(675d) Acid Gas Adsorption on Metal–Organic Framework Nanosheets As a Model of an “All-Surface” Material

To establish a model of metal–organic framework (MOF) surfaces and build an understanding of surface-specific ligand adsorption phenomena in MOFs, we present a computational study exploring multiple models of a series of MOF-2 nanosheet materials, “M-BDCs”, with M = Zn, Cu, and Co and BDC = benzene-1,4-dicarboxylate. We study and assess the appropriateness of a series of models ranging from small clusters (18 atoms) to fully periodic sheet models. We additionally study the interactions of these models with acid gases and energy-relevant small molecules (CO, CO₂, H₂O, SO₂, NO₂, and H₂S). We employ computational methods ranging from DFT with various exchange–correlation functionals to perturbative and coupled-cluster methods. For these systems, we present binding energies and enthalpies with the various ligands studied as well as IR frequency shifts for the normal modes of these ligands upon complexation with the open-metal sites of these materials. Our calculations lead to an understanding of phenomena unique to MOF surfaces and the importance of the periodicity in these materials in capturing surface-specific adsorption behaviors.

Furthermore, we study adsorption of a set of acid gases to the exposed OMSs (open metal sites) existing at the surface of these materials in the presence of water, including competing co-adsorption motif and cooperative co-adsorption motif. Our computational results suggest that preadsorbed acid gases (CO₂, SO₂, NO₂, and H₂S) on Cu-BDC nanosheets would be replaced by water in the presence of water, leading to acid gas adsorbed on water coordinated OMSs instead of pure OMSs. For CO₂ and H₂S, the binding energies on water coordinated OMSs are lower than vacant OMSs, while for SO₂ and NO₂, the binding energies on water coordinated OMSs are higher. The strong double hydrogen bonds established by two oxygen atoms in SO₂ and NO₂ and two hydrogen atoms in water contribute to the enhanced binding energies. Even though water has a high probability to compete with acid gas for the same adsorption site, and even replace preadsorbed acid gases, in SO₂ and NO₂ cases, preadsorbing a small amount of water can increase the SO₂ and NO₂ binding strength and attack on Cu-BDC nanosheets.