Study On The Synthesis And Reaction Kinetics Of Melamine Hexaacetic Acid And Stability Of Its Complexes With Metal Ions

A useful and potential chelating agent, 2, 4, 6– triamino - 1, 3, 5– triazine -N, N, N', N', N", N"-hexaacetic acid (melamine hexaacetic acid), was synthesized by nucleophilic substitution through three steps, and the yield of product is high (69 %). The optimal reaction conditions are as follows:①The proportion of iminodiacetic acid disodium salt monohydrate and cyanuric chloride is 1:1.②Feed sequence of material is adding the acetone solution of cyanuric chloride to iminodiacetic acid disodium salt monohydrate, or adding three of iminodiacetic acid disodium salt monohydrate to the acetone solution of cyanuric chloride in every step.③Sodium bicarbonate (5.5 g) is used to counteract acid.④Three steps optimal reaction temperature is 0～5℃, 45℃and 95℃, respectively.⑤Three steps optimal reaction time is 2 h, 3 h and 5.5h, respectively.⑥Final acidified pH is 1~1.5. The structure of melamine hexaacetic acid is confirmed by 1H NMR, 13C NMR, elemental analyses and IR. The melting point of melamine hexaacetic acid is 243~246℃. Melamine hexaacetic acid is hardly dissolved at room temperature. However, hexasodium melamine hexaacetate is soluble easily in water. Hexasodium melamine hexaacetate has better chelating ability, and its chelation value is 271.40 mg CaCO3 /g.For the first time, the reaction kinetics of every step is studied. It is found that reaction order is first-order, first-order and second-order and activation energy is 50.35 kJ/mol, 69.43 kJ/mol and 83.12 kJ/mol, respectively. At last, The stepwise protonation constants and stepwise stability constants of metal ions (Co2+, Cu2+, Pb2+, and Zn2+) complexes have been determined in aqueous solution by pH titration at 25±0.1℃and I=0.1 mol·dm-3 KCl. Their first and second stability constants of metal ions are as follows: Co2+:4.70,2.95;Cu2+:5.85,3.02;Pb2:6.20,5.50;Zn2+:6.35,5.80.In this paper, melamine hexaacetic acid with functional groups was first synthesized through three steps by nucleophilic substitution of cyanuric chloride and iminodiacetic acid disodium salt monohydrate. Its reaction kinetics and stepwise stability constants with metal ions were also studied for the first time.