Greener Nylon: One-pot recipe could eliminate industrial leftovers

Each year, polymer makers around the world produce roughly 4 billion kilograms of nylon-6, a type of nylon used to make items ranging from clothing to carpets to car parts. Now, researchers have devised a one-step process for making the primary ingredient of nylon-6, a simplification that could eliminate an abundant by-product of the industry’s current two-step process.

As its name implies, nylon-6 has six carbon atoms in its basic molecular unit, or monomer, which is called caprolactam. This monomer polymerizes into long chains to form nylon-6. The chemical reactions that produce caprolactam are cumbersome, says John Meurig Thomas, a solid-state chemist at the University of Cambridge in England.

The widely used two-step process requires large amounts of sulfuric acid, which is corrosive. What’s more, for every kilogram of caprolactam manufactured, says chemist Robert Raja, also of the University of Cambridge, 4 kg of ammonium sulfate are generated. Although this by-product can be used as a low-grade fertilizer, much of it ends up in landfills, says Raja.

With industrial demand for nylon increasing, Thomas and Raja set out to design a more efficient and environmentally friendly method for producing caprolactam. “We chose a big problem,” says Thomas.

At the heart of the process that they devised are porous grains of aluminophosphate. Each grain is a few microns in diameter, and each of its pores measures less than a nanometer across.

To turn the grains into catalysts that can convert starting ingredients into caprolactam, the researchers tweaked the pattern in each grain’s crystalline lattice of aluminum and phosphate ions. Every so often, the team substituted a cobalt or a manganese ion for an aluminum ion and replaced a phosphate ion with a silicon ion.

Swapping these ions created miniature reactive areas in each grain. “Each site does one particular part of the job,” says Raja.

The starting material for caprolactam is a petroleum product called cyclohexanone. Thomas and Raja also use pressurized air and ammonia. These three materials diffuse through the catalyst grains’ pores, coming in contact with the reaction centers. There, the cobalt and manganese ions produce an intermediate compound. The silicon ions finish the reaction, converting this intermediate into caprolactam.

In the Sept. 27 Proceedings of the National Academy of Sciences, Thomas and Raja say that their catalyst can produce caprolactam without requiring sulfuric acid or generating ammonium sulfate.

The new catalytic method “could have a large impact on the industrial landscape over the next decade,” says Alan Levy, an organic chemist at Morristown, N.J.–based Honeywell International, which produces caprolactam. He notes, however, that the reaction’s 8-hour run time will need to be decreased and the yield will need to be increased before the process can become commercial.

Aimee Cunningham is the biomedical writer. She has a master’s degree in science journalism from New York University.

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