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

Abstract

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.