UC Davis

Metal-organic frameworks are hybrid materials constructed by inorganic metal nodes and organic linkers. The presence of both organic and inorganic moieties in the structure allows MOFs to have a significant flexibility and tunability in compositions, structures, and functional properties. This in turns enables MOFs to have very desirable and interesting properties, such as very high porosity, very high surface area, multiferroicity, electrical conductivity, proton conductivity and others. Consequently, MOFs have potentials in a wide array of applications, including catalysis, gas storage and separation, energy storage and drug delivery.Despite the significant amount of research performed on the development and synthesis of MOFs, fundamental study of their intrinsic stability is still fragmentary. The thermodynamic properties of materials are crucial in understanding their properties and evaluating their intrinsic stability. To accelerate the discovery of new MOFs, it is necessary to study the energetics of MOFs systematically to further understand how the compositions and structures affect stability. By relating these composition – structure – energetics relationships to other measurable, calculable, or available chemical properties, it should be possible to predict the stability of new MOFs. Therefore, this dissertation work aims to fulfill this goal of understanding and predicting MOF stability. Chapter 1 is a general introduction. Chapter 2 discusses the synthesis, characterization and thermodynamic study of mineral paceite CaCu(OAc)4‧6H2O as well as its cadmium-based analogue CaCd(OAc)4‧6H2O. The enthalpy of formation revealed the destabilization introduced by the Cd to the structure. Additionally, it also explained the most possible synthesis route of paceite, which is through the formation of a calcium acetate – acetic acid solvate intermediate. Chapter 3 presents a systematic study of the energetics of A-site cation substitution on the thermodynamic stability of protonated-amine manganese formate perovskite. This system was chosen representative of a dense MOF system. The measured enthalpy of formation revealed that the stability of hybrid perovskites is not strongly influenced by the structural factor, unlike its inorganic perovskite counterparts. Instead, the stability is governed more by the interaction of the A-site cation with the framework, which further can be explained by the dissolution enthalpy of the cation salts. Chapter 4 reports the interplay between theoretical calculations and experimental measurements to determine the thermodynamic stability of porous metal organic frameworks, in this case fluorinated zeolitic imidazolate frameworks (ZIFs). The enthalpy of formations revealed the strong influence of ligand functionalization and topology on the stability. Chapter 5 presents a systematic study of the energetics of ligand substitution on the stability of ZIFs with sodalite structure. Based on the results obtained in Chapter 4, this study was designed to see only the contribution of ligand substitution to stability, while keeping the topology the same. It was found that the stability trend of sodalite ZIF samples with different ligand functionalization can be explained by a tabulated chemical property from physical organic chemistry, namely Hammett -para(+) constant. Lastly, in Chapter 6, all the results are related with summarizing points and suggestions for future research will be presented.