Adipic acid synthesis

The total hydrogenation of benzene derivatives represents an important industrial catalytic transformation, in particular with the conversion of benzene into cyclohexane, a key intermediate in adipic acid synthesis, which is used in the production of Nylon-6,6 (Scheme 1). This reaction is still the most important industrial hydrogenation reaction of monocyclic arenes [1]. 

The work forms part of intensive efforts to find an adipic acid synthesis based on butadiene. 

This adipic acid synthesis poses environmental and health concerns because it has benzene as a starting material. Benzene is a volatile organic compound and its inhalation can lead to leukemia and cancer. This compound, therefore, is often an occupational hazard to those who work with it or come in contact with it. Moreover, benzene is a byproduct of petroleum manufacture thus, it is produced from a non-renewable source. 

Besides beginning with a nonrenewable chemical feedstock, adipic acid synthesis has other problems. The use of benzene, a carcinogen and liver toxin, is undesirable, especially in a large-scale reaction. Moreover, oxidation with HNO3 in Step [3] produces N2O as a by-product. N2O depletes ozone in the stratosphere in much the same way as the CFCs discussed in Chapter 15. In addition, N2O also absorbs thermal energy from the earth s surface like CO2, and may therefore contribute to global warming, as discussed in Section 4.14. 

Over iron-phthalocyanine encaged in zeolite Y and using tertiary-butylhydroperoxide (t.-BHP) as oxidant, even cyclohexane can be converted to adipic acid. Selectivities of up to 35 % at conversions around 85 % have been reported. Unfortunately, however, a reaction time of 33 hours at 60 °C was required to achieve this conversion. Although the activity of the latter catalyst is certainly much too low to compete with the conventional catalytic systems for adipic acid synthesis, it provides interesting prospects for further developments. For the near future, we perceive that more and more groups will be working in this interesting field of catalysis by zeolite inclusion compounds. 

Chemical synthesis at the cost of generating a hazardous byproduct is not unique to adipic acid production. This and other problems characteristic of the chemical industry are illustrated by further examination of adipic acid synthesis. Benzene, the primary starting material in adipic acid manufacture, is a proven carcinogen (9). Benzene is used widely in the chemical industry, particularly as a feedstock ( ). For example, benzene is used to make phenol (4), the starting material from which a small percentage of adipic acid is currently synthesized. The United States alone product over 12 billion pounds of benzene in 1993 (10). Benzene is derived exclusively from petroleum (4,5), a non-renewable fossil fuel. Of the chemicals in the United States which are produced in excess of 10 million pounds per year, 98% are derived from petroleum feedstocks (4). Finally, extreme reaction conditions which are used in adipic acid manufacture include temperatures up to 250°C and pressures which reach 800 psi. Reaction conditions such as these are used routinely by the chemical industry. 

Synthesis of adipic acid from DHS utilizes both PCA and catechol as biosynthetic intermediates and requires additional expression of catechol 1,2-dioxgenase in the biocatalytic organism (Figure 1). Since catechol 1,2-dioxygenase is not found in E. coli, the ccuA gene encoding the enzyme has been obtained from Acinetobacter calcoaceticus. Catechol 1,2-dioxygenase catalyzes conversion of catechol to cis, c/.y-muconic acid, which is readily reduced in a separate step to afford adipic acid. Perhaps the most important characteristic of a successful catalyst for adipic acid synthesis will be the avoidance of catechol accumulation. 

Figure 13.5. Ideal plug flow reactor for adipic acid synthesis.

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