Understanding Hysteresis in Carbon Dioxide Sorption in Porous Metal-Organic Frameworks

Two new isostructural microporous coordination frameworks [Mn-3(Hpdc)(2)(pdc)(2)] (1) and [Mg-3(Hpdc)(2)(pdc)(2)] (2) (pdc(2-) = pyridine-2,4-dicarboxylate) showing primitive cubic (pcu) topology have been prepared and characterized. The pore aperture of the channels is too narrow for the efficient adsorption of N-2; however, both compounds demonstrate substantially higher uptake of CO2 (119.9 mL.g(-1) for 1 and 102.5 mL.g(-1) for 2 at 195 K, 1 bar). Despite of their structural similarities, 2 shows a typical reversible type I isotherm for adsorption/desorption of CO2, while 1 features a two-step adsorption process with a very broad hysteresis between the adsorption and desorption curves. This behavior can be explained by a combination of density functional theory calculations, sorption, and X-ray diffraction analysis and gives insights into the further development of new sorbents showing adsorption/desorption hysteresis.