(612e) Gas Sorption, Kinetics of Sorption and Characterization of the Gas Permeation in the Pores of Microporous Metal Organic Frameworks (MOFs)

The separation and conversion of CO₂ from industrial processes are an important focus for research aimed at mitigating the effects of greenhouse gases on the environment. To this end, it is important to develop experimental techniques for the characterization and study of the transport in porous crystalline materials that present potential in CO₂ removal at stationary sources. This study reports on the sorption equilibrium and kinetics of CO₂, N₂ and CH₄ on a Zn(II)-based metal organic framework, and the fabrication and characterization of the gas transport of supported MOF membranes. Sorption equilibrium and kinetics were studied to evaluate its efficacy for CO₂ capture under post-combustion capture conditions. Adsorption and desorption equilibria were measured gravimetrically for temperatures between 308 K and 338 K and pressures up to 5 bar. The Langmuir adsorption model was used to fit sorption data and the Ideal Adsorbed Solution Theory (IAST) was used to calculate selectivity from single-component isotherms suggesting that separation can be enhanced by a decrease in temperature. The MOF exhibited preferential CO₂ adsorption based on the high enthalpy of adsorption and adsorption selectivities of CO₂ over N₂ and CH₄. Kinetics of adsorption and desorption of CO₂ (308K–338 K, pressures up to 1 bar) were fitted to the linear-driving force (LDF) kinetic model, showing a relatively fast adsorption and a low activation energy for adsorption and desorption. Diffusion inside the pores was found to be the rate-limiting step based on fits to the LDF model and the micropore diffusion model. Desorption kinetics studies at 1 bar indicated that CO₂ has greater average residence times at all temperatures and lower values of activation energy for desorption than N₂ and CH₄. This suggests the selective adsorption and capture of CO₂ on (1) will be favored. Zn(II)-based membranes were used to develop an approach to study membrane quality and determine the transport mechanisms through the pores of the crystalline membrane. MOF membranes were consistently synthesized via a solvothermal method with structural defects sealed by a low-permeability polymer coating. This polymer coating does not modify the separation properties of the MOF, but acts to seal pin-holes and cracks in the membrane, allowing for the measurement of permeation in materials that do not form uniform, defect-free films. The Zn(II)-based MOF membrane permeation of CO₂, CH₄, N₂ and H₂ was proportional to the inverse square root of the molecular weight of the permeant gases, indicating that diffusion occurs via Knudsen diffusion. Membrane quality was studied by permselectivity measurements of CO₂, CH₄, N₂ and H₂ as a function of temperature showing that the MOF membranes are crack-free. A new approach, based on studying gas permeation through a polymer coated sparse MOF membrane, was used to corroborate that transport occurs through the pores of the MOF, rather than through pinholes or defects in the structure.