UC Berkeley

The availability of coordinatively unsaturated metal sites within a rigid, extended structure has made metal–organic frameworks attractive scaffolds for the adsorption of a wide variety of gases. The work discussed in this dissertation focuses on the role of structural modification in the tuning of adsorption properties of iron-based frameworks, especially with gases of a π-acidic nature. A special emphasis is made on deconvolution of the structural and electronic factors that contribute to adsorption behavior and the insights that can be gleaned through the versatile powder X-ray diffraction technique.Chapter 1 outlines past approaches to the synthesis of frameworks that exhibit selectivity for π-acidic gases, with a focus on carbon monoxide and hydrocarbon separations. This chapter discusses numerous mechanisms to achieve separations including size exclusion and kinetic separations but focuses on the role of unsaturated open metal sites in these separations. Works that employ structural and electronic modification techniques to enhance binding in open metal site MOFs are highlighted. Chapter 2 discusses the synthesis of a series of novel mixed-metal MOFs that cooperatively adsorbs carbon monoxide gas through a spin transition mechanism. Control over the pressure at which the strong cooperative binding event occurs is demonstrated in Fe2-xCrxCl2+x(btdd) (x = 0.2, 0.5, 1) (H2btdd = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin) by introducing varying quantities of chromium(III) sites into the framework lattice. A series of structural and electronic characterization techniques are employed to explain the origin of the mechanistic control. Powder X-ray diffraction shows that the mixed metal variants all undergo the same lattice contraction upon CO binding as the parent framework, but to a lesser degree with increasing Cr(III) content due to a decrease in unit cell volume of the bare framework. This is attributed to the smaller ionic radius of Cr(III) bringing the crystal lattice in its initial state closer to the volume of the lattice post-spin crossover, which decreases the barrier to transition. Infrared and Mössbauer spectroscopies confirm that the electronic contributions to gas binding amongst the variants are minor, making structural changes the dominant factor in controlling this gas adsorption mechanism. Chapter 3 expands on the gas separation applications of mixed metal frameworks to the separation of alkynes, olefins and paraffins. The isoreticular, smaller pore analogue of the Fe2Cl2(btdd) framework discussed in Chapter 2, Fe2Cl2(bbta) (H2bbta = 1H,5H-benzo(1,2-d:4,5-d')bistriazole), was explored as well as a new mixed metal variant Fe1.5Cr0.5Cl2.5(bbta). Acetylene gas adsorption isotherms showed that the frameworks have a total uptake double that expected for a ratio of one gas molecule per open metal site and that they both exhibit a selectivity for acetylene over ethylene at room temperature. The calculated isosteric heats of adsorption for Fe1.5Cr0.5Cl2.5-(bbta) indicated that the presence of chromium increased the binding strength of acetylene relative to the parent framework. The origin of this unexpected uptake was explored through powder X-ray diffraction which showed that the unit cell of the all-iron framework contracts upon exposure to acetylene, which is indicative of a chemisorptive interaction. Rietveld refinement of the gas dosed framework revealed a secondary binding site in the corners of the hexagonal pore channels. Infrared spectroscopy also confirmed the presence of two spectroscopically distinct binding sites, evidenced by two redshifted acetylene stretches in the gas dosed framework spectrum. It is hypothesized that the presence of terminal chloride anions on the Cr(III) sites in Fe1.5Cr0.5Cl2.5-(bbta) enhances acetylene binding through electrostatic interactions. Powder X-ray diffraction plays a key role in deciphering the origin of gas adsorption phenomena in metal–organic frameworks as a whole and was a highlighted technique in Chapter 2 and 3. In Chapter 4, the versatility of powder X-ray diffraction is discussed along with some of the challenges that arise due to the nature of the technique. Contributions of powder X-ray diffraction analysis to several collaborative projects involving in frameworks exhibiting flexibility, ligand insertion and binding at open metal sites are described. Key insights in adsorption mechanisms and origins of selectivity derived from structural analysis are identified.