Understanding Structure, Metal Distribution, and Water Adsorption in Mixed-Metal MOF-74

We present a joint computational and experimental study of Mg−Ni-MOF-74 and Mg−Cd-MOF-74 to gain insight into the mixing of metals and understand how metal mixing affects the structure of the undercoordinated open-metal sites that exist in these materials. Our calculations predict that metal mixing is energetically and entropically preferred in these materials. Recent experimental work has demonstrated that Mg−Ni-MOF-74 shows much greater surface area retention in the presence of water than Mg-MOF-74. To probe this effect, we study H₂O adsorption in Mg−Ni-MOF-74, finding that the adsorption energetics and electronic structure do not change significantly at the metal sites when compared to Mg-MOF-74 and Ni-MOF-74, respectively. We conclude that the increased stability of Mg−Ni-MOF-74 is a result of a M−O bond length distortion in mixed-metal MOF-74, consistent with recent work on the stability of MOF-74 under water exposure.