Tunable coordinative defects in UHM-3 surface-mounted MOFs for gas adsorption and separation: A combined experimental and theoretical study

• Thin, highly oriented films of the MOF UHM-3 were grown on modified Au substrates.
• The defect density of these SURMOFs was determined by XPS and IR-spectroscopy.
• The concentration of Cu(I) ions in these SURMOFs was found to be below 1%.
• Defects (i.e. Cu(I) species) could be created by heating the MOF films in UHV.
• Unexpectedly, the binding energy of CO2 to Cu (I) and Cu(II) is virtually identical.

The metal organic framework (MOF) UHM-3, constructed with Cu(II)-paddle wheel-type nodes and a new tetracarboxylic acid linker, 5,5′-(dimethylsilanediyl)diisophthalate, has a close-packed alignment of open Cu(II) sites which are of interest for applications in gas storage and separation. Here, we first report on the growth of oriented, homogeneous and virtually defect-free (below 1%) UHM-3 MOF thin films on a solid substrate using a room-temperature liquid phase epitaxy (LPE) method. Thermal postsynthetic treatment allowed to induce Cu(I) defect sites in a controlled fashion. The interaction of CO and CO2 with the Cu(II) and Cu(I) sites was then studied using X-ray photoelectron spectroscopy (XPS) and IR-spectroscopy. The binding energy of these two species was determined using temperature-induced desorption. The interaction between the guest molecules and the Cu(I) and Cu(II) sites were also analyzed using density-functional theory (DFT). Surprisingly, both experiment and theory show that the binding energy of CO2 to Cu(I) and Cu(II) sites are essentially identical, in pronounced contrast to CO, which binds much stronger to Cu(I).

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