Calculating geometric surface areas as a characterization tool for metal-organic frameworks

Metal-organic frameworks (MOFs) synthesized in a building-block approach from organic linkers and metal corner units offer the opportunity to design materials with high surface areas for adsorption applications by assembling the appropriate building blocks. In this paper, we show that the surface area calculated in a geometric fashion from the crystal structure is a useful tool for characterizing MOFs. We argue that the accessible surface area rather than the widely used Connolly surface area is the appropriate surface area to characterize crystalline solids for adsorption applications. The accessible surface area calculated with a probe diameter corresponding to the adsorbate of interest provides a simple way to screen and compare adsorbents. We investigate the effects of the probe molecule diameter on the accessible surface area and discuss the implications for increasing the surface area of metal-organic frameworks by the use of catenated structures. We also demonstrate that the accessible surface area provides a useful tool for judging the quality of a synthesized sample. Experimental surface areas can be adversely affected by incomplete solvent removal during activation, crystal collapse, or interpenetration. The easily calculated accessible surface area provides a benchmark for the theoretical upper limit for a perfect crystal.