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The high efficiency of metal-organic-frameworks (MOFs) such as the ZIF, MIL and UiO type species in dye adsorption is well established. Recently, an emerging class of photoresponsive azobenzene-based MOFs has found suitable application in gas adsorption. However, there is a dearth of research on their use in the adsorption of dyes and other water pollutants. In this research, two microporous photoresponsive azobenzene dicarboxylate MOFs of Al 3+ (Al-AZB) and Zr 4+ (Zr-AZB) were synthesized for the adsorption of congo red (CR) dye. The surface and textural properties of the synthesized MOFs were characterized by FTIR, PXRD, SEM, TGA, BET and pore analysis. Both MOFs were crystalline, thermally stable up to 300 °C and stable in aqueous medium at room temperature. The Al-AZB displayed a higher surface area (2718 m 2 /g) than the Zr-AZB (1098 m 2 /g), which significantly impacted the higher adsorption of CR. Besides, pore volumes of 0.86 cm 3 /g and 0.35 cm 3 /g were obtained for Al-AZB and Zr-AZB, respectively. The maximum adsorption capacity of Al-AZB and Zr-AZB was 456.6 mg/g and 128.9 mg/g, respectively, with the former superior to other potent adsorbents. The pseudo-second-order and Langmuir models were well correlated with the dye uptake on the MOFs. Thermodynamics revealed random and endothermic sorption of CR dominated by chemisorption, while efficient regeneration and reuse of both MOFs were achieved using dimethylformamide as eluent. The results proved the potency of the synthesized photoresponsive MOFs, as highly efficient and reusable materials for dye adsorption., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

• Two isoreticular aluminum-based MOFs with azobenzene (Al-AZB) and stilbene (Al-STB) organic linkers, respectively, were synthesized. • Nitrogen functionalization (Al-AZB) enhanced the thermal stability and surface area of the MOFs. • Celestine blue dye adsorption by the MOFs followed the Freundlich isotherm model. • The pseudo-second-order model provided the best fit to the adsorption data and involves a film diffusion mechanism. • The MOFs were successfully regenerated using DMF and effectively reused for dye adsorption. Nitrogen functionalized metal-organic-frameworks (MOFs) exhibit enhanced dye adsorption capacity compared to their counterparts without nitrogen functionalization. Typically, this functionalization is in the form of nitrogen side groups on the benzene/imidazole ring. Research on the effect of nitrogen atoms included within the rigid MOF structure on adsorption is lacking. In this work, two isoreticular aluminum-based MOFs were synthesized, functionalized with azobenzene (Al-AZB) and stilbene (Al-STB) organic linkers, respectively, with the former having nitrogen atoms within its rigid MOF structure. The effect of these nitrogen atoms on the physical properties and dye adsorption of the MOFs was tested. The MOFs were characterized and analyzed by NMR, FTIR, TGA, PXRD, SEM, BET, and UV/Vis. The characterizations confirmed the successful synthesis of the isoreticular MOFs, with Al-AZB exhibiting photoresponsive properties. The nitrogen functionalization enhanced the thermal stability and surface area of the MOFs as Al-AZB exhibited higher thermal stability (380 °C) and surface area (2500 m2/g) than Al-STB (345 °C; 1500 m2/g). The celestine blue dye adsorption by the MOFs followed the Freundlich isotherm model. A maximum monolayer adsorption capacity of 5224 mg/g and 5604 mg/g was obtained for Al-AZB and Al-STB, respectively, as predicted by the Langmuir model. Kinetic analysis showed that the pseudo-second-order model provided the best fit to the adsorption data and involves a film diffusion mechanism. The MOFs were successfully regenerated using dimethylformamide and effectively reused for dye adsorption. The evaluation of the mode of adsorption revealed the participation of van der Waals interactions and H-bonding in which the latter played a major role. The results showed that the nitrogen functionalization within the MOF's structure influenced the physical properties of the MOF as well as its dye adsorption characteristics. [ABSTRACT FROM AUTHOR]

Metal organic frameworks (MOFs) are porous hybrid crystalline materials that consist of organic linkers coordinated to metal centres. The trans–cis isomerisation kinetics of the azobenzene-4,4′-dicarboxylic acid (AZB(COOH)2) precursor, as well as the Al3+ (Al-AZB)- and Zr4+ (Zr-AZB)-based MOFs with azobenzene-4,4′-dicarboxylate linkers, are presented. The photo-isomerization in the MOFs originates from singly bound azobenzene moieties on the surface of the MOF. The type of solvent used had a slight effect on the rate of isomerization and half-life, while the band gap energies were not significantly affected by the solvents. Photo-responsive MOFs can be classified as smart materials with possible applications in sensing, drug delivery, magnetism, and molecular recognition. In this study, the MOFs were applied in the dye adsorption of congo red (CR) in contaminated water. For both MOFs, the UV-irradiated cis isomer exhibited a slightly higher CR uptake than the ambient-light exposed trans isomer. Al-AZB displayed a dye adsorption capacity of over 95% for both the UV-irradiated and ambient light samples. The ambient light exposed Zr-AZB, and the UV irradiated Zr-AZB had 39.1% and 44.6% dye removal, respectively. [ABSTRACT FROM AUTHOR]