Ammonia adsorption and its effects on framework stability of MOF-5 and MOF-177

Ammonia adsorption equilibrium and kinetics on MOF-5 and MOF-177 were measured volumetrically at 298K and ammonia pressures up to 800torr. This study allowed us to determine the ammonia adsorption properties and stability of both MOFs after exposure to ammonia. MOF-177 adsorbed more ammonia than MOF-5 at ammonia pressures below 150torr and the ammonia adsorption capacity on both MOFs at 800torr was about the same (12.2mmol/g). It can be inferred from the adsorption results that physical adsorption of ammonia on both MOFs dominated at ammonia pressures below 150torr after which chemical reactions between ammonia and MOF frameworks take place. After exposure to ammonia both MOFs lost majority of their pore textures as evidenced by a drastic decrease of specific surface area and pore volume. The X-ray diffraction study suggested a complete loss of crystallinity of both MOFs, and the FT-IR and Raman spectral analyses showed the generation of free organic ligands within both MOF-5 and MOF-177 after they were exposed to ammonia. Ammonia molecules are proposed to destroy the MOF frameworks by forming hydrogen bonds with Zn₄O clusters of MOFs.