Photoreactivity of metal-organic frameworks in the decolorization of methylene blue in aqueous solution

The effect of metal ions and functional groups in organic ligands on the photocatalytic properties of metal-carboxylates, such as isostructural MIL-100(M) (M = Al, Fe, V, and Cr), isoreticular UiO-66-R (R = H, NH₂, and NO₂), MIL-47(V), and MIL-125(Ti), was investigated using the decolorization of methylene blue in aqueous solution at pH 7.0 and 20 °C under UV radiation. The metal-carboxylates were characterized by diffuse reflectance UV/vis measurements. The determination of the band gap (E_g) on the basis of DR-UV/vis measurements showed that E_g values correlate with ionic covalent parameters, i.e., the strength of the interaction between metal and oxygen in the "M⁺ⁿ-O^2- pair. According to DR-UV/vis spectroscopic and catalytic investigations, the reactivity of metal-carboxylates correlated with the band-gap value (E_g), which could be adjusted by varying the nature of the metal ions in SBUs or functional groups in the organic ligands.