ABSTRACT
Nylons comes in many types but the most common and widely used in textile and plastic industries are Nylon 6 and Nylon 6,6. Nylon 6,6 is made up of two monomers each containing 6 carbon atoms. One of them is Adipic Acid which is a dicarboxylic acid. It is manufactured by oxidation of Cyclohexanol which is produced by oxidative cleavage of cyclohexane or by hydrogenation of phenol. Commercially Cyclohexane is preferred as raw material because of its lower cost and a two-step mechanism involving Nitric acid is used to produce Adipic Acid. This process has higher selectivity and yield than other processes. The only concern with this process is the emission of nitrogen oxides in the gas effluents which is a major environmental concern.

1. Introduction
Adipic acid is an organic compound, with formula HOOC (CH2)4COOH, which is a white crystalline solid and one of the most important Dicarboxylic acid. It is Odorless, Colorless and freely soluble in Ethanol, Methanol and Acetone. Other Important properties of Adipic acid is shown in Table 1.From Industrial perspective it is used mainly in Nylon-6, 6 production. About 2.5 billion kilograms of this white crystalline powder are produced annually. In US it is mainly produced by three companies at four manufacturing plants, with nearly two-thirds of the total production, 860,000 tons capacity occurring at DuPont's two Texas facilities. The use of adipic acid in food items have started in recent times and this has created a demand of purer product. Properties | Value | Boiling Point (760mm Hg) | 337oC | Melting Point | 152oC | Flash Point (closed cup) | 196oC | Solubility in water (15oC) | 1.4 gm/100ml | Density (20oC) | 1.36gm/cm3 | pKa1 &pKa2 | 4.43 & 5.41 | pH of Satd. Sol. (25oC) | 2.7 | Viscosity (160oC) | 4.54cP | Relative Vapor Density | 5.04 |

2. Manufacturing processes
Adipic acid is manufactured either in a single step process or a two-step process. The starting material is either cyclohexane or phenol, which are oxidized under specific conditions and using specific reagents to produce adipic acid. The industrially favoured route is the 2-step oxidative cleavage of cyclohexane.
2.1 Single-Step Oxidation
The single step oxidation of Adipic acid can be achieved using either Nitric Acid or air as the oxidizing agent. The process involving Nitric Acid is not industrially viable due to high reagent cost and low yield. Using air instead of HN03 presents two advantages:
1) there is no risk of corrosion;
2) no nitrogen oxides need to be recovered, which implies lower investment costs. Here we discuss in detail the process involving air as the oxidizing agent.
2.1.2 Single Step oxidation using air
Since its early discovery and development around 1940, the process has been modified and improved several times. According to the present scheme, cyclohexane is oxidized with air or oxygen in the liquid phase, using acetic acid as a solvent and a cobalt salt as an oxidation catalyst.
It is of immense importance to choose the right conditions for the process due to the following reasons. Under mild conditions of temperature and catalyst, the oxidation of cyclohexane tends to stop at cyclohexanol and cyclohexanone. On the other hand, extreme conditions could lead to the complete combustion of cyclohexane.
The optimum reaction conditions are claimed to be 70-100 oC and a residence time of 2-6 hr.
Some researchers have advised the use of Aldehydes and Ketones as promoters as they enhance the formation of Co3+ , which serves as the oxidising agent. Another group claims that the present of water can improve the yield by around 20%.
The disadvantage and shortcoming of this process is the medium selectivity ( 70 -75 %) and conversion on a slightly lower side (50-75%).
If these disadvantages can be overcome, this process can be highly profitable commercially since an entire step could be eliminated from the production cycle.
2.2 Two-step processes
Due to the lower selectivity of the single step process, two-stage oxidation is used.
2.2.1 Phenol as substrate
In case of Adipic Production by Phenol, it is first hydrogenated to form Cyclohexanol and Cyclohexanone. For the production of Adipic Acid Cyclohexanol is prefered while Cyclohexanone is used for Caprolactum production. For achieving higher yield of alcohol we use catalysts such as Nickel, Copper or Chromium Oxide and vary the operating Condition. More than 99% phenol is converted into Cyclohexanol and small amount of byproducts are removed by distillation.
Cyclohexanol is obtained from the liquid-phase hydrogenation of Phenol with a Pd catalyst at 150 °C and 10 bar. In 1991 the production capacity for adipic acid from phenol was 15000 tonnes, or about 2% of total adipic acid capacity, in the USA. In Western Europe, only 6% of the adipic acid was still produced from phenol in 1991; in Japan, this route is no longer used.
2.2.2 Cyclohexane as substrate
Preferring Cyclohexane over phenol for manufacturing Adipic Acid is because of the lower cost of the former one.
In the first step of the oxidation, cyclohexane is oxidized to a Ketone-Alcohol mixture at 125-165 °C and 8-15 bar. The reaction is conducted in the liquid phase with air and Mn- or Co-salts, e. g., the acetate or the naphthenate, as catalysts.

Cyclohexyl hydroperoxide, the primary product of this radical reaction, reacts further by means of a catalyst to the alcohol and ketone. A small quantity of adipic acid is also formed at this stage, along with glutaric and succinic acid from stepwise oxidative degradation. They are usually present as the cyclohexyl esters. Cyclohexane conversion is therefore limited to 10-12% in order to increase the alcohoVketone selectivity to 80-85%. The unreacted cyclohexane is distilled off and recycled to the oxidation. The acids are extracted with aqueous alkali, the esters being simultaneously hydrolyzed. Cyclohexanone and cyclohexanol (ratio 1:1) are obtained in 99.5% purity by distillation.
The selectivity of the reaction can be increased by the presence of boric acid. In this case, it increases to over 90% with a mole ratio of alcohol to ketone of about 9:1. The cyclohexane conversion remains almost unchanged at 12-13%.
In the second stage of the KA oxidation to adipic acid, products from other processes can also be employed, e.g., pure cyclohexanol as obtained from the liquid-phase hydrogenation of phenol with a Pd catalyst at 150 °C and 10 bar, as depicted in the figure above.
The products are oxidized further:

This second step of oxidation can be achieved by two methods.
1. HNO3 and NH4-metavanadatc/Cu-nitrate
2. Air and Cu-Mn-acetate
In the first process, the KA is oxidized with 60% HNO3 at 50-80°C and atmospheric pressure in the presence of the catalysts cited. The selectivity to adipic acid is up td 96%. In the course of the reaction, the cyclohexanol is first oxidized to cyclohexanone, which then - after a-nitrosation to the intermediate 2-nitrosocyclohexanone - is further oxidized. A mixture of nitrogen oxides which can be reoxidized to HNO3 is emitted from the HNO3.
In the second process, reaction mixtures rich in cyclohexanone are preferred. The air oxidation is conducted in the liquid phase, usually in acetic acid as solvent, at 80-85°C and 6 bar in the presence of Cu- and Mn-acetate. Adipic acid crystallizes from the product solution as it cools. The crude product is purified by recrystallization. The selectivity to adipic acid is said to approach that of the HNO3 process. The advantage of the air oxidation lies mainly with the absence of corrosive HNO3.
Major players operating the manufacturing of Adipic Acid by this technique are BASF, Bayer, Du Pont, ICI, Inventa, Scientific Design, and Vickers-Zimmer.
2.2.3 Industrial Production Process
In the first-stage as described in the process above, Air, catalyst, cyclohexane, and in some cases small quantities of benzene are fed into a multiple-stage column reactor or a series of stirred tank reactors, with a low conversion rate from feedstock to oxidized product. This low rate of conversion necessitates effective recovery and recycling of unreacted cyclohexane through

distillation of the oxidizer effluent.
In the second stage, the conversion of the intermediates cyclohexanol and cyclohexanone to adipic acid uses the oxidation with 45 to 55 percent nitric acid in the presence of copper and vanadium catalysts. This results in a very high yield of adipic acid. The reaction is exothermic, and can reach an autocatalytic runaway state if temperatures exceed 150° C (300° F). Process control is achieved with the use of large amounts of nitric acid. Nitrogen oxides are removed by bleaching with air. Water is removed by vacuum distillation, and the adipic acid is separated from the nitric acid by crystallization. Further refining, typically recrystallization from water, is needed to achieve polymer-grade material. After the above described reactions are carried out, the wet adipic acid crystals are separated from water and nitric acid. The

product is dried and cooled before packaging and shipping. Dibasic acids (DBA) may be recovered from the nitric acid solution and sold as a co-product. The remaining nitric acid is then recycled to the second reactor.

2.3 New developments and methods
Research has been going on for the development of new processes and manufacture of Adipic Acids from other substrates, especially straight chain compounds.
A process worth mentioning is the synthesis from Butadiene. It involves the carbonylation of the intermediate 1,4-dimethoxy-2-butene (Monsanto), or two-step carbonylation of butadiene in the presence of methanol.

A cobalt carbonyl system with, for example, pyridine ligands serves as the catalyst. The first step is run at 130 °C and 600 bar; the second takes place at 170 °C and 150 bar. The yield from both steps is 72%, based on butadiene. BASF has developed this process in a pilot plant stage, but it has not yet been used commercially.
3. Environmental Impacts of the two-stage process
Emissions from the manufacture of adipic acid consist primarily of hydrocarbons and carbon monoxide from the first reaction, oxides of nitrogen from the second reaction, and particulate matter from product cooling, drying, storage, and loading.
The waste gas stream resulting from the oxidation of cyclohexane will, after removal of most of the valuable unreacted cyclohexane by one or more scrubbers, still contain CO, CO2, and organic compounds. In addition, the most concentrated waste stream, which comes from the final distillation column (sometimes called the "nonvolatile residue") will contain metals, residues from catalysts, and volatile and nonvolatile hydrocarbons. Both the scrubbed gas stream and the nonvolatile residue may be used as fuel in process heating units.
The nitric acid oxidation of the KA results in two main streams, one liquid and one gaseous. The liquid effluent contains primarily water, nitric acid and adipic acid, as well as significant quantities of NOx, which are considered a part of the process stream with recoverable economic value. The NOx component is stripped from the stream in a bleaching column using air. The gaseous effluent from oxidation contains NOx, CO2, CO, and DBA. The gaseous effluent from both the bleaching column and the oxidation reactor are typically passed through an absorption tower to recover most of the NOx; this process does not significantly reduce the concentration of nitrous oxide (N2O) in the stream, however.
The absorber offgases and fumes from tanks storing solutions high in nitric acid content are controlled by extended absorption at one of the three plants utilizing cyclohexane oxidation, and thermal reduction at the remaining two. Extended absorption is accomplished by simply increasing the volume of the absorber, extending the residence time of the NOx-laden gases with the absorbing water, and by providing sufficient cooling to remove the heat released by the absorption process. Thermal reduction involves reacting the NOx with excess fuel in a reducing atmosphere.
4 .Conclusion
The present report contains an overview of the current status of single-step and two-step oxidation processes for the production of adipic acid from cyclohexane, which are compared with alternative adipic acid preparation processes. Newer methods of synthesis have been discussed. Further improvents need to be made in imporving the selectivity of the single step process and lowering the environmental impacts of the two-stage traditional process.
5 .Aknowledgements
The authors are thankful to Professor Sreedevi Upadhyayula for her guidance throughout the course. Her motivation and a wonderful & simple way of teaching has been inspiring and helped us to develop a curious and inquisitive approach towards learning industrial processes.
6. References
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