(727g) High-Throughput Screening of MOFs for CO2 Separation

The capture of CO₂ from fossil fuel combustion flue gas is the focus of significant research efforts. Existing CO₂ capture technologies such as amine absorptionare extremely energy intensive. Metal-organic frameworks (MOFs) are potential size-selective, high-capacity  materials for adsorptive and membrane-based capture of CO₂ and other gases. MOFs exhibit nanoporous crystalline structure in which organic linker molecules self-assemble with metal centers to form crystals with well-defined pore size, high surface area, and low framework density. MOFs have attracted attention as possible components of CO₂ capture technologies based on either adsorption or membranes.

In principle, the enormous number of MOFs with various combinations of organic linker and metal cation creates an opportunity to tailor pore structure, size, and functionality for CO₂ capture from flue gas. Significant problems, however, frustrate attempts to discover or design such materials. In addition to the large, under-explored design space, some MOFs are unstable in water vapor, and the principles for designing water-stable MOFs are not yet well known. The adsorption properties of a MOF are critical to its value for CO₂ capture. Measurements for determining adsorption isotherms are tedious and time consuming. Almost all available data for gas adsorption in MOFs reports adsorption of dry single-component gases. For considering practical CO₂ capture applications, the performance of MOFs that have been exposed to water vapor and acid gases like SO₂ and NO₂ is critical, and almost no data addressing this issue are available. There are few published reports of MOF stability after exposure to water and none, to our knowledge, that examine stability as a function of exposure to acid gases.

To address the above challenges, we report here the efficient screening of MOF CO₂/N₂ adsorption and diffusion selectivity, as well as their sensitivity to water vapor and acid-gases, via the a novel parallel high-throughput (HT) sorption screening system. Our HT approach collects adsorption information on the gases directly relevant to CO₂ capture (CO₂ and N₂) in the pressure regime relevant for flue gas applications. A key element of our HT experiments is to collect adsorption data at only one relevant value of pressure, rather than measuring a complete adsorption isotherm. The HT-sorption instrument has an array of 36 chambers, each connected to a pressure sensor and a valve (Supporting Information). By monitoring pressure decay versus time, CO₂ and N₂ adsorption were measured at a single state point at 30 °C.  We illustrate this approach by simultaneously investigating 8 candidate MOF materials, of which the best material was found to have a CO₂/N₂ membrane selectivity of 152 and a CO₂ permeability of 60 barrer for Co-NIC. This approach provides a significant increase in efficiency of obtaining the separation properties of MOFs. While we describe here the identification of novel materials for CO₂ capture, the methodology enables exploration of the performance and stability of novel porous materials for a wide range of applications.  To our knowledge, this is the first high-throughput screening system which enables measurement of adsorption capacities and selectivities on MOFs, in addition to exposing the MOFs to water vapor and acid gases.  The method leads to rapid selection of MOFs with high selectivity and stability to water vapor and acid gases. As a result, this talk reports data for acid gas stability previously unknown for these MOFs.   The HT screening system can handle a variety of gas adsorbates and hence can be applied widely to screen adsorption and diffusion properties of other materials including inorganic compounds, polymers, and low-volatility liquids.