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Industrial preparation method of benzoic acid

Benzoic acid industrial applications

Industrial preparation:

Initially, benzoic acid was produced by dry distillation of benzoin gum or hydrolysis of alkaline water. It can also be produced by hydrolysis of hippuric acid. In industry, toluene, o-xylene or naphthalene are commonly used as raw materials to prepare benzoic acid. The above raw materials can be obtained from coal tar or petroleum. In addition, benzoic acid can be produced as a by-product when benzaldehyde is produced from toluene. The industrial production methods of benzoic acid mainly include toluene liquid phase air oxidation method, trichlorotoluene hydrolysis method, phthalic anhydride decarboxylation method, and benzyl halogen oxidation method.

Toluene liquid phase air oxidation method:

The first company to use this method to produce benzoic acid was the American company Alied. Commonly used catalysts are soluble cobalt salts or manganese salts, with acetic acid as the solvent. The reaction mechanism is a free radical reaction, the reaction temperature is about 165°C, the pressure is 0.6~0.8 MPa, and the reaction is an exothermic reaction. The main by-products are benzaldehyde, benzyl alcohol, o-methylbiphenyl, biphenyl, p-methylbiphenyl and esters. All by-products can be recycled and utilized, especially benzaldehyde and benzyl alcohol, whose unit price is often 4 to 5 times that of benzoic acid, which can greatly increase the output value and profit of the device.

Although there are gas-phase oxidation methods and stoichiometric oxidation methods for producing benzoic acid using toluene as raw material in the world, the air-liquid-phase catalytic oxidation method not only has less waste, but also has mild reaction conditions, so it is still the main production method.

Trichlorotoluene hydrolysis method:

Trichlorobenzylbenzene is obtained by photochlorination reaction of toluene at 100~150°C. It reacts with water in the presence of ZnCl2 (or lime milk and iron powder) to obtain benzoic acid. Based on trichlorobenzylbenzene, the benzoic acid yield is 74% to 80%. Since this method consumes chlorine, the heating corrosion of HCl aqueous solution is extremely serious. Therefore, this method can only be a supplementary method for recycling the by-products of benzaldehyde and benzyl alcohol produced by chlorinated hydrolysis of toluene.

Phthalic anhydride decarboxylation method:

This method can be divided into liquid phase method and gas phase method. The former catalyst is a mixture of equal amounts of chromium phthalate and sodium salt; the latter’s decarboxylation catalyst is equal amounts of copper carbonate and calcium hydroxide. By-products include phthalic acid, small amounts of biphenyl, benzophenone and anthraquinone.

Benzyl halogen oxidation method:

Benzoic acid can also be produced using benzyl halide as raw material and KMnO4 as oxidant. Domestic articles have reported that this method is used to prepare benzoic acid. White benzoic acid crystals can be obtained without decolorization and recrystallization during post-processing, and the yield is as high as 86%. The reaction is carried out in the presence of a phase transfer catalyst.

Among the above four synthetic methods, the toluene liquid phase air oxidation method is the main one; the products obtained by chlorination and hydrolysis of toluene are not suitable for use in the food industry; the benzoic acid obtained by the phthalic anhydride decarboxylation method is difficult to refine and has high cost. It is used in the manufacturing process of pharmaceutical products with small dosage.

Laboratory preparation:

Principle: The methyl group of toluene is oxidized into carboxylic acid by acidic potassium permanganate solution to generate benzoic acid, potassium salt (K2SO4), manganese salt (MnSO4) and water.

The equation is as follows:

In fact, when PH=7, toluene + potassium permanganate → potassium benzoate + potassium hydroxide + manganese dioxide + water
Drugs and dosage: toluene 1.5 g (1.7 mL, 0.016 mol), potassium permanganate 5 g (0.032 mol), cetyltrimethylammonium bromide 0.1 g.

Operational procedure: Use a 100 mL round bottom flask. Install the backflow device. Add 5 g of potassium permanganate, 0.1 g of cetyltrimethylammonium bromide, 1.7 mL of toluene and 50 mL of water to the reaction flask, stir and heat to boiling (stir vigorously, boil violently) to keep the reactant solution stable. boiling.

When a large amount of brown precipitate is generated, the purple color of potassium permanganate becomes lighter or disappears, and the toluene layer disappears, the reaction is basically over. The manganese dioxide precipitate is filtered out, and the filtrate is acidified with concentrated hydrochloric acid to precipitate the benzoic acid precipitate, and the crude product is obtained by suction filtration.

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