Metal-Organic Frameworks for Selective Gas Separation.

摘要

Metal-organic frameworks (MOFs) are highly porous crystalline materials that can be synthesized with a variety of functional groups in the pores. Though their equilibrium uptake of pure gases has been studied extensively, little is known about their ability to separate mixtures of gases. We use fixed-bed breakthrough separation experiments to elucidate the gas separation properties of MOFs. Study of the separation properties of six "benchmark" MOFs when exposed to mixtures containing harmful gases reveals that the strength of interaction between a gas and reactive sites on the framework is the most important consideration for gas separation. In particular, MOFs with coordinatively unsaturated (open) metal sites provide an opportunity for strong binding of gases and thus for effective separation. Based on this information we undertook to find MOFs that are effective for separation of CO2 from other light gases. This separation is an essential piece in any realistic strategy for addressing the critical problem of anthropogenic global climate change. We fmd that Mg-MOF-74, a member of a series of structurally identical MOFs with variable open metal sites, takes up 8.9 wt. % CO2 and releases it under mild heating, a substantial improvement over known porous adsorbent materials. Finally, we attempted to functionalize MOF materials to optimize their gas separation performance. We successfully synthesized MOF-253, the first MOF to contain an uncoordinated 2,2'-bipyridine group, and bound PdCl 2 in high yield, indicating the potential for creation of controlled, accessible metal centers in this MOF. The studies described here establish MOFs as promising materials for various important gas separation applications and demonstrate the possibility of including reactive centers tuned for a particular mixture of gases.