Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2000, 39 (13), 2310 - 2313, 2313 - 2316
Quicker from Crude Oil to Silk Stockings
Specific Oxidation of Hydrocarbons
Adipic acid is an important building block for a series of chemical products, for example synthetic material and fibers like Nylon - the material from which women’s hosiery is made. On a large-scale, adipic acid, a hydrocarbon chain of six carbon atoms with an acid functionality at each end, is usually generated from cyclohexane. This six-membered hydrocarbon ring has to be oxidized and broken open over several process steps.
Saturated hydrocarbons like cyclohexane are important components of crude oil. Unfortunately they are very unwilling to undergo reactions. It takes a costly, polluting oxidation to turn these into valuable starting materials for chemical industry. British researchers working with Sir John Meurig Thomas are not backing down from this challenge: they are searching for a way to convert saturated hydrocarbons with air to the desired products in one step.
Thomas and his coworkers have now developed a new class of catalyst to go directly from cyclohexane to adipic acid with no detours. The catalyst is based on aluminum phosphates which they have designed so as to contain micropores. A small portion of the aluminum is replaced by iron ions. These sit on the interior walls of the tiny cages and are the active centers at which the reaction occurs. It is important to carefully tune the shape and size of the pores in order to obtain the desired selectivity: only products with the correct molecular form and size can leave the cages. Others, such as the cyclic intermediates of the cyclohexane oxidation, remain inside the pore, where they are forced to continue reacting. The linear chains, on the other hand, are sufficiently flexible to leave the cages. In this way adipic acid is obtained as the main product - along with a number of different byproducts.
"The primary goal of our work was to deliver a design prototype. Many new catalysts for the environmentally friendly conversion of organic molecules should be based on this," explains Thomas. His group has already reached an important milestone - a catalyst, whose cages contain cobalt instead of iron, and whose pore-size is tailored to hexane: this allowed them to obtain appreciable quantities of adipic acid directly from hexane.
But this victory is only one step towards the aim to make methane, the main component of natural gas, available as starting material for the chemical industry.