The use of unconventional methods to synthesise MOFs and MTV-MOFs for photocatalysis

Abstract

This study introduces a novel rapid precipitation technique for synthesising metal-organic frameworks (MOFs) and multivariate MOFs (MTV-MOFs) in aqueous solutions, yielding high-quality, and crystallinity in MOFs and MTV-MOFs. A series of Ce-UiO-66-X (X = NH2, OH, H, NO2, COOH) and Ce/Zr-UiO-66(1,4-NDC/BDC), Ce/Zr-UiO-66(1,4-NDC/2,6-NDC), Ce/Zr-UiO-66(1,4-NDC), Ce/Ti-UiO-66(1,4-NDC), and Ce/Ti-UiO-66(NH2) were synthesised, with Ce-UiO-66-OH and all MTV-MOFs presented for the first time. The research mainly delves into synthesising bimetallic UiO-66 derivatives, with varying ratios of Ce to Zr and different linker types, aiming to understand the potential structural modifications. The efficacy of rapid precipitation, microwave, and ultrasonic methods over traditional solvothermal approaches was investigated to explore controling the molar ratio of metals in the bimetallic structures. The unconventional methods yield pure samples of mixed-metal materials with various concentrations, as confirmed by Pawley refinement against powder X ray diffraction (PXRD) and X-ray fluorescence spectroscopy (XRF). The effect of defects is investigated using thermogravimetric analysis (TGA), which reveals similar defect production at lower Ce concentrations. The microwave and rapid precipitation methods can produce non defective structures, specifically for Ce/Zr-UiO-66(1,4-NDC) and Ce/Zr-UiO-66(BDC). Synthesising MOFs by rapid precipitation gives materials with larger unit cell parameters than have been presented so far, which may be due to small crystal domain size and defective structure. Regarding the photocatalytic activities of the synthesised porous materials, Ce-UiO 66-NH2, Ce-UiO-66-OH, and Ce-UiO-66-H show significant photocatalytic activities to decolourise Alizarin Red S and Congo red within 1 to 3 minutes with 6 W/254 nm radiation. All MTV-MOFs, except Ce/Zr-UiO-66(1,4-NDC/BDC), show the highest decolourisation activity, observed between 1 and 15 minutes in four different dyes. The materials demonstrate excellent photostability and recyclability under UV and visible light over 5 cycles of use, with no loss of crystallinity observed through PXRD. All MTV-MOFs and Ce-UiO-66-NH2 are stable under UV irradiation between 16 and 17 hours, respectively