Tuning metal-organic frameworks for flue gas desulfurization
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Wang, Chengzhai
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Abstract
Continued intensive combustion of fossil fuels has caused the increase of sulfur dioxide (SO2) concentration in the air, which has been considered one of the primary pollutants responsible for human health problems like respiratory disease and environmental issues. Metal organic frameworks (MOFs) have attracted considerable interest in the scientific community for gas separations. Due to features such as high surface area, high porosity, and designable chemistries, MOFs show intriguing potential for being used as selective SO2 sorbents. In this dissertation, we focus on three types of MOF modifications: metal substitution in OMS MOFs, ionic liquid coordination to OMS, and ligand functionalization. This objective is to understand the ability of these modifications to tune and enhance adsorption properties of MOFs for SO2 capture.
MOFs possessing coordinatively unsaturated metal sites (referred to as open metal sites, OMSs) have been observed to exhibit high affinity to specific gas molecules. The presence of unsaturated metal sites with accessible coordination vacancies allows for strong interactions, such as coordination bonds, electrostatic interactions, or dipole-dipole interactions, between the gas molecules and the OMSs. This results in enhanced adsorption of gases within the MOF framework. Furthermore, the unique properties of OMSs can facilitate selective gas adsorption. The presence of OMSs with tailored properties enables preferential adsorption of specific gases over others. This selectivity arises from the specific interactions between the gas molecules and the OMSs, such as size, shape, or electronic matching. In Chapter 3, we focus on the open trivalent metal sites on MIL-100-M (where M = Al, Sc, V, Fe, and In.) and aim to reveal the effect of trivalent metal sites on SO2 adsorption. Moreover, the water stability problem of MOFs featuring OMSs is also addressed by a post-synthetic metal insertion process.
Post-synthetic modification through OMSs provides a versatile and effective approach to tailor MOFs for specific applications. It involves introducing additional functional groups or species onto the MOF structure after its synthesis. This strategy allows for the customization and enhancement of MOF properties for gas separation. It enables the introduction of additional functional groups or active sites, which can enhance the MOF's adsorption capacity, selectivity, or catalytic activity. These functional groups can interact with target molecules, such as gases or pollutants, improving the performance of the MOF. Additionally, post modification through OMSs offers a way to address stability concerns associated with MOFs containing exposed metal sites. OMSs can initiate framework degradation or unwanted reactions with moisture or other reactive species. By post-synthetic modifying the MOF structure, stability can be improved through the coordination or bonding of additional functional groups, protecting the OMSs and enhancing the overall robustness of the MOF material. In Chapter 4, ionic liquids (ILs) containing nitrogen functional sites are introduced post-synthesis into MIL-101 to improve the capacity and selectivity for SO2 capture. Furthermore, by coordinating with the open metal sites, the ILs effectively protect the framework from the intrusion of "invader" molecules, further enhancing the material's stability.
Lastly, the ligand functionalization of MOFs is one of the most common and efficient approaches that can enhance MOFs properties over the parent material for adsorption-based separations. The decorated functional sites on ligands play a crucial role in determining the adsorptive capacity and selectivity by dictating the type of adsorbent-adsorbate interaction, and consequently the enthalpy of adsorption. Amongst this field, incorporating nitrogen-based sites on MOF ligands has proven a feasible approach for achieving the desired affinity for CO2 or water molecules at low relative pressures. Chapter 5 investigates the effect of a variety of nitrogen sites on MOFs ligands on SO2 capture and provides insights into establishing basic rules for the design of MOFs with N-containing functional groups for SO2 adsorption.
In summary, by exploring three distinct categories of modifications for Metal-Organic Frameworks (MOFs), namely the substitution of metals in OMS MOFs, coordination of ionic liquids to OMS, and functionalization of ligands, this dissertation delves into the intricacies of these modifications and their impact on enhancing the adsorption properties of MOFs and develops the possible strategies that can be applied to tune MOFs properties effectively and designedly for SO2 adsorption.
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2023-07-25
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Dissertation