1. Field of the Invention
The invention relates to an improved process for the continuous preparation of .alpha.- and/or .beta.-ionone or homologs of these compounds by cyclizing pseudoionones using concentrated sulfuric acid in the presence of organic solvents or diluents and by diluting the reaction mixture with water.
2. Description of the Background
It is known that a mixture of .alpha.- and .beta.-ionones is obtained on cyclization of pseudoionone in the presence of acids such as sulfuric acid or phosphoric acid. The ratio of the amounts in which these compounds are produced depends greatly on the conditions under which the reaction takes place.
Since both .alpha.-ionone and .beta.-ionone are of great industrial importance, there has been no lack of attempts to develop a maximally advantageous process for their preparation.
Processes for cyclizing pseudoionone with concentrated sulfuric acid have proven particularly suitable. Since this reaction is highly exothermic, it is very important to remove the heat produced in the reaction as quickly as possible in order to avoid local overheating. For this purpose, in the known processes, diluents have been added to the reaction mixture. Thus, for example, DE Patents 10 80 105 and 16 68 505 disclose the use of aliphatic or cycloaliphatic hydrocarbons. The disadvantage of this process is that resins are relatively quickly deposited in the reaction vessels in the procedure described therein, and interfere with continuous operation.
According to IN patent 77 225, the reaction is carried out in the presence of aliphatic chlorinated hydrocarbons such as methylene chloride, ethylene dichloride, chloroform and tetrachloromethane at temperatures from -10.degree. C. to +10.degree. C.
According to the description in DE-A 15 68 108, this Indian process is disadvantageous because the aliphatic chlorinated hydrocarbons eliminate hydrogen chloride with sulfuric acid, leading within a short time to corrosion of the apparatus used. To avoid these disadvantages, it is recommended to carry out the cyclization at -25.degree. C. to +10.degree. C. in a mixture of low-boiling hydrocarbons and chlorinated hydrocarbons. The disadvantage of the two last-mentioned processes is that the reaction temperature must be kept low with costly coolants in order to obtain good ionone yields.
Further known processes entail removing the considerable heat of cyclization by evaporative cooling with liquid gases. Thus, liquid sulfur dioxyde [sic] is used in the process of DE patent 16 68 496, propane, butane or isobutane is used in the process of DE patent 16 68 505, and methyl chloride is used in the process of DE patent 19 17 132, at temperatures from -25.degree. C. to room temperature, preferably temperatures below +10.degree. C.
The results obtained in these processes are generally quite good. The disadvantage thereof is the large expenditure necessary for reliquefying the gas vaporized in the reaction.
Further processes for preparing .beta.-ionone are disclosed in CS patent 179 046, SU patent 458 540 and SU patent 547 445, wherein thorough mixing of the reactants and rapid removal of heat are achieved by using a thin film reactor. The disadvantage of the two last-mentioned processes is that only about 3 to 6 kg of .beta.-ionone are obtained per m.sup.2 of thin film area and hour, and thus transfer to the industrial scale would result in enormous apparatus. The disadvantage of the process in the Czech patent is that temperatures between 10 and 15.degree. C. must be used to obtain good yields, which makes costly coolants necessary once again.
In all the known processes there is always formation of a mixture of .alpha.- and .beta.-ionones. According to DE patents 10 80 105, 16 68 496 and 16 68 505, .beta.-ionone is preferentially obtained at reaction temperatures from -20 to 0.degree. C., while the .alpha.-ionone content increases at temperatures from -10 to 25.degree. C.
In addition, EP 133 668 discloses a process for the continuous preparation of ionones, in which the pseudoionone is mixed with concentrated sulfuric acid, in a hydrocarbon which boils at 25 to 65.degree. C. under the reaction conditions, with vigorous mixing and evaporative cooling by partial or complete vaporization of the solvent, in such a way that the temperature of the reaction mixture is between 25 and 65.degree. C., and the residence time until the reaction mixture is diluted with water is 0.05 to 20 seconds. The disadvantage of this process, which functions very well in small systems, is that difficulties arise on transfer to the industrial scale.
Finally, EP 628 544 A1 discloses a process for preparing .beta.-ionone by sulfuric acid-catalyzed cyclization of pseudoionone in a two-phase solvent system consisting of concentrated sulfuric acid and a second solvent which is essentially immiscible with water, where liquid carbon dioxide under pressure is used as second solvent. The disadvantages of this process are that very high pressures and rather low temperatures must be used, and that the complexity of the apparatus for carrying out the process is considerable.
.beta.-Ionone is an essential precursor for the industrial preparation of vitamin A. In this case, a high content of .alpha.-ionone reduces the yield. Pure .alpha.-ionone and alkyl-substituted ionones are, on the other hand, in demand as fragrances in which a high content of .beta.-ionone would have an interfering effect.