Captured Molecules in Coordination Frameworks
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Daisuke Tanaka and Susumu Kitagawa Abstract In recent years, a new class of porous materials based on a combination of organic components and metal centers has emerged, namely, microporous coordination polymers (MCPs), in which the chemical properties as well as the pore dimensions affect the incorporation of “guest” molecules within the pores. In this article, we describe the ability of MCPs to store gas molecules, which is ascribed to framework regularity and high porosity, and the unique capacity of certain MCPs to capture molecules selectively by well-defined interactions with organic functional groups.
Introduction In recent years, substantial progress has been made in the synthesis of a new class of porous materials, referred to as microporous coordination polymers (MCPs) or microporous metal-organic frameworks (MMOFs). The attention to these materials has been well deserved, as their porous frameworks can be manipulated to produce a diverse range of topologies with fascinating properties in areas such as gas storage, ion exchange, heterogeneous catalysis, and selective guest adsorption.1–8 MCPs have unique characteristics, including high porosity and surface area. Large surface area is one of the most important factors for evaluating pore capacity and is associated with the number of guest molecules accommodated by direct contact with the pore wall. Recently, the specific surface areas attainable with MCPs have increased from 500 m2 g−1, comparable to zeolites, to a very large value: more than 4500 m2 g−1.9,10 In the case of inorganic zeolites, the pore walls are constructed with a thickness of at least several Si, O, and Al atoms, whereas coordination polymers afford thin walls—for instance, only one carbon atom thick— when the wall is of an aromatic group such as 4,4′-bipyridine (bpy), which shows that almost all the atoms constructing porous frameworks can be used as a surface. In addition, regular pore size distribution can be readily realized for coordination polymers as well as for inorganic
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porous materials. Especially, MCPs afford designable pore surfaces that can be adjusted systematically and rationally through synthetic modification of organic and inorganic components.7,11 The pore walls of MCPs enable the introduction of various functional groups capable of imparting specific properties such as selective adsorption and catalytic activity. Two types of strategies are used to functionalize channel surfaces: immobilization of coordinatively unsaturated (open) metal sites and introduction of organic groups to provide guest-accessible functional organic sites. There is growing interest in the use of open metal sites and functional organic sites because they can show highly selective incorporation of guest molecules.12,13 In this article, we present the gas storage properties ascribed to the framework regularity and high porosity of MCPs and the selective guest adsorption properties attributed to their functional organic sites.
tional adsorbents afford insufficient CH4 storage to meet the condi
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