Improvements in the oxidation or cyclohexane to produce partial oxidation products thereof

The present invention relates to a process for the manufacture of adipic acid by reoxidation of the oxidation products of cyclohexane after the cyclohexane has been separated from the oxidation mixture. 5 It is known that for the manufacture of adipic acid by direct oxidation with a gas containing molecular oxygen, the oxygenated mixtures originating from the oxidation of cyclohexane at low temperature are of superior quality to the oxygenated mixtures resulting from oxidation. high temperature cyclohexane. However, cyclohexane oxidations carried out at low temperature are slow to initiate and, even after initiation, are lazy. Consequently, the manufacture of precursors of adipii5 acid only by oxidation of cyclohexane at low temperature takes a long time and is to a certain extent dangerous because of the possible presence of oxygen in explosive concentration, the latter being consumed only slowly. In order to take advantage of the superior properties of oxygen mixtures from the oxidation of cyclohexane at low temperatures, these difficulties must be overcome.

In order to overcome these difficulties, the present invention relates to a process for the manufacture of adipic acid, in which cyclohexane is brought into contact with molecular oxygen as an essential oxidant in a first oxidation zone in order to to produce a mixture of cyclohexanone of so cyclohexanol and other oxygenated products, the unprocessed cyclohexane is separated and returned to said first zone, said mixture is oxidized in a second oxidation zone, in the presence of a catalyst and preferably by means of nitric acid, ss or then also by means of molecular oxygen in the general presence of a manganese and / or copper catalyst, and the adipic acid is separated from the mixture oxide. This process is characterized in that the first operation is carried out first at a temperature of 140-170 ° C and under a pressure ω of 5.3 to 35 kg / cm² with a pressure gauge, for an initial period of 15 to 45 minutes, then at a temperature of 120-140 ° C for a longer period.

Thus in the first oxidation zone, the initial temperature can be 150 ° C for a period of 30 mm and the subsequent temperature is preferably 130 ° C for a period of 90 min, the reaction pressure being 10.5 kg / cm² for both periods.

Molecular oxygen is used in the first oxidation. The weight ratio of cyclohexanone to cyclohexanol is preferably increased to at least 0.75.

In the second oxidation, a manganese and copper catalyst 55 or a manganese catalyst are advantageously used.

In the examples which follow, the parts and percentages are by weight, unless otherwise indicated.

Example 1

Cyclohexane is introduced with an eo catalyst (for example 100 parts per million of cobalt naphthenate) in a first oxidation zone maintained at 150 ° C for 30 min, then at 125 ° C for 90 min, and at a gauge pressure 3.5 kg / cm² (in order to maintain the cyclohexane in its liquid phase). Air is bubbled through cyclohexane at a normal flow rate of 7.11 rpm. About 10% of cyclohexane is transformed into an oil containing cyclohexanone, cyclohexanol

406 181 and oxygenated compounds. The average concentration of oxygen in the outgoing gas is 2.5% by volume. All the unprocessed cyclohexane is separated from the reaction mixture by distillation and returned to the first oxidation zone so that it is again subjected to oxidation.

25 parts of the oil thus obtained are mixed with 75 parts of acetic acid, 0.15 part of manganese acetate and 1.0 part of copper acetate and the mixture is introduced into a second maintained reaction zone at 80 ° C. Air is passed through the mixture at a flow rate of 28.3 to 56.61 rpm normal under a pressure of 7.03 kg / cm². Then, the reaction mixture is cooled and the adipic acid is separated, thus obtaining a yield of 70.5% by weight of adipic acid, relative to the weight of the oil.

Comparative example A

Example 1 is repeated, except that the first oxidation is carried out at 150 ° C. for 90 min. The average concentration of oxygen in the outgoing gas is 2% and the yield of adipic acid is 64%.

Comparative example B

Example 1 is repeated, except that the first oxidation is carried out at 234 ° C. To obtain comparable amounts of oxygenated oil, the first oxidation stage must be extended from 120 to 450 min. The average concentration of oxygen in the outgoing gas is 18% (danger of explosion) and the yield of adipic acid is 71.3%.

These results clearly show that the process according to the invention gives a better yield of product and eliminates the danger of explosion of the outgoing gas.

In the oxidation of cyclohexane, the mixture is maintained at a temperature between 140 ° C and 170 ° C under a pressure of 5.3 to 35, preferably 8.8 to 12.3 kg / cm² with a pressure gauge for 40 a priming period of 15 to 45 min, preferably 20 to 30 min, then at a temperature of 120 to 140 ° C, preferably of 125 to 135 ° C for a longer period. Thus, cyclohexane is maintained in the liquid phase. The conversion rate of the cyclohexane can be from 3 to 20%, advantageously from 10 to 20% and preferably from about 10%. If the conversion rate of the cyclohexane is less than about 3-5%, it becomes difficult to separate the cyclohexane from the oil. Conversion rates 50 exceeding 20% ​​are undesirable, because in general the relative yield of cyclohexanone and cyclohexanol compared to the transformed cyclohexane decreases and the relative yield of unwanted by-products increases when the conversion rate increases in this first stage of treatment. cyclohexane. In addition, the proportion of solids increases with increasing conversion rates. These solids cause practical difficulties due to their limited solubility in the reaction mixture.

A catalyst may be present, for example 1 to 1000, preferably 5 to 200 parts per million of cobalt naphthenate.

To cool the hydrocarbon in reaction, the pressure in the reactor can be lowered so as to vaporize part of the hydrocarbon. Coils or cooling jackets can also be used.

In the second oxidation zone, the oil is oxidized at a temperature between 60 and 105 ° C, advantageously between 65 and 85 ° C 70 and preferably between 70 and 80 ° C, and the pressure is between atmospheric pressure and 70.3 kg / cm² with a pressure gauge, and at a value sufficient for the partial pressure of oxygen to be at least Vs of atmosphere. The pressure can be higher and sometimes it can reach a value such that the partial pressure of the oxygen in the mixture subjected to oxidation is about 14 kg / cm². The feed rate of the air or oxygen-containing gas, both for the oxidation of cyclohexane, and so for the oxidation of the oil which comes from it, is at least partly determined by the geometrical configuration of the particular reaction zone employed.

Obviously, it should not be so high as to cause undesirable overflow or entrainment of the material being oxidized.

A variable proportion of the oil originating from the oxidation of cyclohexane can be used for initiating the oxidation of cyclohexane. In general, it is not necessary to use more than 0.5 to 98 parts of oil per 100 parts of the cyclohexane subjected to oxidation, it being understood that this proportion may be greater.

Although the reaction medium consists of acetic acid in the preceding examples, equally good results are obtained by using as solvent for the reaction any of the saturated carboxylic acids of 2 to 6 carbon atoms in the molecule. In addition, various other reaction media can be used, including chlorobenzene and tertiary butyl alcohol, and generally all of the substances which act as a solvent for the reactants and which are not themselves oxidized, or at least not easily oxidized. Obviously, a change in the solvent may require a change in the anion of the salt used for the usual introduction of manganese and copper, preferably serving as an oxidation catalyst. Thus, while these metal ions are advantageously in the form of acetate in acetic acid, the insolubility of the acetates in chlorobenzene requires that these metal ions be in the form of naphthenates in the latter solvent.

It is surprising that the process according to the invention retains the reaction rate which is characteristic in of a high temperature oxidation process, while providing an oil whose quality has hitherto been obtainable only by oxidation of the cyclohe3 xane at low temperature, expensive and potentially dangerous.

The oxidation process according to the invention, distinct temperature stages, gives better yields as precursors of adipic acid formed by the oxidation of cyclohexane by air. The transformation of these precursors into adipic acid can be carried out by means of a new supply of molecular oxygen or by means of other oxidizing agents, for example nitric acid.