(30c) Competitive and Cooperative Co-Adsorption in Metal Organic Frameworks Materials: The Role of Hydrogen Bonding

Understanding co-adsorption in nanoporous materials such as metal organic framework (MOFs) is important for most applications since they are rarely used solely for pure gases and they are typically subject to gas contamination. Co-adsorption, however, leads to a variety of processes that complicate the analysis, such as molecular competition for adsorption and diffusion. Due to a lack of in situ characterization within these 3D nanoporous structures, these processes remain largely unknown. We report here a novel synergistic effect involving co-adsorption in a prototypical metal organic framework (MOF) material, i.e. MOF-74. We find that the addition of NH₃ or H₂O to MOF-74 previously loaded with a variety of small gases (e.g., CO, CO₂, SO₂) first displaces a certain amount of molecules always and then prevents the removal of the remaining small gases upon evacuation. We further show, with support of first-principles modeling, that this phenomenon is not due to guest-guest binding as usually being regarded as “cooperative binding effect”, but instead to an increase in diffusion barrier for these small molecules. Combining in situ infrared spectroscopic measurements and first-principles modeling, we demonstrate that hydrogen bonding is primarily responsible for the large increase of the diffusion barrier (by a factor of ~7 for CO and CO₂) along the transport pathway. These relevant findings contribute to fundamental understanding of co-adsorption in porous materials and to dispel common assumptions. For instance, H₂O and NH₃ are usually regarded as impurities that are detrimental to the performance of gas adsorption and capture by poisoning active sorption sites. This is clearly not the case here. Therefore, the effect of H₂O and NH₃ (or other hydrogen containing molecules prone to hydrogen bonding) on the gas adsorption performance of MOFs materials needs to be reevaluated.