With the help of a cleverly designed catalyst, chemists at the University of Cambridge, in England, have developed a one-step procedure for making -caprolactam from cyclohexanone (Proc. Natl. Acad. Sci. USA, published online, www.pnas.org/cgi/doi/10.1073/pnas.0506907102). Because the new route is solvent-free and doesn't generate any unwanted ammonium sulfate by-products, it could offer industry an environmentally benign process for preparing -caprolactam--the immediate precursor to nylon-6.
Each year, chemical companies make several billion pounds of -caprolactam. All but a fraction of the material is polymerized to make nylon-6, a popular polymer used in carpets, automotive parts, sporting goods, films, and packaging. The standard method for synthesizing -caprolactam involves conversion of cyclohexanone to its corresponding oxime, followed by a Beckmann rearrangement to generate the cyclic amide. This route has one major drawback: It produces immense quantities of unwanted ammonium sulfate--as much as 2.3 lb for every pound of -caprolactam.
John Meurig Thomas and Robert Raja thought they could eliminate this by-product with judicious use of a bifunctional catalyst. They designed a nanoporous aluminophosphate catalyst with redox-active cobalt sites and acidic silicon, magnesium, or zinc sites integrated into the catalyst's framework.
The cobalt redox centers generate hydroxylamine in situ from ammonia and air. After hydroxylamine and cyclohexanone form the intermediate oxime, the acidic centers nudge the Beckmann rearrangement along via acid catalysis. The nanoporous nature of the catalyst allows products and reactants to move freely throughout the system, a strategy that's been evolving in Thomas' lab for 15 years.
"We're proud of the fact that we've been able to tackle an environmentally harmful process using genuinely benign reagents and catalysts," Thomas says.