Clean-up catalysts convert biodiesel waste into chemicals

Researchers from Rice University have found that palladium-gold nanoparticles, found to be suitable catalysts for cleaning polluted water, are also able to convert biodiesel waste into valuable chemicals. Their research has been published in the Royal Society of Chemistry’s journal Chemical Science.

Catalysts cause other compounds to react with one another faster than they would normally, often by bringing them into close proximity. Palladium and gold - and mixtures of the two - have been recognised as extremely effective catalysts; gold is valued because it doesn’t tarnish or oxidise, and palladium is especially good at binding and inducing molecules to reduce or oxidise.

For several years, chemical engineer Michael Wong and his team have focused on using the tiny metallic nanoparticles to break down carcinogenic and toxic compounds. In his latest study, Wong examined whether palladium-gold nanocatalysts could convert glycerol, a waste by-product of biodiesel production, into high-value chemicals.

Rice University scientists Michael Wong, Zhun Zhao and James Clomburg. Credit: Jeff Fitlow/Rice University.

The team build their catalysts on gold spheres that are about four nanometres in diameter. The spheres are partially covered with palladium, so that the particles’ surface contains some gold and some palladium. This was demonstrated to bring the two metals together with better control; covering 60-80% of the gold’s surface area with palladium typically produced the ideal catalyst.

In the latest study, Wong, Rice graduate student and lead author Zhun Zhao and colleagues from Rice, Argonne National Laboratory and the University of Groningen in Holland used high-powered X-ray spectroscopy and other techniques to show that the ideal coverage area for glycerol catalysis was about 60%. The researchers’ volcano plot depicted a balance of palladium and gold that is 10 times faster at converting glycerol than catalysts of either metal alone.

“Palladium by itself oxidises, which is not good because it slows down the catalysis,” Zhao said. “We found that the gold in our catalysts helps stabilise the palladium and prevents it from degrading.”

In previous studies, the nanocatalysts were used in reduction reactions - chemical processes marked by the addition of hydrogen. In the latest tests on glycerol conversion, the nanocatalysts spurred an oxidation reaction, which involves adding oxygen. Wong remarked that oxidation and reduction are “often thought of as being in opposite directions”.

And yet, said Zhao, “The catalysts in our tests had extremely high durability. Our best catalyst produced a glycerol product with higher purity and in less time than anything else we found in the literature.”

Wong said the research opens up an exciting new area of exploration for his lab, noting, “Now that we understand how these work with glycerol, we can study reactions of other biomass molecules like glucose, a building block of plants.”

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