Metal-Organic Frameworks

Applications from Catalysis to Gas Storage

David Farrusseng, David Farrusseng

• Editore: Wiley-VCH
• Anno: 2011
• ISBN:
  9783527635863
• Edizione: 1
• Argomento: Sciences physiques
• Categoria: Chimie inorganique

An international and interdisciplinary team of leading experts from both academia and industry report on the wide range of hot applications for MOFs, discussing both the advantages and limits of the material. The resulting overview covers everything from catalysis, H2 and CH4 storage and gas purification to drug delivery and sensors. From the Contents:
- Design of Porous Coordination Polymers/Metal-Organic Frameworks: Past, Present and Future
- Design of Functional Metal-Organic Frameworks by Post-Synthetic Modification
- Thermodynamic Methods for Prediction of Gas Separation in Flexible Frameworks
- Separation and purification of gases by MOFs
- Opportunities for MOFs in CO2 capture from flue gases, natural gas and syngas by adsorption
- Manufacture of MOF thin films on structured supports for separation and catalysis
- Research status of Metal-Organic Frameworks for on-board cryo-adsorptive hydrogen storage applications
- Separation of xylene isomers
- Metal-Organic Frameworks as Catalysts for Organic Reactions
- Biomedical applications of Metal Organic Frameworks
- Metal Organic Frameworks for Biomedical Imaging
- Luminescent Metal-Organic Frameworks
- Deposition of thin films for sensor applications
- Industrial MOF Synthesis
- MOF shaping and immobilisation A must-have for every scientist in the field.

Part One

Design of Multifunctional Porous MOFs

Chapter 1

Design of Porous Coordination Polymers/Metal–Organic Frameworks: Past, Present and Future

Satoshi Horike and Susumu Kitagawa

1.1 Introduction

At the end of the 1990s, a new porous compound with an inorganic–organic hybrid framework had an impact on the field of porous materials and represented a new family for porous chemistry. Porous coordination polymers (PCPs), also known as metal–organic frameworks (MOFs), have regular pores ranging from micro- to mesopores, resulting in a large pore surface area, and a highly designable framework, pore shape, pore size, and surface functionality. Their structures are based on organic ligands as linkers and metal centers as the connectors. The rich functionality and designability of the organic ligands and the directability and physical properties of the metal ions are fascinating for the design of various functions, not only conventional adsorptive functions such as storage, separation, and catalysis, but also other physical/chemical functions that can be integrated in the frameworks. Whereas the components of PCPs are connected by coordination bonds and other weak interactions or noncovalent bonds (H-bonds, π-electron stacking, or van der Waals interactions), the interactions lead to structural flexibility and dynamics in the crystalline state, which also promotes the unique character of PCPs in the field of porous materials. As synthetic techniques and knowledge have increased in the last decade, we are now ready to design advanced porous functions by making full use of the chemical components and structural topologies. In this chapter, we introduce the background of PCPs/MOFs with some of the main framework designs and describe the unconventional porous properties of multifunctional porous materials based on ligand–metal networks.

1.2 Background and Ongoing Chemistry of Porous Coordination Polymers

Coordination polymers (CPs) are a family of compounds with extended structures formed by metal ions and organic and/or inorganic ligands with coordination bonds. They can provide various frameworks constructed from one-, two-, and three-dimensional networks. The late transition metal elements (Cu, Ag, Zn, and Cd) tend to provide this type of framework and the chemistry of CPs has been elucidated with the development of single-crystal X-ray crystallography. The term “coordination polymer” was used in a paper in 1916 [1], but there was no means of demonstrating infinite frameworks without single-crystal X-ray crystallography. A three-dimensional coordination framework connected by a CN bridge was realized in 1936 [2], namely the well-known Prussian Blue compounds. Currently, coordination polymers having porous properties are termed PCPs or porous MOFs, and therefore we suggest “coordination framework” as an all-inclusive term because the chemistry of the background is defined as “chemistry of coordination space.” To understand the background of this chemistry, there are three important concepts: (1) framework, (2) molecular metal–organic hybrid, and (3) porosity.

1. Concept of Framework

It is well known that CPs provide us with one-, two-, and three-dimensional motifs. In particular, the structural concept of a framework was demonstrated by Hofmann and Küspert [3], whose compounds are known as the family of Hofmann compounds having a two-dimensional layer-based architecture. The first X-ray crystallographic structure was obtained in 1949 [4]. The complete three-dimensional framework, the so-called Prussian Blue complex, appeared in 1936 and a comprehensive study was performed by Iwamoto et al. in 1967 [2, 5].

2. Molecular Metal–Organic Hybrid

Hofmann and Prussian Blue compounds have structures bridged by the inorganic ion CN⁻, and therefore have a restricted variety of structures. On the other hand, frameworks having organic linkers afford not only designability but also functionality of frameworks. The X-ray crystal structure of the metal–organic coordination framework of [Cu(adiponitrile)₂]·NO₃ appeared in 1959 [6]. Since then, many compounds in this category have been synthesized and characterized crystallographically. Yaghi et al. termed these compounds “metal–organic frameworks (MOFs)” in 1995 [7]. [Cu(adiponitrile)₂]·NO₃ contains the NO₃⁻ anion in the voids. Such compounds are regarded as clathrate-type CPs, however, which are not categorized as “porous” compounds. By the late 1990s, many clathrate-type CPs/MOFs had been synthesized.

3. Porosity

Porosity means “the quality or state of being a porous entity, which has many small holes that allow water, air, and so on, to pass through.” The porosity is antithesis to Aristotle's proposition, “Nature abhors a vacuum.” Indeed, closely packed solid structures formed by molecules and ions can easily form. Researchers have often misunderstood that the crystallographic structure of MOFs having guest species in their voids is a porous material. In 1997, “porosity” was demonstrated to give a compound that maintains a porous structure without guests in the pores; gas sorption experiments under ambient conditions were carried out for stable apohosts [8, 9]. Reversible gas storage properties were identified and the PCPs have attracted wide attention as new porous materials. Since that point, the number of reports on PCPs has been increasing rapidly, and many researchers have been developing strategies for the design of porosity, some of which are intrinsically unique to PCP materials.

1.2.1 Frameworks with High Surface Area

One of the great advantages of PCPs/MOFs is their high surface area, attributable to the low density of the porous structure. An MOF composed of Zn₄O clusters connected by benzenedicarboxylate (bdc), [Zn₄O(bdc)₃] (MOF-5), was synthesized in 1999 and possesses a cubic structure with an ordered three-dimensional (3D) porous system (Figure 1.1a) [10]. This compound has a BET surface area of 3800 m² g⁻¹ [11]. Many porous compounds have been synthesized on the basis of this structural motif, and this approach has been intensively developed to design important porous frameworks. Some related frameworks, [Zn₄O(btb)₂] (MOF-177) and [Zn₄O(bbc)₂] (MOF-200) {btb = 1,3,5-benzenetribenzoate; bbc = 4,4′,4″-[benzene-1,3,5-triyltris(benzene-4,1-diyl)]tribenzoate)} also possess high porosity; the reported BET surface areas for these compounds are 4746 and 6260 m² g⁻¹, respectively [12, 13]. The self-assembly process of structure growth often faces network interpenetration, which precludes a high surface area, but further improvements in the design of pore network topologies could avoid interpenetration to achieve extremely high surface areas.

Figure 1.1 Partial crystal structures of (a) [Zn₄O(bdc)₃] (MOF-5, BET surface area = 3800 m² g⁻¹) and (b) Zn₄O(t₂dc)(btb)_4/3 (UMCM-2, t₂dc = thieno-3,2-bithiophene-2,5-dicarboxylate, BET surface area = 5200 m² g⁻¹) constructed from Zn₄O clusters.

Porous frameworks constructed from two or more kinds of ligands are in some cases effective in the design of high surface area compounds. Zn₄O(t₂dc)(btb)_4/3 (UMCM-2) (t₂dc = thieno-3,2-bithiophene-2,5-dicarboxylate) (Figure 1.1b) is also made up of Zn₄O clusters and two distinct ligands contribute to the construction of the porous framework [14]. There is a narrow distribution of micropores at 1.4–1.6 and 1.6–1.8 nm and a mesopore at 2.4–3.0 nm and the calculated BET surface reaches 5200 m² g⁻¹.

Another framework, [Cr₃F(H₂O)O(bdc)₃] (MIL-101), is made from the linkage of terephthalate and chromium trimer units that consist of three Cr cations and the µ₃O oxygen anion [15]. The pore space is constructed from two cages with diameters of 2.9 and 3.4 nm which are connected with windows with diameters of 1.2 and 1.45 nm, respectively. The compound has a BET surface area of 4100 m² g⁻¹ and, compared with the Zn₄O-type metal cluster, the framework is more stable against water and other chemical species and it has also been utilized as a porous matrix for post-synthesis or hybridization with other species such as metal particles [16].

A paddle-wheel-type dimetal cluster is a popular building unit to construct frameworks. Many transition metals can form this type of cluster and it affords square grid extended networks. [Cu(H₂O)]₃(ntei) (PCN-66) is prepared by the combination of 4,4′,4″-nitrilotris(benzene-4,1-diyl)tris(ethyne-2,1-diyl)triisophthalate (ntei) and a Cu²⁺ paddle-wheel cluster and the ...