Isoreticular aluminium-based metal-organic frameworks with structurally similar organic linkers as highly efficient dye adsorbents

Abstract

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.