Enhanced photoreduction efficiency of Cr(VI) driven by visible light in a new Zr-based metal-organic framework modified by hydroxyl groups.

While metal-organic framework (MOF) photocatalysts have demonstrated a unique Cr(VI) photoreduction capability in recent decades, their performance is still insufficient for practical applications because of their low Cr(VI) uptake and poor visible light response. To cope with these drawbacks, a new OH-modified Zr-based MOF, termed HCMUE-1, was successfully prepared via a solvothermal method in this work. The complete characterization of HCMUE-1 was performed through various techniques, including powder X-ray diffraction (PXRD), Raman spectroscopy, Fourier transform infrared (FT-IR), thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), scanning electron microscopy combined with energy-dispersive X-ray (SEM-EDX), and X-ray photoelectron spectroscopy (XPS). The obtained data exhibited the excellent Cr(VI) photoreduction efficiency of HCMUE-1, reaching up to 98% after 90 min and almost 100% after 120 min under visible light illumination in a low acidic medium. Noteworthily, HCMUE-1 retained the same Cr(VI) removal rate for at least seven cycles without considerable loss. Further experimental investigations demonstrated that the structural stability and surface morphology of HCMUE-1 were retained after photoreduction. Moreover, the photocatalytic reduction mechanism of Cr(VI) to Cr(III) was interpreted through a series of systematic experimental measurements. These results indicate that HCMUE-1 possesses potential as an efficient photocatalyst for reducing toxic Cr(VI) species from wastewater in real-life conditions.

Engineering of appropriate pore size combined with sulfonic functionalization in a Zr-MOF with reo topology for the ultra-high removal of cationic malachite green dye from an aqueous medium.

A Zr-based metal-organic framework with reo topology, denoted as Reo-MOF-1, was fabricated through a solvothermal method capable of efficiently removing the cationic MG dye from an aqueous medium. The effect of pH solution, adsorbent content, adsorption isotherm, and kinetics on the MG capture was observed to determine the optimal conditions. Accordingly, the maximum adsorption capacity of MG over H+⊂Reo-MOF-1 reaches the value of 2532.1 mg g-1 at neutral pH, which is much greater than the published materials. Moreover, the results of the MG process on H+⊂Reo-MOF-1 fit with the Langmuir isotherm and pseudo second order kinetic model. Hence, MG removal is a chemical adsorption process. Remarkably, H+⊂Reo-MOF-1 can maintain the uptake for MG at about 94% over eight cycles. The MG adsorption mechanism is interpreted via the incorporated analyses and experiments. In detail, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA-DSC) of MG⊂Reo-MOF-1 in comparison with H+⊂Reo-MOF-1 indicate that the electrostatic attraction and π-π stacking interaction are found via the interaction between the cationic MG ions and SO3 - moieties within MOF as well as the π electron clouds in the benzene ring of the adsorbent and adsorbate, resulting in significant improvement the MG adsorption uptake. These data prove that acidified Reo-MOF-1 possesses promising application as an effective absorbent of toxic dye in practical conditions.