Insights into the Stability and Activity of MIL-53(Fe) in Solar Photocatalytic Oxidation Processes in Water

Abstract

MIL-53(Fe) is a metal organic framework that has been recently considered a heterogeneous photocatalyst candidate for the degradation of water pollutants under visible or solar radiation, though stability studies are rather scarce in the literature. In this work, MIL-53(Fe) was successfully synthesized by a solvothermal method and fully characterized by X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), N2 adsorption–desorption isotherm, Thermogravimetric analysis coupled with mass spectrometry (TGA-MS), UV-visible diffuse reflectance spectroscopy (DRS), elemental analysis and wavelength dispersive X-ray fluorescence (WDXRF). The effects of pH, temperature, solar radiation and the presence of oxidants (i.e., electron acceptors) such as ozone, persulfate and hydrogen peroxide on the stability of MIL-53(Fe) in water were investigated. The as-synthetized MIL-53(Fe) exhibited relatively good stability in water at pH 4 but suffered fast hydrolysis at alkaline conditions. At pH 4–5, temperature, radiation (solar and visible radiation) and oxidants exerted negative effect on the stability of the metal–organic framework (MOF) in water, resulting in non-negligible amounts of metal (iron) and linker (terephthalic acid, H2BDC) leached out from MIL-53(Fe). The photocatalytic activity of MIL-53(Fe) under simulated solar radiation was studied using phenol and metoprolol as target pollutants. MIL-53(Fe) on its own removed less than 10% of the pollutants after 3 h of irradiation, while in the presence of ozone, persulfate or hydrogen peroxide, complete elimination of pollutants was achieved within 2 h of exposure to radiation. However, the presence of oxidants and the formation of some reaction intermediates (e.g., short-chain carboxylic acids) accelerated MIL-53(Fe) decarboxylation. The findings of this work suggest that MIL-53(Fe) should not be recommended as a heterogeneous photocatalyst for water treatment before carrying out a careful evaluation of its stability under actual reaction conditions.

Sensitive Detection of Cadmium Using Amine-Functionalized Fe-MOF by Anodic Stripping Voltammetry

“…Furthermore, the bands around 1339 and 1250 cm –1 are due to ν­(C–N) amine stretching vibrations . A band around 770 cm –1 refers to the C–H bending vibrations of the benzene rings . An adsorption peak around 573 cm –1 reflects the vibration of metal-oxo (Fe–O) bond formation, which again verifies the bonding of Fe­(III) ions with the carboxylic groups of aminoterephthalic acid. , …”

Product selectivity and mass transport in levulinic acid transfer hydrogenation by monolithic MIL-100, MIL-88B and ZIF-8@Pd MOFs

“…In the case of MIL-100 the drop in reactivity over five cycles is significant, with a permanent decrease in conversion seen after two cycles. This sudden loss of reactivity is due to the hydrolytic breakdown of the hydrophilic framework, via base hydrolysis as a result of the high pH ( Bezverkhyy et al, 2016 ), reducing the surface area and access to catalytic centres ( López et al, 2021 ); see Supplementary Information for post-reaction (5 cycles) XRD. The same process likely occurs in MIL-88B, but at a lower rate.…”

Photocatalytic Aerobic Oxidation of Biomass-Derived 5-HMF to DFF over MIL-53(Fe)/g-C3N4 Composite

“…Simultaneously, part of the electrons on the CB of g-C 3 N 4 can be transported to the Fe 3+ clusters via interfacial charge transfer and reduce Fe 3+ to Fe 2+ due to the more negative E CB of g-C 3 N 4 compared with the redox potential of Fe 3+ /Fe 2+ (0.77 eV) (Equation (8)). The obtained Fe 2+ ions are unstable and react with hydrogen peroxide via a Fenton reaction to form Fe 3+ , accompanied by the generation of ·OH (Equation (9)) [ 45 , 46 ]. The formed ·OH with high reactivity can also oxidize alcohol hydroxyl (Equation (10)), and thus it enhances the photocatalytic activity and partially suppresses the overoxidation of DFF.…”