Sorption, Transport and Gas Separation Properties of Zn-Based Metal Organic Frameworks (MOFs) and their Application in CO2 Capture
Landaverde Alvarado, Carlos Jose
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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 permeant 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.
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