Quantitative Characterization of Breathing upon Adsorption for a Series of Amino-Functionalized MIL-53

This work presents the synthesis of MIL-53(Al) metal organic frameworks with different terephthalate/2-aminoterephthalate linker ratios incorporated into the framework during hydrothermal synthesis. In all cases, the materials show prominent S-shaped isotherms upon CO2 adsorption that can be attributed to sorbate-induced breathing phenomena involving two narrow-to-large-pore phase transitions. However, the breathing properties of the MIL-53(Al) materials (i.e., the relative stability of the np phase) are modified by adjusting the terephthalate/2-aminoterephthalate ratio. As a consequence, the pressure at which the second np -> lp phase transition occurs, as well as Henry's constant for CO2 adsorption, are directly associated with the number of amino-modified linkers. The different N-2 and CO2 adsorption/desorption patterns have been successfully reproduced through the formulation of thermodynamic isotherms, providing direct information about energy heterogeneity of the solid/sorbate system and a direct characterization of the phase transition zone for breathing systems.