Adsorption, separation and conversion of CO₂ from industrial processes are among the priorities of the scientific community aimed at mitigating the effects of greenhouse gases on the environment. One of the main focuses is the capture of CO₂ at stationary point sources from fossil fuel emissions using porous crystalline materials. Porous crystalline materials can reduce the energy costs associated with CO₂ capture by offering high adsorption rates, low material regeneration energy penalties and favorable kinetic pathways for CO₂ separation. MOFs consist of polymeric inorganic networks with adjustable chemical functionality and well-defined pores that make them ideal for these applications. The objective of this research was to test the potential for CO₂ capture on Zn-based MOFs by studying their sorption, transport and gas separation properties as adsorbents and continuous membranes.
Three Zn-based MOFs with open Zn-metal sites were initially studied. Zn4(pdc)4(DMF)2•3DMF (1) exhibited the best properties for CO₂ capture and was investigated further under realistic CO₂ capture conditions. The MOF exhibited preferential CO₂ adsorption based on a high enthalpy of adsorption and selectivity of CO₂ over N₂ and CH₄. Sorption dynamics of CO₂ indicated fast adsorption and a low activation energy for sorption. Diffusion inside the pores is the rate-limiting step for diffusion, and changes in the process temperature can enhance CO₂ separation. Desorption kinetics indicated that CO₂ has longer residence times and lower activation energies for desorption than N₂ and CH₄. This suggests that the selective adsorption of CO₂ is favored.
MOF/Polymer membranes were synthesized via a solvothermal method with structural defects sealed by a polymer coating. This method facilitates the permeation measurements of materials that cannot form uniform-defect-free layers. The membrane permeation of CO₂, CH₄, N₂ and H₂ exhibited a linear relation to the inverse square root of the molecular weight of the permanent gases, indicating that diffusion occurs in the Knudsen regime. Permselectivity was well-predicted by the Knudsen model with no temperature dependence, and transport occurs inside the pores of the membrane. MOF (1) exhibits ideal properties for future applications in CO₂ capture as an adsorbent. / Ph. D. / Separation and conversion of carbon dioxide from industrial processes are among the priorities of the scientific community aimed at mitigating the effects of greenhouse gases on the environment. One of the main focuses is the retention and capture of harmful atmospheric gases at stationary point sources from fossil fuel emissions (such as power plants). Research using materials formed by porous crystalline structures where gases can travel at different rates is key for these applications. Porous crystalline materials can reduce the energy costs associated with carbon dioxide capture by offering high adsorption rates, low material regeneration energy penalties and favorable differences in gas velocities and transport properties for the separation of greenhouse gases. MOFs consist of polymeric networks linked by transition metal ions, and have adjustable chemical functionalities and well-defined pores that make them ideal for these applications. The objective of this research was to test the potential for carbon dioxide capture on MOF materials containing Zinc ions by studying their gas adsorption and desorption, the transport of gases through their crystalline pores and their gas separation properties as adsorbents and continuous membranes.
Three crystalline materials were initially tested, with Zn4(pdc)4(DMF)2•3DMF, (1) for simplicity, exhibiting the best properties for the capture and retention of CO<sub>2</sub> among the materials. (1) was investigated further under realistic CO<sub>2</sub> capture conditions – the conditions of pressure and temperature common in flue gases generated from the production of energy in power plants. MOF (1) exhibited preferential CO<sub>2</sub> adsorption based on a higher bonding energy between the gas molecules and the surface of the material and the preferential adsorption of CO<sub>2</sub> molecules over other relevant species present in combustion gases. It was determined that CO<sub>2</sub> molecules are transported rapidly through the inside of the pores of the material to reach sites where they are adsorbed, and the energetic requirements to start this process are low. The traveling velocities of gases inside the pores of the material are limited by the physical characteristics of the pores of the crystals, and changes in the process temperature can enhance the separation of carbon dioxide. The regeneration of the material was studied to understand the energy required to take the material back to its original state and reuse it. It was determined that, on average, carbon dioxide spends more time on the surface before going back to the bulk gas and it needs less energy to leave the surface of the MOF when compared to nitrogen and methane. This suggests that the adsorption of CO<sub>2</sub> is selective over other typical products of combustion on (1).
Porous crystalline materials can also be applied as selective barriers to gas molecules in a membrane configuration. MOF/Polymer membranes were synthesized and their structural defects were sealed using a polymer coating. This method facilitates the measurements of the transport of gas molecules on materials that cannot form uniform-defect-free layers. The transport of gases through a membrane of (1) is dependent upon the weight of the gas molecules and the relative transport of gases inside the membrane is independent of the temperature in the system. It was concluded that MOF (1) exhibits ideal properties for future applications in CO<sub>2</sub> capture as an adsorbent.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/73214 |
| Date | 13 October 2016 |
| Creators | Landaverde Alvarado, Carlos Jose |
| Contributors | Chemical Engineering, Whittington, Abby R., Davis, Richey M., Martin, Stephen M., Morris, Amanda J. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
Page generated in 0.0028 seconds