CHEMPHYSCHEM MINIREVIEWS DOI: 10.1002/cphc.201300976

Targeted Manipulation of Metal–Organic Frameworks To Direct Sorption Properties Andreas Schneemann,[a] Sebastian Henke,[b] Inke Schwedler,[a] and Roland A. Fischer*[a] Metal–organic frameworks are promising materials for manifold applications. This Minireview highlights approaches for the fine-tuning of specific sorption properties (e.g. capacity, selectivity, and breathing behavior) of this interesting class of materials. Central aspects covered are the control over the crystal morphology, the targeted tuning of sorption properties

by judicious choice of metal centers and linkers, and the preparation of host–guest systems. We want to introduce the reader to these topics on the basis of the manipulation of a handful of outstanding prototypical metal–organic frameworks.

1. Introduction Sorption at metal–organic frameworks (MOFs) is a topic of high interest. Manifold related applications have been investigated extensively, for example, CO2 capture and sequestration,[1, 2] hydrogen storage,[3] separation of volatile organic compounds from gas streams,[4] chemical sensing,[5] catalysis,[6, 7] and biomedical applications.[8] MOFs are highly porous organic/inorganic materials consisting of inorganic units (single metal cations, dinuclear and oligonuclear metal ion clusters; often referred to as secondary building units, SBUs), which are interconnected by multivalent organic linkers, most prominently through the O and N ligator atoms of anionic carboxylate and neutral amine linkers.[8–10] The modular construction principle is based on a library of chemically robust and more or less conformationally rigid building blocks, which arise from the knowledge of the metal ion’s coordination chemistry (Figure 1). So-called reticular synthesis makes the conceptual design and the tailoring of MOFs very attractive, and seemingly unlimited interchanging of linkers and SBUs offers a huge variety of different coordination network topologies.[11–15] MOFs feature crystalline order, extremely high surface areas,[16] exceptional pore volumes,[17] and very low crystal densities[18] with the possibility to control the functionality of the coordination space at the internal surface and pore structure independently from the chosen network topology (isoreticular MOFs). This combination of features, including adaptivity and softness, is not matched by any other porous material, for example, zeolites, mesoporous silica, and activated carbons. [a] A. Schneemann, I. Schwedler, Prof. R. A. Fischer Lehrstuhl fr Anorganische Chemie II Ruhr-Universitt Bochum Universittsstraße 150, 44780 Bochum (Germany) Fax: (+ 49) 234-321-4174 E-mail: [email protected] [b] Dr. S. Henke Functional Inorganics and Hybrid Materials Group Department of Materials Science and Metallurgy University of Cambridge 27 Charles Babbage Road, Cambridge CB3 0FS (UK)

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 1. Depiction of MOF synthesis and architecture. Blue spheres, red rods, and the yellow sphere represent the SBUs, the linkers, and the accesible void volume, respectively.

However, applications of MOFs in industry on a large scale are still to come. Limited thermal, mechanical, and chemical stability remain critical issues in this respect. Nevertheless, the crystalline nature of MOFs enables a precise characterization of adsorption states on a molecular level and pushes supramolecular host–guest science to new frontiers.[19] The first generation of porous coordination polymers (PCPs) was comparably labile and collapsed upon removal of solvent molecules accommodated in the pores during synthesis. The second generation (then specifically called MOFs), however, exhibited substantial robustness and allowed for reversible adsorption/desorption of guest molecules with full conservation of the crystalline order, similar to the competing zeolite materials. Much in contrast to zeolites, however, the third generation of PCPs or MOFs, which will be a focus of this article, is characterized by exceptional adaptivity and responsiveness. Compounds of this family undergo defined (and reversible) phase transitions induced by external stimuli, such as guest molecule adsorption/desorption,[20] thermal[21] and mechanical stress,[22] or light.[23] The effect of large framework flexibility in response to specific host–guest interaction is also known as the “breathing effect”[24] and PCPs or MOFs undergoing such kinds of phase transitions are also called soft porous crystals (SPCs).[25] This Minireview aims to highlight a selection of important concepts and examples for the targeted manipulation, that is, ChemPhysChem 2014, 15, 823 – 839

823

CHEMPHYSCHEM MINIREVIEWS rational control and tailoring of the sorption properties of MOFs. We want to give an idea on how the gas sorption properties of MOFs can be adjusted by the choice of metal centers (SBUs) and by the (functionalized) linkers. Also, we will comment on the effects of mesostructure, such as crystallite size and morphology. The opportunities of tailoring by employing mixed-component (solid solution) MOFs of multivariate complexity will be discussed, as well as the influence of additional co-adsorbed guests in the pores (e.g. metal nanoparticles, organic molecules). We will not include network polymorphism in our discussion. Constitutional isomers that possess different topologies have different sorption properties (e.g. kagome[26] and square grid[27] isomers of [Zn2(bdc)2(dabco)]n ; bdc = benzenedicarboxylate, dabco = diazabicyclo[2.2.2]octane). Furthermore, we think that it is also naturally understood that by increasing linker size and thus pore metrics in isoreticular frameworks the sorption properties change. Thus, we will

www.chemphyschem.org not discuss the effects of isoreticular “expansion” of frameworks.

2. Morphology Crystal morphology and crystallite size can impact sorption properties of materials drastically. Also, hybridization of different MOF crystals leading to isotropic core–shell structures, the organization of MOF crystallites on surfaces (MOF thin films), and hierarchical and heterostructured MOF crystallites modulates the sorption performance. Uehara et al.[28] deposited crystals of HKUST-1 (Hong Kong University of Science and Technology; [Cu3(btc)2]n, btc = 1,3,5-benzenetricarboxylate) solvothermally on the surface of gold-coated quartz crystal microbalance (QCM) substrates. Addition of some coordination modulator during synthesis enabled control of the crystal size. The substrates were mounted in an environmentally controlled

Andreas Schneemann was born in 1986 and originates from Sprockhçvel, Germany. He achieved his B.Sc. in Industrial Chemistry in 2009 under the supervision of Prof. W. Grnert at the Ruhr-University Bochum and his M.Sc. in Inorganic Chemistry under the guidance of Prof. R. A. Fischer in 2011. He spent time abroad in the labs of Dr. I. A. Fallis at Cardiff University (Erasmus scholarship) and Prof. S. M. Cohen at the University of California, San Diego (DAAD fellowship). He is currently a graduate student in the group of Prof. R. A. Fischer at the Ruhr-University Bochum. His current research interest revolves around flexible and functionalized metal– organic frameworks.

Inke Schwedler was born in 1988 in Bochum, Germany. Working on organometallics, she obtained her B.Sc. degree in 2011 and her M.Sc. in 2013, concentrating on MOFs under the supervision of Prof. R. A. Fischer. Since her early Bachelor studies, she has been a fellow of the German National Academic Foundation. During the course of her studies she spent 6 months at Cardiff University, working in the group of Dr. S. Pope (Erasmus scholarship). Part of her Masters thesis was performed in the labs of Prof. A. K. Cheetham at the University of Cambridge (funded by the German National Academic Foundation). She joined the group of Prof. R. A. Fischer in November 2013 at the Ruhr-University of Bochum for her doctoral studies. Her research concentrates on functionalized metal–organic frameworks.

Sebastian Henke studied Chemistry at Ruhr-Universitt Bochum (B.Sc. in 2006) and received his Dr. rer. nat. as a scholar of the Ruhr-University Research School and the German Chemical Industry Fund under the supervision of Roland A. Fischer from the same institution in 2011. Currently he is a Feodor-Lynen Fellow of the Alexander von Humboldt Foundation working together with Antony K. Cheetham at the University of Cambridge. His research interests comprise stimuli-responsive materials, gas adsorption in flexible MOFs, as well as the thermo- and piezomechanical properties of functional hybrid frameworks.

Roland A. Fischer studied Chemistry at Technische Universitt Mnchen (TUM) and received his Dr. rer. nat. in 1989 under the guidance of Wolfgang A. Herrmann. After a postdoctoral collaboration with Herb D. Kaesz at the University of California, Los Angeles (UCLA), he returned to TUM in 1990, where he obtained his habilitation in 1995. In 1996 he was appointed Associate Professor at Ruprecht-Karls Universitt Heidelberg. In 1998 he moved to Ruhr-Universitt Bochum, where he took the chair in Inorganic Chemistry II. He was Dean of the Ruhr University Research School (2006–2009). His research interests focus on group 13/transitionmetal compounds, precursor chemistry for inorganic materials, chemical vapor deposition (CVD), thin films, nanoparticles, colloids, and in particular the supramolecular chemistry and property tailoring of porous coordination network compounds (MOFs).

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemPhysChem 2014, 15, 823 – 839

824

CHEMPHYSCHEM MINIREVIEWS

www.chemphyschem.org

Figure 3. Uptake of cetane and isocetane in core–shell MOFs. 1: Zn2(bdc)2(dabco)]n ; 2: [Zn2(adc)2(dabco)]n ; 1/2: [Zn2(adc)2(dabco)]n@Zn2(bdc)2(dabco)]n. Reprinted with permission from ref. [39]. Copyright 2011 Wiley-VCH.

Figure 2. Top: Illustration of the crystal size control of HKUST-1. L = large, S = small. Bottom: Room-temperature MeOH (circles) and n-hexane (triangles) adsorption in small (red) and large (blue) HKUST-1 crystals. Reprinted with permission from ref. [28]. Copyright 2011 American Chemical Society.

QCM instrument to record the sorption kinetics of MeOH and n-hexane vapor. The sample containing smaller crystals showed much faster vapor uptake (Figure 2). This is attributed to the differences in the (external) surface-to-volume ratios. Bennett et al.[29] reduced the size of ZIF-8 ([Zn(mIm)2]n mIm = 2-methyl-imidazolate) particles from the micrometer to nanometer range, which resulted in an increase of Brunauer– Emmett–Teller (BET) surface area (determined by N2 adsorption at 77 K) from 1006 to 1630 cm2 g 1. This increase is attributed to the higher overall surface area (internal and external) of the nanosized material. Note that nanosized MOF crystallites are likely to exhibit substantial defects at the external surface, which may contribute as additional (potentially strong) adsorption centers. So far, there have been hardly any comprehensive investigations on the crystal size dependency of sorption properties, despite a great number of studies related to the synthesis of nanosized MOF materials.[30–35] Another way to manipulate the sorption properties of a given MOF crystal is the fabrication of (homoepitaxial) core– shell structures.[36–38] The core–shell-type hybridization of [Zn2(bdc)2(dabco)]n with [Zn2(adc)2(dabco)]n (adc = 9,10-anthracenedicarboxylate) was reported by Hirai et al. to separate n-alkanes from branched alkanes.[39] The comparatively bulky adc  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

linkers block the pores for branched alkanes, thus pure [Zn2(adc)2(dabco)]n adsorbs n-alkanes selectively. Due to the bulky linkers the sorption capacity is limited. The core–shell approach combines a core crystal with a higher pore volume, but without selectivity, with an outer shell crystal featuring the (size exclusion) sorption selectivity (Figure 3). The specific capacity for the linear alkane is more than doubled compared to the pure [Zn2(adc)2(dabco)]n, while the specific selectivity of the shell is retained. Quite similarly, selectivity control of an analogous core–shell heterostructure towards polar molecules (dimethylaniline over benzene) was achieved by a two-step process involving postsynthetic linker modification.[40] Rosi and co-authors reported bio-MOF-11/14 core–shell crystals.[41] The bio-MOF structures consist of Co2 paddlewheels that are coordinated by adeninates and monocarboxylates. Each di-cobalt node is coordinated by four adeninates and two monocarboxylates. The monocarboxylates possess an aliphatic side chain of varying length, which points into the pores of the framework. In bio-MOF-11 and bio-MOF-14 the monocarboxylates are acetate and valerate, respectively. The chain length has a distinct influence on the properties. Bio-MOF-11 shows high CO2 uptake but no selectivity towards CO2 over N2, whereas bio-MOF-14 has a low adsorption capacity but shows selectivity (Figure 4). Combination of both materials yielded the composite denoted as bio-MOF-14@bio-MOF-11/14. Sorption selectivity increased by 30 % and the valerates in the shell also increased the water stability of the composite with respect to pure bio-MOF-11. MOF thin films and, in particular, highly oriented layer-bylayer grown surface-mounted metal–organic frameworks (SURMOFs) have gathered broad attention in terms of sorption and sensing.[42] The control of the preferred growth direction allows the precise assembly of the layers in heterostructures similar to the freestanding core–shell crystallites discussed above. Termination of the substrate surface with a suitably functionalized self-assembled monolayer (SAM) allows growth of a MOF along a certain crystal direction onto this surface.[43, 43] Liu et al. demonstrated the control of sorption kinetics at differently oriented SURMOFs of the tetragonal, anisotropic ChemPhysChem 2014, 15, 823 – 839

825

CHEMPHYSCHEM MINIREVIEWS

www.chemphyschem.org Meilikhov et al.[46] demonstrated programming of the sorption properties of SURMOF heterostructures by the deposition sequence of the respective blocks of differently functionalized layers. Two different heterostructures were fabricated by layerby-layer heteroepitaxial growth at the solid/liquid interface. The first heterostructure consisted of a [Cu2(NH2-bdc)2(dabco)]n (A) thin film on top of a [Cu2(ndc)2(dabco)]n (B) film. By postsynthetic modification 50 % of the NH2 groups were modified with succinic anhydride, which resulted in the formation of an amide-containing layer [Cu2(HOOC(CH2)2OCNH-bdc)(NH2-bdc)(dabco)]n (C) on top of a B layer. The control sample was the inverted heterostructure B@C. Sample B@C takes up methanol and n-hexane, whereas C@B selectively takes up only methanol (Figure 6). In co-adsorption experiments employing methanol/

Figure 4. CO2 sorption isotherms recorded at 273 K (A) and N2 sorption isotherms recorded at 77 K (B) of bio-MOF-11 (navy blue), bio-MOF-14 (green), and bio-MOF-14@bio-MOF-11/14 (dark red). Comparison of N2 (C) and CO2 (D) sorption isotherms of bio-MOF-14@bio-MOF-11/14 (black) and ground bio-MOF-14@bio-MOF-11 (red). Adsorption and desorption branches are shown as filled and open symbols, respectively. Reprinted with permission from ref. [41]. Copyright 2013 American Chemical Society.

[45]

[Cu2(ndc)2(dabco)]n (ndc = 1,4-naphthalenedicarboxylate). Pyridyl-terminated SAMs initiate growth along the [001] direction, and COOH-terminated SAMs initiate growth along the 908 rotated orientation in the [100] direction (Figure 5). Films grown in the [100] direction have much faster sorption kinetics. The adsorption through the [100] surface facing up is rate determining. When grown in the [001] direction the pore aperture facing up is blocked by the ndc linkers. These observations also imply very dense and homogeneously grown SURMOFs without many cracks and macro defects at grain boundaries.

Figure 5. A) Illustration of the crystal structure of [Cu2(ndc)2(dabco)]n frameworks. Gray, C; red, O; green, N; blue, Cu. B,C) Depiction of the frameworks grown in the [001] (B) and [100] directions (C) and illustration of the sorption behavior. Reprinted with permission from ref. [43]. Copyright 2013 WileyVCH.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 6. Single-component methanol (black) and hexane (green) adsorption for C@B (a) and B@C (b). Reprinted with permission from ref. [46]. Copyright 2013 Wiley-VCH.

hexane mixtures, the hexane was repelled only by the C@B. In C the succinic acid groups dangle into the pores and decrease and polarize the pore space. Thus, adsorption of nonpolar hexane molecules is prevented, whereas small, polar methanol molecules get adsorbed. If the B framework is on top, both volatile organic compounds enter the top layer. Many studies exist on (homostructured) MOF thin films on macroporous substrates deposited typically by secondary growth, which were fabricated to serve as membranes for gas separation applications.[47–50] However, the above concept of programmed and heterostructured MOFs has not been systematically employed for MOF membranes, so far. Diffusion limitation is one drawback of microporous materials, for example in the case of applications in heterogeneous catalysis. MOFs, which feature structurally well-defined micropores, and at the same time some kind of mesopores, are thus a valid target. Such hierarchically structured MOFs can be achieved by mesostructure control of growing the microporous parent MOF around a template that can be removed after synthesis.[51, 52] Qiu et al.[53] transferred the concept of fabricating mesoporous silica to MOFs.[54] HKUST-1 was chosen for the proof of principle, and it was grown around micelles formed by a structure-directing agent (SDA). Removal of the SDA after synthesis of the framework generated mesopores besides the micropores of HKUST-1. By adjusting the synthesis conditions, tuning of the mesopore diameter in a range of 3.8 to 5.6 nm was possible. The N2 isotherms (77 K) of these materials reveal type I and type IV characteristics. The observed hysteresis loop suggested connections between the micropores and mesoChemPhysChem 2014, 15, 823 – 839

826

CHEMPHYSCHEM MINIREVIEWS pores. Interestingly, the authors were able to control the hysteresis shape, the overall uptake, and the BET surface areas of the material by the size regime of the mesopores. In related studies, MOFs have been transformed to hierarchically structured aerogels.[55] Choi et al.[56] generated hierarchically well defined meso- and macropores within the structure of MOF-5 ([Zn4O(bdc)3]n). This is achieved by a mixed linker approach; during solvothermal synthesis, besides the H2bdc linker, a monocarboxylate with a long alkyl chain (4-(dodecyloxy)benzoic acid, DBA) is added. Depending on the DBA concentration during synthesis, the morphology of the crystals is controlled. During the nucleation process the alkyl chains of DBA are interacting and framework growth occurs around these micelles. If the ratio of bdc/DBA is 50:50, the formed crystallites contain additional macro- and mesopores, distributed over the entire MOF crystal. When the DBA content is decreased (bdc/DBA = 70:30), the macro- and mesopores are only found in the bulk and connection to the crystal surface is only achieved by micropores (pmg-MOF-5). The DBA is completely flushed out of the systems during activation, which makes the macro- and mesopores fully accessible for gaseous guest molecules. At 195 K pmg-MOF-5 shows an improved CO2 uptake (  2000 mg g 1) in comparison to MOF-5 (  1500 mg g 1), despite having a smaller BET surface area (Figure 7). The CO2 isotherm now contains a hysteresis, which

www.chemphyschem.org

Figure 8. Illustration of two isoreticular frameworks containing different metal SBUs.

3. Influence of the Metal Ion Centers on the Adsorption Properties By choosing the metal ion centers of a MOF, one can tune the gas adsorption properties over a considerably wide range (Figure 8). In the following paragraphs we want to discuss adsorption in MOFs with coordinatively unsaturated metal sites (CUSs) as well as the influence of the metal ion on breathing properties of flexible MOFs. 3.1. Coordinatively Unsaturated Sites in Rigid MOFs

Figure 7. CO2 sorption isotherm of pmg-MOF-5 (red) and MOF-5 (blue). Filled symbols represent adsorption, open symbols represent desorption. Reprinted with permission from ref. [56]. Copyright 2011 American Chemical Society.

is attributed to the meso- and macropores. CO2 is adsorbed within the micropores until a certain pressure is reached and then the meso- and macropores start filling. The adsorption process is thus controlled by the macro-, meso-, and micropores of the crystal. The desorption process is solely controlled by the micropores (this behavior is identical to the adsorption usually observed for ink-bottle pores).[57]

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A metal ion center of the framework is called coordinatively unsaturated if the actual coordination number by ligating to the linkers is lower than typically expected for the actual ligand field, charge, or oxidation state. These CUSs are excellent adsorption sites for certain guest molecules. The choice of the metal center modulates the affinity for adsorption of certain guests. In a combinatorial effort of neutron diffraction and theoretical calculations Wu and co-workers[58] identified the CUSs in the two prototypical frameworks HKUST-1 and MOF74[59] as strong adsorption sites for polar adsorbate molecules such as CO2. In the following we will discuss these two prominent examples: [M3(btc)2]n (with M = Ru2/3 + ,[60] Mo2 + ,[61] Ni2 + ,[62] Cr2 + ,[63] and Cu2 + , i.e. HKUST-1) and [(M2(dobdc)H2O]n (with M = Zn2 + , Co2 + ,[64] Ni2 + ,[65] Mg2 + ,[66] and Mn2 + ;[67] dobdc4 = 2,5dioxido-benzenedicarboxylate; i.e. MOF-74 also known as CPO27). In the HKUST-1 type of framework, M2 paddlewheel units are coordinated by four btc ligands forming square-planar [M2btc4] units axially coordinated by solvent molecules. Activation at elevated temperatures removes the solvent molecules, thereby giving rise to CUSs. Wade et al.[68] showed that the isosteric heat of adsorption Qads for CO2 is strongly dependent on the metal center (Figure 9). However, note that not all of the materials could be activated properly. [Ni3(btc)2]n remained coordinated by one H2O and two Me2NH molecules and in [Ru3(btc)2]n 0.5 mol equivalent btc ions are left as a counter ChemPhysChem 2014, 15, 823 – 839

827

CHEMPHYSCHEM MINIREVIEWS

www.chemphyschem.org interaction with CO2 guest molecules leading to a higher Qads. [Ni2(dobdc)]n and [Co2(dobdc)]n have similar electronegativities and similar Qads (41 and 37 kJ mol 1, respectively). The sorption isotherms show remarkable differences in the slopes of the CO2 adsorption (Figure 11), which cannot only be explained by

Figure 9. Top left: Representation of the HKUST-1 crystal structure. Top right: Heats of adsorption in different HKUST-1 analogues. Bottom: Depiction of the M2 paddlewheel coordination environment during the activation process (dehydration). Color scheme: blue, M2 + ; red, O; gray, C. Violet polyhedra represent the coordination environment of M2 + ions. Adapted from ref. [68] with permission from The Royal Society of Chemistry.

charge to stabilize the mixed valence paddlewheel. Qads has its highest value for [Ni3(btc)2]n, although one has to keep in mind that other studies showed that 1) amines and 2) H2O can increase the heat of adsorption for CO2 significantly. [69, 70] [Ru3(btc)2]n has a higher Qads than the remaining frameworks, since the Ru paddlewheel unit has a higher positive charge and binds more strongly to the CO2. However, free btc ions can also have an influence on Qads. The other materials could be activated properly, and the Qads decreases from [Cu3(btc)2]n over [Cr3(btc)2]n to [Mo3(btc)2]n. In MOF-74 the M2 + cations are sixfold coordinated by oxygen atoms from dobdc linkers and H2O. These hexaoxo clusters form one-dimensional helical chains that are bridged by dobdc linkers to form a honeycomb-like framework with one-dimensional channels. Activation removes the coordinating H2O molecules, leaving channels with exposed CUSs (Figure 10). Caskey et al.[66] evaluated the CO2 adsorption of [M2(dobdc)]n derivatives with different M2 + ions (M = Co2 + , Ni2 + , Zn2 + , Mg2 +). The highest heat of CO2 adsorption is observed for [Mg2(dobdc)]n (47 kJ mol 1). This is attributed to the very electropositive Mg2 + CUSs in the framework, which have a strong

Figure 10. Illustration of the activation (dehydration) of [M2(dobdc)]n generating CUS in the framework. M = Zn, Co, Ni, Mg, Mn. Blue, M2 + ; red, O; gray, C.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 11. CO2 sorption isotherms at 296 K of different MOF-74 materials. Filled symbols indicate adsorption, open symbols desorption. Reprinted with permission from ref. [66]. Copyright 2008 American Chemical Society.

the much lower density of the Mg compound. For [Zn2(dobdc)]n no heat of adsorption is given, due to framework collapse after activation of the material. Glover et al.[71] studied the same series of [M2(dobdc)]n derivatives and measured their capability of capturing different toxic molecules by micro-breakthrough experiments. The nature of the metal center leads to different sorption kinetics. The trend shown by the CO2 adsorption experiments by Caskey et al. is maintained. The Mg-containing MOF has a higher polarity and shows faster uptake for polar adsorptives (cyanogen chloride), whereas nonpolar adsorptives (octane or sulfur dioxide) adsorb faster in the frameworks with exposed Co, Ni, and Zn CUSs. 3.2. Breathing, Flexible MOFs Flexible MOFs distinguish themselves from other porous materials by their ability to transform between two or more crystal phases triggered by specific outer stimuli. We want to discuss the phase transitions in several prototypical flexible MOFs induced by gas adsorption. The MIL-53/MIL-47 system (MIL-53: [M(bdc)(OH)]n, M3 + = Al3 + ,[72] Fe3 + ,[73] Cr3 + ,[74] Ga3 + ,[75] In3 + ,[76] Sc3 + ;[77] MIL-47: [M(bdc)(O)]n, M4 + = V4 + ;[78] MIL = Matriaux de l’Institut Lavoisier) is one of the best-studied flexible systems and this series shows the influence of the metal center on the frameworks’ sorption properties. The MIL-53 framework shows flexibility, but the related MIL-47 framework is known to be rigid. MIL-53 consists of M3 + ions, sixfold coordinated by four bdc linkers and two OH groups (Figure 12). The closely related MIL-47(V) structure exhibits a VO6 moiety, that is, V4 + ions, and the bridging m2-OH (hydroxy) groups known from the MILChemPhysChem 2014, 15, 823 – 839

828

CHEMPHYSCHEM MINIREVIEWS

www.chemphyschem.org

Figure 13. Illustration of the structure of DUT-8. Color scheme: Gray, C; red, O; blue, N; coordination polyhedra around M are depicted in green.

Figure 12. A) Illustration of a single pore of MIL-53. B) Depiction of the onedimensional metal backbone in MIL-53. Blue octahedra represent the coordination environment of the metal center. Red, O; gray, C.

53 family are replaced by m2-O (oxo) groups. It is speculated that the bridging m2-O groups are the origin of the missing breathing effect for this analogous material. Leclerc et al.[79] found a method to isolate a MIL-53(V) type of material with V3 + centers that are bridged by OH groups [V(III)(OH)(bdc)]n. Indeed, this MOF is flexible again. MIL-53 can be present in different crystal phases, depending on the activation state and the incorporation of guest molecules. After activation, MIL-53 consists of a high-temperature phase with a large pore (lp), which contracts to a narrow-pored phase (np) when low amounts of polar molecules are adsorbed. If the partial pressure of the adsorptive is increased, MIL-53 switches back to the lp form. For the MIL-53(Fe) and MIL-53(Sc) even a closedpore phase (cp) is observed after activation. The mechanism of flexibility in MIL-53 was investigated thoroughly. There are in situ studies describing the different stages of the framework transition by in situ IR, powder X-ray diffraction (PXRD), and NMR data.[72, 80, 81] Apart from the famous MIL-53, some M2-paddlewheel pillared-layered MOFs also show flexibility upon guest adsorption.[82–85] A notable example is the DUT-8 series of frameworks introduced by Kaskel et al. (DUT = Dresden University of Technology; [M2(2,6-ndc)2(dabco)]n, with M2 + = Ni2 + , Co2 + , Cu2 + , and Zn2 + ).[86] In this class of frameworks a dinuclear paddlewheel is fourfold bridged by linear 2,6-naphthalenedicarboxylate (2,6-ndc) building up two-dimensional [M2(2,6-ndc)2]n layers that are axially interconnected by dabco pillars (Figure 13). The first compound of this series was DUT-8(Ni).[87] The PXRD patterns of this compound showed that DMF containing as-synthesized DUT-8(Ni) shows a drastic pore contraction after solvent removal, accompanied by a color change. This indicates a change of the nickel coordination environment. Naturally, the O-Ni-O angle in the nickel coordination sphere must change, which results in a different ligand field of the metal centers. The contraction is attributed to interactions between adjacent 2,6-ndc ligands that share an O-Ni-O angle. The authors extended their studies to an isoreticular series  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

of [M2(2,6-ndc)2(dabco)]n with M2 + = Ni2 + , Co2 + , Cu2 + , and Zn2 + .[86] To rationalize this flexibility one has to consider the preferred coordination environment of the metal ion centers. As model complexes for the comparison of these compounds we may choose the metal acetates of the respective metal cations, since the coordination bond of the acetate is similar to 2,6-ndc bonding. The Cu2 + cations (d9 electron count) form dimeric [Cu2(AcO)4] of paddlewheel structure with acetate (water free form) and accommodates the preferred square-planar structure (water molecules would coordinate at a longer distance in the apical position resulting in a square pyramid).[88] The energy barrier to deviate from this preferred square-planar CuO4 coordination and allow attractive p–p interactions between adjacent 2,6-ndc groups is high. The Zn2 + cation (d10 electron count) is known to preferentially form tetrahedral complexes; the anhydrous form of zinc(II) acetate is a tetramer of the formula [Zn4O(AcO)6][89] and the zinc(II) acetate dihydrate features a distorted octahedral geometry.[90] Considering this, one can recognize that in case of di-zinc paddlewheel structures Zn2 + is forced into a non-preferred coordination environment in the lp form, so that structural flexibility is facilitated and the structure irreversibly transforms from the lp to a np form upon desolvation. It is suggested that the contracted form (np) leads to bond angles more similar to the preferred tetrahedral coordination sphere of the Zn2 + ion in a ZnO4 environment. DFT calculations of simple, nonperiodic model structures for such kinds of [M2(2,6-ndc)2(dabco)]n frameworks (M = Cu and Zn) done by Bureekaew et al.[91] nicely confirm this qualitative reasoning. Co and Ni acetate form octahedral complexes of the type [M(AcO)2(H2O)4].[92] For DUT-8(Co) and DUT-8(Ni) seemingly neither the expanded nor the contracted form of the materials is at a global energetic minimum, thus switching between the two forms is reversibly possible. Taking such kinds of metal ion and electron count dependent ligand field factors into account, the gas adsorption properties and responsiveness in this series of compounds can be rationalized (Figure 14). For all analyzed gases (N2, CO2, and nbutane) DUT-8(Zn) shows the lowest uptake and Langmuir type I behavior. DUT-8(Zn) stays in the contracted np form, the pores fill at low pressure completely, and no additional pore volume is added. For DUT-8(Cu) the isotherms show a similar shape, but due to the rigidity of the Cu SBU the framework is permanently in the lp form resulting in a considerably higher uptake. DUT-8(Ni) is the only derivative showing large hysteresis upon adsorption of all three analytes, N2, CO2, and nChemPhysChem 2014, 15, 823 – 839

829

CHEMPHYSCHEM MINIREVIEWS

www.chemphyschem.org

Figure 14. Sorption isotherms of the four derivatives of DUT-8 with M = Co, Ni, Cu, and Zn. A) N2 isotherms at 77 K. B) n-Butane isotherms at 293 K. C) CO2 isotherms measured at 196 K. Filled symbols represent adsorption and open symbols desorption. Adapted from ref. [86] with permission from The Royal Society of Chemistry.

butane. At the beginning of the adsorption experiments the framework exhibits a completely closed np form and, when a certain threshold pressure of the adsorptive is reached, the framework expands to the lp form leading to a rapid filling of the pores with adsorbate. This feature of a complete pore contraction and expansion triggers selectivity for CO2 adsorption over CH4 (isotherms not shown). All other frameworks show CH4 uptake due to their partially open form. DUT-8(Co) shows a sorption behavior similar to that of DUT-8(Zn). For nonpolar adsorptives, low uptake with type I isotherm behavior is observed (N2 and n-butane). However, for the polar CO2 molecule a large hysteresis loop is observed.

4. Linker Functionalization We will focus on functionalized linkers containing side groups on the aromatic core of 1,4-benzenedicarboxylate (bdc) and call these linkers fu-bdc (Figure 15). The side groups can be introduced by either postsynthetic methods[93–97] or direct synthesis prior to MOF formation.[98, 99] The functionality can be of varying complexity, but small changes, such as NH2 or halide groups, may alter the framework’s properties significantly or even drastically. In this section we will discuss the influence of such a kind of linker functionalization on guest adsorption in rigid as well as flexible MOFs.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4.1. Functionalized Linkers in Rigid MOFs The seminal conceptual study on isoreticular MOFs (IRMOFs, possessing the same network topology as MOF-5) with a range of substituted bdc linkers was carried out by Eddaoudi et al.[100] The influence on CH4 adsorption was analyzed and the highest uptake was observed for the analogue containing a DHC-bdc linker (Figure 15). The attached four-membered saturated ring on the bdc adds extra adsorption sites for the nonpolar CH4. There are plenty of studies on the adsorption properties of these prototypic isoreticular frameworks;[101–103] however, due to the poor moisture stability of the Zn4O-based IRMOFs we do not cover these nonetheless interesting materials in this review. In contrast, UiO-66 and related systems (Universitetet i Oslo, [Zr6(OH)4O4(bdc)6]n)[104] have become frameworks of high interest because of exceptional thermal and chemical robustness and a wide variety of functionalized analogues has been reported. We thus chose this MOF system as an example for the tuning of adsorption in rigid MOFs. For rigid MOFs the main influence the linker has on the sorption properties is the addition of further sorption sites, as well as the confinement of the pore entrance to only fit a certain type of molecule. Huang et al.[105] showed the possibility to increase the heat of adsorption for CO2 by employing simple functionalized linkers such as NH2-bdc, NO2-bdc, Br-bdc, and (CH3)2-bdc at UiO-66 (Figure 16). Also, an increased CO2 uptake was found for UiO-66(NH2bdc), UiO-66(NO2-bdc), and UiO-66((CH3)2-bdc). This is expectChemPhysChem 2014, 15, 823 – 839

830

CHEMPHYSCHEM MINIREVIEWS

www.chemphyschem.org

Figure 15. Overview of the functionalized bdc linkers (fu-bdc) discussed within Section 4.

ed, since polar molecules such as amines (e.g. ethanolamine) are used for CO2 capture on an industrial scale.[106] Nevertheless, UiO-66((CH3)2-bdc) shows the highest uptake and Qads. This discrepancy is explained by the reduction of the pore volume, which results in smaller cavities leading to overall stronger CO2–framework interactions. There are many studies that deal with the synthesis of functionalized UiO-66 analogues[107–109] and in general it can be said that the surface area and N2 uptake are mostly influenced by the molar mass and size of the linker, whereas the CO2 uptake is generally influenced by the addition of further adsorption sites (e.g. by utilizing fu-bdc: OH-bdc, NH2-bdc, NO2-bdc, SO3Na-bdc, COOH-bdc, and I-bdc). Cohen and co-workers[110] demonstrated the striking influence of amino-halo fu-bdc regioisomers (NH2X-bdc, X = Cl, Br, I) on the sorption properties of UiO-66 and [Zn2(fu-bdc)2(dabco)]n (DMOF). Depending on the positioning of the two substituents fu on the bdc core (either in 2,3- or 2,5-positions), differences in the N2 adsorption isotherms are observed (Figure 17). For instance, in the rigid UiO-66 materials 2,3-substitution leads to a lower nitrogen uptake than the respective 2,5-substitution of the linker. However, these differences are much more pronounced for the pillared-layered DMOFs. The 2,3-substituted frameworks take up considerable amounts of N2, whereas the  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 16. CO2 sorption isotherms of different UiO-66 derivatives recorded at 293 K (A) and 273 K (B). C) Calculated Qads. Symbols—pink squares: UiO66((CH3)2-bdc); blue triangles: UiO-66(NH2-bdc); red circles: UiO-66(NO2-bdc); green diamonds: UiO-66; black diamonds: UiO-66(Br-bdc). Reprinted from ref. [105] with permission from The Royal Society of Chemistry.

frameworks constructed with the 2,5-substituted linkers show hardly any N2 uptake. It is suggested that in the case of the UiO-66 material the side groups in positions 2 and 3 block the pore entries more efficiently than the side groups in the 2,5position. For the DMOFs the authors suggest on the basis of single-crystal data that the pore is slightly contracted when the linker is substituted in the 2,5-position. Hence, these frameworks show increased structural flexibility, similar to the ChemPhysChem 2014, 15, 823 – 839

831

CHEMPHYSCHEM MINIREVIEWS

Figure 17. N2 adsorption isotherms (77 K) for functionalized UiO-66 (top) and DMOFs ([Zn2(fu-bdc)2(dabco)]n ; bottom). Reprinted with permission from ref. [110]. Copyright 2011 Wiley-VCH.

other 2,5-substituted [Zn2(fu-bdc)2(dabco)]n frameworks discussed below, leading to no or only little N2 uptake.

4.2. Functionalized Linkers in Flexible Frameworks Devic et al.[111] analyzed the influence of fu-bdc (fu = Cl, Br, CH3, NH2, and (COOH)2) on the pore structure of MIL-53(Fe) during CO2 adsorption by in situ PXRD at 230 K (Figure 18). At low pressures and contrary to the case of MIL-53(Cr), the MIL53(Fe) is present in a cp form. When a threshold pressure is reached (< 0.1 bar) the material has an intermediate (int) form,

Figure 18. Illustration of the evolution of the cell volume of MIL-53(Cr) and differently functionalized MIL-53(Fe) materials upon CO2 adsorption. Reprinted from ref. [111] with permission from The Royal Society of Chemistry.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org but if 0.3 bar CO2 pressure is exceeded the material switches to the np form. At higher pressures the transition from a phase mixture, containing np and lp forms, to a phase pure lp form (p(CO2) > 8 bar) occurs. If the framework is prepared with linkers containing very polar groups ((COOH)2-bdc, NH2bdc), no CO2 uptake is found and the material stays in the cp state over the whole pressure range (0 to 10 bar CO2). By using halide substituents (Cl-bdc and Br-bdc), the materials switch from cp to the np form below 0.1 bar. MIL-53-Cl(Fe) shows a phase mixture of np and lp forms and above 8 bar the material is solely present as the lp form. MIL-53-Br(Fe) switches at a CO2 pressure of 8 bar to a phase mixture of np and lp forms. Quantum mechanical calculations lead to the conclusion that the functionality on the bdc linker influences the electronic properties of the bridging OH groups, and the differences were correlated with the breathing mode. Essentially, the more or less weak and cooperative forces inside the pores (i.e. dipole–dipole interactions, hydrogen bonding) are modulated by linker functionalization. The impact of alkoxy side chains, having different length, grade of saturation, and branching, on the breathing behavior and CO2 adsorption of [Zn2(fu-bdc)2(dabco)]n was examined in some detail (fu-bdc = DE-bdc, DP-bdc, DiP-bdc, DB-bdc, BAbdc, or BPY-bdc; see Figure 15 for abbreviations).[112] Methoxyethoxy side chains trigger selective adsorption of CO2 over N2.[113] For [Zn2(BME-bdc)2(dabco)]n a strong dynamic behavior upon sorption/desorption of polar guests was found (Figure 19).[114] The as-synthesized DMOFs accommodate solvent molecules and are present in the lp form. After removal of the solvent, the alkoxy side chains can interact attractively with themselves and contraction to np is found. Loading the activated frameworks with polar molecules, for example, DMF, EtOH, or CO2, leads to re-expansion (lp). All materials show little to no uptake of N2 at 77 K. However, the nature of the side chains of fu-bdc has strong effects on the CO2 adsorption isotherms (195 K). In the low-pressure region of the CO2 isotherms all materials are present in the np form and take up small amounts of CO2. Once a certain threshold pressure is reached a phase transition from the np to the lp form takes place, which is indicated by a step in the isotherm. All materials feature hysteretic behavior on the desorption branch. In the following we want to briefly emphasize some of the observed trends. The longer the attached side chain, the higher the threshold pressure for the np–lp transition and the wider the hysteresis loop. The ethoxy-substituted [Zn2(fu-bdc)2(dabco)]n opens at 0.2 bar, propoxy slightly higher, and butoxy at 0.6 bar. [Zn2(BPY-bdc)2(dabco)]n shows a wide hysteresis and switching to lp at 0.5 bar. The particularly high threshold pressure for the [Zn2(DB-bdc)2(dabco)]n can be reasoned by strong nonpolar interactions of the side chains combined with the inability to offer good adsorption sites for the polar CO2 molecules. In the case of shorter side chains, the polarity of the bridging oxygen atom between the benzene core and the side chain and the weaker nonpolar interactions (van der Waals) of the alkyl chains reduce the barriers for CO2 entering the pore. The somewhat higher polarizability on the unsaturated BA-bdc as ChemPhysChem 2014, 15, 823 – 839

832

CHEMPHYSCHEM MINIREVIEWS

www.chemphyschem.org modulated by the competing interactions between side chains and guest molecules. The pinning of the side chains to the framework is essential for the corresponding triggering of np to lp transitions and the Zn2 + cation facilitates the structural changes at the paddlewheel unit (see Section 3).

5. Solid-Solution Frameworks Solid-solution metal–organic frameworks (SSMOFs), also called mixed-component MOFs (MIXMOFs) or multivariate MOFs (MTV-MOFs), are single-phase materials in which several building blocks of the same connectivity are present in parallel in the framework, but without translational symmetry (longrange ordering).[115–120] This means, for example, that different functionalized linkers or SBUs with varying metal ions but of same connectivity, spatial extension, and coordination environment are employed in one single-phase material. The most common way to prepare MIXMOF systems is by solvothermal synthesis with two or more components present in the reaction mixture. In the following paragraphs we discuss the adsorption properties of MIXMOFs with regard to the parent MOFs. An extensive study on socalled MTV-MOFs (MOFs containing two or more different linkers) was conducted by Yaghi and coworkers.[117] IRMOFs (based on Figure 19. Top: Sketch of the breathing in [Zn2(fu-bdc)2(dabco)]n. Bottom: CO2 and N2 sorption isotherms of differMOF-5) with an ensemble (mixent MOFs of the type [Zn2(fu-bdc)2(dabco)]n measured at 195 and 77 K, respectively. Reprinted with permission ture) of two or more fu-bdc linkfrom ref. [112]. Copyright 2012 American Chemical Society. ers integrated into the framework were obtained (Figure 20). Interestingly, the gas sorption properties of the resulting materials are not necessarily linearly compared to the saturated DP-bdc favors attractive interaction dependent on the incorporated linker ratios, but show strong with CO2, over intramolecular interactions between side chains, nonlinear effects. For example, two MOFs containing binary which leads to a lower threshold pressure. The much stronger mixtures of bdc and BA-bdc (ratio 1:0.46), and bdc and BPhside-chain interactions of [Zn2(BPY-bdc)2(dabco)]n yield a very bdc (ratio 1:0.4) show hydrogen uptakes of 177 and high threshold pressure. However, the triple bonds seem to 172 cm3 g 1, respectively (whereas pure MOF-5 shows an offer additional strong adsorption sites causing the large hysteresis. Thus, the adsorption and the framework flexibility are uptake of 139 cm3 g 1). Remarkably, for a ternary mixture con 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemPhysChem 2014, 15, 823 – 839

833

CHEMPHYSCHEM MINIREVIEWS

www.chemphyschem.org

Figure 20. Representation of the preparation of a MTV-MOF containing eight different functionalities. Reprinted with permission from ref. [117]. Copyright 2010, American Association for the Advancement of Science.

taining all three linkers bdc, BA-bdc, and BPh-bdc (ratio 1:0.48:0.5) the overall hydrogen adsorption increases to 189 cm3 g 1. The authors explain this observation by the presence of discrete linker arrays in the complex framework that can increase the overall uptake.[121] Linear control of the adsorption properties was found by Fukushima et al. for the series [Zn(5-NO2-ip)x(5-MeO-ip)1 x(bipy)]n (5-NO2-ip = 5-nitroisophthalate; 5-MeO-ip = 5-methoxyisophthalate; x = 0–1; bipy = 4,4’-bipyridine).[122] The two parent compounds [Zn(5-NO2-ip)1(bipy)]n (CID-5) and [Zn(5-MeO-ip)1(bipy)]n (CID-6) are members of the class of porous coordination polymers with an interdigitated structure (CIDs; Figure 21). The CO2 adsorption properties at 195 K for both materials are extremely different (Figure 22). Activated methoxy-substituted CID-6

Figure 21. Depiction of the construction of CID frameworks. Color scheme: light blue, M2 + ; gray, C; red, O; blue, N.

shows type I behavior and the guest-accessible voids fill with CO2 at very low pressures; saturation is reached at 1 kPa when two CO2 molecules per unit cell are adsorbed. The nitro-substituted CID-5, however, has a gate opening pressure and starts adsorbing at 1.3 kPa and the overall CO2 uptake of 3.5 CO2

Figure 22. CO2 adsorption at 195 K (A) and H2O sorption at 298 K (B) in a series of solid-solution frameworks of the CID family. Adsorption and desorption are shown with solid or open symbols, respectively. Reprinted with permission from ref. [122]. Copyright 2010 Wiley-VCH.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemPhysChem 2014, 15, 823 – 839

834

CHEMPHYSCHEM MINIREVIEWS

www.chemphyschem.org

molecules per unit cell is higher than for the methoxy analogue. Additionally, water adsorption studies at ambient temperature were performed up to 3 kPa. CID-6 features a slow linear adsorption, whereas CID-5 again exhibits a gate opening pressure. This behavior is attributed to the nonporous nature of activated CID-5, which does not adsorb water until a certain pressure is reached, whereas CID-6 is porous and can fill directly with water molecules. The related solid solutions [Zn(5-NO2-ip)x(5-MeO-ip)1 x(bipy)]n (CID-5/6) allow control of the adsorption properties of CO2 and H2O by the molar fraction of NO2-ip versus MeO-ip. The gate opening pressure decreases linearly with increasing content of 5-MeO-ip in the sample. This can be explained by the interactions, which are triggered by the NO2 group, that cause the framework to be nonporous. Although the H2O isotherm of CID-5 material shows a wide hysteresis, a significant decrease of the hysteresis width is observed with increasing 5-MeO-ip content. The same series of solid-solution CIDs was also used in a later study and the separation properties for CH4/CO2 and CH4/C2H6 were examined.[123] A related study showed how the threshold pressure of a phase transition upon guest inclusion can be triggered in functionalized DMOFs [Zn2(BME-bdc)x(DB-bdc)2 x(dabco)]n (x = 0.5, 1, 1.5).[112] BME-bdc and DB-bdc were chosen because of the similar bulkiness but different polarity of the side chains, that is, methoxyethoxy and butoxy (Figure 23). This small varia-

tion leads to very different CO2 sorption isotherms relative to the parent compounds employing just one type of fu-bdc (Figure 23). [Zn2(BME-bdc)2(dabco)]n has a considerably low threshold pressure of 0.2 bar for the np–lp transition and a small hysteresis. For [Zn2(DB-bdc)2(dabco)]n a much wider hysteresis loop is observed and the threshold pressure initiating the phase transition occurs at around 0.6 bar. The corresponding SSMOFs [Zn2(BME-bdc)x(DB-bdc)2 x(dabco)]n (x = 0.5, 1, 1.5) reveal the following behavior. A fraction of 25 % of DBbdc leads to an increase in the threshold pressure, the pore expansion starts at 0.3 bar, and it shows a wider hysteresis compared to the parent [Zn2(BME-bdc)2(dabco)]n (0 % DB-bdc). Increasing the DB-bdc level to 50 % gives a further shift to 0.5 bar and at 75 %, the threshold pressure of 0.8 bar is even higher than that found for the parent [Zn2(DB-bdc)2(dabco)]n (0.6 bar; see Figure 19). In this system strong nonlinear effects, similar to MTV-MOFs, are found as a consequence of the choice of the substituents at fu-bdc. At the end of this section, we want to present one example of mixed-metal SSMOFs. The MIL-53 class of materials has been thoroughly studied in this respect[119, 124] and the outcome of one mixed-metal approach will be briefly highlighted. Nouar and co-workers[125] prepared a SSMIL-53 containing both Fe3 + and Cr3 + ions. Both parent frameworks, the MIL-53(Cr) and the MIL-53(Fe), are known to be flexible. However, both frameworks show a very different behavior. MIL-53(Cr) is present in the lp form after complete guest removal, whereas MIL-53(Fe) features a cp form after activation. Moreover, MIL-53(Cr) is known to be a responsive framework that undergoes two phase transitions upon CO2 adsorption at 283 K, a lp–np transition at very low CO2 pressure followed by the reverse np–lp transition at around 3 bar CO2 pressure. The iron material is different in this respect and pore opening to the lp form requires a high pressure greater than 20 bar (note that the step in the isotherm at  5 bar is the transition from cp to np). Interestingly, for the SSMIL-53(Cr-Fe) a wide hysteresis and a phase transition to the lp form are observed at an intermediate pressure of 10 bar (Figure 24).

Figure 23. CO2 adsorption at 195 K in a series of mixed-linker pillared-layer frameworks of the type [Zn2(BME-bdc)x(DB-bdc)2 x(dabco)]n. Reprinted with permission from ref. [112]. Copyright 2012 American Chemical Society.

Figure 24. CO2 adsorption at 283 K in MIL-53(Cr-Fe) compared to the pure metal materials MIL-53(Cr) and MIL-53(Fe). Reprinted from ref. [125] with permission from The Royal Society of Chemistry.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemPhysChem 2014, 15, 823 – 839

835

CHEMPHYSCHEM MINIREVIEWS

www.chemphyschem.org

6. Guest-Assisted Adsorption The pores of MOFs can also be used to integrate guests that have a beneficial effect or can tune the sorption behavior of the system. Such a kind of modulation of adsorption properties of MOFs may be called guest-assisted adsorption. Guest incorporation can vary greatly and this includes the introduction of nanoparticles,[126–128] and organic molecules[129] such as photoswitches, in particular.[23] 6.1. Hydrogen Adsorption Modulated by Metal Nanoparticles The introduction of metal nanoparticles into MOFs to yield hybrid systems, often called metal@MOF, has been reviewed recently.[130, 131] We focus on the aspect of possibly enhancing the hydrogen adsorption in MOFs by metal nanoparticle inclusion. Hydrogen spillover is a well-established phenomenon in surface science. H2 molecules are adsorbed and dissociated to monoatomic H by one species (e.g. a metal nanoparticle) and then transported and adsorbed on a second species (e.g. a metal surface, a host matrix, the linkers in a MOF) that would not have conducted this dissociation under the same conditions.[132] The embedding of nanoparticles suitable for H2 dissociation into MOFs and the employment of the hydrogen spillover effect to increase hydrogen adsorption is, however, a topic discussed controversially in the current literature. Initially, Yang and co-workers[133] prepared mechanical mixtures of MOF-5 and a conventional Pt catalyst supported on an activated carbon (AC). Increase of the overall H2 uptake of the material from 0.4 wt % (MOF-5) at room temperature and 10 MPa H2 pressure to 1.5 wt % (Pt/AC/MOF-5) was reported. The hydrogen is first adsorbed on the Pt and dissociated. From the Pt a primary spillover to the AC occurs and from there some kind of secondary spillover to the MOF was suggested. Even though these results seem promising, studies by other authors were not able to reproduce the findings on similar samples.[134, 135] Other researchers estimate the effect to be much lower and the effect is still up for discussion.[136] In a more recent study by Kalidindi et al. palladium nanoparticles were embedded into COF-102 (covalent organic framework 102).[128] The nanoparticles were prepared by decomposition of volatile organometallic precursors. The H2 adsorption was increased from 0.15 wt % excess hydrogen adsorbed at 298 8C and 20 bar to 0.38 wt % for a composite of 3.5 wt % Pd@COF-102 and to 0.42 wt % for 9.5 wt % Pd@COF-102 (Figure 25). The authors explain the increase in adsorption by hydrogenation of organic residues being present on the surface of the palladium nanoparticles as well as the formation of palladium hydrides. In particular, the group of Suh and co-workers[127] carried out numerous related studies on metals@MOFs. For example, Mg nanoparticles were introduced to [Zn4O(atb)2]n (SNU-90; atb = aniline-2,4,6-tribenzoate) based on the solvent-free organometallic gas-phase infiltration technique.[126] Materials with different loadings were prepared, namely Mg@SNU-90 with 1.26, 6.25, and 10.5 wt % Mg inside the pores. As expected the N2 adsorption decreases with increasing magnesium inclusion.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 25. H2 adsorption in COF-102 and Pd@COF-102 materials at 297 K. Reprinted with permission from ref. [128]. Copyright 2012 Wiley-VCH.

Also, the hydrogen taken up by physisorption at 77 K decreases with increasing Mg content. However, the (extrapolated) zero-coverage isosteric heats of adsorption increase with increasing Mg content. The authors claim that the material is chemisorbing hydrogen on the surfaces of the Mg nanoparticles. This was verified by high-pressure, high-temperature adsorption experiments. The material with the highest Mg content (10.5 wt %) was exposed to 30 bar of H2 at 473 K and 0.71 wt % excess hydrogen chemisorption was found. The authors state that under these conditions the H2 is solely bound by chemisorption to the Mg nanoparticles. The mass uptake of H2 per mass of Mg hosted in the matrix of SNU-90 amounts to 7.5 wt %, which is the highest value so far measured for magnesium (nano)particles or molecular complexes. This particular example shows the usefulness of MOFs as (innocent) stabilizing matrix for functional nanoparticles. The MOF matrix modulates the sorption properties of the embedded guest, rather than the guest modulating the sorption properties of the matrix. 6.2. Photoswitching the Adsorption Properties A novel approach to trigger gas adsorption properties was shown by Yanai and co-workers.[23] The photoswitchable molecule azobenzene (AB) was included in [Zn2(bdc)2(dabco)]n. This molecule can undergo a conformational change on exposure to light of a specific wavelength and it switches from the trans to the corresponding cis isomer. As discussed in the previous sections, the [Zn2(bdc)2(dabco)]n framework can undergo phase transitions from the open activated lp to a np form when guest molecules are adsorbed inside the pore. This effect is particularly strong if functional side groups of fu-bdc are acting as “immobilized” guest molecules. When the AB photoswitch is accommodated in its trans form into the pores, to form an AB@[Zn2(bdc)2(dabco)]n host–guest complex, the MOF contracts to a np form (presumably based on p–p interactions). After exposing the framework to UV light the AB switches its conformation from trans to cis. This conformational change is transmitted from the AB to the MOF host, which undergoes a phase transition to the lp form. The trans-AB@ ChemPhysChem 2014, 15, 823 – 839

836

CHEMPHYSCHEM MINIREVIEWS [Zn2(bdc)2(dabco)]n is nonporous towards N2, whereas the cisAB@[Zn2(bdc)2(dabco)]n can take up considerable amounts of N2. Only a fraction of the pore volume is filled with AB, and the lp form can now accommodate other guest molecules such as N2. Thus, the N2 sorption properties can be controlled by exposure of the AB-modified MOF to UV light (Figure 26).

www.chemphyschem.org properties and softness. This may eventually lead to novel designer adsorbents, sensors, intelligent catalysts, and more.

Acknowledgements A.S. acknowledges the Research Cluster SusChemSys (http:// www.cmt.rwth-aachen.de/projects.html) for a doctoral fellowship and the German Academic Exchange Service (DAAD) for support. S.H. is grateful to Tony Cheetham for support and to the European Research Council for funding. Keywords: breathing effect · gas adsorption · host–guest systems · metal–organic frameworks · solid solutions

Figure 26. Top: Schematic depiction of the working principle of the AB@[Zn2(bdc)2(dabco)]n hybrid material. Bottom: Difference of the N2 adsorption (77 K) for AB@[Zn2(bdc)2(dabco)]n with the AB photoswitch in trans (red) and cis (blue) conformation. Reprinted with permission from ref. [23]. Copyright 2012 American Chemical Society.

7. Summary and Perspectives In this Minireview we have discussed general concepts for tuning the gas sorption properties of metal–organic frameworks. Mesostructuring including hybridization as core–shell crystals modulates sorption kinetics and selectivity. The nature of coordinatively unsaturated metal centers plays an essential role and the soft, responsive properties of frameworks can be advantageous for the control of adsorption/desorption phenomena. Functionalization of linker molecules offers another parameter set for introducing diversity and complexity to the available MOF libraries. The concept of solid-solution (or multivariate) MOFs is a powerful method to further fine-tune material properties in a linear or even nonlinear fashion. The co-adsorption of functional guests may allow switching between narrow- and large-pore phases. Clearly, the unique perspectives of these kinds of supramolecular host–guest interactions are associated with the soft porous crystal nature of the third generation of MOFs. One can foresee an emerging “fourth generation” of MOFs that is based on the targeted combination of specific physical properties, including charge transport (electrons, holes, ions), magnetism (i.e. spin-crossover systems), luminescence, and so forth, with the modulation of sorption  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

[1] K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae, J. R. Long, Chem. Rev. 2012, 112, 724. [2] J. Liu, P. K. Thallapally, B. P. McGrail, D. R. Brown, J. Liu, Chem. Soc. Rev. 2012, 41, 2308. [3] M. P. Suh, H. J. Park, T. K. Prasad, D.-W. Lim, Chem. Rev. 2012, 112, 782. [4] J.-R. Li, J. Sculley, H.-C. Zhou, Chem. Rev. 2012, 112, 869. [5] L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne, J. T. Hupp, Chem. Rev. 2012, 112, 1105. [6] D. Farrusseng, S. Aguado, C. Pinel, Angew. Chem. Int. Ed. 2009, 48, 7502; Angew. Chem. 2009, 121, 7638. [7] J. Y. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. B. T. Nguyen, J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450. [8] S. Kitagawa, R. Kitaura, S.-i. Noro, Angew. Chem. Int. Ed. 2004, 43, 2334; Angew. Chem. 2004, 116, 2388. [9] G. Frey, Chem. Soc. Rev. 2008, 37, 191. [10] O. K. Farha, J. T. Hupp, Acc. Chem. Res. 2010, 43, 1166. [11] G. Frey, J. Solid State Chem. 2000, 152, 37. [12] M. O’Keeffe, M. Eddaoudi, H. Li, T. Reineke, O. M. Yaghi, J. Solid State Chem. 2000, 152, 3. [13] M. Eddaoudi, D. B. Moler, H. Li, B. Chen, T. M. Reineke, M. O’Keeffe, O. M. Yaghi, Acc. Chem. Res. 2001, 34, 319. [14] M. O’Keeffe, O. M. Yaghi, Chem. Rev. 2012, 112, 675. [15] D. J. Tranchemontagne, J. L. Mendoza-Cortes, M. O’Keeffe, O. M. Yaghi, Chem. Soc. Rev. 2009, 38, 1257. [16] O. K. Farha, I. Eryazici, N. C. Jeong, B. G. Hauser, C. E. Wilmer, A. A. Sarjeant, R. Q. Snurr, S. T. Nguyen, A. O. Yazaydin, J. T. Hupp, J. Am. Chem. Soc. 2012, 134, 15016. [17] J. An, O. K. Farha, J. T. Hupp, E. Pohl, J. I. Yeh, N. L. Rosi, Nat. Commun. 2012, 3, 604. [18] H. Furukawa, Y. B. Go, N. Ko, Y. K. Park, F. J. Uribe-Romo, J. Kim, M. O’Keeffe, O. M. Yaghi, Inorg. Chem. 2011, 50, 9147. [19] J. L. C. Rowsell, E. C. Spencer, J. Eckert, J. A. K. Howard, O. M. Yaghi, Science 2005, 309, 1350. [20] S. Kitagawa, K. Uemura, Chem. Soc. Rev. 2005, 34, 109. [21] S. Henke, A. Schneemann, R. A. Fischer, Adv. Funct. Mater. 2013, 23, 5990. [22] I. Beurroies, M. Boulhout, P. L. Llewellyn, B. Kuchta, G. Ferey, C. Serre, R. Denoyel, Angew. Chem. Int. Ed. 2010, 49, 7526; Angew. Chem. 2010, 122, 7688. [23] N. Yanai, T. Uemura, M. Inoue, R. Matsuda, T. Fukushima, M. Tsujimoto, S. Isoda, S. Kitagawa, J. Am. Chem. Soc. 2012, 134, 4501. [24] G. Frey, C. Serre, Chem. Soc. Rev. 2009, 38, 1380. [25] S. Horike, S. Shimomura, S. Kitagawa, Nat. Chem. 2009, 1, 695. [26] H. Chun, J. Moon, Inorg. Chem. 2007, 46, 4371. [27] D. N. Dybtsev, H. Chun, K. Kim, Angew. Chem. Int. Ed. 2004, 43, 5033; Angew. Chem. 2004, 116, 5143. [28] H. Uehara, S. Diring, S. Furukawa, Z. Kalay, M. Tsotsalas, M. Nakahama, K. Hirai, M. Kondo, O. Sakata, S. Kitagawa, J. Am. Chem. Soc. 2011, 133, 11932. [29] T. D. Bennett, S. Cao, J. C. Tan, D. A. Keen, E. G. Bithell, P. J. Beldon, T. Friscic, A. K. Cheetham, J. Am. Chem. Soc. 2011, 133, 14546.

ChemPhysChem 2014, 15, 823 – 839

837

CHEMPHYSCHEM MINIREVIEWS [30] S. Hermes, T. Witte, T. Hikov, D. Zacher, S. Bahnmueller, G. Langstein, K. Huber, R. A. Fischer, J. Am. Chem. Soc. 2007, 129, 5324. [31] T. Tsuruoka, S. Furukawa, Y. Takashima, K. Yoshida, S. Isoda, S. Kitagawa, Angew. Chem. Int. Ed. 2009, 48, 4739; Angew. Chem. 2009, 121, 4833. [32] S. Diring, S. Furukawa, Y. Takashima, T. Tsuruoka, S. Kitagawa, Chem. Mater. 2010, 22, 4531. [33] D. Zacher, J. Liu, K. Huber, R. A. Fischer, Chem. Commun. 2009, 1031. [34] J. Cravillon, C. A. Schroeder, R. Nayuk, J. Gummel, K. Huber, M. Wiebcke, Angew. Chem. Int. Ed. 2011, 50, 8067; Angew. Chem. 2011, 123, 8217. [35] S. R. Venna, J. B. Jasinski, M. A. Carreon, J. Am. Chem. Soc. 2010, 132, 18030. [36] S. Furukawa, K. Hirai, K. Nakagawa, Y. Takashima, R. Matsuda, T. Tsuruoka, M. Kondo, R. Haruki, D. Tanaka, H. Sakamoto, S. Shimomura, O. Sakata, S. Kitagawa, Angew. Chem. Int. Ed. 2009, 48, 1766; Angew. Chem. 2009, 121, 1798. [37] S. Furukawa, K. Hirai, Y. Takashima, K. Nakagawa, M. Kondo, T. Tsuruoka, O. Sakata, S. Kitagawa, Chem. Commun. 2009, 5097. [38] K. Koh, A. G. Wong-Foy, A. J. Matzger, Chem. Commun. 2009, 6162. [39] K. Hirai, S. Furukawa, M. Kondo, H. Uehara, O. Sakata, S. Kitagawa, Angew. Chem. Int. Ed. 2011, 50, 8057; Angew. Chem. 2011, 123, 8207. [40] K. Hirai, S. Furukawa, M. Kondo, M. Meilikhov, Y. Sakata, O. Sakata, S. Kitagawa, Chem. Commun. 2012, 48, 6472. [41] T. Li, J. E. Sullivan, N. L. Rosi, J. Am. Chem. Soc. 2013, 135, 9984. [42] A. Btard, R. A. Fischer, Chem. Rev. 2012, 112, 1055. [43] E. Biemmi, C. Scherb, T. Bein, J. Am. Chem. Soc. 2007, 129, 8054. [44] D. Zacher, A. Baunemann, S. Hermes, R. A. Fischer, J. Mater. Chem. 2007, 17, 2785. [45] B. Liu, M. Tu, R. A. Fischer, Angew. Chem. Int. Ed. 2013, 52, 3402; Angew. Chem. 2013, 125, 3486. [46] M. Meilikhov, S. Furukawa, K. Hirai, R. A. Fischer, S. Kitagawa, Angew. Chem. Int. Ed. 2013, 52, 341; Angew. Chem. 2013, 125, 359. [47] A. Huang, N. Wang, C. Kong, J. Caro, Angew. Chem. Int. Ed. 2012, 51, 10551; Angew. Chem. 2012, 124, 10703. [48] B. Zornoza, C. Tellez, J. Coronas, J. Gascon, F. Kapteijn, Microporous Mesoporous Mater. 2013, 166, 67. [49] H. B. T. Jeazet, C. Staudt, C. Janiak, Dalton Trans. 2012, 41, 14003. [50] A. Btard, S. Wannapaiboon, R. A. Fischer, Chem. Commun. 2012, 48, 10493. [51] L.-B. Sun, J.-R. Li, J. Park, H.-C. Zhou, J. Am. Chem. Soc. 2012, 134, 126. [52] S. Cao, G. Gody, W. Zhao, S. Perrier, X. Peng, C. Ducati, D. Zhao, A. K. Cheetham, Chem. Sci. 2013, 4, 3573. [53] L.-G. Qiu, T. Xu, Z.-Q. Li, W. Wang, Y. Wu, X. Jiang, X.-Y. Tian, L.-D. Zhang, Angew. Chem. Int. Ed. 2008, 47, 9487; Angew. Chem. 2008, 120, 9629. [54] C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 1992, 359, 710. [55] L. Li, S. Xiang, S. Cao, J. Zhang, G. Ouyang, L. Chen, C.-Y. Su, Nat. Commun. 2013, 4, 1774. [56] K. M. Choi, H. J. Jeon, J. K. Kang, O. M. Yaghi, J. Am. Chem. Soc. 2011, 133, 11920. [57] K. Morishige, N. Tateishi, J. Chem. Phys. 2003, 119, 2301. [58] H. Wu, J. M. Simmons, G. Srinivas, W. Zhou, T. Yildirim, J. Phys. Chem. Lett. 2010, 1, 1946. [59] N. L. Rosi, J. Kim, M. Eddaoudi, B. Chen, M. O’Keeffe, O. M. Yaghi, J. Am. Chem. Soc. 2005, 127, 1504. [60] O. Kozachuk, K. Yusenko, H. Noei, Y. Wang, S. Walleck, T. Glaser, R. A. Fischer, Chem. Commun. 2011, 47, 8509. [61] M. Kramer, U. Schwarz, S. Kaskel, J. Mater. Chem. 2006, 16, 2245. [62] P. Maniam, N. Stock, Inorg. Chem. 2011, 50, 5085. [63] L. J. Murray, M. Dinca, J. Yano, S. Chavan, S. Bordiga, C. M. Brown, J. R. Long, J. Am. Chem. Soc. 2010, 132, 7856. [64] P. D. C. Dietzel, Y. Morita, R. Blom, H. Fjellvaag, Angew. Chem. Int. Ed. 2005, 44, 6354; Angew. Chem. 2005, 117, 6512. [65] P. D. C. Dietzel, B. Panella, M. Hirscher, R. Blom, H. Fjellvag, Chem. Commun. 2006, 959. [66] S. R. Caskey, A. G. Wong-Foy, A. J. Matzger, J. Am. Chem. Soc. 2008, 130, 10870. [67] W. Zhou, H. Wu, T. Yildirim, J. Am. Chem. Soc. 2008, 130, 15268. [68] C. R. Wade, M. Dinca, Dalton Trans. 2012, 41, 7931.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org [69] T. M. McDonald, D. M. D’Alessandro, R. Krishna, J. R. Long, Chem. Sci. 2011, 2, 2022. [70] A. . Yazaydin, A. I. Benin, S. A. Faheem, P. Jakubczak, J. J. Low, R. R. Willis, R. Q. Snurr, Chem. Mater. 2009, 21, 1425. [71] T. G. Glover, G. W. Peterson, B. J. Schindler, D. Britt, O. M. Yaghi, Chem. Eng. Sci. 2011, 66, 163. [72] T. Loiseau, C. Serre, C. Huguenard, G. Fink, F. Taulelle, M. Henry, T. Bataille, G. Ferey, Chem. Eur. J. 2004, 10, 1373. [73] F. Millange, N. Guillou, R. I. Walton, J.-M. Greneche, I. Margiolaki, G. Ferey, Chem. Commun. 2008, 4732. [74] C. Serre, F. Millange, C. Thouvenot, M. Nogues, G. Marsolier, D. Loueer, G. Ferey, J. Am. Chem. Soc. 2002, 124, 13519. [75] C. Volkringer, T. Loiseau, N. Guillou, G. Ferey, E. Elkaim, A. Vimont, Dalton Trans. 2009, 2241. [76] E. V. Anokhina, M. Vougo-Zanda, X. Wang, A. J. Jacobson, J. Am. Chem. Soc. 2005, 127, 15000. [77] J. P. S. Mowat, V. R. Seymour, J. M. Griffin, S. P. Thompson, A. M. Z. Slawin, D. Fairen-Jimenez, T. Dueren, S. E. Ashbrook, P. A. Wright, Dalton Trans. 2012, 41, 3937. [78] K. Barthelet, J. Marrot, D. Riou, G. Ferey, Angew. Chem. Int. Ed. 2002, 41, 281; Angew. Chem. 2002, 114, 291. [79] H. Leclerc, T. Devic, S. Devautour-Vinot, P. Bazin, N. Audebrand, G. Ferey, M. Daturi, A. Vimont, G. Clet, J. Phys. Chem. C 2011, 115, 19828. [80] S. Bourrelly, B. Moulin, A. Rivera, G. Maurin, S. Devautour-Vinot, C. Serre, T. Devic, P. Horcajada, A. Vimont, G. Clet, M. Daturi, J.-C. Lavalley, S. Loera-Serna, R. Denoyel, P. L. Llewellyn, G. Ferey, J. Am. Chem. Soc. 2010, 132, 9488. [81] C. Serre, S. Bourrelly, A. Vimont, N. A. Ramsahye, G. Maurin, P. L. Llewellyn, M. Daturi, Y. Filinchuk, O. Leynaud, P. Barnes, G. Ferey, Adv. Mater. 2007, 19, 2246. [82] H. Chun, D. N. Dybtsev, H. Kim, K. Kim, Chem. Eur. J. 2005, 11, 3521. [83] B.-Q. Ma, K. L. Mulfort, J. T. Hupp, Inorg. Chem. 2005, 44, 4912. [84] K. Uemura, Y. Yamasaki, Y. Komagawa, K. Tanaka, H. Kita, Angew. Chem. Int. Ed. 2007, 46, 6662; Angew. Chem. 2007, 119, 6782. [85] B. Chen, S. Ma, E. J. Hurtado, E. B. Lobkovsky, C. Liang, H. Zhu, S. Dai, Inorg. Chem. 2007, 46, 8705. [86] N. Klein, H. C. Hoffmann, A. Cadiau, J. Getzschmann, M. R. Lohe, S. Paasch, T. Heydenreich, K. Adil, I. Senkovska, E. Brunner, S. Kaskel, J. Mater. Chem. 2012, 22, 10303. [87] N. Klein, C. Herzog, M. Sabo, I. Senkovska, J. Getzschmann, S. Paasch, M. R. Lohe, E. Brunner, S. Kaskel, Phys. Chem. Chem. Phys. 2010, 12, 11778. [88] J. N. Van Niekerk, F. R. L. Schoening, Nature 1953, 171, 36. [89] W. Clegg, I. R. Little, B. P. Straughan, Acta Crystallogr. Sect. C 1986, 42, 1701. [90] J. N. Van Niekerk, F. R. L. Schoening, J. H. Talbot, Acta Crystallogr. 1953, 6, 720. [91] S. Bureekaew, S. Amirjalayer, R. Schmid, J. Mater. Chem. 2012, 22, 10249. [92] J. N. Van Niekerk, F. R. L. Schoening, Acta Crystallogr. 1953, 6, 609. [93] S. M. Cohen, Chem. Rev. 2012, 112, 970. [94] M. Savonnet, D. Bazer-Bachi, N. Bats, J. Perez-Pellitero, E. Jeanneau, V. Lecocq, C. Pinel, D. Farrusseng, J. Am. Chem. Soc. 2010, 132, 4518. [95] Y. Goto, H. Sato, S. Shinkai, K. Sada, J. Am. Chem. Soc. 2008, 130, 14354. [96] G. Tuci, A. Rossin, X. Xu, M. Ranocchiari, J. A. van Bokhoven, L. Luconi, I. Manet, M. Melucci, G. Giambastiani, Chem. Mater. 2013, 25, 2297. [97] K. Hindelang, A. Kronast, S. I. Vagin, B. Rieger, Chem. Eur. J. 2013, 19, 8244. [98] T. Gadzikwa, B.-S. Zeng, J. T. Hupp, S. T. Nguyen, Chem. Commun. 2008, 3672. [99] S. Henke, A. Schneemann, S. Kapoor, R. Winter, R. A. Fischer, J. Mater. Chem. 2012, 22, 909. [100] M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffe, O. M. Yaghi, Science 2002, 295, 469. [101] S. T. Meek, S. L. Teich-McGoldrick, J. J. Perry, J. A. Greathouse, M. D. Allendorf, J. Phys. Chem. C 2012, 116, 19765. [102] D. Y. Siberio-Prez, A. G. Wong-Foy, O. M. Yaghi, A. J. Matzger, Chem. Mater. 2007, 19, 3681. [103] J. L. C. Rowsell, O. M. Yaghi, J. Am. Chem. Soc. 2006, 128, 1304.

ChemPhysChem 2014, 15, 823 – 839

838

CHEMPHYSCHEM MINIREVIEWS [104] J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga, K. P. Lillerud, J. Am. Chem. Soc. 2008, 130, 13850. [105] Y. Huang, W. Qin, Z. Li, Y. Li, Dalton Trans. 2012, 41, 9283. [106] G. T. Rochelle, Science 2009, 325, 1652. [107] S. Biswas, P. Van Der Voort, Eur. J. Inorg. Chem. 2013, 2154. [108] S. J. Garibay, S. M. Cohen, Chem. Commun. 2010, 46, 7700. [109] G. E. Cmarik, M. Kim, S. M. Cohen, K. S. Walton, Langmuir 2012, 28, 15606. [110] M. Kim, J. A. Boissonnault, P. V. Dau, S. M. Cohen, Angew. Chem. Int. Ed. 2011, 50, 12193; Angew. Chem. 2011, 123, 12401. [111] T. Devic, F. Salles, S. Bourrelly, B. Moulin, G. Maurin, P. Horcajada, C. Serre, A. Vimont, J.-C. Lavalley, H. Leclerc, G. Clet, M. Daturi, P. L. Llewellyn, Y. Filinchuk, G. Ferey, J. Mater. Chem. 2012, 22, 10266. [112] S. Henke, A. Schneemann, A. Wuetscher, R. A. Fischer, J. Am. Chem. Soc. 2012, 134, 9464. [113] S. Henke, R. Schmid, J.-D. Grunwaldt, R. A. Fischer, Chem. Eur. J. 2010, 16, 14296. [114] S. Henke, D. C. F. Wieland, M. Meilikhov, M. Paulus, C. Sternemann, K. Yusenko, R. A. Fischer, CrystEngComm 2011, 13, 6399. [115] W. Kleist, F. Jutz, M. Maciejewski, A. Baiker, Eur. J. Inorg. Chem. 2009, 3552. [116] A. D. Burrows, CrystEngComm 2011, 13, 3623. [117] H. Deng, C. J. Doonan, H. Furukawa, R. B. Ferreira, J. Towne, C. B. Knobler, B. Wang, O. M. Yaghi, Science 2010, 327, 846. [118] T. Fukushima, S. Horike, H. Kobayashi, M. Tsujimoto, S. Isoda, M. L. Foo, Y. Kubota, M. Takata, S. Kitagawa, J. Am. Chem. Soc. 2012, 134, 13341. [119] M. I. Breeze, G. Clet, B. C. Campo, A. Vimont, M. Daturi, J.-M. Greneche, A. J. Dent, F. Millange, R. I. Walton, Inorg. Chem. 2013, 52, 8171. [120] O. Kozachuk, K. Khaletskaya, M. Halbherr, A. Betard, M. Meilikhov, R. W. Seidel, B. Jee, A. Poeppl, R. A. Fischer, Eur. J. Inorg. Chem. 2012, 1688. [121] X. Kong, H. Deng, F. Yan, J. Kim, J. A. Swisher, B. Smit, O. M. Yaghi, J. A. Reimer, Science 2013, 341, 882.

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org [122] T. Fukushima, S. Horike, Y. Inubushi, K. Nakagawa, Y. Kubota, M. Takata, S. Kitagawa, Angew. Chem. Int. Ed. 2010, 49, 4820; Angew. Chem. 2010, 122, 4930. [123] S. Horike, Y. Inubushi, T. Hori, T. Fukushima, S. Kitagawa, Chem. Sci. 2012, 3, 116. [124] O. Kozachuk, M. Meilikhov, K. Yusenko, A. Schneemann, B. Jee, A. V. Kuttatheyil, M. Bertmer, C. Sternemann, A. Poeppl, R. A. Fischer, Eur. J. Inorg. Chem. 2013, 4546. [125] F. Nouar, T. Devic, H. Chevreau, N. Guillou, E. Gibson, G. Clet, M. Daturi, A. Vimont, J. M. Greneche, M. I. Breeze, R. I. Walton, P. L. Llewellyn, C. Serre, Chem. Commun. 2012, 48, 10237. [126] D.-W. Lim, W. Y. Ji, Y. R. Keun, M. P. Suh, Angew. Chem. Int. Ed. 2012, 51, 9814; Angew. Chem. 2012, 124, 9952. [127] Y. E. Cheon, M. P. Suh, Angew. Chem. Int. Ed. 2009, 48, 2899; Angew. Chem. 2009, 121, 2943. [128] S. B. Kalidindi, H. Oh, M. Hirscher, D. Esken, C. Wiktor, S. Turner, G. Van Tendeloo, R. A. Fischer, Chem. Eur. J. 2012, 18, 10848. [129] H. J. Park, M. P. Suh, Chem. Commun. 2012, 48, 3400. [130] H. R. Moon, D.-W. Lim, M. P. Suh, Chem. Soc. Rev. 2013, 42, 1807. [131] M. Meilikhov, K. Yusenko, D. Esken, S. Turner, G. Van Tendeloo, R. A. Fischer, Eur. J. Inorg. Chem. 2010, 3701. [132] R. Prins, Chem. Rev. 2012, 112, 2714. [133] Y. Li, R. T. Yang, J. Am. Chem. Soc. 2006, 128, 8136. [134] C. I. Contescu, C. M. Brown, Y. Liu, V. V. Bhat, N. C. Gallego, J. Phys. Chem. C 2009, 113, 5886. [135] N. P. Stadie, J. J. Purewal, C. C. Ahn, B. Fultz, Langmuir 2010, 26, 15481. [136] R. Campesi, F. Cuevas, M. Latroche, M. Hirscher, Phys. Chem. Chem. Phys. 2010, 12, 10457.

Received: October 23, 2013 Published online on March 11, 2014

ChemPhysChem 2014, 15, 823 – 839

839