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Journal of Nanomaterials
Volume 2013 (2013), Article ID 616501, 11 pages
http://dx.doi.org/10.1155/2013/616501
Review Article

Recent Advances as Materials of Functional Metal-Organic Frameworks

1College of Biology, Chemistry and Material Science, East China Institute of Technology, Fuzhou, Jiangxi 344000, China
2Fundamental Science on Radioactive Geology and Exploration Technology Laboratory, East China Institute of Technology, NanChang, Jiangxi 330013, China

Received 30 January 2013; Accepted 22 March 2013

Academic Editor: Jianmin Ma

Copyright © 2013 Xiao-Lan Tong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Metal-organic frameworks (MOFs), also known as hybrid inorganic-organic materials, represent an emerging class of materials that have attracted the imagination of solid-state chemists because MOFs combine unprecedented levels of porosity with a range of other functional properties that occur through the metal moiety and/or the organic ligand. The purpose of this critical review is to give a representative and comprehensive overview of the arising developments in the field of functional metal-organic frameworks, including luminescence, magnetism, and porosity through presenting examples. This review will be of interest to researchers and synthetic chemists attempting to design multifunctional MOFs.

1. Introduction

Metal organic frameworks (MOFs) are a new class of crystalline materials [1], the structures of which are composed of metal-oxide units or metal ions joined by organic linkers through strong covalent bonds. It may be defined as supramolecular solids but consists of strong bonding providing robustness, linking units that are available for modification by organic synthesis, and a geometrically well-defined structure. The latter property further implies that these solids should be highly crystalline. Specifically, the chemistry of MOFs has provided an extensive class of crystalline materials with high stability, tunable metrics, organic functionality, and porosity.

An extensive body of research results [2] currently exists from the synthesis of metal-organic frameworks (MOFs), an area that has attracted widespread attention due to the facility with which well-defined molecular building blocks can be assembled into periodic frameworks and the promise that such a process holds for the logical design of materials. The synthesis of MOFs generally involves the copolymerization of organic links and metal ions in a polar solvent under mild temperatures (up to 200°C) and autogenous pressures (up to 100 atm). Since most products can be considered kinetically driven and lie on local thermodynamic minima, factors such as solubility of the organic link and metal salt, solvent polarity, ionic strength of the medium, temperature, and pressure play critical roles in determining the character of products. Indeed, slight perturbations in synthetic parameters have been the basis for the preparation of what seems to be a flood of new MOF compounds.

The designed construction of extended metal-organic frameworks from soluble molecular building blocks represent one of the most challenging issues facing synthetic chemistry today. Also metal-organic frameworks (MOFs) are widely studied because of their potential applications in many areas such as luminescence, magnetism, hydrogen storage, and gas adsorption and separation. We will give a representative and comprehensive overview of the arising developments of the functional MOFs.

2. Luminescent Metal-Organic Frameworks

The luminescent properties of metal-organic frameworks (MOFs) have attracted much attention for a long time, although these types of materials were frequently referred to as metal coordination polymers in the literature before the term “MOF” was widely adopted. The first report of luminescence, in which the structure was termed “MOF” that we are aware of, appeared in 2002. Since then, nearly 200 articles have appeared reporting light emission by MOFs, and a few reviews covering certain aspects of MOF luminescent properties have been published [310].

The hybrid nature of MOF materials, which include both an organic ligand and a metal ion, enables a wide range of emissive phenomena found in few other classes of material; the metal coordination can increase the conjugation and rigidity of the linkers. MOFs offer a unique platform for the development of solid-state luminescent materials as they have a degree of structural predictability, in addition to well-defined environments for lumophore in crystalline form. The lanthanoid (Ln) ions have spectrally narrow emission, even in solution, and nearly all of the lanthanoids exhibit photoluminescent properties [1117].

He et al. [18] synthesized the first open-framework heterometallic MOF structure (Figure 1) based on the assembly of infinite rod building units from the solvothermal reaction of zinc nitrate hexahydrate, sodium hydroxide, and 1,3-benzene-dicarboxylic (m-BDC) acid. The strong fluorescent emission (Figure 2) of the above MOF may make it a potential useful photoactive material.

616501.fig.001
Figure 1: Framework of the MOF with DMF templating cations occupying the pores.
616501.fig.002
Figure 2: Photoluminescent spectrum of the MOF in the solid state at room temperature.

Chen et al. [19] report a prototype luminescent MOF (Figure 3), Tb (BTC)·G (MOF-76: BTC = benzene-1,3,5-tricarboxylate, G = guest solvent), for the recognition and sensing of anions, exhibiting a high-sensitivity sensing function with respect to fluoride (Figure 4). Such a recognition event can be readily transformed into an external luminescence intensity change once luminescent metal site and/or organic linkers have been incorporated into the luminescent MOFs.

616501.fig.003
Figure 3: Single crystal X-ray structure of MOF-76b activated in methanol containing NaF.
616501.fig.004
Figure 4: 5D47F5 transition intensities of MOF-76b activated in different types of 10-2  NaX and Na2X methanol solution.

3. Magnetic Metal-Organic Frameworks

One of the many interesting properties of metal-organic frameworks (MOF) is magnetism [2022]. It can be implemented by incorporating magnetic moment carriers such as paramagnetic metals, open-shell organic ligands or both [23, 24]. Magnetic MOFs and molecular magnets reported with their designs, synthetic approaches, structures, and physical properties [2533] are both branches of coordination chemistry where metals are bound in a solid by coordination bonds to organic linkers. There exist several reviews dealing with the different aspects of magnetism [3438].

Lanthanoid metals are attractive as magnetic materials because of their high spin numbers and strong magnetic anisotropies on the 4f orbitals. Magnetic materials based on lanthanoid metals or lanthanoid metal oxides have extensive applications [3942].

Sun et al. [43] have reported a novel 2D coordination polymer 1 (Figure 5(a)) consisting of ferromagnetic Ni(II) chain with alternating double EO-azide bridges and (EO-azide)bis(carboxylate) triple bridges, and the material exhibits solvent sensitive metamagnetism; it can become 2 by dehydrating. The reversible dehydration/hydration processes are accompanied by significant changes in critical temperature, critical field and hysteresis (Figure 5(b)).

fig5
Figure 5: (a) The repeating unit (left) and the 2D layer formed by organic ligands interlinking 1D chains (right) in 1. (b) The and 1/versus plots at 1 kOe for 1 and 2.

Liu et al. [44] have successfully synthesized a high nuclearity cubic cage involving 64 Fe3+ ions (Figure 6), which displays strong antiferromagnetism (Figure 7). They also demonstrate that the combination of the small stereohindrance HCOO- and polypodal ligands can obtain high nuclearity magnetic clusters in order to explore their novel and interesting magnetism.

fig6
Figure 6: The structure of Fe64 cubic. ((a) and (b)) Top and side view of the Fe8 corner. (c) The cubic cage with the corner Fe8O3 cores and HCOO edges highlighted.
616501.fig.007
Figure 7: versus plots under 1 KOe field and versus plot (inset) for Fe64 and Fe8a. Data are for one Fe3+.

4. Porous Metal-Organic Frameworks

The designed construction of extended porous frameworks from soluble molecular building blocks represents one of the most challenging issues facing synthetic chemistry today. A large number of wonders and advances have been made possible by the synthesis of porous metal-organic frameworks (MOFs) because they have a wide array of applications, ranging from hydrogen storage to gas adsorption and separation to catalysis and so on. Thanks to several effective engineering strategies, systematic fabrication of porous MOFs can be achieved through designer assembly from judiciously selected molecular building blocks.

4.1. Hydrogen Storage in Metal-Organic Frameworks

New materials capable of storing hydrogen at high gravimetric and volumetric densities are required if hydrogen is to be widely employed as a clean alternative to hydrocarbon fuels in cars and other mobile applications. With exceptionally high surface areas and chemical-tunable structures, microporous metal-organic frameworks have recently emerged as some of the most promising candidate materials; they can display outstanding performance characteristics for cryogenic hydrogen storage at 77 K and pressures up to 100 bar.

One of the first metal-organic frameworks investigated for hydrogen storage was the cubic carboxylate-based framework Zn4O(BDC)3 (see Figure 8), and its gas storage properties were found to depend very large on the methods utilized in preparation and activation, with Langmuir surface areas ranging between 1010 and 4400 m2·g−1 and H2 uptake capacity varying accordingly [4549]. Inspired by the performance of compounds such as Zn4O(BDC)3, researchers have thus far reported hydrogen storage data for over 150 other microporous metal-organic frameworks [5055], containing carboxylate-based frameworks, heterocyclic azolate-based frameworks, mixed-ligand/functionality systems, metal cyanide frameworks, and so on.

616501.fig.008
Figure 8: A portion of the crystal structure of Zn4O (BDC)3 (MOF-5).

Wang et al. [56] reported a porous MOF, PCN-20 (Figure 9) with a twisted boracite topology based on a designed planar TTCA ligand. PCN-20 possessed a large Langmuir surface area of over 4200 m2·g−1 as well as demonstrated a high hydrogen uptake capacity of 6.2 wt% at 77 K and 50 bar (Figure 10).

fig9
Figure 9: (a) Octahedral cage in PCN-20. (b) Cuboctahedral cage with alignment of the unsaturated metal center (UMCs) in PCN-20. (c) Cuboctahedral cage with open metal sites aligned orthogonally in PCN-20. (d) Square channels of PCN-20.
616501.fig.0010
Figure 10: H2 sorption isotherms of PCN-20 at 77 K, at high pressure.

Rowsell et al. [57, 58] have obtained remarkably detailed information on the primary binding sites of hydrogen in a series of metal-organic frameworks (Figures 11, 12, and 13) composed of Zn4O (O2C)6 secondary building units with the use of inelastic neutron scattering from the hindered rotations of the adsorbed molecule. And this paper underlines the need to explore new topologies composed of novel secondary building units from metal cations that have received less attention to increase the building energies for H2 on all sites. In particular, the use of more polarizing centers or the installment of open metal sites should enhance hydrogen physisorption by this case of materials.

616501.fig.0011
Figure 11: Metal-organic frameworks analyzed in this study.
616501.fig.0012
Figure 12: Hydrogen adsorption isotherms measured at 77 K for the MOFs analyzed in this study.
616501.fig.0013
Figure 13: Inelastic neutron scattering spectra for each material loaded with 4 H2 per formula unit.

However, significant further advances will be required in order to meet the US DoE targets for an onboard hydrogen system.

4.2. Gas Adsorption and Separation

In industry, the adsorptive separation needs efficient porous materials. Preparing as traditional porous solid material, the porous metal-organic frameworks with tailored structures and tunable surface properties are becoming promising candidates for gas adsorption and separations, also they have commendable thermal stability, in most cases, these structures are robust enough to allow the removal of the included guest species resulting in permanent porosity, so MOFs are ideal for research and practical applications, such as in gas separation and purification as adsorbents [5963].

An et al. [64] have prepared a porous MOFs Co2(ad)2(CO2CH3)2·DMF·0.5H2O (bio-MOF-11) (Figure 14) via a solvothermal reaction between cobalt acetate tetrahydrate and adenine in DMF, which has a high heat of adsorption for CO2, high CO2 capacity, and impressive selectivity for CO2 over N2 (Figure 15).

fig14
Figure 14: Crystal structure of bio-MOF-11.
616501.fig.0015
Figure 15: Adsorption isotherms for CO2 (circles) and N2 (triangles) at 273 (black) and 298 K (red).

In 2009, Britt et al. [65] reported that a known MOF, Mg-MOF-74 (Figures 16 and 17), with open magnesium sites, rivals competitive materials in CO2 capture, with 8.9 wt.% dynamic capacity, and undergoes facile CO2 release at significantly lower temperature, 80°C. Mg-MOF-74 offers an excellent balance between dynamic capacity and regeneration. These results demonstrate the potential of MOFs with open metal sites as efficient CO2 capture media.

616501.fig.0016
Figure 16: Single crystal structure of Mg-MOF-74.
616501.fig.0017
Figure 17: A 20% mixture of CO2 in CH4 is fed into a bed of Mg-MOF-74. Effluent concentrations are shown, indicating complete retention of CO2 until saturation. CO2 breakthrough occurs at the dashed line.

Li et al. [66] reported a novel three zeolite-like chiral guest-free MOF material (Figure 18), [Zn(dtp)] (H2dtp = 2,3-di-1H-tetrazol-5-ylpyrazine), which has high thermal stability and permanent porosity. It exhibits rare gas-adsorption selectivity for O2 and CO2 over N2 gas (Figure 19), and could be useful in the separation of air.

616501.fig.0018
Figure 18: Structure of [Zn(dtp)] showing open channel along the axis.
616501.fig.0019
Figure 19: Adsorption isotherms of N2, O2 and CO2 for [Zn(dtp)].

Besides, MOFs also have some other properties that occur through the metal moiety and/or the organic ligand, such as catalysts [67], enantioselective catalysis [68], and applications in industry [69].

5. Conclusions Remarks

We exhibited the variety applications as materials of few MOFs here; in fact although a large number of MOFs have been reported until now, we only have seen the tip of the iceberg with respect to the application potential of MOFs. In order to investigate more potential applications of MOFs, an extensive body of researchers should pay for further effort.

Acknowledgments

This work was supported by Project of Jiangxi Provincial Department of Education (Grant no. GJJ12396 and no. GJJ11140), the open fund of Fundamental Science on Radioactive Geology and Exploration Technology Laboratory (REGT1212), the National Science Foundation of Jiangxi Province (no. 2010GQH0007), and the Start-up Fund of East China Institute of Technology.

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