Ultramicroporous Al(iii) MOFs with selective CO2 adsorption, acid resistance, and efficient Cr(vi) sorption properties

Abstract

In the present work, we report the hydrothermal synthesis and detailed characterization of a series of ultramicroporous, luminescent, multivariate metal–organic frameworks (MOFs) with the general formula [Al(OH)(IATP)_1−x(NH₂-BDC)_x], constructed from two different dicarboxylate ligands, H₂IATP = 2-(((1H-imidazol-4-yl)methyl)amino) terephthalic acid and NH₂-H₂BDC = 2-aminoterephthalic acid. Structural analysis, using both powder X-ray diffraction (PXRD) and microcrystal electron diffraction (MicroED), confirms that the reported MTV-Al-MOFs are topological analogues of the related NH₂-MIL-53(Al) MOF. Introducing ultramicroporosity (∼5.2 Å) together with bulky imidazole functional groups significantly enhances the chemical stability of these MOFs under acidic conditions, compared to NH₂-MIL-53(Al). It also imparts interesting carbon dioxide physisorption properties, with MTV-Al-MOF-4 [Al(OH)(IATP)_0.39(NH₂-BDC)_0.61] demonstrating a maximum CO₂ adsorption capacity of 2.33 mmol g⁻¹ at 273 K and a selectivity CO₂/N₂ of ∼76 at the same temperature (273 K) and low-pressure limit. Among the series, MTV-Al-MOF-4 exhibits the most promising combination of chemical stability and Cr(VI) sorption capacity. This material was extensively studied for eliminating Cr₂O₇²⁻ anions from acidic aqueous solutions, indicating high sorption capacity (174 mg Cr₂O₇²⁻ per g), rapid uptake kinetics (equilibrium reached within 5 minutes), and decent selectivity against competing anions such as Cl⁻, NO₃⁻, and SO₄²⁻. For practical applications in wastewater treatment, a MOF-calcium alginate (CA) composite material was fabricated in beads and effectively removed Cr(VI) from diluted electroplating waste under dynamic flow conditions. Finally, all MTV-Al-MOFs displayed fluorescence with quantum yields in the range of 4.64–9.53%, while Cr(VI)-loaded MTV-Al-MOF-4 shows a pronounced reduction in fluorescence quantum yield by a factor of four, thereby confirming the presence of ligand-sorbent interactions leading to photoinduced energy and/or electron transfer processes. Overall, this study highlights a versatile approach to designing multifunctional MOFs with potential applications in environmental remediation, industrial wastewater treatment, luminescence, and gas separation.