(353h) A Novel Metal–Organic Coordination Polymer for Selective Adsorption of CO2 Over CH4

Porous coordination polymers or metal-organic frameworks (MOFs) have attracted great interest recently for potential applications in adsorption separations, gas storage, sensing, and catalysis. In contrast to conventional microporous materials, these organic-inorganic hybrids have the potential for synthesis using a rational design approach by flexible control of the architecture and functional group. In materials such as those assembled from the paddle-wheel structure, as-synthesized MOFs have solvent molecules attached as ligands to the metal centers. Removal of these solvent ligands by thermal activation generates unsaturated metal centers (UMC) or open metal sites. Thus, the metal atoms are exposed on the interior surfaces of the material and are open to direct approach by sorbate molecules. Open metal sites greatly increase the ability of the material to selectively adsorb particular molecules. Consequently, the incorporation of UMCs into MOFs is a practical strategy for manipulating the adsorption behavior. In this work, we present the synthesis and characterization of a new metal-organic coordination polymer for selective adsorption of carbon dioxide over methane. This material assembles in a unique two-dimensional interpenetrating network structure with uncoordinated carboxylic functional groups and unsaturated metal sites. The selectivity of this material for CO_2 at low pressure is among the highest reported for any porous material. The broader implications of our results are two-fold: (1) constructing MOFs with unsaturated metal centers is of paramount importance for selective adsorption of polar molecules over nonpolar molecules and (2) while somewhat counterintuitive, three-dimensional interconnected pores can actually be detrimental to adsorption selectivities due to the increase in van der Waals interactions.

/Chem. Commun./, 2009, (18), 2493-2495.