Control over the Free Space within Poly(ionic liquid)s for Selective Adsorption of “Size-Matching” Dyes

Here we report a strategy to effectively control the free volumes within poly(ionic liquid)s (PILs). The initial parent imidazolium-based PILs were synthesized by copolymerization using three size-variant aryl borates—trifluorophenyl borate (BPhF3–), tetraphenyl borate (BPh4–), and tetranaphthyl borate (BNap4–)—as couteranions. These counteranions were found to aggregate and play the role of templates and patterning holes. After exchange of the larger anions to chloride or acetate, the smaller anions occupied less inner space of holes, generating the porous structures. Impressively, depending on the types of the initial template anions, the average sizes of holes within these PILs were accurately regulated from 0.78 to 22 nm, showing various high values of specific surface areas. Their successful formation, porosity, and morphology were fully characterized by nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FT-IR), thermogravimetry analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), and elemental analysis. These porous PILs were then used for the adsorption of anionic dyes, such as Congo red, methyl blue, Sirius red, and sodium lignosulfonate. PILs were observed to show the highest adsorption capacities only toward the dyes bearing similar sizes to their average pore sizes, highlighting the size-matching effect. This phenomenon allowed us to not only separate the anionic dyes from cationic and neutral dyes based upon their charges but also selectively isolate the “size-matching” one from a mixture of anionic dyes. The adsorbed dye could be removed by washing with hydrochloric acid aqueous solution or sodium acetate methanol solution, and the PIL adsorbents could be recovered and reused at least five times without obvious decrease of adsorption efficiency.