(194f) Characterization of Microporous Adsorbents: A Lattice DFT Study of the “Surface Area” of Metal-Organic Frameworks

Surface area per unit mass is one quantity used to characterize adsorbent materials, as it provides an easily understood description of the adsorption capacity of a particular material. For porous materials, surface areas are defined through either direct geometric analysis (Connolly area, surface-accessible area) or indirect model-based methods such as BET[1-4]. In microporous materials, such as metal-organic frameworks (MOF) and zeolites, the true surface is far from flat and the standard metrics may not provide a reliable characterization of the material. Nonetheless, the BET theory (which was derived for multilayer adsorption on a flat surface)[1] has proven useful in describing the adsorption of nitrogen in certain isoreticular MOFs[5]. What is not understood, however, is whether this acceptable description of microporous materials via the BET theory is a chance event or has a more profound interpretation. We reexamine adsorption in MOFs using the recently derived Lattice Density Functional Theory[6], and use the resultant 3D density profiles to quantify where gases adsorption actually occurs and whether true gas monolayers form. We compare these results to those of the BET theory and identify why the BET theory is successful in some MOFs but fails in others. Finally, we discuss the utility of surface area as a characterization tool for microporous materials such as MOFs.

[1] Brunauer, Emmet, and Teller, J. Am. Chem. Soc., 60, 309 (1938)

[2] Connolly, J. Appl. Crystallogr. 16, 548 (1983)

[3] Leach, Molecular Modeling: Principles and Applications (2001)

[4] Gelb and Gubbins, Langmuir, 14, 2097 (1998)

[5] Walton and Snurr, J. Am. Chem. Soc., 129, 8552 (2007)

[6] Siderius and Gelb, Langmuir, 25, 1296 (2009)