(159f) Atomic-Level Modeling of Separation CO2 Using Metal-Organic Framework Membranes

Metal-organic frameworks (MOFs) are promising nanoporous materials that may be useful for efficient and cost effective capture of CO2 from fuel or flue gases. Experimental work is critical to progress in developing and testing new materials, but the number of possible candidate materials makes it impractical to use an Edisonian approach to materials discovery. Molecular modeling is a tool that can be used to help guide experiments and screen materials more cost effectively than experiments alone. We present results from semi-empirical statistical mechanical models and first-principles density functional theory for evaluating the effectiveness of MOF materials as a membrane for CO2 capture. The membrane selectivity for gas mixture is a function of both adsorption selectivity and diffusion selectivity. In most cases the adsorption selectivity and diffusion selectivity are opposite to each other. For example, the adsorption selectivity is typically > 1 and the diffusion selectivity is < 1. A simple estimate for the membrane selectivity is the product of the adsorption and diffusion selectivities. Hence, the opposite trends in selectivities lead to overall low membrane selectivity. To increase the membrane selectivity, it is crucial to make both selectivities as large as possible. We present simulation results for an amine-functionalized MOF that shows high membrane selectivity for CO2/CH4 gas mixtures as a result of having both high adsorption and diffusion selectivities.