While virtually all process technologies are based on oxidation of KA with nitric acid, there are many variations on this theme. And, in addition to these established approaches, there are have been many attempts over the years to improve adipic acid production technologies by eliminating the need for nitric acid by either using air, oxygen or hydrogen peroxide as the oxidant. Alternatively, if butadiene could be used as the key feedstock, oxidation chemistry would not be needed at all. The Figure below outlines the many commercial, historical and developing routes to adipic acid. 
Commercial TechnologyCurrently operating commercial production processes for adipic acid depend on the production or purchase of KA oil (a mixture of cyclohexanone, the ketone or K component, and cyclohexanol, the alcohol or A component), or of pure cyclohexanol, and its subsequent oxidation in solution to adipic acid using an excess of strong nitric acid. This report deals with KA oil/cyclohexanol production by various routes, followed by the common step of nitric acid oxidation. KA oil production from cyclohexane by the cobalt catalyst and boric acid air oxidation routes is reviewed. Production of KA oil from phenol is also covered, as is the Asahi process for cyclohexanol production from benzene via cyclohexene. Appropriately adjusted nitric acid oxidation is appended to each of these processes. Air oxidation of cyclohexane to KA oil was developed in the 1940s. The original process using a cobalt catalyst has been improved and is still in use by some producers. The process consists of molecular air oxidation of liquid cyclohexane containing a dissolved cobalt catalyst such as cobalt naphthenate. A well-established alternative route for liquid cyclohexane oxidation is the boric acid- modified oxidation of cyclohexane. Scientific Design introduced this route in the 1960s. The process is characterized by the addition of substantial quantities of anhydrous meta-boric acid, added as a slurry in cyclohexane to the first of a staged series of air-sparged oxidation reactors. No other catalyst is necessary. meta-Boric acid reacts with cyclohexanol as it is formed to give a borate ester that stabilizes the cyclohexanol and reduces its tendency to be oxidized further to form either cyclohexanone or degradation products. The result is a significant improvement in reaction selectivity to KA oil at any given conversion and also a greatly increased cyclohexanol to cyclohexanone ratio in the KA oil. The borate ester formed in the liquid reaction product is very easily hydrolyzed by hot water to boric acid and cyclohexanol. The chemistry of nitric acid oxidation of KA oil is quite complicated. Nitrous acid is an essential intermediate while the major gaseous product, perhaps surprisingly, is nitrous oxide. The surplus nitrous acid produced by various reactions eventually decomposes into nitric oxide and nitrogen dioxide and leaves the reactors in the gas stream. Nitric oxide and nitrogen dioxide can be recovered almost completely. The vent gases from the reactors are mixed with air and scrubbed with water to produce a weak nitric acid stream, which is concentrated by distillation and returned to the process. Nitrous oxide, on the other hand, cannot be recovered as nitric acid and is either vented to the atmosphere or subjected to downstream treatment. Nitrous oxide is a radiatively and chemically active trace gas which is believed to contribute to global warming by absorbing reflected infrared radiation. Most adipic acid producers, including Asahi, BASF, Bayer, and Invista, are employing catalytic or thermal processes to destroy the N2O. Recovery of waste heat from the exothermic abatement reactions is more effective with thermal systems due to their higher operating temperatures, but producers report that as little as 60 percent of the operating costs may be recovered through steam generation. More efficient systems can cover more of the operating costs or may actually provide a marginal net cost saving. Adipic acid in the effluent from the nitric acid oxidation reactors is recovered and purified by two stages of crystallization and centrifugation. Phenol is an alternative raw material in KA oil production and has some advantages, particularly for smaller scale manufacturers and for companies that are themselves large-scale manufacturers of phenol. The main advantages are that the equipment for KA oil manufacture from phenol is simpler and the process is safer than that for KA oil based on cyclohexane oxidation, resulting in reduced investment cost that helps to mitigate any scale diseconomies. Solutia, formerly Monsanto, working with the Boreskov Institute of Catalysis (BIC) in Russia, has developed a one step process to manufacture phenol from benzene, using nitrous oxide for the oxidation step. Thus, by coupling phenol production and adipic acid production, Solutia has developed a process with no net production of nitrous oxide and potentially very good production economics for both phenol and adipic acid. Asahi Chemical has commercialized a process based on partial hydrogenation of benzene to cyclohexene, which is subsequently hydrated to cyclohexanol. The partial hydrogenation reaction product is a mixture of unreacted benzene, product cyclohexene, and byproduct cyclohexane. Asahi has been active in investigating solvent mixtures suitable for extractive distillation to make this separation practical. Developing TechnologiesSingle stage air oxidation of cyclohexane has been studied by a number of research groups, typically involving systems in which the effective catalyst is Co+3. Perhaps the best small scale selectivity result to date is 88.2 mole percent to adipic acid, which exceeds that attained by air/nitric acid oxidation. Oxidation of cyclohexene with hydrogen peroxide offers reduced reaction temperature and greatly reduced pressures, but at the expense of a more costly oxidizing agent. Very low cost hydrogen peroxide would likely be necessary for this process to attain economic viability. Reaction of butadiene with carbon monoxide and water has always appeared to be an attractive option for adipic acid production because of the potential savings in raw material costs. Hydroxycarbonylation of butadiene to primarily 3-pentenoic acid using a palladium/crotyl chloride catalyst system has shown a 3-pentenoic acid selectivity of 92 mole percent. Further conversion of pentenoic acids by reaction with carbon monoxide and methanol by the use of a palladium, ferrocene, phosphorous ligand catalyst system has demonstrated a selectivity to dimethyl adipate of 85 mole percent. The dimethyl adipate is then readily hydrolyzed to adipic acid. Methyl-5-formyl valerate (M5FV), which is prepared from butadiene via sequential carbonylation, isomerization, and hydroformylation reactions, will undergo non-catalytic oxidation to monomethyl adipate. Monomethyl adipate, in turn, is known to be readily hydrolyzed to adipic acid. M5FV can be converted with a 95.6 percent yield of monomethyl adipate. Methyl acrylate can be dimerized to give a material hydrogenatable to dimethyl adipate. The dimethyl adipate, in turn, can be hydrolyzed to adipic acid. This development could possibly represent a route to adipic acid from propylene as starting material. However, this overall scheme requires five reaction steps, and thus investment requirements may be large. Heat removal from any exothermic reactions conducted at low ambient temperatures would require refrigeration. Dimerization of acrylonitrile to adiponitrile can also lead to adipic acid. Brief discussions are included for air/oxygen oxidation of cyclohexane, cyclohexanone, and n-hexane. Hydrogen peroxide oxidation of cyclohexane and cyclohexanol is also discussed. Finally, a "green" route from renewable glucose to adipic acid via muconic acid is described. EconomicsDetailed cost of production estimates are given for 5 process variations to KA oil, showing the phenol-based route from no-cost nitrous oxide to have the lowest cost + return on capital employed (ROCE) value. When phenol is made from cumene, the highest cost KA oil results, with the difference being about 13 percent between these two cases. The remaining three cases are cyclohexane oxidation by the cobalt-catalyzed and boric-acid routes and the cyclohexene hydration route. A cost of production comparison is developed for seven competing adipic acid processes. These represent nitric acid oxidation of KA oil from the five processes mentioned above plus air-only oxidation of cyclohexane and butadiene hydrocarbonylation. The latter two processes are yet to be commercialized. Commercial AnalysisThe major use of adipic acid is to produce nylon 6,6, a linear polyamide made by condensing adipic acid with hexamethylene diamine. The market for nylon 6,6 is predominantly in fibers. Nylon 6,6 fiber is used in apparel, especially ladies' hosiery, sleepwear, and underwear, carpets, and home furnishings. Other nylon 6,6 fiber uses include tire cord, fishing line, brush bristles, and in tough fabrics for parachutes, backpacks, luggage, and business cases. Engineering resins, chiefly for parts molding applications, provide the next largest outlet. Nylon 6,6 engineering resins are used in auto parts and small parts used throughout industry for such items as self-lubricating bearings, gears, and cams. Resins are often sold as compounds, which incorporate fillers and modifiers to further enhance properties. The major non-nylon uses of adipic acid are in plasticizers (dioctyl adipate, diisodecyl adipate, etc. for vinyl chloride, nitrocellulose, and cellulose acetate polymers), unsaturated polyesters, and polyester polyols (for polyurethane resins). Polyester polyols are made by reacting adipic acid with ethylene glycol or propylene glycol. Polyurethanes produced by reacting these polyester polyols with diisocyanates have higher oil and abrasion resistance when compared to polyurethanes made from polyether polyols, the other major polyol type. Demand by application, producer capacities, and supply/demand balances are provided for the United States, Western Europe, and Japan for the period 2001-2010. Significant adipic acid capacity outside of these regions is also noted. | 
