(719c) Sorption and Transport Properties of Zn-Based Metal Organic Frameworks (MOFs)

Increasing concern over the deleterious effects of greenhouse gas emissions on the environment has driven the search for novel technologies that can minimize their environmental impact. Carbon dioxide separation is a critical step when addressing ways to mitigate global warming effects. From an energy point of view, a material that is efficient, inexpensive and selective for CO₂ capture and conversion is desirable for these applications. Adsorption separation processes by porous materials are among the most promising methods for CO₂ removal from flue gas. Metal Organic Framework (MOF) materials have shown potential for gas separations, specifically in power plant emissions, as their chemical functionality, surface area and pore sizes can be tuned by choosing metal centers and organic ligands that are specific to each capture application. A Zn-based metal organic framework (Zn₄(pydc)₄(DMF)₂•3DMF (1)) has been prepared solvothermally and its sorption properties, transport properties and selectivity for CO₂ has been investigated. At low pressures and low temperatures, CO₂ and N₂ isotherms were obtained and surfaces areas were reported with (1) showing a type-I adsorption isotherm. The zero coverage heat of adsorption of CO₂ into the framework was calculated, and the binding into (1) was found to be exothermic and consistent with a mechanism of adsorption that involves CO₂ binding to the unsaturated Zn(II) metal centers present in the crystal structures. These metal sites are available without the need for an activation step. As this framework was the most promising material for CO₂ capture based on the preliminary sorption study, adsorption and desorption of CO₂, N₂ and CH₄ in framework (1) were further investigated under high pressure conditions at temperatures that mimic post-combustion CO₂­ capture applications. (1) showed a type-I adsorption isotherm for CO_2­ and CH₄ while it showed a linear behavior for N₂ with no hysteresis present in any of the isotherms. Thermodynamic adsorption data was obtained from the isothermal behavior at three different temperatures confirming the physisorption of gases on the Zn metal sites of (1). The selectivity of the framework for CO₂ was estimated using the ideal adsorbed solution theory (IAST). Adsorption and desorption dynamics of CO₂ on (1) were studied to obtain activation energies for adsorption and average residence times of the molecules on the framework. CO₂ showed fast adsorption dynamics with a low threshold to overcome for adsorption, and the adsorption kinetics are well described by modeling the linear difference in concentration between the bulk gas and the adsorbed gas. Desorption dynamics were fast and data for N₂ and CH₄ was obtained to conclude that CO₂ shows better desorption dynamics with longer residence times but lower activation energies for desorption. Finally, a continuous film of (1) to be used as a gas separation membrane was synthesized, and the surface morphology, presence of defects, and preferred orientation were characterized. Single gas permeability experiments of CO₂, CH₄ and N₂ were performed at different temperatures and feed pressures to examine the potential of (1) as a material for gas separation membranes.