Reactive adsorption of ammonia and ammonia/water on CuBTC metal-organic framework: A ReaxFF molecular dynamics simulation

We report ReaxFF molecular dynamics simulations for reactive adsorption of NH₃ on dehydrated CuBTC metal-organic framework. If the temperature is moderate (up to 125 °C), the dehydrated CuBTC demonstrates a good hydrostatic stability for water concentrations up to 4.0 molecules per copper site. However, if the temperature increases to 550 K, the dehydrated CuBTC will collapse even at a small water concentration, 1.0 H₂O molecule per copper site. When NH₃ molecules are adsorbed in the channel and micropores of CuBTC, they prefer to chemisorb to the copper sites rather than forming a dimer with another NH₃ molecule. The formation of equimolar Cu₂(NH₂)₄ and (NH₄)₃BTC structures is observed at 348 K, which is in good agreement with previous experimental findings. The dehydrated CuBTC framework is partially collapsed upon NH₃ adsorption, while the Cu-Cu dimer structure remains stable under the investigated conditions. Further calculations reveal that the stability of CuBTC is related to the ammonia concentration. The critical NH ₃ concentration after which the dehydrated CuBTC starts to collapse is determined to be 1.0 NH₃ molecule per copper site. Depending on whether NH₃ concentration is below or above the critical value, the dehydrated CuBTC can be stable to a higher temperature, 378 K, or can collapse at a lower temperature, 250 K. H₂O/NH₃ mixtures have also been studied, and we find that although water molecules do not demonstrate a strong interaction with the copper sites of CuBTC, the existence of water molecules can substantially prevent ammonia from interacting with CuBTC, and thus reduce the amount of chemisorbed NH₃ molecules on CuBTC and stabilize the CuBTC framework to some extent.