The construction of amorphous metal-organic cage-based solid for rapid dye adsorption and time-dependent dye separation from water

For the first time, an amorphous solid contains large cationic M12L24 metal–organic cage (MOC) based on Pd(II) and 3,5-di-pyridin-4-yl-benzaldehyde (L), namely aMOC-1, was constructed and characterized. Although it is an amorphous structure, regular cavity that created by MOC still exists in the solid and performs excellent dye adsorption and separation ability in water. The aMOC-1 can rapidly adsorb the anionic dyes including methyl orange (MO), fluorescein disodium salt (FS), acid fuchsin (AF) and methyl blue (MB) only in several minutes to about 30 min. The adsorption capacities for them are 359, 464, 116 and 150 mg/g, respectively. On the contrary, the adsorption rate of aMOC-1 towards cationic dyes is significantly slower than that of anionic dyes. The adsorption of cationic dyes shows a size-selectively behavior, in which the adsorption of rhodamine B (RhB) is more favorable than the other selected smaller or bigger cationic dyes. The adsorption capacity of RhB is up to 454 mg/g. According to the differences in the adsorption rates and capacities for different charged dyes, aMOC-1 displays diverse dye separation behaviors. It can selectively adsorb RhB from a mixed solution of MLB and RhB by size selectivity. Moreover, the selective separation of anionic and cationic dyes shows an interesting new time-dependence mode that the anionic dye can be selectively adsorbed from a mixed solution of anionic and cationic dyes by controlling the adsorption time. The good adsorption and separation performance of aMOC-1 is caused by the large cavity and the high positive Zeta potential of the material, which are created by the Pd12L24 metal-organic cages.