Surface Area Determination of Porous Materials 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 nonreactive nitrogen
or argon. Recently, 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 of over 200 geometrically diverse
real MOFs as well as 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 of the
surface from molecular simulation snapshots. Furthermore, we found
that the excess sorption work (ESW) method or a combination of ESW
and BET methods can potentially help facilitate a more accurate estimation
of the surface area, particularly in cases where the structures show
relatively less complex isotherms having distinct steps.