Analysis of a Cu-Doped Metal-Organic Framework, MFM-520(Zn 1-x Cu x ), for NO 2 Adsorption.

MFM-520(Zn) confines dimers of NO ₂ with a high adsorption of 4.52 mmol g ^-1 at 1 bar at 298 K. The synthesis and the incommensurate structure of Cu-doped MFM-520(Zn) are reported. The introduction of paramagnetic Cu ²⁺ sites allows investigation of the electronic and geometric structure of metal site by in situ electron paramagnetic resonance (EPR) spectroscopy upon adsorption of NO ₂ . By combining continuous wave and electron-nuclear double resonance spectroscopy, an unusual reverse Berry distorted coordination geometry of the Cu ²⁺ centers is observed. Interestingly, Cu-doped MFM-520(Zn _0.95 Cu _0.05 ) shows enhanced adsorption of NO ₂ of 5.02 mmol g ^-1 at 1 bar at 298 K. Whereas MFM-520(Zn) confines adsorbed NO ₂ as N ₂ O ₄ , the presence of monomeric NO ₂ at low temperature suggests that doping with Cu ²⁺ centers into the framework plays an important role in tuning the dimerization of NO ₂ molecules in the pore via the formation of specific host-guest interactions.
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