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 NH(3) 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 H(2)O molecule per copper site. When NH(3) 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 NH(3) molecule. The formation of equimolar Cu(2)(NH(2))(4) and (NH(4))(3)BTC structures is observed at 348 K, which is in good agreement with previous experimental findings. The dehydrated CuBTC framework is partially collapsed upon NH(3) 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 NH(3) molecule per copper site. Depending on whether NH(3) 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. H(2)O∕NH(3) 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 NH(3) molecules on CuBTC and stabilize the CuBTC framework to some extent.