Trisodium N-(1-carboxylatoethyl)iminodiacetate, methylglycinediacetic acid trisodium salt (MGDA-Na3) or trisodium α-DL-alanine diacetate (α-ADA), is the trisodium anion of N-(1-carboxyethyl)iminodiacetic acid and a tetradentate complexing agent. It forms stable 1:1 chelate complexes with cations having a charge number of at least +2, e.g. the "hard water forming" cations Ca2+ or Mg2+. α-ADA is distinguished from the isomeric β-alaninediacetic acid by better biodegradability and therefore improved environmental compatibility.[1]
The patent literature on the industrial synthesis of trisodium N-(1-carboxylatoethyl)iminodiacetate describes the approaches for solving the key requirements of a manufacturing process that can be implemented on an industrial scale, characterized by
An obvious synthesis route to α-alaninediacetic acid is from racemic α-DL-alanine, which provides racemic α-ADA by double cyanomethylation with methanal and hydrogen cyanide, hydrolysis of the intermediately formed diacetonitrile to the trisodium salt and subsequent acidification with mineral acids in a 97.4% overall yield.[2] In a later patent specification, however, only an overall yield of 77% and an NTA content of 0.1% is achieved with practically the same quantities of substances and under practically identical reaction conditions.[3]
This later patent specification also indicates a process route via alaninonitrile, which is obtained by Strecker synthesis from hydrogen cyanide, ammonia and methanal and converted to methylglycinonitrile-N,N-diacetonitrile by double cyanomethylation (step 1). The three nitrile groups are then hydrolyzed with sodium hydroxide to α-ADA (step 2). The total yield is given as 72%, the NTA content as 0.07%.
One variant of the reaction involves iminodiacetonitrile or iminodiacetic acid (step 1'), which reacts in a weakly acidic medium (pH 6) with hydrogen cyanide and ethanal to form methylglycinonitrile-N,N-diacetic acid, the nitrile group of which is hydrolyzed with sodium hydroxide to trisodium N-(1-carboxylatoethyl)iminodiacetate (step 2'). The reactant iminodiacetic acid is accessible at low cost by dehydrogenation of diethanolamine. Again, the total yield is given as 72%, the NTA content as 0.07%.
A further variant is suitable for continuous production, in which ammonia, methanal and hydrogen cyanide react at pH 6 to form iminodiacetonitrile, which in a strongly acidic medium (pH 1.5) reacts with ethanal to produce trinitrile methylglycinonitrile-N,N-diacetonitrile in a very good yield of 92%. (step 1).
Alkaline hydrolysis (step 2) results in a total yield of 85% trisodium N-(1-carboxylatoethyl)iminodiacetate with an NTA content of 0.08%. This process variant seems to fulfil the above-mentioned criteria best.
A low by-product synthesis route for trisodium N-(1-carboxylatoethyl)iminodiacetate has recently been described, in which alanine is ethoxylated with ethylene oxide in an autoclave to form bis-hydroxyethylaminoalanine and then oxidized to α-ADA at 190 °C with Raney copper under pressure.[4]
The yields are over 90% d.Th., the NTA contents below 1%. The process conditions make this variant rather less attractive.
The commercially available trisodium N-(1-carboxylatoethyl)iminodiacetate (84% by weight) is a colourless, water-soluble solid whose aqueous solutions are rapidly and completely degraded even by non-adapted bacteria. Aquatic toxicity to fish, daphnia and algae is low.[5] Trisodium N-(1-carboxylatoethyl)iminodiacetate is described as readily biodegradable (OECD 301C) and is eliminated to >90 % in wastewater treatment plants.[6] Trisodium N-(1-carboxylatoethyl)iminodiacetate has not yet been detected in the discharge of municipal and industrial sewage treatment plants. In addition to their very good biodegradability, trisodium N-(1-carboxylatoethyl)iminodiacetate solutions are characterized by high chemical stability even at temperatures above 200 °C (under pressure) in a wide pH range between 2 and 14 as well as high complex stability compared to other complexing agents of the aminopolycarboxylate type.[7] [8]
The following table shows the complexing constants log K of α-ADA compared to tetrasodium iminodisuccinate and ethylenediaminetetraacetic acid (EDTA) versus polyvalent metal ions:
| Metal ions | MGDA | IDS[9] | EDTA[10] | |
|---|---|---|---|---|
| Ba2+ | 4.9 | 3.4 | 7.9 | |
| Ag+ | 3.9 | 7.3 | ||
| Sr2+ | 5.2 | 4.1 | ||
| Ca2+ | 7.0 | 5.2 | 10.6 | |
| Mg2+ | 5.8 | 6.1 | 8.7 | |
| Mn2+ | 8.4 | 7.7 | 13.8 | |
| Fe2+ | 8.1 | 8.2 | 14.3 | |
| Cd2+ | 10.6 | 8.4 | 16.5 | |
| Cr3+ | 9.6 | |||
| Co2+ | 11.1 | 10.5 | 16.3 | |
| Zn2+ | 10.9 | 10.8 | 16.5 | |
| Pb2+ | 12.1 | 11.0 | 18.0 | |
| Ni2+ | 12.0 | 12.2 | 18.6 | |
| Cu2+ | 13.9 | 13.1 | 18.8 | |
| Al3+ | 14.1 | 16.1 | ||
| Hg2+ | 14.9 | 21.8 | ||
| Fe3+ | 16.5 | 15.2 | 25.1 |
The complex formation constants of the biodegradable chelators α-ADA and IDS are in a range suitable for industrial use, but clearly below those of the previous standard EDTA.
In solid preparations, trisodium N-(1-carboxylatoethyl)iminodiacetate is stable against oxidizing agents such as perborates and percarbonates, but not against oxidizing acids or sodium hypochlorite.
Like other complexing agents in the aminopolycarboxylic acid class, trisodium N-(1-carboxylatoethyl)iminodiacetate (α-ADA) finds due to its ability to form stable chelate complexes with polyvalent ions (in particular the water hardening agents Ca2+ and Mg2+, as well as transition and heavy metal ions such as Fe3+, Mn2+, Cu2+, etc.) use in water softening, in detergents and cleaning agents, in electroplating, cosmetics, paper and textile production. Due to its stability at high temperatures and pH values, α-ADA should be particularly suitable as a substitute for the phosphates banned in the EU from 2017, such as sodium tripolyphosphate (STPP)[11] in tabs for dishwashers.
BASF SE is the most important manufacturer of α-ADA under the brand name Trilon M, has large-scale plants in Ludwigshafen and Lima, Ohio, and is currently expanding its existing capacities with another large-scale plant at Evonik's site in Theodore, Alabama.[12]