Size-exclusive and coordination-induced selective dye adsorption in a nanotubular metal-organic framework

A new metal-organic framework, {[(CH3)(2)NH2][Co2NaL2(CH3COO)(2)]center dot xS}(n) (BUT-51, H2L = 5-(pyridine-4-yl) isophthalic acid) with two types of nanotubular channels, was synthesized and used in dye adsorption. It was found that cationic dyes, including Methylene Blue (MB), Acridine Red (AR) and Acriflavine Hydrochloride (AH), can be rapidly adsorbed by BUT-51, but larger Methylene Violet (MV) and anionic Methyl Orange (MO), as well as neutral Solvent Yellow 2 (SY2), can hardly be adsorbed. These results imply that the adsorption behaviour of BUT-51 with the cationic dyes follows not only a charge-but also size-and shape-exclusive effect. Surprisingly, BUT-51 also preferentially adsorbs (or separates) AH from MB and AR, respectively, in their mixed acetone solution, probably due to the coordination between AH and Co2+ in its framework. Furthermore, AH uptake can change the porosity and framework stability of BUT-51 to give a robust partner.