Functionality and Functionalization of Metal--Organic Frameworks

Abstract

Metal—organic frameworks (MOFs) are a relatively new class of materials based upon organic linkers bridging metal clusters or ions to create a 2D or 3D structure. The most exceptional materials among MOFs possess large surface areas extending up to 6000 m2/g and are geared towards applications in gas storage, separations, and catalysis. In these applications, the chemical environment of the MOF interior is important as it can allow for selective interactions with guests as necessary. In the functionalization of MOFs, postsynthetic exchange (PSE), a suite of methods for introducing chemical functionality to a preformed MOF, has arisen as a key method for incorporation of new functional groups into MOFs that allow for further tailoring of these materials towards applications. This dissertation explores the use of functional environments in MOFs towards applications in separation and probes the mechanism of PSE in MOFs and the effects this has on the chemical environment. Chapter 2 describes the utility of MOFs with coordinatively unsaturated metal centers for purification of the industrially important gas chloromethane; this was achieved through selective interactions with the impurity dimethyl ether thus offering an alternative to the current practice of reactive separation. The MOFs, HKUST-1, MIL-100(Fe), and Co/DOBDC showed not only high capacities for dimethyl ether adsorption, but Co/DOBDC was shown to be easily regenerable with mild heating under a flow of inert gas. Chapter 3 describes mechanistic studies of how PSE effects the chemical environment inside the MOF. The microstructure of MOFs after PSE was examined using Raman microscopy which revealed exchanged ligand was concentrated at the edges of the crystal and decreased in concentration towards the center of the crystal resulting in a gradient core-shell behavior. Diffusion studies of carboxylate based ligands into MOF-5 showed that lack of uniform exchange is the result of slow diffusion kinetics. This core-shell behavior was also observed in UiO-66 and UMCM-8 showing the general applicability of PSE in generating core-shell materials. Finally, in Chapter 4 PSE methods were applied in a different class of MOFs, zeolitic imidazolate frameworks, to incorporate a primary amine in order to increase the CO2 uptake of the material. By using the readily available biomolecule histamine, incorporation into ZIF-8 saw a marked increase in CO2 capacity compared to the unmodified material.