5 Methods to Get Synthetic Vanillin

4-Hydroxy-3-methoxybenzaldehyde, commonly known as vanillin. It is an important flavor, widely used in cosmetics, soaps, beverages, cigarettes and baked goods. Meanwhile, vanillin is also an important chemical intermediate and raw material, which is widely used in pesticide synthesis and electroplating. At present, vanillin is mainly obtained by chemical synthesis, and domestic manufacturers mainly adopt the nitrosation process of guaiacol and urotropin reaction, which has low yield and serious pollution to the environment. Overseas, guaiacol is mainly used in the condensation process with glyoxylate. At present, there are other processes in the production, such as lignin method and eugenol method. The following is an overview of the chemical synthesis of vanillin.

1 Synthesis of vanillin

1.1 Coniferin method

The first chemical synthesis of vanillin was in 1847 by hydrolysis of pine resin, and then isolated from the hydrolysis product of the pine alcohol oxidation obtained, the reaction formula for because of the source of raw material pine resin, the reaction in the current industrial production has been rarely used.

1.2 Eugenol method

The route to produce vanillin from eugenol: under alkaline condition, isomerize eugenol to produce sodium isoeugenol, then oxidize the sodium salt of isoeugenol to sodium salt of vanillin by oxidizing agent, and then acidify it to get vanillin, and the reaction formula of oxidizing agent is hydrogen peroxide, potassium permanganate, oxygen, potassium perferrate, etc. There are also methods to synthesize vanillin by electrolysis of sodium isoeugenol. In addition, there is a method of electrolysis of sodium isoeugenol to synthesize vanillin.

Li Lihua et al[1] used ozone to oxidize sodium isoeugenol to produce vanillin. Sodium isoeugenol and water were added into the oxidation reaction tower, and ozonated air was passed into the tower from the top of the tower to carry out the gas-liquid oxidation reaction with the stirring of air flow. After the oxidation reaction, the organic layer was separated, and sodium sulfite solution was added to decompose the ozonized products, and the decomposed products were neutralized and washed to obtain the crude vanillin.

LamPman et al[2] reported that isomerization of eugenol to isoeugenol, interaction of isoeugenol with ethanoic anhydride to form isoeugenol acetate, and hydrolysis of isoeugenol in acidic medium to form vanillin after oxidation. They used tetraoctyl ammonium chloride as phase transfer catalyst and potassium permanganate as oxidizing agent. The yield of vanillin in this method was 4o%.

LamPman et al[3] synthesized vanillin from eugenol or sawdust, using nitrobenzene as oxidant and dimethyl sulfate as solvent, with 38% yield of vanillin.

1.3 Lignin method

Direct production of vanillin from lignin of wood (e.g. sulfite pulp waste and wood chips, etc.) is an important method for vanillin production at present. The route of vanillin production from paper waste liquid: firstly, oxidation by air under heating and alkaline condition, and then refinement of vanillin through extraction and acidification. The process was improved by Wu Guoxiong et al[4] by treating wood chips with water vapor at 22o℃ for 2 min, then washing with water at 1oo℃ for two times and filtration. The insoluble solids were treated with sodium hydroxide solution at pH=13 for 3omin and filtered.

The filtrate was treated with sulfuric acid at pH=2 for 1omin at 80℃ and filtered to obtain acid-insoluble lignin. Add 1og of lignin and 1oomL of sodium hydroxide solution into a 5-mL autoclave, add a certain amount of copper oxide, add oxygen, keep the pressure at 1.4Mpa, stirring rate at 2oor/min, heat to 17o℃, hold the reaction for 3 min, after the reaction is finished, filter and remove the copper oxide, and then the mother liquor is acidified to obtain vanillin, and get the optimal reaction conditions at 17o℃: lignin, 1og, sodium hydroxide, 14g, lignin, sodium hydroxide, sodium hydroxide, sodium hydroxide, sodium hydroxide, sodium hydroxide, sodium hydroxide, sodium hydroxide. 1og, sodium hydroxide 14g, oxygen 7. 5g, copper sulfate o. 5g, iron chloride o. 5g, reaction time 1omin.

The production of vanillin from lignin as raw material opens a new way for the comprehensive development and utilization of paper waste liquid. However, due to the fact that copper salts are usually added in the oxidation process, the copper oxide generated after the reaction is difficult to remove, and the pre-treatment of wood residue and pulp waste liquid is more complicated, the process needs to be further researched and improved.

1.4 p-Hydroxybenzaldehyde method

Using p-hydroxybenzaldehyde as raw material, vanillin can be synthesized in only two steps. Firstly, 3-bromo-4-hydroxybenzaldehyde was produced by bromination of p-hydroxybenzaldehyde, and then it reacted with methanol under the catalyst of sodium alcohol to produce vanillin, and the reaction formula is Nobel D[5] Copper carbonate was used as the catalyst, and 3-bromo-4-hydroxybenzaldehyde was reacted with methanol and sodium methanol solution, and the conversion rate was 98%; copper chloride was used as the catalyst[6] , and 3-bromo-4-hydroxybenzaldehyde was converted to vanillin with a conversion rate of 94%. The conversion rate was 94%.

Para-hydroxybenzaldehyde can be produced by the Reimer-Tiemann reaction of phenol or by oxidation of p-cresol. This method is simple, with low environmental pollution and high yield.

1.5 Guaiacol method

1) Condensation with Glyoxalic Acid When guaiacol condenses with glyoxalic acid to form 3-methoxy-4-hydroxyphenylglycolic acid under alkaline condition, 3-methoxy-4-hydroxyphenylglycolic acid was converted into 3-methoxy-4-hydroxyphenylglycolic acid at a conversion rate of 82% by adding air to a solution of sodium hydroxide under a maintained pressure of O.2Mpa.

In addition to oxidation by air or oxygen with the participation of catalysts such as copper salts, electrochemical oxidation methods have been developed both domestically and internationally. For example, UBE[11] used electrolytic oxidation to treat 3-methoxy-4-hydroxyphenylglycolic acid by electrolysis at 60℃ for 6h, then cooled the solution to below 30℃, adjusted the PH value to 3, and obtained 92.5% vanillin. The method of Beijing University of Technology in China is to add sodium hydroxide into the condensation solution to make the solution strongly alkaline, electrolyze the solution at current density O.3A/dm2, after the reaction, directly add strong acid to adjust the reaction solution to weak acidity, decarboxylate the solution, and then extract it with organic solvent to obtain vanillin.

The process of raw materials are easy to obtain, simple equipment, "three wastes" less and easy to manage, high yield, in recent years, also developed the electrochemical oxidation of mandelic acid method, there is a great prospect for development.

2) Reaction with formaldehyde: Guaiacol, formaldehyde (can be replaced by urotropin) and aryl hydroxylamine are condensed to produce Schiff base, which is then hydrolyzed and introduced into the aldehyde group to produce vanillin. This process is used for the production of vanillin in China. In China, Zhao Xiuzhen[12] studied this process, and the main factors affecting the reaction yield are reaction time and reaction temperature. Nitrosation should be carried out at low temperature and the reaction time should not be too long, otherwise the product will be peroxidized. In addition, the titration rate of nitroso-N,N-dimethylaniline also has an effect on the reaction.

Overseas, vanillin has also been synthesized directly by the reaction of guaiacol and formaldehyde. For example, sunku venkataIah[13] et al. used the condensation reaction of guaiacol sodium salt and formaldehyde to produce 4-hydroxymethyl guaiacol mixture, and then according to the addition of 5% pt/C and bismuth sulfate as a catalyst, and the reaction was carried out at 5O℃ with the introduction of pure oxygen, the hydroxymethyl group was oxidized to aldehyde group. It was also reported in the literature [14] that the reaction of guaiacol and formaldehyde under acidic conditions with aluminum and m-nitrobenzene as catalysts produced vanillin.

Nitrosation process for the production of vanillin, guaiacol conversion rate is low, the separation and purification of the product is complex, "three wastes", high consumption of raw materials, foreign countries have long been eliminated.

3) ReImer-Tiemann method Guaiacol reacts with trichloromethane under alkaline conditions to form vanillin.

Jiang Yuren et al[15] used guaiacol and chloroform reaction, using ultraviolet lamp irradiation, the conversion rate of guaiacol was 39.2%, and the reaction time was shortened.Divakar[16] in order to improve the conversion rate of guaiacol and reduce the content of vanillin isomers, using β-cyclodextrin as a phase-transfer catalyst, the results of the yield of vanillin than the non-use of cyclodextrin increased by 32%. Li Zhongzhu[17] et al. used triethylamine as the phase transfer catalyst and industrial ethanol as the solvent, and the crude yield of vanillin was 78%.

The Reimer-Tiemann method is simple, but the conversion rate is low and the neighboring product is more selective than the para product.

1.6 Other methods

1) Synthesis of vanillin from safrole Treat safrole with alkali to get isosafrole, oxidize it to produce piperonal, treat it with phosphorus pentachloride to get protocatechuic aldehyde, and then methylate it with dimethyl sulfate to get the mixture of isovanillin and vanillin. This method is seldom used in practical production because of the long process and the low yield of dimethyl sulfate as methylation reagent.

2) Composition of 2-methoxy-4-bromophenol According to literature

[18] reported that 26% vanillin and 20% guaiacol could be obtained from 2-methoxy-4-bromophenol in a toluene solution of 2-methoxy-4-bromophenol by adding carbon monoxide and hydrogen (3OMpa) and reacting with palladium acetate as a catalyst at 3OO C. This reaction is only in the laboratory stage because of the raw materials and catalyst. This reaction is only in the laboratory research stage because of the raw materials and catalysts.

3) Oxidized vanillin Yoshiko et al[19] oxidized vanillin with potassium pertechnetate as oxidant to obtain vanillin. Potassium pertechnate was produced by oxidizing trivalent iron salts with sodium hypochlorite and then treating with potassium hydroxide. The reaction was carried out at room temperature, and after completion of the reaction, vanillin was acidified with dilute sulfuric acid and extracted with benzene to obtain crude vanillin with a yield of 90%. Due to the high cost of potassium pertechnetate, it is difficult to realize the industrial production of this reaction.

2 Conclusion

Guaiacol is currently used in the production of vanillin in our country,formaldehyde and aryl hydroxylamine condensation nitrosation process. The process route is long, separation process is complicated, serious pollution to the environment, high consumption of raw materials, foreign countries have long been eliminated. Overseas mostly use guaiacol and glycolic acid condensation - oxidation route, the process equipment is simple, "three wastes" less and easy to manage, high yield. But at present, the domestic is still in the research stage, has not been able to achieve industrial production, from the actual situation in China, should focus on the development of glyoxylate process, oxidation and extraction methods can be improved, such as; can be used in electrochemical oxidation and carbon dioxide supercritical extraction. In addition, there are many paper mills in China, the development of lignosulfonate synthesis of vanillin can turn waste into treasure and improve economic efficiency.

References 

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