This invention relates to a process for the preparation of N-phosphonomethylglycine, and more particularly to the preparation of N-phosphonomethyliminodiacetic acid-N-oxide from N-phosphonomethyliminodiacetic acid using peroxides, which is converted to N-phosphonomethylglycine.
N-Phosphonomethylglycine, known also by its common name glyphosate, is a highly effective, commercially important, phytotoxicant useful in controlling a large variety of weeds. It is applied to the foliage of a very broad spectrum of annual and perenial grasses and broadleaf plants. Industrial uses include control of weeds along roadsides, waterways, transmission lines, in storage areas, and in other nonagricultural areas. Usually, N-phosphonomethylglycine is formulated into herbicidal compositions in the form of its various salts in solution, preferably water.
U.S. Pat. No. 3,950,402 to Franz discloses a process for the production of N-phosphonomethylglycine by forming an admixture of N-phosphonomethyliminodiacetic acid, water, and a metallic catalyst selected from the noble metals, heating the admixture to an elevated temperature (greater than 70.degree. C. to avoid low yields) and contacting the admixture with a free oxygent-containing gas.
U.S. Pat. No. 3,954,848 to Franz discloses a process for the production of N-phosphonomethylglycine by reacting N-phosphonomethyliminodiacetic acid with an oxidizing agent, such as hydrogen peroxide, in an aqueous acidic medium in the presence of a strong acid at a temperature of from about 70.degree. C. to about 100.degree. C. It is disclosed that one should employ at least 2 moles of the hydrogen peroxide for each mole of the N-phosphonomethyliminodiacetic acid, and preferably more.
Hungarian Patent Application No. 187,347 discloses a process for the preparation of N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid with peroxides using a catalytic amount of a metal compound selected from compounds of silver, iron, tin, lead, manganese or molybdenum. Molybdates are preferred. At temperatures lower than 80.degree. C., usually a contaminated end product is obtained. Typically, the reaction is carried out at a temperature of above 80.degree. C. and preferably above 100.degree. C. at pressures exceeding atmospheric, wherein the intermediate N-oxide is decomposed as rapidly as it forms. It is further disclosed that at least two mole equivalents of peroxide should be used for each mole equivalent of N-phosphonomethyliminodiacetic acid.
A publication entitled "Peroxygen Compounds in Organic Synthesis" published by Interox as a technical bulletin discloses that transition metal compounds, like sodium tungstate or molybdate or the relevant oxides, promote oxidation reactions by a non-radical mechanism. Peroxidic complexes also play a transient role in the metal catalyzed oxidation of olefins and acetylenes by alkyl hydroperoxides.
Although satisfactory results are obtained by the above processes to make N-phosphonomethylglycine, all of them suffer from one or more disadvantages, such as the use of excessive amounts of peroxide, and/or reaction at elevated temperatures and pressures. Now, there is disclosed a process which provides N-phosphonomethylglycine in high yields at modest temperatures and at atmospheric pressure using substantially stoichiometric amounts of peroxide to oxidize the N-phosphonomethyliminodiacetic acid to the desired N-phosphonomethylglycine.