New Approach Could Help Nickel Replace Costlier Metals In Chemical Manufacturing

Postdoctoral researcher Joseph Kuchta is exploring a new way to make nickel catalysts—materials that help chemical reactions happen faster and more efficiently—more predictable and effective.

Scientists at UNC-Chapel Hill are exploring a new way to make nickel catalysts—materials that help chemical reactions happen faster and more efficiently—more predictable and effective, potentially allowing the abundant, relatively inexpensive metal to take on jobs now performed by costlier metals such as palladium.

In the paper “Controlling Nickel Catalyst Reactivity by Controlling Molecular Catalyst Speciation,” published in Chemistry, researchers compared different ways of attaching molecular nickel catalysts to solid materials. They found that the way a catalyst is attached, and even the material it is attached to, can determine how long it remains active, which chemical reactions it performs and which products it makes. The challenge comes from one of nickel’s most useful characteristics: its chemical flexibility.

“Nickel can be a lot of different things,” said Joseph Kuchta, a postdoctoral researcher in the lab of Gerald Meyer, Arey Distinguished Professor in UNC-Chapel Hill’s Department of Chemistry. “During a reaction, nickel can exist in several electronic forms, known as oxidation states. They sometimes all have different rates. They can produce different products, so you lose out on this activity because you have what is called a cocktail of species.”

Palladium, a workhorse catalyst used to manufacture pharmaceuticals, specialty chemicals and other products, is generally more predictable. Nickel is much less expensive and more abundant but controlling its behavior can be difficult.

Kuchta and his colleagues tried to “wrangle” nickel by attaching individual catalyst molecules to metal oxide surfaces. Normally, molecular catalysts float freely through a liquid. That allows them to work under relatively mild conditions, but it also allows nickel molecules to encounter one another, join together and sometimes become inactive. Anchoring them to a surface keeps them apart.

“You’re effectively keeping that molecular structure, that fine structure, but you’re putting it onto a surface that is now preventing them from flying around in solution,” said Kuchta.

The researchers compared three chemical strategies for fastening nickel catalysts to a silica surface: carboxylate, ester and silanol attachments. Ester groups proved particularly interesting because they are relatively easy to use and form strong chemical bonds with the surface. They found that these covalent attachments could also pack substantially more catalyst onto the surface.

Kuchta compares the difference to receivers on a football field. A quarterback trying to hit one receiver has a much smaller target than one surrounded by dozens of receivers.

“With our covalent loading, we’re able to get 60 times more catalysts on the surface,” he said. “More available catalysts give molecules moving through the solution more opportunities to encounter one and react.”

One experiment demonstrated the advantage dramatically. A nickel catalyst that was inactive while floating freely in solution became active after researchers attached it to silica. Isolating the molecules prevented them from joining together and shutting down the reaction. But the researchers also discovered that the solid surface is more than a parking place for catalysts. It can change their chemistry.

An ester-linked catalyst attached to silica was highly active but produced several unwanted products. When the researchers attached the same type of molecular catalyst to zirconia instead, it produced the desired product with a 61% yield and continued working through three consecutive reaction cycles, producing yields of 61%, 56% and 54%. The results indicated that zirconia influenced the electronic behavior of nickel in a way that favored the desired reaction. That gives chemists another tool for catalyst design.

“Typically, when we think about a catalytic reaction, you would go: What metal, what molecules surrounding it and what’s its available oxidation state?” said Kuchta.

Chemists can fine-tune a metal’s behavior by changing the molecules surrounding it. This research adds another tool: changing the material to which the catalyst is attached can also influence how it behaves.

“The approach could ultimately help researchers develop nickel catalysts capable of replacing palladium in some reactions,” said Kuchta. “But the broader lesson is about looking beyond conventional approaches.”