Adsorption of pyridine over amino-functionalized metal-organic frameworks: Attraction via hydrogen bonding versus base-base repulsion

Pyridine (Py) was adsorbed over metal-organic frameworks (MOFs) (UiO-66 and UiO-66-NH₂ with different incorporated amino group content) in both vapor and liquid phases to understand the interactions between the basic adsorbate Py and a basic or neutral adsorbent. Py was adsorbed more favorably over UiO-66-NH₂ than over the pristine UiO-66. Not only the adsorbed amount of Py but also adsorption kinetics increased with increasing amino group content in UiO-66s, showing that amino groups in the MOFs benefit the adsorption of Py in both vapor and liquid phases. To understand the favorable interaction between Py and basic UiO-66, calculations were also carried out. The results, including calculations, suggested that hydrogen bonding is important for improving the adsorption of Py over UiO-66s. However, the expected repulsive interaction between basic Py and amino groups of UiO-66-NH₂ was not observed. Therefore, it is vital that specific interaction mechanisms should be considered in order to understand selective adsorption processes. Moreover, the amount of adsorbed Py in the vapor phase increased with increasing adsorption temperature, suggesting that the window size of UiO-66 is very similar to the kinetic diameter or critical dimension of Py and that the window size probably is slightly increased with increasing temperature because of lattice vibrations.