Review pubs.acs.org/cm
Functional Hybrid Porous Coordination Polymers Maw Lin Foo,†,‡ Ryotaro Matsuda,*,†,‡ and Susumu Kitagawa*,†,‡,§ †
Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ERATO Kitagawa Integrated Pores Project, Kyoto Research Park Bldg #3, Shimogyo-ku, Kyoto 600-8815, Japan § Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ‡
ABSTRACT: Porous coordination polymers (PCPs) have attracted vast interest in recent years because of their possibility for rational design of crystal structures and functional properties. An emerging trend in PCP research is hybridization, which is the subject of this review. We have divided hybrid PCPs into three broad classes: class I, isomorphous mixed metal/ligand PCPs; class II, core@shell PCPs; class III, PCP⊃guest or PCPs with accommodated guests. In this Review, we examine 62 representative hybrid PCPs in the area of gas adsorption/separation/ storage, luminescence, catalysis, drug delivery, and ionic conductivity with representative examples of each class to understand the underlying principles of hybridization for PCPs and their advantages over conventional PCPs. The future directions and applications of hybrid PCPs are postulated. KEYWORDS: porous coordination polymers, metal−organic frameworks, hybrid materials, porous frameworks
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dicarboxylic acid, L1 = pyrazine or dipyridyl pillar ligands), IRMOF,17 HKUST-1,18 MOF-74,19 and UIO-66.20 Recently, another approach, hybridization, has been in the forefront to increase diversity and functions of PCPs. From past experiences with other traditional materials such as metals, ceramics and polymers, the hybridization (mixing) of different components at the atomic, molecular, nanoscale, and mesoscale allows the physical properties of the resultant composite to be tuned. PCPs are no exceptions and recent works have demonstrated that hybridization allows for the exquisite control of physical properties between that of pure end members (law of mixing), and sometimes intriguingly, synergism (sum of the parts is greater than sum of individual) may occur. It is the purpose of this Review to cover this exciting new frontier of functional PCPs. The scope of this review is conceptual and not meant to be exhaustive. We define hybridization for the scope of this Review to belong to three broad classes (Figure 1). Class I: Isomorphous Mixed Metal/Ligand PCPs. Since a PCP consists of metal centers and organic ligands, there are two possible routes for formation of mixed frameworks. An isomorphous mixed metal PCP (IMM-PCP), is defined as a PCP in which there are more than one type of metal in equivalent sites, that is, [M1−xM′x(L)] (0 ≤ x ≤ 1), where M(L) is the parent framework and M′ is the metal ion “dopant”. It is noted that different metals need to occupy
orous coordination polymers (PCPs) or metal−organic frameworks (MOFs) are porous, polymeric (1-D, 2-D, or 3-D) solids held by coordination bonds between metal centers and organic ligands. Growth in this area has been explosive since the first PCPs were reported in the late 20th century.1 Their potential applications have been explored in diverse areas, such as gas storage/separations,2 drug delivery,3 catalysis,4 luminescence,5 sensing,6 and ionic/electronic conductivity.7 The synthesis of new PCPs for new/improved functions is still an ongoing activity. There are currently a few approaches to the synthesis of new PCPs. One approach is the design and synthesis of new functional ligands.8 Another approach to PCP synthesis is to obtain isomorphous structures of a certain crystal structure type by using either different metals with the same charge, coordination environment, or different ligands with the same denticity. Prototypical examples of structure types include (but are certainly not confined to) MOF-59 or [M4O(1,4BDC)3] (M = Zn2+, Co2+, Be2+; H2BDC = benzendicarboxylic acid), MOF-7410 series or [M2(DOBDC)] (M = Mg2+, Zn2+, Mn2+, Fe2+, Ni2+, Co2+; H2DOBDC = 2,5-dihydroxyterephthalic acid), MIL-5311 or [M(OH)(BDC)] (M = Al3+, Fe3+, In3+, Cr3+, Sc3+, V3+, Ga3+), MIL-10112 or [M3F(H2O)2O(BDC)]3 (M = V3+, Cr3+, Al3+, Fe3+, In3+), HKUST-113 series or [M3(BTC)2] (M = Cr2+, Fe2+, Ni2+, Cu2+, Mo2+, Ru2+; H3BTC = trimesic acid). Another family utilizes both carboxylates and pillaring pyridyls such as JAST-1 or [M(1,4-BDC)(Pillar)]14 (M = Cu2+, Zn2+, Ni2+), CID-1 or [M(1,3-BDC)(bipy)]15 (M = Zn2+, Mn2+, Cd2+, Ni2+). Isoreticular synthesis is yet another approach: ligands of the same denticity but with longer girths are employed to obtain the same crystal structure type with increased pore dimensions. This approach has been used successfully to extend the pore dimensions of structure types such as CPL16 or [Cu2(pzdc)2(L1)](H2pzdc = pyrazine-2,3© 2013 American Chemical Society
Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: July 1, 2013 Revised: October 15, 2013 Published: November 4, 2013 310
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MOF-5 structure type has been reported with BDC-NO2 (H2BDC-NO2 = 2-nitro-1,4-benzene-dicarboxylic acid) as a ligand. However, on adding BDC as a coligand, an IML-PCP [Zn4O(BDC)2.14(BDC-NO2)0.86] 1, with MOF-5 structure type could be synthesized. Another key concept is the control of interpenetration with doping level. For example, [Zn4O(L2)3] (H2L2 = 9,10-bis(triisopropylsilyloxy)phenanthrene-2,7dicarboxylic acid or H2TPDC) is noninterpenetrated because of the bulky TIPS (triisopropylsilyloxy) group, and [Zn4O(L3)3] (H2L3 = 3,3′,5,5′-tetramethyl-4,4′-biphenyldicarboxylic acid or H2Me4BPDC) is interpenetrated because of the length of the linker. Thus, both these frameworks have relatively low surface areas due to pore blockage and framework interpenetration respectively. Using an IML-PCP, [Zn4O{(L2)x(L3)1−x}3] 2 (Figure 2), the level of framework interpenetration and the degree of pore blockage is modulated, resulting at x = 0.4−0.5 the largest surface areas.22
Figure 1. Schematic of the three classes of hybrid PCPs discussed in this Review.
equivalent sites, thus PCPs which contain metallo-ligands21c or different metals occupying distinct sites are not inherently IMM-PCPs. An isomorphous mixed ligand PCP (IML-PCP) is defined as a PCP in which more than one ligand is present with equivalent denticity, that is, [M(L)x(L′)1−x] (0 ≤ x ≤ 1), in this case L′ is the ligand dopant. It is stressed that the doping ligand has to be of equivalent denticity as there are a significant number of pillared PCPs14,15 that inherently contain two ligands of unequivalent denticity and thus are not IML-PCPs. It is noted that the terminology is still unclear in this arena, depending on the authors it can be referred to as solid solution, mixed component, mixed MOF, or multivariate MOF (MTV). We prefer to use the definition isomorphous mixed (IM) because it denotes that mixing of the metal/ligand sites has occurred with preservation of topology. The term solid solution, in theory can only be used when rigorous structural evidence is present, that is, Vegard’s Law (see Characterization section) is obeyed, and this may or may not be detected for porous PCPs compared to purely close-packed inorganic compounds. The field of mixed PCPs has been the subject of recent comprehensive reviews.21 Class II: Core−Shell (A@B). Another effective way of hybridizing PCPs is the formation of core−shell structures. Unlike the formation of IMM/IML-PCPs, where mixing is present at the intimate (atomic/molecular) scale, there are two or more distinct phases in a core−shell system. The convention A@B where A is the core and B is the shell will be used. The core and shell may be both coordination polymers (i.e., PCP@ PCP) or only one of them is a coordination polymer and the other is wholly inorganic (i.e., Au nanoparticle). Class III: PCP w ith A ccommodated Guest (PCP⊃guest). Because of the inherent porosity of PCPs with well-defined pores and cavities, PCPs are well suited for accommodation of functional guests, such as monomers, polyoxometallates, and enzymes upon removal of solvent guests. Because of the uniform, typically angstrom sized channels of the PCPs, the guests are confined in nanospace and do not interact with each other, creating unique properties via nanoconfinement. This form of accommodation is represented by the convention PCP⊃guest whereupon the guest is residing in the pores/cavities of the PCP.
Figure 2. Variation in surface area with x for [Zn4O{(L2)1−x(L3)x}3] 2, where H2L2 = H2Me4BPDC and H2L3 = H2TPDC. Reprinted with permission from ref 22. Copyright 2011 American Chemical Society.
For core−shell frameworks, it allows for the incorporation of PCPs with two different ligands of different lengths but identical denticity, which is not possible in an IML-PCP. For example, in JAST type frameworks, a shell layer of [Zn(1,4NDC)(DPNI)] (H2NDC = naphthalene dicarboxylic acid, DPNI = 1,4-dipyridyl-naphthalenediimide) could be grown on top of a [Zn(1,4-NDC)(DABCO)] core,23 that is, [Zn(1,4NDC)(DABCO)]@[Zn(1,4-NDC)(DPNI)] 3. Another function of a PCP shell, especially those with a nanoparticle core is to serve as a size selective molecular sieve, only allowing target molecules of a certain size or shape to reach the nanoparticle core. Nanoparticle cores also endow on the PCP hybrid properties of inorganic nanoparticles such as catalytic activity, magnetism and light emission. The fabrication and applications of inorganic nanoparticles in PCPs/MOFs has been recently reviewed.24 For frameworks with functional guests, it allows the incorporation of other classes of materials with different properties from typical coordination compounds, such as porrphyrins, polymers, polyoxometallates, and enzymes. Because of nanospace confinement of guests in angstromsized pores, physical properties such as magnetic/dielectric/ catalysis/luminescence/transport properties may be strongly enhanced. In addition, monomeric guests can be polymerized in the pores to yield polymers with unique properties strictly dictated by pore size, pore geometry, and pore chemistry of the host PCP.25
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UTILITY OF HYBRID PCPS IMM or IML-PCPs allow the chemist to obtain intermediate properties between its end members (the law of mixing). In other instances, the reason is purely synthetic: for IML-PCPs, sometimes with a particular ligand, the PCP with the required structure type does not result despite being of suitable denticity, perhaps due to steric or electronic constraints. For example, no 311
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SYNTHESIS OF HYBRID PCPS Direct Addition. Direct addition is perhaps the most straightforward method for obtaining IMM/IML-PCPs (Class I, Figure 3). The metals and ligand dopants are added directly
concept can be applied to ligands, using MOF-5 but exposure to a solution of organic ligands can yield IML-PCPs. [Zn4O{(BDC)1−x(BDC-Br)x}3] 5 (H2BDC-Br = 2-bromo-1,4benzene-dicarboxylic acid) can be synthesized via treatment of the ligand solution in DMF at 85 °C with no significant degradation.27 Up to 50% ligand exchange is possible. Covalent Postsynthetic Modification. Another alternative to the synthesis of IML-PCPs is covalent postsynthetic modification on the ligands, typically on aromatic −NH2 “handles”.28 Some examples of postsynthetic modification reactions include the formation of amide29 and urea30 linkages via reaction with acetic anhydrides and isocyanates respectively; ring-opening reactions via reaction with 1,3-propanesultone or 2-methylaziridine.31 The aromatic −NH2 group can also be transformed into isocyanides32 and azido functional groups33 for further reactions. IML-PCPs can be produced by postsynthesis as such reactions typically does not usually go to completion, either due to steric hindrance, or the reaction time being controlled for preservation of crystallinity, Thus, almost any postsynthetic reaction will result in IML-PCPs. This best manifestation of this concept is for [Zn4O(BDC-NH2)3] (H2BDC-NH2 = 2-amino-1,4-benzene-dicarboxylic acid) or IRMOF-3, in which four additional ligands is grafted by postsynthesis. Thus, the resultant IML-PCP 6 contains f ive distinct ligands, corresponding to the unmodified NH2−BDC, two amide-modified linkers, and two urea-modified linkers.34 For core−shell PCPs consisting of coordination polymers in both core and shell or PCP@PCP, they are usually synthesized via postsynthesis (Class II, Figure 3), that is, adding crystals of one PCP into the mother liquor of another PCP. This approach allows the epitaxial growth of the second PCP on top of the first.18,35 Direct one-pot addition of metal ions, ligands to form core−shell PCP structures is also possible, albeit rare as illustrated in [Zn(5-NO2-ip)(bpy)]@ [Mn(5-NO2-ip)(bpy)] 7 or Zn-CID-5@Mn-CID-5 (5-NO2-ip = 5-nitroisophthalate, bpy =4,4′-bipyridine).36 For nanoparticles(NPs)@PCP hybrids, the preformed nanoparticles can be placed in the growth solution and the PCP will grow around the nanoparticles. Alternatively, deposition of nanoparticle precursors by chemical deposition, followed by decomposition can be performed. We note that another powerful method of forming core− shell PCP thin films is via step-by-step liquid phase epitaxial (LPE) approach for the fabrication of surface attached MOF crystallites forming PCP thin films or SURMOFs (surface mounted metal−organic frameworks).37 As the synthesis of SURMOFs is in a layer-by-layer fashion, it is possible to achieve a high degree of control the PCP architecture. Because of space constraints, this Review will focus only on bulk materials. Light-induced structural change with pore surface modification is a powerful tool for both syntheses of Class I and II hybrid PCP. In this method, the PCP is synthesized with a light responsive module and then subsequent light irradiation achieves structural change or pore surface activation. This method renders it possible to generate highly active sites which are otherwise impossible to synthesize directly. In addition, the conversion ratio can be controlled by irradiation time, and moreover anisotropic conversion is also possible. Recently the postsynthetic synthesis of a reactive nitrene intermediate that could activate oxygen was demonstrated (Figure 4).38 UV activation of [Zn2(5-N3-isop)2(bpy)] (5-N3isop = 5-azido-isophtalate) occurred in a single-crystal to singlecrystal fashion, converting some of the azide groups into nitrenes, and giving a product that was crystallographically
Figure 3. Synthesis of classical PCP and class I, II, and III hybrid PCPs.
to the reactant solution. As the actual ratio of metals/ligands in the obtained product may not have a composition similar to the feed ratio, it is of utmost importance to characterize the actual composition of the obtained product (see Characterization section). Postsynthesis. Postsynthesis is another powerful synthetic method or obtaining IMM/IML-PCPs if direct addition is not successful. The precursor PCP is synthesized first, in single crystal or powder form, and then the metal sites/and or ligand sites modified by soaks in solutions, either at room temperature or elevated temperatures. This method exploits the inherent dynamic nature of the coordination bond. Depending on the system, there may be complete or partial exchange of ligands or metals. For example, MOF-5 crystals could be soaked in a NiCl2/ DMF solution to obtain IMM-PCP, [(Zn 1−x Ni x ) 4 O(BDC)3(DEF)2x] 4, a pale-yellow solid can be obtained with a maximum value of x = 1 after one year of soaking.26 The same 312
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not as pronounced compared to close packed metals/inorganic solids. Nevertheless, this consistent shift in lattice parameters has been observed in the literature. Baiker and co-workers used high-resolution synchrotron radiation to verify that Vegard’s Law was observed in the IML-PCP or MIXMOF [Zn4O{(BDC)1−x(BDC-NH2)x}3] 10 (x = 0, 0.2, 0.3, 0.9).45 A linear correlation between BDC-NH2 content and peak shift was observed (Figure 5). A linear shift in the lattice parameter versus x was observed for the IML-PCPs of UiO-66 derivative [Zr6O4(OH)4{(BDC-HSO3)x(BDC)1−x}6] 11 (x = 0, 0.18, 0.40, 0.69, and 1.0).46
Figure 4. Photoactivation of [Zn2(5-N3-isop)2(bpy)] to obtain IMLPCP with nitrene moiety, followed by reaction with oxygen to afford a mixture of NO2 and NO groups. From ref 38. Reprinted with permission from AAAS.
characterized as an IML-PCP: [Zn2(5-N3-isop)1.38(5-N-isop)0.62(bpy)] 8 (5-N-isop = 5-nitrene-isophtalate) The photogenerated nitrene reacts with oxygen to form a mixture of nitro (−NO2) and nitroso (−NO) groups (Figure 4). Reaction with carbon monoxide is also possible. Thus removal of these gases with photoirradiation is rendered possible. [2 + 2] cycloaddition reaction between C−C double bonds is also another useful photochemical module.39 For the synthesis of PCPs with accommodated guests in pores (Class III, Figure 3), one-pot synthetic reaction of the preformed guest and PCP growth solution is possible. It has been observed in certain cases such as Cu-HKUST1⊃H3PW12O40 9 that the Keggin cluster actually facilitates the formation of the hybrid PCP at room temperature.40 More commonly, the inf iltration route is used. The guest or precursor is infiltrated into the guest-free PCP framework via sublimation,41 vacuum assisted infiltration from melt42 or solution.43
Figure 5. High resolution synchrotron X-ray of 10 for x = 0, 0.2, 0.3, and 0.9. (inset) A linear relationship is established between BDC-NH2 content and peak position, proving random mixing of ligands in crystal structure. Reprinted with permission from ref 45. Copyright 2009 Wiley VCH.
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CHARACTERIZATION For hybrid PCPs, meticulous characterization is important to prove that mixing has occurred at the molecular level for class I materials; for class II materials, the core−shell architecture has been successfully formed; and for class III materials, there are no extraneous guests located outside the pore. Class I. By mixing two or more components of metal or ligands, the onus is to show that the resultant frameworks are truly mixed at the molecular level instead of a mixture of phases at the microscale. Hence a variety of characterization techniques are employed as detailed below and can be employed on both single crystal and powders. Optical Microscopy. Optical microscopy is perhaps the easiest to perform given the easy access of optical microscopes. This is most useful on single-crystals which exhibit a color change on metal or ligand exchange. A uniform color change occurring in a single crystal is strong evidence that exchange has occurred homogeneously and only a single domain is present. SEM-EDX and TEM-EDX. SEM-EDX and TEM-EDX is a useful characterization technique for IMM-PCPs, especially for metallic elements. Energy dispersive X-ray maps of elements can be constructed to show that the distribution of metals in each PCP particle is identical and uniform if IMM-PCPs are synthesized. Powder XRD. When powder X-ray diffraction is used as a characterization technique, the fulfillment of Vegard’s Law44 is a good indication that solid-solution formation has occurred. Commonly employed in metallurgy and solid-state chemistry, Vegard’s law is an empirical rule that there is a linear relationship between crystal lattice parameters (or cell volume for low symmetry solids) and the degree of substitution. If no isomorphous substitution has occurred, each peak may be split into two individual peaks belonging to the pure end members. As PCPs are porous materials, the shift in lattice parameters are
Liquid and Solid-State NMR. From the previous techniques, if it is proven that the metals or ligands are truly mixed at the molecular scale with no contaminating side phases, liquid NMR can be employed to examine digested powders/crystals to yield the actual proportion of organic ligands present in the crystal. As mentioned previously, this actual composition may be different from the starting (feed) composition depending on the system. For example, even though equimolar concentrations of both BDC and BDC-NH2 were used in the synthetic solution, the composition of the resultant multivariate (MTV) MOF-5 crystal obtained is [Zn4O(BDC)1.92(BDC-NH2)1.08] 10a. To prove that no macroscopic domains of pure members exist, separate sections of a large single crystal were analyzed via liquid NMR of digested samples. All had the same composition of ligands, suggesting that no macroscopic domains of pure end members exist.19 Solid-state 13C NMR was also employed to prove the MTV-MOFs had different chemical shifts from the free ligand. In addition, for the copper paddlewheel system [(ZnxCu1−x)3(BTC)2] 12 (x ≤ 0.07), solid-state 1H NMR was used to quantify the amount of Zn2+ doped into the parent CuHKUST-1 structure.47 Mass Spectrometry. For powders, ATOFMS (aerosol timeof-flight mass spectrometry) can be used to analyze the chemical composition of powders on the single particle level, which is very useful for determining whether an IML-PCP has been formed. This technique was used to prove that a mixture of UIO-66-Br or [Zr6(OH)4O4(BDC-Br)6] and UIO-66-NH2 or [Zr6(OH)4O4(BDC-NH2)6] in water gave [Zr6(OH)4O4{(BDC-Br)x(BDC-NH2)1−x}6] 13 by examining 1976 particles in total, out of which 97% had both −Br and −CN fragments detected. ATOFMS can also be employed to prove that ligand exchange has occurred instead of dissolution by monitoring particle size.48 313
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Class II. In a core−shell system, direct visualization of the core/shell heterostructure is important. Optical microscopy was used to demonstrate that MOF-5@IRMOF-3 14 core−shells have been obtained as the color of MOF-5 is colorless compared to orange IRMOF-3.35 In the same fashion, elemental maps using SEM/TEM can be used to prove core−shell formation in powder samples of Zn-CID-5@MnCID-5 or 7.36 NPs@PCPs can be visualized directly by TEM, however it is noted that PCPs are extremely sensitive to electron beam imaging and agglomeration of nanoparticles may occur. Confocal laser scanning microscopy (CLSM) to verify the presence of face-selective monolayer growth of BODIPY (boron dipyrromethene)carboxylate shell on [Zn2(BDC)2(DABCO)] or Zn-JAST-1 (Figure 6).72
Hybridization offers a powerful method to enhance gas adsorption selectivity/amounts from the existing library of PCPs. An early example of IML-PCP was published with JAST structure [Zn2(BDC)2(TMDC)(DABCO)] 17 (H2TMDC = 2,3,5,6-tetramethyl-1,4-dicarboxylic acid) via direct synthesis, for investigation of its hydrogen uptake.14e The IML-PCP had a slightly higher hydrogen uptake (20.8 mg/g) at 77 K compared to its end members Zn-JAST-1 (20.1 mg/g) or [Zn2(TMBDC)2(DABCO)](18.5 mg/g). This is probably due to fine turning of pore apertures as Zn-JAST-1 has larger pore apertures (7.5 Å) than [Zn2(TMDC)2(DABCO)] (narrowest portion over 3 Å).
In an exhaustive study, 18 different MOF-5 derivatives, termed multivariate (MTV) frameworks were synthesized using a combination of terepthalate linkers with different functionalities attached to 2-position or 2,5-positions on the same zinc framework. 19 Up to 8 different linkers with different functionalities can be incorporated into a single framework. The isotherms demonstrate that the uptake capacity of [Zn4O(BDC)1.52(L4)0.73(L5)0.75] 18 or MTV-MOF-5-AHI for H2 at 77 K is greater than that of [Zn4O(BDC)2.04(L4)0.96] 19 or MTV-MOF-5-AH, [Zn4O(BDC)2.13(L5)0.87] 20 or MTVMOF-5-AI, and MOF-5 by a maximum of 84% (Figure 7). For Figure 6. Representations of surface-modified crystals (left), CLSM images (middle), and transmission images (right) (a) c-axis orientation and (b) a-axis orientation. Reprinted with permission from ref 72. Copyright 2010 Wiley VCH.
Class III. For guests that are accommodated in the pores of the PCP, the main thrust of characterization is to prove that all guests are inside the pores and negligible amounts of guests are left outside/on the surface of the PCP. From SEM, the presence of guest agglomerates can be observed. If guests are inside the pores, the N2 BET surface area should be greatly reduced compared to the pristine PCP. In addition, as the guest is trapped inside the pores and interacting with the walls of the PCP, the thermal characteristics of the guests are markedly different. In the Zn-JAST-1⊃PEO 15 (PEO = polyethylene glycol) hybrid, the glass transition temperature of the polymeric guest is dramatically different from bulk depending on the pore size, chemistry and molecule weight.42 2D solid state NMR was used in the Zn-JAST-1⊃PSt 16 (PSt = polystyrene) to prove unambiguously that intermolecular correlations exists between the single chain PSt guest and PCP host, thus demonstrating successful accommodation of the guest molecule in the PCP.49
Figure 7. Hydrogen uptake at 77 K of MOF-5 and MTV-MOF-5-(AI, AH, AHI). From ref 19. Reprinted with permission from AAAS. (right) Crystal structure of MOF-5 with emphasis on the Zn4O tetrahedra (blue). Carbon atoms are represented by gray spheres; oxygen atoms represented by red spheres. Hydrogen atoms are omitted for clarity.
selectivity of CO2 over CO at 298 K, 400% better selectivity was observed in the case for [Zn4O(BDCNO2)1.19(L4)1.07(L5)0.74] 21 compared with MOF-5. This shows that the performance of some MTV-MOF-5 derivatives is more than a linear combination of its constituents. The authors hypothesized that this may be due to the synergistic interactions between proximal functional groups or formation of nanodomains which may be beneficial for gas adsorption. Another utility of the formation of IML-PCPs, namely, the stabilization of the evacuated framework was recently demonstrated. [Zr6O4(OH)4(HSO3-BDC)6] or UiO-66-SO3H is not stable to evacuation but [Zr6O4(OH)4(BDC)6] is, thus by mixing the two ligands during synthesis affords the series [Zr6O4(OH)4{(HSO3-BDC)x(BDC)1−x}6]. At x = 0.18, the
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TYPES OF FUNCTIONS Gas Adsorption, Separation, and Storage. Because of their porosity, the most prominent property of PCPs investigated is undoubtedly their gas adsorption behavior. Since the discovery of PCPs being able to store gases which can be used for alternative energy purposes such as methane,50 gas storage in PCP has been a heavily researched topic. 314
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channel compared to that of the original compound, resulting in change in sorption behavior. IML-PCPs of [Zn{(mtz)x(mim)1−x}2] 25 (Hmim = 2methylimidazole, Hmtz = 3-methyl-1,2,4-triazole) with sodalite topology can be synthesized for x = 0, 0.23, 0.49, 0.76, 1.0 by direct synthesis.55 Because of the difference between hydrophobic [Zn(mIm)2] (MAF-4 or ZIF-8) and hydrophilic [Zn(mtz)2] (MAF-7), the gate opening pressure for water adsorption of the resultant mixed PCPs can be systematically turned from p/p0 = 0.33, 0.43, and 0.63 for x = 0.23, 0.49, and 0.76 (Figure 9). No hysteretic behavior was also observed in the water adsorption for the mixed members, compared to pure MAF-7.
resultant IML-PCP 11a was robust to evacuation with incorporation of the sulfonate group. 11a compared to its parent UiO-66 had higher initial heats of adsorption for CO2.46 The gate opening phenomena refers to the abrupt increase in gas adsorption of PCP upon achievement of a certain gate opening pressure (Pgo), this is correlated to a structural transition occurring. As the amount of gas adsorption is negligible before the gate opening pressure, these “soft” PCPs are ideal for selective gas capture with low energy penalty for release.51 Fine tuning the Pgo is thus of importance both for fundamental research and industrial applications. One way of achieving this is via IMM/IML-PCPs. By using an IML-PCP, [Cu(bpy)(BF4)(CF3BF3)] 22 or ELM-12/13, its gate pressure is lower than that of its end members, [Cu(bpy)(CF3SO3)2] (ELM-12) and [Cu(bpy)(CF3BF3)2] (ELM-13).52 Zn-CID-5 or [Zn(5-NO2-ip)(bpy)] exhibits gate opening behavior whereas Zn-CID-6 or [Zn(5OMe-ip)(bpy)] (5-OMe-ip = 5-OMe-isophtalate) does not. By formation of solid solution Zn-CID-5/6 or [Zn(5-NO2ip)1−x(5-OMe-ip)x(bpy)] 23 with 0.13 ≤ x ≤ 0.92, the Pgo of carbon dioxide and water can be tuned. Separation of CO2 from CO2/CH4 mixture (1:1) under breakthrough conditions at 273 K with good capacity is achieved using the x = 0.13 member because of the combination of the properties of CID-5 (selective gas adsorption for CO2) and CID-6 (high capacity for gases). The same principle was utilized for CH4/ C2H6 separations as well. In this case, C2H6 can be selectively separated from a 9:1 (v/v) mixture under breakthrough conditions at 273 K using the x = 0.1 member.53 [2 + 2]-Cycloaddtion reaction is a useful method for tuning of surface properties.54 UV irradiation of [Zn2(5-OMeisop)2(bpe)] (bpe = 1,2-bis(4-pyridyl)ethylene) occurred in a single-crystal to single-crystal fashion (Figure 8), coupling between adjacent pillar bpe pillars, and giving not only a perfect converted product but also partially converted PCP characterized as an IML-PCP 24. The converted PCP had a smaller
Figure 9. Water adsorption isotherms at 298 K for [Zn{(mtz)x(mim)1−x}2] for x = 0.0 (1), 0.23, (2) 0.49(3), 0.76 (4), 1.0 (5). Reprinted with permission from ref 55. Copyright 2011 Wiley VCH. (right) Crystal structures of MAF-4 and MAF-7. Zinc atoms are represented by blue spheres, carbon atoms are represented by gray spheres, nitrogen atoms are represented by brown spheres and oxygen atoms represented by red spheres. Hydrogen atoms are omitted for clarity.
For IMM-PCPs, Co-doped MOF-5 [(Zn 1−x Co x ) 4 O(BDC)3(DEF)2x] 26 for x ≤ 1 can be synthesized via direct synthesis.56 At maximum doping level, only one of the Zn atoms in the Zn4O cluster can be substituted for Co. The Co ion is incorporated into the cluster in an octahedral geometry with two coordinated DEF molecules but changes its geometry to tetrahedral upon evacuation as manifested by a dramatic color change from pink to blue. The resultant IMM-PCP had higher adsorption capacity for H2 (7.4% more) than MOF-5 at 77 K at a pressure of 10 bar. [Cr0.6Fe0.4(OH)0.7F0.3(BDC)]57 or (Cr,Fe)-MIL-53 27 and demonstrated from high pressure CO2 adsorption isotherms that the NP to LP transformation occurs at a pressure (∼10 bar at 283 K) intermediate between the pure Cr-MIL-53 (∼3 bar)58 and pure Fe-MIL-5359 (>20 bar) solid (Figure 10). Thus, in terms of ease of pore opening behavior, the mixed phase is intermediate between the pure Cr and Fe solids. As a consequence, the control of the metal ratio in mixed-cation MIL-53 materials might appear as a promising method to rationally fine-tune the sorption behavior. Several postsynthetically modified PCPs were examined for hydrogen uptake at 77 K.60 It was found that IRMOF-3 PCPs modified with amide/urea linkages had increased hydrogen uptake capacity and heat of adsorption compared to unmodified IRMOF-3. For example, with 63% converted
Figure 8. Change of pore interior in PCP via [2 + 2] cyloaddition. From ref 54, reproduced by permission of The Royal Society of Chemistry. 315
dx.doi.org/10.1021/cm402136z | Chem. Mater. 2014, 26, 310−322
Chemistry of Materials
Review
adsorption and chemisorption properties, and exhibits synergistic effects with increased isosteric heat of the H 2 physisorption62 (Qst = 11.4 vs 4.55 kJmol−1 of unhybridized SNU-90). Inorganic hydrides such as NH3BH3 (amine borane, AB), LiBH4, Mg(BH4)2 and NaAlH4 are potentially useful as solidstate hydrogen carriers for automobile and on-site hydrogen delivery applications.63 However, they suffer from sluggish dehydrogenation kinetics
