Understanding the structure-dependent adsorption behavior of four zirconium-based porphyrinic MOFs for the removal of pharmaceuticals

The adsorptive removal of typical pharmaceuticals, carbamazepine (CBZ) and tetracycline (TC), was investigated with four zirconium-based porphyrinic metal-organic frameworks (MOFs) (PCN-221, PCN-222, PCN-223, and PCN-224). For both CBZ and TC, the adsorption capacities of the PCNs were ranked in the following order: PCN-221 < PCN-223 < PCN-224 < PCN-222. Notably, this order aligns with the BET surface areas of the PCNs. Cage-type PCN-221 showed a lower adsorption capacity than the other three channel-type MOFs due to its small apertures, which were not conducive to the activation of PCN-221 and space utilization. The amount of adsorption was proportional to the cumulative pore volume in PCNs with a width larger than that of the CBZ/TC molecule. Therefore, with high pore volume and large one-dimensional channels formed by ordered linker vacancies, PCN-222 was the best adsorbent in this work. According to the Langmuir model, PCN-222 exhibited exceptional adsorption capacities for CBZ and TC, reaching 305 mg/g and 280 mg/g, respectively. Additionally, the mechanisms for CBZ removal involved hydrogen bonding and pi-pi interactions/stacking, while electrostatic interactions, hydrogen bonding, and pi-pi interactions/stacking played crucial roles in TC adsorption onto MOFs. These findings establish a profound comprehension of the interplay between pore structure and performance, providing helpful guidance for designing MOFs to remove emerging contaminants effectively.