Breathing effects of CO₂ adsorption on a flexible 3D lanthanide metal-organic framework

This work reports a new three-dimensional flexible lanthanide metal-organic framework [La(BTB)(H₂O)·3DMF]_n (BTB = 1,3,5-tris(4-carboxyphenyl) benzene) that possesses a rare chiral space group (P6₅22), very high thermal stability (560 °C), and high surface area (1014 m² g^-1). This new MOF demonstrates a large adsorption hysteresis loop on the N₂ isotherm at 77 K after activation at 300 °C. Powder X-ray thermodiffraction experiments reveal that structural transformation occurs upon removal of guest molecules. The as-synthesized material is the "open" form, while the activated sample is "closed". Adsorption isotherms of CO₂ exhibit a dramatic step near a relative pressure of 0.3, which is similar to the location of the step in the N₂ isotherm at 77 K. Upon desorption, the material transforms to the closed form near a reduced pressure of 0.15, where it collapses back to the adsorption isotherm. Adsorption kinetic studies show that equilibrium is reached more slowly during the opening/closing steps compared to adsorption loadings measured on the static portion of the isotherm.