Claims
- 1. A process for the preparation of an aqueous solution of glyoxylic acid, comprising:
- reacting alyoxal in an aqueous solution with a reactant consisting essentially of molecular oxygen and in the presence of a catalyst consisting essentially of nitrogen monoxide, said catalyst being present in a catalytic amount, and said aqueous solution of glyoxal containing an amount sufficient of a strong mineral acid to provide a pH value less than 1.
- 2. Process according to claim 1, characterized in that the proportion of nitrogen monoxide is between 75 and 150 millimoles per mole of glyoxal to be oxidized.
- 3. Process according to claim 2, characterized in that it is carried out under a molecular oxygen pressure of between 10.sup.5 and 8.times.10.sup.5 Pa.
- 4. Process according to claim 2, characterized in that it is carried out at a temperature between 35.degree. and 75.degree..
- 5. Process according to claim 1, characterized in that it is carried out under a molecular, oxygen pressure of between 10.sup.5 and 8.times.10.sup.5 Pa.
- 6. Process according to claim 5, characterized in that it is carried out at a temperature between 35.degree. and 75.degree..
- 7. Process according to claim 1, characterized in that it is carried out at a temperature between 35.degree. and 75.degree..
- 8. Process according to claim 7, wherein said temperature is between 40.degree. and 70.degree. C.
- 9. Process according to claim 1, wherein said strong mineral acid is hydrochloric acid.
- 10. Process according to claim 9, characterized in that the proportion of nitrogen monoxide is between 75 and 150 millimoles per mole of glyoxal to be oxidized.
- 11. Process according to claim 10, characterized in that it is carried out at a temperature between 35.degree. and 75.degree..
- 12. Process according to claim 9, characterized in that it is carried out under a molecular oxygen pressure of between 10.sup.5 and 8.times.10.sup.5 Pa.
- 13. Process according to claim 12, characterized in that it is carried out at a temperature between 35.degree. and 75.degree..
- 14. Process according to claim 9, characterized in that it is carried out at a temperature between 35.degree. and 75.degree..
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 88 08957 |
Jul 1988 |
FRX |
|
Parent Case Info
The present invention relates to a new process for obtaining aqueous solutions of glyoxylic acid industrially.
Oxidizing aqueous solutions of glyoxal into aqueous solutions of glyoxylic acid by nitric acid, possibly in the presence of either a strong mineral acid such as hydrochloric acid, or of oxalic acid, or of a soluble cobalt salt, or finally, of molecular oxygen, is known. (H. Debus, Annalen, 1857, 102, 28; French Patents Nos. 1,326,605, 2,372,141 and 2,516,506; Federal Republic of Germany Patent Nos. 932,369, 933,987, 1,002,309 and 31,32,006; Japanese Patents No. 73-103,517, 76-29441, 76-80821, 77-105121 and 80-129440).
These processes produce aqueous solutions of glyoxylic acid containing, apart from glyoxal and unchanged nitric acid, relatively large quantities of oxalic acid, and they release large quantities of nitrogen oxides which require expensive absorption means to prevent their being discharged into the atmosphere.
The presence in the aqueous glyoxylic acid solutions of glyoxal and nitric acid is particularly disturbing because of their reactivity with glyoxylic acid. Therefore their elimination has to be carried out quickly and in such a way as to obtain a residual concentration of glyoxal less than 2% in molar proportions with respect to glyoxylic acid and a residual concentration of nitric acid less than 0.05% in molar proportions with respect to the glyoxylic acid. The elimination of the glyoxal from the aqueous solutions of glyoxylic acid is particularly delicate and expensive, because these two products have at least one aldehyde function.
Recently, in the French Patent No. 2,516,506, a process was proposed for the oxidation of aqueous solutions of glyoxylic acid by an oxidizing agent, obtained starting from nitric acid and a strong nonoxidising acid present at a concentration by weight of 6 to 40% in the reactive mixture. In spite of its interest for the weak residual concentrations of nitric acid present in the aqueous solutions of glyoxylic acid obtained, this process, however, does not correspond to commercial requirements because of the high concentrations of unconverted glyoxal.
The Applicant has now discovered a new process for the industrial manufacture of aqueous solutions of glyoxylic acid which avoids these inconveniences.
According to the process of the present invention, aqueous solutions of glyoxylic acid are prepared by reacting, at a pH value less than 1, an aqueous solution of glyoxal with oxygen, in the presence of catalytic quantities of nitrogen monoxide.
As aqueous solutions of glyoxal, there can be used the aqueous solutions of glyoxal available in industry and containing 5 to 40% by weight of glyoxal, advantageously those containing from 10 to 30% of glyoxal and preferably an aqueous solution of glyoxal at 17.+-.3% by weight.
The process according to the invention is carried out at a pH value less than 1. To achieve this pH, the starting aqueous solution of glyoxal is acidified with a strong mineral acid such as hydrochloric acid, sulphuric acid or their mixtures in variable proportions. Advantageously, the aqueous solutions of glyoxal are acidified with 2 moles of hydrochloric acid per liter of solution.
The process according to the invention is carried out by bubbling in molecular oxygen (dioxygen), at a pressure between 10.sup.5 and 8.times.10.sup.5 Pa, advantageously at a pressure of 4.times.10.sup.5 Pa. The process is carried out at the most desirable temperature, generally at a temperature between 35.degree. and 75.degree. C., advantageously between 40.degree. and 70.degree. C.
The oxidation catalyst is nitrogen monoxide, NO. Usually between approximately 75 and 150 mmoles of nitrogen monoxide is used per mole of glyoxal used. At the end of the reaction, this catalyst is easily eliminated from the reactive medium by means known per se. In the presence of oxygen, this catalyst oxidizes spontaneously into nitrogen dioxide, NO.sub.2, or its dimer, nitrogen tetroxide, N.sub.2 O.sub.4. On contact with glyoxal, the nitrogen dioxide is reduced to nitrogen monoxide and oxidizes the glyoxal into glyoxylic acid.
The reaction scheme can be written as follows:
The overall reaction equation is therefore as follows:
These reactions are quick and exothermic. As a consequence, the reaction temperature has to be kept at the desired level by an appropriate means of heat elimination.
Theoretically, there is no consumption of nitrogen monoxide. However, because of the imperfection of the apparatus and/or certain secondary reactions, from time to time there is a slight consumption of nitrogen monoxide; however this is less than 80 mmoles per mole of glyoxal used, and can be compensated for when the process is carried out continuously, by the addition of the corresponding quantity of fresh nitrogen monoxide.
One of the interesting features of the process of the present invention is that it takes place without dilution of the aqueous solution of glyoxal used.
The process according to the present invention is selective. Therefore, for a transformation rate of 99%. a selectivity of 80.+-.5% is noted, that is to say that at the end of the reaction 80.+-.5% of glyoxal used is recovered in the form of glyoxylic acid. The oxidation of glyoxal either into oxalic acid or into carbon dioxide, is at the most 25% for oxalic acid and 1% for carbon dioxide, expressed in molar proportions with respect to the glyoxylic acid obtained.
In continuous operation, the oxidation reaction can be followed by analysing, by means known per se, the gases evolved, notably oxygen, carbon dioxide, and nitrogen oxides and it can be regulated by adjustment of one or more of its reaction parameters such as temperature, pressure, the rate of introduction of oxygen, and the rate of supply of glyoxal.
The process according to the present invention allows aqueous solutions of glyoxylic acid to be obtained, which are nearly free from nitric acid, and contain, in molar proportions with respect to the glyoxylic acid present, less tan 2.5% glyoxal. Furthermore, very little or no nitrogen oxides are released into the environment.
At the end of the reaction, the oxalic acid possibly present in the aqueous solutions of glyoxylic acid obtained, is eliminated by means known per se. Advantageously it is recovered by the crystallisation of its hydrate with two molecules of water and the final traces are eliminated either by electrodialysis or by treatment with an ion exchange resin, at the same time, if desired, as the elimination of the mineral acid or acids used to acidify the starting aqueous solution of glyoxal to a pH value less than 1. In effect, the mineral acid or acids introduced into the starting solution of glyoxal can be recovered at the end of the reaction by submitting the aqueous solutions of glyoxylic acid obtained either to a treatment by suitably selected ion exchange resins or to a treatment by electrodialysis, well known in other applications. To obtain an aqueous solution having a concentration of glyoxylic acid higher than that which can be obtained directly by the process of the present invention, the aqueous solution of glyoxylic acid can optionally be concentrated by known techniques.
Advantageously, the process of the present invention is carried out continuously, either in a set of agitated reactors, arranged in a series, and equipped with an adequate recycling system for the gases, or in a liquid gas column. The apparatus, kept at the selected temperature and pressure, is supplied continuously with the necessary quantities of oxygen, nitrogen monoxide and glyoxal in aqueous solution, acidified to a pH value of less than 1. The reaction is controlled by the analysis of the gases evolved. On leaving the apparatus, the aqueous solution of glyoxylic acid is cooled to ambient temperature and it is optionally left to crystallise in order to eliminate, if necessary, the oxalic acid present, then, if desired, it is advantageously submitted to an electrodialysis, according to known techniques, in order to obtain on the one hand an aqueous solution of glyoxylic acid free from all other mineral and/or organic acids and on the other hand an aqueous solution of the mineral acid or acids used.
The following examples illustrate the invention without however limiting it.
US Referenced Citations (1)
| Number |
Name |
Date |
Kind |
| 2298387 |
Kenyon et al. |
Oct 1942 |
|
Foreign Referenced Citations (2)
| Number |
Date |
Country |
| 0132336 |
Sep 1978 |
DEX |
| 2001621 |
Feb 1979 |
GBX |
Patent Grant
5091566
