The Ubiquity of Catalysis

Catalysis plays a role at some point in the process of an estimated 90 percent of all commercially produced chemical products. More importantly, life itself would not be possible without the biological catalysts known as enzymes.

Events at the 71^st Lindau Nobel Laureate Meeting showcased recent research by both Nobel Laureates and young scientists that involves different aspects of catalysis.

How Catalysis Contributes to Green Chemistry

On Wednesday 29 June, the panel discussion ‘Catalysis and Green Chemistry’ covered different approaches to green chemistry, particularly those involving catalysis. Green chemistry is defined as the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances. It works to reduce pollution at its source by minimizing hazards associated with chemical feedstocks, reagents, solvents, and products.

Panelist Richard R. Schrock received the 2005 Nobel Prize in Chemistry, along with Robert H. Grubbs and Yves Chauvin, “for the development of the metathesis method in organic synthesis.” In 1990, he produced a metallic compound that effectively facilitates metathesis, which has enabled more effective and environmentally sound processes in industry.

“Catalysis is half the message here, and that is green by the very nature of it because it generally does not produce waste or the waste can be readily recycled,” he said.

Sir David MacMillan, who received the 2021 Nobel Prize in Chemistry “for the development of asymmetric organocatalysis,” agreed. In particular, he doesn’t adhere to the belief that some types of catalysis are superior to others. For diverse problems in green chemistry to be solved, chemists should have a plethora of different tools at their disposal.

“All forms of catalysis are really important for green chemistry,” he said. “We have what we already have, which is sustainable, but we need to keep inventing new forms of catalysis as well.”

He also believes that gaps in the current approaches to green chemistry can be addressed by thinking about issues that aren’t necessarily popular or trendy. For the field of catalysis to continue advancing forward, young scientists should not give up on unconventional ideas that excite them.

“For me, I actually think that’s the most important question. What is the thing that you’re excited about that no one else is thinking about?” said MacMillan.

Young scientist Jiangnan Li of the University of Manchester described her work on metal-organic frameworks that can transform waste or residues into energy, fuels, or other value-added materials. She developed a zinc-based porous material that could selectively capture the nitrogen oxides from exhaust gas under real-life conditions.

“In general, waste-to-chemicals should be another ideal process for the sustainable future, and MOFs can play a key role in this process,” said Li.

Another young scientist on the panel, Carla Casadevall Serrano of the University of Cambridge, spoke about her research in photoredox catalysis and artificial photosynthesis. The former discipline works to develop milder and more sustainable reaction conditions to access chemical products necessary for industry and our daily lives. Artificial photosynthesis, on the other hand, aims to mimic plants and use sunlight to convert water and carbon dioxide to fuels and chemical feedstock.

“Both these disciplines are really important to green chemistry and catalysis, and they kind of complement each other,” said Serrano. “Actually, this is very important for the transition towards a more sustainable society because our carbon system still relies a lot on fossil fuels.”

Solving a 60-Year-Old Puzzle

Metathesis, a word that means “change places”, refers to reactions where double bonds are broken and made between carbon atoms with the help of special catalyst molecules. The result is that the atom groups end up changing places with one another. Today, metathesis reactions are crucial for the development of pharmaceuticals and advanced plastic materials using more efficient and environmentally friendly methods.

“Metathesis is one of the most complicated metallo-organic puzzles – or rather, one of the most complicated puzzles of any kind in chemistry,” said Schrock. “And we still haven’t solved it completely.”

In his Agora Talk on Tuesday 28 June titled ‘Tungstacyclopentane Ring-Contraction Yields Olefin Metathesis Catalysts and More’, Schrock summarised his previous work with metathesis, as well as recent enlightening findings that his laboratory released only two weeks ago. Alkene (olefin) metathesis is now roughly 60 years old and has transformed the landscape of synthetic chemistry. The technology employs metal catalysts to rearrange carbon-carbon double bonds and create compounds useful for diverse areas such as polymer chemistry, synthetic organic chemistry, and bioorganic chemistry.

However, the question of how catalysts are formed from olefins has not been answered definitely until this past year.

“How is an alkylidene formed in the absence of an acid? That is a question I asked about two years ago,” he said. “We answered that. The paper appeared about two weeks ago and probably opens up a whole new field or a new way of looking at the field, at least, from my perspective.”

Forty-three years after his initial work in this field, Schrock and his colleagues reported the first documented examples of forming a metallacyclobutane ring from a metallacyclopentene ring through a process known as ring-contraction. They established how metathesis-active methylene and metallacyclobutane complexes can be formed and reformed in the presence of ethylene.

Rotating the Cytochrome C Oxidase Catalytic Reaction Cycle

Oxygen was absent from the earth’s atmosphere for close to half its 4.5 billion year old lifespan. The Great Oxygenation Event, a period of climate change when oxygen became permanently abundant in the atmosphere, changed everything.

“This was the most significant extinction event in Earth’s history, and this was caused by the invention of the oxygenic photosynthesis by the ancestors of the present day’s cyanobacteria,” said Nobel Laureate Hartmut Michel. “How did nature cope with that? The organisms around had to survive in oxygen-free niches or remove the dangerous intermediates by superoxide dismutases or catalases, or they could remove molecular oxygen via reduction to water by oxygen reductases.”

In his Agora Talk on Wednesday 29 June titled ‘Structures of Intermediates of the Cytochrome c Oxidase Reaction Cycle Suggest a Revolution’, Michel outlined his recent work on cytochrome c oxidases, which belong to the protein superfamily of heme-copper containing terminal oxidases. He received the 1988 Nobel Prize in Chemistry “for the determination of the three-dimensional structure of a photosynthetic reaction center” and has continued to reveal the structure of essential molecules, atom by atom, throughout his esteemed career.

“Cytochrome c oxidases are the terminal enzymes of the aerobic respiratory chain, and they use the electrons from cytochrome c to reduce molecular oxygen to water,” said Michel. “These enzymes are known since quite some time, and they were discovered already in 1886, and still people are finding about the mechanism of what it does.”

While their atomic structures have been known for more than 25 years, the mechanism of action had remained a matter of debate. In a 2021 study, Michel and his colleagues determined the structure of four intermediate states of its catalytic cycle by cryo-EM. It appears that the oxidized form of the enzyme contains a peroxide bridging the atoms of its catalytic center, whereas the so-called peroxide state contains a neutral dioxygen molecule and the ferryl state a bound superoxide, meaning that the accepted models for the catalytic cycle may have to be rotated by 180 degrees.