GB2154588A - A process for preparation of N-phosphonomethyl glycine - Google Patents

Abstract

A process for preparation of N-phosphonomethylglycine by reacting phosphorous acid, formaldehyde and iminodiacetic acid in stoichiometric ratio and in the presence of hydrochloric acid and using elevated pressure which may be created by heating the reaction mixture under a closed system. The product yield can be readily crystallised in situ without addition of ethanol for salting it out and the hydrochloric acid may readily be recovered and reused.

Description

SPECIFICATION A process for preparation of N-phosphonomethyl glycine N-phosphonomethylglycine is a very effective plant growth control agent. Many processes have been disclosed for its preparation, but none are entirely satisfactory. One such process is disclosed in British patent publication No. GB 2021589A, and is carried out by having phosphorus trichloride react with water under a cooled process to initially liberate phosphorous acid and hydrochloric acid. When the reaction is terminated, the mixture is mixed with iminodiacetic acid and heated to boiling and then excessive amount of aqueous formaldehyde, about 3 or 4 times as much as the theoretical quantity, is added and boiled under reflux.However the yield contains only a small amount of N phosphonomethylglycine, and most of the yield is in the form of byproduct, N-phosphonomethyliminodiacetic acid, which is a precursor for preparation of N-phosphonomethylglycine. Furthermore, due to the presence of excess formalin, which has a tendency to polymerise, the resultant mixture of N-phosphonomethyliminodiacetic acid and N-phosphonomethylglycine becomes viscous and the glycine cannot be easily crystallised in situ. As a result, a substantially amount of ethanol has to be added for salting out. In addition, because the filtrate contains hydrochloric acid, ethanol and formalin as well as derivatives of formaldehyde, the recovery of formalin and ethanol is rather difficult.

Even when formalin is recovered its reuse involves further problems since it tends to result in precipitates of N-phosphonomethyliminodiacetic acid and N-phosphonomethylglycine gradually forming and firmly adhering to the wall of the reactor on stand ing, which renders the removal of the product difficult. Such situation becomes worse when the number of times of reuse increase.

No process is known which in practice produces predominantly N-phosphonomethylglycine directly from a reaction of phosphorous acid, iminodiacetic acid and formaldehyde, though a number of attempts for preparing N-phosphonomethylglycine from N-phosphonomethyliminodiacetic acid have been made. Some of these attempts have used aqueous hydrogen peroxide to oxidise N-phos phonomethyliminodiacetic acid. For example, as disclosed in U.S. Patent No. 3,950,402, the oxidation of N-phosphonomethyliminodiacetic acid is implemented by using hydrogen peroxide and a metal catalyst. U.S. Patent No. 4,147,719, discloses another kind of catalytic agent, a platinum on activated carbon catalyst. In U.S. Patent No. 3,969,398, the oxidation of N-phosphonomethyliminodiacetic acid is carried out in the presence of a catalyst consisting essentially of activated carbon.The above approaches are all aimed at finding out an economic way to prepare N-phosphonomethylglycine from N-phosphonomethyliminodiacetic acid.

It is a primary object of the present invention to provide an improved process for preparation of Nphosophonomethylglycine in which the P-C-N product can be easily crystallised in situ from the reaction medium from phosphorous acid.

The present invention provides a process for the preparation of N-phosphonomethylglycine in which formaldehyde, phosphorous acid and iminodiacetic acid are reacted together in the presence of hydrochloric acid under conditions of elevated pressure.

Carrying out the process under raised pressure improves the recoverablility of the products, viz the P-C-N compounds and HCI. The reactions mixture has much less tendency to become viscous and the resultant N- phosphonomethylglycine and other P-C-N compound can readily be crystallised out without introducing ethanol and the hydrochloric acid can easily be recovered for later use.

The process according to the invention is generally carried out at a pressure in the range of from 1.1 to 6.0 atmospheres, preferably in the range of from 2.0 to 4.0 atmospheres. The process of the invention is generally carried out at a raised temperature, usually in the range 110"C to 180 C and preferably in the range 125"C to 160 & . Suitably the pressure is autogenic at a temperature in the range 110 C to 1800C, preferably in the range 125"C to 160 C. That is to say the pressure is the same as the pressure obtained by heating to 110 C to 180 C, preferably 125"C to 1600C, in a closed vessel containing the reaction mixture. It is advantageous for the reaction to be carried out in the presence of hydrochloric acid at a concentration higher than its saturation at reaction conditions. Best results are often obtained at temperatures above 140 C, preferably above 150 C.

It is especially preferred to have the starting materials formaldehyde, phosphorous acid and iminodiacetic acid present initially in substantially stoichiometric amounts, for example, at a ratio in the ranges 0.5-2.0 : 0.5-2.0 1, preferably in the ranges 1-2.0 : 0.8-1.2 1.

Formaldehyde used in the present invention can be in either crystal or solution form, for example, paraformaldehyde or formalin. In case of the latter, phosphorus trichloride is utilised instead of phosphorous acid, so that the water of the formalin will react with phosphorus trichloride to produce phosphorous acid and hydrochloric acid in situ. In order to keep the concentration of hydrochloric acid higher than its saturated state, hydrochloric acid can be added as needed. If phosphorous acid is used, paraformaldehyde is preferably chosen for the reaction to avoid the diluting effect of water contained in formalin which is disadvantageous in maintaining the high concentration of hydrochloric acid.

The overall reaction can be much easier to control if phosphorus trichloride is used. Phosphorus trichloride is very stable in the presence of ice water, and so ice water with ice cubes is introduced into the reactor after the addition of iminodiacetic acid and paraformaldehyde to keep the reaction mixture cold. The phosphorus trichloride is subsequently added and the whole reaction system is closed immediately. The temperature of the reactor is gradually raised the ice melts and the water reacts with phosphorus trichloride to pro duce hydrochloric acid and phosphorous acid. The reaction of the reagents is usually completed, if they are added initially in substantially stoichiometric ratio, and agitated for 3 hours under 125130"C in the presence of hydrochloric acid at a concentration greater than 25% by weight.

In a UK Patent Application, publication No. GB 2021589A (see supra), it is disclosed that N-phosphonomethylglycine can be obtained by reaction of phosphorous acid, iminodiacetic acid and formaldehyde. However, it is apparent that the reaction condition of that process is not adequately set out and thus the N- phosphonomethylglycine has to be salted out by ethanol which would result in a small yield. It is believed by the present inventors that the yield of 95% in glycine derivative claimed in the above noted UK patent application, is stated incorrectly. If it were correct, the resultant N-phosphonomethyl compound would not require further treatment by hydrogen peroxide to implement the complicated oxidation from iminodiacetic acid derivative to glycine derivative which results in only limited improvement on the calculated purity of Nphosphonomethylglycine.It is unlikely that the decomposition temperatures of 95% and 98% pure glycine derivative would differ by 28"as stated in the citation.

Additional details of the present invention will be further illustrated by the following Examples.

Example I To a 50ml long-neck and ball-shaped pyrex glass bottle, 259 H20, 10.6g (0.08 mole) iminodiacetic acid and 3g of 80% by weight (0.08 mole) paraformaldehyde are added. After cooling to -5 C, 11g (0.08 mole) phosphorus trichloride are added into the bottle and the bottle is closed and heated in a silicon oil bath. As the temperature increases, the ice melts and the water reacts with phosphorus thrichloride to form phosphorous acid and hydrochloric acid, and therefore the solution is supersaturated with hydrogen chloride.

While stirring, the reaction mixture inside of the reaction bottle continues to be heated until the temperature reaches about 128"C, which is higher than the boiling point of the mixture under normal atmospheric pressure. Since the reaction system is closed, the reaction mixture is under a pressure higher than 1 atm. The reaction mixture is maintained at 128+3 C for 3 hours. Thereafter, the reaction bottle is taken out of the oil bath and cooled at room temperature. After cooling, the bottle is carefully opened, and the resultant solution is transferred to a 250 ml beaker and evaporated by heating to recover hydrochloric acid. Evaporation is ended as the solution volume becomes 25ml.

After cooling in air for minutes, P-C-N compounds are crystallised in large granular size without saturation with ethanol. Separated from mother solution, the P-C-N compound is dried in an oven and analysed to find its contents containing 1.4% by weight of N-phosphonomethylglycine and the balanced N-phosphonomethyliminodiacetic acid in the total weight of 159 P-C-N compound. The filtrate is desirably clear, not viscous, and readily re-usable.

Example 2 The procedure followed in Example 1 is repeated except 6.5g of 37% by weight formalin (0.08 mole formaldehyde), 22.09 water, 11.09 (0.08 mole) phosphorus trichloride and 10.69 (0.8 mole) iminodiacetic acid are used and reacted at 130-135"C for three hours. The product yield is 15.19 and contains 1.6% N-phosphonomethlyglycine and 98.4% N-phosphonomethyliminodiacetic acid by weight.

Example 3 The procedure followed in Example 1 is repeated except that 39 of 75% by weight paraformaldehyde (0.075 mole), 12.39 of 50% by weight phosphorous acid (0.075 mole), 16.2 ml of 23% by weight hydrochloric acid recovered from Examples 1 and 2, and 8.8 ml of 35% by weight hydrochloric acid and 10g (0.075 mole) iminodiacetic acid are used and reacted 130-135"C for 3 hours. The product yield is 14.79 and contains 1.8% N-phosphonomethylglycine and 98.2% N-phosphonomethlyiminodiacetic acid by weight.

Example 4 The procedure followed in Example 1 is repeated except that 3.0g of 75% by weight paraformaldehyde (0.075 mole), 12.3g of 50% by weight phosphorous acid (0.075 mole), 30.0 ml of 30% by weight hydrochloric acid and 10.0g (0.075 mole) iminodiacetic acid are added and reacted at 145+3 C for 4 hours. The product yield is 13.8g and contains 14% N-phosphonomethylglycine and 86% N-phosphonomethyliminodiacetic acid by weight.

The results show that the higher the temperature applied, the greater the yield of N-phosphonomethylglycine.

Example 5 The procedure followed in Example 1 is repeated except that 0.6g of 75% by weight paraformaldehyde (0.015 mole), 2.5g of 50% by weight phosphorous acid (0.015 mole), 5.0 ml of 35% by weight hydrochloric acid and 2.09 (0.015 mole) iminodiacetic acid are added and reacted at 155-160"C for 12 hours. The P-C-N product yield is 2.259 and contains 62.27% N-phosphonomethylglycine and 37.73% N-phosphonomethyliminodiacetic acid by weight It can be noted that, by using higher temperature and longer reaction time, the product yield is predominantly N-phosphonomethylglycine.

Claims (14)

1. A process for the preparation of N-phosphonomethlyglycine in which formaldehyde, phosphorous acid and iminodiacetic acid are reacted together in the presence of hydrochloric acid under conditions of elevated pressure.

2. A process according to claim 1 in which the pressure is in the range of from 1.1 to 6.0 atmospheres.

3. A process according to claim 1 in which the pressure is in the range of from 2.0 to 4.0 atmos pheres.

4. A process according to any one of claims 1 to 3 carried out at a temperature in the range of from 1100 to 1800C.

5. Process according to any one of claims 1 to 4 carried out in a closed system.

6. Process according to claim 5 in which the pressure is autogenic pressure.

7. Process according to any one of claims 1 to 6 in which the hydrochloric acid is present at a concentration greater than saturation under the conditions of the reaction.

8. Process according to any one of claims 1 to 7 in which the phosphorous acid and hydrochloric acid are formed in situ by the reaction of water on phosphorus trichloride.

9. Process according to claim 8 in which formaldehyde is added as formalin.

10. Process according to any one of claims 1 to 9 in which the formaldehyde, phosphorous acid and iminodiacetic acid are present in the relative proportions in the ranges of 0.5-2.0 : 0.5-2.0 1.

11. Process according to claim 10 in which the relative proportions are 1-2 : 0.8-1.2 : 1.

12. Process according to claim 10 or claim 11 carried out at an autogenic pressure equivalent to at least 1500C.

13. Process according to any one of claims 1 to 12 carried out in a closed system at a temperature in the range of from 125 to 1600C for at least 4 hours.

14. Process carried out substantially as described in any one of Examples herein.