Enhanced Structural Organization and Sorption Properties of Covalent Organic Frameworks Through Rational Monomer Design
Alahakoon, Sampath Bandara
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Covalent Organic Frameworks (COFs) are a class of crystalline porous polymer networks which have attracted much attention over the past decade due to their unique properties. COFs consist of lightweight elements (C, N, H, O, B, etc.) and have a higher surface area, crystallinity, permanent porosity and low density. The formation of these materials primarily depends on the interplay between dynamic covalent bond formation and supramolecular interactions between the monomer units. Over the past decade, boron ester, imine, azine, and hydrazone formation reactions have been intensively utilized as dynamic covalent reactions to overcome the crystallization problem with the aid of dipolar and π-π stacking interactions in the synthesis of crystalline COFs. Depending on the geometry of the starting monomers these materials can be divided into 2D- and 3D-COFs. Even though the 3D-COFs exhibit higher degree of order, most 2D-COFs show moderate crystallinity. So developing strategies to improve the long-range order, surface area, and sorption capabilities are of prime importance to make the COFs promising candidates in gas storage and separation, electrical energy storage, heterogeneous catalysis, sensing, drug delivery, etc. Thus, the overall goal of this research effort is to improve the material properties of 2D-COFs aiming at developing general synthetic rules to support future materials development. The first chapter of the dissertation provides a literature review of efforts to develop 2D-COFs in gas and electrical energy storage. Some of the design strategies for developing the gas sorption properties of COFs and mechanistic studies of their formation are also discussed. In the second chapter, the use of a six-fold symmetric hexaphenylbenzene based aldehyde (HEX) in the synthesis of an azine-linked COF with triangular micropores and excellent sorption capability for CO2 (20 wt%) and CH4 (2.3 wt%) is discussed. This is the first report of the study of sorption capability of a hexaphenylbenzene based COF in literature. In the third chapter, the use of fluorines in the COF monomers to improve the structural organization of COFs is discussed. Furthermore, a detailed synthesis and an intensive characterization of a series of fluorine-containing COFs are also discussed with their mechanism of formation. Further insight into the effect of fluorines in the COF synthesis through mixed linker studies and computational calculations is discussed in the fourth chapter.