Metal organic frameworks (MOFs) have emerged as a versatile materials for gas adsorption, storage, separation, and even catalysis. The ability to modify their structure and composition makes them easily tailorable to optimize the desired properties and their crystalline nature makes them amenable to first principles calculations. Thus, they are model systems to examine the fundamental interactions of small gases in nanoporous materials, such as diffusion, co-adsorption, and reaction in constrained environments.
We have combined in situ IR spectroscopy and first-principles calculations to examine the behavior of small molecules in one of the best known and characterized MOF: M2(dobdc) [dobdc=2,5-dioxido-1,4-benzenedicarboxylate, M=Mg, Zn, Co, Ni] or MOF-74 composed of 1-D linear pores and unsaturated metal centers. For this talk, we focus on the interaction of H2O in MOF-74, in which the internal structure of the metal oxide node mimics surfaces with exposed cations, as a function of vapor pressure and temperature. We find that, while adsorption is reversible below the water condensation pressure (~19.7 Torr) at room temperature, a reaction takes place at ~150 ˚C even at low water vapor pressures. This important finding is unambiguously demonstrated by a clear spectroscopic signature of reaction using D2O, positively identified first-principles calculations that also derive a pathway and kinetic barrier for the reaction. Furthermore, the reaction appears to be facilitated by a cooperative effect of several H2O molecules. Once water is dissociated, we show that CO can react to form formic acid that can be removed by evacuation at 200°C. We have also been able to cap the MOF-74 crystals with a monolayer of ethylenediamine (EDA) that acts as a selective barrier. While it effectively traps molecules such as CO, CO2, SO2 and C2H4, this EDA layer completely lets through water molecules, which can then force these trapped molecules out.