Trisodium dicarboxymethyl alaninate

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.[3]

Trisodium dicarboxymethyl alaninate
Names
Other names
* N,N-Bis(carboxymethyl)-DL-alanin trisodium salt
  • N-(1-Carboxyethyl)-iminodiacetic acid
  • α-Alanindiacetic acid
  • α-ADA
  • MGDA-Na3
  • Trilon M
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.120.943
EC Number
  • 605-362-9
UNII
Properties
C7H8NNa3O6
Molar mass 271.111 g·mol−1
Density * 0.690 g/cm3[1] as powder
  • 1.31 g/cm3[1] as ~40% aqueous solution at 20 °C
  • 1.464 g/cm3[2] as 56-58% aqueous solution at 20 °C
Hazards
GHS pictograms
GHS Signal word Warning
GHS hazard statements
 H290
P234, P390, P404
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Production

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

  • Achieving the highest possible space-time yields
  • Simple reaction control at relatively low pressures and temperatures
  • Realization of continuous process options
  • Achieving the lowest possible levels of impurities, particularly nitrilotriacetic acid, which is suspected of being carcinogenic
  • Use of inexpensive raw materials, e.g. instead of pure L-alanine the raw mixture of Strecker synthesis from methanal, hydrogen cyanide and ammonia
  • Avoidance of complex and yield-reducing isolation steps; instead, direct further use of the crude reaction solutions or precipitates in the following process step.

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.[4] 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.[5]

This later patent specification[5] 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.[6]

The yields are over 90% d.Th., the NTA contents below 1%. The process conditions make this variant rather less attractive.

Properties

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.[7] Trisodium N-(1-carboxylatoethyl)iminodiacetate is described as readily biodegradable (OECD 301C) and is eliminated to >90 % in wastewater treatment plants.[8] 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.[1][9]

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 ionsMGDAIDS[10]EDTA[11]
Ba2+4,93,47,9
Ag+3,97,3
Sr2+5,24,1
Ca2+7,05,210,6
Mg2+5,86,18,7
Mn2+8,47,713,8
Fe2+8,18,214,3
Cd2+10,68,416,5
Cr3+9,6
Co2+11,110,516,3
Zn2+10,910,816,5
Pb2+12,111,018,0
Ni2+12,012,218,6
Cu2+13,913,118,8
Al3+14,116,1
Hg2+14,921,8
Fe3+16,515,225,1

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.

Use

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)[12] 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.[13]

References
  1. BASF SE, Technical Information: Trilon M Types
  2. National Industrial Chemicals Notification and Assessment Scheme (NICNAS): Full Public Report "Methyl glycine diacetic acid, trisodium salt", File No: STD/1092, August 2004.
  3. Environmental Protection Agency, DfE's Safer Chemical Ingredients List, Chelating Agents, Alanine, N,N-bis(carboxymethyl)-, sodium salt (1:3).
  4. WO 9429421, J. Schneider et al., "Use of glycine-N,N-diacetic acid derivatives as biodegradable complexing agents for alkaline earth metal ions and heavy metal ions, and methods of preparing them", issued 1994-12-22, assigned to BASF AG
  5. US 5849950, T. Greindl et al., "Preparation of glycine-N,N-diacetic acid derivatives", issued 1998-12-15, assigned to BASF AG
  6. EP 2547648, R. Baumann et al., "Verfahren zur Herstellung nebenproduktarmer Aminocarboxylate", issued 2013-01-23, assigned to BASF SE
  7. BASF, Sicherheitsdatenblatt: Trilon M Powder MSDS
  8. Hessisches Landesamt für Umwelt und Geologie, 6.12 Komplexbildner. 2003, S. 12/6.
  9. Kołodyńska, Dorota; Hubicka, Halina; Hubicki, Zbigniew (2009). "Studies of application of monodisperse anion exchangers in sorption of heavy metal complexes with IDS". Desalination. 239 (1–3): 216–228. doi:10.1016/j.desal.2008.02.024..
  10. Lanxess AG, General Product Information: Baypure
  11. BASF SE, Technical Information: Trilon B Types (Dec 2013)
  12. SEPAWA, Rückblick 2013, Abstracts: Wasch- und Reinigungsmittel Session Reinigen und Hygiene, Jürgen Kielholz: Phosphatfreie Reiniger für maschinelle Geschirrspüler
  13. BASF SE: No more tea stains and chalky deposits
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