Unusual adsorption behavior on metal-organic frameworks.

Metal-organic frameworks (MOFs) have shown adsorption behavior that is not observed in other microporous materials such as zeolites or activated carbons. This study used grand canonical Monte Carlo simulation to evaluate a particular form of behavior, which corresponds to the presence of unusual type V adsorption isotherms. Study of a series of MOFs in the IRMOF family, containing chemically similar linkers of different length, showed that the presence of type V adsorption depends on a fine balance between the strength of the fluid-fluid and fluid-solid interactions, which in turn is a strong function of the length of the linker and therefore the pore size. A transition from type V behavior to the more common type I behavior is observed as the temperature increases. The temperature at which this transition occurs increases, and the transition becomes more diffuse, as the length of the linker increases. This type V behavior leads to an interesting possibility in the design of MOF adsorbents for use in gas separation and gas storage applications.