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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2024 год > № 110

№ 110

 

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  • XI Международный форум технологического развития «Технопром-2024»
  • Защита диссертаций в области катализа
  • За рубежом
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XI Международный форум технологического развития «Технопром-2024»

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Scorching heat turns MOF into discerning hydrogenation catalyst

By selectively turning alkynes into alkenes, the porous material could pave the way for more efficient purification of polymer feedstocks

A catalyst that helps convert alkynes into alkenes could offer a more efficient and greener way to achieve the tricky industrial transformation (ACS Catal. 2024, DOI: 10.1021/acscatal.4c00310).

The alkenes used to manufacture polymers often contain troublesome traces of alkynes. To purify these feedstocks, chemists use semihydrogenation to turn carbon-carbon triple bonds into double bonds. This process requires a catalyst that preferentially binds and hydrogenates alkynes but shuns alkenes, sparing double-bonded carbons from further hydrogenation into unwanted alkanes.

One option is a palladium-based Lindlar catalyst, which contains additives such as lead that dial down palladium’s activity. But this type of catalyst sometimes suffers from poor stability and can cause overhydrogenation. And using toxic lead on an industrial scale is far from ideal.

A team led by Pascual Oña Burgos at the Institute of Chemical Technology (ITQ) in Valencia, Spain, has now created a semihydrogenation catalyst based on a metal-organic framework (MOF), a type of porous material containing a scaffold of metal-based nodes holding together organic molecular struts.

Researchers had already found that when MOFs are pyrolyzed at several hundred degrees Celsius, they can form metal nanoparticles embedded in porous carbon, potentially making stable and active catalysts. Oña Burgos’s team has now shown that chemical pretreatment of a MOF before pyrolysis can produce an even more effective catalyst.

The researchers reacted a palladium-indium MOF with aniline and then pyrolyzed the material at 800 °C. That caused the MOF’s metals to form palladium-indium nanoparticles and its organic molecules to degrade into porous carbon peppered with nitrogen atoms. Compared with pyrolyzed MOFs that didn’t get the pretreatment, the strategy produced smaller nanoparticles—and hence a higher density of catalytic sites. Indium improves palladium’s preference for binding alkynes, and nitrogen helps to activate incoming hydrogen molecules.

The catalyst semihydrogenates phenylacetylene to styrene with 96% selectivity, and with 96% conversion at ambient temperature and pressure. In contrast, a commercial palladium-on-carbon catalyst offers only 50% selectivity, generating loads of the unwanted alkane product.

Having proved that their catalyst works well with liquid alkynes, “the next step is to move to the gas phase for industrial applications like the semihydrogenation of acetylene to ethylene,” Oña Burgos says.

 

Natural minerals catalyze phosphorus cycling

New research shows that natural iron oxides actively catalyze organic phosphorus into its inorganic form

Phosphorus is a key element for all life on Earth. For decades, researchers thought that, in nature, only enzymes could transform organic phosphorus—phosphates within biomolecules—into its bioavailable, inorganic form, free phosphate ions. Minerals in sediment and soil were thought to only adsorb phosphorus, not participate in the dephosphorylation reaction. But now new research shows that naturally occurring iron oxide minerals also act as catalysts (Nat. Commun. 2024, DOI: 10.1038/s41467-024-47931-z).

The work began years ago, when lead researcher Ludmilla Aristilde, an environmental chemist at Northwestern University, designed an initial experiment to follow the dephosphorylation products that developed when aden-osine triphosphate was mixed with pure ferrihydrite, an iron oxide mineral commonly found in soil. Using high-resolution mass spectrometry, the team was able to search for more than just phosphorus, the only target in past experiments. This expertise paid off; the team found organic products—adenosine diphosphate, adenosine monophosphate, and adenosine—but no inorganic phosphate. “If you were only following phosphate,” Aristilde says, “you would say there is no catalysis,” and might incorrectly assume iron oxides are noncatalytic.

A serendipitous encounter with Sharon Bone, a former lab mate and now beamline scientist at SLAC National Accelerator Laboratory, helped Aristilde search for the missing phosphate. Bone agreed to collaborate and initiated an urgent request for beam time so the researchers could analyze the surface of the ferrihydrite with advanced X-ray scattering techniques. Not only did they find phosphate adsorbed to the pure mineral, Aristilde says they also found even more organic, dephosphorylated products. Their findings indicated that the pure minerals were clearly acting as catalysts.

But other researchers remained unconvinced after Aristilde published the work in 2019 (J. Colloid Interface Sci. 2019, DOI: 10.1016/j.jcis.2019.03.086). Fellow scientists asked her, What if real-world samples are too complex to behave in the same way? So Aristilde searched for more collaborators. Two field scientists sent her samples: sediment from the bottom of a lake and soil from a forest floor. Both contained a fraction of iron oxide minerals.

With those real-world samples, Aristilde and her team repeated their initial experiments. After they analyzed the mass spectra and X-ray data, the role of minerals was clear. “Iron oxide in a soil matrix and sediment matrix is also acting as a catalyst,” Aristilde says. And not only are the minerals cata-lyzing dephosphorylation, she says, but the rate of mineral catalysis is comparable to that of an enzyme.

Finding abiotic pathways of producing bioavailable phosphorus in the natural environment has important implications for how researchers understand the phosphorus cycle, says Elizabeth Herndon, an environmental geochemist at Oak Ridge National Laboratory. “With some of the quantitative information that they provide here,” she says, “there’s the potential to incor-porate mineral-catalyzed pathways into biogeochemical models.”

Herndon thinks there are plenty more ecosystems to investigate. “This study opens the door to looking at this catalysis across more environments,” she says, “Maybe it’ll be important in some places and less in other places.”

 

Chemists make the popular fragrance compound ambrox via asymmetric catalysis

Synthetic route uses an imidodiphosphorimidate catalyst and a fluorinated alcohol solvent

Chemists have made (–)-ambrox—a compound that’s popular in perfumes—starting from (3E,7E)-homofarnesol. A team led by the Max Planck Institute for Kohlenforschung’s Benjamin List developed the route, which achieves asymmetric cyclization of a polyene using a scant amount of a chiral imidodiphosphorimidate catalyst in a fluorinated alcohol solvent (Nature 2024, DOI: 10.1038/s41586-024-07757-7).

This type of asymmetric polyene cyclization has long eluded synthetic chemists, who have been looking for alternatives to gathering (–)-ambrox from ambergris, a waxy substance vomited by sperm whales. List calls the reaction “a provocation by nature to us chemists” because nature can guide the polyene to fold itself in a way that it easily makes the desired isomer, but synthetic chemists struggle to achieve the same selectivity. Previous attempts have used stoichiometric amounts of chiral acids and were not as successful as the new route.

David Sarlah, a synthetic chemist at Rice University who was not involved in the work, calls the synthesis a significant leap forward. “It exemplifies the best case of achieving catalytic and highly selective classical polyene cyclization that mimics nature,” he says in an email. “Although they reported a limited substrate scope, this work can pave the way for translating biomimetic polyene cyclizations toward asymmetric synthesis of many important molecules.”

In recent years, fragrance scientists have developed enzymes that can make (–)-ambrox, and List says his group’s route might be competitive with that biocatalytic synthesis. The team demonstrates that its synthesis works on a multigram scale. The synthesis uses a chiral imidodiphosphorimidate catalyst, which possesses an enzyme-like microenvironment. This catalyst coaxes (3E,7E)-homofarnesol into the perfect position to become (–)-ambrox upon protonation. List says using a fluorinated alcohol solvent was key to the reaction’s success because it helped boost the ionizability of the reactants. The use of fluorinated solvents raises environmental concerns, but Mathias Turberg, a graduate student in List’s lab, says it was easy to recover and recycle the solvent.

The chemists say the approach might also be used on the polyene cycliza-tion of squalene, which is an important step in sterol synthesis. “We’ve barely scratched the surface of this exciting type of transformation,” Turberg says. “It holds great promise for efficiently producing natural products” that are currently made only by enzymes.

Chemical & Engineering News


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