阅读说明：本技术 一种制备对甲氧基肉桂酸的方法 (Method for preparing p-methoxycinnamic acid ) 是由 刘启发 王荣 杨健 王中孝 张亚楠 于 2018-09-05 设计创作，主要内容包括：本发明涉及一种制备对甲氧基肉桂酸的方法,该方法包括以下步骤：成盐反应：丙烯酸与碱性无机盐反应,获得丙烯酸盐；偶联反应：4-甲氧基卤苯与丙烯酸盐反应,酸化后得到对甲氧基肉桂酸。本发明中将有机相反应体系变更为水相反应体系,采用全新高效低廉的催化剂进行催化,制备得到高品质、低成本的对甲氧基肉桂酸,HPLC含量≥99.5％；单个杂质≤0.2％,总杂质≤0.5％。(The invention relates to a method for preparing p-methoxycinnamic acid, which comprises the following steps: salt forming reaction: reacting acrylic acid with alkaline inorganic salt to obtain acrylate; coupling reaction: 4-methoxy halogeno benzene reacts with acrylate, and p-methoxy cinnamic acid is obtained after acidification. According to the invention, an organic phase reaction system is changed into a water phase reaction system, and a brand-new high-efficiency low-cost catalyst is adopted for catalysis to prepare high-quality low-cost p-methoxycinnamic acid, wherein the HPLC content is more than or equal to 99.5%; the single impurity is less than or equal to 0.2 percent, and the total impurity is less than or equal to 0.5 percent.)

1. A method for preparing p-methoxycinnamic acid, which is characterized by comprising the following steps:

Step 1), reacting acrylic acid with alkaline inorganic salt to obtain acrylate;

and step 2), reacting the 4-methoxy halogeno benzene with acrylate, and acidifying to obtain the p-methoxy cinnamic acid.

2. The method according to claim 1, wherein in step 1), the basic inorganic salt is selected from one or more of sodium carbonate, sodium bicarbonate, potassium carbonate or potassium bicarbonate, preferably sodium carbonate or sodium bicarbonate, more preferably sodium carbonate.

3. The method according to claim 1 or 2, wherein the reaction solvent in step 1) comprises water or an alcohol-water solvent, preferably water,

Wherein the alcohol-water solvent comprises methanol-water, ethanol-water or propanol-water with different concentrations.

4. The process according to any one of claims 1 to 3, wherein in step 2), the 4-methoxyhalobenzene is selected from 4-methoxybromobenzene and/or 4-methoxyiodo, preferably 4-methoxybromobenzene;

The dosage ratio of the 4-methoxy halogenobenzene to the acrylic acid is 1 (1.1-1.3), and preferably 1 (1.15-1.25).

5. The process according to any one of claims 1 to 4, wherein in step 2), the coupling reaction of 4-methoxyhalogenobenzene with an acrylic acid salt is carried out in the presence of a catalyst,

The catalyst is a mixed catalyst of catalysts I and II, wherein the catalyst I is selected from one or more of basic copper carbonate (Cu2(OH)2CO3), cuprous chloride (CuCl) or cupric chloride (CuCl2), and is preferably Cu2(OH)2CO 3;

catalyst II is selected from one or more of sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, ammonium carbonate or ammonium bicarbonate, preferably sodium carbonate or sodium bicarbonate, more preferably sodium carbonate.

6. The preparation method according to any one of claims 1 to 5, wherein in step 2), the catalyst I is used in a molar ratio of the catalyst I to the 4-methoxy halogenobenzene, wherein the molar ratio of the catalyst I to the 4-methoxy halogenobenzene is (100-250) x 10-6:1, preferably (150-200) x 10-6: 1;

The molar ratio of the acrylic acid to the catalyst II is 1 (0.5-0.6), preferably 1 (0.5-0.55).

7. The production process according to any one of claims 1 to 6, wherein in the step 2), an antioxidant is added to the reaction system,

The antioxidant is selected from phenolic compounds or compositions containing phenolic compounds, preferably one or more of Butylated Hydroxyanisole (BHA), 2, 6-di-tert-butyl-4-methylphenol (BHT), Propyl Gallate (PG) or tert-butylhydroquinone (TBHQ), and more preferably BHT.

8. The process according to any one of claims 1 to 7, wherein the process further comprises post-treating the p-methoxycinnamic acid obtained in step 2), the post-treating comprising: dissolving p-methoxycinnamic acid in a proper amount of organic solvent, heating to dissolve, slowly cooling to a temperature T1, crystallizing after crystal precipitation, continuously cooling to a temperature T2, preserving heat, crystallizing, and filtering to obtain p-methoxycinnamic acid;

Preferably, the temperature rise and dissolution temperature is 140-145 ℃;

Cooling to the temperature T1, namely 90-100 ℃ after 1-2 h, and growing crystals for 0.5-1.5 h after crystals are precipitated;

And continuously cooling to the temperature T2, namely 23-35 ℃, and keeping the temperature for grain growth for 0.5-1.5 h.

9. The production method according to any one of claims 1 to 8, wherein the obtained p-methoxycinnamic acid is a white crystal, and the HPLC content (area normalization method): not less than 99.5%; the content of single impurity (HPLC) is less than or equal to 0.2 percent, and the content of total impurity (HPLC) is less than or equal to 0.5 percent.

10. p-methoxycinnamic acid obtainable by a process according to one of claims 1 to 9, characterized in that it is a white crystal with an HPLC content (area normalization): not less than 99.5%; the content of single impurity (HPLC) is less than or equal to 0.2 percent, and the content of total impurity (HPLC) is less than or equal to 0.5 percent.

Technical Field

The invention belongs to the technical field of daily chemical industry, relates to a preparation method of an organic intermediate, and particularly relates to a preparation method of an intermediate p-methoxycinnamic acid.

Background

Isooctyl p-methoxycinnamate (OMC) is one of the most commonly used sun-screening agents in the world at present, is a good ultraviolet absorbent in a UVB region, can effectively prevent ultraviolet rays of 280-330nm, has high absorptivity, no stimulation to skin, good safety and good solubility to the solubility of oily raw materials, is almost an ideal sun-screening agent, and is widely applied to the fields of daily chemicals, plastics, rubber, coatings and the like.

one of the current routes for industrially preparing OMC is to prepare p-methoxycinnamic acid and isooctanol by esterification under the condition of an acid catalyst, and the synthesis of a key intermediate p-methoxycinnamic acid is the key point for realizing industrialization of the route. The p-methoxycinnamic acid has CAS number of 830-09-1, is white crystal, has melting point of 173.5 deg.C and relative molecular mass of 178.18, and is mainly used for synthesis of cosmetic ultraviolet absorbent isooctyl p-methoxycinnamate, enoxate, isoamyl p-methoxycinnamate, etc. The product is also used as an organic synthetic medical intermediate, such as esmolol which is an anti-epinephrine drug.

P-methoxy cinnamic acid is synthesized by a plurality of methods. The early preparation method is that the p-methoxycinnamic acid is prepared by Perking reaction (CN 102060832; Asian J.chem.2007,19(6): 4945-4947; Rev.Roum.Chim.2005,50(7-8):627-631), which takes p-methoxybenzaldehyde and acetic anhydride as raw materials, and condenses at the temperature of 180-200 ℃ under the catalysis of equivalent or excessive acetate, then neutralizes the reaction liquid by carbonate, and then acidifies by hydrochloric acid to obtain the p-methoxycinnamic acid. The method has the following defects that the reaction temperature is higher, more energy is consumed, and at such a high temperature, the byproduct acetic acid has strong corrosivity on equipment, so that the design of the equipment is a great challenge, and greater potential safety hazard exists in the production process. A large amount of acetate, carbonate and inorganic acid are used in the reaction process, so that a large amount of wastewater with high salt and high COD value is generated, and serious problems are brought to pollution treatment. Most importantly, the yield of the p-methoxycinnamic acid prepared by the method is lower than 70%.

Knoevenagel reaction preparation (bioorg. Med. chem.2011,19(10): 3192-4. 3203; Chin. chem. Lett.2011,22 (6): 667-670; Chin. chem. Lett.2009,20(3): 279-282; J.Agric. food chem.,2009,57(8): 3261-3265; Med. chem.2007,3(5): 475-479; Tetrahedron Lett.2007,48(26): 4505-4508; Eur.J.Pharm. Sci.1103, 23(4-5): 363-369; agro-pharmaceutical letters, Med 2002, (4): 28-32; J.Fluor. chem.1990,48(2): 159-67; zh.org.1989; Het.1879; 1984-24. J.1108; Eur. 19823; Eur.31-24; Eur.31: 27; Eur.8). The method comprises the steps of taking p-methoxybenzaldehyde and malonic acid as raw materials, piperidine as a catalyst and aromatic hydrocarbon as a solvent to carry out condensation reaction, continuously releasing carbon dioxide in the reaction process, washing the catalyst piperidine by acid water after the reaction is finished, undoubtedly generating a large amount of wastewater and carbon dioxide, and seriously polluting the environment. Piperidine is expensive and not suitable for industrial application. The synthesis yield of the method is reported to be 88-92%.

P-methoxy benzaldehyde and malonate are used as raw materials, and toluene is used as a water-carrying agent under the catalysis of ammonium acetate or secondary amine to synthesize the p-methoxy cinnamic acid (appl.Catal.A: Gen.2010,381(1-2): 226-232.). The method has the following defects that the prices of the raw material malonate and the catalyst secondary ammonium are high, and the utilization rate of the malonate is low, so that the method has no advantage in cost; after the reaction is finished, alkali is needed to carry out saponification reaction to generate sodium salt, and then acid acidification and decarboxylation are carried out to obtain p-methoxycinnamic acid, so that ethanol can be produced as a byproduct, extra manufacturing cost is increased by separating and recovering ethanol, extra cost is increased by using a large amount of alkali and acid, a large amount of three wastes are generated, and the environment is polluted; due to the use of ammonium acetate and the like, side reaction is generated in the reaction process to generate p-methoxy cinnamamide, and the substance is difficult to remove from the product, thereby bringing difficulty to separation and purification; in addition, in the production process, amine substances are inevitably overflowed, so that adverse effects are brought to the production environment and operators. However, the yield of this method can be up to 80% or more.

Horn-Wozhous-Emont reaction (Synthesis, (21), 3654) -3660; 2006). P-methoxy benzaldehyde is used as a raw material, and p-methoxy cinnamic acid is prepared with phosphonate ester reagent under the catalytic action of organic strong base DBU and zinc trifluoromethanesulfonate with the yield of 98%. This method is not suitable for industrial production because it uses expensive and unavailable phosphine reagent, strong organic base and catalyst, although the yield is extremely high.

The Witting reaction was prepared (Synth. Commun.2002,32(2): 195-201). Triphenylphosphine firstly reacts with ethyl bromoacetate to prepare a unstable Witting reagent under the action of strong alkali, the Witting reagent is coupled with p-methoxybenzaldehyde under the catalysis of titanium tetrachloride to prepare ethyl p-methoxycinnamate, and the p-methoxycinnamate is obtained through saponification reaction and acid neutralization, wherein the yield is 68%. The method has the advantages of multiple reaction steps, lower yield, higher cost, serious three wastes and unsuitability for industrial production.

Aldol condensation reaction (CN 1218794; CN 1170401; CN 102627559). P-methoxy benzaldehyde and methyl acetate, ethyl acetate or isooctyl acetate are used as raw materials, and the raw materials are subjected to aldol condensation reaction under the action of sodium alkoxide such as sodium methoxide, potassium methoxide and the like to prepare corresponding p-methoxy cinnamate, and then the p-methoxy cinnamate is prepared through saponification reaction and acid neutralization. However, the above process has the following disadvantages in view of practical production. The utilization rate of the reaction raw material, namely the acetate is only 30%, the acetate is decomposed by about 66.6% under the alkaline condition to cause raw material loss, more side reactions are generated in the condensation reaction and the saponification reaction, the crude product has complex components, the product needs to be crystallized and purified for many times, the product is difficult to separate, and after the reaction is finished, a large amount of water is needed to wash the material, so that the generated high salt and high COD waste water bring pressure to environmental protection treatment and cause pollution to the surrounding environment.

Diazotization-coupling reaction preparation (EP553668), firstly, p-methoxyaniline is subjected to diazotization reaction to prepare diazonium salt, and then the diazonium salt and acrylic acid are subjected to coupling reaction under the catalytic action of copper salt, palladium salt and organic phosphine ligand, so that p-methoxycinnamic acid can be prepared with a yield of 67%. The raw material p-methoxyaniline used by the method is not easy to obtain, the used catalyst palladium salt and the organic phosphine ligand are expensive, and the method has potential danger in the process of preparing the diazonium salt and generates a large amount of three wastes, so that the industrial production is difficult to realize.

heck reaction preparation (CN 102010279; molecular catalysis, 2010, 24 (3): 247-. Acrylic acid and 4-methoxy halogeno benzene are used as raw materials to carry out Heck coupling reaction to prepare the p-methoxy cinnamic acid.

in conclusion, the above methods all have advantages and disadvantages, and therefore, a new synthetic method still needs to be continuously explored to meet the requirement of the industry on high-quality and low-price p-methoxycinnamic acid, and simultaneously reduce the risk of environmental pollution caused by production.

Disclosure of Invention

The inventor of the invention has made extensive research, changes the form of the raw materials, changes the organic phase reaction system into the water phase reaction system, and adopts a brand-new high-efficiency and low-cost catalyst to catalyze to prepare the p-methoxycinnamic acid with high quality and low cost, and the preparation process is low (no) pollution, thereby completing the invention.

The invention aims to provide the following technical scheme:

(1) a method for preparing p-methoxycinnamic acid, comprising the steps of:

Step 1), reacting acrylic acid with alkaline inorganic salt to obtain acrylate;

And step 2), reacting the 4-methoxy halogeno benzene with acrylate, and acidifying to obtain the p-methoxy cinnamic acid.

(2) p-methoxycinnamic acid prepared according to the above (1) as white crystals with HPLC content (area normalization): not less than 99.5%; the content of single impurity (HPLC) is less than or equal to 0.2 percent, and the content of total impurity (HPLC) is less than or equal to 0.5 percent.

The method for preparing p-methoxycinnamic acid provided by the invention has the following beneficial effects:

(1) the cheap catalyst (Cu2(OH)2CO 3/CO2 in essence) adopted by the invention can efficiently realize Heck reaction of p-bromoanisole and acrylic acid, the needed key intermediate p-methoxycinnamic acid of the sun-screening agent is prepared with high yield, the use of expensive palladium salt catalyst is avoided, and the manufacturing cost of the p-methoxycinnamic acid is greatly reduced;

(2) The invention adopts the carbon dioxide CO2 for concerted catalysis, avoids the use of expensive organic phosphine ligand which is commonly used in Heck reaction, not only greatly reduces the manufacturing cost of p-methoxycinnamic acid, but also reduces the pollution to products and environment;

(3) The water adopted by the invention is used as a solvent, so that the use of solvents commonly used in Heck reaction, such as N, N-Dimethylformamide (DMF), N-Dimethylacetamide (DMA), 1, 3-dimethyl-2-imidazolidinone (DMI), N-methylpyrrolidone (NMP) and the like, is avoided, the manufacturing cost of p-methoxycinnamic acid is greatly reduced, and the pollution to the environment caused by the recovery of a water-soluble solvent and the volatilization of the solvent is avoided;

(4) the invention also has the characteristics of simple operation, easy control and easy realization of industrial production.

Drawings

FIG. 1 shows the HPLC chromatogram of the product obtained after crystallization in example 1;

FIG. 2 shows the HPLC chromatogram of the product obtained after recrystallization in example 1;

FIG. 3 shows the HPLC chromatogram of the product obtained after recrystallization-elution in example 1.

Detailed Description

Acrylic acid and 4-methoxy halogeno benzene are used as raw materials to carry out Heck coupling reaction to prepare p-methoxy cinnamic acid, the method is one of the simplest and direct methods for synthesizing p-methoxy cinnamic acid, has potential industrial value, and gradually becomes a hotspot of research in the industrial and academic fields. However, the inventor of the invention finds that the method has the following defects through research: (i) the 4-methoxy halogenobenzene as the raw material needs to use the very expensive p-methoxy iodobenzene, and the cheap p-methoxy bromobenzene or chlorobenzene is difficult to activate; (ii) the addition of toxic organophosphine ligands is generally required to maintain the stability and activity of the palladium catalyst; (iii) organic solvents such as acetonitrile, DMF, DMI, NMP and the like are required to be used in the reaction process; thereby placing a pressure on cost and environmental protection.

In order to combine the advantages of the Heck coupling reaction and solve the disadvantages of the method, the inventor conducts extensive research and provides a method for preparing p-methoxycinnamic acid, which comprises the following steps:

step 1), reacting acrylic acid with alkaline inorganic salt to obtain acrylate;

And step 2), reacting the 4-methoxy halogeno benzene with acrylate, and acidifying to obtain the p-methoxy cinnamic acid.

The present invention is specifically described below.

Step 1), salt forming reaction: reacting acrylic acid with alkaline inorganic salt to obtain acrylate.

Specifically, a reaction solvent and an alkaline inorganic salt are added into a reaction vessel, and after stirring and dissolving, acrylic acid is added in portions until no carbon dioxide is discharged, and the material becomes clear and transparent.

In step 1) of the present invention, the reaction solvent comprises water or an alcohol-water solvent (such as methanol-water, ethanol-water or propanol-water with different concentrations), preferably water.

in step 1) of the present invention, the basic inorganic salt is a water-soluble inorganic salt to be in sufficient contact with acrylic acid to promote the reaction, and is selected from one or more of sodium carbonate, sodium bicarbonate, potassium carbonate or potassium bicarbonate, preferably sodium carbonate or sodium bicarbonate, and more preferably sodium carbonate.

In step 1), the molar ratio of the acrylic acid to the basic inorganic salt is 1 (0.5-0.6), preferably 1 (0.5-0.55).

Step 2), coupling reaction: 4-methoxy halogeno benzene reacts with acrylate, and p-methoxy cinnamic acid is obtained after acidification.

Adding a catalyst into an acrylate solution, reacting with 4-methoxy halogenobenzene under the catalysis action, wherein the catalyst is a mixed catalyst of catalysts I and II,

Wherein, the catalyst I is selected from one or more of basic copper carbonate (Cu2(OH)2CO3), cuprous chloride (CuCl) or cupric chloride (CuCl2), and is preferably Cu2(OH)2CO 3;

catalyst II is selected from one or more of sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, ammonium carbonate or ammonium bicarbonate, preferably sodium carbonate or sodium bicarbonate, more preferably sodium carbonate.

In essence, it is the CO2 that is produced by the carbonate and bicarbonate in catalyst II that catalyzes the reaction. CO2 has synergistic catalysis effect on catalyst I, and CO2 can be generated in situ when catalyst II is carbonate and bicarbonate, no additional addition is needed, the production process is simplified, and the production operation is easy. Further, the catalyst II is added into the reaction system in the form of carbonate and bicarbonate, so that CO2 is generated from the reaction system (liquid), the contact between the catalyst and the reaction raw materials is increased, and the reaction efficiency is improved.

In the reaction process, the reaction container is a closed container to avoid the overflow of CO2 generated by adding the catalyst II, and if the in-situ generated CO2 is removed from the reaction system, the coupling reaction efficiency is greatly reduced.

In a preferred embodiment, the molar ratio of the catalyst I to the 4-methoxy halogenobenzene is (100-250). times.10-6: 1, preferably (150-200). times.10-6: 1.

In a preferred embodiment, the molar ratio of the acrylic acid to the catalyst II is 1 (0.5-0.6), preferably 1 (0.5-0.55).

In a preferred embodiment, the reaction temperature of step 2) is 145 ℃ to 160 ℃, preferably 150 ℃ to 155 ℃; the pressure is 1.4-1.7 MPa; the reaction time is 1-4 h, preferably 2-3 h.

in the invention, the temperature is 145-160 ℃, the reaction rate is reduced when the temperature is lower than 145 ℃; the reaction raw material acrylic acid (sodium acrylate) is easy to generate condensation reaction or decarboxylation reaction at the temperature higher than 160 ℃, so that the generation of side reaction is promoted, and the utilization rate of the raw material and the yield of the product are reduced.

In the invention, the pressure is 1.4-1.7 MPa, the pressure is formed in a closed container by the vaporization of the reaction solvent at the temperature and the CO2 generated by the catalyst II, no additional gas is needed to be introduced to increase the reaction pressure, and the pressure generated in situ is favorable for promoting the conversion of the raw materials and the improvement of the reaction rate.

In the present invention, the 4-methoxyhalobenzene is selected from 4-methoxybromobenzene and/or 4-methoxyiodobenzene, preferably 4-methoxybromobenzene. According to the invention, the catalyst can effectively activate the 4-methoxy halogenobenzene, so that the selection range of the 4-methoxy halogenobenzene is expanded, the cost of reaction raw materials is reduced, and the industrial production requirement is more easily met.

Wherein the dosage ratio of the 4-methoxy halogenobenzene to the acrylic acid is 1 (1.1-1.3), and preferably 1 (1.15-1.25). In order to promote the reaction, one reaction raw material is selected to be excessive, preferably, the acrylic acid is slightly excessive compared with 4-methoxy halogenobenzene, the acrylate is easily dissolved in a reaction solvent, and the acrylate is a halogen-free substance and has small pollution; the excessive amount of the acrylic acid is less, so that the impurities generated in the polymerization reaction and the useless consumption of reaction raw materials are reduced.

the present inventors have studied this reaction and found that the reaction efficiency is extremely high in the specific catalysis, but the product yield cannot meet the industrial demand.

for this purpose, an antioxidant for inhibiting polymerization of the reaction raw material acrylic acid (acrylate) is added in step 2) in the present invention. The antioxidant is selected from phenolic compounds or compositions containing phenolic compounds, preferably one or more of Butylated Hydroxyanisole (BHA), 2, 6-di-tert-butyl-4-methylphenol (BHT), Propyl Gallate (PG) or tert-butylhydroquinone (TBHQ), and more preferably BHT.

In the present invention, the reaction raw material acrylic acid (acrylate) is easily polymerized at the reaction temperature, and the polymerization reaction is more easily caused as the temperature is increased, and the phenolic compound or the composition containing the phenolic compound has a polymerization inhibiting effect on the olefin substance, and particularly, BHT, BHA, PG and TBHQ are excellent in effect, and the occurrence of polymerization side reactions can be effectively suppressed. Meanwhile, as p-methoxycinnamic acid is used for the preparation of the sunscreen agent in the invention, the antioxidant is preferably BHT (butylated hydroxytoluene) in view of safety to human bodies.

in a preferred embodiment, the antioxidant is used in a molar ratio of (40-80) x 10-6:1, preferably (50-60) x 10-6:1, based on the molar ratio of the antioxidant to the 4-methoxy halogenobenzene.

In the invention, before the temperature rise reaction in the step 2), the gas in the reaction system is replaced by inert gas, so that the atmosphere of the reaction system is inert atmosphere, wherein the inert gas is selected from nitrogen, CO2 gas or helium, and is preferably nitrogen. Meanwhile, CO2 generated by the catalyst II in the reaction process is inert gas, so that the inert atmosphere of the reaction system is favorably maintained.

And after the coupling reaction is finished, adding activated carbon, and stirring for 30-60 min at 80-90 ℃ for removing impurities such as polymers generated by the reaction. And after stirring, filtering, and mechanically using a filter cake for 5-15 times.

And adding an acidifying agent into the filtrate until the system pH is 1-2, maintaining stirring for 30-60 min, and filtering to obtain the p-methoxycinnamic acid. The acidifying agent can be sulfuric acid, hydrochloric acid, or nitric acid, etc., such as 55 wt% H2SO 4.

In the invention, stirring is always carried out in the coupling reaction stage, the stirring speed is kept at 1000-1200 r/min, and the water-soluble reaction raw material (acrylate) and the oil-soluble reaction raw material (4-methoxy halobenzene) are fully mixed.

In the invention, the method also comprises the step of carrying out post-treatment on the p-methoxycinnamic acid crude product obtained in the step 2) and purifying to obtain the p-methoxycinnamic acid.

According to the invention, the post-processing comprises: dissolving the p-methoxycinnamic acid crude product in a proper amount of organic solvent, heating to dissolve, slowly cooling to a temperature T1, crystallizing after crystal is separated out, continuously cooling to a temperature T2, preserving heat and growing crystal, and filtering to obtain p-methoxycinnamic acid; preferably, the organic solvent is leached to obtain the p-methoxycinnamic acid product.

Preferably, the temperature rise and dissolution temperature is 140-145 ℃;

Cooling to the temperature T1 (90-100 ℃) for 1-2 h, and growing crystals for 0.5-1.5 h after crystals are precipitated;

And continuously cooling to the temperature T2 (23-35 ℃), and keeping the temperature for grain growth for 0.5-1.5 h.

By adopting the recrystallization method, particularly the sectional crystallization, the product with high purity and high yield can be obtained.

Wherein the organic solvent is selected from toluene, ethylbenzene, p-xylene, o-xylene, acetonitrile, ethyl acetate or isooctanol, preferably isooctanol. The weight ratio of the organic solvent to the p-methoxycinnamic acid crude product is (2-6): 1, preferably (2-4): 1; after the p-methoxycinnamic acid is obtained by filtration, the filtrate is used for reuse.

Through the steps, particularly, the methods of salifying, acidifying, crystallizing, dissolving a solvent such as isooctanol, recrystallizing, leaching and the like are comprehensively adopted, and finally the high-quality p-methoxycinnamic acid can be prepared at low cost, and is a white crystal, and the HPLC content (area normalization method): not less than 99.5%; the content of single impurity (HPLC) is less than or equal to 0.2 percent, and the content of total impurity (HPLC) is less than or equal to 0.5 percent.