(708d) Adsorption Characteristics of Coordinatively Unsaturated Metal Sites Containing Dhtp Series of Metal Organic Frameworks

During last decade, a new class of crystalline materials commonly known as metal organic frameworks (MOFs) has been widely investigated for their potential use in gas storage, gas separation and catalysis. 2,5-dihydroxyterephthalate (dhtp) series of MOFs is one of the most interesting MOF series due to presence of large number of coordinatively unsaturated metal sites.   

  In this study, we report high pressure adsorption isotherms of four industrially relevant gases (CO₂, CO, CH₄ and N₂) on Mdhtp (M = Mg, Mn, Co, and Ni) MOFs at three temperature over a wide pressure range. These gases were purposefully chosen to cover a wide range of adsorpbate polarity and polarizability and observe their effects on the adsorption characteristics in unsaturated metal sites containing MOFs.  For gases like CO₂ and CO which have significant polarity, these MOFs exhibit two different adsorption sites and a DSL model was necessary to describe the adsorption behavior; however for other relatively non polar gases a variant of Langmuir formulation was sufficient.   

 The MOF containing, Ni has shown the strongest lateral interactions (Henry’s constant) for CO; this can be readily attributed to the small ionic radius of Ni²⁺. On the other hand, the effect of metal atoms on adsorption characteristics of relatively non-polar gases like CH₄ and N₂ was found to be negligible, indicating the role of polarity for specific interaction with unsaturated metal sites. In contrast to CO, CO₂ has the highest Henry’s Constant for Mg²⁺. Mg²⁺ has a fixed coordination number of 6 with an exceptional preference for oxygen-containing ligands (and hence exhibits high Henry constants); other metal atoms studied in this work have coordination number ranging from 4 to 6.  In addition, it was observed that the metals in the framework have negligible effect on the saturation capacity of the gases. Any slight differences observed in saturation loadings can be readily attributed to difference in pore volume (due to presence of different metal atoms) of these samples.