Elucidating Lewis acidity of metal sites in MFU-4l metal-organic frameworks: N2O and CO2 adsorption in MFU-4l, CuI-MFU-4l and Li-MFU-4l

• Zn–Cl units within the MFU-4l framework possess only very weak Lewis acidity.
• Introduction of Li+ leads to a considerable increase of Lewis acidity.
• Li-MFU-4l shows weak and unselective binding of both, CO2 and N2O.
• CuI-MFU-4l shows relatively strong and selective N2O binding.

The interaction strength of N2O and CO2 molecules with different Lewis-acidic sites within MFU-4l–type metal-organic frameworks was studied via gas sorption measurements and density-functional theory calculations. MFU-4l comprising Zn–Cl units shows only physisorption of both gases. Introduction of Li into the parent MFU-4l framework leads to a remarkable increase of binding strength of both N2O and CO2 showing considerable Lewis acidity of LiI centers. CuI-MFU-4l shows even stronger binding of N2O, as compared to Li-MFU-4l, whereas CO2 doesn't bind to CuI centers. Preferential binding of N2O to CuI centers was also confirmed by in situ synchrotron X-ray powder diffraction measurements. These results show that CuI-MFU-4l can be considered as a material for selective N2O adsorption.

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