Dye adsorption performance of an anionic Cd2+ MOF material based on semi-rigid hexacarboxylic acid†

Abstract

We have selected a semi-rigid hexacarboxylic acid ligand H₆L enriched with oxygen-active sites with Cd²⁺ salt, successfully synthesising a 3-D anionic material MOF-1 ([(CH₃)₂NH₂]₂[Cd₂L]·3DMA·2H₂O, H₆L = 5,5′,5′′-(benzene-1,3,5-triyltris(oxy))triisophthalic acid). The structure of this carboxylic acid ligand is triangular in shape, which contributes to the formation of the 3-D framework structure, and its semi-rigidity facilitates entry of dye molecules by slightly adjusting the pore size. The larger pore size of MOF-1 is 1.45 nm × 0.86 nm. The oxygen spacer groups are uniformly distributed throughout the framework, promoting the adsorption of MOF-1 for dyes. Based on these factors, there is a significant advantage in that the anionic material MOF-1 containing [(CH₃)₂NH₂]⁺ selectively adsorbs cationic organic dyes from aqueous contaminants for recycling. It was discovered that the adsorption process' equilibrium and kinetics followed the Langmuir isothermal model and pseudo-secondary kinetics, respectively. And we measured the adsorption rate of MOF-1′ on MB⁺ to be 1.77 × 10⁻³ g mg⁻¹ min⁻¹. Its relatively high capacity is ca. 1220 mg g⁻¹. After five cycles of adsorption and desorption, the dye adsorption performance was reduced to ca. 80% and the framework stability remained well.