Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-06-22
2001-08-14
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
Reexamination Certificate
active
06274772
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a continuous process for the manufacture of 3,5,5-trimethylcyclohexa-3-en-1-one, hereinafter &bgr;-isophorone, by isomerization of 3,5,5-trimethylcyclohexa-2-en-1-one, hereinafter &agr;-isophorone, in the liquid phase in the presence of a homogeneous catalyst.
BACKGROUND OF THE INVENTION
&bgr;-Isophorone is a synthetic intermediate in the manufacture of carotenoids, of vitamins, such as vitamin E, and of pharmaceuticals.
It is also directly involved in syntheses of fragrances and natural products, such as astaxanthin and abscisic acid and derivatives.
&bgr;-Isophorone is an isomer of &agr;-isophorone, obtained by trimerization of acetone in alkaline medium, which is distinguished from the latter by the position of the double bond: the double bond is no longer conjugated with the carbonyl, as represented hereinbelow:
The isomerization of &agr;-isophorone to &bgr;-isophorone is an equilibrium deconjugation reaction of the double bond and of the carbonyl and, for this reason, the thermodynamic equilibrium is weighted towards &agr;-isophorone.
Numerous processes for the isomerization of &agr;-isophorone to &bgr;-isophorone have been described but exhibit numerous disadvantages, such as high consumption of chemicals (in particular catalysts), mediocre yields or formation of &agr;-isophorone condensation products (heavy products), which results in a rise in the temperature of the reaction mixture,. thus accelerating the formation of heavy products, which destabilize the system.
Patent EP 832,871, Example 7, discloses a process for the preparation of &bgr;-isophorone by catalytic isomerization of &agr;-isophorone which consists, in a first step, in continuously extracting from the reaction mixture, by distillation, a primary mixture comprising from 20 to 22% of &bgr;-isophorone and in then isolating from this mixture, by distillation, a &bgr;-isophorone with a purity equal to approximately 98%. It is found, when proceeding in this way, that a mixture composed of approximately 90% of &agr;-isophorone and 10% of heavy products is collected in the recirculation evaporator. In view of the duration of operation (approximately 15 hours) and of the amount of product collected in the said evaporator (615 g), this corresponds to an hourly production of heavy products of approximately 4 g/h.
In addition, a large amount by weight of catalyst is used: 0.6% with respect to the &agr;-isophorone employed.
If the thermodynamics of the reaction are considered, it is found that the temperature at which isomerization is carried out fixes the concentration of &bgr;-isophorone at equilibrium: the higher the temperature, the greater the concentration at equilibrium of &bgr;-isophorone. As &bgr;-isophorone is more volatile than &agr;-isophorone, the equilibrium can be shifted by distillation of the most volatile product. In addition, &bgr;-isophorone, on heating, reverse isomerizes to &agr;-isophorone, even in the absence of catalyst: the kinetics of this thermal reverse isomerization are slow.
It is therefore advantageous to carry out the isomerization at high temperature.
The objective of obtaining pure &bgr;-isophorone (&bgr;-isophorone/&agr;-isophorone≧2 99%) cannot be achieved in a single column at the isomerization temperature. This is because the &bgr;-isophorone/&agr;-isophorone separation requires not only a column with many theoretical plates but also a high reflux ratio. The reflux ratio R is defined as being the ratio of the total amount of condensate obtained at the distillation column top to the amount of product extracted. The increase in the reflux ratio makes it possible to increase the concentration of the &bgr;-isophorone extracted up to a certain threshold. This is because a high reflux ratio increases the mean residence time of the &bgr;-isophorone in the distillation column, that is to say at high temperature, promoting thermal retrogression.
Thus, a high reflux ratio does not make it possible to produce &bgr;-isophorone referred to as pure.
Furthermore, to carry out the isomerization, it is necessary to pass through the intermediate enol or enolate, which are also intermediates in the aldolization/crotonization reaction, which results in the formation of isophorone polycondensates which constitute heavy products, the formation of which is to be prohibited since they represent a loss of isophorone in a process for the manufacture of &bgr;-isophorone. The formation of the heavy products is promoted, on the one hand, by the concentration of catalyst and, on the other hand, by the temperature.
Thus, the use of heterogeneous catalysts does not appear to be very favorable as it can increase the concentration of heavy products at the surface of the solid catalyst, which can deactivate the said catalyst.
SUMMARY OF THE INVENTION
A continuous process for the manufacture of 3,5,5-trimethylcyclohexa-3-en-1-one (&bgr;-isophorone) by isomerization under homogeneous catalysis of 5 3,5,5-trimethylcyclohexa-2-en-1-one (&agr;-isophorone), obtained by trimerization of acetone in alkaline medium, has now been found, the said process being characterized in that the following stages are carried out:
a) &agr;-isophorone and a solution of an alkaline hydroxide are continuously introduced into a reaction region,
b) the reaction mixture is brought to a temperature at least equal to 150° C. and preferably to a temperature ranging from 185° C. to 216° C.,
c) the following are continuously and simultaneously removed from the said reaction mixture:
1) by distillation, at a temperature between 150° C. and 216° C. and under a pressure ranging from 350 mbar to atmospheric pressure, a fraction comprising an amount by weight of &bgr;-isophorone ranging from 30% to 90% and preferably ranging from 50% to 70%; and
2) by drawing off, heavy products, so that their content is at most equal to 7% by weight in the reaction mixture,
d) the fraction distilled in c) 1) is rectified continuously and in parallel under reduced pressure at a temperature ranging from 100° C. to 150° C., so as to obtain &bgr;-isophorone with a purity as defined by the &bgr;-isophorone/&agr;-isophorone ratio of greater than 99%, and,
e) the bottoms from the rectification are continuously returned to the reaction region.
According to the present invention, the alkaline hydroxide is NaOH or KOH. It is preferable to use KOH. This alkaline hydroxide can be dissolved in water or an aliphatic alcohol of low molecular weight, such as methanol, ethanol, propanol or isopropanol. Methanol or ethanol is generally used.
According to the present invention, use is made of an amount by weight of alkaline hydroxide at most equal to 0.03% with respect to the &agr;-isophorone introduced into the reaction region.
Use will preferably be made of an amount by weight of alkaline hydroxide ranging from 0.010% to 0.020%.
The reflux ratio for the distillation of the fraction comprising the &bgr;-isophorone, the parameter which regulates the concentration of &bgr;-isophorone in his fraction, is chosen so as to minimize the thermal retrogression for a maximum &bgr;-isophorone purity.
According to the invention, a mixture comprising a percentage by weight of heavy products at most equal to 7%, the remainder being a mixture of &agr;- and &bgr;-isophorones, is continuously extracted. These heavy products are composed essentially of dimers and trimers of isophorone and also of catalytic residues. These heavy products can advantageously be introduced into a line for the manufacture &agr;-isophorone by basic catalysis, after the trimerization reaction of acetone and before the neutralization of the basic catalyst.
The continuous process according to the present invention, characterized by continuously drawing off the heavy products, exhibits the advantage, by limiting the concentration of the said heavy products in the boiler, of stabilizing the temperature of the isomerization.
Another advantage of the process of the invention is that it is not necessary to carry out expensive treatment operations on the said heavy products since t
Martino-Gauchi Georges
Teissier Remy
Atofina
Millen White Zelano & Branigan P.C.
Richter Johann
Witherspoon Sikarl A.
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