The effects of functionalization on adsorption properties of microporous materials
Cmarik, Gregory E.
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The theme of this work is the observation and understanding of the effects of pore functionalization on adsorption properties of stable Metal-Organic Frameworks (MOFs). Over the first two sections of this work, sets of materials with representative pore sizes and functional groups are studied for adsorption properties. Observed trends are used to identify the best pore properties achievable via functionalization for adsorption systems. The third section of this work provides perspective on MOF materials and proposes target pore features for an efficient adsorbent for carbon dioxide capture from flue gas. First, the highly stable UiO-66 series of materials was selected for a pure-component adsorption study. The selectivity and capacity for CO2 can be best enhanced with the smallest, most polar functional group, such as an amino group, but significantly enhance water adsorption. Large, non-polar groups do not yield a completely hydrophobic material, but may be useful for humid gas separations as pore filling with water is inhibited. Next, a breakthrough study was conducted using CO2:CH4 and CO2:N2 mixtures on a set of stable MOFs. UiO-66-NH2 and UiO-66-DM, where DM=dimethyl, outperform predictions based on published isotherms and have dynamic CO2:CH4 selectivity comparable to zeolite NaY. UiO-66-DM may be a good candidate for further study due to the combination of partial hydrophobicity and good selectivity. Finally, by combining a review of literature with observations made in this work, a perspective on MOFs as efficient humid gas separation materials is provided. The presence of water vapor prevents use of current high performance adsorbents, but several MOF pore features show promise for these separations. The designable nature of MOFs allows for targeted design of size-matched pores and single-molecule traps which can selectively or cooperatively adsorb CO2 in the presence of water. Also, many MOF materials would be well suited for advanced pressure swing adsorption cycles and engineered sorbents, which enables greater material utilization and system efficiency.