UC Berkeley

This dissertation presents the design, synthesis, and characterization of amine-functionalized metal–organic frameworks for applications in gas separations and gas storage. Of particular focus is the separation and capture of carbon dioxide from point sources, like natural gas flue emissions, as well as directly from air. Adsorption analysis, through collection of isotherms and isobars, along with a number of characterization techniques, like infrared and nuclear magnetic resonance spectroscopies, and single crystal and powder X-ray diffraction are utilized.

Chapter one provides a brief introduction to carbon dioxide separations. Emissions of CO2 are broken down by sector and potential strategies to capture CO2 are portrayed. Specifically, methods using adsorbents as well as the different types of adsorbent classes are examined closely. A qualitative assessment of each of the adsorbent classes using key performance metrics is given. Last, the emergence, development, and usage of amine-functionalized M2(dobpdc) (M2+ = Mg2+, Mn2+, Fe2+, Co2+, and Zn2+, dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) materials, which exhibit unique cooperative adsorption through the formation of ammonium carbamate chains, is highlighted.

Chapter two describes the synthesis of a new family of amine-functionalized M2(dobpdc) materials, where the framework is appended with tetraamine molecules rather than diamine molecules. Typically, a single diamine molecule coordinates each metal in the material, but in this new system one tetraamine molecule coordinates two metals adjacent to each other across the wall of a pore. This is done such that cooperative adsorption through the formation of ammonium carbamate chains is retained. The structures of these materials were studied in depth using single-crystal and powder X-ray diffraction. In addition, the adsorption mechanism was confirmed through spectroscopic study and computations. In particular, the Mg2(dobpdc)(3-4-3) (3-4-3 = N,N′-bis(3-aminopropyl)-1,4-diaminobutane) material was found to have enhanced stability, arising from the multi-metal coordination of the tetraamines, as compared to the diamine-functionalized variants. This material is a good candidate for usage in capturing CO2 efficiently from the emissions of natural gas-fired power plants and the enhanced stability allowed for steam desorption, a potentially cost effective process.

Chapter three extends the work done in the previous chapter by using a number of tetraamine-functionalized Mg2(dobpdc) materials for direct air capture. Removal of CO2 from air is typically very difficult due to the ultralow concentration of CO2 present. Utilizing the extremely high CO2 adsorption enthalpies found in these materials, these materials were found to be capable of capturing CO2 from concentrations relevant to air. Specifically, the Mg2(dobpdc)(3-3-3) (3-3-3 = N,N′-bis(3-aminopropyl)-1,3-diaminopropane) was found to have one of the lowest adsorption step pressures (by isotherm measurement) and highest adsorption step temperatures (by isobar measurement) of all of the amine-appended materials. Another candidate material, Mg2(dobpdc)(3-2-3) (3-2-3 = 1,2-bis(3-aminopropylamino)ethane), was also studied by spectroscopy and computation to understand its unique mechanism for CO2 adsorption and desorption. Furthermore, these two materials along with Mg2(dobpdc)(3-4-3) were closely examined for their applicability in air capture conditions, with a specific focus on their performance in the presence of water.

Chapter four describes the usage of the technique of amine-functionalization in the M2(dobdc) (dobdc4– = 2,5-dioxido-1,4-benzenedicarboxylate) and M2(dobpdc) systems for the storage of methane. Effectively using natural gas under ambient conditions for applications in transportation requires improving its volumetric energy density. To achieve this, materials that could exhibit step-shaped adsorption of CH4 through a pore-gating mechanism were sought. A series of phenylalkylamine-functionalized metal–organic frameworks, specifically M2(dobpdc), were synthesized and characterized to control the accessibility of CH4 to the pore through the use of phenylalkylamine intermolecular interactions. However, the flexibility of long alkyl chain phenylalkylamines and the insufficient bulkiness of short alkyl chain phenylalkylamines were found to impede pore-gating. Preliminary work has shown that by using the smaller pore analogue M2(dobdc), these issues may be addressable, but further work is necessary. Last, it was hypothesized that this same approach allows for an effective method of easily tuning the pore environment of a metal–organic framework for potential usage in the separation of gases, such as olefins and paraffins.