Ultramicroporous Al(III) MOFs with selective CO2 adsorption, acid resistance, and efficient Cr(VI) sorption properties.

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.