(339a) Applicability of Consistency Criteria to Calculate BET Areas in Metal-Organic Frameworks (MOFs) with Simultaneous Micro- and Meso-Porosity

Metalâ??organic frameworks (MOFs) are extremely attractive porous materials for application such as gas storage, chemical separations and catalysis in large part due to the exceptionally high surface areas that they can provide. Indeed, the performance of a MOF for certain applications can be somewhat anticipated by the value of this property. MOF surface areas are typically estimated in experiments by applying the BET theory to measured nitrogen isotherms, where an estimated Brunauer, Emmett, and Teller (BET) nitrogen monolayer loading is converted to a â??BET area.â? However, the meaning of MOF BET areas remains a subject of debate, especially in regard to the relation between the BET area and the geometrically calculated surface area, which can only be obtained computationally.

Since there is sometimes ambiguity in terms of selecting the so-called â??BET range,â? most recent emphasis has been placed on the utilization of four so-called â??BET consistency criteriaâ? of Rouquerol during the BET calculation. To elucidate the general applicability of these four criteria without the caveat of imperfect (or improperly activated) MOF samples or measurement uncertainties, we simulated nitrogen adsorption in â??perfect crystalsâ? of graphene and 25 MOFs having different pore-size distributions and evaluated the effectiveness of these criteria in directing the calculation of BET areas from the simulated isotherms. Additionally, the BET areas thus obtained were compared with the corresponding geometrically calculated, nitrogen-accessible surface areas (NASAs).

We also analyzed molecular configurations from the simulations to elucidate the contributions of â??pore fillingâ? and â??monolayer formationâ? to the nitrogen adsorption loadings in different MOF pores, revealing the origin of inaccuracies in BET-calculated monolayer loadings that largely explain discrepancies between BET areas and NASAs. We find that the BET calculation can significantly overestimate the true monolayer loading even if all consistency criteria are satisfied. This scenario is especially common in MOFs combining mesopores (d â?¥ 20 Ã?) and large micropores (d = 10â??20 Ã?), due to the overlap of pore-filling and monolayer-formation regimes of these two kinds of pores.

It is important to note, however, that while it is not always possible to satisfy all consistency criteria, we find that it is critical to minimize the deviation from these criteria during the BET range selection for a consistent comparison of BET areas of different MOFs, as well as for comparing simulated and experimental BET areas of a particular MOF. Furthermore, to accurately assess the quality of a MOF sample, it is best to compare experimental BET areas with simulated BET areas rather than with calculated NASAs.

References:

[1] D.A. Gomez-Gualdron, P.Z. Moghadam, J.T. Hupp, O.K. Farha, R.Q. Snurr, J. Am. Chem. Soc. 2016, 138, pp. 215-224.