(631b) Surface Area Determination of Metal-Organic Frameworks (MOFs) Using the Brunauer-Emmett-Teller (BET) Method: Limitations and Improvements
Surface areas of metal-organic frameworks (MOFs) have been commonly characterized using the Brunauer-Emmett-Teller (BET) method based on adsorption isotherms of non-reactive nitrogen or argon. Recently, however, some discrepancies between surface areas computed from the BET method and those from geometric methods were reported in the literature. In this study, we systematically evaluated the BET and geometric surface areas for hundreds of geometrically diverse MOFs as well as carbon nanotubes (CNTs) with varying pore sizes as model systems to achieve a comprehensive understanding of the limitations of the BET and geometric methods. We compared the BET and geometric surface areas to the true monolayer area which is determined by directly counting the number of molecules included in the monolayer from molecular snapshots. An important concern with the use of the BET method is that the predicted area is highly dependent on the region of the isotherm that is chosen for the computation. The choice of this region is often confusing and ambiguous. Currently, this choice is informed by consistency criteria proposed by Rouquerol et al. While these criteria are useful in general, there are scenarios where their applicability is limited. In this study, we systematically evaluated the implementation of the consistency criteria and their influence on the predicted surface areas. Additionally, in order eliminate the dependence of predicted areas on chosen regions, we also employed the excess sorption work (ESW) method to determine the surface areas. With the ESW method, one simply needs to identify the first minima of a transformation of the isotherm in order to determine the surface area. In this study, we found that the excess sorption work (ESW) method, or a combination of ESW and BET methods, can potentially provide a more accurate estimation of the surface area, particularly in cases where the BET method overestimates the surface area significantly.