Beyond Equilibrium: Metal–Organic Frameworks for Molecular Sieving

Perspective pubs.acs.org/crystal

Beyond Equilibrium: Metal−Organic Frameworks for Molecular Sieving and Kinetic Gas Separation Published as part of a Crystal Growth and Design virtual special issue on Crystal Engineering of Nanoporous Materials for Gas Storage and Separation Yuxiang Wang and Dan Zhao* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore ABSTRACT: Metal−organic frameworks (MOFs) are a class of crystalline inorganic−organic hybrid materials that have demonstrated huge potential in gas separation due to their ultrahigh porosity, boundless chemical tunability, as well as surface functionality. Most gas separations realized in MOFs are under an equilibrium state and are dependent on the difference in thermodynamic affinities of gases to MOFs, whereas nonequilibrium separation such as kinetic and molecular sieving separation attracting growing attention in the past decade is achieved based on the difference in the size and diffusivity of gas molecules. In this perspective, we first discuss the pore size, temperature, and pressure effect on gas diffusion as well as nonequilibrium gas separation in MOFs. Second, we introduce current techniques reported to measure intracrystalline gas diffusivity. Third, we review recent progress in MOF-based nonequilibrium N2/O2 separation, CO2 capture, and hydrocarbon separation. In addition, we describe the hydrogen isotope separation based on kinetic quantum sieving in MOFs as a special scenario of kinetic gas separation. Lastly, we summarize general design strategies toward MOF-based nonequilibrium gas separation and propose several directions to advance the study in this exciting area.

1. INTRODUCTION Separation processes are important operations in the chemical industry, accounting for 10−15% of the U.S. total energy consumption (Figure 1).1,2 Gas separation, such as olefin/ paraffin separation and CO2 capture, is gaining increasing attention recently due to its profound impact on advanced manufacturing, clean energy, and environmental sustainability.3−6 The conventional gas separation technologies such as cryogenic distillation and absorption have the limitations of extensive energy consumption, large carbon footprint, and high operation cost.7 On the contrary, adsorption-based separation processes such as pressure swing adsorption (PSA), vacuum swing adsorption (VSA), and temperature swing adsorption (TSA) have been proposed as promising gas separation techniques due to their features of easy operation and low cost.8 One of the key components in adsorption-based gas separation processes is the adsorbent materials. Apart from conventional adsorbent materials such as silicates, zeolites and carbon molecular sieves, metal−organic frameworks (MOFs), a type of crystalline porous materials composed of inorganic metal clusters and organic ligands, have been extensively studied over the past decade as potential adsorbent materials for adsorptive separation.9−12 Compared with aforementioned conventional adsorbent materials, the structure, pore environment, and pore size of MOFs can be more readily tuned by © 2017 American Chemical Society

crystal engineering to cater to various needs of gas separation.13−16 The mechanisms of adsorptive separation can be roughly categorized into equilibrium separation, kinetic separation, and molecular sieving. In equilibrium separation, all components in gas mixtures can enter the pores of adsorbents, and separation is based on the relative thermodynamic affinities of gases to the adsorbents. A common strategy to realize equilibrium separations in MOFs is introducing strong interaction sites into the porous materials to enhance the framework affinity to certain component(s) of the gas mixture.17 For instance, uncoordinated open metal sites in MOFs are well-known for binding CO2 and olefins owing to their Lewis acidity.18,19 On the other hand, amine groups can form carbamic acid with CO2 and have been widely employed in MOFs for CO2 capture.4,20 A potential shortcoming of equilibrium separation employing strong interactions between gases and MOFs is that extra energy input will be required to desorb gases in order to regenerate MOFs.21 As for kinetic separation, adsorption is governed by the diffusivity of each gas component in porous materials, and Received: February 26, 2017 Revised: April 8, 2017 Published: April 13, 2017 2291

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equilibrium separation can be more energy-efficient due to the absence of strong binding sites. Besides, study in nonequilibrium gas separation can provide useful information and profound insights for designing practical adsorptive gas separation processes considering that industrial PSA, VSA, and TSA processes are mainly operated under nonequilibrium conditions.8 Although kinetic separation using carbon molecular sieves (CMS) and zeolites has been well established,26−28 reports on MOFs for kinetic separation or molecular sieving are relatively fewer.17,21,22 In this perspective, we will first discuss the pore size, temperature, and pressure effect on gas diffusion as well as nonequilibrium gas separation in MOFs. After that, we will briefly introduce different techniques to measure the intracrystalline gas diffusivity. Next, we will highlight recent advances in MOFs showing potentials in nonequilibrium N2/ O2 separation, CO2 capture, and hydrocarbon separation. Apart from traditional nonequilibrium gas separation, quantum kinetic separation based on the difference in the de Broglie wavelength of H2 isotopes will be discussed as a special extension of traditional kinetic separation. Last but not least, we will summarize general strategies to engineer MOFs suitable for nonequilibrium gas separation as well as possible directions to improve the performance of MOFs to satisfy the needs of these interesting and useful applications.

Figure 1. Chemical separations account for about half of U.S. industrial energy use and 10−15% of the nation’s total energy consumption. Developing alternatives that reduce energy input can make most of these separations more energy efficient. Modified and reproduced with permission from ref 1. Copyright 2016 Nature Publishing Group.

2. GAS DIFFUSION IN MICROPOROUS MOFs 2.1. Pore Size, Temperature, and Pressure. Pore size reduction by crystal engineering of MOFs is of critical importance to achieve nonequilibrium separation.29,30 When the pore size of MOFs is sufficiently large, gas diffusion is unrestricted, and the diffusivity of each component roughly equals each other. In this case, the equilibrium gas adsorption selectivity can be defined as the ratio of the Henry’s constants of the gas components (Sk = K1/K2). As the pore size reduces, gas diffusion becomes increasingly hindered by the confined space, and molecular diffusion will gradually evolve to Knudsen diffusion or surface diffusion, wherein diffusivities of different components become distinguishable because they are related to kinetic diameter and the molecular weight of gas components.31 In this case, both Henry’s constant Ki and diffusivity Di will join together to determine the kinetic separation factor: S k = K1/K 2 D1/D2 .8 It should be noted that sometimes the equilibrium adsorption selectivity will be reinforced by the kinetic effect,32 but in other cases the selectivity may be compromised by the gas diffusion behavior if the more

22

hence selectivity is inherently transient. If the pore aperture size of an adsorbent is so narrow that one component (or several components) in the gas mixture cannot enter, the adsorption rate of such component(s) can be regarded as zero, and kinetic separation will evolve to steric separation, which is also called size-selective separation.23 It should be noted that occasionally the gas components, although smaller than the pore aperture size, still cannot diffuse into the MOFs. Instead of steric effects, this can be attributed to the extremely low diffusion kinetics of these gas molecules under cryogenic conditions used during gas sorption measurements (e.g., 77 K).24 This phenomenon has also been exploited for gas separation,25 which together with steric separation can be categorized as molecular sieving considering the inaccessibility of the pores to certain gas components under a sufficiently long period of time (e.g., the equilibrium time set for gas sorption measurements). Molecular sieving and kinetic separation can be summarized as nonequilibrium separation to distinguish them from thermodynamic equilibrium separation. Compared with equilibrium separation, activation of adsorbents in non-

Figure 2. (a) 1D zigzag channels with narrow necks (red arrow) in Mn(HCO2)2. (b) O2, N2, and Ar uptakes of Mn(HCO2)2 as a function of temperature. Modified and reproduced from ref 34. Copyright 2010 Royal Society of Chemistry. 2292

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the gas pressure, which is demonstrated by its higher adsorption amount of H2 at 77 K and 60 bar than that at 87 K. These examples indicate that nonequilibrium separation may only be effective for specific gas diffusion behaviors regulated by corresponding temperature and pressure. 2.2. Techniques To Measure Gas Diffusivities in MOFs. As nonequilibrium gas separation is based on the difference in diffusion behavior of different gases, it is important to study gas diffusivities in MOFs to evaluate the feasibility of conducting nonequilibrium gas separation. Methods to measure gas diffusivities in MOFs can be roughly classified into two categories: microscopic methods and macroscopic methods. Microscopic methods need advanced instruments for tracking the movement of gas molecules or direct observation of concentration profiles of gas molecules in MOFs (normally using single crystals). One of such techniques is pulsed field gradient nuclear magnetic resonance (PFG-NMR), which is able to measure the displacement of gases in the system due to transport processes.38 Mean square displacement ⟨r2(Δ)⟩ of gases during interval Δ can be calculated and self-diffusion coefficients defined by Ds = ⟨r2(Δ)⟩/(6Δ) can be determined.39 Self-diffusivities can also be obtained by another microscopic method called quasi-elastic neutron scattering (QENS) if gas molecules contain hydrogen whose scattering is essentially incoherent. Besides, transport diffusivities Dt relating a concentration gradient to the flux originated from this gradient (J = −Dt∇c) can be determined by QENS as well when molecules are deuterated with coherent scattering.39−41 Recently, the development of microimaging using interference microscopy (IFM) and infrared microscopy (IRM) enables the direct monitoring of guests in individual single crystals and records the concentration profiles of guests in the observation direction.42−45 These techniques can be used not only for the calculation of self-diffusivities and transport diffusivities from the concentration profiles, but also for the determination of the governing mass transfer mechanism of gas molecules diffusion into the crystals.46 Despite the powerful functions to obtain the gas diffusivities in MOFs, microscopic methods have limitations such as the requirement of high quality single crystals and sophisticated instruments, which are not easily available. Macroscopic methods such as the batch adsorption method, frequency response, and breakthrough experiments determine the gas diffusivity based on the overall gas adsorption behavior of bulk MOF powders. The batch adsorption method is a facile, well-developed, and most popular method that has been adopted to determine gas diffusivities in MOFs.32,33,47−49 By fitting the experimental adsorption kinetics data obtained from the gas sorption analyzer with established mathematical diffusional models based on the assumptions for calculation simplicity, the apparent gas diffusivities depending on the properties of porous adsorbents such as surface permeability and crystal dimensions can be obtained.50 It should be noted that batch adsorption reflects the macroscopic gas diffusion behaviors consisting of macropore diffusion, diffusion from the crystal surface into internal crystal structure, and intracrystalline diffusion.46 Macropore or interparticle resistance can be excluded if the uptake rate is independent of sample mass.51 In an intracrystalline diffusion dominant model, where an adsorbent at an initial concentration C0 experiences a step change ΔC at r = Rc and held constant from then on, the analytical solution is

adsorbable component diffuses much more slowly than the less adsorbable component.33 When the pore size reduces to a critical value so that the diffusion barriers generated thereof become too high for the crossing of some gas components, the other component(s) can be enriched in the internal cavity of adsorbents and molecular sieving separation will occur. The corresponding Sk approaches infinity because the less adsorbed component has a diffusivity literally equaling zero. To this sense molecular sieving can be regarded as the ideal separation method. It should be noted that these diffusion barriers may not necessarily originate from a pore aperture smaller than the dimension of gas molecules. For instance, Mn(HCO2)2 possesses channels with narrow necks of small diameter (3.64 Å) but larger than the size of Ar (3.4 Å) (Figure 2a), which theoretically should allow the diffusion of Ar.34 Nevertheless, this MOF exhibits a negligible amount of Ar uptake at 77 K possibly due to the high diffusion barrier.35 Another point worth noting is that reducing pore size may also enhance the host−guest interactions due to increased overlapping of wall potential of the framework, which may affect the performance of equilibrium gas separation.22 In this perspective, we will not discuss these scenarios in detail to avoid redundancy with the existing reviews.21,22 Instead, we will focus on the effects on gas diffusional behaviors exerted by pore size reduction. Apart from the effect of pore size on gas diffusion, temperature and pressure can also affect the mobility of the gas molecules in microporous frameworks. On the one hand, gas diffusion is a process related to the energy provided by the environment (related to temperature) and the concentration gradient (related to pressure); on the other hand, framework structures of MOFs may change in response to the external stimuli such as variation of temperature and pressure, which will lead to the changes of diffusion barriers. These two aspects may convolute with each other to affect the gas diffusion, corresponding accessible pore volumes, and gas adsorption amount. For example, the uptakes of N2 and Ar in Mn(HCO2)2 at about 140 K are significantly greater than those at 77 K as shown in Figure 2b, which is opposite to the common trend that the gas adsorption amount decreases as temperature increases due to the weakened adsorbent−adsorbate affinity at elevated temperatures.35 MAF-2 (MAF = metal azolate frameworks) reported by Zhang et al. exhibits similar gas adsorption properties.36 Prepared from Cu(NH3)2OH and 3,5-diethyl-1,2,4-triazole (Hetz), MAF-2 possesses an NbO network featuring large cavities (d = 9 Å) interconnected by eight hexagonal [Cu6(etz)6] rings whose apertures are dependent on the conformation and movement of pendent ethyl groups. Interestingly, the ring aperture of MAF-2 is too small to allow the permeation and adsorption of N2 into MAF-2 at 77 K, while at 195 K the aperture expands and allows the entrance of N2. Such a temperature-dependent gate-opening phenomenon can be ascribed to the activated motions of ethyl groups at elevated temperatures. Similarly, a porous coordination cage CuTEI synthesized from Cu(NO3)2·2.5H2O and 5-((triisopropylsilyl)ethynyl)isophthalic acid (TEI) reported by Zhao et al. shows a thermosensitive gating effect by which raising the temperature yields an increase in gas uptake under certain temperature ranges, suggesting the existence of kinetic barriers at those temperature ranges.37 Apart from raising the temperature, such kinetic barriers of CuTEI can also be overcome by increasing 2293

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Crystal Growth & Design Mt 6 =1− 2 M∞ π

∞

∑ n=1

Perspective

⎛ Dn2π 2t ⎞ 1 ⎜− ⎟ exp n2 Rc2 ⎠ ⎝

diffusivity are rather limited.23,61−64 By purging a packed bed of MOFs with mixture gas of helium and the gas of interest, one can obtain the single-component breakthrough curve of the selected gas. Fitting this breakthrough curve with the linear driving force (LDF) model will afford the mass transfer coefficient k related to macropore resistance, surface resistance, and micropore resistance. If macropore resistance and surface resistance are negligible compared with micropore resistance under selected experimental conditions, the microcrystalline gas diffusivity can be calculated by Dc = krc2/15, where rc is the average size of crystals.8

(1)

where Mt is mass uptake at time t, M∞ is mass uptake at equilibrium, D is intracrystalline diffusivity, and Rc is the radius of the crystallites. If the concentration in gas phase is not constant, which means the boundary conditions are not constant, the solution for the uptake becomes

(

exp −

∞

Mt = 1− 6∑ M∞ n=1

9α 1−α

DPn2t R c2

)

+ (1 − α)Pn2

(2)

3. APPLICATIONS OF NONEQUILIBRIUM SEPARATION In this section, we will highlight the recent studies of MOFs with potential in nonequilibrium N2/O2 separation, CO2 capture, and hydrocarbon separation. Although some studies discussed herein conducted multicomponent gas sorption or breakthrough experiments to demonstrate nonequilibrium gas separation performance, it should be noted that other studies only suggest the potential of the MOFs in nonequilibrium gas separation according to the single component gas sorption results. However, testing temperature and framework rigidity of MOFs should be taken into consideration for such evaluations. On one hand, it is worth noting that gas diffusion behavior and corresponding nonequilibrium gas separation performance can be significantly affected by temperature. Therefore, the gas adsorption preference of MOFs determined by gas sorption studies under cryogenic conditions may not keep identical trends at ambient temperatures. On the other hand, the nonadsorbed gas in single component gas sorption experiments may coadsorb with other gas components in real gas separation processes, especially in flexible MOFs that can undergo phase transitions stimulated by sorption, temperature, and/or pressure.65 Such phase transitions may alter the pore aperture size of MOFs and the diffusion barrier of gas, leading to remarkably different gas uptake and gas separation performance of the MOFs.66 For instance, Schneemann et al. studied the coadsorption of CO2 and other adsorbates (N2, CH4, C2H6, C3H8) in a series of flexible pillared-layered MOFs denoted as [Zn2(fu-bdc)2(dabco)]n (fu-bdc = 2,5-funcitonalized-1,4-benzenedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane).67 They found that in the presence of C2H6 and C3H8, the CO2 uptake is significantly higher than that measured in pure component sorption tests even though the partial pressure of CO2 is below the phase transition threshold. More detailed discussions about gas sorption in flexible MOFs can be found in relevant reviews.65,68,69 3.1. Separation of N2 and O2. 3.1.1. Kinetic Separation of N2 and O2. As one of the widely used chemical commodities in the world, O2 not only plays a crucial role in sustaining respiration of most life on Earth, but also has vast applications in areas such as medical and life support, synthetic chemistry, oxyacetylene welding, military usage, and oxy-fuel combustion in fossil-fuel power plants.70 Currently, industrial separation of O2 from air is mainly implemented by cryogenic distillation. This process can provide O2 in very high purity (>99%) but consumes considerable energy and capital cost.71 Zeolites or CMS based adsorptive separations used in industrial scale and portable medical devices are more energy efficient alternatives, though the purity of O2 obtained by this process is slightly lower (∼94%).72 Though kinetic N2/O2 separation using CMS and 4A zeolite has already been developed,73 few studies on kinetic separation of N 2 /O 2 using MOFs have been

where Pn are the nonzero roots of tan Pn =

3Pn 3+

(

1 α

)

− 1 Pn2

(3)

and α is the fraction of the adsorbate added in the step which is finally taken up by the adsorbent. In the case of surface barrier model where the process of diffusion through the crystal surface is the rate-determining step of the whole adsorption process, the model can be described as ⎛ Mt 3 ⎞ = 1 − exp⎜ −α t ⎟ M∞ ⎝ Rc ⎠

(4)

where α is the surface permeability of the crystals. The governing mass transfer mechanism can be determined by comparing the degree of fitting of experimental data obtained by gas sorption analyzer with the fitted results of different diffusion models. For instance, Tanaka et al. engineered the size of ZIF-8 crystals and studied the size dependence of the diffusion of n-butanol inside the crystals.51 By fitting the fractional uptake curve with mathematical models, they found that uptake behavior of large crystals (88 μm) can be well described by intracrystalline diffusion model, while that of smaller crystals is better described by the surface resistance model. This result indicates that crystal downsizing can lead to increased portion of diffusion surface barrier in the overall mass transfer resistance. Frequency response (FR) is another widely adopted macroscopic technology for gas diffusion measurement.52 In typical FR experiments, one parameter of the adsorption system, such as volume,53,54 flow rate,55 concentration56 or pressure,57 will be perturbed sinusoidally, and another system parameter will respond periodically with a different phase lag and amplitude.58 The information from the perturbed parameter and responding parameter together with the frequency of perturbation help to extract gas diffusion information in the porous adsorbents, and the governing mass transfer mechanism can be further determined by comparing the FR data with predictions from mathematical models corresponding to different mechanisms.59 In a recent work, concentration swing FR (CSFR) is extended to study the CO2 diffusion in millimeter-scale Cu-BTC single crystals, indicating a micropore diffusion coefficient of 1.7 × 10−9 m2/s with little dependence on CO2 concentration from 0.1% to 10%.60 Although there are several studies using breakthrough experiments (or column dynamics) to characterize the gas separation performance of MOFs, the reports of using breakthrough experiments to study the microcrystalline gas 2294

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Figure 3. (a) A cup unit in STAM-1 with a green ball representing the hydrophilic cavity. (b) Six cups are arranged with alternating up and down orientations to form porous layers with hydrophobic chambers between the cups (indicated by the yellow sphere). (c) The overall structure of STAM-1 viewing from the [001] direction. Color: Cu, blue; O, red; C, gray; H, white. Modified and reproduced with permission from ref 74. Copyright 2011 Nature Publishing Group.

Figure 4. (a) The structure of CUK-1 viewed from the c direction. (b) The extended asymmetric unit of CUK-1. (c) Gas isotherms of CUK-1 activated at 573 K. (d) Adsorption capacity of N2 (77 K), O2 (77 K), and CO2 (196 K) of CUK-1 dehydrated at different temperatures. Modified and reproduced with permission from ref 77. Copyright 2007 Wiley-VCH.

reported.21,22 Mohideen and coauthors reported a Cu-based MOF in which N2 diffuses much slower than O2.74 Starting from benzene-1,3,5-tricarboxylic acid (BTC) and Cu(NO3)2· 3(H2O), BTC undergoes selective monoesterification during reaction to obtain STAM-1 (St Andrews MOF-1) rather than the expected 3D CuBTC (aka HKUST-1),75 which was confirmed by single-crystal X-ray diffraction, infrared spectroscopy, and solid-state NMR. Composed of a copper “paddlewheel” connected by monoesterified ligands, STAM-1 contains hydrophilic channels lined by metal ions which are filled with H2O molecules as well as hydrophobic cavities lined only by organic functional groups (Figure 3a,b). The hydrophilic channels lie along the crystallographic c-axis with pore apertures of ∼4 Å, while the hydrophobic chambers with pore apertures of similar size (∼4 Å) sit among hydrophilic channels (Figure 3c). Interestingly, water in the pores can be removed upon heating at 393 K, after which only hydrophilic channels remain

open for the adsorption of N2, N2O, and CO2. On the contrary, the interactions between polar molecules such as H2O and CH3OH and the hydrophilic pores are so strong that they can reverse the crystal structure change upon activation, which enables the access of H2O and CH3OH to the hydrophilic channels along with hydrophobic chambers. Moreover, the adsorption kinetics of O2 is significantly higher than that of N2, leading to an O2/N2 selectivity of 4. Though this value is lower than that of the best CMS used in industry (20),74 this proofof-concept work opens a new door for developing porous adsorbents for kinetic N2/O2 separation. 3.1.2. Molecular Sieving of N2 and O2. Considering the size difference between O2 (3.467 Å) and N2 (3.64 Å), molecular sieving is a promising method to separate N2 from O2,22 and a number of microporous MOFs have been proven effective for this application thanks to the pore size engineering. An early example is Mg3(NDC)3(DEF)4 (NDC = 2,6-naphthalenedi2295

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carboxylate, DEF = N,N-diethylformamide), which is composed of Mg3 units stacking orderly and NDC bridging the Mgoxygen clusters to form a 3D network.76 One-dimensional channels defined by the Mg-containing secondary building units (SBUs) and NDC ligands extend along the (101) direction with DEF guests residing inside. Activation at 190 °C removed the DEF molecules and resulted in Mg3(NDC)3 with a Brunauer−Emmett−Teller (BET) surface area of about 190 m2 g−1 determined by O2 sorption tests. Interestingly, although Mg3(NDC)3 can adsorb H2 and O2 at 77 K, it cannot adsorb gases of a relatively larger molecular size such as N2 and CO, suggesting a possible molecular sieving effect that can be used in air separation. Another early example is CUK-1, a MOF prepared by Co(II) source and an unsymmetrical ligand 2,4pyridinedicarboxylate under hydrothermal conditions.25 X-ray crystallography showed that the distance between two walls in CUK-1 is 11.148 Å, and the corner-to-corner distance across the channels (Co2−Co2′) is 14.334 Å, indicating possible N2 uptake at 77 K due to wide pore dimensions (Figure 4a,b). Nevertheless, gas adsorption analyses showed that CUK-1 preferentially adsorbs H2 and O2 over N2 (Figure 4c) even though the crystallinity was confirmed to be the same after thermal activation, which suggests a molecular sieving effect under cryogenic conditions that can be used to separate N2 from O2. A follow up study showed that dehydration temperature has a critical influence on the gas adsorption performance, especially for O2 adsorption.77 The O2 adsorption at 87 K is very low when the dehydration temperature is less than 450 K but increases abruptly when the dehydration temperature is higher than 523 K (Figure 4d). Moreover, after degassing at 573 K, CUK-1 exhibits an irreversible O2 adsorption isotherm at 87 K, which was attributed to the flexibility of the porous framework. An effective strategy of pore size reduction in MOFs for molecular sieving is using bulky organic ligands to construct MOFs. For instance, Ma and coauthors reported PCN-13 constructed by Zn cations and 9,10-anthracenedicarboxylate with a crystal structure identical to that of MOF-5.78 No interpenetration of networks was observed in PCN-13 due to the bulkiness of the ligand. Gas sorption tests demonstrated that activated PCN-13 can selectively adsorb O2 and H2 but exclude N2 and CO, suggesting a pore opening between 3.46 and 3.64 Å matching well with the crystallography data (3.5 × 3.5 Å). Another general strategy to reduce the pore size of MOFs is using longer ligands to render network interpenetrations. Cheon and Suh reported a 4-fold interpenetrated MOF, [[Ni(cyclam)]2-mtb]n·8nH2O·4nDMF, assembled from [Ni(cyclam)][ClO4]2 and methanetetrabenzoic acid (H4MTB) (MTB = methanetetrabenzoate, cyclam = 1,4,8,11-tetraazacyclotetradecane).79 This MOF possesses 1D channels that are occupied by water and DMF guest molecules with an effective window size of 2.05 × 2.05 Å. After removal of guest molecules, it preferentially adsorbs H2 and O2 and excludes N2 and CH4, suggesting promising applications in air separation. Using similar strategies, Ma and coauthors reported a ytterbium MOF PCN-17, which is constructed from Yb(III), 4,4′,4″-S-triazine2,4,6-triyl tribenzoate and sulfate anions (Figure 5).80 PCN-17 with a pore size around 3.5 Å features coordinatively linked double-interpenetrated (8,3)-nets based on Ln4(μ4-H2O) SBUs. Being stable up to 480 °C, PCN-17 selectively adsorbs H2 and O2 over N2 and CO (Figure 5d), demonstrating potential applications in N2/O2 separation and H2/CO

Figure 5. (a) A Ln4(μ4-H2O) SBU connecting four SO42− anions in PCN-17. Color: C, gray; N, blue; O, red; S, yellow; Yb, green. (b) A octahedral cage in PCN-17. (c) The 2-fold interpenetrated (8,3) net composed of octahedral cages connected by SO42− anions in PCN-17. (d) H2, O2, N2, and CO isotherms at 77 K of PCN-17. Modified and reproduced with permission from ref 80. Copyright 2008 Wiley-VCH.

separation. Notably, PCN-17(Er), PCN-17(Dy), and PCN17(Y) that have the same structure with PCN-17(Yb) but different lanthanide metal cations fail to show a molecularsieving capability similar to that of PCN-17(Yb), indicating the subtle role of metal cations in determining the pore size of MOFs.81 3.2. Separation of CO2 from N2 or CH4. 3.2.1. Kinetic Separation of CO2 from N2 or CH4. As the global warming situation intensifies, growing awareness of reducing CO2 emission associated with human activities has been aroused.1 In order to curb the atmospheric CO2 level, effective approaches are the capture, storage, and utilization of emitted CO2, where adsorbent materials with excellent CO2 selectivity and capacity are of crucial importance.3,4 There are mainly two approaches for CO2 capture from fuel combustion: precombustion CO2 capture and postcombustion CO2 capture. Precombustion CO2 capture refers to the CO2 capture from H2/CO2 mixture obtained by the conversion of hydrocarbon fuels such as biomass, coal, or natural gas, so that water will be the final product after H2 combustion. Postcombustion CO2 capture, on the other hand, stands for the process of CO2 capture from the flue gas (mainly N2) after direct combustion of fossil fuels. Different from precombustion CO2 capture which is conducted at high temperatures (above 200 °C) and high pressures (∼10 atm), postcombustion CO2 capture is operated at ambient temperatures (up to 60 °C) and ambient pressures.3 In addition to the aforementioned CO2 capture, natural gas upgrading conducted at high pressures is another application that requires CO2 separation from mixture gas, as raw natural gas contains CO2 concentrations higher than the allowed concentration (