Preparing a Sustainable Path to Rubber Material

A staple in car tires and sneaker soles can be made sustainably, after a new analysis of an ancient catalytic process. Butadiene, an essential component of synthetic rubber, is currently produced by the petrochemical industry from fossil reserves.

But it can be made efficiently in a one-step reaction from renewable ethanol using a modern version of an unusually old catalyst. “Butadiene is currently produced as a by-product of the petrochemical industry, which could lead to a shortage in its supply,” explains Sang Ho Chung, a research scientist in the laboratories of Javier Ruiz Martinez, with whom he co-led the work. “It’s also clear that these methods are not sustainable,” he adds.

These challenges in producing butadiene have sparked renewed interest in the Lebedev process, first developed in the 1930s, which converts ethanol to butadiene in a single catalytic reactor. “Sustainable butadiene can be made using bioethanol in the Lebedev process or even ethanol made using state-of-the-art CO2-to-ethanol processes,” says Chung.

The Lebedev process is carried out by means of magnesia silica catalysts which are produced in an unusual way called wet kneading. The method involves combining solid catalyst precursors into water under continuous mixing.

“Wet kneading is uncommon in catalyst preparation, but is often applied to prepare high-performance silicumnesia catalysts for the Lebedev process,” says Ruiz Martinez.

However, the wet-kneading-enhanced catalysts were mainly discovered by trial and error. Why some catalysts outperform others is not fully understood, says Ruiz-Martinez, and “understanding how these materials form is the first step to making better catalysts.”

Chung, Ruiz-Martinez and their colleagues at KAUST used solid-state nuclear magnetic resonance spectroscopy to study the composition of a magnesia-silica catalyst under real wet kneading conditions. “We found that two different catalyst particles form, based on the cross-deposition of silicon species on magnesium and magnesium species on silica,” says Chung. The results have been published in the journal stimulate nature.

Crucially, they showed that magnesium silicate on silica particles tended to produce ethylene as an unwanted side product. “With this understanding, we can only manufacture active catalyst particles to produce butadiene and avoid particles that produce ethylene,” Chung says.

The team also showed that the best catalysts also had certain active sites very close. “This has helped us work on the next generation of catalysts for this process,” says RuizMartinez. “We’re making good progress and already have a more selective version, which could be a major step to commercialize the process.”