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Library of Congress Cataloging-in-Publication Data
ISBN 9781119640431
Cover image: Pixabay.Com
Cover design by Russell Richardson
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
Preface
As a subclass of coordination polymers and porous materials, metal– organic frameworks (MOFs) are composed of a dual organic–inorganic structure based on organic (organic linkers) and inorganic (metal ions/clusters) building blocks. In structural view, a unique kind of connection between organic linkers and inorganic nodes leads to construction of a three-dimensional framework with vacant spaces between building blocks. Owing to unlimited possibility in selection of organic ligands and metal ions/clusters, theoretically it is feasible to synthesis an unlimited number of frameworks.
In recent decades, MOFs received lots of attention in the world of material science and chemistry. Such tremendous attention is owing to their unique chemical characters such as hybrid organic–inorganic nature, high porosity and surface area, tunability in chemical functionality, highly ordered and crystalline structure and moderate-to-high chemical and thermal stabilities. Each one of these chemical properties enable MOFs to apply for specific purpose, but the ability to functionalize MOFs is a specific character to improve the capability of MOFs in different field of applications.
There are three ways for functionalization of MOFs including: (I) using functional organic linker, (II) pore functionalization through immobilization of other functional materials and (III) functionalization of inorganic nodes of the framework. Owing to versatile kind of organic functional groups, linker functionalization is recognized as a favorite strategy to tailor the chemical properties and enrich the host-guest chemistry of functional metal–organic frameworks (FMOFs).
In this book, we tried to review the literature to gain deep insight about the effects of linker functionalization on structure and host-guest chemistry of FMOFs. The content of this book is useful for gaining better understanding of the structural and chemical properties of FMOFs. Considering our strategy in this book, we believe that this book is interesting for diverse group of scientists like chemists, material engineers and anyone who is working on supramolecular chemistry of MOFs and designing functional materials.
The authors
October 2020
1
Introduction to Functional Metal–Organic Frameworks
Abstract
In this chapter, we discuss about the advantages of porous materials and crystalline materials and explain that what kinds of benefits are attainable if these advantages combine together in the structure of functional materials like metal-organic frameworks. Then, functional metal-organic frameworks are discussed and classified based on the roles of organic functions in the structure and application of MOFs.
Keywords: Porous materials, crystalline materials, functional metal-organic frameworks, coordination polymers, host-guest chemistry, function-application properties, function-structure properties
1.1 Coordination Polymers
Solid materials are generally classified in amorphous and crystalline (single-crystalline or poly-crystalline) solids in chemistry and material science. Crystalline solids are constructed based on periodic symmetrical arrays of constituents giving rise to definite, regular and repeating pattern of the solid in three dimensions over a large distance. Such long-range structural order rises in the beneficial fact that crystalline solids represent specific and repeatable chemical properties. This is a very pivotal advantage which is not observed in amorphous solids. For example, crystalline solids are of sharp melting point and definite heat of fusion while amorphous solids have not a characteristic heat of adsorption and sharp melting point. As a result, crystalline solids benefit from repeatable structure and chemical properties which are fitting characters in application of novel materials.
Another classification of materials is based on their porosity. Porosity, which also is called void fraction, is defined as the ratio of vacant space (void) in material to the total volume that the materials occupy. This fraction is always between 0 and 1. Porous materials encompass vacant spaces in their structure based on accessible pore volume (vacant cavities or channels) for guest molecules. This is a unique advantage of porous material in which not only can they interact with guest molecules on their surface, but also they can adsorb and interact with guests within their pores inside the bulk material. The characteristics of a porous material define by their surface area (m2·g−1), accessible pore volume (m3·g−1), shape, size and distribution of pores. Based on pore size, porous materials are classified in three major groups including microporous (in the range of 2 nm and below), mesoporous (in the range of 2 to 50 nm) and macroporous (above 50 nm) [1]. Another way to classify porous materials is pursuant to uniformity in the pore size, volume and distribution [2]. In this approach, porous materials are classified as ordered (uniform) and non-ordered groups. Uniform porous materials are developed based on same pore size, shape and distribution. To observe such uniformity in porosity, a porous material must be founded on uniform and repeatable structural patterns. This uniformity in the structure and porosity is essential for some of superior applications like size selective separation of a small molecule from a mixture containing large molecules. In size selective applications, guest molecules with smaller size (or kinetic diameter) than pore aperture of the host are able to diffuse into the pores of ordered porous material while molecules with larger size cannot. Definitely, porous materials without uniformity in their pore size and distribution could not be applied in size-selective applications because they cannot differentiate guest molecules with different sizes. These contents indicate that crystalline porous solids with regular and repeatable structure and porosity are very efficacious in molecular-sieving and also other kinds of applications.
Crystalline porous solids can be extended by different types of interactions (ionic and hydrogen bonds, covalent interactions and coordination interactions) between their individual molecular building blocks [3]. Especially, crystalline porous materials which are developed by coordination interactions are coordination polymers (CPs). In structural view, CPs could be extended in different dimensions, so they could be 1-dimensional (1D), 2D or 3D. Also, they are synthesized based on linkers and metal ion/clusters when a polydentate linker is able to associate multiple metal centers through coordination bonds in self-assembly process (Figure 1.1) [4]. As a subclass of CPs, metal–organic frameworks (MOFs)
