Solvent Effects in Metal-Organic Framework Activation, Resolvation, Synthesis, and Linker Exchange

Abstract

Metal-organic frameworks (MOFs) are the largest, highest performing class of porous materials, with applications including gas storage, catalysis, separations, and controlled release, but their widespread application has been hindered by a number of factors. Several challenges in the field are 1) structural degradation during relevant conditions or processing steps, 2) an overreliance on expensive, toxic precursor chemicals and synthesis solvents, and 3) difficulty achieving desired MOF phases and microstructures. This dissertation focuses on addressing all three of these challenges. In Chapter 2, MOF degradation during the process of activation (solvent removal), a major barrier to the preparation of many potentially useful frameworks, is explored. The process is demonstrated to have potential to generate a range of products along the crystalline-amorphous continuum, including materials that have increased polycrystallinity with no decrease in accessible surface area. In addition to revealing new insights into the scope of outcomes possible from activation-induced damage, these findings highlight the importance of proper sample characterization during optimization of MOF activation conditions, an important consideration as MOFs move towards large-scale production. In Chapter 3, the same characterization techniques are applied to probe the reverse of activation: resolvation of activated MOFs. MOF resolvation is commonly performed when characterizing catalytically active frameworks, and the utility of high-performing MOF systems may be masked if resolvation can beget MOF collapse. This work reveals that this process indeed has destructive potential, which can be mitigated by stepwise resolvation starting with low surface tension solvents. These findings inform best practices for treatment of MOFs intended for liquid-phase catalysis as well as in other solvated applications, such as molecular sensing and pollutant capture. In Chapter 4, the problem of (alkyl formamide) MOF synthesis solvent toxicity is addressed. In particular, a greener, safer route to the creation of MOFs is explored through the use of N,N-diethyl-3-methylbenzamide (DEET), the most common and effective insect repellant, as a MOF synthesis solvent. MOFs synthesized in DEET are demonstrated to have potential as a component of controlled-release DEET formulations which operate via vapor pressure suppression. This work to make MOF syntheses safer is vital as MOF syntheses are translated from the research scale (grams) to the industrial scale (kilograms-tons). Finally, in Chapter 5, the process of MOF linker exchange, a technique useful for generation of otherwise inaccessible MOFs and MOF microstructures, is studied. Specifically, the role of solvent in the process of MOF linker exchange is examined, and the possibility of modulating linker distributions within MOFs (microstructural control) by careful choice of solvent is demonstrated. This work lays the foundation for efficient generation of core–shell MOFs, and provides valuable chemical insight into the process of MOF linker exchange, increasing the potential widespread utility of these materials.