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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2022 год > № 101

№ 101



  • IV Российский конгресс по катализу «РОСКАТАЛИЗ»
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IV Российский конгресс по катализу «РОСКАТАЛИЗ»


IV Российский конгресс по катализу «РОСКАТАЛИЗ»- продолжение


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How to optimize precious metal usage in catalytic converters

A study that tracks structural changes that deactivate and regenerate catalysts suggests ways to improve these important auto parts

Catalytic converters may become deactivated by a reaction between rhodium nanoparticles and an alumina support (left) that forms inactive rhodium aluminate (right).

Catalytic converters efficiently strip pollutants and smog-forming compounds from engine exhaust by reacting these species on the surface of precious metals such as rhodium. The expensive metal may last longer and catalyst makers may be able to use it more sparingly thanks to a study that details atomic-level changes that deactivate and reactivate the catalytic metal (Chem. Mater. 2022, DOI: 10.1021/acs.chemmater.1c03513). Gasoline engine emissions are scrubbed by a three-way catalyst (TWC), which takes its name from the system’s ability to scrub three pollutants. TWCs oxidize hydrocarbons and carbon monoxide and reduce nitrogen oxides. Modern TWCs rely on rhodium nanoparticles typically supported on alumina. Earlier studies showed that exposure to oxidizing exhaust gases at high temperature can deactivate the catalyst and that reducing conditions can help restore its activity. But details of these processes have remained elusive. So Cheng-Han Li and Joerg R. Jinschek of the Ohio State University and coworkers at Ford Motor scrutinized TWCs using atomic-resolution microscopy, X-ray spectroscopy, and other methods. They exposed the catalysts to high temperatures and typical exhaust streams, which vary from oxygen rich to oxygen poor during normal engine operation. They showed that oxidizing conditions can cause rhodium nanoparticles to dissolve into the support and form rhodium aluminate, a catalytically inactive material. Reducing conditions help reverse the process. The study suggests that rhodium usage can be optimized by chemically anchoring the nanoparticles more tightly and by using alloys that resist dissolution.


New enzyme catalyzes biaryl cross-coupling reactions

Researchers engineered an artificial P450 enzyme to selectively catalyse the formation of a key motif

Using directed evolution, researchers have developed an enzyme that catalyses the formation of biaryl bonds (Nature 2022, DOI: 10.1038/s41586-021-04365-7). This new enzyme makes carbon-carbon bonds between aromatic moieties in a very selective manner, providing a new tool for the preparation of chiral ligands, pharmaceuticals, and materials.

Traditionally, chemists have synthesized biaryl bonds using metal-catalyzed reactions, such as Suzuki and Negishi cross-couplings. However robust, these processes require additional prefunctionalization steps to yield the target molecule. “It’s still challenging to make sterically hindered biaryl bonds,” explains lead author Alison R. H. Narayan, a chemist at the University of Michigan. Other structures, like electron deficient aromatic rings, also pose problems. “We envisioned a biocatalytic alternative could solve both issues and become a viable alternative for synthetic chemists,” she adds. The team used directed evolution, a technique that intentionally mutates the genes for a parent enzyme in successive rounds to evolve new catalytic functions.

The team explored different enzymes from secondary metabolic pathways involved in natural product formation to find a starting point. “Sometimes, these proteins already forge molecules that resemble our targets,” explains Narayan. “It’s all about finding a hint of reactivity, even 0.1%. Then it’s often possible to further optimize,” she adds. In this case, the team identified a cytochrome P450 enzyme that naturally catalyzes dimerization of coumarin into biaryls in Aspergillus fungi, and they engineered it to couple a broader range of substrates.

The team created over 2,000 variants of the natural enzyme and screened for those that could catalyze a cross-coupling reaction to produce chiral biaryl compounds. They used the best-yielding enzymes for successive rounds of engineering to improve yield, site selectivity and stereoselectivity. The team’s final enzyme gave a 92-fold improvement in yield.

“The catalytic and selective construction of carbon-carbon bonds is one of the most important tasks in organic chemistry,” says Ania Fryszkowska, a biocatalysis expert at Merck & Co. “Nature generates molecular complexity with ease and elegance, avoiding protecting groups and oxidative state readjustments, which is usually unachievable using traditional chemistry,” she adds.

Fryszkowska highlights how forming certain biaryl bonds is hard with the currently available tools: “It needs chiral ligands, protecting groups, auxiliary moieties,” she says. Biocatalysis offers a simple synthetic approach, which is typically safer and greener, too, since it minimizes the number of purifications and isolations along the way.

Further engineering resulted in an enzyme that offers unprecedented atroposelectivity—a preference between stereoisomers with axial chirality—in their target compound (shown). “This is a unique case in biocatalysis,” says Fryszkowska.

Atroposelectivity is key to preparing chiral ligands, which are used in asymmetric catalysis, and to synthesize certain commercial drugs like the antibiotic vancomycin and the antimalarial ancistrocladine.

By comparing the sequence of the engineered enzyme with other natural proteins, Narayan’s team has already found other promising leads: “We discovered a treasure trove of enzymes that have impressive cross-coupling activity on different classes of substrates, some with sufficient activity that engineering is not required,” says Narayan. “This panel of enzymes offers a solid starting point for others interested in planning this transformation into a synthesis, as biaryl bond formation is a bread-and-butter transformation.”


Zeolite intermediates offer new possibilities in catalysis

Amidst a well-known zeolite phase transformation, researchers have found active species that accelerate acid-catalyzed reactions

By stopping a well-known zeolite phase transformation before it completes, researchers can use the active intermediates (top, center) to accelerate acid-catalyzed reactions such as a Friedel-Crafts alkylation (bottom).

To make stable zeolites—porous inorganic materials used as industrial catalysts—researchers often convert the zeolites’ internal structure, transforming looser phases into more stable, denser forms. Now, chemists have caught these zeolites mid-transformation and discovered that the resulting intermediates supercharge three reactions important in industry (J. Am. Chem. Soc. 2022, DOI: 10.1021/jacs.2c00665). The process is highly tunable, giving chemists more control over the properties of their catalysts.

“Interzeolite transformation is a well-known process in the field,” says Javier García Martínez, whose lab at the University of Alicante conducted the study with collaborators at the National University of Colombia. “We decided to stop the process at different times and test the catalytic activity of the intermediates,” he says.

García Martínez’s team used three established methods to obtain the interzeolite transformation intermediates (ITIs): using an organic template, a surfactant, or a combination of both strategies. All are standard procedures for manufacturing mesoporous zeolites on industrial scales. The combination method creates a competition between the organic template and the surfactant, transforming zeolites considerably more slowly, which could be an advantage for controlling and monitoring the formation of ITIs, García Martínez says.

The researchers tried out the ITIs as catalysts in three popular acid-catalyzed reactions, including a Friedel-Crafts alkylation, a Claisen-Schmidt condensation, and polystyrene cracking. Overall, the turnover rate of the new catalysts was up to six times as fast as commercial zeolites.

Because the materials aren’t given time to fully set into the new structure, their pores are larger, thus allowing molecules to easily penetrate the porous network. Plus, the ITIs’ strongly acidic groups are more exposed, accelerating the acid catalysis, García Martínez says. “This is exactly why the ITIs showcase great activity.”

“Some people had tried to engineer similar catalytic materials, but this process looks more reproducible,” says Raúl Lobo, an expert in zeolites and materials engineering at the University of Delaware who was not involved in the study. Lobo points to the advantage of the improved control over the transformation and the uniformity of the new catalysts’ pores.

Matteo Cargnello, who studies materials for catalysis at Stanford University, says, “This synthesis could be easily adapted to large scales.” Companies could consider the added cost of the surfactant if their productivity increases, he adds.

In the meantime, García Martínez and collaborators have applied for a patent on the technology. “The possibilities of processing polystyrene into hydrocarbons have already attracted a few suitors,” he says.

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