CN110922316B

CN110922316B - Method for preparing L-menthone from R-citronellal

Info

Publication number: CN110922316B
Application number: CN201911223677.0A
Authority: CN
Inventors: 王亚新; 王联防; 董菁; 李文滨; 孙启魁; 迟森森; 曹娜; 程英; 王文
Original assignee: Wanhua Chemical Group Co Ltd
Current assignee: Wanhua Chemical Group Co Ltd
Priority date: 2019-12-04
Filing date: 2019-12-04
Publication date: 2022-11-08
Anticipated expiration: 2039-12-04
Also published as: CN110922316A

Abstract

The invention discloses a method for preparing L-menthone from R-citronellal, wherein the R-citronellal is subjected to heterogeneous catalytic reaction under the action of a Pd-Co-MOF-MMT catalyst to generate the L-menthone, the conversion rate of the R-citronellal is 90-99.9%, the yield of the L-menthone can reach 85-98%, and the ee value of the menthone can reach 95-99.99%.

Description

Method for preparing L-menthone from R-citronellal

Technical Field

The invention relates to a method for preparing L-menthone from R-citronellal, belonging to the field of organic chemical synthesis.

Background

Menthone, also known as menthone, has the cooling characteristic aroma of natural mint. Menthone exists in the form of two stereoisomers: menthone and isomenthone, each of which exists in the form of two enantiomers, are mainly used for preparing mint-type essences, and the preparation methods disclosed at present include the following:

U.S. Pat. No. 3,3124614 reports that menthone can be obtained by hydrogenating thymol under the action of Pd catalyst, but the source of thymol as a raw material is in short supply, which causes great limitation on continuous production, and the reaction has high requirements on equipment materials and harsh reaction conditions, which causes high equipment cost.

Chinese published patent CN104603095A employs a metal complex containing a phosphine ligand as a catalyst. The process can achieve a menthone yield of more than 85%, but cannot achieve high revolution per minute (TON), has short catalyst life, and is not suitable for industrial synthesis of menthone in consideration of high cost of the catalyst.

Chinese published patent CN106068160A describes a ruthenium-phenol catalyst for transfer hydrogenation reaction, and the catalyst has excellent performance in transfer hydrogenation reaction, and the catalyst is used for preparing menthone from isopulegol, and has higher conversion rate and selectivity. However, the process has a limited increase in the number of revolutions per minute (TON), the catalyst life is still short, and a large amount of phenol derivative is used, which has adverse effects on the post-treatment and the environment. Meanwhile, the process cannot well solve the problem of poor purity of the L-menthone, and the complexity of the whole process is increased.

Chinese published patent CN106061933A discloses a method for preparing menthone by contacting isopulegol in gas phase with an activated oxidized copper catalyst, firstly, the copper catalyst needs to be activated in advance in the method, the activation effect has a large influence on the reaction yield, so the quality difference of different batches of products is large, and the process is not suitable for large-scale industrial production.

Therefore, a method which is simple in process, mild in reaction conditions, economical, efficient, environment-friendly and easy to realize industrialization is urgently needed to realize the preparation of menthone.

Disclosure of Invention

The invention aims to provide a method for preparing high-purity L-menthone, so that a plurality of problems in the existing menthone preparation process are solved. The method for directly preparing the L-menthone from the R-citronellal greatly reduces reaction steps, optimizes a reaction process, can recover a catalyst in a convenient mode, has a simpler reaction process, lower reaction cost and good environmental friendliness, does not need hydrogen through a hydrogenation transfer process, and has better process safety and industrial prospect.

The term "menthone", if not otherwise specified, refers to any possible stereoisomer, including:

wherein the R-citronellal has the following structure:

the structure of the L-menthone is as follows:

the structure of the D-menthone is as follows:

if not otherwise stated, the term "ee value" means an enantiomeric excess, which means the excess of one enantiomer over the other, and is used herein to refer to the difference in the percentage of L-menthone in the gas phase compared to D-menthone.

In order to achieve the purpose of the invention, the technical scheme adopted by the invention is as follows:

a method for preparing L-menthone from R-citronellal is characterized in that under the action of a Pd-Co-MOF-MMT catalyst, the R-citronellal is subjected to a heterogeneous catalytic reaction to generate the L-menthone.

The ee value of the R-citronellal ranges from 95 to 99.99 percent, and is preferably more than 98 percent.

The high ee value means that the ee value can reach 95 to 99.99 percent.

In the invention, the mole ratio of palladium element to cobalt element in the Pd-Co-MOF-MMT catalyst is 1-1.5, preferably 1; the construction monomer of the MOF is methyl p-aminobenzoate, the molar ratio of methyl p-aminobenzoate to palladium element is 1-3, preferably 1.8-2.2; the mass ratio of the methyl p-aminobenzoate to the kaolin is 1-4: 1, preferably 1.5 to 2.

In the method, the preparation method of the Pd-Co-MOF-MMT catalyst comprises the following steps:

(1) Adding methyl p-aminobenzoate and kaolin into ethanol and water for ultrasonic dissolution, stirring for 20-24 h at the temperature of 55-65 ℃, then performing suction filtration and washing, and performing vacuum drying to obtain an MMT precursor;

(2) Putting the palladium metal precursor, the cobalt metal precursor and the MMT precursor into N, N-dimethylformamide for ultrasonic treatment until all the precursors are dissolved, dropwise adding water, pouring the obtained solution into a reaction kettle, heating for reaction for 45-50 h, cooling to 0-5 ℃ by a program, filtering crystals by using a centrifugal machine, washing, and drying in vacuum to obtain a catalyst finished product.

As a preferable scheme, the preparation method of the Pd-Co-MOF-MMT catalyst comprises the following steps:

adding a certain proportion of methyl p-aminobenzoate and kaolin (MMT) into a proper amount of ethanol and deionized water for ultrasonic dissolution, stirring at 55-65 ℃ for 20-24 h, then carrying out suction filtration and washing, and drying in a vacuum oven at 55-65 ℃ for 5-8 h to obtain the MMT precursor.

Putting a palladium metal precursor, a cobalt metal precursor and an MMT precursor into N, N-dimethylformamide according to a certain proportion, performing ultrasonic treatment until all the precursors are dissolved, dropwise adding a proper amount of deionized water, pouring the obtained solution into a high-pressure reaction kettle with a proper volume and a polytetrafluoroethylene lining, screwing the solution, putting the solution into a program-controlled oven, heating and reacting at a certain temperature for 45-50 h, performing program cooling to 0-5 ℃, filtering crystals by using a centrifugal machine, washing by pure water, drying at 45-55 ℃ for 12-15 h in a vacuum drying oven, and cooling to room temperature to obtain a catalyst finished product.

As a preferable scheme, the mass ratio of the methyl p-aminobenzoate to the kaolin (MMT) is 1 to 4:1, preferably 1.5 to 2.

As a preferable scheme, the mass ratio of the total mass of the methyl aminobenzoate and the kaolin (MMT) to the deionized water is 1.

As a preferable scheme, the mass ratio of the total mass of the methyl aminobenzoate and the kaolin (MMT) to the mass of the ethanol is 1.

Preferably, in the preparation process of the Pd-Co-MOF-MMT catalyst: the palladium metal precursor is selected from one or more of palladium iodide, palladium dibromide, palladium acetate, palladium sulfate, bis (triphenylphosphine) palladium dichloride and benzyl bis (triphenylphosphine) palladium (II) chloride, and preferably palladium dibromide and/or palladium acetate.

The cobalt metal precursor is selected from one or more of cobalt sulfate heptahydrate, anhydrous cobalt chloride, cobalt nitrate hexahydrate, cobalt bromide and cobalt acetate tetrahydrate, and preferably cobalt nitrate hexahydrate and/or cobalt acetate tetrahydrate.

In the method of the present invention, the amount of N, N-dimethylformamide added is 20 to 50ml/1mmol (based on the molar amount of palladium element), preferably 20 to 30ml/1mmol (based on the molar amount of palladium element).

Water is added dropwise in a proportion of 0.5 to 10ml/100mlN, N-dimethylformamide, preferably 1 to 6ml/100mlN, N-dimethylformamide.

The preparation method of the Pd-Co-MOF-MMT catalyst comprises the following steps: in the step (2), the reaction temperature in the reaction kettle is 130-150 ℃, and is reduced to 0-5 ℃ at the speed of 0.5-2 ℃/min.

In the invention, the dosage of the Pd-Co-MOF-MMT catalyst is 0.01 to 0.5mol percent, preferably 0.1 to 0.3mol percent of the amount of R-citronellal calculated by the molar weight of palladium element.

In some preferred embodiments, the heterogeneously catalyzed reaction is carried out at atmospheric pressure and is divided into two stages;

the first stage reaction temperature is-30-15 ℃, preferably 0-10 ℃, and the reaction time is 2-8 hours, preferably 4-6 hours;

the second stage reaction temperature is 50-100 ℃, preferably 70-90 ℃, and the reaction time is 0-24 hours, preferably 6-12 hours;

in the method, in the process of generating the L-menthone by the heterogeneous catalytic reaction of the R-citronellal, the R-citronellal is subjected to cyclization reaction to prepare the L-isopulegol, and then the generated L-isopulegol is subjected to intramolecular hydrogenation transfer reaction to generate the L-menthone.

According to the method, the conversion rate of the raw material R-citronellal is 98-99.9%, and the yield of the final product L-menthone of the heterogeneous catalytic reaction is 90-98%.

The method for preparing the L-menthone from the R-citronellal adopts Pd-Co-MOF-MMT for catalysis, kaolin has the characteristics that a stable layered structure supported catalyst has high catalytic performance and good cyclic utilization rate, after methyl p-aminobenzoate is modified, the crystalline MOF material can uniformly and regularly grow, and meanwhile, the formed spatial pore size structure is beneficial to the change of Pd and Co ions from a free state to a fixed state. Meanwhile, preferably, the MOF material can be controlled to grow according to the designed size by adjusting factors such as solvent, temperature and crystallization conditions, and the utilization rate of the catalyst is more favorable. Meanwhile, the methyl p-aminobenzoate has a good effect on inhibiting cyclization side reactions, and the catalytic performance of the methyl p-aminobenzoate is further improved.

The method has the advantages that:

1) Under the action of a Pd-Co-MOF-MMT catalyst, high-purity L-menthone can be efficiently prepared from R-citronellal with high yield under mild reaction conditions, and the method has remarkable operability and economy, reduces the process flow, improves the economic benefit, and reduces the investment of equipment, public engineering and the like.

2) The reaction system does not need to add a solvent, so that the introduction of other impurities is avoided, and the generated waste liquid is less and has good environmental friendliness.

3) The L-isopulegol is subjected to intramolecular hydrogenation transfer to generate the L-menthone, so that hydrogen is prevented from being introduced as a hydrogen source, oxidation operation is not required, and the process safety is greatly improved.

4) The adopted Pd-Co-MOF-MMT catalyst has higher catalytic activity in both aqueous phase and organic phase solvents, and has the advantages of easy recovery, high catalytic activity and the like.

Detailed Description

In order to better understand the technical solution of the present invention, the following examples are further provided to illustrate the present invention, but the present invention is not limited to the following examples.

1. The main raw materials used in the examples of the present invention are described below:

palladium acetate, alatin, product number P111486, purity 99.98%;

palladium dibromide, alatin, product number P100503, purity 99%;

cobalt acetate tetrahydrate, alatin, product number C11085, purity 99.5%;

cobalt nitrate hexahydrate, alatin, product number C112729, purity 99%;

p-aminobenzoic acid methyl ester, alatin, product number P108506, purity 99%;

n, N dimethylformamide, alatin, product number D112002, purity 99.9%;

kaolin, alatin, product number K100133;

r-citronellal, wuhan, is far from the Co-creation science and technology Limited company, the chemical purity is more than 99 percent, and the ee value range is 95-100 percent.

2. The reaction product test apparatus and method in the examples:

gas chromatograph: shimadzu GC-2010plus, chromatographic column DB-WAX UI, injection port temperature: feeding 0.1 mu L at 220 ℃; the flow splitting ratio is 100; carrier gas flow: 1.0ml/min; temperature rising procedure: holding at 80 ℃ for 2min, heating to 150 ℃ at 2.5 ℃/min, holding for 10min, detector temperature: at 250 ℃ to obtain a mixture. Hydrogen flow rate: 40 mL/min, air flow rate: 400mL/min, tail-blow flow rate: 30mL/min.

ICP-AES (inductively coupled plasma emission Spectroscopy), optima 2000DV.

Example 1

Adding 1.249g of methyl p-aminobenzoate and 0.6245g of kaolin (MMT) into 112g of ethanol and 112g of deionized water, ultrasonically dissolving, stirring for 24 hours at 55 ℃, then carrying out suction filtration washing, and drying for 5 hours at 55 ℃ in a vacuum oven. Named MMT precursor.

1g of palladium dibromide, 1.09g of cobalt nitrate, hexahydrate and the obtained MMT precursor are placed in 75ml of N, N-dimethylformamide to be subjected to ultrasonic treatment until the palladium dibromide, 34.5ml of deionized water are dropwise added, the obtained solution is poured into a reaction kettle with a 250ml of polytetrafluoroethylene lining, the reaction kettle is screwed and placed into a program-controlled oven, heating reaction is carried out at 130 ℃ for 45h, the temperature is reduced to 0 ℃ at 0.5 ℃/min, crystals are filtered by a centrifugal machine after the temperature is reduced to 0 ℃, the crystals are washed by the deionized water, dried in a vacuum drying oven at 45 ℃ for 12h, cooled to room temperature, and the catalyst is weighed to obtain 3.22g. ICP detected that the loading amount of palladium element was 9.92wt%, the loading amount of cobalt element was 5.49wt%, and the molar ratio of palladium element to cobalt element was 1.

Example 2

1.02g of methyl p-aminobenzoate and 0.681g of kaolin (MMT) are added into 110g of ethanol and 110g of deionized water for ultrasonic dissolution, stirred for 22 hours at 60 ℃, then filtered, washed and dried for 6.5 hours at 60 ℃ in a vacuum oven. Named MMT precursor.

1g of palladium dibromide, 1.127g of cobalt acetate, tetrahydrate and the obtained MMT precursor are placed in 112ml of N, N-dimethylformamide to be subjected to ultrasonic treatment until the palladium dibromide, 1.12ml of deionized water are dropwise added, the obtained solution is poured into a 250ml of polytetrafluoroethylene-lined reaction kettle, the reaction kettle is screwed and placed into a program-controlled oven, the reaction kettle is heated at 150 ℃ for 50 hours, the temperature is reduced to minus 5 ℃ at the rate of 2 ℃/min, crystals are filtered by a centrifuge, the crystals are washed by the deionized water, dried in a vacuum drying oven at 55 ℃ for 15 hours, cooled to room temperature, and 3.142g of the catalyst is obtained by weighing. The load amount of palladium element detected by ICP is 10.178wt%, the load amount of cobalt element is 6.764wt%, and the molar ratio of palladium element to cobalt element is 1.2.

Example 3

1.347g of methyl p-aminobenzoate and 0.77g of kaolin (MMT) are added into 137.5g of ethanol and 137.5g of deionized water for ultrasonic dissolution, stirred for 20 hours at 65 ℃, then filtered and washed, and dried for 8 hours at 65 ℃ in a vacuum oven. Named MMT precursor.

1g of palladium acetate, 1.49g of cobalt nitrate, hexahydrate and the obtained MMT precursor are placed in 111.4ml of N, N-dimethylformamide to be subjected to ultrasonic treatment until the palladium acetate, the cobalt nitrate, the hexahydrate and the obtained MMT precursor are completely dissolved, 6.68ml of deionized water is dropwise added, the obtained solution is poured into a reaction kettle with a 250ml of polytetrafluoroethylene lining, the reaction kettle is screwed up and placed into a program-controlled oven, heating and reaction are carried out at 140 ℃ for 45 hours, the temperature is reduced to 0 ℃ at 1 ℃/min, crystals are filtered by using a centrifugal machine, the crystals are washed by deionized water, dried in a vacuum drying oven at 55 ℃ for 12 hours, cooled to room temperature, and the catalyst is weighed to obtain 3.7g. ICP (inductively coupled plasma) detects that the loading amount of the palladium element is 10.252wt%, the loading amount of the cobalt element is 6.81wt%, and the molar ratio of the palladium element to the cobalt element is 1.2.

Example 4

Adding 141.4g ethanol and 141.4g deionized water into 1.212g methyl p-aminobenzoate and 0.808g kaolin (MMT), ultrasonically dissolving, stirring at 60 ℃ for 22h, then performing suction filtration and washing, and drying in a vacuum oven at 65 ℃ for 6.5h. Named MMT precursor.

1g of palladium acetate, 1.331g of cobalt acetate, tetrahydrate and the obtained MMT precursor are placed in 133.6ml of N, N-dimethylformamide to be ultrasonically dissolved completely, 4.67ml of deionized water is dropwise added, the obtained solution is poured into a 250ml of reaction kettle with a polytetrafluoroethylene lining, the reaction kettle is screwed and placed into a program-controlled oven, the reaction kettle is heated at 150 ℃ for 50h, the temperature is reduced to-5 ℃ at the speed of 2 ℃/min, crystals are filtered by a centrifuge, the crystals are washed by the deionized water, dried in a vacuum drying oven at 55 ℃ for 15h, cooled to room temperature, and 3.577g of the catalyst is obtained by weighing. The load amount of palladium element detected by ICP is 10.6wt%, the load amount of cobalt element is 7.045wt%, and the molar ratio of palladium element to cobalt element is 1.2.

Example 5

The reaction kettle was flushed to 0.3MPaG with nitrogen, vented to atmospheric pressure, and repeated three times, 3.688g of the catalyst prepared by the method described in example 1 was added to the reaction kettle, 200g of R-citronellal was added to a 500ml closed reaction kettle, the stirring was started, the temperature was reduced to 0 ℃, the reaction temperature was kept stable, and the reaction was continued for 6h.

Then, the reaction temperature was raised to 70 ℃ and kept stable for 12 hours.

The chromatographic analysis calculates that the final conversion rate of the R-citronellal reaction is 98.01 percent, the yield of the L-menthone is 97.38 percent, and the ee value of the L-menthone is 99.88 percent

Example 6

The reaction kettle was flushed with nitrogen to 0.3MPaG, vented to atmospheric pressure, and repeated three times, 1.209g of the catalyst prepared by the method described in example 2 was added to the reaction kettle, 200g of R-citronellal was added to a 500ml closed reaction kettle, the stirring was started, the temperature was reduced to 5 ℃, the reaction temperature was kept stable, and the reaction was continued for 5h.

Then, the reaction temperature was raised to 80 ℃ and kept stable for 9 hours.

The final conversion rate of the R-citronellal reaction is 98.95% and the yield of the L-menthone is 95.8% by chromatographic analysis calculation. The ee value of L-menthone is 99.12 percent

Example 7

The reaction kettle was flushed to 0.3MPaG with nitrogen, vented to atmospheric pressure, and repeated three times, 2.839g of the catalyst prepared by the method described in example 3 was added to the reaction kettle, 200g of R-citronellal was added to a 500ml closed reaction kettle, the stirring was started, the temperature was reduced to 10 ℃, the reaction temperature was kept stable, and the reaction was continued for 4h.

Then, the reaction temperature was raised to 90 ℃ and kept stable for 6 hours.

The chromatographic analysis calculates that the final conversion rate of the R-citronellal reaction is 99.79 percent, and the yield of the L-menthone is 92.38 percent. The ee value of L-menthone is 97.32 percent

Example 8

The reaction kettle was flushed to 0.3MPaG with nitrogen, vented to atmospheric pressure, and repeated three times, 1.155g of the catalyst prepared as described in example 4 was added to the reaction kettle, 200g of R-citronellal was added to a 500ml closed reaction kettle, the stirring was started, the temperature was reduced to 0 ℃, the reaction temperature was kept stable, and the reaction was continued for 6h.

Then, the reaction temperature was raised to 90 ℃ and kept stable for 12 hours.

The chromatographic analysis calculates that the final conversion rate of the R-citronellal reaction is 98.51 percent, and the yield of the L-menthone is 98.822 percent. The ee value of L-menthone is 97.72 percent

Example 9

The catalyst is filtered and recovered on the basis of the example 6, and is mechanically applied for 30 times, and the experimental results are as follows:

group of	R-citronellal conversion/%)	L-menthone yield/%	L-menthone ee value/%
				Example 9	98.89	95.89	99.12
Apply it 10 times	98.81	95.91	99.06
				Apply it 20 times	98.79	94.85	99.11
Apply it for 30 times	98.63	94.47	99.13

COMPARATIVE EXAMPLE 1 (CN 104603095 preparation example 4)

Under inert conditions, 404mg of [ Ru (PnOct) ₃ ) ₄ (H) ₂ ]3.6g of isopulegol and 10ml of o-xylene (anhydrous) are weighed into a 50ml glass autoclave. The reaction mixture was then stirred at an oil bath temperature of 130 ℃ under autogenous pressure (0.5 bar positive pressure) for 12 hours. After the reaction, the conversion and yield (% by area) of menthone (sum of isomers) were determined by gas chromatography. Conversion of isopulegol 64.5%, with menthol (65.8)The selectivity of% (-) -menthol, 34.2% (+) -isomenthesia isomer mixture) was 46.3%. The selectivity of the secondary components is 30.2 percent of menthol, 14.4 percent of isopulegone and 90.9 percent of the total selectivity (the menthol plus the isopulegone).

COMPARATIVE EXAMPLE 2 (CN 106061933A example 1)

X540T 1/8 (l 50g,30-40% copper oxide, 10-25% aluminum oxide, 10-25% magnesium oxide and 30-40% aluminum copper (Al) ₂ CuO ₄ ) Loaded into a gas phase reactor and the catalyst is activated at a temperature of from 170 to 180 ℃ in a specific gas-containing stream (20 to 40 NL/h). The evaporator and reactor were subsequently operated at a temperature of 170 ℃ and at atmospheric pressure with a nitrogen stream (20 NL/h). Isopulegol (water content 3.7% by weight, 15g/h,97.2 mmol/h) is continuously introduced into the evaporator. The product mixture was condensed at the reactor outlet and analyzed for composition using gas chromatography. In each case, after a test time of 5 hours, the reactor and the evaporator were cooled under a stream of nitrogen (20 NL/h), and the test was continued after 18h without changing the catalyst.

The reaction of isopulegol is completed over the entire reaction time.

Table 1: conversion, mass balance and product composition in the isopulegol reaction.

* Values are based on area percent.

Isopulegol is reacted to menthone in good to very good yields of up to 88.5%. For all experiments, the ratio of menthone to isomenthone was 65/35 to 70/30 (menthone/isomenthone) (see table 1).

Claims

1. A method for preparing L-menthone from R-citronellal comprises the steps of carrying out heterogeneous catalysis reaction on R-citronellal under the action of a Pd-Co-MOF-MMT catalyst to prepare L-menthone, wherein MMT is kaolin; the ee value range of the R-citronellal is 95-99.99%; the mole ratio of palladium element to cobalt element in the Pd-Co-MOF-MMT catalyst is 1; the MOF is prepared from p-aminobenzoic acid methyl ester as a construction monomer, wherein the molar ratio of the p-aminobenzoic acid methyl ester to palladium is 1-3, and the mass ratio of the p-aminobenzoic acid methyl ester to kaolin is 1-4: 1; the preparation method of the Pd-Co-MOF-MMT catalyst comprises the following steps:

(1) Adding methyl p-aminobenzoate and kaolin into ethanol and water for ultrasonic dissolution, stirring for 20-24 h at 55-65 ℃, then performing suction filtration and washing, and performing vacuum drying to obtain an MMT precursor;

(2) Putting the palladium metal precursor, the cobalt metal precursor and the MMT precursor into N, N-dimethylformamide for ultrasonic treatment until all the palladium metal precursor, the cobalt metal precursor and the MMT precursor are dissolved, dropwise adding water, pouring the obtained solution into a reaction kettle, heating for reaction for 45-50 h, carrying out programmed cooling to 0-minus 5 ℃, filtering crystals by using a centrifugal machine, washing, and carrying out vacuum drying to obtain a catalyst finished product.

2. The method of claim 1, wherein the Pd-Co-MOF-MMT catalyst has a molar ratio of palladium element to cobalt element of 1; the MOF is prepared from p-aminobenzoic acid methyl ester as a construction monomer, wherein the molar ratio of the p-aminobenzoic acid methyl ester to palladium element is 1.8-2.2, and the mass ratio of the p-aminobenzoic acid methyl ester to kaolin is 1.5-2.

3. The method according to claim 1, wherein the ee value of R-citronellal is in the range of 98 to 99.99%.

4. The method according to claim 1, wherein the mass ratio of methyl p-aminobenzoate to kaolin in the step (1) is 1-4: 1; the mass ratio of the total mass of the methyl aminobenzoate and the kaolin to the water is 1; the mass ratio of the total mass of the methyl aminobenzoate and the kaolin to the ethanol is 1.

5. The method according to claim 4, wherein the mass ratio of methyl p-aminobenzoate to kaolin in the step (1) is 1.5 to 2; the mass ratio of the total mass of the methyl aminobenzoate and the kaolin to the water is 1; the mass ratio of the total mass of the methyl aminobenzoate and the kaolin to the mass of the ethanol is 1.

6. The method according to claim 1, wherein, in step (2),

the palladium metal precursor is selected from one or more of palladium iodide, palladium dibromide, palladium acetate, palladium sulfate, bis (triphenylphosphine) palladium dichloride and benzyl bis (triphenylphosphine) palladium (II) chloride;

the cobalt metal precursor is selected from one or more of cobalt sulfate heptahydrate, anhydrous cobalt chloride, cobalt nitrate hexahydrate, cobalt bromide, cobalt acetate tetrahydrate.

7. The method according to claim 6, wherein in step (2), the palladium metal precursor is selected from palladium dibromide and/or palladium acetate; the cobalt metal precursor is selected from cobalt nitrate hexahydrate and/or cobalt acetate tetrahydrate.

8. The method according to claim 1, wherein in the step (2), the N, N-dimethylformamide is added in a proportion of 20 to 50ml/1mmol based on the molar amount of the palladium element; water is added dropwise according to the proportion of 0.5-10 ml/100mlN, N-dimethylformamide.

9. The method according to claim 8, wherein in the step (2), the N, N-dimethylformamide is added in a proportion of 20 to 30ml/1mmol based on the molar amount of the palladium element; water is added dropwise according to the proportion of 1-6 ml/100mlN, N-dimethylformamide.

10. The method as claimed in claim 1, wherein the reaction temperature in the reaction vessel in the step (2) is 130 to 150 ℃ and is decreased to 0 to-5 ℃ at a rate of 0.5 to 2 ℃/min.

11. The method of claim 1, wherein the temperature of the vacuum drying in the step (1) is 55-65 ℃ and the time is 5-8 h, and the temperature of the vacuum drying in the step (2) is 45-55 ℃ and the time is 12-15 h.

12. A process according to any one of claims 1 to 3, characterized in that the amount of the Pd-Co-MOF-MMT catalyst, calculated as the molar amount of the palladium element, is between 0.01 and 0.5mol% of the amount of R-citronellal.

13. The method of claim 12, wherein the amount of the Pd-Co-MOF-MMT catalyst is 0.1 to 0.3mol% based on the molar amount of the palladium element.

14. The method according to any one of claims 1 to 3,

the heterogeneous catalytic reaction is a normal pressure reaction;

the heterogeneous catalytic reaction is divided into two sections; the first stage reaction temperature is-30-15 ℃, and the reaction time is 2-8 h;

the second stage reaction temperature is 50-100 deg.c and the reaction time is 0-24 hr.

15. The method according to claim 14, wherein the first stage reaction temperature is 0-10 ℃ and the reaction time is 4-6 h;

the second stage reaction temperature is 70-90 deg.c and the reaction time is 6-12 hr.