A common catalyst works by cycling between two different forms, overturning long-standing assumptions

The catalytic process in which materials speed up chemical reactions is essential for the production of many of the chemicals used in our daily lives. However, these catalytic processes are widespread, but researchers often don’t understand exactly how they work.

A new analysis by MIT researchers shows that the production of vinyl acetate, a critical industrial synthesis process, requires two different forms of catalysts.

Previously, it was thought that only one of two forms was needed. The new findings include MIT graduate students Deiaa Harraz and Kunal Lodaya, Bryan Tang, Ph.D. and presented today in a paper by Professor MIT of MIT Chemical Engineering Yogesh Surendranath.

There are two broad catalysts: homogeneous catalysts made up of dissolved molecules and heterogeneous catalysts, which are solid materials that provide sites of chemical reactions to the surface.

“For a long time, there’s been a general view that the catalyst is happening on these surfaces or that it’s happening on these soluble molecules,” Surendranath says.

However, new research shows that vinyl acetate has an interaction between both catalysts, a critical material that falls into many polymer products, such as rubber in shoe soles.

“What we discovered is that in cyclic dance, these solid metal materials actually have the conversion into molecules and then return to the material,” he explains.

He says, “This work raises questions about this paradigm with a catalyst flavor. In reality, there can be an interaction between both in certain cases, which is really advantageous to have a selective and efficient process.”

The synthesis of vinyl acetate has been a major industrial reaction since the 1960s and has been well studied and refined over the years to improve efficiency. This occurred primarily through a trial-and-error approach, researchers say, without accurate understanding of the underlying mechanisms.

Chemists are often familiar with uniform catalytic mechanisms, and chemical engineers are often familiar with surface catalytic mechanisms, but fewer researchers study both. This is probably part of the reason why the complete complexity of this reaction was not captured previously. However, Haraz says that he and his colleagues work in the interdisciplinary interface.

“We appreciated both sides of this reaction and found that both types of catalysts are important,” he says.

The reaction that produces vinyl acetate requires one of the components of the reaction, which activates oxygen molecules, and other components, acetic acid, something else that activates ethylene. Researchers have discovered that the optimal catalyst form for part of the process is not optimal for the other part. It can be seen that the molecular type of the catalyst performs important chemistry between ethylene and acetic acid, while it is the surface that activates the oxygen.

They found that the underlying process involved in the interconversion of two forms of catalyst is indeed corrosion, as is the rust process.

“In rust, you’ve actually found that you pass through a soluble molecular species somewhere in the sequence,” Surendranath says.

The team borrowed techniques traditionally used in corrosion research to study the process. They used electrochemical tools to study the reaction, even though the overall reaction did not require any power. By making potential measurements, the researchers determined that corrosion of palladium-catalyzed materials to soluble palladium ions is driven by an electrochemical reaction with oxygen and converted to water.

Corrosion is “one of the oldest topics in electrochemistry,” says Rodaya.

By correlating catalytic corrosion measurements with other measurements of the chemical reactions taking place, the researchers proposed that corrosion rates are limiting the overall reaction.

“That’s the chokepoint that controls the rate of the entire process,” Surendranath said.

The interaction between the two catalytic actions “actually and selectively because it uses the synergistic effect of the material surface that does good and uses molecules that do what it is good at,” says Surendranath.

This finding suggests that, rather than focusing on either solid materials or soluble molecules, researchers should consider how both interactions open up new approaches when designing new catalysts.

“Understanding why this catalyst is so effective now, you can design a specific interface that promotes a particular material or desired chemistry,” says Haraz.

Because this process has been working on for a long time, these findings may not necessarily lead to improvements in this particular process that makes acetate vinyl, but they may lead to better understanding of why the material works and improving other catalytic processes.

“The ability of catalysts to pass between molecules, materials and backs, and the role electrochemistry plays in their transformations are concepts that we are truly excited to expand,” says Rodaya.

Harraz said, “With this new understanding that both types of catalysts can play a role, is there something else out there that actually involves both?

The study states that it “imposes lighting hindered and is worth teaching at the undergraduate level.”

“This work highlights a new way of thinking. … (IT) is not only remarkable in the sense that it not only coordinates homogeneous and heterogeneous catalysts, but also describes these complex processes as half-reactions, and that electron transfer circulates between different entities.”

This story has been republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and education.