Preparation method of p-hydroxybenzaldehyde

1. There are many process routes for the production of p-hydroxybenzaldehyde. At present, industrial production mainly includes raw materials such as phenol, p-cresol and p-nitrotoluene. 1. Phenol method Phenol method is divided into Reimer-Tiemann reaction, Gattermann reaction, phenol-trichloroacetaldehyde route, phenol-glyoxylate route, phenol-formaldehyde route and other synthetic routes. The process characteristic of the phenol method is that the raw materials are easily available, and the manufacturing process is simple, but the yield is low and the cost is high.

(1) Reimer-Tiemann reaction phenol and chloroform are heated in an aqueous alkali solution at 60-100 ° C for 2-4 h, simultaneously producing p-hydroxybenzaldehyde and o-hydroxybenzaldehyde (commonly known as salicylaldehyde), total yield About 50%, the yield of p-hydroxybenzaldehyde is only 17%. This process is mainly used to synthesize salicylaldehyde, which is a by-product of p-hydroxybenzaldehyde, but it is one of the existing main process production methods. The process conversion rate and product yield of this process are both low, and a large amount of tar is produced. The chloroform must be in excess, the unreacted phenol is not easily recovered, and the separation and purification of the product are difficult. Therefore, it is necessary to vigorously develop new catalysts, improve the selectivity of the reaction, and develop simple and effective product separation and purification methods in order to reduce costs and increase product yield.

(2) Gattermann reaction phenol and HCN, in the presence of AlCl3, pass dry HCl, carry out catalytic reaction, and decompose in ice water to obtain p-hydroxybenzaldehyde, the product yield is higher. If zinc cyanide is used instead of HCN, the yield is almost theoretical. The selectivity of this process product is high, but the disadvantage is that the cyanide is highly toxic, the operation technology is high and difficult, and the second is due to the use of waterless operation, the reaction equipment is strict and the cost is high; the third is that a small amount of salicylaldehyde is accompanied. The separation and purification of the product is difficult, thus limiting mass production.

2. The process of producing p-hydroxybenzaldehyde by the p-nitrotoluene method to p-nitrotoluene is divided into three steps of redox, diazotization and hydrolysis.

(1) p-Nitrotoluene Redox p-Nitrotoluene is simultaneously oxidized and reduced with sodium polysulfide to obtain p-aminobenzaldehyde. The specific process is: mixing p-nitrotoluene, ethanol solvent and surfactant (such as OP Tween, etc.) by mass ratio 1:5: 0.02-0.04, and adding sodium polysulfide aqueous solution at 80-85 ° C. Reaction 2-3h. The product was steam distilled to remove p-nitrotoluene and p-aminotoluene. The p-aminobenzaldehyde was obtained by extraction with diethyl ether. The reaction conversion rate and yield were both above 90%. Sodium polysulfide can be prepared from sodium hydrosulfide, caustic soda and sulfur.

(2) Diazotization and hydrolysis. Treat p-aminobenzaldehyde with 40% sulfuric acid, add 30% sodium nitrite solution at 0-3 °C, react for about 30 minutes, and decompose excess sodium nitrite with a small amount of urea to obtain p-amino group. Benzaldehyde diazonium salt solution. This solution was hydrolyzed in the presence of sulfuric acid at a temperature of 80-85 ° C for a period of about 30 min. The product is extracted, purified and dried to obtain a p-hydroxybenzaldehyde product with a yield of more than 90%. The advantage of this process is that the raw material is cheap, but the disadvantage is that the process route is long, the equipment is huge, and the intermediate product is toxic to aminobenzaldehyde, the reaction temperature of diazotization is low, and the freezing condition is high. At present, the Shanxi Jingxian Fine Chemical Plant uses this process to produce p-hydroxybenzaldehyde.

3. P-cresol catalytic oxidation method The process is to directly oxidize p-cresol to p-hydroxybenzaldehyde by air or oxygen under the action of a catalyst. In the 1980s, Japan, the United States, and Germany conducted in-depth research and reports on this process route. In the late 1980s and early 1990s, several research and production units in Jiangsu, Shanghai, Dalian and other places also researched and developed this process and used it for industrial production. The specific process is as follows: adding p-cresol, sodium hydroxide and methanol to a stainless steel autoclave, stirring until dissolved, adding cobalt acetate to seal the reaction vessel, heating to 55 ° C and starting to pass oxygen, so that the pressure in the kettle is maintained at 1.5. MPa condition reaction 8-10h, the oxygenation rate is strictly controlled during the reaction process, and the coil cooling system is arranged in the kettle. When the temperature rises during the reaction, the jacket can pass through the cooling water. At this time, the cooling water is started in the coil. Control the total amount of oxygen and keep the temperature in the kettle at around 60 °C. At the end of the reaction, the material was placed in the initial steamer, and the solvent was distilled off to recover the methanol. After adding water, hydrochloric acid was added to carry out salting out. The solid-liquid material is filtered by a centrifuge, and the obtained solid is placed in a vacuum oven and dried at about 60 ° C for 3-5 hours to obtain p-hydroxybenzaldehyde having a content of more than 98%.