Preparation and Function of Adipic Acid

Adipic acid, also known as fatty acid, is a carboxylic acid organic compound. It is a white crystalline powder, slightly soluble in water, and acidic when dissolved in water. As the basic unit of various chemical products, adipic acid is by far the most important aliphatic dicarboxylic acid in industrial production. About 3 million tons of adipic acid are used in the synthesis of nylon-66 monobasic every year worldwide. In addition, adipic acid is also used as a gelling aid in synthetic polyesters, polyurethanes, lubricants, plasticizers, and adiponitrile.

1. The conventional preparation method of adipic acid is divided into two steps to complete

First, in the presence of catalyst cobalt or catalyst cobalt and anhydrous metaboric acid, cyclohexane is oxidized to a mixture of cyclohexanol and cyclohexanone (KA oil) by air, and then KA oil is oxidized to obtain adipic acid. It has matured in the 1940s, which involves the oxidation of KA oil by 40%-60% nitric acid under the catalysis of copper and vanadium. After eliminating nitrogen oxides and water, adipic acid is crystallized in nitric acid to obtain adipic acid. The by-products glutaric acid, succinic acid, valeric acid and caproic acid produced in the process are further refined to obtain adipic acid. The gaseous by-products generated in this process include carbon oxides and nitrogen oxides. Among them, NO and NO2 can be completely recovered and returned to the process in the form of nitric acid, while N2O cannot be recovered and must be processed downstream.

Although adipic acid producers have adopted technologies to reduce N2O emissions since 1996, the production of adipic acid is still the main industrial source of N2O emissions. In 1998, it was reported that cyclohexene was directly oxidized to adipic acid with an aqueous hydrogen peroxide solution without organic solvent. The method used Na2WO4˙2H2O as a catalyst, [CH3(n-C8H17)3N]HSO4 ( PTC) as a phase transfer catalyst, using 1 mol% catalyst, 1 mol% PTC and 4.4 equiv (10% excess) of 30% H2O2 aqueous solution, cyclohexene can be converted to adipic acid within 8 hours at 90 °C.

However, the direct oxidation of cyclohexene to adipic acid by hydrogen peroxide has certain obstacles in industrial application. Ammonium bisulfate PTC needs to achieve sufficient contact between the hydrophobic substrate, the hydrophilic reagent and the catalyst (tungsten peroxide). acid salts are soluble in water but immiscible with cyclohexene), however, this demonstrates that PTC can be replaced by organic ligands coordinated on tungstate catalysts. In addition, it has been reported that adipic acid can also be prepared from cyclohexene and aqueous H2O2 in the absence of any PTC. This conversion was carried out on a pilot scale in a continuous stirred tank reactor using tungstic acid as catalyst and H2SO4 and H3PO4 as cocatalysts. Unfortunately, the preparation of adipic acid requires 10 hours of reaction at 90°C, thus requiring a rather large reactor to achieve industrially relevant productivity.

2. The use of adipic acid

Adipic acid is the most valuable dibasic acid among the aliphatic dibasic acids. Adipic acid has the general properties of aliphatic dibasic acid, including salt formation reaction, esterification reaction, amidation reaction, etc. The main use of adipic acid is as a raw material for three major categories of products, namely: synthesizing nylon 66 salts, and then manufacturing polyamide resins and fibers (nylon 66). Nylon 66 is the largest end market for adipic acid; synthetic polyester polyols , for the production of polyester polyurethane; adipic acid diester plasticizer. Polyurethane is an important application field of adipic acid, and adipic acid-based polyester diols (polyols) are used to produce polyurethane elastomers, polyurethane leather, polyurethane shoe materials, and polyurethane adhesives.

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