties and reproducibility are addressed AT


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pure CO2 permeances (0.8 nmol·m−2·s−1·Pa−1) and a H2/CO2 permselectivity of about 4, approaching the Knudsen threshold. 3.3.1.2. MOFs Based on Zn(II) Clusters. A second class of MOF membranes is constituted by materials with Zn(II) clusters. An example of a Zn-based membrane with partial H2/ CO2 permselectivity is IRMOF-1 (MOF-5)395grown on a hollow-fiber support. The materials showed competitive roomtemperature H2/CO2 permselectivity compared to Knudsen selectivity. In addition to this material, a comprehensive series of studies have been devoted to the preparation of Zn-based ZIF-type membranes, showing promising hydrothermal and chemical stability. The most systematic body of results on ZIF membranes with H2 selectivity has been reported by Caro’s group. In a pioneering study, this team reported the preparation of well-intergrown c-oriented ZIF-8 films on titania supports by a microwave (MW)-assisted solvothermal process in methanol after impregnating the support with PEI.573 After activation at room temperature, the membrane showed an equimolar H2/ CO2 separation factor of 7.0 overcoming the Knudsen selectivity threshold, but with a reduced H2 permeance (15 nmol·m−2·s−1·Pa−1). In addition, some H2/CH4 separation experiments revealed partial CH4 passage through the pore network of the membrane, even if the window size of ZIF-8 (3.4 Å) is smaller than the kinetic diameter of CH4 (3.8 Å). The authors attributed this observation to the framework flexibility. As a matter of fact, the ZIF-8 framework is flexible due to the flip-over of the imidazole ring, involving an expansion of the window size from 3.4 to 4.0 Å. Xu et al.574 also reported high H2/CO2 permselectivities (up to 55) and H2 permeances as high as 1 μmol·m−2·s−1·Pa−1 on ZIF-8−alumina hollow fibers. Figure 61 plots the characteristic permeation trends with time for H2, N2, CH4, and CO2 on this material. Among the different gases tested, CO2 showed the longest stabilization times, while H2 showed a practically instantaneous steady-state permeation. Membrane pretreatment protocols for ZIF-8 activation before gas permeation/ separation tests are usually stricter than for zeolite membranes, especially in the presence of open metal sites, involving heating at 473−573 K under inert gas flow for 24−72 h. Related to this study, Lai and co-workers577 and Tao et al.578 recently reported the preparation of ZIF-8 films supported on the outer surface of YSZ and ceramic hollow fibers with very high H2 permeances (>10 μmol·m−2·s−1·Pa−1), promising room-temperature H2/

Figure 59. Pure gas permeances at 473 K as a function of the kinetic diameter for a series of Ge-ZSM-5 membranes grown on SS supports. Image adapted from Li et al.559

Figure 60. H2 permeance and equimolar H2/CO2 selectivity as a function of temperature for a B-ZSM-5 membranes ubjected to postsynthetic silylation. Image adapted from Hong et al.533

also prepared HKUST-1 membranes on porous alumina supports by seeded solvothermal synthesis, but with lower H2/CO2 permselectivities (3.5) at room temperature. Ranjan and Tsapatsis587 prepared oriented [Cu(hfipbb)(H2hfipbb)0.5] (MMOF) films on porous α-alumina substrates by seeded solvothermal synthesis, the material showing extremely low

Table 14. Summary of Reported Data on CO2 Separation Using MOF and ZIF Membranes

AU


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Table 15. Summary of Reported Data on H2 Separation using MOF and ZIF Membranes

by ethanolamine enhanced the equimolar H2/CO2 separation factor to 16 at 498 K, involving the formation of imine surface groups.584 The authors attributed such improvement to a slight reduction of the pore size and to an increase of the layer intergrowth (see section 4.4.1.1 for more details). 3.3.1.3. MOFs Based on Al(III) Clusters. The third family of MOF membranes displaying selectivity to H2 relying on preferential diffusion is based on MIL-53(Al)-NH2. Two very recent studies have been published showing H 2 /CO 2 permselectivities overcoming the Knudsen threshold.570,571 Interestingly, the former study also addressed H2 separation from equimolar H2/CO2 mixtures at near room temperature, achieving a separation factor of ca. 31 and a H2 permeance >1 μmol·m−2·s−1·Pa−1. The membrane was operated at nearambient pressure with a transmembrane pressure of 101 kPa, far enough from the np → lp phase transition. The H2/CO2 separation factor showed a slight decrease with the temperature in the range of 290−350 K, whereas the H2 permeance increased slightly, as expected for a membrane governed by preferential diffusion (Figure 63). 3.3.2. CO2 Separation via Preferential Adsorption. MOF materials with partial CO2 separation from CO2/N2 and CO2/CH4 mixtures have been developed in the past 5 years focusing on the enhanced CO2 strength of the material. The best trade-off between CO2 permeance and selectivity in ZIFtype membranes was reported by Venna and Carreon566 (ZIF-8 on α-alumina) in the separation of equimolar CO2/CH4 mixtures. These authors reported room-temperature CO2 permeances and CO2/CH4 separation factors as high as 240

CO2 permselectivities (3.8 and 5.5, respectively), and high equimolar H2/propane separation factors (474). Unfortunately, no separation results were provided to assess the relative role of potential mesoporous intercrystalline defects on the membrane selectivity. Caro and co-workers579 manufactured ZIF-7 membranes on porous α-alumina supports by seeded MW-assisted solvothermal synthesis with a high equimolar H2/CO2 separation factor (6.5) after activation at 473 K driven by a molecular sieving mechanism. In a further development, a ZIF-7 membrane with an equimolar H2/CO2 separation factor of 14 at 493 K was reported with an increasing trend of the H2 permeance with the temperature and a remarkable hydrothermal stability.580 Figure 62 plots the time behavior of the membrane, showing a stable permeation and separation performance. Finally, the same team reported the preparation of c-oriented ZIF-7 membranes with a promising equimolar H2/CO2 separation factor at 473 K (8.4), but at the expense of a low H2 permeance, most likely due to the anisotropy of the pore structure.588 Relying on these results, Caro and co-workers582 developed ZIF-22 membranes on titania supports by solvothermal synthesis using APTES as a covalent linker. The equimolar H2/CO2 separation factor was 7.2 at 323 K driven by a molecular sieving effect. Likewise, ZIF-90 membranes were prepared on α-alumina supports by chemical bonding between ZIF-90 crystals and the support via the APTES linker. The equimolar H2/CO2 separation factor was 7.3 at 473 K with a stable permeation and separation performance even when water vapor was added into the feed stream.583 The covalent postsynthetic functionalization of the former ZIF-90 membrane AV


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Figure 63. Permeation and separation performance as a function of temperature of a H2/CO2 equimolar mixture for an MIL-53(Al)-NH2 membrane. Image adapted from Zhang et al.570

permeation properties of ZIF-8 and ZIF-7 membranes reported so far by Caro and co-workers (Figure 64).

Figure 61. Time evolution of the pure H2, CO2, N2, and CH4 permeances, as well as the H2/CO2, N2/CO2, and CH4/CO2 permselectivites, for a ZIF-8 film grown on an α-alumina hollow fiber. Images adapted from Xu et al.574

Figure 64. Comparison of pure gas permeation behavior of ZIF-7 (493 K) and SIM-1 (303 K) membranes as a function of the molecular kinetic diameters. Image adapted from Li et al.579and Aguado et al.590

Besides ZIF membranes, Bétard et al.562 recently reported the preparation of membranes based on [Cu 2 (BMEbdc)2(dabco)] with BME-bdc = 2,5-bis(2-methoxyethoxy)1,4-benzenedicarboxylate. The membrane displayed a roomtemperature equimolar CO2/CH4 separation factor of 4.5, which is larger than the Knudsen selectivity (0.6). Moreover, Lin and co-workers395 developed an MOF-5 membrane displaying preferential CO2 separation from CO2/H2 and CO2/N2 mixtures at near room temperature for a CO2 molar composition in the feed stream >0.8. At equimolar concentrations, the membrane only displayed a moderate CO2/H2 separation factor of 2.5 and a CO2 permeation of 0.2 μmol·m−2· s−1·Pa−1. In addition to the above-stated MOF films with a polycrystalline structure, Takamizawa et al.591 prepared a Cu2(bza)4(pyz) (bza = benzoate; pyz = pyrazine) single-crystal membrane with a 1D chain structure constituted of oriented narrow channels (

Porous Inorganic Membranes for CO2 Capture: Present and Prospects

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