Gas separations are economically important for industrial applications, such as natural gas purification and CO2 capture. Porous materials provide a unique challenge and opportyinity to materials design, as adsorbent properties can be tailored to achieve selective gas adsorption. Metal-organic frameworks represent a rapidly expanding new class of porous adsorbents. Their three-dimensional structure based on metallic nodes and organic linkers provide a large range of possibilities for designing materials that incorporate desired functionalities. Given the large number of possible framework structures, quantum mechanical computations can provide useful guidance in prioritizing the synthesis of the most useful materials for a given application. We report a quantum chemical study that predicts a new metal-organic framework of potential utility for the separation of dinitrogen from methane, a particularly challenging separation of critical value for utilizing natural gas. We propose that an under- coordinated V(II) site within a metal-organic framework would yield a hypothetical material with a considerably higher enthalpy of adsorption for dinitrogen than for methane. This predicted behavior is explained based on the stronger selective back bonding of V(II) with N2 compared with CH4. This phenomena is not exhibited by the existing analogous topologies with different metal sites e.g. Fe(II). Additionally, by grafting amine functionalities to the under-coordinated Mg(II) site of Mg2(dobpdc), mmen-Mg2(dobpdc) is formed,1 and an unexpected two amine to two CO2 reaction stoichiometry is achieved with a higher capacity and weaker CO2 binding energy than that of the two to one stoichiometry observed in the conventional carbamate/ammoniaum CO2 adsorption chemistry in solution. Given the parasitic energy cost associated with amine scrubbing for industrial CO2 capture applications, an understanding of this reactivity for less energy intensive alternatives is of fundamental interest. We have characterized the mechanism of CO2 adsorption using density functional theory, and the unique reaction stoichiometry is explained by the formation of a hydrogen-bonded complex, involving two carbamic acid moieties resulting from the adsorption of CO2 onto the secondary amines.   1 T. M. McDonald, W. Lee, J. A. Mason, B. M. Wiers, C. S. Hong, and J. R. Long. J. Am. Chem. Soc. 2012, 134, 7056-7065.

Exploiting under-coordinated metal sites in porous materials for unique reactivity and novel gas separations

247th National Meeting (2014)

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