Method for Selective Hydrogenation of Citral to Synthesize Nerol and Geraniol

Abstract

The present invention belongs to the technical field of organic synthesis, and discloses a method for selectively hydrogenating citral to synthesize nerol and geraniol, which comprises the following steps: mixing raw material citral and a transition metal catalyst in a solvent, introducing hydrogen, and carrying out a hydrogenation reaction under alkaline or acidic conditions by heating and pressurizing to generate nerol and geraniol; after the reaction is completed, cooling the reaction solution, evaporating the solvent, and then performing rectification to obtain finished products of nerol and geraniol. The present invention uses an efficient transition metal catalyst to selectively hydrogenate the carbon-oxygen double bond of citral, and prepares nerol and geraniol with a relatively high conversion rate and selectivity. The operation of the present invention is simple, and it has a very high industrial application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of organic synthesis, and relates to a method for synthesizing nerol and geraniol, and particularly relates to a method for selectively hydrogenating citral to synthesize nerol and geraniol. Background Art

[0002] Citral is an α,β-unsaturated aldehyde with a polyene structure, which exists in Cymbopogon winterianus oil and Litsea cubeba oil. Natural citral is a mixture composed of neral and geranial, and its selective hydrogenation products are nerol and geraniol. Nerol and geraniol are cis-trans isomers of each other, and are colorless to light yellow oily liquids, which naturally exist in neroli oil, geranium oil, citronella oil, palmarosa oil, rose oil, etc., and have a soft and sweet rose aroma, and have a wide range of functions and application values. As a precious fragrance, it is widely used in the preparation of flavors such as food, soap and daily cosmetics. It is an important fragrance and pharmaceutical intermediate, and is used as a key raw material for manufacturing citronellol, vanillin, ionone and vitamin A, etc. It can also be used as medicine for antibacterial and anthelmintic, and has a relatively significant therapeutic effect on chronic bronchitis clinically, with advantages such as fast onset and small side effects.

[0003] Nerol and geraniol are products obtained by selectively hydrogenating the C=O double bond of citral. Citral has two C=C double bonds and one C=O double bond, and the bond energy of the C=O double bond is greater than that of the C=C double bond. Therefore, the difficulty of the method for selectively hydrogenating citral to synthesize nerol and geraniol lies in that it is difficult to meet the high requirements for selectivity and conversion rate. By-products such as citronellol and dihydrocitronellol will be produced during the reaction process. Since the boiling points of the products and by-products are close, they are not easy to separate, which brings difficulties to production.

[0004] At present, the methods for selectively hydrogenating citral to synthesize nerol and geraniol mainly focus on the research and development of various types of catalysts to selectively hydrogenate the C=O double bond without hydrogenating the C=C double bond. The most reported is the use of supported catalysts for hydrogenation synthesis of nerol and geraniol. For example, CNl01747152 reported platinum supported on iron oxide as a catalyst, and CN02155367.X reported a ruthenium catalyst doped with iron supported on carbon.

[0005] In addition, CN110963888 describes the synthesis of MOFs materials from ruthenium salts and other metal salts and organic ligands by solvothermal method, and introducing a passivating component Si as a catalyst by gas-phase or liquid-phase deposition method. The passivating component Si weakens the catalytic activity of the outer surface, and at the same time changes the pore structure of the metal framework catalyst, enhancing the space-selective ability of MOFs. Due to the small steric hindrance of the terminal C=O double bond in citral, the C=O double bond in the citral molecule is preferentially adsorbed on the metal active center, so as to achieve the purpose of increasing the contents of nerol and geraniol in the hydrogenation product of citral. The conversion rate of citral can reach 99.5%, and the total selectivity of nerol and geraniol is more than 98.9%. However, the preparation of the catalyst by this method is complex and time-consuming, and the amount of catalyst used is large (the molar ratio of citral to catalyst is 100:1 to 5).

[0006] CN111925275 describes a method for preparing zirconium 2-hydroxyphosphonoacetate (Zr-HPAA) catalyst by a solvent method for selectively catalytic hydrogenation of citral to geraniol. This reaction is a heterogeneous catalytic reaction system. The selective hydrogenation of citral to geraniol does not use hydrogen, and isopropyl alcohol is used as a hydrogen donor, and a large amount of solvent is required. This reaction has high requirements for temperature, which has an important impact on the conversion rate and selectivity. At the same time, the catalyst is prepared by a solvent method, and the process is relatively complex and time-consuming, and the amount of catalyst used is large. Summary of the Invention

[0007] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a method for selectively hydrogenating citral to synthesize nerol and geraniol. The present invention uses an efficient transition metal catalyst to selectively hydrogenate the carbon-oxygen double bond of citral, and prepares nerol and geraniol with a high conversion rate and selectivity. The operation of the present invention is simple and has a high industrial application prospect.

[0008] The present invention is realized by the following technical solutions:

[0009] A method for selectively hydrogenating citral to synthesize nerol and geraniol, comprising the following steps:

[0010] Mix the raw material citral and the transition metal catalyst in a solvent, introduce hydrogen, and carry out a hydrogenation reaction under alkaline or acidic conditions by heating and pressurizing to generate nerol and geraniol. After the reaction is completed, the reaction solution is cooled and the solvent is evaporated, and then rectified to obtain the finished products of nerol and geraniol; the transition metal catalyst is composed of a metal nickel salt or a copper salt and an organic ligand.

[0011] The chemical reaction equation is as follows:

[0012]

[0013] A further improvement of the present invention is:

[0014] The metal nickel salt is one or a mixture of two or more of nickel bromide, nickel chloride, nickel carbonate, nickel acetate tetrahydrate, anhydrous nickel acetate, nickel acetylacetonate, tetrakis(triphenylphosphine)nickel or nickel trifluoromethanesulfonate.

[0015] Furthermore, the copper salt is one or a mixture of two or more of copper nitrate, bis(triphenylphosphine)copper nitrate, cuprous chloride, copper chloride, copper bromide, copper trifluoromethanesulfonate, copper acetate or copper acetylacetonate.

[0016] Furthermore, the organic ligand is one or a mixture of two or more of triphenylphosphine, 1,2-bis(diphenylphosphino)ethane, 1,4-bis(diphenylphosphino)butane, 1,1'-bis(diphenylphosphino)ferrocene, (±)-2,2'-bis-(diphenylphosphino)-1,1'-binaphthalene or rac-QuinoxP.

[0017] Furthermore, the acidic condition is adding one or a mixture of two or more of formic acid, acetic acid or propionic acid to the reaction system.

[0018] Furthermore, the basic condition is adding one or a mixture of two or more of potassium hydroxide, sodium hydroxide, lithium hydroxide, potassium tert-butoxide, sodium tert-butoxide, lithium tert-butoxide, potassium carbonate, sodium carbonate, cesium carbonate or sodium methoxide to the reaction system.

[0019] Furthermore, the solvent is one or a mixture of two or more of methanol, ethanol, n-propanol, isopropanol, 2,2,2-trifluoroethanol, hexafluoroisopropanol, toluene, tetrahydrofuran or 1,2-dichloroethane.

[0020] Furthermore, the molar ratio of the transition metal to the organic ligand in the transition metal catalyst is 1-5:1; the molar ratio of citral to the organic ligand is 5000:1-5.

[0021] Furthermore, when the reaction is carried out under acidic conditions, the volume ratio of the solvent to the acid is 2-100:1; when the reaction is carried out under basic conditions, the molar ratio of the organic ligand to the base is 1:2-25.

[0022] Furthermore, the temperature of the hydrogenation reaction is 25-80 °C, the time is 12-24 h, and the hydrogen pressure is 20-60 MPa.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] The transition metal nickel catalyst or copper catalyst is a catalyst based on abundant metals. The raw materials are easily available, and the pre-prepared catalyst can be obtained by in-situ generation or simple treatment. It can achieve good conversion rates with a small amount of catalyst, and at the same time, the selectivity for nerol and geraniol is relatively high. It has the advantages of simple operation, low requirements for equipment, and high product quality, and has great prospects for industrial application. Embodiment

[0025] The present invention will be described in detail below with reference to specific embodiments. Example 1

[0026] In a nitrogen glove box, citral (1 mmol), nickel acetate tetrahydrate (0.5 mol%), rac -QuinoxP (0.5 mol%), trifluoroethanol / acetic acid (2 mL / 0.5 mL) were added to a 10 mL reaction flask, and the reaction flask was placed in a high-pressure reactor. After the autoclave was removed from the glove box, it was pressurized and purged three times with hydrogen (10 bar) to displace the gas, and then pressurized with hydrogen to 20 bar. The autoclave was placed in an oil bath at 40 °C and reacted for 12 h. After the reaction was completed, the temperature was lowered, and hydrogen was slowly released. The reaction solution was detected by GC. The conversion rate of citral was 98%, and the selectivity of the product was 97%. Example 2

[0027] In a nitrogen glove box, citral (1 mmol), nickel trifluoromethanesulfonate (0.5 mol%), triphenylphosphine (0.2 mol%), trifluoroethanol / acetic acid (2.3 mL / 0.2 mL) were added to a 10 mL reaction flask, and the reaction flask was placed in a high-pressure reactor. After the autoclave was removed from the glove box, it was pressurized and purged three times with hydrogen (10 bar) to displace the gas, and then pressurized with hydrogen to 40 bar. The autoclave was placed in an oil bath at 40 °C and reacted for 18 h. After the reaction was completed, the temperature was lowered, and hydrogen was slowly released. The reaction solution was detected by GC. The conversion rate of citral was 91%, and the selectivity of the product was 94%. Example 3

[0028] In a nitrogen glove box, citral (1 mmol), nickel chloride (0.1 mol%), 1,4-bis(diphenylphosphino)butane (0.1 mol%), 2,2,2-trifluoroethanol / formic acid (1.6 mL / 0.8 mL) were added to a 10 mL reaction flask, and the reaction flask was placed in a high-pressure reactor. After the autoclave was removed from the glove box, it was pressurized and purged three times with hydrogen (10 bar) to displace the gas, and then pressurized with hydrogen to 50 bar. The autoclave was placed in an oil bath at 60 °C and reacted for 20 h. After the reaction was completed, the temperature was lowered, and hydrogen was slowly released. The reaction solution was detected by GC. The conversion rate of citral was 95%, and the selectivity of the product was 98%. Example 4

[0029] In a nitrogen glove box, citral (5 mmol), nickel acetate (0.1 mol%), 1,1'-bis(diphenylphosphino)ferrocene (0.1 mol%), trifluoroethanol / acetic acid (10 mL / 0.1 mL) were added to a 50 mL reaction flask, and the reaction flask was placed in a high-pressure reactor. After the autoclave was removed from the glove box, it was pressurized and purged with hydrogen (10 bar) three times to displace the gas, and then pressurized with hydrogen to 40 bar. The autoclave was placed in an oil bath at 50 °C and reacted for 20 h. After the reaction was completed, the temperature was lowered, and hydrogen was slowly released. The reaction solution was detected by GC. The conversion rate of citral was 90%, and the selectivity of the product was 95%. Example 5

[0030] In a nitrogen glove box, citral (5 mmol), nickel acetylacetonate (0.1 mol%), (±)-2,2'-bis-(diphenylphosphino)-1,1'-binaphthalene (0.05 mol%), trifluoroethanol / propionic acid (10 mL / 2 mL) were added to a 50 mL reaction flask, and the reaction flask was placed in a high-pressure reactor. After the autoclave was removed from the glove box, it was pressurized and purged with hydrogen (10 bar) three times to displace the gas, and then pressurized with hydrogen to 60 bar. The autoclave was placed in an oil bath at 80 °C and reacted for 24 h. After the reaction was completed, the temperature was lowered, and hydrogen was slowly released. The reaction solution was detected by GC. The conversion rate of citral was 94%, and the selectivity of the product was 97%. Example 6

[0031] In a nitrogen glove box, citral (10 mmol), copper acetate (0.1 mol%), 1,4-bis(diphenylphosphino)butane (0.1 mol%), sodium hydroxide (1 mol%), ethanol (15 mL) were added to a 50 mL reaction flask, and the reaction flask was placed in a high-pressure reactor. After the autoclave was removed from the glove box, it was pressurized and purged with hydrogen (10 bar) three times to displace the gas, and then pressurized with hydrogen to 20 bar. The autoclave was placed in an oil bath at 50 °C and reacted for 16 h. After the reaction was completed, the temperature was lowered, and hydrogen was slowly released. The reaction solution was detected by GC. The conversion rate of citral was 97%, and the selectivity of the product was 93%. Example 7

[0032] In a nitrogen glove box, citral (10 mmol), copper trifluoromethanesulfonate (0.1 mol%), 1,4-bis(diphenylphosphino)butane (0.1 mol%), potassium tert-butoxide (1 mol%), and methanol (15 mL) were added to a 50 mL reaction flask, and the reaction flask was placed in a high-pressure reactor. After the high-pressure reactor was removed from the glove box, it was pressurized and purged three times with hydrogen (10 bar) to displace the gas, and then pressurized with hydrogen to 20 bar. The high-pressure reactor was placed in an oil bath at 50 °C and reacted for 18 h. After the reaction was completed, the temperature was lowered, and hydrogen was slowly released. The reaction solution was detected by GC. The conversion rate of citral was 95%, and the selectivity of the product was 94%. Example 8

[0033] In a nitrogen glove box, citral (10 mol), copper chloride (0.1 mol%), 1,4-bis(diphenylphosphino)butane (0.1 mol%), sodium methoxide (0.5 mol%), and ethanol (15 mL) were added to a 50 mL reaction flask, and the reaction flask was placed in a high-pressure reactor. After the high-pressure reactor was removed from the glove box, it was pressurized and purged three times with hydrogen (10 bar) to displace the gas, and then pressurized with hydrogen to 50 bar. The high-pressure reactor was placed in an oil bath at 50 °C and reacted for 24 h. After the reaction was completed, the temperature was lowered, and hydrogen was slowly released. The reaction solution was detected by GC. The conversion rate of citral was 97%, and the selectivity of the product was 95%. Example 9

[0034] In a nitrogen glove box, citral (10 mol), copper acetylacetonate (0.2 mol%), (±)-2,2'-bis-(diphenylphosphino)-1,1'-binaphthalene (0.1 mol%), sodium methoxide (2 mol%), and tetrahydrofuran (20 mL) were added to a 50 mL reaction flask, and the reaction flask was placed in a high-pressure reactor. After the high-pressure reactor was removed from the glove box, it was pressurized and purged three times with hydrogen (10 bar) to displace the gas, and then pressurized with hydrogen to 50 bar. The high-pressure reactor was placed in an oil bath at 40 °C and reacted for 18 h. After the reaction was completed, the temperature was lowered, and hydrogen was slowly released. The reaction solution was detected by GC. The conversion rate of citral was 95%, and the selectivity of the product was 92%.

[0035] Example 10

[0036] In a nitrogen glove box, citral (10 mol), bis(triphenylphosphine) copper nitrate (0.1 mol%), 1,4-bis(diphenylphosphino)butane (0.02 mol%), sodium hydroxide (0.5 mol%), and ethanol (15 mL) were added to a 50 mL reaction flask, and the reaction flask was placed in a high-pressure reactor. After the autoclave was removed from the glove box, it was pressurized and purged three times with hydrogen (10 bar) to displace the gas, and then pressurized with hydrogen to 50 bar. The autoclave was placed in an oil bath at 60 °C and reacted for 24 h. After the reaction was completed, the temperature was lowered, and hydrogen was slowly released. The reaction solution was detected by GC. The conversion rate of citral was 96%, and the selectivity of the product was 95%.

[0037] The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose is to enable those who are familiar with this technology to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. All equivalent transformations or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for selectively hydrogenating citral to synthesize nerol and geraniol, characterized in that, It includes the following steps: Mix the raw material citral and the transition metal catalyst in a solvent, introduce hydrogen, and under acidic conditions, increase the temperature and pressure to carry out a hydrogenation reaction to produce nerol and geraniol. After the reaction ends, the reaction solution is cooled and the solvent is distilled off, and then rectified to obtain the finished products of nerol and geraniol; the transition metal catalyst is composed of a metal nickel salt and an organic ligand; The metal nickel salt is one or a mixture of two or more of nickel bromide, nickel chloride, nickel carbonate, nickel acetate tetrahydrate, anhydrous nickel acetate, nickel acetylacetonate, tetrakis(triphenylphosphine)nickel or nickel trifluoromethanesulfonate; The organic ligand is one or a mixture of two or more of triphenylphosphine, 1,2-bis(diphenylphosphino)ethane, 1,4-bis(diphenylphosphino)butane, 1,1'-bis(diphenylphosphino)ferrocene, (±)-2,2'-bis-(diphenylphosphino)-1,1'-binaphthalene or rac-QuinoxP; 2. The method for selectively hydrogenating citral to synthesize nerol and geraniol according to claim 1, wherein: The acidic condition is to add one or a mixture of two or more of formic acid, acetic acid or propionic acid to the reaction system.

3. The method for selectively hydrogenating citral to synthesize nerol and geraniol according to claim 1, characterized in that: The solvent is one or a mixture of two or more of methanol, ethanol, n-propanol, isopropanol, 2,2,2-trifluoroethanol, hexafluoroisopropanol, toluene, tetrahydrofuran or 1,2-dichloroethane.

4. The method for selectively hydrogenating citral to synthesize nerol and geraniol according to claim 1, characterized in that: The molar ratio of the transition metal to the organic ligand in the transition metal catalyst is 1 to 5:1; the molar ratio of citral to the organic ligand is 5000:1 to 5.

5. The method for selectively hydrogenating citral to synthesize nerol and geraniol according to claim 1, wherein: The volume ratio of the solvent to the acid is 2 to 100:

1.

6. The method for selectively hydrogenating citral to synthesize nerol and geraniol according to claim 1, wherein: The temperature of the hydrogenation reaction is 25 to 80 °C, the time is 12 to 24 h, and the hydrogen pressure is 20 - 60 MPa.

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