A method for preparing citral

Abstract

The application provides a preparation method of citral. By controlling the content of isopentenoic acid in a citral intermediate composition in the range of 10-2000 ppm, and then performing a thermal rearrangement reaction, citral product is prepared. The method can effectively reduce the selectivity of isomerization side reactions, and the yield of the final product citral is increased to more than 95%.

Description

Technical Field

[0001] This invention belongs to the field of chemical synthesis technology, specifically relating to a method for preparing citral. Background Technology

[0002] Citral, also known as 2,6-dimethyl-3,7-octadien-1-aldehyde, is a widely used fragrance ingredient. It is also widely used as a fragrance raw material in daily chemical products such as dishwashing detergents, soaps, and toilet waters. At the same time, it is a key chemical intermediate and a raw material for the synthesis of ionones, methyl ionones, dihydrodamascone, and other fragrances. In nutritional chemicals, it is used to manufacture vitamins A and E.

[0003] The organic synthesis of citral is technically challenging and is considered an oligopolistic chemical within the fragrance industry. BASF (Germany) described a method in EP0021074A1 and WO2008037693 A1 involving the pyrolysis of 3-methyl-2-butenal diisoprendyl acetal under high temperature and acidic conditions, followed by thermal rearrangement of the citral-containing intermediate to obtain citral. However, this method produces numerous byproducts and has a low rearrangement yield; the addition of medium-boiling agents and inhibitors is necessary to improve the rearrangement yield. RHONEPOULENC SANTE (France) described a method in EP0344043 involving the pyrolysis of 3-methyl-2-butenal diisoprendyl acetal under halide conditions in a vacuum distillation column, using hydroquinone as an auxiliary agent to suppress pyrolysis side reactions, followed by thermal rearrangement of the citral-containing intermediate to obtain citral. However, this method has a long process flow and, while improving the pyrolysis yield, the rearrangement yield remains low. Chinese patent CN112058313B discloses an amine-based composite metal supported catalyst for catalyzing the condensation, cracking, and rearrangement reactions of C5 aldehydes and C5 alcohols and citral intermediates. It mentions that the yield of the rearrangement reaction can reach more than 97%. However, according to the nature of amine catalysts, the number of times they can be reused is not high, and they are easily deactivated by the blockage of the internal catalyst channels by the by-product citral heavy component.

[0004] Therefore, it would be of great significance to effectively improve the reaction yield of citral intermediate to citral through thermal rearrangement. Summary of the Invention

[0005] The mainstream synthesis process for citral involves a two-step process: pyrolysis and rearrangement of 3-methyl-2-butenal diisoprene acetal. Pyrolysis of 3-methyl-2-butenal diisoprene acetal yields a composition containing citral intermediates. This composition inevitably contains trace components. Separation and purification of this composition can increase the total purity of the new composition to approximately 99.0 wt%. The inventors of this application have discovered that the key component affecting the rearrangement reaction to obtain citral from the new composition is isoprene acid, with the following structure: This component originates from the synthesis of 3-methyl-2-butenal diisoprendyl acetal, and its boiling point is comparable to that of citral intermediates during subsequent separation, making separation relatively difficult. Furthermore, the presence of isoprenic acid causes isomerization of the citral intermediate, generating isocitral, which adversely affects the selectivity of the rearrangement reaction of the citral intermediate. Further research by the inventors revealed that by controlling the content of the key component, isoprenic acid, within a certain range, the occurrence of side reactions can be reduced, thus improving the yield of the rearrangement reaction, thereby completing this invention.

[0006] The purpose of this invention is to provide a method for preparing citral, which effectively reduces the selectivity of isomerization side reactions and improves the yield of the final product citral by controlling the content of isopentenyl acid in the citral intermediate composition within a certain range.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0008] A method for preparing citral, using a citral intermediate composition as raw material, and preparing the product citral through a thermal rearrangement reaction, wherein the content of isoprenic acid in the citral intermediate composition does not exceed 2000 ppm, preferably 10-2000 ppm, for example 10 ppm, 15 ppm, 20 ppm, 25 ppm, 30 ppm, 35 ppm, 40 ppm, 50 ppm, 60 ppm, 70 ppm, 80 ppm, 90 ppm, 100 ppm, 150 ppm, 200 ppm. 250ppm, 300ppm, 350ppm, 400ppm, 450ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, 1600ppm, 1700ppm, 1800ppm, 1900ppm, 2000ppm, preferably 20 to 1200ppm, more preferably 40 to 400ppm.

[0009] The citral intermediate composition of this invention is obtained by pyrolyzing 3-methyl-2-butenal diisopreneyl acetal to obtain a crude citral intermediate composition, which is then separated and purified to a purity of over 99.0%. The content of the key component, isoprene acid, is then controlled within the aforementioned range. The isoprene acid content in the purified crude citral intermediate composition typically exceeds 2000 ppm and needs to be reduced to 10–2000 ppm. In practice, the lower the isoprene acid content, the smaller the impact on the selectivity of the subsequent rearrangement reaction. However, controlling it below 10 ppm often requires higher costs and is less economical. Therefore, this invention preferably controls the isoprene acid content between 10 and 2000 ppm. Those skilled in the art should understand that values ​​below 10 ppm are also within the scope of this invention. This invention does not impose any particular limitations on the method for reducing the isoprene acid content; conventional methods such as distillation and adsorption can be used, but this is not a limitation. Any method that can control the isoprene acid content within the aforementioned range is acceptable. Typically, purification is achieved through distillation, with a distillation temperature of 70–110°C, a pressure of 400–1500 PaA, and a reflux ratio of 2:1–5:1.

[0010] The citral intermediate composition of the present invention mainly comprises component A and component B, except for a small amount of isopentenylic acid, and their structural formulas are as follows: The corresponding chemical names are, in order: 3-methyl-1-(3-methyl-2-butenoxy)-1,3-butadiene, 3-methyl-2-(1,1-dimethyl-2-butenyl)3-butenal. In the citral intermediate composition of the present invention, the total mass percentage of component A and component B is not less than 99%, wherein the content of component A and component B is approximately 1:1, which is well known to those skilled in the art.

[0011] In this invention, a citral intermediate composition undergoes thermal rearrangement to obtain the citral product, specifically through a two-step thermal rearrangement reaction involving claisen and cope rearrangement. The two-step thermal rearrangement reaction is not particularly limited in this invention; existing processes can be used. Specifically, for example, the claisen and cope rearrangement reactions of the citral intermediate composition are carried out in series in the same reactor; for example, the reaction temperature of the claisen and cope rearrangement reactions of the citral intermediate composition is 50–200°C, including but not limited to 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, and 200°C. 0°C, preferably 100–200°C; for example, the pressure of the two-step thermal rearrangement reaction of the citral intermediate composition involving claisen rearrangement and cope rearrangement is 10–50 kPaA, for example, including but not limited to 10 kPaA, 20 kPaA, 30 kPaA, 40 kPaA, 50 kPaA, preferably 20–30 kPaA; for example, the reaction time of the two-step thermal rearrangement reaction is 20–50 min, for example, 20 min, 30 min, 40 min, 50 min, etc., preferably 25–40 min.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] The method for preparing citral of the present invention, by controlling the content of the key component isopentenylic acid, greatly reduces the isomerization side reaction and significantly increases the yield of the rearrangement main reaction to over 95%. Detailed Implementation

[0014] The method provided by the present invention will be further illustrated by the following embodiments, but the present invention is not limited to the listed embodiments and should also include any other known modifications within the scope of the claims of the present invention.

[0015] Raw material source:

[0016] The citral intermediate composition was obtained from Wanhua Chemical Group Co., Ltd.

[0017] The purity and isopentenyl acid content of the raw materials in this invention were determined by gas chromatography.

[0018] Analytical instruments:

[0019] GC gas chromatograph: Agilent 7890, column DB-5, injection port temperature: 260℃; split ratio 40:1; carrier gas flow rate: 30ml / min; temperature program: 80~230℃, 3℃ / min; detector temperature: 280℃.

[0020] Example 1

[0021] A glass condenser was connected to the top of a 1000mL glass reactor, through which circulating cooling water was introduced. 700g of a citral intermediate composition was added to the reactor, in which the citral intermediate content was 99.5% and the content of the key component isoprenic acid was 10ppm. Nitrogen was used to purge the reactor three times, and the reaction pressure was maintained at 20kPaA. Stirring was started, and the temperature was raised to 200℃. The product yield of citral was continuously monitored (every 10min). The highest yield was 97.5%, and the selectivity of isomerization side reaction was 2.2%.

[0022] Example 2

[0023] A 1000mL glass reactor was connected to a glass condenser tube at the top, through which circulating cooling water was introduced. 700g of a citral intermediate composition was added to the reactor, containing 99.4% citral and 40ppm isoprenic acid (a key component). The reactor was purged with nitrogen three times, maintaining a reaction pressure of 20 kPaA. Stirring was started, and the temperature was raised to 200℃. Continuous dynamic monitoring (10 min / time) was performed. The highest citral product yield was 97.1%, with a selectivity of 2.4% for isomerization side reactions.

[0024] Example 3

[0025] A 1000mL glass reactor was connected to a glass condenser tube at the top, through which circulating cooling water was introduced. 700g of a citral intermediate composition was added to the reactor, containing 99.2% citral and 400ppm isoprenic acid (a key component). The reactor was purged with nitrogen three times, maintaining a reaction pressure of 20 kPaA. Stirring was started, and the temperature was raised to 200℃. Continuous dynamic monitoring (10 min / time) was performed. The highest citral product yield was 96.8%, with a selectivity of 2.7% for isomerization side reactions.

[0026] Example 4

[0027] A 1000mL glass reactor was connected to a glass condenser tube at the top, through which circulating cooling water was introduced. 700g of a citral intermediate composition was added to the reactor, containing 99.2% citral and 1200ppm isoprenic acid (a key component). The reactor was purged with nitrogen three times, maintaining a reaction pressure of 20 kPaA. Stirring was started, and the temperature was raised to 200℃. Continuous dynamic monitoring (every 10 minutes) was performed. The highest citral product yield was 96.5%, with a selectivity of 3.0% for isomerization side reactions.

[0028] Example 5

[0029] A 1000mL glass reactor was connected to a glass condenser tube at the top, through which circulating cooling water was introduced. 700g of a citral intermediate composition was added to the reactor, containing 99.1% citral and 2000ppm isoprenic acid (a key component). The reactor was purged with nitrogen three times, maintaining a reaction pressure of 20 kPaA. Stirring was started, and the temperature was raised to 200℃. Continuous dynamic monitoring (every 10 minutes) was performed. The highest citral product yield was 96.2%, with a selectivity of 3.5% for isomerization side reactions.

[0030] Comparative Example 1

[0031] A glass condenser was connected to the top of a 1000mL glass reactor, through which circulating cooling water was introduced. 700g of a citral intermediate composition was added to the reactor, containing 99.7% citral intermediate and 2500ppm isoprenic acid, a key component. The reactor was purged with nitrogen three times, and the reaction pressure was maintained at 20kPaA. Stirring was started, and the temperature was raised to 200℃. Continuous dynamic monitoring (10min / time) was performed. The highest yield of citral product was 94.6%, and the selectivity of isomerization side reactions was 5.2%.

[0032] Compared to Example 5, although the total purity of the citral intermediate composition was higher and other trace components were less, the content of the key component isopentenyl acid was higher than the control value of the present invention, resulting in a decrease in the yield of the final product citral and an increase in the selectivity of isomerization side reactions.

Claims

1. A method for preparing citral, characterized in that, Using a citral intermediate composition as raw material, citral is prepared through a thermal rearrangement reaction. The citral intermediate composition is obtained by cracking 3-methyl-2-butenal diisoprendyl acetal to obtain a crude citral intermediate composition, which is then separated and purified to increase its purity to more than 99.0% by mass, with the content of isoprenic acid being 10-2000 ppm. In addition to isopentenyl acid, the citral intermediate composition also includes component A and component B, whose structural formulas are as follows:

2. The preparation method according to claim 1, characterized in that, The citral intermediate composition contains 20 to 1200 ppm of isoprenic acid.

3. The preparation method according to claim 2, characterized in that, The citral intermediate composition contains 40 to 400 ppm of isoprenic acid.

4. The preparation method according to claim 1, characterized in that, The total mass percentage of components A and B in the citral intermediate composition is not less than 99%.

5. The preparation method according to claim 4, characterized in that, The citral intermediate composition was subjected to a two-step thermal rearrangement reaction of claisen rearrangement and cope rearrangement to obtain the citral product.

6. The preparation method according to claim 5, characterized in that, The two-step thermal rearrangement reaction is carried out in series in the same reactor.

7. The preparation method according to claim 5, characterized in that, The reaction temperature for the two-step thermal rearrangement reaction is 50–200 °C.

8. The preparation method according to claim 7, characterized in that, The reaction temperature for the two-step thermal rearrangement reaction is 100–200°C.

9. The preparation method according to claim 5, characterized in that, The reaction pressure for the two-step thermal rearrangement reaction is 10–50 kPaA.

10. The preparation method according to claim 9, characterized in that, The reaction pressure for the two-step thermal rearrangement reaction is 20–30 kPaA.

11. The preparation method according to claim 5, characterized in that, The reaction time for the two-step thermal rearrangement reaction is 20–50 min.

12. The preparation method according to claim 11, characterized in that, The reaction time for the two-step thermal rearrangement reaction is 25–40 min.

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