Process for the preparation of p-hydroxybenzaldehyde

Abstract

This invention discloses a method for preparing p-hydroxybenzaldehyde, relating to the field of electrochemical synthesis. The preparation method includes the following steps: mixing p-cresol, acetic anhydride, a first catalyst, and a non-water-soluble solvent, and carrying out an esterification reaction to obtain p-cresol acetate; mixing p-cresol acetate, an electrolyte, and an alcohol solvent, and carrying out an electrocatalytic reaction to obtain a first solution; mixing the first solution, a second catalyst, and water, and carrying out a hydrolysis reaction to obtain a second solution; purifying the second solution to obtain p-hydroxybenzaldehyde. This preparation method uses a direct electro-oxidation method to synthesize p-hydroxybenzaldehyde, featuring a simple process flow, high efficiency, environmental friendliness, and a high yield of p-hydroxybenzaldehyde.

Description

Technical Field

[0001] This invention relates to the field of electrochemical synthesis technology, and more specifically, to a method for preparing p-hydroxybenzaldehyde. Background Technology

[0002] p-Hydroxybenzaldehyde is an important organic chemical intermediate widely used in pharmaceuticals, fragrances, pesticides, petroleum, textiles, liquid crystal materials, electroplating, and other fields. Currently, methods for synthesizing p-hydroxybenzaldehyde include the Reimer-Tiemann process, the phenol-formaldehyde process, the p-nitrotoluene process, the p-aminobenzaldehyde process, electrochemical synthesis, and the catalytic oxidation of p-cresol. Among these, the electrochemical synthesis method typically employs indirect electrooxidation. This method first converts the hydroxyl group (-OH) of p-cresol into an ether bond (COC) through an etherification reaction to protect the hydroxyl group and reduce the dissolution loss of the aldehyde during electrooxidation. Then, a strong oxidizing intermediate (e.g., Mn) is formed on the anode surface through an electrolytic reaction. 3+ The method involves using a strong oxidizing intermediate to oxidize the methyl group (-CH3) on the etherification product to an aldehyde group (-CHO). After the oxidation reaction is complete, the ether bond on the oxidation product is broken, and a hydroxyl group is reformed to obtain p-hydroxybenzaldehyde. However, the high chemical stability of the ether bond in this method makes the subsequent breaking of the ether bond and reformation of the hydroxyl group quite demanding and may trigger unnecessary side reactions, resulting in a low yield of p-hydroxybenzaldehyde. Furthermore, this method is inefficient (low current efficiency, low separation efficiency), complex to operate, and requires large amounts of water and metal ions during electro-oxidation, making it environmentally unfriendly. Summary of the Invention

[0003] The main objective of this invention is to provide a method for preparing p-hydroxybenzaldehyde, thereby solving the problems of low yield, harsh preparation conditions, low efficiency, complex operation, and poor environmental performance in the prior art when using electrochemical synthesis to prepare p-hydroxybenzaldehyde.

[0004] To achieve the above objectives, according to one aspect of the present invention, a method for preparing p-hydroxybenzaldehyde is provided, comprising the following steps:

[0005] p-Cresol, acetic anhydride, a first catalyst, and a non-aqueous solvent are mixed and subjected to an esterification reaction to obtain p-cresol acetate;

[0006] Acetic acid, p-cresol ester, electrolyte, and alcohol solvent are mixed and subjected to an electrocatalytic reaction to obtain the first solution;

[0007] The first solution, the second catalyst, and water are mixed and a hydrolysis reaction is carried out to obtain the second solution.

[0008] The second solution was purified to obtain p-hydroxybenzaldehyde.

[0009] Furthermore, the molar ratio of p-cresol, acetic anhydride, and the first catalyst is 1:(1~1.1):(0.002~0.004).

[0010] Furthermore, the mass ratio of acetic acid to p-cresol ester and electrolyte is 1:(0.011~0.0132).

[0011] Furthermore, the electrolyte includes a first electrolyte and a second electrolyte, wherein the first electrolyte includes an alkali metal halide and the second electrolyte includes a sulfonate compound.

[0012] Furthermore, alkali metal halides include at least one of potassium fluoride, sodium fluoride, potassium bromide, and sodium bromide.

[0013] Furthermore, the sulfonate compound includes at least one of sodium p-toluenesulfonate, sodium benzenesulfonate, and sodium mesitylenesulfonate.

[0014] Furthermore, the mass ratio of alkali metal halide to sulfonate compound is (0.01~0.012):(0.001~0.0012).

[0015] Furthermore, the first catalyst is selected from at least one of 4-dimethylaminopyridine, pyridine, and triethylamine.

[0016] Furthermore, the non-aqueous solvent is at least one of acetic acid, dichloromethane, toluene, and n-heptane.

[0017] Furthermore, the alcohol solvent is at least one of methanol and ethanol.

[0018] Furthermore, the mass ratio of p-cresol to the non-water-soluble solvent is 1:(2~5).

[0019] Furthermore, the mass ratio of acetic acid to p-cresol ester and alcohol solvent is 1:(5~10).

[0020] Furthermore, the esterification reaction time is 2h~5h.

[0021] Furthermore, the esterification reaction is carried out at a temperature of 55℃~65℃.

[0022] Further, after the esterification reaction is completed, a third solution is obtained. The non-water-soluble solvent in the third solution is removed, and the solution is washed with acid to obtain p-cresol acetate.

[0023] Furthermore, the current density of the electrocatalytic reaction is 3 A / dm³. 2 ~5A / dm 2 .

[0024] Furthermore, the temperature for the electrocatalytic reaction is 35℃~50℃.

[0025] Furthermore, the electrocatalytic reaction time is 3 to 5 hours.

[0026] Furthermore, the cathode used in the electrocatalytic reaction is titanium, and the anode is graphite.

[0027] Furthermore, the mass ratio of acetic acid p-cresol ester, the second catalyst, and water is 1:(0.05~0.08):(0.24~0.288).

[0028] Furthermore, the second catalyst includes at least one of hydrochloric acid, sulfuric acid, and nitric acid.

[0029] Furthermore, the hydrolysis reaction occurs at temperatures ranging from 20°C to 50°C.

[0030] Furthermore, the hydrolysis reaction takes 1 to 4 hours.

[0031] Further purification is performed by washing the second solution with alkali to obtain a fourth solution, removing the alcohol solvent and water from the fourth solution to obtain a crude product, and sublimating the crude product to obtain p-hydroxybenzaldehyde.

[0032] Furthermore, the alkaline washing method is as follows: the second solution is washed with an alkaline aqueous solution until the pH value is 6.9-7.1, wherein the alkaline aqueous solution contains an alkaline agent, which includes at least one of sodium bicarbonate and sodium carbonate.

[0033] Furthermore, the sublimation pressure is 300 Pa to 400 Pa.

[0034] Furthermore, the sublimation temperature is 110℃~120℃.

[0035] The technical solution of this invention, which uses direct electro-oxidation to synthesize p-hydroxybenzaldehyde, has several advantages: First, it helps reduce the occurrence of side reactions and improves the conversion rate of raw materials, thereby increasing the yield of p-hydroxybenzaldehyde. Second, it helps simplify the process flow and improve current efficiency and separation efficiency, thereby reducing energy consumption and improving the overall efficiency of the process. Third, it helps reduce the use of water resources and avoids the use of metal ions as oxidants, thus protecting the environment. Attached Figure Description

[0036] Figure 1 The synthetic route of p-hydroxybenzaldehyde when methanol is used as the alcohol solvent;

[0037] Figure 2 The liquid chromatogram of p-hydroxybenzaldehyde standard;

[0038] Figure 3 The liquid chromatogram of p-hydroxybenzaldehyde prepared in Example 1 is shown. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0040] As described in the background section of this invention, existing technologies for preparing p-hydroxybenzaldehyde using electrochemical synthesis suffer from low yields, stringent preparation conditions, low efficiency, complex operation, and poor environmental friendliness. To address these issues, in a typical embodiment of this invention, a method for preparing p-hydroxybenzaldehyde is provided, comprising the following steps:

[0041] p-Cresol, acetic anhydride, a first catalyst, and a non-aqueous solvent are mixed and subjected to an esterification reaction to obtain p-cresol acetate;

[0042] Acetic acid, p-cresol ester, electrolyte, and alcohol solvent are mixed and subjected to an electrocatalytic reaction to obtain the first solution;

[0043] The first solution, the second catalyst, and water are mixed and a hydrolysis reaction is carried out to obtain the second solution.

[0044] The second solution was purified to obtain p-hydroxybenzaldehyde.

[0045] In this invention, firstly, p-cresol and acetic anhydride undergo an esterification reaction under the action of a first catalyst and a non-water-soluble solvent. During the reaction, the oxygen atom of the phenolic hydroxyl group (-OH) of p-cresol nucleophilically attacks the carbonyl carbon of the acetic anhydride, converting the phenolic hydroxyl group into an acetoxy group (-OOCCH3), yielding p-cresol acetate. This step effectively protects the phenolic hydroxyl group and inhibits the occurrence of side reactions during subsequent oxidation. Then, p-cresol acetate undergoes an electrocatalytic reaction under the action of an electrolyte and an alcohol solvent, converting it into an intermediate. During the reaction, p-cresol acetate loses electrons at the anode surface and is oxidized. The methyl group on its benzene ring reacts with the alcohol solvent to generate a dialkoxymethyl intermediate, thus yielding a first solution containing this intermediate. Next, the dialkoxymethyl intermediate in the first solution undergoes hydrolysis under the action of a second catalyst and water, yielding a second solution containing p-hydroxybenzaldehyde. After purification, p-hydroxybenzaldehyde is obtained. The aforementioned preparation method utilizes a direct electro-oxidation mechanism and has the following technical advantages:

[0046] Firstly, it helps reduce the occurrence of side reactions and improves the conversion rate of raw materials, thereby increasing the yield of p-hydroxybenzaldehyde. Secondly, it helps simplify the process flow and improve current efficiency and separation efficiency, thereby reducing energy consumption and improving the overall efficiency of the process. Thirdly, it helps reduce the use of water resources and avoids the use of metal ions as oxidants, thus protecting the environment.

[0047] In some embodiments, the molar ratio of p-cresol, acetic anhydride, and the first catalyst is 1:(1~1.1):(0.002~0.004). Preferably, the molar ratio of p-cresol, acetic anhydride, and the first catalyst is 1:(1.02~1.06):(0.003~0.004).

[0048] In the above embodiments of this application, by limiting the molar ratio of p-cresol, acetic anhydride and the first catalyst to the above range, it is helpful to improve the selectivity of the reaction path and the conversion rate of the raw materials, while improving the efficiency of the esterification reaction.

[0049] In some embodiments, the first catalyst is selected from at least one of 4-dimethylaminopyridine, pyridine, and triethylamine.

[0050] In the above embodiments of this application, the first catalyst has good acylation catalysis, which can significantly improve the esterification reaction rate and selectivity between p-cresol and acetic anhydride, thereby effectively improving the yield and stability of p-cresol acetic acid.

[0051] In some embodiments, the non-water-soluble solvent is at least one of acetic acid, dichloromethane, toluene, and n-heptane.

[0052] In the above embodiments of this application, limiting the type of non-water-soluble solvent helps to adjust the polarity and / or pH of the reaction system to a suitable range, thereby providing a suitable reaction environment for the esterification reaction and improving the conversion rate.

[0053] In some embodiments, the mass ratio of p-cresol to the insoluble solvent is 1:(2~5).

[0054] In the above embodiments of this application, by limiting the mass ratio of p-cresol and non-water-soluble solvent within the above range, it is helpful to control the reactant concentration within a suitable range, thereby improving the mass transfer efficiency and temperature stability of the reaction system.

[0055] In some implementations, the esterification reaction takes 2 to 5 hours.

[0056] In the embodiments described above in this application, limiting the esterification reaction time to the above range helps to promote the full conversion of p-cresol and acetic anhydride and increase the yield of p-cresol acetic acid.

[0057] In some embodiments, the esterification reaction is carried out at a temperature of 55°C to 65°C.

[0058] In the above embodiments of this application, limiting the temperature of the esterification reaction within the above range helps to optimize the kinetic environment of the reaction system, improve reaction efficiency and selectivity, thereby increasing the purity and yield of p-cresol acetic acid.

[0059] In some embodiments, after the esterification reaction is completed, a third solution is obtained. The non-water-soluble solvent in the third solution is removed, and the solution is then acid-washed to obtain p-cresol acetate. The methods for removing the non-water-soluble solvent include, but are not limited to, vacuum distillation, washing with water, and separation to remove the aqueous phase. The reagent used for acid washing is an acidic aqueous solution, which includes, but is not limited to, at least one of dilute hydrochloric acid, dilute sulfuric acid, and dilute nitric acid.

[0060] In the embodiments described above in this application, the above-described scheme helps to remove residual reactants, alkaline impurities, and solvents from the third solution, resulting in p-cresol acetate with high purity. Acetic anhydride decomposes into acetic acid upon contact with water, and acetic acid is soluble in the aqueous phase, thus it can be removed during the liquid-liquid separation process.

[0061] In some embodiments, the alcohol solvent is at least one of methanol and ethanol. Preferably, the alcohol solvent is methanol.

[0062] In the embodiments described above, alcohol solvents not only serve as solvents but also as reactants in the electrocatalytic reaction. Limiting the types of alcohol solvents to the aforementioned range helps to regulate the polarity and solvation ability of the electrolyte, improving ion migration efficiency. Furthermore, this range helps to control the molecular size and carbon chain length of the alcohol solvent within an optimal range, thereby reducing steric hindrance and improving electro-oxidation efficiency and conversion rate. Methanol, with its relatively small molecular size and single carbon atom, reacts more readily with p-cresol acetate to obtain an intermediate, thus improving conversion rate and product purity. When methanol is used as the alcohol solvent, the synthetic route for p-hydroxybenzaldehyde is as follows: Figure 1 As shown, the methyl group on the benzene ring of p-cresol acetic acid is converted to dimethoxymethyl during electro-oxidation, and then the dimethoxymethyl group is converted to an aldehyde group through hydrolysis.

[0063] In some embodiments, the mass ratio of p-cresol acetic acid to the alcohol solvent is 1:(5-10). Preferably, the mass ratio of p-cresol acetic acid to the alcohol solvent is 1:(6-7).

[0064] In the above embodiments of this application, by limiting the above mass ratio, it is helpful to ensure that acetic acid cresol ester is fully dissolved and its concentration in the electrocatalytic reaction system is controlled within a suitable range so as to fully react and improve reaction efficiency.

[0065] In some embodiments, the electrolyte includes a first electrolyte and a second electrolyte, wherein the first electrolyte is an alkali metal halide and the second electrolyte is a sulfonate compound.

[0066] In the embodiments described above in this application, the electrolyte is a combination of alkali metal halide and sulfonate compound, which helps to synergistically optimize the ionic environment at the electrode interface, suppress the occurrence of side reactions, and thereby improve the yield of p-hydroxybenzaldehyde.

[0067] In some embodiments, the alkali metal halide includes at least one selected from potassium fluoride, sodium fluoride, potassium bromide, and sodium bromide. Preferably, the alkali metal halide is at least one selected from potassium fluoride and sodium fluoride.

[0068] In the embodiments described above in this application, limiting the types of alkali metal halides to the aforementioned range helps to improve the conductivity of the electrolyte system. Simultaneously, halide anions can promote the oxidative activation of the CH bonds on the methyl group, thereby improving the reaction efficiency. Further preferably using potassium fluoride and / or sodium fluoride as the types of alkali metal halides further helps to improve the reaction efficiency.

[0069] In some embodiments, the sulfonate compound includes at least one selected from sodium p-toluenesulfonate, sodium benzenesulfonate, and sodium mesitylenesulfonate. Preferably, the sulfonate compound is sodium p-toluenesulfonate.

[0070] In the above embodiments of this application, limiting the types of sulfonate compounds to the above range helps to improve the nucleophilic activity of alcohol solvents and the diffusion mass transfer efficiency of acetic acid-cresol ester, thereby improving reaction selectivity and promoting the formation of dialkoxymethyl intermediate.

[0071] In some embodiments, the mass ratio of alkali metal halide to sulfonate compound is (0.01~0.012):(0.001~0.0012). Preferably, the mass ratio of alkali metal halide to sulfonate compound is (0.01~0.011):(0.001~0.0011).

[0072] In the above embodiments of this application, limiting the mass ratio of the two electrolytes to the above range helps to effectively suppress the occurrence of side reactions while enhancing the conductivity of the system and promoting charge transfer, thereby further improving the reaction efficiency and the yield of p-hydroxybenzaldehyde.

[0073] In some embodiments, the mass ratio of p-cresol acetic acid to electrolyte is 1:(0.011~0.0132). Preferably, the mass ratio of p-cresol acetic acid to electrolyte is 1:(0.011~0.0121).

[0074] In the above embodiments of this application, limiting the mass ratio of p-cresol acetic acid to the electrolyte within the above range helps to control the concentration of the electrolyte within a suitable range and promotes efficient mass transfer of p-cresol acetic acid.

[0075] In some embodiments, the current density of the electrocatalytic reaction is 3 A / dm³. 2~5A / dm 2 Preferably, the current density of the electrocatalytic reaction is 4 A / dm³. 2 ~4.5A / dm 2 .

[0076] In the above embodiments of this application, limiting the current density to the above range helps to balance the electro-oxidation reaction rate and the selectivity of the reaction to generate the dialkoxymethyl intermediate, thereby increasing the reaction rate while maintaining a high yield.

[0077] In some embodiments, the electrocatalytic reaction temperature is 35°C to 50°C. Preferably, the electrocatalytic reaction temperature is 35°C to 40°C.

[0078] In the embodiments described above, limiting the temperature of the electrocatalytic reaction to the aforementioned range helps to maintain the stability of the product in the system while improving mass transfer efficiency and reaction efficiency. Furthermore, a suitable operating temperature also helps to extend the lifespan of the electrode.

[0079] In some embodiments, the electrocatalytic reaction takes 3 to 5 hours. Preferably, the electrocatalytic reaction takes 3 to 4 hours.

[0080] In the above embodiments of this application, limiting the electrocatalytic reaction time to the above range helps to fully convert p-cresol acetate into the dialkoxymethyl intermediate, while saving energy and reducing electrode wear.

[0081] In some implementations, the cathode used in the electrocatalytic reaction is titanium, and the anode is graphite.

[0082] In the above embodiments of this application, the electrode combination has good electrocatalytic oxidation performance. Titanium has good chemical stability and corrosion resistance, which can effectively reduce hydrogen evolution corrosion or metal dissolution under electrocatalytic reduction conditions. Graphite has good conductivity and chemical inertness, and can provide abundant active sites, which is beneficial to promoting the efficient oxidation of p-cresol acetate.

[0083] In some embodiments, the mass ratio of p-cresol acetic acid, the second catalyst, and water is 1:(0.05~0.08):(0.24~0.288). Specifically, the second catalyst is an acidic catalyst. More specifically, the second catalyst includes at least one of hydrochloric acid, sulfuric acid, and nitric acid. Preferably, the mass ratio of p-cresol acetic acid, the second catalyst, and water is 1:(0.05~0.06):(0.26~0.288).

[0084] In the embodiments described above in this application, limiting the aforementioned mass ratio helps to fully hydrolyze the dialkoxymethyl intermediate in the first solution; on the other hand, it helps to balance the hydrolysis rate and product stability, reducing the dissolution loss of the product p-hydroxybenzaldehyde in the aqueous phase. Furthermore, the acidic catalyst can promote the protonation of the oxygen atom in the dialkoxymethyl group, thereby accelerating the hydrolysis reaction.

[0085] In some implementations, the hydrolysis reaction is carried out at a temperature of 20°C to 50°C.

[0086] In the above embodiments of this application, limiting the temperature of the hydrolysis reaction to the above range helps to promote the efficient catalysis of the second catalyst and increase the rate of the hydrolysis reaction.

[0087] In some implementations, the hydrolysis reaction takes 1 to 4 hours.

[0088] In the above embodiments of this application, limiting the hydrolysis reaction time to the above range helps to suppress the occurrence of side reactions and improve the yield of p-hydroxybenzaldehyde.

[0089] In some embodiments, the purification method is as follows: the second solution is washed with alkali to obtain a fourth solution, the alcohol solvent and water in the fourth solution are removed to obtain a crude product, and the crude product is sublimated to obtain p-hydroxybenzaldehyde.

[0090] In the embodiments described above, alkaline washing helps neutralize residual second catalyst and acidic byproducts in the second solution, improving the stability of p-hydroxybenzaldehyde in subsequent processing. Sublimation helps obtain high-purity p-hydroxybenzaldehyde.

[0091] In some embodiments, the alkaline washing method involves washing the second solution with an alkaline aqueous solution until the pH value reaches 6.9 to 7.1, wherein the alkaline aqueous solution contains an alkaline agent, including at least one of sodium bicarbonate and sodium carbonate.

[0092] In the embodiments described above in this application, limiting the pH range helps to further improve the stability of p-hydroxybenzaldehyde during subsequent processing. Furthermore, sodium bicarbonate and sodium carbonate are relatively weakly alkaline, making the alkaline washing process gentler and more controllable, and the sodium salts generated in the reaction are easily removed, which helps to improve product purity.

[0093] In some embodiments, the sublimation pressure is 300 Pa to 400 Pa. Preferably, the sublimation pressure is 300 Pa to 350 Pa.

[0094] In the above embodiments of this application, limiting the sublimation pressure to the above range is beneficial to promoting the rapid sublimation of p-hydroxybenzaldehyde.

[0095] In some embodiments, the sublimation temperature is 110°C to 120°C. Preferably, the sublimation pressure is 110°C to 115°C.

[0096] In the above embodiments of this application, limiting the sublimation temperature to the above range is beneficial to promoting the rapid sublimation of p-hydroxybenzaldehyde while maintaining its stability.

[0097] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0098] Some of the materials used in the embodiments and comparative examples of this invention are as follows:

[0099] p-Hydroxybenzaldehyde standard: purity 99.57%, purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0100] Example 1

[0101] An embodiment of the preparation method of p-hydroxybenzaldehyde in this invention includes the following steps:

[0102] Step S1: p-Cresol, acetic anhydride, 4-dimethylaminopyridine (first catalyst), and acetic acid (a non-water-soluble solvent) are mixed and esterified at 60°C for 3 hours to obtain a third solution. Acetic acid in the third solution is removed by vacuum distillation. Then, 10% (w / w) dilute hydrochloric acid is added for acid washing. After standing and separating the layers, the aqueous phase is removed to obtain p-cresol acetate. The molar ratio of p-cresol, acetic anhydride, and 4-dimethylaminopyridine is 1:1.04:0.003. The mass ratio of p-cresol to acetic acid is 1:3.

[0103] In step S2, p-cresol acetate, potassium fluoride (first electrolyte), sodium p-toluenesulfonate (second electrolyte), and methanol are mixed and subjected to an electrocatalytic reaction to obtain a first solution containing a dialkoxymethyl intermediate. The mass ratio of p-cresol acetate, potassium fluoride (first electrolyte), sodium p-toluenesulfonate (second electrolyte), and methanol is 1:0.01:0.001:6. The electrocatalytic reaction conditions are: current density 4.5 A / dm³. 2 The reaction was carried out at a temperature of 35°C for 3 hours, with titanium as the cathode and graphite as the anode. In this invention, the contents of p-cresol acetate and dialkoxymethyl intermediate in the system were monitored in real time using liquid chromatography. The electrocatalytic reaction was terminated after the p-cresol acetate reaction was complete, and the selectivity of the dialkoxymethyl intermediate was calculated.

[0104] In step S3, the first solution, concentrated hydrochloric acid (second catalyst) with a mass concentration of 36.5% and water are mixed and hydrolyzed at 50°C for 1 hour to obtain the second solution; wherein the mass ratio of acetic acid, concentrated hydrochloric acid and water is 1:0.05:0.26.

[0105] Step S4: The second solution is washed with alkali. Specifically, the second solution is washed with a 10% sodium bicarbonate aqueous solution until the pH value is 7.0 to obtain the fourth solution. Then, methanol and water in the fourth solution are removed by vacuum distillation, and sublimation is carried out at 350 Pa pressure and 115 °C to obtain p-hydroxybenzaldehyde.

[0106] Example 2

[0107] The difference between this embodiment and Embodiment 1 is that sodium p-toluenesulfonate (the second electrolyte) is not added in step S2.

[0108] Example 3

[0109] The difference between this embodiment and Embodiment 1 is that in step S2, potassium fluoride (the first electrolyte) is replaced with potassium bromide, and sodium p-toluenesulfonate (the second electrolyte) is replaced with sodium benzenesulfonate.

[0110] Example 4

[0111] The difference between this embodiment and Embodiment 1 is that methanol in step S2 is replaced with ethanol.

[0112] Example 5

[0113] The difference between this embodiment and Embodiment 1 is that in step S2, the mass ratio of p-cresol acetate, potassium fluoride (first electrolyte), sodium p-toluenesulfonate (second electrolyte), and methanol is 1:0.011:0.0011:6.

[0114] Example 6

[0115] The difference between this embodiment and Embodiment 1 is that, in step S2, the mass ratio of p-cresol acetate, potassium fluoride (first electrolyte), sodium p-toluenesulfonate (second electrolyte), and methanol is 1:0.012:0.0012:6.

[0116] Example 7

[0117] The difference between this embodiment and Embodiment 1 is that in step S2, the mass ratio of p-cresol acetate, potassium fluoride (first electrolyte), sodium p-toluenesulfonate (second electrolyte), and methanol is 1:0.01:0.001:10.

[0118] Example 8

[0119] The difference between this embodiment and Embodiment 1 is that, in step S2, the current density of the electrocatalytic reaction is 5 A / dm³. 2 .

[0120] Example 9

[0121] The difference between this embodiment and Embodiment 1 is that, in step S2, the temperature of the electrocatalytic reaction is 50°C.

[0122] Example 10

[0123] The difference between this embodiment and Embodiment 1 is that in step S2, the reaction time of the electrocatalytic reaction is 5 hours.

[0124] Example 11

[0125] The difference between this embodiment and Embodiment 1 is that in step S3, the mass ratio of acetic acid to p-cresol ester, concentrated hydrochloric acid, and water is 1:0.05:0.288.

[0126] Example 12

[0127] The difference between this embodiment and Embodiment 1 is that in step S3, the mass ratio of acetic acid to p-cresol ester, concentrated hydrochloric acid, and water is 1:0.05:0.24.

[0128] Example 13

[0129] The difference between this embodiment and Embodiment 1 is that in step S2, the mass ratio of acetic acid to p-cresol ester, concentrated hydrochloric acid, and water is 1:0.08:0.26.

[0130] Example 14

[0131] The difference between this embodiment and Embodiment 1 is that in step S2, the temperature of the electrocatalytic reaction is 60°C.

[0132] Example 15

[0133] This embodiment provides a method for preparing p-hydroxybenzaldehyde, including the following steps:

[0134] Step S1: p-cresol, acetic anhydride, 4-dimethylaminopyridine (first catalyst), and dichloromethane (non-water-soluble solvent) are mixed and esterified at 55°C for 2 hours to obtain a third solution. The dichloromethane in the third solution is removed by vacuum distillation. Then, 5% (w / w) dilute sulfuric acid is added for acid washing. After standing and separating the layers, the aqueous phase is removed to obtain p-cresol acetate. The molar ratio of p-cresol, acetic anhydride, and 4-dimethylaminopyridine is 1:1:0.002. The mass ratio of p-cresol to dichloromethane is 1:2.

[0135] In step S2, p-cresol acetate, sodium fluoride (first electrolyte), sodium p-toluenesulfonate (second electrolyte), and methanol are mixed and subjected to an electrocatalytic reaction to obtain a first solution containing a dialkoxymethyl intermediate. The mass ratio of p-cresol acetate, potassium fluoride (first electrolyte), sodium p-toluenesulfonate (second electrolyte), and methanol is 1:0.01:0.001:5. The electrocatalytic reaction conditions are: current density 3 A / dm³.2 The reaction was carried out at a temperature of 40°C for 3.5 hours, with titanium as the cathode and graphite as the anode. In this invention, the contents of p-cresol acetate and dialkoxymethyl intermediate in the system were monitored in real time using liquid chromatography. The electrocatalytic reaction was terminated after the p-cresol acetate reaction was complete, and the selectivity of the dialkoxymethyl intermediate was calculated.

[0136] In step S3, the first solution, 15% concentrated sulfuric acid (second catalyst), and water are mixed and hydrolyzed at 20°C for 4 hours to obtain the second solution; wherein the mass ratio of acetic acid, p-cresol ester, concentrated sulfuric acid, and water is 1:0.06:0.26.

[0137] Step S4: The second solution is washed with alkali. Specifically, the second solution is washed with a 9% sodium carbonate aqueous solution until the pH value is 6.9 to obtain the fourth solution. Then, methanol and water in the fourth solution are removed by vacuum distillation, and sublimation is carried out at 300 Pa pressure and 110 °C to obtain p-hydroxybenzaldehyde.

[0138] Example 16

[0139] This embodiment provides a method for preparing p-hydroxybenzaldehyde, including the following steps:

[0140] Step S1: p-cresol, acetic anhydride, 4-dimethylaminopyridine (first catalyst), and toluene (non-water-soluble solvent) are mixed and esterified at 65°C for 5 hours to obtain a third solution. Toluene in the third solution is removed by vacuum distillation. Then, 7% (w / w) dilute nitric acid is added for acid washing. After standing and separation, the aqueous phase is removed to obtain p-cresol acetate. The molar ratio of p-cresol, acetic anhydride, and 4-dimethylaminopyridine is 1:1.1:0.004. The mass ratio of p-cresol to toluene is 1:5.

[0141] In step S2, p-cresol acetate, sodium fluoride (first electrolyte), sodium p-toluenesulfonate (second electrolyte), and methanol are mixed and subjected to an electrocatalytic reaction to obtain a first solution containing a dialkoxymethyl intermediate. The mass ratio of p-cresol acetate, potassium fluoride (first electrolyte), sodium p-toluenesulfonate (second electrolyte), and methanol is 1:0.01:0.001:5. The electrocatalytic reaction conditions are: current density 4.5 A / dm³. 2 The temperature was 40℃, the electrolysis time was 4h, the cathode was titanium, and the anode was graphite. In this invention, the contents of p-cresol acetate and dialkoxymethyl intermediate in the system were monitored in real time using liquid chromatography. After the p-cresol acetate reaction was completed, the electrocatalytic reaction was stopped, and the selectivity of the dialkoxymethyl intermediate was calculated.

[0142] In step S3, the first solution, 15% concentrated nitric acid (second catalyst), and water are mixed and hydrolyzed at 40°C for 2 hours to obtain the second solution; wherein the mass ratio of p-cresol acetic acid, concentrated nitric acid, and water is 1:0.06:0.26.

[0143] Step S4: The second solution is washed with alkali. Specifically, the second solution is washed with an 11% sodium carbonate aqueous solution until the pH value is 7.1 to obtain the fourth solution. Then, methanol and water in the fourth solution are removed by vacuum distillation, and sublimation is carried out at 400 Pa pressure and 120 °C to obtain p-hydroxybenzaldehyde.

[0144] Comparative Example 1

[0145] The difference between this comparative example and Example 1 is that p-hydroxybenzaldehyde was prepared using an indirect electro-oxidation method, including the following steps:

[0146] Step S1: p-cresol, acetic anhydride, 4-dimethylaminopyridine, and acetic acid are mixed and esterified at 60°C for 3 hours to obtain a third solution. Acetic acid in the third solution is removed by vacuum distillation. Then, 10% hydrochloric acid is added for acid washing. After standing and separation, the aqueous phase is removed to obtain p-cresol acetate. The molar ratio of p-cresol, acetic anhydride, and 4-dimethylaminopyridine is 1:1.04:0003, and the mass ratio of p-cresol to acetic acid is 1:2.

[0147] Step S2: Prepare 200 mL of a 0.8 mol / L MnSO4 aqueous solution and add it to the electrolytic reactor. Next, add 20 g of a 50% sulfuric acid solution, followed by 10 g of p-cresol acetate and 50 mL of methanol as organic solvents to initiate the electrocatalytic reaction. Monitor the contents of p-cresol acetate and p-hydroxybenzaldehyde in the system in real time using liquid chromatography. Once the p-cresol acetate has completely reacted, terminate the electrocatalytic reaction, obtaining the post-reaction solution. Based on the liquid chromatography results, the selectivity for p-hydroxybenzaldehyde is calculated to be 75%.

[0148] Step S3: Extract the reaction solution obtained in step S2 with ethyl acetate to obtain an ethyl acetate solution of p-hydroxybenzaldehyde. Wash the ethyl acetate solution of p-hydroxybenzaldehyde with sodium bicarbonate aqueous solution until the pH value is 7.0. Then remove the ethyl acetate by vacuum distillation to obtain the crude product. Sublimate the crude product at 400 Pa pressure and 120 °C to obtain p-hydroxybenzaldehyde.

[0149] Test and calculation methods

[0150] 1. In Examples 1-16 and Comparative Example 1 of this invention, the solutions obtained from the electrocatalytic reaction were detected using an Agilent 7890A liquid chromatograph. Then, the peak area ratio of each component in the liquid chromatogram was calculated using the normalization method to obtain the contents of p-cresol acetate and dialkoxymethyl intermediate. The detection conditions for liquid chromatography are as follows:

[0151] Column: Agilent Prosholl EC-C18, 250mm × 4.6mm, 0.5μm;

[0152] Mobile phase: Methanol: Water = 75:25;

[0153] Detection wavelength: 278nm;

[0154] Column flow rate: 1 mL / min;

[0155] Injection volume: 10 μL.

[0156] The selectivity of the dialkoxymethyl intermediate can be further calculated based on the contents of p-cresol acetate and the dialkoxymethyl intermediate, using the following formula:

[0157] Selectivity of dialkoxymethyl intermediate = [content of dialkoxymethyl intermediate / (content of 1-acetic acid p-cresol ester)] × 100%.

[0158] 2. The p-hydroxybenzaldehyde prepared in Example 1 was detected by liquid chromatography, and the obtained liquid chromatogram was compared with the liquid chromatogram of the p-hydroxybenzaldehyde standard for qualitative analysis.

[0159] 3. Weigh the mass of p-hydroxybenzaldehyde obtained in Examples 1 to 16 and Comparative Example 1, and calculate the yield of p-hydroxybenzaldehyde using the following formula:

[0160] Yield = (actual mass of p-hydroxybenzaldehyde / theoretically calculated mass of p-hydroxybenzaldehyde) × 100%.

[0161] Figure 2 This is the liquid chromatogram of a p-hydroxybenzaldehyde standard. Figure 3 The liquid chromatogram of p-hydroxybenzaldehyde prepared in Example 1 is shown in Table 1. The test data are shown in Table 1.

[0162] Table 1

[0163]

[0164] contrast Figure 2 and Figure 3 As shown in Table 1, the product prepared in Example 1 is p-hydroxybenzaldehyde with a purity of 99.96%.

[0165] The selectivity of the dialkoxymethyl intermediate, the actual mass of p-hydroxybenzaldehyde, and the yield of p-hydroxybenzaldehyde obtained by testing and calculation are shown in Table 2.

[0166] Table 2

[0167]

[0168] As shown in Table 2, the yields of p-hydroxybenzaldehyde in Examples 1-16 are significantly higher than those in Comparative Example 1. This indicates that the direct electrocatalytic preparation method for p-hydroxybenzaldehyde provided in this application has a higher conversion rate than the indirect electrocatalytic preparation method, which helps to improve the production efficiency of p-hydroxybenzaldehyde and reduce production costs. Furthermore, compared to the indirect electrocatalytic preparation method, this application uses less water, eliminates the need for heavy metal ions, and exhibits higher current efficiency and selectivity, demonstrating significant advantages.

[0169] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing p-hydroxybenzaldehyde, characterized in that, Includes the following steps: p-Cresol, acetic anhydride, a first catalyst, and a non-aqueous solvent are mixed and subjected to an esterification reaction to obtain p-cresol acetate; The acetic acid p-cresol ester, electrolyte, and alcohol solvent are mixed and subjected to an electrocatalytic reaction to obtain the first solution; The first solution, the second catalyst, and water are mixed and subjected to a hydrolysis reaction to obtain the second solution. The second solution was purified to obtain the p-hydroxybenzaldehyde.

2. The method for preparing p-hydroxybenzaldehyde according to claim 1, characterized in that, The method for preparing p-hydroxybenzaldehyde satisfies at least one of the following conditions: (1) The molar ratio of p-cresol, acetic anhydride and the first catalyst is 1:(1~1.1):(0.002~0.004); (2) The mass ratio of p-cresol acetate to the electrolyte is 1: (0.011~0.0132).

3. The method for preparing p-hydroxybenzaldehyde according to claim 1 or 2, characterized in that, The electrolyte includes a first electrolyte and a second electrolyte, wherein the first electrolyte includes an alkali metal halide and the second electrolyte includes a sulfonate compound.

4. The method for preparing p-hydroxybenzaldehyde according to claim 3, characterized in that, The electrolyte satisfies at least one of the following conditions: (1) The alkali metal halide includes at least one of potassium fluoride, sodium fluoride, potassium bromide and sodium bromide; (2) The sulfonate compound includes at least one of sodium p-toluenesulfonate, sodium benzenesulfonate, and sodium mesitylenesulfonate; (3) The mass ratio of the alkali metal halide to the sulfonate compound is (0.01~0.012):(0.001~0.0012).

5. The method for preparing p-hydroxybenzaldehyde according to claim 2, characterized in that, The method for preparing p-hydroxybenzaldehyde satisfies at least one of the following conditions: (1) The first catalyst is selected from at least one of 4-dimethylaminopyridine, pyridine and triethylamine; (2) The non-water-soluble solvent is at least one of acetic acid, dichloromethane, toluene, and n-heptane; (3) The alcohol solvent is at least one of methanol and ethanol; (4) The mass ratio of the p-cresol to the non-water-soluble solvent is 1:(2~5); (5) The mass ratio of the p-cresol acetic acid to the alcohol solvent is 1: (5~10).

6. The method for preparing p-hydroxybenzaldehyde according to claim 5, characterized in that, The esterification reaction satisfies at least one of the following conditions: (1) The esterification reaction takes 2 to 5 hours; (2) The temperature of the esterification reaction is 55℃~65℃; (3) After the esterification reaction is completed, a third solution is obtained. The non-water-soluble solvent in the third solution is removed, and the solution is acid-washed to obtain the p-cresol acetate.

7. The method for preparing p-hydroxybenzaldehyde according to claim 6, characterized in that, The electrocatalytic reaction satisfies at least one of the following conditions: (1) The current density of the electrocatalytic reaction is 3 A / dm³. 2 ~5A / dm 2 ; (2) The temperature of the electrocatalytic reaction is 35℃~50℃; (3) The electrocatalytic reaction takes 3 to 5 hours; (4) The cathode used in the electrocatalytic reaction is titanium and the anode is graphite.

8. The method for preparing p-hydroxybenzaldehyde according to claim 7, characterized in that, The hydrolysis reaction satisfies at least one of the following conditions: (1) The mass ratio of p-cresol acetate, the second catalyst, and the water is 1:(0.05~0.08):(0.24~0.288); (2) The second catalyst includes at least one of hydrochloric acid, sulfuric acid, and nitric acid; (3) The temperature of the hydrolysis reaction is 20℃~50℃; (4) The hydrolysis reaction takes 1 to 4 hours.

9. The method for preparing p-hydroxybenzaldehyde according to any one of claims 5 to 8, characterized in that, The purification method is as follows: the second solution is washed with alkali to obtain a fourth solution, the alcohol solvent and water in the fourth solution are removed to obtain a crude product, and the crude product is sublimated to obtain the p-hydroxybenzaldehyde.

10. The method for preparing p-hydroxybenzaldehyde according to claim 9, characterized in that, The method for preparing p-hydroxybenzaldehyde satisfies at least one of the following conditions: (1) The alkaline washing method is as follows: the second solution is washed with an alkaline aqueous solution until the pH value is 6.9~7.1, wherein the alkaline aqueous solution contains an alkaline agent, and the alkaline agent includes at least one of sodium bicarbonate and sodium carbonate; (2) The sublimation pressure is 300 Pa to 400 Pa; (3) The sublimation temperature is 110℃~120℃.

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