Hydrogen storage in microporous metal-organic frameworks with exposed metal sites

Owing to their high uptake capacity at low temperature and excellent reversibility kinetics, metal-organic frameworks have attracted considerable attention as potential solid-state hydrogen storage materials. In the last few years, researchers have also identified several strategies for increasing the affinity of these materials towards hydrogen, among which the binding of H(2) to unsaturated metal centers is one of the most promising. Herein, we review the synthetic approaches employed thus far for producing frameworks with exposed metal sites, and summarize the hydrogen uptake capacities and binding energies in these materials. In addition, results from experiments that were used to probe independently the metal-hydrogen interaction in selected materials will be discussed.     

High gas sorption and metal-ion exchange of microporous metal-organic frameworks with incorporated imide groups

Metal-organic frameworks (MOFs), {[Cu(2)(bdcppi)(dmf)(2)]·10DMF·2H(2)O}(n) (SNU-50) and {[Zn(2)(bdcppi)(dmf)(3)]·6DMF·4H(2)O}(n) (SNU-51), have been prepared by the solvothermal reactions of N,N'-bis(3,5-dicarboxyphenyl)pyromellitic diimide (H(4)BDCPPI) with Cu(NO(3))(2) and Zn(NO(3))(2), respectively. Framework SNU-50 has an NbO-type net structure, whereas SNU-51 has a PtS-type net structure. Desolvated solid [Cu(2)(bdcppi)](n) (SNU-50'), which was prepared by guest exchange of SNU-50 with acetone followed by evacuation at 170 °C, adsorbs high amounts of N(2), H(2), O(2), CO(2), and CH(4) gases due to the presence of a vacant coordination site at every metal ion, and to the presence of imide groups in the ligand. The Langmuir surface area is 2450 m(2) g(-1). It adsorbs H(2) gas up to 2.10 wt% at 1 atm and 77 K, with zero coverage isosteric heat of 7.1 kJ mol(-1), up to a total of 7.85 wt% at 77 K and 60 bar. Its CO(2) and CH(4) adsorption capacities at 298 K are 77 wt% at 55 bar and 17 wt% at 60 bar, respectively. Of particular note is the O(2) adsorption capacity of SNU-50' (118 wt% at 77 K and 0.2 atm), which is the highest reported so far for any MOF. By metal-ion exchange of SNU-51 with Cu(II), {[Cu(2)(bdcppi)(dmf)(3)]·7DMF·5H(2)O}(n) (SNU-51-Cu(DMF)) with a PtS-type net was prepared, which could not be synthesized by a direct solvothermal reaction.     

Mixed-ligand metal-organic frameworks with large pores: gas sorption properties and single-crystal-to-single-crystal transformation on guest exchange

Two isomorphous 3D metal-organic frameworks, {[Cu2(BPnDC)2(bpy)].8 DMF.6 H2O}n (1) and {[Zn2(BPnDC)2(dabco)].13 DMF.3 H2O}n (2), have been prepared by the solvothermal reactions of benzophenone 4,4'-dicarboxylic acid (H2BPnDC) with Cu(NO3)(2).2.5 H2O and 4,4'-bipyridine (bpy), and with Zn(NO3)(2).6 H2O and 4-diazabicyclo[2.2.2]octane (dabco), respectively. Compounds 1 and 2 are composed of paddle-wheel {M2(O2CR)4} cluster units, and they generate 2D channels with two different large pores (effective size of larger pore: 18.2 A for 1, 11.4 A for 2). The framework structure of desolvated solid, [Cu2(BPnDC)2(bpy)]n (SNU-6; SNU=Seoul National University), is the same as that of 1, as evidenced by powder X-ray diffraction patterns. SNU-6 exhibits high permanent porosity (1.05 cm3 g(-1)) with high Langmuir surface area (2910 m2 g(-1)). It shows high H2 gas storage capacity (1.68 wt % at 77 K and 1 atm; 4.87 wt % (excess) and 10.0 wt % (total) at 77 K and 70 bar) with high isosteric heat (7.74 kJ mol(-1)) of H2 adsorption as well as high CO2 adsorption capability (113.8 wt % at 195 K and 1 atm). Compound 2 undergoes a single-crystal-to-single-crystal transformation on guest exchange with n-hexane to provide {[Zn2(BPnDC)2(dabco)].6 (n-hexane).3 H2O}n (2hexane). The transformation involves dynamic motion of the molecular components in the crystal, mainly a bending motion of the square planes of the paddle-wheel units resulting from rotational rearrangement of phenyl rings and carboxylate planes of BPnDC2-.     

Modifying cage structures in metal-organic polyhedral frameworks for H2 storage

Three isostructural metal-organic polyhedral cage based frameworks (denoted NOTT-113, NOTT-114 and NOTT-115) with (3,24)-connected topology have been synthesised by combining hexacarboxylate isophthalate linkers with {Cu(2)(RCOO)(4)} paddlewheels. All three frameworks have the same cuboctahedral cage structure constructed from 24 isophthalates from the ligands and 12 {Cu(2)(RCOO)(4)} paddlewheel moieties. The frameworks differ only in the functionality of the central core of the hexacarboxylate ligands with trimethylphenyl, phenylamine and triphenylamine moieties in NOTT-113, NOTT-114 and NOTT-115, respectively. Exchange of pore solvent with acetone followed by heating affords the corresponding desolvated framework materials, which show high BET surface areas of 2970, 3424 and 3394 m(2) g(-1) for NOTT-113, NOTT-114 and NOTT-115, respectively. Desolvated NOTT-113 and NOTT-114 show high total H(2) adsorption capacities of 6.7 and 6.8 wt%, respectively, at 77 K and 60 bar. Desolvated NOTT-115 has a significantly higher total H(2) uptake of 7.5 wt% under the same conditions. Analysis of the heats of adsorption (Q(st)) for H(2) reveals that with a triphenylamine moiety in the cage wall, desolvated NOTT-115 shows the highest value of Q(st) for these three materials, indicating that functionalisation of the cage walls with more aromatic rings can enhance the H(2)/framework interactions. In contrast, measurement of Q(st) reveals that the amine-substituted trisalkynylbenzene core used in NOTT-114 gives a notably lower H(2)/framework binding energy.     

Understanding adsorption and interactions of alkane isomer mixtures in isoreticular metal-organic frameworks

Novel metal-organic frameworks (MOFs) may lead to advances in adsorption and catalysis owing to their superior properties compared to traditional nanoporous materials. A combination of the grand canonical Monte Carlo method and configurational-bias Monte Carlo simulation was used to evaluate the adsorption isotherms of C4-C6 alkane isomer mixtures in IRMOF-1 and IRMOF-6. The amounts of adsorbed linear and branched alkanes increase with increasing pressure, and the amount of branched alkanes is larger than that of the linear ones. The locations of the alkane isomer reveal that the Zn4O clusters of the IRMOFs are the preferential adsorption sites for the adsorbate molecules. The interaction energy between the Zn4O cluster and the adsorbate is larger than that between the organic linker and the adsorbate. It was further confirmed that the Zn4O cluster plays a much more important role in adsorption by pushing a probe molecule into the pore at positions closer to the Zn4O cluster. It is difficult for branched alkane molecules to approach the Zn4O cluster of IRMOF-6 closely owing to strong spatial hindrance. In addition, the adsorption selectivity is discussed from the viewpoints of thermodynamics and kinetics, and the diffusion behavior of n-butane and 2-methylpropane were investigated to illustrate the relationship between diffusion and adsorption.     

Synergetic effect of host-guest chemistry and spin crossover in 3D Hofmann-like metal-organic frameworks [Fe(bpac)M(CN)4] (M=Pt, Pd, Ni)

The synthesis and characterization of a series of three‐dimensional (3D) Hofmann‐like clathrate porous metal–organic framework (MOF) materials [Fe(bpac)M(CN)₄] (M=Pt, Pd, and Ni; bpac=bis(4‐pyridyl)acetylene) that exhibit spin‐crossover behavior is reported. The rigid bpac ligand is longer than the previously used azopyridine and pyrazine and has been selected with the aim to improve both the spin‐crossover properties and the porosity of the corresponding porous coordination polymers (PCPs). The 3D network is composed of successive {Fe[M(CN)₄]}_n planar layers bridged by the bis‐monodentate bpac ligand linked in the apical positions of the iron center. The large void between the layers, which represents 41.7 % of the unit cell, can accommodate solvent molecules or free bpac ligand. Different synthetic strategies were used to obtain a range of spin‐crossover behaviors with hysteresis loops around room temperature; the samples were characterized by magnetic susceptibility, calorimetric, Mössbauer, and Raman measurements. The complete physical study reveals a clear relationship between the quantity of included bpac molecules and the completeness of the spin transition, thereby underlining the key role of the π–π stacking interactions operating between the host and guest bpac molecules within the network. Although the inclusion of the bpac molecules tends to increase the amount of active iron centers, no variation of the transition temperature was measured. We have also investigated the ability of the network to accommodate the inclusion of molecules other than water and bpac and studied the synergy between the host–guest interaction and the spin‐crossover behavior. In fact, the clathration of various aromatic molecules revealed specific modifications of the transition temperature. Finally, the transition temperature and the completeness of the transition are related to the nature of the metal associated with the iron center (Ni, Pt, or Pd) and also to the nature and the amount of guest molecules in the lattice.     

Reversible transformation of ZnII coordination geometry in a single crystal of porous metal-organic framework [Zn3(ntb)2(EtOH)2].4 EtOH

A 3D porous metal-organic framework [Zn3(ntb)2(EtOH)2]n.4nEtOH (1) that generates 1D channels of honeycomb aperture has been prepared by the solvothermal reaction of Zn(NO3)(2).6 H2O and 4,4',4'-nitrilotrisbenzoic acid (H3NTB) in EtOH at 110 degrees C. Framework 1 exhibits reversible single-crystal-to-single-crystal transformations upon removal and rebinding of the coordinating EtOH as well as the EtOH guest molecules, which give rise to desolvated crystal [Zn3(ntb)2]n (1') and resolvated crystal [Zn3(ntb)2-(EtOH)2]n.4nEtOH (1'). The X-ray structures indicate that 3D host framework is retained during the transformations from 1 to 1' and from 1' to 1', but the coordination geometry of ZnII ions changes from/to trigonal bipyramid to/from tetrahedron, concomitant with the rotational rearrangement of a carboxylate plane of the NTB3- relative to its associated phenyl ring. To retain the single crystal integrity, extensive cooperative motions must exist between the molecular components throughout the crystal. Framework 1' exhibits permanent porosity, thermal stability up to 400 degrees C, and blue luminescence, and high storage capabilities for N2, H2, CO2, and CH4.     

Structure and charge control in metal-organic frameworks based on the tetrahedral ligand tetrakis(4-tetrazolylphenyl)methane

Use of the tetrahedral ligand tetrakis(4-tetrazolylphenyl)methane enabled isolation of two three-dimensional metal-organic frameworks featuring 4,6- and 4,8-connected nets related to the structures of garnet and fluorite with the formulae Mn(6)(ttpm)(3)5 DMF3 H(2)O (1) and Cu[(Cu(4)Cl)(ttpm)(2)](2)CuCl(2)5 DMF11 H(2)O (2) (H(4)ttpm=tetrakis(4-tetrazolylphenyl)methane). The fluorite-type solid 2 displays an unprecedented post-synthetic transformation in which the negative charge of the framework is reduced by extraction of copper(II) chloride. Desolvation of this compound generates Cu(4)(ttpm)(2)0.7 CuCl(2) (2 d), a microporous material exhibiting a high surface area and significant hydrogen uptake.     

New metal-organic frameworks with large cavities: selective sorption and desorption of solvent molecules

Five novel transition metal complexes [Cd(II) (3)(tpba-2)(2)(SCN)(6)].6 THF.3 H(2)O (1), [Cu(II) (3)(tpba-2)(2)(SCN)(6)].6 THF.3 H(2)O (2), [Ni(II) (3)(tpba-2)(2)(SCN)(6)].6 THF.3 H(2)O (3), [Cd(II) (2)(tpba-2)(SCN)(3)]ClO(4) (4), [Cu(I) (3)(SCN)(6)(H(3)tpba-2)] (5) [TPBA-2 = N',N',N'-tris(pyrid-2-ylmethyl)-1,3,5-benzenetricarboxamide, THF=tetrahydrofuran] were obtained by reactions of the corresponding transition metal salts with TPBA-2 ligand in the presence of NH(4)SCN using layering or solvothermal method, respectively. The results of X-ray crystallographic analysis showed that complexes 1, 2 and 3 are isostructural and have the same 2D honeycomb network structure with Kagomé lattice, in which all the M(II) (M = Cd, Cu, Ni) atoms are six-coordinated, and the TPBA-2 ligands adopt cis,cis,cis conformation while the thiocyanate anions act as terminal ligands. Capsule-like motifs are found in 1, 2 and 3, in which six THF molecules are hosted, and the results of XPRD and solid-state (13)C NMR spectral measurements showed that the compound 1 can selectively desorb and adsorb THF molecules occurring along with the re-establishment of its crystallinity. In contrast to 1, 2 and 3, complex 4 has different 2D network structure, resulting from TPBA-2 ligands with cis,trans,trans conformation, thiocyanate anions serving as end-to-end bridging ligands, and the incomplete replacement of perchlorate anions, which further link the 2D layers into 3D framework by the hydrogen bonds. In complex 5, the Cu(II) atoms are reduced to Cu(I) during the process of solvothermal reaction, and the Cu(I) atoms are connected by thiocyanate anions to form a 3D porous framework, in which the protonated TPBA-2 ligands are hosted in the cavities as templates.     

Elucidating gating effects for hydrogen sorption in MFU-4-type triazolate-based metal-organic frameworks featuring different pore sizes

A highly porous member of isoreticular MFU‐4‐type frameworks, [Zn₅Cl₄(BTDD)₃] (MFU‐4l(arge)) (H₂‐BTDD=bis(1H‐1,2,3‐triazolo[4,5‐b],[4′,5′‐i])dibenzo[1,4]dioxin), has been synthesized using ZnCl₂ and H₂‐BTDD in N,N‐dimethylformamide as a solvent. MFU‐4l represents the first example of MFU‐4‐type frameworks featuring large pore apertures of 9.1 Å. Here, MFU‐4l serves as a reference compound to evaluate the origin of unique and specific gas‐sorption properties of MFU‐4, reported previously. The latter framework features narrow‐sized pores of 2.5 Å that allow passage of sufficiently small molecules only (such as hydrogen or water), whereas molecules with larger kinetic diameters (e.g., argon or nitrogen) are excluded from uptake. The crystal structure of MFU‐4l has been solved ab initio by direct methods from 3D electron‐diffraction data acquired from a single nanosized crystal through automated electron diffraction tomography (ADT) in combination with electron‐beam precession. Independently, it has been solved using powder X‐ray diffraction. Thermogravimetric analysis (TGA) and variable‐temperature X‐ray powder diffraction (XRPD) experiments carried out on MFU‐4l indicate that it is stable up to 500 °C (N₂ atmosphere) and up to 350 °C in air. The framework adsorbs 4 wt % hydrogen at 20 bar and 77 K, which is twice the amount compared to MFU‐4. The isosteric heat of adsorption starts for low surface coverage at 5 kJ mol^?1 and decreases to 3.5 kJ mol^?1 at higher H₂ uptake. In contrast, MFU‐4 possesses a nearly constant isosteric heat of adsorption of ca. 7 kJ mol^?1 over a wide range of surface coverage. Moreover, MFU‐4 exhibits a H₂ desorption maximum at 71 K, which is the highest temperature ever measured for hydrogen physisorbed on metal–organic frameworks (MOFs).     

Pillaring of CdCl2-like layers in lanthanide metal-organic frameworks: synthesis, structure, and photophysical properties

A hydrothermal reaction of lanthanide salts, pyridine-2,3-dicarboxylic acid, benzene-1,4-dicarboxylic acid, and water gave rise to a new series of three-dimensional mixed carboxylates (homocyclic and heterocyclic) of lanthanides with the general formula [M2(H2O)4][{C5H3N(COO)2}2{C6H4(COO)2}], M=La (I), Pr (II), and Nd (III). The structure consists of M2O14N2 dimeric units connected by pyridine-2,3-dicarboxylate moieties to form two-dimensional layers that are pillared by terephthalate units. The structures also possess two co-ordinated water molecules, which are arranged to form one-dimensional helical chains and can be reversibly adsorbed. The connectivity within the layers closely resembles that of the CdCl2 layered structure with 3(6) topology. To the best of our knowledge, this is the first observation of CdCl2 topology in lanthanide metal-organic framework compounds. Partial substitution of La3+ in I by Eu3+ and Tb3+ (2 and 4 %) gives rise to characteristic red/pink or green emission, which suggests a ligand-sensitized metal-centered emission. The Nd compound III shows interesting UV and blue emission through an up-conversion process.