Metal-Organic Framework Structure-Property Relationships for Selective Sulfur Dioxide Adsorption
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Evans, Tania Gabrielle
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Abstract
Adsorption-based separations offer a relatively energy-efficient approach to flue gas desulfurization, but the development of materials that are selective, stable, and regenerable under realistic conditions remains a challenge. Metal–organic frameworks (MOFs) are promising candidates due to their high surface area, high porosity, and uniquely tunable structures. However, many MOFs are susceptible to degradation or deactivation when exposed to acidic gases, which limits their widespread adoption. This dissertation investigates three MOF design strategies to improve SO2 adsorption performance: mixed-metal incorporation, linker functionalization, and post-synthetic metalation. Each approach is evaluated in terms of its effect on SO2 uptake, selectivity against CO2, and framework stability, with the goal to understand underlying structure–property relationships that can inform better material design.
Chapter 3 explores bimetallic MOF-74 frameworks, incorporating Ni2+ or Cu2+ into MOF-74(Mg) to tune SO2 adsorption behavior. Each of the mixed-metal frameworks exhibited a linear relationship between the two parent frameworks for CO2 adsorption capacity. Increasing Cu content correlated with decreasing SO2 capacity and SO2/CO2 selectivity, but also showed better retention of accessible surface area post-SO2 exposure, likely attributed to stabilization due to distortions at the Cu metal centers. In contrast, Mg–Ni materials displayed irregular behavior with no clear relationship between Ni content and performance. A combination of PXRD,
BET, and XPS showed that across all materials there were significant reductions in porosity with a maintenance of crystallinity, attributed to irreversible binding of SO2. While both Mg and Mg-Ni MOFs irreversibly bound SO2, increasing Cu content was aligned with a smaller amount of SO2 found in the post-desorption samples. These results suggest that while metal substitution can be a strategy to alter adsorptive performance, its results are highly metal-specific.
Chapter 4 investigates the effect of linker functionalization on SO2 and CO2 adsorption in UiO-66 frameworks. A series of derivatives with –CH3, –CH32, –NH2, –NO2, and –COOH groups were synthesized to examine how modifications to surface chemistry and pore size via functional group influence adsorptive performance. Functional groups –NO2,–COOH, and NH2 exhibited the most improved SO2 uptake and SO2/CO2 selectivity in both unary and binary adsorption experiments. The relatively neutral methyl group led to capacity and selectivity improvements as well, though more modest. while –NH2 showed intermediate behavior. Additionally, installation of dual methyl or carboxylic groups led to a larger increase in adsorptive performance. All samples retained crystallinity after SO2 exposure, along with small surface area loss. Combined with the fact that none of the functionalized UiO-66 materials retained detectable SO2 following desorption, SO2 binding in this series appears to be reversible under traditional desorption conditions. These results suggest that functionalization can be used to significantly tune selectivity and uptake and offers potential advantages for regenerability.
Chapter 5 examines the impact of post-synthetic Cu2+ coordination on MIL- 101 frameworks functionalized with –COOH and –SO3H groups. The goal was to introduce alternative adsorption sites and assess whether metalation could improve SO2 uptake or selectivity. Cu coordination to the –COOH group led to a moderate increase in SO2 capacity, while coordination to –SO3H groups resulted in decreased capacity. Both COOCu and SO3Cu materials showed clear evidence of degradation
upon SO2 exposure - PXRD patterns revealed loss of crystallinity, surface areas declined post-exposure, and breakthrough capacity dropped over multiple cycles. Though SO2 was detected in the non-metalated frameworks, it was not detected in either metalated framework after desorption. These results indicate can enable reversible binding in place of MIL-101’s usual irreversible binding sites, but only if the chosen anchoring group does not compromise framework stability. Overall, post- synthetic metalation can alter adsorption behavior, but both the choice of functional group and the local coordination environment must be considered when designing alternative metal binding sites.
Chapter 6 discusses the overall conclusions of this work and outlines future directions to advance MOF design. Overall, this dissertation demonstrates that tuning MOFs for SO2/CO2 separations, like any separations process, requires balancing the competing priorities of capacity, selectivity, and stability. Each design strategy explored (metal substitution, linker functionalization, and post-synthetic metalation) offered distinct advantages, but also introduced tradeoffs. Taken together, the findings highlight how specific structural features influence acid gas adsorption, offering insight into more effective MOF design.
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2025-08-25
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Dissertation (PhD)