CO2 Capture and Separations Using MOFs ... - ACS Publications

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CO2 Capture and Separations Using MOFs: Computational and Experimental Studies Jiamei Yu,† Lin-Hua Xie,‡ Jian-Rong Li,‡ Yuguang Ma,§ Jorge M. Seminario,§ and Perla B. Balbuena*,§ †

Institute of Circular Economy, and ‡Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, People’s Republic of China § Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States ABSTRACT: This Review focuses on research oriented toward elucidation of the various aspects that determine adsorption of CO2 in metal−organic frameworks and its separation from gas mixtures found in industrial processes. It includes theoretical, experimental, and combined approaches able to characterize the materials, investigate the adsorption/desorption/reaction properties of the adsorbates inside such environments, screen and design new materials, and analyze additional factors such as material regenerability, stability, effects of impurities, and cost among several factors that influence the effectiveness of the separations. CO2 adsorption, separations, and membranes are reviewed followed by an analysis of the effects of stability, impurities, and process operation conditions on practical applications.

CONTENTS 1. Introduction 2. Adsorption Capacity of CO2 in MOFs 2.1. Computational Exploration 2.1.1. Methodologies and Models 2.1.2. Computational CO2 Adsorption 2.2. Experimental Exploration 3. CO2 Selective Adsorption and Separation in MOFs 3.1. Computational Exploration 3.1.1. Methodologies 3.1.2. Structural Effects on Selectivities 3.1.3. Specific MOF Designs To Improve Selectivities 3.1.4. Effect of Linkers on Selectivity 3.1.5. Kinetic-Based Separations 3.1.6. Industrial Applications 3.2. Experimental Exploration 3.2.1. Postcombustion 3.2.2. Precombustion 3.2.3. Oxy-Fuel Combustion 3.3. Strategies for Enhancing CO2 Capture Ability in MOFs 3.3.1. Utilizing Open Metal Sites 3.3.2. Introducing Polar Functional Groups 3.3.3. Alkylamine Incorporation 3.3.4. Pore Size and Function Control 3.4. Other Evaluation Parameters for CO2 Capture in MOFs 4. MOF-Based Membranes for CO2 Capture 4.1. Computational Exploration

© 2017 American Chemical Society

4.1.1. Membrane-Based Selectivity Calculations 4.1.2. Pure MOF Membrane 4.1.3. Mixed-Matrix Membrane 4.2. Experimental Exploration 4.2.1. Pure MOF Membranes for CO2 Capture 4.2.2. MOF-Based Mixed Matrix Membranes for CO2 Capture 4.2.3. Ideal Selectivity 5. Additional Considerations for CO2 Capture in MOFs toward Practical Applications 5.1. Stability of MOFs 5.2. Effect of Impurities on CO2 Capture in MOFs 5.3. Shaping of MOFs 5.4. Effect of Process Operation on CO2 Capture in MOFs 6. Concluding Remarks Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References Note Added after ASAP Publication

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Special Issue: Carbon Capture and Separation Received: September 11, 2016 Published: April 10, 2017 9674

DOI: 10.1021/acs.chemrev.6b00626 Chem. Rev. 2017, 117, 9674−9754

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1. INTRODUCTION Anthropogenic CO2 emissions to the atmosphere are one of the most urgent climate issues of our age, which makes the development of materials for capturing the carbon dioxide produced by fossil fuels and sequestering it away from the atmosphere one of the grand challenges of the 21st century.1 According to the U.S. National Oceanic and Atmospheric Administration (NOAA),2 the atmospheric CO2 concentration has increased from approximately 310 to 400 ppm over the period 1960−2016 and is expected to reach more than 500 ppm by 2050 even if its emission stabilizes. The main anthropogenic source of CO2 emission is the burning of fossil fuels like coal, oil, and natural gas. These emissions will continue to increase in the future due to the rises in energy demands associated with the increase of the global population and industrial development. It is widely accepted that the increase of CO2 concentration in the atmosphere is a primary contributor to global warming. CO2 thus has often been regarded as the primary anthropogenic greenhouse gas. Reducing or eliminating CO2 emission from various industrial processes to minimize its influence on climate change is urgently required.3 If no necessary actions are taken, inevitably our future generations will be adversely affected. The transition of energy dependence from fossil fuels to cleaner alternatives such as hydrogen fuel or solar energy would be ideal; however, the realization of sufficiently developed new energy sources in large-scale industrial implementation is a long-term goal, which will need many more years of development. Thus, carbon capture and storage (CCS) technologies will play a vital role to reduce CO2 emissions in the interim period until environmentally cleaner energy sources are deployed. CCS embodies a group of technologies for the capture of CO2 from emission sources, such as power plants, followed by CO2 transportation to a storage site, and then its permanent sequestration. The transportation and permanent storage of CO2 are associated to relatively mature and inexpensive technologies. However, the considerable cost of capture, approximately two-thirds of the total cost for CCS,4 is reducing the deployment of commercial CCS efforts, which basically comes from the large energy requirement for the recovery of the captured CO2. Depending on the generation of CO2, there are mainly three basic CO2 separation and capture options: precombustion capture, oxy-fuel combustion, and postcombustion capture.5 The operational conditions for each process are different in terms of temperature, pressure, and optimized materials. Postcombustion capture is the most feasible because it can be used by just retrofitting existing power plants and saving costs of new constructions. The key challenge for the postcombustion is a large gas stream at low pressure and low CO2 content. Usually, the flue gas in power plants contains 15% CO2 and 75% N2 by volume, as well as other components such as water, SO2, and O2 at ambient conditions. The separation of CO2 from N2 is indeed the central issue in the postcombustion CO2 capture. The second alternative, oxy-fuel combustion, requires pure oxygen rather than air for the burning; therefore, the production of pure oxygen significantly increases the cost of this technology, thereby hindering the application of this option. Instead, a key challenge for the precombustion alternative is the separation of the CO2/H2 mixture produced from the water gas shift reaction. Because fossil fuels are decarbonated prior to combustion, precombus-

tion does not produce CO2 in the combustion step. In contrast to postcombustion capture where a low pressure gas stream has to be treated, the precombustion stream is a gas mixture at up to 40 bar. Precombustion holds the advantage of lower energy requirements and easier separation of CO2/H2 than those of CO2/N2 or O2/N2 separations. However, the high temperature and low efficiency make the precombustion a challenging problem. Moreover, the expensive cost and the community resistance for new construction of plants are other concerns. Despite extensive studies, no single technology has been demonstrated to meet the Department of Energy (DOE)/ National Energy Technology Laboratory (NETL) requirements: 90% CO2 capture at less than 35% increase in the cost of electricity.6,7 Currently, the most highly developed technology uses aqueous alkanolamine solutions to capture CO2 from the postcombustion flue gas.3 However, this process involves the formation of C−N bonds via chemical interactions between amine functionalities and CO2, suffering from the high cost for the regeneration of the sorbent (the amine solution). Thus, there is a critical demand to develop CO2 capture materials and associated processes that can effectively lower the regeneration cost. In this regard, CO2 capture by physical sorbents provides a promising energy-efficient alternative to the current amine-based absorption systems. For this purpose, a lot of physical sorbents, such as activated carbon, zeolites, and porous polymers for CO2 capture, have been evaluated. Simultaneously, some relatively new developed materials are being explored. Detailed descriptions for CO 2 capture processes and exploited materials can be found in a couple of well-written books and comprehensive review articles.7−11 As an emerging new class of porous solids, metal−organic frameworks (MOFs) have been recognized as highly promising porous materials for various potential applications. MOFs are formed by the assembly of metal-containing nodes (metal ions or metal-based clusters) that function as structural building units and organic linkers.12 An example of constructing a MOF is shown in Figure 1. MOFs are generally robust enough to allow the removal of guest species resulting in permanent porosity, therefore promising for adsorption-related applications. CO2 capture is one of the most active and attractive research areas in MOF’s applications. Because of the virtually limitless combinations of metals and ligands, there is an incalculable number of MOFs that can be designed and synthesized. Particularly, the physicochemical properties of MOFs can be judiciously designed and systematically tuned by the predesign in synthesis and postsynthetic modifications.13−15 Several strategies have been employed to finely tune the structures of MOFs, including incorporation of exposed metal cation sites,16,17 functional groups attachment,18,19 contraction of pore sizes by ligand shortening,20−22 or framework interpenetration.21,23 This remarkable tunability of their structures and chemical functionality is quite different from that of traditional porous materials, such as zeolites and activated carbon, allowing the direct optimization of pore structures, surface functions, and other properties for specific applications. Additionally, MOFs often exhibit a higher pore volume and surface area than those in zeolites and activated carbon, some of which possess the highest record of internal surface areas of >7000 m2/g.24 MOFs have thus been recognized as promising candidates for application in gas adsorptions and separations, particularly for CO2 capture. CO2 adsorption and separations in MOFs have been studied intensely through experimental techniques and computational 9675

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Figure 1. Schematic representation of a MOF construction. Adapted with permission from ref 8. Copyright 2011 Elsevier.

and thermal stability could be overcome by identifying the key issues via an integrated experimental and theoretical approach. The emphasis of this Review is on how theoretical efforts including both molecular simulations and quantum mechanical modeling can help in elucidating certain phenomena that are otherwise experimentally inaccessible or very difficult to obtain, and how these studies can be used to guide the design of these materials for CO2 capture. On the experimental side, a group of novel MOF materials and new experimental explorations of MOFs as potential new solid adsorbents to be used within CO2 capture systems have been published after a comprehensive review of CO2 capture in MOFs by Long et al. was published in 2012. Therefore, this Review also discusses and analyzes the most recent progress made in the experimental investigation of MOFs in CO2 capture from 2012 to 2016. This Review is organized as follows: after the introduction given as section 1, section 2 addresses several aspects related to the adsorption of CO2 within MOFs, including models and methods used to evaluate CO2 adsorption in MOFs, both computational and some new experimental explorations. Sections 3 and 4 review CO2 separations based on selective adsorption and membrane developments in MOFs from both computational and experimental perspectives. Three technologically viable capture options, CO2 precombustion capture, oxy-fuel combustion, and postcombustion capture, are discussed, respectively. Section 5

simulation methods in the past decade (Figure 2). Recently, a number of high-qualified reviews were published with main emphasis on experimental explorations of CO2 capture in MOFs,7−9,13,25,26 in which notwithstanding some computational simulations are described. However, to the best of our knowledge, a thorough analysis of theoretical studies focusing on CO2 adsorption and separation in MOFs and their impact on synthesis, characterization, and applications has not yet been reviewed. Computational simulations have been recognized as crucial tools to investigate and predict MOFs’ properties and potential applications. Computational analyses of MOFs are regularly performed to estimate molecular adsorption and diffusion in MOFs and can provide detailed microscopic information that is usually not accessible or is difficult to obtain experimentally.27−29 In this regard, simulations complement and augment the information obtained from the experiments. Moreover, computational simulation is a strong and attractive tool for large-scale screening not only of real materials but also of hypothetical materials based on the atomistic model of their structures.30−32 Simulations can predict the most promising candidates, such as MOFs for specific applications, thus guiding experimental explorations.33 On the basis of these stimulating advantages, computer simulations have been widely used to study MOFs applications. A number of comprehensive reviews are available describing the development of theoretical modeling and their applications in the MOF field.9,34−38 Sholl and co-workers have reviewed the application of molecular simulations and quantum mechanical modeling in MOFs.34 Snurr and co-workers have discussed applications of molecular simulations of methane, hydrogen, and acetylene storage in MOFs.35 Focusing on molecular simulations, CO2 separations with other gases and some explorations of liquid separation in MOFs have been analyzed by Jiang.36 More recently, the traditional and new developments of computational methodologies for MOFs and their application to gas separations have been reviewed by Zhong and co-workers.37 However, in all of these reviews, most of the attention was paid to either the development of computational methodologies in MOFs or to their specific applications in gas storage or separations. Even when some theoretical reviews partially discuss CO2 capture in MOFs, it has not been their main focus. In this Review, we center our attention on the advances of computational studies of CO2 capture in MOFs and their role in elucidating mechanisms of adsorption and diffusion, thus contributing to the parallel progress of advanced MOF synthesis and applications. Moreover, we investigate how specific bottlenecks such as the problem of MOF chemical

Figure 2. Year-by-year number of publications studying “the CO2 capture in MOFs” in the last 10 years (by SciFinder with the keywords: “carbon dioxide capture” or “CO2 capture” or “carbon capture”, and “metal−organic framework” or “porous coordination polymer”. 9676

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components were simulated. More recently, some attempts have been made to develop first-principles polarizable force fields for CO2 based on symmetry adapted perturbation theory,46 which have been demonstrated to yield very accurate CO2 adsorption isotherms in a wide range of MOFs.47−50 With charge−quadrupole interactions considered, two new potentials for CO2 named CO2−PHAST and CO2*−PHAST have also been introduced.51 Both of them are rigid five-site models, where LJ parameters are located on the carbon atom and phantom sites from the carbon along the C∞ axis in each direction. In addition to charge−quadrupole interactions, CO2*−PHAST considers polarization effects as well. Both of them have been successfully applied for exploration of CO2 adsorption mechanisms in rht-MOF.52 In fact, the models and force fields potentials for CO2 molecules have been relatively well established; the main challenge for simulating CO2 adsorption in MOFs lies in force fields for accurately describing CO2−MOF interactions. LJ parameters for framework atoms are generally taken from generic force fields, such as the universal force field (UFF),53 DREIDING,54 and optimized potentials for all-atom liquid simulations (OPLS-AA)55 models, which cover most of the elements in the full periodic table. Although the generic force fields work well for many typical MOFs56−60 (all CO2 adsorption), these empirical force fields were mainly developed for covalent bonds between atoms, so they give only approximate predictions of interaction energy for nonbonded interactions, leading to an inaccurate description of adsorption properties. For example, due to the inability of generic force fields to describe the chemically specific interactions between the unsaturated metal sites and CO2, the simulated isotherms with generic force fields are systematically underestimated when compared to experimental results for CO2 adsorption as shown for a couple of MOFs with coordinatively unsaturated metal (CUM) sites.31,61−63 In addition, due to the large number of diverse MOF structures, the above force fields may not yield accurate data. To achieve quantitative predictions of adsorption isotherms in MOFs when generic force fields fail to do well, some studies resorted to scaling or reparametrizing force field parameters or partial charges of the framework to match simulation results with experiments. As a typical example, by adjusting the standard UFF parameters with a scaling factor of 0.51, excellent agreement was observed between simulated and experimental isotherms for N2, CH4, and CO2 adsorption on ZIF-76 at 303 K over a wide range of pressures.64 However, this parametrization strategy is purely empirical. Its ability to correctly capture the underlying physics of the system as well as the transferability of the parameters to other systems is questionable. To overcome the force fields limitations noted above, considerable efforts have been made to develop better models and improve the accuracy of existing models using firstprinciples quantum mechanical (QM) calculations. As compared to empirical force fields, force fields derived from first-principles QM approaches can precisely describe the nonbonded interactions between gas molecules and MOFs, and therefore lead to a more accurate prediction of the adsorption properties of adsorbates in MOFs. To develop first-principlesbased force fields, the interaction energies and interatomic distances between adsorbates and MOFs are first obtained using ab initio or density functional theory (DFT)methods, followed by fitting to appropriate potential functions to get the improved force field parameters. The approach combines first-

deals with the critical properties that must be considered along with the more obvious high adsorption loadings and selectivities for MOFs as adsorbents in CO2 capture from an applications-based perspective. These properties include the thermal and chemical stability of MOFs, the effect of impurities, shaping of MOFs, and the effect of process operations. Finally, we present the conclusions and outlook for the future of this research topic in section 6, focusing on the high selectivity for CO2 over the other flue gas components, high adsorption capacities under the operating conditions, and minimal energy penalty for regeneration, which should be considered as the ideal properties of an adsorbent for a postcombustion.

2. ADSORPTION CAPACITY OF CO2 IN MOFs 2.1. Computational Exploration

2.1.1. Methodologies and Models. 2.1.1.1. Force Field Potentials. Reliable force fields are prerequisites for simulating adsorption properties of CO2 in MOFs, including individual models for CO2 and MOFs, and force fields to describe the interactions between them. In general, a force field consists of bonded and nonbonded interactions, where the bonded interactions include bond-stretching, angle-bending, and sometimes dihedral and improper angle terms, while nonbonded interactions include van der Waals (vdW, normally modeled by LJ potential) and Coulombic components. In most simulations, it is generally assumed that the frameworks are rigid, and periodic boundary conditions are normally applied to mimic the crystalline periodicity to avoid boundary or finite size effects.39 Because the adsorption phenomena are governed by interactions between adsorbates and framework sites, it is acceptable not to consider the intraframework interactions. The combination of Lennard-Jones (LJ) potential and Coulombic potentials is normally used to describe the adsorbate−adsorbate and adsorbate−adsorbent nonbonded interaction energies. ⎡⎛ ⎞12 ⎛ ⎞6 ⎤ qiqj σij σij Vij = 4εij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ + r ⎝ rij ⎠ ⎦ 4ε0rij ⎣⎝ ij ⎠

(1)

where i and j are interacting atoms and rij is the distance between them. εij is the depth of the potential well, and σij is the collision diameter at which the interparticle potential is zero, respectively. qi and qj are the partial charges of atoms i and j. The force field parameters are generally obtained by fitting the LJ potential to experimental vapor−liquid equilibrium, viscosity, or second virial coefficient data. To model CO2 molecules, various well-determined force fields are available. The early investigations have used pure LJ potential functions to describe intermolecular interactions between supercritical CO2 and aromatic compounds.40 More accurate potential models with both vdW and Coulombic interactions have also been proposed. For example, Moller and Fischer developed a two-center Lennard-Jones plus a point quadrupole model.41 Later, some newer models for CO2 have been developed, which generally consist of three LJ sites (two O and one C atom sites) with charges located on each site to describe its intrinsic quadrupole moment. The models in this category include the MSM model,42,43 “elementary physical models” (EPM and EPM2),44 and the TraPPE model,45 among others. TraPPE is one of the most widely used models for modeling CO2 adsorption in MOFs. In addition to CO2 molecules, TraPPE was also developed for various other species, such as N2, making it a convenient choice when mixtures of these 9677

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Figure 3. (a) Experimental (green) and simulated with MP2-derived force field (black), and UFF (red) adsorption isotherms for CO2 in Mg-MOF74 structure at T = 313 K. (b) CO2 single point energy distribution: MP2-derived force field (black) and UFF (red), UFF shifted (blue) values based on 8 million randomized insertions. The green arrows indicate the location of maximum points within the single point energy distribution, corresponding spatially to the pore centers in the Mg-MOF-74 structure. Adapted with permission from ref 75. Copyright 2014 American Chemical Society.

demonstrated a good agreement with the experimental isotherms as well. Several useful reviews relevant to the applications of first-principles-derived force fields for nanoporous media35,37,72,73 and a procedure to develop these FFs74 have been reported recently. Developing accurate firstprinciples-derived force fields is usually computationally expensive because it demands numerous quantum mechanical calculations. To accelerate these calculations while keeping accuracy, Kim and co-workers75 introduced a systematic method to develop force fields efficiently and accurately, as shown in Figure 3. Their approach starts from an initial trial force field, which can generate a reasonably accurate adsorption picture of the porous materials, followed by a single point energy correction by quantum mechanical calculations. The results showed accurate adsorption data in a diverse selection of guest molecules in porous materials, such as CO2 in Mg-MOF74 and Fe-MOF-74. The procedure works well for structures with strong binding sites because the adsorption properties are dominated by the low energy regions. However, the method is not robust for structures with large pore sizes without strong adsorption sites when the trial force field is much different from the reference case.75 In addition to MOFs with CUMs, flexible MOFs have also found potential applications for CO2 capture. Those MOFs can exhibit intriguing flexible behavior, in which the frameworks can shrink and swell reversibly (also called “breathing”) upon adsorption of gas molecules76,77 or other external stimuli such as temperature78 or pressure changes.79 Some of the typical examples showing this flexible or breathing behavior include the series of zeolitic imidazolate frameworks (ZIFs) and Matériaux Institut Lavoisier (MIL) frameworks. Developing simulation models for flexible MOFs represents a major challenge primarily arising from the treatment of the forces to represent chemical bonds.80,81 To investigate the properties of flexible MOF systems, an additional force field for describing the framework itself has to be included in the simulations. Failure to do so may lead to lattice collapse or to a completely rigid framework. The first attempt to develop force fields in flexible MOF-5 was made by Greathouse and Allendorffor.82 Following their work, more attempts have been made to model the properties of flexible frameworks including MOF-5,81,83,84 ZIFs,85−87 and MIL-53 series.80,88−95 Among MOFs that can breathe, the series of MIL-53 frameworks are certainly the most

principles QM calculations and molecular simulations. Because of significantly enhanced accuracy in comparison with generic empirical FFs, first-principles-based force fields have been applied to model the adsorption of various gases including H2, CH4, and CO2, in nanoporous materials. As compared to H2 and CH4, the existence of more parameters in the CO2 potential models makes the fitting of force fields more complex. Some specific force fields based on this method have been developed, especially for MOFs with strongly specific interacting sites such as CUMs or those with framework flexibility, two of the most important subsets of MOFs for CO2 capture. MOFs with CUMs have been one of the focuses for the application of carbon capture in both experimental and computational studies because these electron-deficient metal atoms can act as strong yet often reversible binding sites for CO2. As noted above, generic force fields generally lead to underestimation of simulated adsorption isotherms. To accurately capture the CO2−metal interactions, first-principles QM has been employed to develop force fields capable of predicting CO2 adsorption in MOFs with CUMs.62,65−69 Because of the lower computational costs, finite-sized cluster models were adopted in most of these studies to represent the whole frameworks in the QM calculations. For example, along given paths that were well-designed to probe the pairwise shortrange interactions based on cluster models, Smit, Gagliardi, and co-workers63 have employed MP2 calculations to derive the force field parameters for CO2 adsorption in Mg-MOF-74, also referred to as Mg2(dobdc) or CPO-27-Mg, one of the most promising MOFs for CO2 capture due to the high density of CUM sites in its framework. The derived force field has been proved to yield good agreement between the simulated and the experimental isotherm data. Furthermore, this approach has been found transferable to other MOFs with CUMs, such as MOF-5 (also called IRMOF-1). More recently, Lin, Lee, Smit, and co-workers developed force fields using DFT calculations with periodic boundary conditions; the derived force fields can correctly predict not only adsorption isotherms but also binding geometries and transport properties.70,71 Moreover, Duren and co-workers62 have successfully derived a force field for the adsorption of CO2 and CH4 in Mg-MOF-74 by using a multiobjective genetic algorithm to accurately fit over a thousand of single point energies. The simulated results 9678

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been applied successfully in some MOFs,59,60 although the atomic charges extracted from this analysis are known to depend on the basis set.117 Employing QM calculations to fit the charges makes the modeling process quite time-consuming. Toward the goal of computationally screening MOFs more rapidly, Wilmer and Snurr118 reported that the charge equilibration method (Qeq)119 yielded atomic charges similar to those of DFT calculations for finite clusters of selected MOFs, with much lower computational cost. Later, with the inclusion of a lattice effect on charge distribution, Sholl and coworkers120 used the modified Qeq to assign atomic charges for periodic frameworks. By simulating CO2 and N2 adsorption in the periodic structures of ∼500 MOFs, they demonstrated that the PQeq charges adequately describe the electrostatic interactions. Also, on the basis of the assumption that the atoms with the same bonding connectivity have identical charges in different MOFs, Xu and Zhong121 proposed a strategy named the connectivity-based atom contribution (CBAC) method to quickly estimate the atomic partial charges for dozens of MOFs. Results nearly identical to those from the QM charges, as well as good reproduction of experimental data, were obtained. The limitation of this method lies in the fact that it can only be applicable for materials with the same kinds of connectivities observed in the initial training set. 2.1.1.2. Simulating CO2 Adsorption with Grand Canonical Monte Carlo. Grand Canonical Monte Carlo (GCMC) has been recognized as one of the most extensively used techniques to study gas adsorption properties of MOFs, where simulations are conducted at a fixed chemical potential of the molecules, temperature, and volume.122 GCMC is suitable to study guest molecule adsorptions such as gas uptakes and heats of adsorption because it models a system at equilibrium, where the temperature and chemical potentials of the adsorbate in adsorbed and bulk phases are equal by allowing the particles of two phases to exchange. Each Monte Carlo simulation involves four types of movement of adsorbates including insertions, deletions, displacements, and rotations to achieve equilibrium of the system. The attempted movements of particles are either accepted or rejected depending on the change in potential energy, temperature of the system, and the chemical potentials.123 The properties calculated on the basis of these acceptable movements are then averaged over the entire trajectory. The adsorption isotherms can be obtained by relating the chemical potential and gas-phase pressure through an equation of state. 2.1.1.3. Quantum Mechanical Calculations. Quantum mechanical (QM) calculations employ approximations to the multibody Schrödinger equation, providing detailed information regarding chemical and physical interactions for a system of interest. During the past decade, QM calculations have been employed extensively to study, for example, structures, electronic properties, interaction mechanisms, and adsorption enthalpy for various gas adsorptions in MOFs, such as CO2. An enormous variety of QM approaches including ab initio and DFT methods exist that vary in both their accuracy and their computational complexity. The challenge for QM calculations is how to select the suitable methods while balancing accuracy and computational costs. In quantum-based simulations, the DFT method has been widely used from small clusters to large systems because it can give good results for some properties, such as binding geometries with exceptional computational efficiency.124 Traditional density functionals such as B3LYP,125 PBE,126 and others do not appropriately describe long-range

studied structures. To model the breathing of MIL-53, various flexible force fields models based on existing generic force fields like CVFF,96 DREIDING,54 UFF,53 or MM397 with missing parameters filled in or refined using empirical fitting and/or quantum mechanical approaches were developed. For example, Maurin and co-workers80 first developed a robust force field for MIL-53(Cr) based on the CVFF and DREIDING force fields. In a more sophisticated case, Verstraelen, Speybroeck, and coworkers proposed a new force field for MIL-53(Al) with three contributions to the force fields considered, respectively, an electrostatic term, a vdW term from the existing MM3 model, and a valence force field derived from DFT calculations on cluster models. It has been found that the derived force field predicts geometries and cell parameters that compare well with the experimental values for both phases before and after breathing.98 In addition to the flexible force field method, other approaches to describe the flexibility of MOFs include the phase mixture model method,99,100 the osmotic thermodynamic model method,101−103 and the stress-based model method.104 Several attempts have been made to model CO2 adsorption in flexible MOFs, particularly in prototype and functionalized MIL-53 using these methods.89,94,101,105−107 The detailed description of these methods and the development of various force fields for describing both rigid and flexible frameworks can be referred to in a few reviews.37,73,74,108 Because of the large quadrupole moment of CO2, charge− charge interactions are important. To incorporate Coulombic potentials for intermolecular interactions, the assignment of partial charges for the framework atoms is a prerequisite. Several strategies for calculating atomic charges of the MOFs are available nowadays. The most popular methods are electrostatic potential (ESP) methods such as CHelpG, where a set of point charges centered on the atomic nuclei are fitted to reproduce the ab initio-calculated ESP around the molecule as best as possible. Electrostatic potential (ESP) charge methods minimize the root-mean-square (rms) electrostatic potential difference between the point charge model and electrostatic potential for a grid of points outside the material’s vdW surface. Its applicability for CO2 adsorption has been verified in various MOFs with different structural features.31,109 Special attention needs to be paid to the termination of finite clusters and the appropriate cluster boundaries when using this approach. To avoid this issue, the repeating electrostatic potential extracted atomic (REPEAT) charge method has been developed for assigning point charges in porous periodic materials without building nonperiodic cluster models, which has been shown to give excellent predictions of atomic charges for periodic porous materials such as MOF-5, ZIFs, etc.110−113 Extensive tests have led to the conclusion that these methods do not necessarily assign sensible charges to the buried atoms.114 This problem has been addressed by new approaches such as the densityderived electrostatic and chemical (DDEC) methods that were shown to reproduce chemical and physical properties across a wide range of material types.111,115,116 Sholl and co-workers114 showed recently that it is not necessary to assign point charges to MOF atoms to describe adsorption electrostatics in calculations where the MOF is assumed to be rigid. Instead, an electrostatic potential energy surface (EPES) computed from a periodic DFT calculation can be applied directly to define the electrostatic interactions for adsorption. In addition, on the basis of DFT calculations, charges extracted from Mulliken population analysis present an alternative choice for charge calculations of periodic structures of MOFs that has 9679

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(CuBTC). Although it slightly overestimates the metal−CO2 bond length, the revPBE-based functional showed good accuracy with an average error of only ∼2 kJ mol−1 (4%) relative to experiment. Lin, Lee, and co-workers70 also presented a systematic and efficient methodology to derive accurate force fields from periodic DFT calculations with vdWDF2 functional for use in classical molecular simulations, which can nicely predict the CO2 and H2O adsorption isotherms inside Mg-MOF-74, and is transferable to Zn-MOF-74. Rubes and co-workers144 conducted an accurate ab initio description on unsaturated Cu2+ and Fe3+ sites in MOFs. They evaluated the accuracy of DFT exchange-correlation functionals for the description of adsorption in MOFs referenced by a high level ab initio method (CCSD(T)/CBS), DFT(CC), and experimental data. The accuracy was found to heavily depend on specific adsorbates and adsorbents. They concluded that there is no universal functional that can describe all of the systems accurately: for example, B2PLYP-D shows the highest accuracy for the Cu2+ cluster model but fails in the description of the Fe3+ cluster. For the periodic models, the GGA functional with semiempirical correlation for the dispersion, PBE-D2 and PBED3, produces decent accuracy for the periodic model of HKUST-1. Therefore, it is crucial to select suitable functionals for a particular adsorption system when using DFT methods. To get reliable binding energy results, the basis set superposition error (BSSE)145 generally needs to be corrected to avoid overestimating the interaction energies between MOFs and CO2.130,138,146 It has been found that BSSE shows strong effects on adsorption energy obtained from the MP2 method.147 However, DFT only shows relatively small dependences on BSSE.148 2.1.2. Computational CO2 Adsorption. 2.1.2.1. Electrostatic Effects on CO2 Adsorption. The first attempts to simulate CO2 adsorption in a MOF were reported by Kawakami and co-workers.149 They tested the influences of framework changes derived from three different methods and found that the values of the charges made a substantial difference for the calculated saturation loading of CO2. They attributed the discrepancy of adsorption behavior to different Coulomb interactions induced by charge densities. In addition, the adsorption amounts predicted by simulations were 3 times larger than the experimental results. They attributed this inconsistency to the unsaturated adsorption in the experiment caused by irregular stacking of crystal cells. In the following years, the electrostatic effects on CO2 adsorption have been explored further by a couple of groups. Snurr and co-workers150 simulated the adsorption isotherms of CO2 in MOF-5 at 195, 208, 218, 233, 273, and 298 K, revealing the necessity of inclusion of electrostatic interactions between CO2 molecules to reproduce the experimental results. In addition, they also simulated CO2 adsorption isotherms in MOF-177 and IRMOF3, which agree excellently with the experimental ones. Zhong and co-workers151 have investigated the effect of framework charges in MOFs on the amount CO2 adsorbed in IRMOF-14, IRMOF-17, and MOF-177 at 298 K and revealed that the electrostatic interactions between MOF atoms and CO2 contribute to the increase of CO2 uptake notoriously by ∼20−30% at low pressures but only by a few percent at high pressures. A similar conclusion was drawn in their follow-up study, where the influences of framework charges on CO2 uptakes in a total of 20 MOFs with different topologies, pore sizes, and chemical characteristics were examined.152 They demonstrated that at moderate operating pressures, the

dispersion forces (i.e., van der Waals), although they include some dispersion correlation through the use of an exchangecorrelation potential, which only accounts for local contributions to the electron correlation.127 The good description of gas molecules and MOFs depends heavily on long-range dispersion corrections because they may play a dominant role in gas adsorption. For example, Sauer and co-workers reported that dispersion has a substantial share on the calculated adsorption energies (46−77%) for CO2 adsorption in M-MOF-74 (M = Mg, Ni, Zn).128 In comparison with DFT, high level ab initio methods, such as second-order Møller−Plesset (MP2),129 show superiority in treating a dispersion interaction in weakly bonded systems with any degree of precision.130 However, higher level calculations are extremely computationally expensive and only affordable for small clusters representing MOFs. Considering both calculation time and accuracy, the hybrid combinations of high level ab initio and DFT, or low level ab initio, offer an optimal choice.130 For example, with a hybrid high level (MP2 with complete basis set extrapolation):low level DFT + dispersion corrections (B3LYP+D*) method, Sauer and coworkers128 calculated adsorption energies for CO2 adsorption in M-MOF-74 (M = Mg, Ni, Zn). The accuracy of methods was tested in comparison with experimental heats of adsorption. B3LYP+D* underestimated heats of adsorption by about 5 kJ mol−1, whereas hybrid MP2:B3LYP+D* slightly overestimates them by about 2 kJ mol−1. With MP2:B3LYP+D*, also the mean absolute error is somewhat smaller, 3.8 kJ mol−1 as compared to 5.6 kJ mol−1 for B3LYP+D*. Both methods predict the same sequence of binding energies for CO2 (Mg > Ni > Zn) adsorption on CUMs in the MOF-74 frameworks. In addition, some DFT approaches including dispersion-corrected DFT (DFT-D)131−133 and the ab initio van der Waals density functional (vdW-DF)134−137 are available to treat systems with long-range dispersion interactions and provide good results for binding geometries and energies.138−141 In addition, the reduced computational cost of DFT makes the derivation of accurate force fields from a fully periodic framework of MOFs possible. A recent report by Smit, Neaton, and co-workers demonstrates how DFT-D can be applied to successfully reproduce the experimentally determined structures and isosteric heats of adsorption for various gases including CO2 within a series of M-MOF-74 frameworks, promising materials due to the high density of CUMs in their frameworks, using periodic structures.141 They142 also employed several vdWcorrected DFT methods to study the geometries and binding energies of CO2 within Mg-MOF74. At a reduced computational cost as compared to high level quantum approaches, the calculated enthalpies with vdW-DF, PBE+D2, and vdW-DF2 (40.5, 38.5, and 37.4 kJ mol−1, respectively) compare extremely well with the experimental value of 40 kJ mol−1 for MgMOF74, whereas PBE underestimates adsorption enthalpies by about 50%. In addition, CO2−MOF bond lengths with vdWDF and vdW-DF2 agree well with experiments, while vdWC09x results in the best agreement with lattice parameters. The excellent agreement of calculated adsorption enthalpies and geometries verified the feasibility of vdW-DF and vdW-DF2 for calculating adsorption enthalpies of CO2 in MOFs with CUMs. Meanwhile, Siegel and co-workers143 have benchmarked five newer vdW density functionals including optB86b, optB88, optPBE, revPBE, and rPW86, along with the DFT-D2 method and conventional LDA and PBE-GGA functionals, with respect to experimental enthalpies for CO2 adsorption in five MOFs with CUMs: M-MOF-74 (M = Mg, Ni, and Co) and HKUST-1 9680

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Figure 4. Location of CO2 molecule in configurations (left) and within the Znbpetpa structure (right), marked by a blue arrow, and the corresponding change in system enthalpy from (top) evacuated structure. Hydrogen atoms and some fragments have been removed for clarity. Adapted with permission from ref 28. Copyright 2013 Royal Society of Chemistry.

series of ZIF materials ZIF-8, ZIF-69, and ZIF-76.64 More recently, CO2 adsorption sites in the flexible MOF zinc 1,2bis(4-pyridyl) ethane tetrafluoroterephthalate (Znbpetpa) were identified by Chang and co-workers.28 Two types of sites with heats of adsorption of 126 and 68 kJ mol−1 (depicted in Figure 4) were derived from DFT and confirmed by canonical Monte Carlo calculations. The simulations demonstrated that both adsorption sites displayed a “breathing” of the structure with gas uptakes. 2.1.2.3. Effects of CUMs and Ion Doping on CO 2 Adsorption. Simulations can provide important information on the influence of different structural features of MOFs on CO2 adsorption. These structure−function correlations are useful for a priori prediction of CO2 capacity and for the rational design of MOFs for higher adsorption capacities of CO2. Through screening of a diverse collection of 14 MOFs for low-pressure CO2 uptake, Snurr and co-workers31 found that the CO2 uptake correlates with the heat of adsorption at low pressures. The best performing MOFs for CO2 uptakes including M-MOF-74, HKUST-1, UMCM-150, and UMCM150(N)2 all possess CUMs. Therefore, MOFs possessing a high density of CUMs such as M-MOF-74 series are found to adsorb significant amounts of CO2 even at a pressure of 0.1 bar. The interaction of CO2 with CUM sites in Mg-MOF-74 has been characterized by Valenzano et al.138 Different from the linear configurations found for CO and N2 with Mg, an angular Mg2+···OCO complex was formed for CO2 adsorption. The importance of CUMs in M-MOF-74 (M = Mg and Zn) for CO2 adsorption was further explored by Hou, Li, and coworkers.154 Using the combination of DFT and GCMC, they found that the high CO2 adsorption ability of M-MOF-74 is due to the strong Lewis acid and base interactions between metal ions and oxygen atom of CO2, as well as from the carbon atom of CO2 with oxygen atoms in the organic linkers. To clarify the effects of CUMs in MOFs on CO2 adsorption, CO2 binding strengths between CO2 and the different transition metals in M-MOF-74 (M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) were calculated with an MP2-based QM/MM method.155 The

framework charge contribution in MOFs can be ignored in large-scale prescreening such as in the natural gas upgrading process, whereas the contribution cannot be ignored in computational screening of MOF materials for CO2 adsorption under low-pressure conditions. Nieto-Draghi and co-workers64 have reported that the electrostatic interactions in ZIFs are crucial to reproduce the behavior observed on experimental CO2 adsorption isotherms, particularly in the low-pressure regime, where host−guest interactions practically dominate CO2 adsorption. Different from CH4 and N2 where adsorption is totally controlled by the vdW interactions, Henry’s constants and isosteric heats of adsorption revealed by the PES indicate both vdW and electrostatic interactions play a role for CO2 adsorption in ZIF-8, ZIF-69, and ZIF-76, with the latter being dominant. 2.1.2.2. Adsorption Sites Identification. One important contribution from the simulations is to determine the preferred adsorption sites and the degree to which they are filled at low and high pressures. The early work by Snurr and co-workers153 has identified the adsorption sites and energies for various gases including CO2 in MOF-5 from molecular simulations. They found that the most preferred site in MOF-5 is near the zinc− oxygen cluster in the cavities where the linkers point outward and the least energetically favorable site is above and beneath the linker molecule for all gases evaluated. On the basis of cluster models using quantum mechanics calculations, Hannongbua and co-workers130 have also investigated the optimal binding sites and orientation as well as the binding energy for CO2 and CH4 adsorption in MOF-5. Either parallel or perpendicular adsorbates to both linker and corner domains of the cluster were considered, and the corner domains were identified as the optimal binding sites for both adsorbates with the stronger adsorption for CO2. In addition, by employing potential-energy surfaces of the different host−guest contributions, Nieto-Draghi and co-workers confirmed the main adsorption sites for CH4 on ZIF-8 obtained by neutron powder diffraction (NPD), difference Fourier analysis, and Rietveld analysis. They further identified adsorption sites for CO2 in a 9681

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good agreement between binding energies from the MP2-based QM/MM method and isosteric heats of adsorption from GCMC simulation for CO2 indicated the feasibility and accuracy of QM/MM method for adsorption of guest molecules in MOFs with CUMs. The relative CO2 binding strengths for the different transition metals can be explained by the relative strength of electrostatic interactions caused by the effective charge of the metal atom in the direction of the CUMs induced by incomplete screening of 3d electrons. In addition, using four MOFs of MOF-5, IRMOF-3, HKUST-1, and Zn2[bdc]2-[dabco] as model materials, Karra and Walton investigated the interplay of factors including pore size, heat of adsorption, open metal sites, electrostatics, and ligand functionalization contributing to CO2 adsorption by GCMC. They demonstrated that HKUST-1 shows the highest adsorption capacity among these MOFs at low pressures. MOFs with smaller pores can have an impact on CO2 adsorption similar to that of larger pore MOFs with CUMs.156 Subsequently, focusing on different adsorption sites and the corresponding enthalpies, CO2 adsorption in HKUST1 at various coverages up to 13 mmol g−1 has been studied using the DFT/CC method and microcalorimetry. The simulation work clearly showed that adsorption sites in HKUST-1 are rather heterogeneous. CUMs are the most favorable sites for CO2 adsorption, which were occupied preferentially at low coverage. CO2 adsorbs in cage window sites at higher coverages up to CO2/Cu = 5:3 as well as in cage centers and in large cages at even higher coverages. The detailed explorations of CO2 adsorption sites successfully explained a rather unexpected dependence of the adsorption enthalpies on the CO2 coverage by microcalorimetry.157 Interestingly, Snurr and co-workers158 found that the CO2 uptakes in HKUST-1 were significantly increased by the presence of water molecules coordinated to CUMs in the framework using molecular simulations, which was later validated by experiments. In addition to introducing CUMs onto MOF frameworks, the study by Cao, Wang, and coworkers159 revealed that doping of metals in MOFs serves as an efficient approach for enhancing CO2 adsorption capacities. By the combination of first-principles calculations and GCMC simulations, they comprehensively explored the effects of the doping of a series of alkali (Li, Na, and K), alkaline-earth (Be, Mg, and Ca), and transition (Sc and Ti) metals on CO2 adsorption in nanoporous covalent organic frameworks (COFs). Because of the weak interactions, Be, Mg, and Ca fail to dope in COFs, whereas Li, Sc, and Ti can bind with COFs stably. Furthermore, it has been found that doping of Li, Sc, and Ti can improve the uptakes of CO2 in COFs significantly. However, doping with Sc and Ti suffers from the difficulty of desorption because the binding energy of a CO2 molecule with Sc and Ti exceeds the lower limit of chemisorptions. In contrast, Li serves as the best surface modifier of COFs for CO2 adsorption among all of the metals studied. The further investigation of the uptakes of CO2 in the two Li-doped COFs verified that high CO2 uptakes can be achieved by Li doping in MOFs. The excess CO2 uptakes of the Li-doped COF-102 and COF-105 reached 409 and 344 mg/g, which are about 8 and 4 times those in the nondoped ones, respectively. The CO2 uptakes of the Li-doped COF-102 and COF-105 reached uptakes as high as 1349 and 2266 mg/g at 40 bar and 298 K, respectively, which are among the reported highest scores so far. More recently, Ha and co-workers160 conducted DFT calculations for investigating the CO 2

adsorption in alkali-metal (Li, Na, and K) doped MOF-5s. They found out that doping can significantly enhance the CO2 adsorption capacity due to strong interactions between CO2 and the metals. Li doping shows the best results for the enhancement. 2.1.2.4. Effects of Ligands on CO2 Adsorption. Because the interaction of CO2 with organic ligands is also very important, ligands often play significant roles in CO2 adsorption in MOFs. A few works have been published on the interaction of CO2 with organic ligands.161−167 Lewis, Ruiz-Salvador, and coworkers168 reported an important role of the BDC ligands in MOF-5 for CO2 adsorption. The ligands were found to be among the favorable sites for CO2 binding. A scheme for decomposition of the relative contributions of the metal oxoclusters and the organic ligands was proposed on the basis of GCMC simulations. The CO2 adsorption in MOF is ascribed to the short-range interactions with equal contribution from the zinc oxoclusters and the BDC ligands, and the nanoconfinement of CO2. Neaton and co-workers140 showed that the adsorption energy can be 3 times more sensitive to the choice of the bridging ligand than to metal cation choice in BTT-type frameworks with CUMs. First-principles calculations indicated that the binding energy of CO2 to the frameworks can be tuned from 34.8 kJ mol−1 (for CaBTTri) to a maximum of 64.5 kJ mol−1 (MgBTT) by a judicious choice of the organic linker and the metal center. The functionalization of organic ligands is a well-accepted route to improve CO2 adsorption capacity in MOFs. To find suitable functional groups for the design of linker molecules to form MOFs with enhanced CO 2 adsorption, a systematic exploration of the impact of ligand functionalization on ligand−CO2 affinity, using the benzene molecule as a representation of the aromatic ligands extensively used in MOF synthesis, has been made by Torrisi and coworkers.146,169 They started the evaluation of halogen and polymethyl substitution on the CO2 adsorption with a series of substituted benzenes, using PW91/DNP, and found that functionalizing organic ligands with electron-donating groups such as methyl may increase the calculated CO2−ligand binding energy, although the impact of methyl substituents is rather limited. The increase of binding energy can be attributed to the inductive effect of the methyl groups, which strengthens the quadrupole interaction between the aromatic ring and the CO2 molecule. By contrast, halogen substitution has been found unlikely to result in substantial improvements of adsorption affinity.169 Focusing on lone pair electrons (amino, nitro, and carbonyl) and acidic hydrogens (hydroxyl and sulfonate), they further evaluated the impact of the various polar groups including NO2, NH2, COOH, OH, and SO3H on CO2 adsorption. It has been found that CO2−ligand affinity was enhanced significantly for ligands with polar groups, demonstrating the benefit of using substituents containing lone pair electrons or acidic hydrogens.146 Similarly, taking ZIFs as material representations, Soujanya, Sastry, and co-workers170 have systematically evaluated the influence of various factors on the CO2 binding affinity with imidazolate (Im) linkers of ZIFs substituted by CH3, Cl, CN, OH, NH2, and NO2 functional groups at C2, C4, and C5 positions of the Im ring using DFT calculations. Their study demonstrated that both the nature and the position of the functional group affect CO2 adsorption capacities. Im linkers with asymmetrical substitution, NO2/OH, CN/OH, and Cl/OH combinations, are found highly promising as linkers of ZIFs for CO2 adsorption. Localized molecular orbital energy decomposition analysis (LMO-EDA) 9682

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are the dominant factors at high pressures. Synergistic effects of both amino functionalization and framework interpenetration on improving CO 2 uptakes have been also analyzed experimentally and computationally on the basis of two isostructural MOFs.174 In addition, for the MOFs incorporated with multiple functional groups, Wilson and co-workers175 have shown that the functional groups on neighboring layers can adsorb CO2 synergistically. Vaidhyanathan, Shimizu, Woo, and co-workers have revealed that the combination of appropriate pore size, strongly interacting amine functional groups and the cooperative binding of CO2 guest molecules is responsible for the low-pressure binding and large uptake of CO2 in Zn2(Atz)2(ox) (Atz, 3-amino-l,2,4-triazole; ox, oxalate).112 2.1.2.7. Mechanism Exploration of CO2 Adsorption in Flexible MILs. As noted above, the MIL series of hybrid porous materials can exhibit the unusual structural transformation called “breathing” or “gate opening” induced by adsorption of guest species or the temperature and pressure stimuli. A distinct step in the isotherm was found during the adsorption of CO2 on MIL-53 at 304 K. Such behavior is neither observed during the adsorption of CH4 on MIL-53 nor during the adsorption on the isostructural MIL-47.76 The mechanisms for this intriguing phenomenon were studied in a series of works by Maurin’s group. They first studied CO2 adsorption in a hybrid MOFs of MIL-53(Al).176 It has two structural forms named MIL-53np (Al) and MIL-53lp (Al), which have the same chemical composition and differ only in their pore width. They used DFT calculations to derive charges of the adsorbent frameworks for the GCMC calculations that followed. The simulations showed that the enthalpies of adsorption for CO2 at low coverage in the large and narrow pore forms reproduced experimental data obtained by microcalorimetry quite well. The calculated enthalpies of adsorption for CO2 in the region below 6 bar match well those calculated for the MIL-53np (Al), while those at high pressures around 6 bar are close to those for the large pore MIL-53lp (Al) analogue. The agreement between experiment and simulation validated the assumption that the MIL-53(Al) framework switches between two forms during the adsorption process. Subsequently, the same group focused on CO2 adsorption in two different metal centers containing MILn, MIL-53(Al) and MIL-47 (V). Although isostructural to MIL53, MIL-47 does not breathe.76 The explorations of the adsorption mechanism revealed that CO2 adsorption in MIL53(Al) is mainly governed by the interactions between CO2 and the μ2-OH group in the frameworks. When the CO2 loading increases past the point where no more preferred adsorption sites are available, the preferential interactions in the narrow pore framework are broken, and the structure switches to the large pore form. In contrast, CO2 adsorption in MIL-47 (V) experiences a homogeneous adsorption environment because there are no preferential adsorption sites.177 Maurin and co-workers have developed the mixture model method (also called “composite” method) to a system of CO2 adsorption in MIL-53(Al), demonstrating the feasibility of this method for exploring CO2 adsorption in flexible MOFs. They began with GCMC simulations to first obtain adsorption isotherms of CO2 at 300 K in both the NP and the LP forms of MIL-53(Al) with rigid structures. The “composite” isotherm can be obtained by following the curve in MIL-53np (Al) up to a transition pressure measured experimentally and then switching over to the MIL-53lp (Al) curve. The simulated isotherm reproduces the experimental characteristics well, only slightly overestimating the experimental values. Moreover, both

reveals that CO2 binding is governed by a combination of Hbonding, electrostatic, and dispersion interactions. Beyond introducing functional groups on organic linkers, Wang and coworkers demonstrated that CO2 adsorption capacities at room temperature were further improved by tetraethylenepentamine (TEPA) grafted on the CUMs of NH2-MIL-101, particularly with high concentrations of TEPA incorporation, where the CO2 binding affinity was enhanced due to the existence of abundant amine groups.171 2.1.2.5. Effects of Framework Topology on CO2 Adsorption. Zhong and co-workers151 have investigated the effects of surface area, pore size and topology, and electrostatic fields on the adsorption of CO2 in various MOFs. They found that a pore size between 1.0 and 2.0 nm is suitable to achieve high CO2 uptakes. For MOFs with pore sizes located in this range, the large accessible surface area and free volume generally lead to higher CO2 adsorption capacity at pressures resembling practical industrial conditions. The effects of pore size on CO2 adsorption capacities have been also explored by Snurr and coworkers using combined experimental−computational techniques. 172 They revealed the smaller pores benefit CO 2 adsorption at low pressures. The effect of framework topology on CO2 adsorption was further investigated by Laird and coworkers113 based on a series of ZIF with different topology but fixed metal ion and imidazolate functionality: ZIF-7, ZIF-11, ZIF-93, and ZIF-94 as model materials. The simulation isotherms for CO2 adsorption in the above MOFs reproduce reasonably experimental results, except ZIF-7 at higher pressure due to the presence of structural changes. They found that the topologies with the smaller pores (ZIF-7 and 94) have larger adsorptions than their counterparts (ZIF-11 and 93, respectively) at low pressures (25000 1818 140 770e 600e 327 314.7 294e 225e 200 148.1 98 94 87.7 75e 57.8 56i 31.5 30

2.34d

3.85 4.20 2.38 2.37

3.13 5.28

2.55 5.41 4.35 5.00 4.20

96 34.6 32.5 26 71 42 27.6 32

1.12 4.41 3.86 2.10

2.21 1.67 4.1

45

2.39

34.8h 25.9 42

1.82

3.84

CO2/CH4 3300 231 33

29.8 257e 5.8e

13.8 9.0

temp (K)

ref

293 298 298 298 298 298 298 296 298 298 298 313 298 298 296 298 296 303 298 298

296 295 21 21 196 196 292 184 298 299 186 198 300 301 184 285 184 289 291 302

a

Selectivity obtained from IAST calculations unless specially noted. bCalculated from 195 K CO2 isotherm. cCalculated from 298 K CO2 isotherm. CO2 uptake at 0.1 bar. eMolar selectivity. fLangmuir surface area. gTheoretically accessible surface area. hBased on real coadsorption experiment using a 15/85 CO2/N2 gas mixture at 303 K and 1.0 bar. iBased on microcalorimetry experiments. d

mixture with a composition of 21% O2 and 79% N2 show that the nitrogen broke through the column immediately after injection (1.8 H2: 1.45 H2: 1.3 H2: 266 H2: 210 H2: 20.14 CO2: 0.0977 H2: 20.5 H2: 201 CO2: 0.344 CO2: 0.344 H2: 3.62b H2: 3.58b H2: 140.5 N2: 38.1 CO: 35.9 CH4: 51.7 H2: 74.3 CO2: ∼0.236 CO2: ∼1.034 CO2: ∼0.236 CO2: ∼1.031 CO2: ∼0.236 CO2: ∼1.023 H2: ∼1.1 H2: 0.97 CO2: 1.06b

CO2/CH4

CO2: 1.06b

1.5a

H2/CO2

H2: 1.91b

1.8a

H2/CO2 H2/CO2: 50/50 H2/CO2

H2: 3.08 H2: 2.91 H2: 2.96

7.51a 7.20 21.6a

H2/CO2: 50/50 H2/CO2 CO2/CH4 CO2/CH4: 50/50 H2/CO2 H2/CO2: 50/50 H2/CO2 H2/CO2: 50/50 H2/CO2: 50/50 H2/CO2 H2/CO2: 50/50

H2: 2.82 H2: 2.85 CO2: 0.135 CO2: 0.126 H2: 24.6 H2: 19.6 H2: ∼0.63 H2: ∼0.58 H2: 9.0b H2: 39.5 H2: 26.5 9713

20.1 21a 3.7a 4.7 34.9a 25.7 77a 72 291 16.9a 12.1

498 498 498 498 598 598 298 298 298 453 453

K, K, K, K, K, K, K, K, K, K, K,

1 1 1 1 1 1 1 1 1 1 1

bar bar bar bar bar bar bar bar bar bar bar

support

ref

PVDF PVDF

382 383

PSF

384

PVDF PVDF Al2O3

383

Al2O3

390

stainless steel nets

391

PVDF Al2O3 BPPO PVDF-TiO2 AAO

382 392 395 396 397

SiO2

444

Al2O3

445

Al2O3 Al2O3

446 447

ZnO

406

Torlon

399

Al2O3

400

389

Al2O3

Al2O3

401

Al2O3

403

Al2O3

404

Al2O3 Al2O3

385 431

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Table 7. continued MOF

gas pair

[COF-300][Zn2(bdc)2(dabco)]

gas permeance (10−7 mol m−2 s−1 Pa−1)

selectivity

testing conditions

H2/CO2

H2: 4.65b

13.3a

298 K, 1 bar

H2/CO2: 50/50

H2: 4.42b

12.6

298 K, 1 bar

support SiO2

ref 444

Ideal separation factor. Permeability is reported in barrer in the references and converted to 10−7 mol m−2 s−1 Pa−1 in this table by 1 GPU = 3.35 × 10−10 mol m−2 s−1 Pa−1. a

b

4.2. Experimental Exploration

length of about 600 nm and a uniform thickness of 1.12 nm, which corresponds to the thickness of a monolayer of Zn2(bim)4. The membrane was obtained by dropwise depositing a colloidal dispersion of the Zn2(bim)4 nanosheets onto the surface of a hot (120 °C) α-Al2O3 disk (Figure 20). The obtained membrane achieved a H2 permeance of 9.0 × 10−7 mol m−2 s−1 Pa−1 (2700 GPU, 1 GPU = 3.35 × 10−10 mol m−2 s−1 Pa−1) with a H2/CO2 selectivity of 291. This H2/CO2 separation performance represents the highest result for all reported membranes. The authors also demonstrated a high thermal and hydrothermal stability of the Zn2(bim)4 nanosheet membrane. Because of its ultramicroporous structure, thermal, and chemical stability, ZIF-8 is one of the most studied membrane materials for gas separation.386−388 Huang and co-workers reported a highly reproducible and permselective ZIF-8 membrane on Al2O3 support by using polydopamine (PDA) as a novel covalent linker.389 It was believed that dopamine could attract and bound the ZIF-8 nutrients to the substrate surface, promoting the heterogeneous nucleation of ZIF-8 crystals. As shown in Figure 21, a uniform and well intergrown ZIF-8 layer can be obtained by this synthesis strategy. The prepared ZIF-8 membrane displayed high gas selectivity and thermal stability. At 423 K and 1 bar, the gas mixture separation factor of H2/CO2 was 8.9 with a high H2 permeance of 1.8 × 10−7 mol m−2 s−1 Pa−1. Later, the same group developed a bicontinuous ZIF-8@GO membrane on PDA-modified Al2O3 support, through layer-by-layer deposition of a graphene oxide (GO) suspension on a semicontinuous ZIF-8 layer.390 The gaps between ZIF-8 crystals were effectively sealed by the deposited GO layer, which significantly improved the gas separation performance of the membrane. At 523 K and 1 bar, the mixture separation factor of H2/CO2 was 14.9 with a H2 permeance of 1.3 × 10−7 mol m−2 s−1 Pa−1. Huang and co-workers also prepared a ZIF-8 membrane on a polydopamine-functionalized stainless steel net.391 The macroporous stainless steel net is much thinner and has a higher void volume than the Al2O3 support. The resulted ZIF-8 membrane exhibited an exceptionally high H2 permeance of 210 × 10−7 mol m−2 s−1 Pa−1, while maintaining a high H2/CO2 selectivity of 8.1 at 373 K and 1 bar. Liu, Caro, and co-workers reported a ZIF-8-ZnAl-NO3 layered double hydroxide (LDH) composite membrane on γAl2O3 support by partial conversion of the LDH membrane to ZIF-8.392 LDHs consisted of regularly arranged, positively charged brucite-like 2D layers and charge-compensating anions in interlayer galleries, which exhibit size-based selectivity when their gallery height is comparable with the kinetic diameters of gas molecules.393,394 The 2D ZnAl-NO3 LDH membrane served as metal source, reacted with 2-methylimidazole (2mIm), forming a well-intergrown ZIF-8 layer firmly attached to the remaining ZnAl-NO3 LDH membrane. The ZIF-8-ZnAlNO3 LDH composite membrane shows high gas separation performance. The gas mixture separation factor for CO2/CH4 of the membrane reaches 12.9, which largely exceeded the

In this section, we summarize recent experimental works on MOF-based membranes, which are projected for gas separations in hydrogen recovery, natural gas purification, and CO2 capture in flue gas. Some gas separation results for pure MOF membranes and MOF-based MMMs are listed in Tables 7 and 8, respectively. 4.2.1. Pure MOF Membranes for CO2 Capture. ZIFs constructed from tetrahedral metal ions bridged by imidazolates are a subclass of MOFs, which combine highly desirable properties from MOFs and zeolites, such as designable structures, high thermal, and chemical stability. Most reported MOF-based membranes are fabricated with this type of MOF materials. ZIF-7 is formed by bridging benzimizazolate (bim) ligand and Zn(II) centers with the sod topology.381 The pore window size of ZIF-7 is about 0.3 nm, which is between the kinetic diameters of H2 (0.29 nm) and CO2 (0.33 nm). ZIF-7 membranes are thus expected to display a good molecular sieving effect for the separation of H2 and CO2. Zhang and coworkers fabricated a ZIF-7 layer on a polyvinylidene fluoride (PVDF) hollow fiber membrane, which was modified by a ZnO array.382 A ZnO array was directly grown on a PVDF hollow fiber membrane, which proved to be an outstanding buffering layer for the synthesis of crack-free and uniform ZIF-7 layer without any activation procedure. The prepared ZIF-7 membrane exhibits an ideal H2/CO2 selectivity of 18.43, which far exceeds the Knudsen separation factor (4.7), and the permeance of H2 reaches 23.54 × 10−7 mol m−2 s−1 Pa−1. The same group later developed a new ammoniation-based chemical modification method for the synthesis of a continuous and uniform MOF layer on PVDF hollow fiber membrane.383 It was suggested that the modification of PVDF hollow fiber by ethylenediamine increases the solvent resistance of the PVDF membrane and provides dense reactive sites for heterogeneous nucleation of ZIF-7 crystals. The resulted ZIF-7 membrane shows high H2/CO2 selectivity (16.32 for single-component gases, 15.52 for gas mixture) at 298 K and 1 bar. Coronas and co-workers also fabricated a ZIF-7 membrane on the inner surface of a polysulfone (PSF) hollow fiber using a microfluidics approach.384 The ZIF-7 membrane displays a high separation factor of CO2/CH4 (13.5) and of CO2/N2 (13.6) in binary gas permeation tests, which are the highest results for ZIF-7 membranes. However, the CO2 permeance of the membrane is relatively low (0.0076 × 10−7 mol m−2 s−1 Pa−1). Li, Yang, and co-workers prepared ultrathin membranes (several nanometers thick) on α-Al2O3 support with 1nanometer-thick sheets of a isomer of ZIF-7, Zn2(bim)4, which is a 2D layer structure with about 0.21 nm aperture size.385 A soft-physical process was developed for exfoliation of the MOF nanosheets, where pristine Zn2(bim)4 crystals were first wet ball-milled at very low speed (60 rpm), and then ultrasonicated in a mixture of methanol and propanol for exfoliation. The obtained Zn2(bim)4 nanosheet has a side 9714

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Table 8. Selected Gas Separation Results Related to CO2 Capture for MOF-Based MMMs dispersion phase

continuous phase

loading (wt %)

gas separation

NH2-MIL-125 NH2-MIL-125(Ti)

Matrimid PSF

NH2-MIL-125(Ti)

PSF

10

NH2-MIL-125(Ti)

PSF

20

NH2-MIL-125(Ti)

PSF

30

PEI@MIL-101(Cr) PEI@MIL-101(Cr) PEI@MIL-101(Cr) PEI@MIL-101(Cr) TSIL@NH2-MIL-101 (Cr) NH2-MIL-53(Al)

SPEEK SPEEK SPEEK SPEEK PIM-1

0 0 40 40 5

CO2/CH4: 50 CO2/CH4: 50 CO2/CH4: 50 CO2/CH4: 50 CO2/CH4 CO2/N2 CO2/CH4 CO2/N2 CO2/N2

10

CO2/CH4

MIL-53(Al) MIL-53(Al) NH2-MIL-53(Al)

6FDA-DAM-HAB (FDH-11) Matrimid Matrimid Matrimid 5218

MIL-53 MIL-53-as MIL-53-ht NH2-MIL-53(Al)

Matrimid Matrimid Matrimid 6FDA-DAM

0 37.5 37.5 20

NP-NH2-MIL-53(Al)

6FDA-DAM

20

NH2-MIL-53(Al)

PSF

25

NH2-MIL-101(Al)

PSF

25

NH2-MIL-53(Al)

Matrimid 5218

20

NH2-MIL-101(Al)

Matrimid 5218

8

NH2-UiO-66

Matrimid 9725

0

NH2-UiO-66

Matrimid 9725

30

NH2-UiO-66-BA

Matrimid 9725

30

NH2-UiO-66-ABA

Matrimid 9725

30

UiO-66

Matrimid 9725

30

UiO-66-BA

Matrimid 9725

30

UiO-66-ABA

Matrimid 9725

30

0

0 15 25

NH2-UiO-66 NH2-UiO-66 NH2-UiO-66-PA NH2-UiO-66 NH2-UiO-66 NH2-UiO-66 NH2-UiO-66 nZIF-11 nZIF-11 ZIF-8(M) ZIF-8(M) ZIF-8

Matrimid 5218 Matrimid 5218 Matrimid 5218 PSF PSF PSF PSF Matrimid Matrimid SEBS SEBS Ultem 1000

0 23 23 0 40 0 40 25 15 30 30 13

ZIF-8

PBI

30

gas permeance (barrer)

selectivity

testing conditions

substrate

ref

50/

CO2: 9.5

22.0

303 K, 3.0 bar

443 491

50/

CO2: 18.5

28.3

303 K, 3.0 bar

491

50/

CO2: 29.3

29.5

303 K, 3.0 bar

491

50/

CO2: 40.0

29.2

303 K, 3.0 bar

491

CO2: CO2: CO2: CO2: CO2:

545 545 2490 2490 2979

24.7a 36.0a 71.8a 80.0a 37

298 K, 1.0 bar 298 K, 1.0 bar 298 K, 1.0 bar 298 K, 1.0 bar 298 K, 3.0 bar

455 455 455 455 456

CO2: 47.1

78.5a

451

28.2 51.8a ∼40

308 K, 150 psi 308 K, 3.0 bar 308 K, 3.0 bar 308 K, 3.0 bar

448 448 450

39.4a 90.1a 47.0a 26.3

308 K, 2.0 bar 308 K, 2.0 bar 308 K, 2.0 bar 298 K, 1.0 bar

492 492 492 452

CO2: 660

28

298 K, 3.0 bar

452

CO2: ∼5.6

∼27.5

308 K, 3.0 bar

454

CO2: ∼8.8

∼29.0

308 K, 3.0 bar

454

CO2: ∼8.8

∼43.5

308 K, 3.0 bar

454

CO2: ∼10.1

∼35.0

308 K, 3.0 bar

454

CO2: 5.9

31.2

308 K, 9.0 bar

457

CO2: 17.8

37.3

308 K, 9.0 bar

457

CO2: 17.4

39.3

308 K, 9.0 bar

457

CO2: 37.9

47.7

308 K, 9.0 bar

457

CO2: 15.7

35.8

308 K, 9.0 bar

457

CO2: 17.8

42.9

308 K, 9.0 bar

457

CO2: 13.6

45.1

308 K, 9.0 bar

457

RT, 20 psia RT, 20 psia RT, 20 psia 308 K, 3.0 bar 308 K, 3.0 bar 308 K, 3.0 bar 308 K, 3.0 bar 308 K, 2.0 bar 473 K, 2.0 bar 308 K 308 K 308 K, 100 psia 503 K, 2 atm

458 458 458 459 459 459 459 467 467 460 460 493

CO2/CH4 CO2/CH4 CO2/CH4: 50/ 50 CO2/CH4 CO2/CH4 CO2/CH4 CO2/N2: 15/ 85 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/CH4: 50/ 50 CO2/N2 CO2/N2 CO2/N2 CO2/CH4 CO2/CH4 CO2/N2 CO2/N2 H2/CO2 50/50 H2/CO2 50/50 CO2/N2 CO2/CH4 CO2/N2 H2/CO2: 50/ 50

9715

CO2: 6.2 CO2: 12.43 CO2: ∼14.5 CO2: CO2: CO2: CO2:

8.4 40.0 51.0 737

CO2: ∼8.5 CO2: ∼24 CO2: ∼29 CO2: 5.6 CO2: 46 CO2: 5.6 CO2: 46 H2: 95.9 H2: 535 CO2: 454.6 CO2: 454.6 CO2: 26 GPU H2: 470.5

a

∼29 ∼36.5a 37.0a 26.0 24.0a 29.0a 26.0a 4.4 9.1 12.0a 5.4a 36a a

26.3

asymmetric hollow fiber

494

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Table 8. continued dispersion phase

continuous phase

loading (wt %)

gas separation

gas permeance (barrer)

selectivity

H2: 2014.8

12.3

CO2: 16.63 CO2: 16.63 CO2: ∼25

35.8a 19.0a ∼22 ∼52.5

35 35 15 15 15

H2/CO2: 50/ 50 CO2/CH4 CO2/N2 CO2/N2 CO2/CH4: 50/ 50 CO2/N2 CO2/CH4 CO2/N2 CO2/CH4 CO2/CH4

CO2: CO2: CO2: CO2: CO2:

32.3a 9.0a 83.9a 34.8a ∼38.5a

33.3

H2/CO2

H2: 283.4

ZIF-8

PBI

60

ZIF-8 ZIF-8 ZIF-8 ZIF-8

Matrimid Matrimid Matrimid Matrimid

5218 5218 5218 5218

20 20 10 30

ZIF-8 ZIF-8 [bmim][Tf2N]@ZIF-8 [bmim][Tf2N]@ZIF-8 ZIF-8-ambz-(30)

Pebax-2533 Pebax-2533 Pebax-1657 Pebax-1657 Matrimid 5218

ZIF-8

6FDA-durene

1293 1293 104.9 104.9 ∼10.4

303 K 303 K 298 K, 5 bar

PSF Udel-P3500

469 469 470

∼35.5a

298 K, 5 bar

PSF Udel-P3500

470

∼27

308 K, 5 bar

471

30a 13a 19.1a 78.7a 26.7a

308 RT 298 298 308

471 476 498 488 489

CO2/CH4 CO2/CH4 CO2/CH4 CO2/N2 H2/CO2

CO2: ∼3944 CO2: ∼2615 CO2: 37.8 CO2: 37.8 H2: 24.5

26.8 32.5a 44.8a 35.1a 25.0a

ZIF-90

PBI

45

H2: 226.9

13.3

sonicated HKUST-1 sonicated HKUST-1 HKUST-1

PPO PPO PDMS

10 10

H2/CO2: 50/ 50 CO2/N2 CO2/CH4 CO2/N2

HKUST-1

PDMS

Fe(BTC)

Matrimid 5218

30

Fe(BTC) CAU-1-NH2 Mg-MOF-74 ns-CuBDC [Cu2(ndc)2(dabco)]n

Matrimid 5218 PMMA PIM-1 PI PBI

30 15 20 8 20

a

CO2: 13.5 H2: 11000 CO2: 21000 CO2: 4.09 H2: 6.13

308 K, 5.0 bar

495 495 496 462

∼24a ∼28a ∼29.5a

20 20 10 10 45

CO2/CH4: 50/ 50 CO2/CH4 H2/CO2 CO2/CH4 CO2/CH4 H2/CO2

ref 494

468

12.0a

TOX-PIM-1 TOX-PIM-1 6FDA-ODA 6FDA-ODA PBI

CO2/CH4

substrate

503 K, 2 atm

RT, 6 bar RT, 6 bar 298 K, 1.0 bar 298 K, 1.0 bar 308 K, 3.45 bar 308 K, 3.5 atm 295 K, 4 bar 295 K, 4 bar 298 K, 2 atm 298 K, 2 atm 308 K, 3.5 atm 453 K, 7 atm

ZIF-8 SiO2 Cd-6F Cd-6F ZIF-90

CO2: ∼89 CO2: ∼89 CO2: ∼109.2 GPU CO2: ∼109.2 GPU CO2: ∼12.5

testing conditions

a

K, 5 bar K, 2 bar K, 3 bar K, 5 bar

463 463 464 464 465 497 466 466 475 475 468

Ideal selectivity.

Figure 20. (A) SEM image of a bare porous α-Al2O3 support. (B) SEM top view and (C) cross-sectional view of a Zn2(bim)4 nanosheet layer on αAl2O3 support. Adapted with permission from ref 385. Copyright 2014 Science.

11.98 at 298 K and 1 bar. The ZIF-8 film deposited on ZnO array modified PVDF membrane exhibits an ideal H2/CO2 selectivity of 16.29 with a H2 permeance of 20.14 × 10−7 mol m−2 s−1 Pa−1 at 298 K and 1 bar. Wang and co-workers prepared a ZIF-8 membrane with a thickness of around 200 nm on bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) substrate modified by ethylenediamine vapor. Treatment of the substrate with ethylenediamine vapor not only reduced the pore size of the substrate but also enriched the

corresponding Knudsen values (0.6), and the CO2 permeance is 0.0977 × 10−7 mol m−2 s−1 Pa−1. Zhang and co-workers reported the fabrication of ZIF-8 membrane on ZnO array modified PVDF membrane and ammoniated PVDF membrane (as mentioned above for ZIF7).382,383 The resulted ZIF-8 membranes show high selectivities of H2/CO2 and high H2 permeances. The ZIF-8 layer prepared on ammoniated PVDF membrane shows a high H2 permeance of 19.03 × 10−7 mol m−2 s−1 Pa−1 with a H2/CO2 selectivity of 9716

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Figure 21. Top view (a) and cross-section (b) SEM images of the ZIF-8 membrane prepared on PDA-modified porous Al2O3 disk. Adapted with permission from ref 389. Copyright 2013 American Chemical Society.

synthetic functionalization of ZIF-90 membranes with organosilica APTES by an imine condensation reaction between the free aldehyde groups of ZIF-90 and the amino groups of APTES was an effective method to minimize the intercrystalline defects of the membrane.400,401 After APTES modification, the separation performances of the ZIF-90 membrane were remarkably enhanced. The separation factor of CO2/CH4 for the APTES modified ZIF-90 membrane reaches 4.7 with a CO2 permeance of 0.126 × 10−7 mol m−2 s−1 Pa−1 for the separation of equimolar CO2/CH4 mixture at 498 K and 1 bar. ZIF-95 and ZIF-100 with previously unknown topological structures are highly thermally stable (up to 500 °C) with a high adsorption affinity and capacity for CO2.402 Huang, Caro, and co-workers developed a highly permeable and selective ZIF-95 membrane on APTES-modified macroporous α-Al2O3 disk.403 At 598 K and 1 bar, the obtained ZIF-95 membrane shows a H2/CO2 gas mixture separation factor of 25.7 with a H2 permeance of 19.6 × 10−7 mol m−2 s−1 Pa−1. The ZIF-95 membrane retains its H2/CO2 separation performance when the tested gas mixture is mixed with water vapor, displaying a hydrothermal stability of the membrane. Wang, Huang, and coworkers fabricated a ZIF-100 membrane on PDA-modified macroporous α-Al2O3 support.404 The single gas permeance of H2 at room temperature for the ZIF-100 membrane is 6.3 × 10−7 mol m−2 s−1 Pa−1, and the single gas permeances follow the order H2 > N2 > CH4 > CO2. For binary mixtures, the separation factor of H2/CO2 reaches 72 at room temperature and 1 bar. It was believed that CO2 can be retained in the pore structure of ZIF-100, while other gas molecules easily permeate the membrane because of the high CO2 adsorption affinity and small window aperture (0.335 nm) of ZIF-100. ZIF-78 is a GME topological structure with a cage size of 0.71 nm and a pore window size of 0.38 nm.405 Jin and coworkers fabricated a ZIF-78 membrane on porous ZnO support by the reactive seeding method.406 The authors used a new solvent activation-exchange method to activate the ZIF-78 layer, eliminating formation of the macroscopic defects and the intercrystalline gaps in the membrane. The ideal selectivity factor of H2/CO2 is 11.0 for the ZIF-78 membrane at 298 K. The low permeance of CO2 relative to that of H2 was attributed to the strong interaction between CO2 and ZIF-78 frameworks. At 298 K and 1 bar, the binary gas mixture separation factor of H2/CO2 was found to be 9.5, with a H2 permeance of 0.97 × 10−7 mol m−2 s−1 Pa−1. Eddaoudi and co-workers reported a unique subset of MOFs topologically related to pure inorganic zeolites, zeolite-like metal−organic frameworks (ZMOFs), which exhibit accessible extra-large cavities, chemical stability, and cation exchange

heterogeneous nucleation density of ZIF-8 crystals. H2 permeance of the obtained ZIF-8 membrane reaches a high value of 20.5 × 10−7 mol m−2 s−1 Pa−1, and the ideal separation factor for H2/CO2 was calculated to be 12.8. The authors claimed that the high H2 permeance is attributed to the ultrathin ZIF-8 layer and the highly porous structure of the BPPO support.395 More recently, ultrathin, continuous ZIF-8 membranes were fabricated by Chen et al. on APTES-functionalized (APTES = 3-aminopropyltriethoxysilane) TiO2-coated PVDF hollow-fiber membranes by using a facile immersion technique.396 APTESfunctionalized TiO2 nanoparticles provide reactive sites with Zn2+ ions to form a complex metal cation by the terminal amine groups, which promote the nucleation density of ZIF-8 on the polymer membrane surface. The prepared ZIF-8 membranes exhibited an ideal H2/CO2 selectivity of 7 and an ultrahigh H2 permeance of 201 × 10−7 mol m−2 s−1 Pa−1. Wang and coworkers prepared a ZIF-8/GO membrane with a thickness of 100 nm on anodic aluminum oxide (AAO) support by using 2D ZIF-8/GO hybrid nanosheets as seeds.397 The 2D ZIF-8/ GO hybrid seeds were prepared as a sandwich-like structure with ZIF-8 nanoparticles grown on both sides of the GO nanosheet surface. AAO support then was seeded by the ZIF-8/ GO/ZIF-8 nanosheets by spin-coating. The secondary growth of the ZIF-8/GO membrane was carried out with the contradiffusion method (also known as the interfacial synthesis). The authors believed that this special seeding method provides a 2D confined space and uniform coating of ZIF-8 nanocrystals to facilitate fast crystal intergrowth. However, crystal overgrowth and defects were effectively avoided during the contra diffusion process because the seeding layer acted as a barrier between two different synthesis solutions, which led to a defect-free and ultrathin membrane. At 298 K and 1 bar, the ZIF-8/GO membrane shows high CO2/N2 (7.0) and CO2/CH4 (7.1) ideal selectivities with a CO2 permeance of 0.344 × 10−7 mol m−2 s−1 Pa−1. ZIF-90 is an analogue of ZIF-8 with a narrow pore window of 0.35 nm and free aldehyde groups, which could be postmodified by an imine condensation reaction with organic molecules with amine groups.315,398 Nair and co-workers fabricated continuous ZIF-90 membranes on a poly(amide− imide) (Torlon) macroporous hollow fiber outer surface by the secondary seeded growth method.399 The ZIF-90/Torlon membranes exhibit good separation properties for linear over cyclic hydrocarbons, but the CO2/N2 (3.5) and CO2/CH4 (1.5) ideal selectivities from single-gas permeation tests are only slightly above the corresponding Knudsen selectivities (0.8 and 0.6). Huang, Caro, and co-workers found that post9717

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Figure 22. (a) SEM image of Ni foam; (b) SEM image of Ni3S2 nanoarrays on treated Ni foam; (c) SEM image of Cu/Ni3S2 microstructure; and (d) HKUST-1 membrane on the modified Ni foam. Adapted with permission from ref 413. Copyright 2016 Wiley-VCH.

capability.407 They also fabricated a continuous sod-ZMOF-1 membrane on a porous alumina substrate.408 The sod-ZMOF-1 has a sodalite topological anionic framework with extraframework cations in pore. The obtained sod-ZMOF-1 membrane shows permselectivity for CO2 over many other gases. The ideal selectivities of CO2/He, CO2/H2, CO2/N2, CO2/O2, and CO2/CH4 are 3.2, 2.6, 8.7, 5.1, and 3.6, respectively, with a low CO2 permeance of 0.0063 × 10−7 mol m−2 s−1 Pa−1. The mixture separation factor of CO2/CH4 is 4.0 with a CO2 permeance of 0.0049 × 10−7 mol m−2 s−1 Pa−1 at 308 K and 1 bar. The authors believed that the anionic character of the sod-ZMOF-1 framework prone to interactions with the CO2 quadrupole and its small pore window size (0.41 nm) account for the observed CO2 permselectivity. Besides ZIFs and ZMOFs, some metal-carboxylate-type MOFs have also been investigated in membrane fabrication and separation applications. Peng and co-workers reported a pressure-assisted room-temperature growth strategy to fabricate continuous and well-intergrown HKUST-1 films on a PVDF hollow fiber.409 One end of the PVDF hollow fiber was connected to the pump, while the other side was sealed by Teflon tape. Copper hydroxide nanostrands (CHNs) were filtered onto the PVDF hollow fiber under vacuum and acted as a copper source for HKUST-1 synthesis. The PVDF hollow fiber with a CHN layer was then placed in a solution of ligand for reaction under room temperature for about 40 min, which resulted in a continuous HKUST-1 layer firmly adhered on the PVDF hollow fiber surface. The ideal separation factor of H2/ CO2 for the HKUST-1 membrane from single component gas permeation tests is 4.2, which is close to the corresponding Knudsen selectivity. The result was explained arguing that the pore size of HKUST-1 (0.9 nm) is much larger than the kinetic diameters of H2 (0.29 nm) or CO2 (0.33 nm). However, the separation factor of H2/CO2 for 50:50 binary gases is 8.1 with a H2 permeance of 19.7 × 10−7 mol m−2 s−1 Pa−1. The authors attributed this selectivity to sorption and size effect. The research group also prepared a free-standing HKUST-1 membrane,410 and HKUST-1 membranes on other sup-

ports.411,412 Li and co-workers developed a facile and versatile ONACF (oriented nanomicrostructure assisted controllable fabrication) approach to fabricate some MOF membranes.413 By this approach, a HKUST-1 membrane was in situ prepared on oriented Ni3S2 nanomicrostructure arrays modified Ni foam. During the in situ membrane growth process, a new phase, Cu, was formed in the early stage on the Ni3S2 nanoarrays (Figure 22), which was believed to play a key role in the fabrication of continuous HKUST-1 layer. For gas mixture permeation tests, H2 permeance of the HKUST-1 membrane reaches 27.2 × 10−7 mol m−2 s−1 Pa−1 with a H2/CO2 selectivity of 6.8. Zhang, Meng, and co-workers developed a well intergrown HKUST-1 membrane on stiff polyacrylonitrile (PAN) hollow fibers by a chemical modification technique.414 The support was premodified by hydrolyzing −CN groups of PAN into −COO− groups, which provides nucleation sites for growing crystals into a compact layer and improves the adhesion between the MOF layer and the PAN hollow fiber support. The prepared membrane shows an ultrahigh H2 permeance of 705 × 10−7 mol m−2 s−1 Pa−1 with a H2/CO2 separation factor of 7.14 for binary mixtures as well as high thermal and pressure stability. The authors also prepared a continuous HKUST-1 membrane on an ammoniated PVDF membrane.383 The obtained HKUST-1 membrane shows a high H2 permeance of 84.60 × 10−7 mol m−2 s−1 Pa−1 and an ideal H2/CO2 selectivity of 7.3 from single-component gas permeation tests at 298 K and 1 bar. The mixture separation factor of H2/CO2 was measured to be 7.05 with a H2 permeance of 60.12 × 10−7 mol m−2 s−1 Pa−1 at 298 K and 1 bar. The same research teams also developed a novel trinity HKUST-1-based membrane.415 A continuous HKUST-1 membrane was formed on polysulfone hollow fiber membrane, which was precovered with a polydimethylsiloxane (PDMS)/HKUST-1 layer. The authors proposed that the mixed PDMS/HKUST-1 layer has four main purposes: (1) serving as seeds for the growth of a continuous and well-intergrown MOF layer, (2) enhancing the adhesion of the MOF layer, (3) increasing the capacity of gas separation, and (4) decreasing the mass transfer resistance as compared to 9718

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Figure 23. SEM image of the surface and cross-section of the CAU-1 membrane (a,b) and EDS mapping of the CAU-1 membrane: green, C; red, Al (c,d). Adapted with permission from ref 423. Copyright 2014 Royal Society of Chemistry.

preferential CO2 adsorption over CH4. Later, the research group also prepared cobalt-adeninate MOF (bio-MOF-13 and bio-MOF-14) membranes on tubular alumina porous supports via secondary seeded growth.420 Bio-MOF-13 and bio-MOF-14 both display high CO2 adsorption capacities, and their pore sizes are close to the size of CO2 and CH4 molecules, making them highly appealing for CO2/CH4 molecular gas separation.421 The obtained bio-MOF-13 and bio-MOF-14 membranes show CO2 permeances in range of (31−46) × 10−7 mol m−2 s−1 Pa−1 with CO2/CH4 separation selectivities in the range of 3.1−4.8 at 295 K and 1.38 bar. CAU-1 constructed of Al(III) metal centers and 2-amino-1,4benzenedicarboxylic acid is an amino-functionalized MOF, which is particularly interesting for CO2 capture due to the favorable acid−base interaction between CO2 and the amino group.422 Yang and co-workers prepared a CAU-1 membrane on asymmetric α-Al2O3 tube by the secondary seeded growth method (Figure 23).423 For the single gas permeation, the permeance of CO2 for the CAU-1 membrane is 13.2 × 10−7 mol m−2 s−1 Pa−1, which is higher than those of H2, CH4, N2, and SF6. The ideal selectivities of CO2/H2, CO2/N2, CO2/CH4, and CO2/SF6 systems are 2.6, 26.2, 14.8, and 79.3, respectively, indicating that the permeation of CO2 through the CAU-1 membrane is governed by preferential adsorption. The authors further evaluated the separation performance of the CAU-1 membrane for CO2/N2 mixtures at various CO2 concentrations. At a CO2 molar fraction of 0.1−0.2, which is the flue gas composition, the CO2 permeance is (5.0−5.7) × 10−7 mol m−2 s−1 Pa−1 with the CO2/N2 separation factor of about 17.4. At a high CO2 molar fraction of 0.9, a CO2 permeance of 13.4 × 10−7 mol m−2 s−1 Pa−1 and a CO2/N2 separation factor of 22.8 were observed. Zhu, Zou, and co-workers also prepared a CAU-1 membrane on an α-Al2O3 hollow ceramic fiber by the

entire MMMs or pure polymer layers. The ideal separation factors of H2/CO2 and N2/CO2 for the trinity HKUST-1 membrane reach 21.03 and 7.23, respectively, with higher permeances of H2 (4.85 × 10−7 mol m−2 s−1 Pa−1) and N2 (1.66 × 10−7 mol m−2 s−1 Pa−1). Zhu and co-workers prepared continuous NH2-MIL-53(Al) membranes on macroporous glass frit discs with colloidal seeds.416 At 288 K and 1 atm, single-component gas permeances for the NH2-MIL-53(Al) membrane are in the order of H2 > CH4 > N2 > CO2. The H2 permeance is 26.71 × 10−7 mol m−2 s−1 Pa−1, and the H2/CO2 ideal selectivity is 27.3. The mixture separation factor of H2/CO2 is 30.9 with a H2 permeance of 15 × 10−7 mol m−2 s−1 Pa−1. Zhang, Meng, and co-workers prepared a NH 2 -MIL-53(Al) layer on an ammoniated PVDF hollow fiber.417 The obtained NH2-MIL53(Al) membrane is well intergrown with a thickness of about 8 μm, and exhibits a H2 permeance of 54.2 × 10−7 mol m−2 s−1 Pa−1 with an ideal H2/CO2 separation factor of 30.37. The membrane could also efficiently separate the H2/CO2 mixture with a H2 permeance of 42.13 × 10−7 mol m−2 s−1 Pa−1 and a separation factor of 32.35, which represents the highest H2/ CO2 separation factors for reported polymer supported MOF membranes. Bio-MOF-1 constructed from infinite zinc-adeninate columnar units and biphenyldicarboxylate linkers is an attractive membrane material for CO2 capture.418 Carreon and coworkers prepared a Bio-MOF-1 membrane on a porous stainless steel tube by the secondary seeded growth method.419 The obtained Bio-MOF-1 membrane shows a high CO2 permeance (42.13 × 10−7 mol m−2 s−1 Pa−1) with a CO2/ CH4 mixture separation factor of 2.5 at 298 K and 1.38 bar. The observed CO2/CH4 selectivities are above the Knudsen selectivity, indicating that the separation is promoted by 9719

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10−7 mol m−2 s−1 Pa−1 from the single-component gas permeation tests. The mixture separation factor of H2/CO2 achieved 38.7. The JUC-150 membrane also displays high thermal and mechanical stability and reusability. Jin and co-workers synthesized an intergrown [Ni2(mal)2(bpy)] (Ni-MB) homochiral membrane on αAl2O3 by the secondary seeded growth method, in which high energy ball milling was employed to prepare nanosized MOF seeds.433 At 298 K and 1 bar, the single gas permeances of H2, N2, CH4, and CO2 for the Ni-MB membrane are 1.549, 0.481, 0.446, and 0.017 × 10−7 mol m−2 s−1 Pa−1, respectively. The ideal selectivities of H2/CO2, N2/CO2, and CH4/CO2 were calculated to be 89, 27, and 25, respectively. Strong interactions between CO2 and Ni-MB framework were suggested to be responsible for this permselectivity. Because of the tunable pore sizes, low cost, and facile synthesis, Cu(bipy)2(SiF6) and its analogues are promising candidates to fabricate membranes for gas separation.21,434 Sun, Zhu, and co-workers explored a new route and obtained a continuous Cu(bipy)2(SiF6) membrane on a macroporous glass-frit disk.435 In this route, the glass-frit disk used as a substrate was first modified by (NH4)2SiF6, in which the SiF62− was expected to be integrated into the substructures of the substrate surface. The Cu(bipy)2(SiF6) membrane on the modified substrate was formed after a solvothermal reaction without a SiF62− source. The Cu(bipy)2(SiF6) membrane shows a H2/CO2 separation factor of 8.0 at 293 K and 1 bar with a H2 permeance of 2.7 × 10−7 mol m−2 s−1 Pa−1. 4.2.2. MOF-Based Mixed Matrix Membranes for CO2 Capture. The MOF-based hybrid membrane or mixed matrix membranes have also attracted considerable research interest for CO2 capture besides pure MOF membranes. MOF-based MMMs are obtained from mixing polymers and MOF particles, which usually exhibit advantageous mechanical properties and superior separation performances, benefiting from combining advantages of both organic and inorganic materials. Remarkable advances for this type of hybrid materials have been made in the past years, some of which show high separation performance for CO2 capture. MIL-53(Al), an important member of the MILs family, has a good chemical stability as well as high porosity. Especially, this MOF has a special breathing character upon varying pressures of CO2, which is desirable in CO2 storage or separation. This MOF has been employed as a filler to fabricate MMMs for CO2 capture or separation by several research groups. For example, Omidkhah and co-workers fabricated Matrimid/MIL-53(Al) membranes and investigated their potential for CO2/CH4 separation.448 The separation performance was evaluated by the permeability of pure CO2 and CH4 gases for all membranes, and the ideal CO2/CH4 selectivities were calculated. The results show that CH4 permeabilities of the membranes slightly increase with elevating the loading of MIL-53(Al). However, at 20 wt % loading, a void forms, which results in a significant increase of CH4 permeability (300% over pure Matrimid). The permeability of CO2 also exhibits the same trend. A 94% increase in permeability as compared to pure Matrimid was observed when the MOF loading was at 15 wt % in the MMMs. When the loading of MIL-53(Al) is 15 wt %, the highest CO2/ CH4 selectivity of 51.8 is achieved with a CO2 permeability of 12.43 barrer. Clearly, the pore sizes of MOFs have an important effect on the gas separation peformances of the resulted MMMs. Musselman and co-workers investigated the effect of pore

secondary seeded growth method.424 In contrast, the obtained CAU-1 membrane shows preferential permeance of H2 over CO2, N2, and CH4. The mixture separation factor of H2/CO2 is 12.34 with a H2 permeance of 1.08 × 10−7 mol m−2 s−1 Pa−1 at 298 K. Li, Yang, and co-workers lately prepared a CAU-10-H membrane with a thickness of about 6 μm on α-Al2O3 disc by in situ solvothermal synthesis.425 CAU-10-H is built from Al(III) metal centers, and the V-shaped linker 1,3-benzenedicarboxylate exhibits an excellent hydrothermal stability.426,427 The obtained CAU-10-H membrane shows a H2/CO2 mixture separation factor of 10.5, and a long-term hydrothermal stability. The effect of feed pressure and temperature on the membrane performance was investigated for the ternary mixture of 41:41:18 H2/CO2/H2O. Overall, the maximum H2/CO2 and H2/H2O mixed gas separation factors were 11.1 and 5.67, respectively, with a H2 permeance of 0.153 × 10−7 mol m−2 s−1 Pa−1 at 423 K and 1 bar. It is well-known that M-MOF-74 materials exhibit high CO2 adsorption capacity and selectivity due to their uncoordinated metal centers.31,198 Kim and co-workers prepared a Ni-MOF-74 membrane on α-Al2O3 support via a layer-by-layer seeding technique followed by secondary growth crystallization.428 The ideal selective factors of H2/CO2, CH4/CO2, and N2/CO2 for the Ni-MOF-74 membrane are 9.1, 3.2, and 3.0, respectively. The result was explained by arguing that Ni-MOF-74 exhibits stronger adsorption affinity to CO2 than other gases, and thus the permeation of CO2 through the Ni-MOF-74 membrane is dominated by surface diffusion. Caro, Huang, and co-workers used magnesium oxide as seeds to synthesize a Mg-MOF-74 membrane.429 After postmodification of the Mg-MOF-74 membrane with ethylenediamine, the separation performance of the Mg-MOF-74 membrane could be enhanced. The H2/ CO2 mixture selectivity increased from 10.5 to 28 at room temperature. It was believed that the modification of the MgMOF-74 membrane with ethylenediamine enhanced the CO2 adsorption affinity of the MOF membrane, and in turn reduced the permeance of CO2. Lin and co-workers fabricated a MOF-5 membrane on porous α-Al2O3 disks by the secondary seeded growth method.430 The MOF-5 membrane shows selective permeation for CO2 over H2 or N2 with a separation factor of close to 5 for CO2/H2 and a separation factor greater than 60 for CO2/N2 at 298 K. Huang and co-workers prepared a highly hydrophobic and permselective Zn(BDC)(TED)0.5 membrane for H2/CO2 separation through the secondary seeded growth method.431 For the separation of an equimolar H2/CO2 mixture, the H2 permeance of the membrane is 27 × 10−7 mol m−2 s−1 Pa−1 and the selectivity of H2/CO2 was 12.1. It was believed that the Zn(BDC)(TED)0.5 membrane exhibits a H2/CO2 permselectivity because of the preferential adsorption affinity and capacity to CO2 as well as a highly porous structure with large channels of the MOF materials. A high thermal and hydrothermal stability of the Zn(BDC)(TED)0.5 membrane was also demonstrated. Tunable pore size is one of the most important advantages for MOF membranes in gas separation. Xue, Qiu, and coworkers fabricated an ultramicroporous JUC-150 membrane (0.25 nm × 0.45 nm) on a nickel screen by the secondary seeded growth method.432 The JUC-150 membrane shows preferential permeance of H2 relative to other gases based on size-sieving effect. It was claimed that only H2 could pass through the JUC-150 membrane with a permeance of 2.96 × 9720

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Figure 24. TEM micrographs of (a) NH2-MIL-53(Al) nanoparticles, (b) NH2-MIL-53(Al) nanorods, and (c) NH2-MIL-53(Al) microneedles. Adapted with permission from ref 452. Copyright 2016 Wiley-VCH.

CO2 over CH4 while maintaining CO2 permeabilities of the MMMs as high as those of the unfilled copolyimides. At 35 °C and 150 psi, a NH2-MIL-53/FDH-11 membrane with 10 wt % NH2-MIL-53(Al) loading gives an ideal selectivity for CO2 over CH4, being 4 times greater than that of the neat FDH-11. Such a high CO2/CH4 ideal selectivity was attributed to the simultaneous increase in CO2/CH4 diffusivity selectivity and solubility selectivity of the MMMs. These works show that both the functionality and the pore size of MIL-53(Al) have impacts on the gas separation performances of the MMMs. Apart from these factors, morphologies of MOFs have also been found to affect the functionality of the MMMs. Seoane, Gascon, and co-workers synthesized three types of NH2-MIL-53(Al) with different morphologies, nanoparticles (NP), nanorods (NR), as well as microneedles (MN) (Figure 24), and doped them into the Matrimid or 6FDA-DAM to fabricate MMMs, respectively.452 As compared to nanorods and microneedles, incorporation of NH2-MIL-53(Al) nanoparticles with 8 wt % NH2-MIL-53(Al) loading achieved the largest improvement of CO2/CH4 separation performance at 3 bar and 25 °C. Furthermore, by using the highly permeable polyimide 6FDA-DAM instead of Matrimid, the permeability was increased up to 85% upon NH2-MIL-53(Al) nanoparticles addition. NP-NH2-MIL-53(Al) @6FDA-DAM membranes with a 20 wt % NH2-MIL-53(Al) loading show a CO2 permeability of 660 barrer with a CO2/ CH4 selectivity of 28 at 25 °C and 3.0 bar. Besides NH2-MIL-53(Al), another amine-functionalized MOF NH2-MIL-125(Ti) as one of the limited titanium-based MOFs, with a high surface area and excellent stability, has also been used to fabricate MMMs for CO2 separation by Yang, Li, and co-workers.453 In the study, NH2-MIL-125(Ti) was incorporated into polysulfone (PSF) matrix to fabricate NH2MIL-125(Ti)/PSF MMMs through a mixing-casting process. Permeation tests with CO2/CH4 gas mixture reveal that the incorporation of NH2-MIL-125(Ti) significantly improves the CO2 permeability as compared to the pure PSF membrane, which is mainly reflected by the increase of CO2/CH4 selectivity. A NH2-MIL-125(Ti)/PSF membrane with a 20 wt % MOF loading gives the highest CO2/CH4 selectivity (29.5) at 30 °C and 3.0 bar. The CO2/CH4 selectivity of the pure PSF membrane is 22.0 under a similar operation condition. NH2functionalized MIL-53(Al) and MIL-101(Al) have also been incorporated within PSF polymer to prepare MMMs for the separation of CO2 and CH4 by Rodenas, Kapteijn, et al.454 The functionality of MOFs and/or their particles could also be tailored through post modification before fabricating MMMs for the CO2 capture or separation. For example, Wu and coworkers immobilized polyethylenimine (PEI) using MIL-

size of MIL-53(Al) on the CO2/CH4 separation performance of the MOF-based MMMs.449 MMMs made of Matrimid and MIL-53(Al)-ht (open-pore phae), MIL-53(Al)-lt (closed-pore phase), or MIL-53(Al)-as (as-synthesized) were fabricated by mixing dispersions of the MOFs in chloroform and chloroform solutions of the Matrimid polymer, respectively. The resulted dispersions were then cast onto a Mylar substrate, and dried at 200 °C. The open-pore structure of MIL-53(Al)-ht was found to be unchanged in the MMM. In contrast, the closed-pore structure of MIL-53(Al)-lt transformed into the open-pore structure, similar to the MIL-53(Al)-ht. The authors attribute this phase change to the exchange of water present in the MIL53(Al) pores with chloroform solvent molecules during membrane casting and pore penetration and confinement by Matrimid polymer chains. The separation performances of the membranes were measured by single gas permeabilities of H2, CO2, and CH4, at 35 °C and 2 bar, and the MIL-53(Al)-ht/ Matrimid and MIL-53(Al)-as/Matrimid MMMs both exhibit higher permeability for all gases tested as compared to pure Matrimid, with increased CO2/CH4 selectivities and decreased H2/CO2 selectivities. Yet only MIL-53(Al)-as/Matrimid MMM shows higher selectivities than pure Matrimid for gas pairs with kinetic diameters differing by ≥0.5 Å, including H2/O2, CO2/ CH4, H2/CH4, and H2/N2, suggesting the presence of excess benzene dicarboxylic acid molecules within the pores that reduced its diameter, enabling the material to discriminate between smaller and larger gases. To improve the CO2 separation performance of polymeric membrane, functionalized MOFs have been used to prepare MMMs. For example, NH2-functionalized MIL-53(Al) (NH2MIL-53(Al)) was employed as filler to fabricate MMMs with polyimide (PI) material (Matrimid 5218) by Rodenas, Gascon, and co-workers.450 At 35 °C and 3 bar, the NH2-MIL-53(Al)@ PI membrane effectively enhances CO2 permeability with respect to membranes based on pure Matrimid 5218, while preserving the relatively high selectivity. This work also domonstrated that fast solvent removal could contribute to contraction of the MOF structure into its narrow pore phase, which further led to enhanced selectivity and, particularly, CO2 permeability. In an optimal condition, the NH2-MIL-53(Al)@ PI membrane with a 25 wt % MOF loading shows a CO2 permeability of ∼14.5 barrer and a selectivity of ∼40 for CO2 over CH4. In another study, Kaliaguine et al. incorporated NH2-MIL53(Al) into hydroxyl-functionalized homo- and copolyimides 6FDA-(DAM)x-(HAB)y (with x:y molar ratios of 1:0, 2:1, 1:1, and 1:2) (denoted FDH-xy) to prepare MMMs.451 The incorporation of NH2-MIL-53(Al) in the hydroxyl-copolyimides was found to significantly improve the selectivity toward 9721

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Figure 25. Higher magnification SEM cross-section images of (a) polysulfone homopolymer and (b−f) hybrid membranes containing (b) 10 wt %, (c) 20 wt %, (d) 30 wt %, (e) 40 wt %, and (f) 50 wt % UiO-66-NH2, respectively. The network polymer region (brighter regions) signifies good interfacial contact between the MOF and polysulfone. Adapted with permission from ref 459. Copyright 2015 Royal Society of Chemistry.

101(Cr) (∼550 nm) via a facile vacuum-assisted method.455 The obtained PEI@MIL-101(Cr) was then doped into sulfonated poly(ether ether ketone) (SPEEK) to prepare PEI@MIL-101(Cr)/SPEEK MMMs via a mixing-casting process. The authors believed that the PEI both in the pore channels and on the particle surface of MIL-101(Cr) improved the filler−polymer interface compatibility due to the electrostatic interaction and hydrogen bond between sulfonic acid group and PEI, and simultaneously rendered abundant amine carriers to facilitate the transport of CO2 through reversible reaction. The as-obtained PEI@MIL-101(Cr)/SPEEK membrane shows increased gas permeability and selectivity. The experimental results showed that the highest ideal selectivities of 71.8 and 80.0 for CO2/CH4 and CO2/N2 with a CO2 permeability of 2490 barrer were achieved with 40 wt % PEI@ MIL-101(Cr) loading at 1.0 bar and 25 °C, respectively.

Moreover, the mechanical property and thermal stability of PEI@MIL-101(Cr)/SPEEK MMMs were enhanced as compared to the unfilled SPEEK membrane. In another study, postmodification of NH2-MIL-101(Cr) in MMMs was reported by Liu, Zhong, and co-workers.456 In their work, a material was first prepared through the loading of a ionic liquid [C3NH2bim][Tf2N] (TSIL) into a NH2-MIL-101(Cr). They further doped the resulted material into PIM-1 to fabricate MMMs with different loadings. The introduction of aminecontaining TSIL was believed to be beneficial for the improvement of CO2 permeability and the selectivity for CO2 over N2. Meanwhile, NH2-MIL-101(Cr) is an appropriate porous host material, which ensures well dispersion of TSIL with effectively exposed more active adsorption sites. In the operation condition of 25 °C and 3 bar, the TSIL@NH2-MIL101(Cr)/PIM-1 membrane with a 5 wt % MOF loading gives a 9722

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largely improved CO2 permeability of 2979 barrer and a selectivity of 37 for CO2 over N2. Similar to MILs, most Zr-MOFs also have exceptional stability, being a class of promising materials for fabricating membranes. Vankelecom and co-workers synthesized MMMs composed of PI Matrimid as the polymer and Zr-MOF as the filler.457 To enhance the intrinsic separation performance of MOFs and improve the compatibility between PI and MOF fillers, they used amine-functionalized linkers to synthesize MOF fillers. For example, the use of 2-aminoterephthalic acid as linker and 4-aminobenzoic acid (ABA) as modulator in preparation generated a modulated NH2-UiO-66-ABA material. The as-prepared membranes with NH2-UiO-66-ABA as fillers show high stability, which was attributed to the covalent linking between the fillers and Matrimid. Moreover, the presence of amine groups and linker vacancies inside the pore of the MOF positively influenced CO2 transport. A NH2-UiO-66-ABA/PI MMM with 30 wt % MOF loading shows high separation performance for 50 vol % CO2/CH4 mixtures at 35 °C and 9 bar. As compared to the unfilled Matrimid membrane (CO2/ CH4 selectivity, 31.2; CO2 permeability, 5.9 barrer), a significant increase in the CO2/CH4 selectivity (47.7) and CO2 permeability (37.9 barrer) was observed. Rosi, Albenze, and co-workers fabricated MMMs containing Matrimid polymer and particle surface functionalized NH2-UiO-66, which exhibit improved thermal and mechanical properties.458 The separation performances of these membranes were tested by single gas permeation experiments. The results showed that MOF particle modification could produce MMM with 200% increased CO2 permeability and 25% elevated CO2/N2 ideal selectivity. Urban and co-workers prepared a hybrid NH2-UiO66/PSF membrane with dual transport pathways.459 NH2-UiO66/PSF membranes with high NH2-UiO-66 loading up to 50 wt % were successfully obtained without losing mechanical integrity of the membranes (Figure 25). Remarkably, dual transport pathways of the MMMs were found when the NH2UiO-66 loading is between 30 and 40 wt %, where gas transport through the MOF acts as a molecular transport highway. CO2 permeability of the MMM with dual transport pathways is 46 barrer, an 8-fold increase over pure polysulfone (5.6 barrer), and the selectivity over CH4 and N2 remains near that of polysulfone (22−25). Apart from MILs and Zr-MOFs, ZIFs have also widely been used as fillers to prepare MMMs for CO2 separation. For example, ZIF-8 was incorporated into a polystyrene-blockpoly(ethylene-ran-butylene)-block-polystyrene (SEBS) block copolymer matrix to prepare ZIF-8/SEBS MMMs for CO2/ N2 separation by Chi and co-workers.460 They synthesized ZIF8 particles with different sizes (ZIF-8(S), 88 nm; ZIF-8(M), 240 nm; and ZIF-8(L), 533 nm) first by altering the precursors. Next, the three types of ZIF-8 particles were incorporated into the SEBS to investigate the effect of particle sizes on the gas separation performance of these MMMs. The separation behaviors of these membranes were evaluated by single gas permeation tests. As shown in Figure 26, the introduction of ZIF-8 led to considerable enhancement in gas permeability regardless of the dimensions of the particles. Gas separation performances of the MMMs are in the following order: SEBS/ ZIF-8(M) > SEBS/ZIF-8(L) > SEBS/ZIF-8(S). The low performance of SEBS/ZIF-8(S) MMM was believed to be a result of the large mass transfer resistance at enhanced sites of the SEBS/ZIF-8(S) interface, which could hinder CO2 gas permeation. The SEBS/ZIF-8(L) MMMs are more permeable

Figure 26. (a) Pure gas permeabilities and (b) ideal gas selectivities through neat SEBS membranes and SEBS/ZIF-8 MMMs containing ZIF-8 nanoparticles of different sizes at 35 °C. Adapted with permission from ref 460. Copyright 2015 Elsevier.

toward all gases because of a reduction in interfacial contact area, but show declined selectivities. As a result, the SEBS/ZIF8(M) MMM has the best gas separation performance among the three MMMs. When the gas permeability was measured at 35 °C, a MMM loaded to 30 wt % with ZIF-8(M) exhibits an approximately 2.5-fold enhancement with regard to CO2 permeability, increasing from 170.6 to 454.6 barrer without significant loss of CO2/N2 selectivity (from 12.4 to 12.0), and shows a CO2/CH4 selectivity enhancement (from 4.3 to 5.4) as compared to unfilled SEBS membranes as well. Sholl, Koros, and co-workers demonstrated that ZIF-8 could also be incorporated into polyetherimide (Ultem 1000) matrix to prepare dual-layer asymmetric hollow fiber membranes via the dry jet-wet quench method.461 The outer separating layers of these MMMs contain 13 wt % of ZIF-8 filler. The obtained MMMs were evaluated for the CO2/N2 separation over a range of temperatures and pressures. Gas permeation tests revealed an increase in CO2/N2 separation performance of the MMMs for experiments from both single-component gases and mixed gas feeds. At 35 °C and 100 psi, a 13 wt % MOF loading ZIF-8/ Ultem hybrid hollow fiber MMM gives a CO2 permeability of 26 GPU and a selectivity of 36 for CO2 over N2. 9723

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MMMs incorporating MOF fillers frequently suffer from agglomeration of the MOF particles and insufficient adhesion between the polymer matrixes and the fillers, which results in nonselective voids at the filler/polymer interface. Quemener and co-workers demonstrated that a two-step MMM preparation approach could effectively avoid this problem. The first step is in situ growth of MOF in polymer particle suspension, and a particle fusion step is followed to prepare the MMMs. They prepared ZIF-8/PI MMMs via ZIF-8 nanoparticles in situ grown on surface modified Matrimids particles.462 SEM images show an excellent dispersion of the ZIF-8 nanoparticles in Matrimids matrix without agglomeration, even at high loading of the ZIF-8, which leads to a significant improvement in both CO2 permeability and CO2/ CH4 selectivity. As compared to unfilled Matrimids, the CO2 permeability of a ZIF-8/PI MMM increases from ∼10 to ∼25 barrer, and the selectivity for CO2 over CH4 increases by 65% at 35 °C and 5 bar. Nafisi and Hägg reported a new self-supported dual layer MMM using ZIF-8 as filler and Pebax-2533 polymer as matrix.463 The gas separation properties of dual layer flat sheet MMMs were tested using a mixture of CO2 and N2 in dry and humidified conditions. With the increasing of ZIF-8 loading in Pebax-2533 polymer matrix, CO2 permeabilities of the MMMs increased dramatically from 299 to 1293 barrer at room temperature under 6 bar. However, the ideal selectivity for CO2 over N2 decreased slightly from 33.8 for the pure Pebax-2533 membrane to 32.3 for the mixed matrix membrane with 35% ZIF-8 loading. To avoid insufficient adhesion and nonselective interfacial voids existing in MMMs, He and co-workers developed a toughened MOF−polymer interface by confining a roomtemperature ionic liquid (IL) [bmim][Tf2N] into ZIF-8 cages.464 The interfacial toughening between the polymer and the IL@ZIF-8 filler was verified by SEM and DSC results. It was found that the mechanical properties and gas separation performance of IL@ZIF-8/Pebax membranes were significantly improved. As compared to ZIF-8/Pebax membranes, IL@ZIF8/Pebax membranes show improved molecular sieving properties, which was attributed to a stiffer interphase between the filler and the polymer and a reduction in the effective aperture size of ZIF-8. At 25 °C under 1 bar, an IL@ZIF-8/Pebax-1657 membrane with 15 wt % filler loading shows an increased CO2 permeability of 104.9 barrer, and selectivities of 83.9 and 34.8 for CO2 over N2 and CO2 over CH4, respectively. MMMs containing mixed-linker ZIF materials with the same topology of ZIF-8 but composed of different organic linker compositions have also been reported.465 Single gas permeation results suggest a higher ideal CO2/CH4 selectivity of the mixed-linker ZIFs MMM than the membrane containing only ZIF-8. Song, Sivaniah, and co-workers reported two MMMs consisting of cross-linked polymer of intrinsic microporosity (PIMs) and nanoscale filler.466 Two materials were used as the filler, which are nonporous inorganic fumed silica nanoparticles with a size of 12 nm and microporous 70−100 nm ZIF-8, respectively. As compared to pure thermo-oxidatively crosslinked TOX-PIM-1 membranes, the gas permeabilities of TOXPIM-1/ZIF-8 and TOX-PIM-1/SiO2 are enhanced, while their ideal gas selectivities apparently decrease. With the increase of filler (SiO2 or ZIF-8) loading, the selectivities for CO2 over CH4 slightly decrease from 38 to 27 for TOX-PIM-1/ZIF-8 MMMs, while an evident loss of that was found from 64 to 33 for TOX-PIM-1/SiO2 MMMs.

Besides ZIF-8, some other ZIFs have also been used for the preparation of MMMs. ZIF-11 with a nanosize of 36 nm was synthesized by Coronas and co-workers,467 which has the same chemical composition and properties similar to those of the conventional microcrystalline ZIF-11. The nanosized ZIF-11 in chloroform forms a stable colloidal suspension. MMMs were prepared by direct blending of the polymer (polyimide Matrimid) with the colloidal suspension, and particle agglomeration resulting from drying was avoided. The fabricated MMM shows improved H2/CO2 separation performance as compared to a pure polymer membrane, with a H2/ CO2 selectivity of 4.4 and a H2 permeation value of 95.9 barrer at 35 °C. When measured at 200 °C, these values increased to 9.1 and 535 barrer, respectively. Yang and Chung incorporated ZIF-90 nanocrystals into polybenzimidazole (PBI)-based nanocomposite membranes for H2/CO2 separation.468 The obtained ZIF-90/PBI nanocomposite membranes exhibited a homogeneous particle dispersion and a fine MOF particle−polymer adhesion. A 45/ 55 (w/w) ZIF-90/PBI MMM with the highest ZIF-90 volume loading of up to 50.9 vol % has the best ideal H2/CO2 separation performance with a moderate H2 permeability of 24.5 barrer and a high selectivity of 25.0 for H2 over CO2 at 35 °C and 3.5 atm. For mixed gas tests at 180 °C, the membrane shows a H2/CO2 selectivity of 13.3 and an H2 permeability of 226.9 barrer. Another well-known MOF, HKUST-1, was used as filler to make MMMs for CO2 separation by Zhu and co-workers.469 In their study, MMMs with dispersed HKUST-1 doped into poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) were fabricated for CO2/N2 separation. Sonication treatment was used to reduce the crystal size of MOFs and improve the affinity between the polymer matrix and HKUST-1. Permeation results indicated that the gas separation performances of the MMMs depend on the crystal size of HKUST-1 fillers. The MMMs incorporated with sonicated HKUST-1 show an enhancement of both gas permeability and CO2/N2 selectivity. At 30 °C, a 10 wt % MOF loading HKUST-1-S2/PPO membrane gives a CO2 permeability of ∼89 barrer and selectivities of ∼24 and ∼28 for CO2 over N2 and CO2 over CH4, respectively. Ismail and co-workers also suspended HKUST-1 in the polydimethylsiloxane (PDMS) solution to prepare HKUST-1/ PDMS MMMs on asymmetric hollow fiber PSF membranes by the dip-coating technique.470 A series of MMMs were prepared for CO2/CH4 and CO2/N2 separations. Single gas permeation experiments show an increase of gas permeation rates with increasing number of HKUST-1/PDMS coating applied. After five consecutive coatings, CO2 permeance increased from 69.7 to 109.2 GPU, and the CO2/CH4 and CO2/N2 selectivities were found to increase as well. The improved separation performance was attributed to the coordinatively unsaturated copper sites in HKUST-1, which provide high adsorptive capability for polar molecules, and hence result in the increase of the overall selectivities and gas permeation rates across the membrane. Fe(BTC) is also used as filler and incorporated into a Matrimid-PI matrix.471 The CO2/CH4 separation performances of the obtained MMMs were investigated at a range of pressures, which shows an increase of CO2 permeability and selectivity for the Fe(BTC)/Matrimid-PI MMMs in comparison with the unfilled polymeric membranes. It should be mentioned that Fe(BTC), MIL-100(Fe), and a mixed-valence iron(II,III) trimesate were not well discriminated in the reference. Fe(BTC), also named Basolite F300, is a 9724

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commercially available material, which has a neither crystalline nor amorphous disordered structure.472 MIL-100(Fe) is a crystalline zeotype structure based on μ3-oxo-centered trimers of FeIII octahedra and BTC3−.473 The mixed-valence iron(II,III) trimesate is built from paddle-wheel Fe2 clusters and BTC3−, and it is isomorphous to HKUST-1.474 Apart from the above-mentioned MOFs, several other MOFs have been used as fillers to make MMMs for CO2 separation. For example, Ge, Zhu, and co-workers used Cd-6F as the MOF filler and the 4,4′-(hexafluoroisopropylidene)diphthalic anhydride-4,4′-oxidianiline (6FDA-ODA) polyimide as the polymer matrix to prepare MOF MMMs.475 A specific interfacial interaction between MOF crystals and polymer chains was observed and achieved through in situ polymerization procedure. The prepared Cd-6F/6FDA-ODA MMM shows a high CO2 permeability of 37.8 barrer and selectivities of 35.1 and 44.8 for CO2 over N2 and CO2 over CH4, respectively. CAU-1-NH2 particles were incorporated into the poly(methyl methacrylate) (PMMA) polymer to prepare thin and compact MMMs for H2/CO2 separation by Kong, Chen, and coworkers.476 The 3D microporous framework of CAU-1-NH2 has small triangular windows with a free aperture of 0.3−0.4 nm, making it a good candidate to separate small gas molecules (such as H2) from large ones. The gas test results showed that with the increasing of CAU-1-NH2 loading, the selectivities of CAU-1-NH2/PMMA MMMS for H2 over CO2, no matter in the single-component gas or in the gas mixture, increased first and then decreased slightly. When the CAU-1-NH2 loading is up to 15 wt %, a highest ideal selectivity of 13 for H2 over CO2 can be achieved. Most MOFs used as fillers in MMMs are of 3D porous framework structure. In the past few years, 2D nanostructures have attracted considerable attention, which are used as components of advanced functional materials for applications including (opto)electronics, energy storage, and gas separation.477−481 2D MOFs have been manufactured on solid substrates via layer-by-layer or epitaxial growth approaches.482−486 However, the synthesis of free-standing MOF nanosheets, which is central to intimately blend them into polymers and produce spatially uniform MMMs, remains challenging.487 In a recent work, Xamena, Gascon, and coworkers prepared a series of MMMs through doping CuBDC nanosheets into a PI matrix at different filler loadings (2−12 wt %).488 CO2/CH4 separation performance of these MMMs was investigated. As shown in Figure 27, the addition of bulk type CuBDC crystals into the PI matrix slightly worsens the separation selectivity in comparison with a neat PI membrane, which was attributed to the disruption of the polymer chains due to the presence of the bulky filler particles. This disruption often worsens the intrinsic separation properties of the polymeric matrix and generates unselective nano- or microvoids between the filler and matrix. The use of smaller, submicrometer-sized CuBDC crystals as fillers could bring a small improvement in the separation selectivity, which nevertheless underperforms the PI membrane. Interestingly, the advantage of the MOF nanosheets as filler is immediately apparent. At every studied transmembrane pressure difference, the separation selectivity for ns-CuBDC(8)@PI with 8 wt % nsCuBDC loading is 30−80% higher than that for the unfilled polymeric membrane and 75% to 8 times higher than that for the b-CuBDC(8)@PI counterpart in the range of operating conditions investigated. Another 2D MOF, [Cu2(ndc)2(dabco)]n, was also selected and doped into

Figure 27. Application of the MOF−polymer composites in a gas separation process. Separation selectivity, defined as the ratio between the permeability of CO2 and CH4, as a function of the pressure difference over the membrane for the MOF−polymer composites when employed as membranes in the separation of CO2 from an equimolar CO2/CH4 mixture at 298 K. For comparison purposes, results for a neat polyimide membrane (PI) are also presented. The data correspond to steady operation conditions, after at least 8 h on stream. CO2 permeabilities spanned in the range of 2.8−5.8 barrer, whereas CH4 permeabilities were lower than 0.3 barrer in all cases. One barrer = 10−10 cm3(STP) cm−1 s−1 cm Hg. Error bars correspond to the standard deviations, as determined from three independent tests with selected membranes. When not shown, error bars are smaller than the symbols. Adapted with permission from ref 488. Copyright 2013 Macmillan Publishers Ltd.: Nature Mater.

polymeric material, such as PBI, to prepare MMMs for gas separation by Zhao et al.489 Pure gas permeation tests showed that MMMs with partially oriented nanosheet MOFs possess the largest improvement as compared to unfilled polymers, with the overall H2/CO2 separation performance exceeding the polymer upper bound proposed by Robeson in 2008.490 At 35 °C under 5 bar, the [Cu2(ndc)2(dabco)]n/PBI-20 MMMs with 20 wt % MOF loading give a H2 permeability of about 6.13 barrer and a selectivity of 26.7 for H2 over CO2. 4.2.3. Ideal Selectivity. Both computational and experimental works have shown that MOF-based MMMs can generally exhibit higher permeability as compared to pure polymeric membranes. Unfortunately, the incorporation of MOFs into polymers does not always improve CO2 selectivity. A high permeability alone is of limited interest if the membrane cannot separate two components efficiently. The identification of correct MOFs that will improve both selectivity and gas permeability of polymeric membranes would be very valuable for industrial applications. Because strongly adsorbed species diffuse more slowly than the weakly adsorbed ones, the low diffusion selectivities compensate the adsorption selectivities. One way to achieve high CO2 selectivity but that does not compensate adsorption and diffusion for MMMs is to incorporate a MOF with high permeation selectivity, as indicated by Erucar and Keskin.373 Recent studies have demonstrated that MOFs with large cages connected by narrow windows exhibit high diffusion-based selectivities for 9725

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CO2 and CH4, taking advantage of their differences of kinetic diameters. For example, Sholl and co-workers339 predicted a MOF, Cu(hfipbb)(H2hfipbb)0.5, showing unprecedented selectivity for membrane-based separation of CO2/CH4 mixtures by the combination of molecular dynamics, transition state theory, and plane wave DFT calculations. Similar to the silica zeolite DDR, molecular modeling of Cu(hfipbb)(H2hfipbb)0.5 demonstrated that this MOF has two crucial properties that make kinetic-based separation possible. First, molecular diffusion of CO2 is far more rapid than that of CH4 because CH4 diffusion is strongly hindered by large energy barriers created by narrow windows in the MOFs’ pores. Second, the presence of the slowly moving CH4 molecules was found at most to have a weak impact on the net diffusion of CO2. Keskin253 found that diffusion of CO2 is several orders of magnitude larger than the diffusivity of CH4 in the pores of the microporous metalimidazolate framework (MMIF). The large differences of diffusion between the two adsorbates can be attributed to the much greater restriction by narrow windows in the framework to larger adsorbate molecules. This makes MMIF a very promising material for kinetic separations with an unprecedentedly high CO2/CH4 selectivity. This suggests that highperformance MMMs can be obtained with these MOFs as filler particles.

Figure 28. CO2 capture capacities of pristine M-MOF-74 and regenerated M-MOF-74 samples after hydration at 70% RH. Adapted with permission from ref 505. Copyright 2011 American Chemical Society.

with those of the pristine materials, and commented that PXRD is not a good diagnostic of material quality with regard to sorption performance. The performances of different types of porous material, one chemisorbent, TEPA-SBA-15 (aminemodified mesoporous silica), and four physisorbents, Zeolite 13X (inorganic), HKUST-1, Mg-MOF-74, and SIFSIX-3-Ni (hybrid ultramicroporous material), on sequestration of CO2, either from flue gas mixture or directly from air (direct air capture, DAC), were evaluated by Zaworotko and coworkers.268 It has been found that the four physisorbents are capable of carbon capture from CO2-rich gas mixtures. However, the competition and reaction with atmospheric moisture significantly reduced their DAC performance. DeCoste et al. found that Mg-MOF-74 nearly completely lost its surface area after just one day of exposure to 90% RH at room temperature; however, the PXRD patterns showed only a change in the [100] peak.506 They hypothesized that the adsorption of water on the metal centers of Mg-MOF-74 led to distortion of the framework and breakage of partial Mg−O bonds; this type of structural degradation broke bonds that are binding the 1D channels parallel to each other, which hinders the diffusion of small molecules to the interior of the MOF, but leaves the channels themselves intact, as observed in the PXRD patterns. In contrast, Thonhauser and co-workers proposed that the instability and the accompanying reduction of the CO2 uptake capacity of M-MOF-74 under humid conditions originate in the water dissociation reaction (H2O → OH− + H+) at the metal centers.507 After this dissociation, the OH− groups coordinate to the metal centers, explaining the reduction in CO2 uptake capacity and leading to the possible disintegration of the M-MOF-74. Experimental proof of this explanation was the observation of a sharp absorption band that appears at 970 cm−1 by in situ IR spectroscopy after D2O was adsorbed by M-MOF-74 at above 150 °C, which could be attributed to an O−D bending vibration on the phenolate linker. They explained that H2O also undergoes a similar dissociation reaction, but the corresponding O−H mode is too strongly coupled to MOF vibrations to be detected.508 ZIF-8, also referred as MAF-4 or Basolite Z1200, represents one of the most studied MOFs, and another interesting example for the study of structural stability of MOFs in CO2 capture.381,509−511 Yaghi et al. first reported that this MOF

5. ADDITIONAL CONSIDERATIONS FOR CO2 CAPTURE IN MOFs TOWARD PRACTICAL APPLICATIONS 5.1. Stability of MOFs

By volume, a typical untreated flue gas consists of 73−77% N2, 15−16% CO2, 5−7% water vapor, 3−4% O2, and some other substances in the ppb or ppm range, such as SO2, SO3, NOX, HCl, CO, hydrocarbons, and Hg vapor.499 The stability of MOFs under such a complex gas flow has been a major concern for their applications in CO2 capture, because many MOFs tend to degrade when they are exposed to water vapor and acidic gases. Water is ubiquitous, and the concentration of water is normally much higher than those of the acidic gases in flue gas; consequently, the stability of MOFs against water is mostly investigated for CO2 capture, and less reports were published for the stability of MOFs against the acid gases present in flue gas.500−503 Water stability of MOFs is actually also a topic of significant importance for other applications, such as natural gas upgrading, water treatment, air purification, catalysis, drug delivery, etc. For a detailed introduction about water stability of most existing MOFs and an in-depth understanding of structural factors governing water stability of MOFs, the reader is referred to a comprehensive review by Walton and coworkers.504 Here, we focus on introducing some recent studies, which investigated the stability of MOFs for CO2 capture. M-MOF-74s (M = Zn, Ni, Co, and Mg) are attractive MOFs for CO2 capture based on their single component gas adsorption isotherms.17,31 However, Matzger et al. found that the CO2 capture capacity of all of these MOFs was all reduced after the MOF samples were hydrated at 70% RH and subsequently regenerated under a stream of Ar at 150 °C by gas breakthrough experiments.505 Especially, the CO2 capture capacity of Mg-MOF-74 was drastically diminished, although this MOF exhibited the highest CO2 capture capacity among these isostructural MOFs in dry surrogate flue gas (Figure 28). The authors also pointed out that PXRD patterns collected from the regenerated samples were unexpectedly consistent 9726

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Du and Liu et al. also discovered a new phenomenon associated with water instability of MOFs.516 The authors synthesized a 3D MOF [Cu(tba)2]·(DMF) with 1D rhombusshaped channels in size of ∼3.0 × 6.0 Å2. Single-crystal XRD studies revealed that [Cu(tba)2]·(DMF) could release guest DMF molecules at 150 °C and readsorb DMF in a singlecrystal to single-crystal (SCSC) fashion with the framework unchanged. The guest-free phase [Cu(tba)2] shows a moderate CO2 uptake (1.96 mmol g−1) at 298 K and 1 bar, but high CO2 adsorption selectivities over CH4/H2/O2/Ar/N2 gases. Singlecrystal XRD analyses also showed that [Cu(tba)2] transformed into [Cu(tba)2]·(H2O) when the crystal was placed in water solution in minutes, and the adsorbed water could be released at 150 °C in a SCSC fashion with the framework integrity retained. From the SCSC transformation results, it seems that the framework of [Cu(tba)2] is quite stable against water as excellent single crystallinity could be maintained during the hydration and dehydration processes (Figure 30). However,

maintained its structure after being treated in boiling water for 7 days, or in 8 M aqueous sodium hydroxide for 24 h at 100 °C.381 However, Kaskel and co-workers observed the occurrence of several additional peaks in the PXRD pattern of a sample of ZIF-8 after it was stored in water for 24 h at 323 K.512 Similarly, Cychosz and Matzger also found additional peaks present in the PXRD pattern of a ZIF-8 sample after being placed in pure water for 12 months, although minimal changes in the PXRD pattern were observed after the sample was placed in pure water for at least 1 week. The authors thus concluded that this MOF is only kinetically stable in water.513 Furthermore, Yang, Li et al. also experimentally observed that ZIF-8 underwent hydrolysis under hydrothermal conditions. It was found that ZIF-8 maintained its structure in 80 °C hot water for 24 h when the ratio of the ZIF-8 to water was 6.0 wt %, but ZIF-8 crystals were completely transformed into ZnO under the same condition except the ZIF-8/water ratio changed to 0.060 wt %, as proved by the SEM and PXRD characterizations.514 Recently, Mottillo and Frišcǐ ć published a significant work, which explained all previous conflicting results about the stability of ZIF-8.515 They found that ZIF-8 chemically reacts with CO2 in the presence of moisture or liquid water to form a complex carbonate with an approximate formula Zn(MeIm)2·ZnCO3·0.66HMeIm, which is a structurally unknown phase so far (Figure 29). In contrast, exposing

Figure 30. A single 3D coordination network (cyan polyhedra for CuII) (a) and 4-fold interpenetrating framework (b) of [Cu(tba)2], and (c) destruction, reconstruction, and rearrangement of coordination interactions around the CuII ion in the SCSC transformation from [Cu(tba)2] (top) to [Cu(tba)2(H2O)4] (bottom) upon thermodynamic hydration. Adapted with permission from ref 516. Copyright 2014 American Chemical Society.

when crystals of [Cu(tba)2] were exposed to humid air for over 1 month, single-crystal XRD analysis of the resulted crystals revealed [Cu(tba)2] with a 3D framework transformed into a 0D mononuclear coordination complex [Cu(tba)2(H2O)4]. The authors proposed that [Cu(tba)2]·(H2O) and [Cu(tba)2(H2O)4] are kinetic and thermodynamic products after exposure of [Cu(tba)2] to water. This is an original finding about the duality and sensitivity of MOFs toward water, which provides new insights for the evaluation of the water stability of MOFs. There are also some MOFs promising in CO2 capture, which were reported to be highly stable against water or moisture.208,214,225,421,517−519 For example, Serre, Weireld, Maurin et al. reported the synthesis of UiO-66(Zr)(COOH)2 with water as solvent, and found that this MOF is

Figure 29. Selected PXRD patterns of aging ZIF-8 in CO2, air, and Ar. ZIF-8 reacts with CO2 in the presence of water and water vapor. A characteristic reflection of the new formed phase is marked by “★”. Adapted with permission from ref 515. Copyright 2014 Wiley-VCH.

ZIF-8 at 100% RH and 45 °C under inert atmosphere (like Ar) did not result in any significant changes in the PXRD pattern after 20 days. On the basis of these results, the authors commented that ZIF-8 is possibly stable in boiling water, because the solubility of CO2 in water drops with increasing temperature, but it slowly degrades at room temperature water that is richer in CO2. This work highlights the necessity to consider the influence of a complex multicomponent gas mixture on the MOF materials in CO2 capture. 9727

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Figure 31. (a) PXRD pattern of UiO-66(Zr)-(COOH)2: (1) theoretical; (2) as-synthesized; and (3) after treatment in H2O at 373 K for 16 h; and (b) water adsorption isotherms in UiO-66(Zr)-(COOH)2 at 303 K. Adapted with permission from ref 520. Copyright 2013 American Chemical Society.

For a 30:70 CO2/H2 gas mixture with 74% relative humidity, the reduction was found to be slight (1.61 mmol g−1 and 191 vs 1.99 mmol g−1 and 237 at wet and dry conditions, respectively). In contrast, a clear reduction could be observed from the breakthrough curves for a 10:90 CO2/N2 binary gas with 74% relative humidity, although the relevant data were not provided.

not only water stable, but also shows good performance for CO2/N2 gas mixture separation with a very good selectivity and relatively high working capacity.520 The high water stability of this MOF was demonstrated by the PXRD pattern of a sample of UiO-66(Zr)-(COOH)2 after being dispersed in boiling water for 16 h, and repetitive water adsorption isotherms at 303 K, which show the same uptakes for each relative pressure (Figure 31). Such solid evidence justifies a clear-cut advantage of this MOF in stability over most of the reported MOFs for CO2 capture. The CO2 capture capacity of this MOF is also high. A CO2/N2 selectivity of 56 was obtained by a real binary gas (CO2:N2 = 15:85) coadsorption experiment at 1.0 bar and 303 K. The working capacity of CO2 adsorption between 0.1 and 1 bar was reported to be approximately 42 cm3 cm−3, which is slightly higher than that reported for the zeolite 13X (34 cm3 cm−3) used commercially. Of course, most MOFs are not as water stable as UiO66(Zr)-(COOH)2, but many MOFs at least tolerate water in low concentrations based on reported experimental results.192,217,294,521−526 For example, Long and co-workers reported a diamine appended MOF mmen-Mg2(dobpdc), which displayed an extremely high affinity for CO2 at extraordinarily low pressures.186 At 25 °C, the CO2 uptakes of mmen-Mg2(dobpdc) are up to 2.0, 2.6, 3.13, and 3.86 mmol g−1 at 0.39 mbar, 5 mbar, 0.15 bar, and 1 bar, respectively. The authors also demonstrated that mmen-Mg2(dobpdc) retained its affinity for CO2 under a humid surrogate flue gas (∼1.5% water) by a breakthrough measurement.293 The breakthrough CO2 capture capacity of mmen-Mg2(dobpdc) remained at 2.7 mmol g−1 in such a humid condition. From dry gas adsorption experiments, SIFSIX-2-Cu-i and SIFSIX-3-Zn reported by Eddaoudi, Zaworotko et al. also showed very good performance in selective CO2 adsorption.21 The CO2 uptakes of SIFSIX-2Cu-I at 298 K and 1 bar are 5.4 mmol g−1, and the experimentally determined adsorption selectivity for CO2 over H2 in a 30:70 CO2/H2 mixture is up to 240. SIFSIX-3Zn uptakes 2.5 mmol g−1 CO2 at 298 K and 0.1 bar, and it shows very high CO2 adsorption selectivities for 10:90 CO2/N2 (495) and 50:50 CO2/CH4 (109) mixtures from breakthrough tests. However, it was found that SIFSIX-3-Zn underwent a reversible phase change when being exposed to relative humidity higher than 35%. In contrast, SIFSIX-2-Cu-i was structurally unchanged by exposure to moisture with relative humidity up to 95%. Breakthrough experimental results revealed that the CO2 uptake and selectivity of SIFSIX-2-Cu-i were all reduced when the tested gas mixtures were humidified.

5.2. Effect of Impurities on CO2 Capture in MOFs

Not only do MOFs suffer from the danger of degrading in harsh industrial conditions, but the effects of these coexisting components on CO2 capture also need to be considered. It is well accepted that the existence of those impurities in gas mixtures could significantly influence MOF adsorption and selectivity properties. Experimental studies focusing on impurity effects on CO2 capture in MOFs to date are limited primarily because quantitative measurements for gas mixture are challenging. In this regard, molecular simulations show a clear advantage. Water, as a major component next to CO2 and N2 of postcombustion flue gases, has drawn special attention. Initial efforts at understanding the effect of water on CO2/N2 separations in MOFs have focused on HKUST-1 and extended to the series of M-MOF-74, with encouraging results. Interestingly, water may have a beneficial effect on the CO2 capture, as illustrated by the calculations of Snurr’s group on HKUST-1.158 They found that the presence of water molecules coordinated to CUM sites in the framework, first predicted by molecular simulations and later validated by experiments, significantly enhanced CO2 uptake and its selectivity over N2 and CH4 in HKUST-1. Additional analysis of simulation data revealed that the interaction between the CO2 quadrupole moment and the water molecules electric field was responsible for the enhanced CO2 uptakes. Following their pioneering work, our group further explored the effects on CO2 capture of more impurity species including H2O, O2, and SO2 in HKUST1.527 Consistent with the results reported by Snurr and coworkers, it was found that both coordinated water and water vapor may induce a significant improvement on CO2 capture. DFT calculations indicated that the improved adsorption can be attributed to the enhanced binding energy between CO2 and the MOF by water coordination. Moreover, it was discovered that the mechanisms for the selectivity improvements depend on the state and structure of water: for coordinated water, it can mainly be ascribed to the increased CO2 adsorption capacities with the hydrated HKUST-1. In contrast, when water vapor is present, the increase of CO2/N2 selectivity is not only related to the increased CO2 adsorption but also to the competition for 9728

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performance than the most traditionally used zeolite NaX. In terms of water effects, however, hydrophobic ZIF-8 is an optimal choice when moisture is in the mixture because water only shows a small influence on breakthrough times for ZIF-8 in comparison with other materials. Additionally, some other MOFs have also been selected as model materials for evaluating impurities effects. Barbarao and Jiang530 evaluated water effects on upgrade efficiency. They showed that even with 0.1% water in a CO2/CH4 mixture, the interaction between CO2 and Na+ is reduced, leading to decreases by 1 order of magnitude of the CO2/CH4 selectivity in rho zeolite-like MOF. Li and co-workers58 conducted a computational study on the adsorption and separation of CO2, CH4, and N2 in an rht-type MOF, Cu-TDPAT having a high density of CUM sites and Lewis basic sites. It was computationally demonstrated that Cu-TDPAT possesses very high adsorption selectivity of CO2 over N2 in the CO2/ N2 mixtures and over CH4 in the binary mixtures of CO2/CH4. For ternary mixtures of CO2/N2/CH4, the selectivities of both CO2 versus N2 and CO2 versus CH4 are higher than that of their binary mixtures. The presence of H2O vapor in the gas mixtures influences the CO2 adsorption selectivity and capacity for Cu-TDPAT. Water molecules were found to compete with CO2 adsorption and led to a decrease of CO2 uptake. However, the selectivity of CO2 over N2 is increased in the presence of H2O. Taking UiO-66(Zr)-X (X = NH2, OH, Br) as model materials, Yu and Balbuena studied the interference of H2O and SO2 with CO2 capture, depending on the properties of the functional groups in MOF.531 It was discovered that the presence of water lowers CO2 adsorption significantly in UiO66(Zr) with −NH2 and −OH groups, due to strong interactions between water and the framework. In contrast, due to the water-phobic effects of −Br, water shows a much smaller effect on CO2 capture in UiO-66-Br. Zhong and coworkers532 performed molecular simulations to study the effect of a trace amount of water on CO2 capture in 25 different MOFs. It was found that the interaction strength between H2O molecules and MOFs significant influences CO2 capture: Weak H2O−MOF interactions allow water molecules to move freely in the MOFs and thus play no obvious effect on the adsorption selectivity of CO2/CH4; on the other hand, the effect can be significant when the interactions are strong enough to fix the water molecules in some specific sites in the MOFs. As H2O is a polar molecule, the electrostatic interactions are dominant for water adsorption. The adsorbed water may either occupy the CO2 adsorption sites or provide additional adsorption sites for CO2, resulting in reduction or enhancement of the CO2 selectivity, respectively. Liu and Smit533 explored the effect of water on the separation performance of the two ZIFs, ZIF-68 and ZIF-69, and found similar effects. The selectivity of CO2 over N2 does not change with the presence of water in both ZIFs at low pressures; however, the selectivity increases at high pressures as a consequence of a competitive adsorption of H2O over CO2 and N2. The presence of much more water decreases both the loadings of CO2 and N2, but to a different extent, leading to an increase of the selectivity. In addition to water, the impact of other trace flue gas contaminants (NOx, SOx, etc.) for Mg-MOF-74 and MIL-101 has been examined by Schmidt and co-workers.503 It was found that the strong binding is large enough to outcompete CO2 for binding sites despite the low concentration of the impurities. Therefore, these contaminants can poison a large portion of CUMs sites. In particular, the sites are very difficult to

adsorption sites between water and CO2/N2. The effects of O2/ SO2 on CO2 capture were also investigated for the first time. The results showed that the CO2/N2 selectivity approximately remains unchanged even with relatively high concentrations of O2 in the flue gas (up to 8%). A slightly lower CO2/N2 selectivity in a CO2/N2/H2O/SO2 mixture was observed as compared to that in a CO2/N2/H2O mixture, especially at high pressures, due to the strong SO2 binding with HKUST-1. To fully understand water incorporation in HKUST-1, the interactions between water molecules and CUM sites were studied via density functional theory at the meta-GGA + U level by Cockayne and Nelson.528 Water molecules were found showing chains of hydrogen-bonded water molecules with a tendency to form closed cages at high concentration. A combination of water−water hydrogen bonding and Cu−water oxygen interactions stabilized the configurations, and they can be improved by electric field enhancement of water−water bonding, van der Waals interactions, and hydrogen bonding of water to framework oxygens. The formation of stable clusters explained the particularly strong affinity of water to HKUST-1 and may be applicable to related MOFs with CUMs. Although the interesting finding that water coordinated in the HKUST-1 frameworks enhances CO2 capture capacity and selectivity over N2 and CH4, negative effects of impurities on CO2 adsorption were reported for most studied MOFs. For example, Yu and Balbuena reported that CO2 adsorption capacity is decreased by water coordinated to the CUMs in MgMOF-74 due to the reduced Coulombic interaction between CO2 and coordinated water molecules.529 DFT calculations revealed that, as compared to the uncoordinated Mg-MOF-74, CO2 interacts much weaker with water-coordinated frameworks, leading to much less adsorption of CO2 molecules. Moreover, although both CO2 and N2 adsorptions decrease with the presence of water coordinated to the MOF framework, CO2/N2 adsorption selectivity increased in the hydrated MgMOF-74 because of more pronounced negative effects on N2 adsorption. Similar results have also been reported by Lin, Lee, and co-workers.70 They addressed the effect of water on the separation of flue gases in Mg-MOF-74 and Zn-MOF-74 using force fields developed from periodic DFT calculations. The results showed a significant reduction in CO2 uptake with the existence of trace amounts of water vapor. However, the effect of water is found to be quantitatively different between the two MOFs. Further exploration revealed that the difference in the H2O-metal binding strength leads to different H2O adsorption mechanisms between Mg-MOF-74 and Zn-MOF-74. Moreover, Zn-MOF-74 showed significantly higher water tolerance than the Mg derivative. More recently, Bahamon and Vega257 studied water effects on CO2 capture in different materials including traditionally used zeolites NaX and LTA-4A, as well as Mg-MOF-74, HKUST-1, ZIF-8, MOF-5, and MOF-177 at process conditions. A first screening was done on the basis of adsorption isotherms, Henry’s constants, and isosteric heats of adsorption calculated from GCMC simulations, followed by evaluations of selectivities, breakthrough curves, and working capacities for a TSA process among the different frameworks, with and without water traces. Considering the requirements for an ideal adsorbent of a postcombustion CO2 capture such as high selectivity for CO2 over the other flue gas components, high adsorption capacities under the operating conditions, and minimal energy penalty for regeneration, Mg-MOF-74 stands up as the most promising material among the 11 studied adsorbents to be used in such TSA processes, with even better 9729

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which CO2 exchange for NH3 rather than for NO, NO2, CH4, O2, and N2 molecules is preferred. The critical parameters unveiled such as kinetic barrier and exchange pathway shed light on the mechanism of competitive coadsorption, selective adsorption, and diffusion. Our group evaluated the effects of water on CO2 adsorption in terms of self-diffusion coefficients in MOF-5, HKUST-1, and MIL-47 at various temperatures using molecular dynamics simulations.537 They found that, while in MOF-5 all species maintain their mobility when they are adsorbed as pure components or in mixtures, the diffusivities of CO2 and water in HKUST-1 are slower for the mixture than individual adsorbate, whereas the opposite behavior is observed in MIL-47 where the species diffuse faster in the mixture. The differences can be attributed to the strong interactions of water with HKUST-1 that slow diffusion of CO2 and water. However, the competition between CO2 and water for the active vanadium sites increases the mobilities of both adsorbates in MIL-47. To the best of our knowledge, only one paper regarding experimental measurement of equilibrium adsorption isotherms for gas mixtures has been reported. Using a high-throughput multicomponent adsorption instrument, Long’s group measured equilibrium adsorption isotherms for mixtures of gases including CO 2 , N 2 , and H 2 O at conditions that are representative of adsorption from a postcombustion flue gas.538 15 different MOFs, zeolites, mesoporous silicas, and activated carbons representative of the broad range of solid adsorbents that have received attention for CO2 capture were investigated. It has been found all of the adsorbents with exposed metal cations or anions take up a significant amount of H2O and negligible amount of CO2 under the simulated equilibrium conditions, while MOF-5 and the AX-21 activated carbon adsorb not only very little H2O but also very little CO2 uptake at low pressures due to the nonpolar surfaces. In contrast, adsorbents functionalized with alkylamines maintain a significant CO2 capacity even in the presence of N2 and H2O. Most significantly, the amine-appended MOF mmenMg 2 (dobpdc) (mmen = N,N′-dimethylethylenediamine, dobpdc4− = 4,4′-dioxido-3,3′-biphenyldicarboxylate) adsorb a record amount of CO2 capacity with 4.2 ± 0.2 mmol g−1 (16 wt %) at 0.1 bar and 40 °C in the presence of a high partial pressure of H2O. It should be pointed out that, in addition to maximizing CO2 adsorption, minimizing H2O adsorption is critical to achieving low regeneration energies in any CO2 capture process because some of the heat supplied to desorb CO2 will also go toward desorbing H2O. Therefore, the evaluation of H2O adsorption in the presence of N2 and CO2 is thus important for identifying adsorbents with the best CO2 capture performance. Moreover, the investigation of effects of other flue gas components, such as O2, NOx, and SOx, on the CO2 capture performance of MOFs is so far virtually limited. Future research on this issue will be valuable in assessing adsorbents for CO2 capture applications.

regenerate in some cases. Furthermore, the structural stability of MOFs in the presence of flue gas contaminants was also investigated: The contaminants may also decrease the stability of MOFs via protonation and hydrolysis of the metal−ligand linkers. Ding and Yazaydin presented a comprehensive computational study of various impurities such as SO2, NO2, NO, CO, and O2 on CO2 adsorption in MOFs of M-MOF-74, HKUST-1, Zn-Atz-Ox, and Biomof-11, which all have been found to exhibit exceptionally high pure CO2 uptake at low pressures.534 It has been demonstrated that CO2 uptake in all of these MOFs does not change significantly with increasing concentrations of NO2, NO, CO, or O2 in the mixture. In contrast, SO2 has been found to be a significant threat to CO2 capture in Mg and Co-MOF-74, while Ni-MOF-74 tolerates it very well. Furthermore, the presence of SO2 might either increase or not affect the CO2 uptake in some MOFs rather than causing a decrease, suggesting the simultaneous capture of CO2 and SO2 from flue gas. They also revealed that NO2, NO, and O2 are a major concern for CO2 adsorption in MOFs with adeninate linkers (e.g., Bio-MOFs) as they are chemisorbed. Subsequently, the same authors investigated CO2 capture performance in the presence of SO2 in ZIFs.535 It was demonstrated that SO2 is preferentially adsorbed over CO2. However, while SO2 competed with CO2 for the adsorption sites, it also showed a cooperative effect on CO2 adsorption. In ZIF-10 and ZIF-71, the effect of SO2 on CO2 adsorption is negligible due to the availability of the preferred adsorption sites to accommodate both species within the simulated range of flue gas compositions. In ZIF-68 and ZIF-69, although large amounts of SO2 were adsorbed, the decrease in CO2 uptake was relatively small due to the cooperative interaction between the SO2 and CO2 molecules. The exploration of water effects indicated that the CO2 capture performance in ZIF-68 dropped under wet conditions. In contrast, ZIF-10, ZIF-71, and ZIF-69 maintained their CO2 adsorption in the presence of up to 4% of water in the flue gas. Instead of thermodynamic considerations, Chabal and coworkers first pointed out that the details of the kinetics have to be taken into account for coadsorption and separation processes in MOFs.536 By examining the competitive adsorption of CO2 with a series of gases (CO2, H2O, NH3, SO2, NO, NO2, N2, O2, and CH4) in M-MOF-74 (M = Mg, Co, Ni) with in situ infrared spectroscopy and ab initio calculations, it was suggested that the prediction of molecular adsorption and stability in MOFs by the binding energy at the most favorable metal site is not a satisfactory indicator. Instead, kinetics directs the exchange process of the CUM sites, whereby the interaction of the guest molecules with the MOF organic linkers controls the reaction barrier for molecular exchange. Despite higher binding energies to CUMs of SO2, NO, and NO2 (∼70−90 kJ mol−1) than that of CO2 (38−48 kJ mol−1) and slightly higher than that of water (∼60−80 kJ mol−1), the exchange of CO2 was mainly observed for H2O and NH3. DFT simulations evaluated the kinetic barriers for H2O → CO2 and SO2 → CO2 exchange to be ∼13 and 20 kJ mol−1, respectively, leading to the slower exchange of CO2 by SO2 in comparison with water. Furthermore, it was found that hydrogen bonding of H2O molecules with the nearby oxygen of the organic linker can facilitate the H2O → CO2 exchange process, characterized by the reduction of the exchange barrier. However, distant benzene sites interact with SO2 molecules, away from the metal center, obstructing the exchange process. Analogous thoughts can be applied to the other molecules, in

5.3. Shaping of MOFs

Mostly, properties of bulk powder samples are investigated for MOFs in academic studies. However, shaping MOFs into desired size and geometry is necessary for easy handling, mechanical integrity, facilitating mass and heat transfer, minimizing pressure drop, volumetric efficiency, and some other requirements for chemical or physical processes in largescale applications.539,540 Porous materials can be shaped into 9730

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granules, pellets, extrudates, spheres, beads, honeycombs, foams, monoliths, or other desired shapes. Although there are mature methods for shaping of some other porous materials, such as zeolites and porous carbon-based materials, the shaping process of MOFs needs additional considerations because of the unique crystalline coordination-bonding frameworks and inorganic−organic constitutions of this type of materials. Shaped porous carbon-based materials are commonly obtained by shaping polymerized precursors into a desired shape and subsequent carbonizing or pyrolysis of the materials at high temperatures.541,542 The shaping processes are completed during the formation of the porous carbon-based materials. In contrast, zeolites are normally synthesized as submicrometer or submillimeter crystals. The shaping of zeolites is classically performed by mixing the powder samples with inorganic oxide (such as silica and metal oxides) or organic (such as tetramethylorthosilicate, methylsiloxane) additives or binders, subsequent shaping the mixtures into the desired shapes by a variety of methods such as extrusion, slip and tape casting, foaming, gel casting, coating, spray drying, and dry pressing, and finally stabilizing the shapes with thermal treatments at high temperatures for removal of the organic substances and enhancement of bonding between the powder particles.540,543 These shaping processes all involve treatments at high temperatures, which are typically higher than 800 °C. MOFs thermally decompose at so high temperatures; thus the existing shaping methods for zeolites and porous carbon-based materials are not suitable for MOFs. Many studies have been carried out, and great progress has been made for the shaping of MOFs. Most relevant works published before 2014 have been reviewed by DeCoste and Peterson.544 Several shaping methods of MOFs developed in last years, including granulation, spray drying, extrusion, and pressing, have been detailed by Lee et al. in a recently published book chapter.545 Here, we introduce the current development trends of this topic and highlight some recent studies with new shaping methods. The research team from BASF SE and Yaghi et al. first initiated the study of shaping powder sample for MOFs.546 Taking the shaping process of MOF-5 as an example, a mixture of 49.9 g of MOF-5 and 0.1 g of graphite is first prepared by thoroughly mixing in a flask. The mixture is then shaped into pellets by an eccentric press in a matrix with a hole diameter of 4.75 mm. Pellets with a circular base of diameter 4.75 mm and a height of 3 mm are obtained by certain eccentric press strength. The whole procedure is carried out under inert gas atmosphere. The shaped pellets of MOF-5 show crush strength (lateral pressure resistance to pressure) of 10 N/pellet, density of 0.583 g cm−3, and a BET surface area of 1532 m2 g−1, which is lower than that of MOF-5 powder sample (1796 m2 g−1). However, the volumetric surface area increases to 607 from 395 m2 L−1 after shaping. They pointed out that introduction of binders or additives into the shaping process is necessary because most MOFs do not show sufficient crush strength after shaping, and that the pressure applied to the MOF samples, ranging from atmospheric pressure to several hundred bars, during the shaping process should be carefully chosen due to low mechanical stability of MOFs as compared to zeolites. As shown in Figure 32, BASF SE manufactured several MOFs in special shaped geometries,539 and some of these shaped MOFs are commercially available with a registered trademark Basolite; for example, HKUST-1 is named as Basolite C300. Most details of shaping processes are patent protected for those shaped

Figure 32. Shaped bodies made from MOF materials by BASF SE. Adapted with permission from ref 539. Copyright 2009 Royal Society of Chemistry.

MOFs from BASF SE.547,548 Many research studies have been reported with those commercially available materials to evaluate the performance of MOFs in hydrogen and methane storage,549−551 hydrocarbon separations,552−558 heterogeneous catalysis,472,559,560 CO2 capture,561−563 and other adsorption and separation applications.562,564−566 Some studies have been reported to make MOF tablets or pellets by compression of powder samples of MOFs.549,567−575 The shaped MOFs made in this way always show decreased surface area due to structural transition or amorphization more or less as compared to the pristine powder samples, but density and volumetric gas adsorption capacity can be elevated. For example, Zacharia mechanically compressed MOF-177 powder into monoliths and evaluated the excess and total gravimetric and volumetric hydrogen storage capacities of the compressed and powder samples of MOF-177.568 The results show that MOF-177 crystals progressively change into to an amorphous phase after being subjected to compressive stress, but excess volumetric hydrogen storage capacity of the MOF-177 monolith (approximately 25.8 g L−1) at ∼6 MPa and 77 K is 78% higher than that of the powder sample, and close to 80% of the theoretical maximum excess volumetric hydrogen storage capacity predicted by its ideal density from single-crystal structure. Peng et al. discovered that HKUST-1 exhibits a room-temperature volumetric methane uptake, 230 cc(STP) cc−1 at 35 bar and 270 cc(STP) cc−1at 65 bar, which exceeds those of most reported MOFs (including PCN-14, UTSA-20, HKUST-1, Ni-MOF-74, NU-111, and NU-125).574 Because of the void space between the MOF particles, a realistic density of a HKUST-1 sample (0.43 g cm3) is much lower than its ideal density (0.883 g cm3) from the single-crystal structure. The HKUST-1 sample was pressed into wafers to increase the packing density. Nitrogen isotherms and PXRD measurements for the compacted wafer samples showed partial collapse of the HKUST-1 framework and significant reduction of the micropore volume, which is also reflected by the fact that a sample subjected to 5 tons of mechanical pressure even shows a density of 1.1 g cm3 higher than the ideal density. Shaping of MOFs by direct compression is simple; however, structural transition or amorphization of the samples induced by high mechanical force is an inevitable drawback. In addition, some MOF crystals are difficult to press into large pellets. Some works have been reported to shape MOFs with binders, such as polyvinyl alcohol (PVA),571,576,577 graphite,578 Teflon,579 9731

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Figure 33. Optical photos of UiO-66 pellets with different diameters of 0.5−15 mm (a) and a drop test setup (b). Adapted with permission from ref 582. Copyright 2015 Elsevier.

Figure 34. From left to right: pure R,F-xerogel, MOF@xerogel composites with 58 wt % MIL-100(Fe), 41 wt % MIL-100(Cr), 50 wt % MIL101(Cr), and 77 wt % MIL-101(Cr) (77 wt %, MIL-101(Cr)@xerogel-H2O). Dimensions (diameter × height) are 13 × 8 mm for pure R-F-xerogel; 10 × 13 mm for both MIL-100@xerogels; 10 × 14 mm for MIL-101(Cr)@xerogel (50 wt %); and 15 × 10 mm for MIL-101(Cr) @xerogel-H2O (77 wt %). Adapted with permission from ref 586. Copyright 2015 Elsevier.

sodium silicate/starch,580 cellulose,581 siloxane ether,581 and sucrose,582 by compression, extrusion, or granulation. Introduction of several percentages of binder improves the adhesion interaction between the MOF crystals, and additional mechanical compression treatment could be avoided in some cases. Grande et al. reported that UTSA-16 extrudates could be produced with PVA as binder, water, and propanol as plasticizer.577 As compared to the powder sample, the UTSA16 extrudates showed no significant reduction of surface area when the content of PVA was 2 wt %. In addition, it was claimed that the crushing strength of extrudates produced with 0.5 wt % is comparable to that of commercial zeolites 4A, and the crushing strength increases significantly at the expense of reduction of surface area when a higher amount of PVA was used. Ren et al. reported a method to shape UiO-66 powder into spherical pellets in the presence of 10 wt % sucrose as a binder by a centrifugal granulator (BZL-300, China).582 The diameters of the resulting spherical pellets could be controlled by varying the shaping time to produce pellets in the range of 0.5−15 mm. N2 adsorption isotherms at 77 K showed that the BET surface area of the shaped MOF (674 m2 g−1) was significantly reduced as compared to that of the UiO-66 powder sample (1367 m2 g−1). It was believed that sucrose blocked partially the micropores of the MOF material. The BET surface area of the crushed UiO-66 pellets (682 m2 g−1) was very close to that of the whole pellets, suggesting that the binder was

homogeneously dispersed throughout the pellet and it did not influence the gas transfer within the pellet (Figure 33). Drop test and simulated tumbler drum test results revealed a high mechanical stability of the UiO-66 pellets. Zero breakage of the pellets after 70 consecutive drops at a height of 0.5 m and 5% breakage after 60 min of tumbling time at a speed of 25 rpm were observed. More recently, shaping MOFs with sol−gel chemistry has attracted considerable attention.583−589 One advantage of this method is that there is no loss of surface area for the MOF materials due to the absence of mechanical compression and blockage of the micropores by binders. Janiak et al. reported that monolithic shaped composite materials could be prepared by embedding MIL-100(Fe,Cr) and MIL-101(Cr) into resorcinol-formaldehyde xerogel (R,F-xerogel).586 To avoid pore blocking by monomers or oligomers of the xerogel precursors, a prepolymerized R,F-xerogel solution was first prepared, which was a honey-like, viscous material in contrast to a clear, almost colorless solution of a starting R,F-xerogel solution. Well-ground MOF materials were then added to the prepolymerized R,F-xerogel. Shaped bodies with MOF contents between 35 and 58 wt % were formed after homogenization, curing, washing, and drying (Figure 34). It is worth noting that R,F-xerogel can be dried under atmospheric conditions with high mechanically stability and negligible shrinking, whereas aerogels require supercritical CO2 drying. Composites with a 9732

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higher MOF content (77 wt %) could be prepared by using additional water during the synthetic procedure. These MOF@ xerogel composites showed expected BET surface areas and water uptakes by their composition. A high mechanical stability of shaped bodies was suggested by abrasion tests in a shaking incubator for 3 h. Tian et al. reported the synthesis of transparent, robust monoliths of ZIF-8 by a simple sol−gel process.587 No binders or high pressure compression were needed in the shaping process. Instead of using a xerogel or aerogel with different contents, 1 mm3 to 1 cm3 shaped ZIF-8 monoliths could be simply obtained by washing 60−70 nm ZIF-8 particles in ethanol with the existence of some precursors (2-methylimidazole and Zn ions) and subsequent slow drying the collected solid in a desired mold at room temperature overnight. The authors hypothesized that the existence of positively charged ZIF-8 particles and residuary reactants within the sample allowed further polymerization of the particles under the mild drying process and the formation of a monolithic structure at last. The Young’s modulus and hardness of the shaped ZIF-8 bodies were measured to be around 7.04 and 0.643 GPa, respectively, higher than those of ZIF-8 single crystal (2.973 and 0.501 GPa). The shaped ZIF-8 monoliths also kept their monolithic morphology and the crystalline structure after being immersed in boiling water for 7 days. Furthermore, N2 adsorption studies at 77 K suggested that the characteristic porosity of ZIF-8 was retained in the monolithic materials. The densities of the ZIF-8 monoliths and powder ZIF-8 were recorded by mercury porosimetry. The measured density for bulk powder ZIF-8 is 0.35 g cm−3, much lower than the calculated density (0.95 g cm−3) from single-crystal structure due to the presence of interparticle spaces. The measured density for ZIF-8 monoliths was high up to 1.19 g cm−3, even slightly higher than the calculated density, which was explained by the presence of impurities or a noncomplete activation. As a result, the shaped ZIF-8 showed a volumetric BET area 3 times higher than that of the powder sample. Clearly, techniques for shaping of MOFs have been largely prompted in the last years due to the need of advancing this type of porous materials to the application stage. However, research about shaping MOFs directed to CO2 capture remains scarce.580,590,591 Hong et al. prepared MIL-101 (Cr) monoliths with square channels with equal wall thickness and channel size of 0.90 mm by paste extrusion techniques for dynamic adsorption breakthrough study in biogas upgrading application (Figure 35).592 MIL-101 (Cr) monoliths containing 60−75 wt % of MIL-101 were manufactured from a paste with MIL-101 (Cr) powder, binding agent bentonite clay, and water, which was extruded into monoliths by a single screw extruder and then stabilized by drying and firing at 150 °C. The MIL-101 (Cr) monoliths showed high adsorption capacity of CO2 at higher pressure and at reduced temperature with good regenerability and repeatability at a moderate temperature of 150 °C according to the pure CO2 sorption isotherm and dynamic adsorption breakthrough curve measurements. Dynamic adsorption properties, such as breakthrough time, stoichiometric time, equilibrium time, and equilibrium adsorption capacity, mass transfer zone velocity, and length were also evaluated from its CO2 adsorption breakthrough curves. The mass transfer zone velocity indicates how fast the adsorption concentration front moves through the adsorbent bed, while the mass transfer zone length indicates the distance of the adsorbent bed being utilized by the CO2 molecules. The

Figure 35. Cross-sectional view of a MIL-101 (Cr) monolith. Adapted with permission from ref 592. Copyright 2016 American Chemical Society.

results suggested that monolithic MOF materials are highly promising in industrial applications for adsorbing high concentrations of CO2 gas. 5.4. Effect of Process Operation on CO2 Capture in MOFs

The cost of industrial CO2 capture processes is high. A 1000 MW coal-fired power plant produces about 1000 tons of dilutestream CO2 per hour. The CO2 capture process is expected to capture CO2 from the flue gas with a purity over 95% and a recovery over 90%. The operating costs for different techniques are high, up to $10−$50 US per ton CO2 captured.593 Energy consumption of the CO2 capture processes is also intensive. The energy penalty of a conventional CO2 capture process using amine scrubbers is about 25−40%.9 It is expected that the energy penalty could be largely minimized with advanced materials and optimized process operation because the theoretical minimum energy needed to separate CO2 from the flue gas is estimated to be 4−5% of the energy produced by the power plant.594 For large-scale CO2 capture with MOFs, the adsorption separation method is the most relevant and feasible, although there are also some other promising methods in CO2 capture, including chemical absorption, membranes separation, cryogenic processes, adsorption, and hydrate formation.595 Adsorption separation with solid adsorbents is mainly based on two major methods: temperature swing adsorption (TSA) and pressure swing adsorption (PSA) (Figure 36). In a TSA process, adsorption equilibrium for the gas components is reached at low temperatures, and the adsorbent is regenerated at high temperatures. In a PSA process, the gas components are adsorbed at high pressures, and the adsorbent is regenerated by lowering the pressure. The adsorption step in a typical PSA process is carried out at pressures higher than 1 atm with desorption at near-ambient pressure. If the regeneration pressure is lower than 1 atm, the process is referred to as vacuum swing adsorption (VSA). In industrial applications, several columns packed with adsorbents are operated under a cyclic variation of the pressure or temperature to make the PSA or TSA process continuous. The effluent stream during the adsorption step that less or no longer contains the preferentially adsorbed species is called the light product or “raffinate”, while the effluent stream at the desorption step that contains the 9733

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7.5 wt %). The presence of both high surface area and strong adsorption sites of Mg-MOF-74 is believed to account for its high performance and potential utility in a TSA postcombustion CO2 capture process. Zhou and co-workers also concluded that TSA is a favorable CO2 capture process as compared to PSA or VSA where high energy demands associated with reducing pressures in large volumes of gas at a low cost are needed, because it is easier to implement, and is already being used in other processes at large scale and can potentially be done cost effectively with low grade steam (a very cheap source of energy).308 However, Bae and Snurr proposed that TSA is limited to the removal of small quantities of strongly adsorbed impurities, because this method has the disadvantage of relatively slow heating and cooling steps, although it is more effective in regenerating the adsorbent.304 For a quick evaluation of porous materials for their practical application in carbon dioxide separation processes by single-component adsorption isotherms of CO2, CH4, and N2, these authors suggested a PSA process with adsorption at 5 bar and desorption at 1 bar for CO2 capture in natural gas with a typical composition of 10:90 CO2/CH4 gas, and a VSA process operated between 1 and 0.1 bar for CO2 capture in flue gas with a typical 10:90 CO2/N2 ratio. PSA is a widely accepted and commercial technology for many gas separation applications. However, Hedin et al. pointed out that conventional PSA processes should be difficult to implement for the capture of CO2 in flue gases from large point sources.597 The technical difficulties mainly lie in the pressurization steps in PSA, and in the integration of the enormous flow in the flue gas stack for a normal power station. PSA processes typically purify weakly adsorbing species from gaseous mixtures with strongly adsorbing species, such as air purification, separation of hydrogen from CO2, and removal of trace amounts of CO2 from air, but it is not inappropriate for recovery of CO2 from stream, which contains >3% CO2.595 However, a PSA process should be suitable for the cases where CO2 capture occurs at high partial pressures. Separating H2 and CO2 in precombustion CO2 capture with MOFs as adsorbents by a PSA process has already been tested by Long and coworkers.269 Single-component CO 2 and H 2 adsorption isotherms at 313 K and pressures up to 40 bar for several MOFs, including MOF-177, Be-BTB, Co(BDP), Cu-BTTri, and Mg-MOF-74, were measured and analyzed by mimicking a PSA process for a 80:20 and 60:40 H2/CO2 gas mixture, where adsorption and desorption pressures are 40 and 1 bar, respectively. Mg-MOF-74 shows very high IAST CO2/H2 selectivities at all pressures, outperforming all other MOFs, zeolite 13X, zeolite 5A, and activated carbons (Figure 37). In comparison with other adsorbents, Mg-MOF-74 as well as CuBTTri show high CO2 capture working capacity, which refers to the difference between the capacity at the high intake pressure and at the lower regeneration pressure. Therefore, it is suggested that separations of CO2/H2 using the PSA method are suitable in MOFs with strongly adsorbing sites; however, the large surface and large CO2 uptake of some MOF are not necessarily optimum for such separations. It has been proposed very early that VSA is more prospective than regular PSA for flue gas separation processes.595,598 In a VSA process, the adsorbents capture CO2 from the flue gas at near 1 atm, and vacuum is then applied to recover the adsorbed CO2 and regenerate the adsorbents. It seems that applying a high vacuum is beneficial to recovery of the adsorbed CO2 and regeneration of the adsorbents. However, Nikolaidis et al.

Figure 36. Scheme of the process for TSA or PSA. In the adsorption step (1), the flue gas is brought into contact with the solid adsorbent. The material selectively adsorbs CO2, and (nearly) pure N2 leaves the adsorber. When the adsorber is saturated, it is regenerated (2) by heating the system and/or applying a vacuum. The purge (3) and cooling or repressurization step (4) brings the system back to its original state (1). Adapted with permission from ref 32. Copyright 2012 Macmillan Publishers Ltd.: Nature Mater.

strongly adsorbed species in larger proportions as compared to the feed stream is often called the heavy product or “extract”. Although a large number of papers with CO2 sorption study by MOFs have been reported, there is no general agreement on which process is better for real CO2 capture application with MOFs. TSA on MOFs is firstly promising for the CO2 capture because of the possibility of using low-grade heat from the power plant as a source of energy for the adsorbent regeneration.596 Long and co-workers evaluated the potential use of MOF-177 and Mg-MOF-74 and in postcombustion CO2 capture by the TSA method.198 The results from low-pressure single-component CO2 and N2 adsorption isotherms recorded from 20 to 200 °C, calculations for isosteric heat of CO2 adsorption and ideal adsorbed solution theory (IAST) CO2/N2 selectivities, working capacities of CO2 capture, and breakthrough simulations suggested that the MOF Mg-MOF-74 with strong CO2 adsorption open metal sites has a CO2 capture performance superior to that of MOF-177 and zeolite NaX in a TSA process. The working capacity, which is calculated as the difference between the amount of CO2 adsorbed at 0.15 bar at a flue gas temperature of 40 °C and the amount of CO2 adsorbed at 1 bar at the regeneration temperature, for MOF-177 is negative at a regeneration temperature of 200 °C, while MgMOF-74 reaches a working capacity of 17.6 wt % under the same condition, much higher than that for zeolite NaX (about 9734

DOI: 10.1021/acs.chemrev.6b00626 Chem. Rev. 2017, 117, 9674−9754

Chemical Reviews

Review

significant effect of the specific separation process conditions on CO2 capture performance of the MOFs. Clearly, the regeneration of adsorbent in a VSA process is the most energy consumable step in CO2 capture. Reducing the cost for the regeneration step has been a hot topic of research.600−609 Zhou and co-workers introduced an azobenzene functional group to the organic linker of a MOF, PCN123, which adsorbs different amounts of CO2 after UV or heat treatment (Figure 38).600 As a typical light-responsive organic

Figure 38. (Top) Trans-to-cis isomerization of the ligand of PCN-123 induced by UV irradiation and the cis-to-trans isomerization induced by heat treatment. (Bottom) Schematic illustration showing the suggested CO2 uptake in MOF-5, PCN-123 trans, and PCN-123 cis. Adapted with permission from ref 600. Copyright 2012 American Chemical Society. Figure 37. IAST-calculated CO2/H2 selectivities and CO2 working capacities for MOF-177, Be-BTB, Co(BDP), Cu-BTTri, Mg-MOF-74, activated carbon JX101, and zeolite 13X with a 80:20 H2/CO2 mixture at 313 K. Adapted with permission from ref 269. Copyright 2011 American Chemical Society.

group, azobenzene can undergo trans-to-cis isomerization reversibly by the stimuli of UV irradiation and heat treatment. Although the azobenzene groups in PCN-123 could not be located in the electron density map from the single-crystal Xray diffraction data due to crystallographically imposed pseudo high symmetry, the NMR spectrum of the acid digested sample confirmed the presence of the azobenzene groups in PCN-123 crystals and excluded any significant ligand decomposition during the solvothermal reaction procedure. Upon light irradiation, PCN-123 shows a decrease of CO2 uptake from 22.9 to 10.5 cm3 g−1 at 1 bar, corresponding to a decrease of 53.9% in CO2 uptake when compared to the pristine sample, which should result from the change of conformation of the azobenzene groups inside the pores of the MOF. The adsorbent returns to its original state when allowed to stay at ambient conditions for a prolonged period of time or under gentle heating. These experimental results demonstrate the possibility of a reversible CO2 adsorption−desorption cycle with a need for only UV light for regeneration of the adsorbent. Because UV light is abundantly available in the form of sunlight, an adsorption separation system with such UV light stimuliresponsive porous materials is of high potential for energy efficient CO2 capture. Another class of representative photochromic organic compounds are diarylethene derivatives, which undergo photoinduced isomerization involving ring-opening/-closing reactions triggered by UV and visible light (Figure 39a). Luo et al. reported a diarylethene-based MOF (DMOF) with a pyridine-nitrogen donor containing diarylethene ligand and biphenyl-4,4′-dicarboxylic acid (H2bpdc).605 The DMOF has a 3D framework with 5-fold interpenetrated dia nets, where 36.9% of the unit cell is solvent-accessible volume. Upon UV irradiation, buff single crystals of the DMOF become blue, whereas the blue color disappears upon irradiation with visible

pointed out that applying deep vacuum levels (