Novel strategies for terephthalic acid processing under soft conditions using ionic liquids

Terephthalic acid (H₂TPA) solubility in several ionic liquids (ILs) at multiple concentrations is higher than for any other known solvents at lower temperatures and pressures which suggests low energy purification of H₂TPA from its major impurity, 4-carboxybenzaldehyde (4-CBA) might be possible. To understand the mechanism several strategies were explored to purify H₂TPA by taking advantage of this high solubilizing power of ILs for H₂TPA in the crystallization of unique salts and cocrystals. Using either zwitterionic carboxylate IL-precursors or direct salt formation with carboxylate ILs or amines, a series of salts of mono, dibasic and two ionic cocrystals were obtained including monobasic, [C₁C₁im][HTPA], [N₄₄₄₁][HTPA], [C₄C₁im][HTPA]•0.5H₂TPA (a cocrystal), and [C₁Him][HTPA] ([C₁C₁im]⁺ = 1,3-dimethylimidazolium, [N₄₄₄₁]⁺ = tribuytlmethylammonium, [C₁Him]⁺ = 1-methyl-3-H-imidazolium), dibasic [C₁C₁im]₂[TPA], [C₄C₁im]₂[TPA], [N₄₄₄₄]₂[TPA], [C₁Him]₂[TPA], [H₂N₂₂]₂[TPA], [H₃N₆]₂[TPA], and [HN(CH₂CH₂OH)₃]₂[TPA] ([C₄C₁im]⁺ = 1-butyl-3-methylimidazolium, [N₄₄₄₄]⁺ = tetrabuylammonium, [H₃N₆]⁺ = hexylammonium, [HN(CH₂CH₂OH)₃]⁺ = triethanolammonium, [H₂N₂₂]⁺= diethylammonium), and a second cocrystal [C₂C₁im]Cl•0.5H₂TPA. The formation of these salts suggest a viable method to purify H₂TPA because of preferred salt formation at low energy conditions. One elegant route using 1-ethyl-3-methylimidazolium chloride ([C₂C₁im]Cl) could be especially promising because the cocrystal [C₂C₁im]Cl•0.5H₂TPA was readily isolated and is easily dissociated when exposed to ambient conditions into crystalline H₂TPA and a liquid of hydrated [C₂C₁im]Cl.