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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2021 год >  № 98

№ 98



  • Усеин Меметович ДЖЕМИЛЕВ – к 75-летнему юбилею
  • Владимир Борисович КАЗАНСКИЙ – к 90-летнему юбилею
    О научно-организационной деятельности в 2020 году
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Усеин Меметович ДЖЕМИЛЕВ – к 75-летнему юбилею

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Владимир Борисович КАЗАНСКИЙ – к 90-летнему юбилею

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Научный совет по катализу

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Научный совет по катализу (продолжение)

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Tandem catalyst converts propane to propylene

Nanoparticles unite platinum and indium oxide catalysts for better yields at lower temperature

The tandem catalyst uses platinum to turn propane (C3H8) into propylene (C3H6)
before handing hydrogen atoms to indium oxide for conversion into water.

By combining the catalytic powers of platinum and indium oxide in finely tuned nanoparticles, researchers have improved a reaction that converts propane into propylene, an important process in the petrochemical industry (Science 2021, DOI: 10.1126/science.abd4441).

The proof-of-concept work shows that nanostructured tandem catalysts—which host different reactions side-by-side on a single nanoparticle—have the potential to play a bigger role in industrial processes, says Justin M. Notestein of Northwestern University, who led the work with his colleague Peter C. Stair.

The team targeted propylene production because of its significance in the chemical industry—global output reached 110 million metric tons last year, much of it destined for polypropylene plastics. Changes to the raw materials supplying steam crackers have shrunk the supply of propylene, while the method developed to replace this supply, propane dehydrogenation (PDH), is energy intensive and expensive. PDH plants convert propane to propylene at temperatures of 600 °C or more, and these conditions generate sooty carbon deposits that quickly deactivate the catalysts.

To solve these problems, researchers have spent decades developing the oxidative dehydrogenation of propane (ODHP), in which the hydrogen freed from propane is immediately combined with oxygen to create water. This pulls the reaction equilibrium in the right direction, requires lower temperatures, and makes less catalyst-corrupting carbon. “Given the scale at which this technology is needed, that could mean tremendous energy and cost savings,” says Ive Hermans of the University of Wisconsin-Madison, who has developed boron-based catalysts for ODHP. However, ODHP catalysts also have a nasty habit of turning propylene into CO and CO2, which means they still cannot beat the propylene output from PDH.

That’s where the new tandem catalyst comes in. It contains two catalysts, each targeting a different stage of the reaction, to increase propylene production and reduce the formation of unwanted byproducts.

To make the catalyst, the Northwestern team dotted 2 nm wide clumps of platinum onto 100 nm particles of alumina. Then they coated each particle with a 2 nm thick shell of indium oxide, using a process called atomic layer deposition. Heating the particles opened up 1.4 nm pores in the shell, exposing about half of the platinum atoms on the surface beneath.

During the 450 °C ODHP reaction, platinum plucks hydrogen from propane to release propylene before indium oxide takes over to combine the hydrogen atoms with oxygen. This process converts about 40% of the available propane, giving a mix of products that is about 75% propylene and 25% CO2 with virtually no carbon. Overall, Notestein says, this system offers the best balance between conversion and selectivity for any ODHP catalyst. “To be honest, I did not expect this system to work nearly as well as it did,” he says.

The indium oxide shell stabilizes the platinum nanoparticles, which improves the catalyst’s longevity. Since the reaction operates at a constant temperature in a single vessel, Notestein says it could enable much simpler reactor designs than typical PDH systems. “From what I’ve seen, it looks very exciting,” Hermans says. Notestein adds that tandem catalysts might also offer a less energy-intensive route to produce ethylene.

The tiny laboratory system used in this study is a long way from a gigantic propylene plant, though, and atomic layer deposition is a laborious way to build a catalyst. “A major challenge with the scale-up process will be the production of the catalyst,” says Jinlong Gong of Tianjin University, who has developed PDH catalysts. Notestein hopes that more straightforward synthesis methods could be developed to create similar nanostructures.


Phosphonium salt boosts electrochemical Haber-Bosch reaction

An electrochemical Haber-Bosch process achieves record efficiency and longevity with the aid of a fast proton shuttle

A phosphonium cation helps transfer protons to the cathode of an electrochemical cell where
nitrogen is reduced to ammonia through a Li3N intermediate. The cation is regenerated at the anode.
R1, R2, and R3 are hexyl groups and R4 is a tridecyl group.

Aphosphonium salt speeds the delivery of protons and boosts the performance of an electrochemical Haber-Bosch process to record levels (Science, 2021 DOI: 10.1126/science.abg2371).

The Haber-Bosch process is one of the most important and energy-consuming chemical reactions in the world. If it could be done electrochemically, the process could be powered by renewable electricity instead of fossil fuels, burned to provide the high temperatures and pressures in most Haber-Bosch reactors. The electrochemical process directly reduces nitrogen to ammonia. It first activates dissolved nitrogen gas with lithium ions at the cathode, forming lithium nitride, an unstable, transient species. Then, protons produced at the anode replace lithium and convert lithium nitride to ammonia.

For decades, this process has been studied only in labs, suffering from low efficiency and slowness. Bryan H. R. Suryanto, Alexandr N. Simonov, Douglas R. MacFarlane, and their colleagues at Monash University identified one target for improvement: if protons could move faster from the anode to the cathode, ammonia production could speed up.

Typically, ethanol serves as a proton shuttle. But ethanol molecules diffuse slowly across the cell and get consumed in the process. Effectively, that means ammonia is synthesized from ethanol, which is not sustainable, Simonov said.

Instead, the team turned to phosphonium salts. Lithium nitride is a strong, proton-seeking base, and phosphonium cations are known to give up a proton from the phosphorus atom’s neighboring carbon atoms to form ylides, molecules with opposite charges on adjacent atoms, MacFarlane explained. If the ylide could in turn pick up protons from the anode, it would regenerate the phosphonium cation. The phosphonium cations thus had the potential to be a recyclable proton shuttle, driven by their charge to move quickly towards the negatively charged cathode.

The researchers tested their idea using trihexyltetradecylphosphonium salt in a simple electrochemical cell. Over a 20-hour experiment, the cell produced ammonia at a rate of 53 nmol per second per square centimeter of the electrode’s surface area with 69% faradaic efficiency, a measure of how efficiently electrons are converted to products in an electro-chemical reaction. In comparison, the previous high record was 30 nmol s–1 cm–2 with 35% faradaic efficiency, which was sustained for under an hour (Nat. Catal., 2020 DOI: 10.1038/s41929-020-0455-8).

These “much-improved” performances represent important steps forward, said Karthish Manthiram at the Massachusetts Institute of Technology, who led the 2020 study. “To get this kind of performance at this duration is really very special.” More importantly, the novel demonstration that phosphonium ions could be effective proton shuttles “opens up a lot of new space,” Manthiram said.

When the researchers extended the reaction time to 93 hours, the overall performance dropped closer to that from Manthiram’s study. Still, the fact the reaction lasted 93 hours was “remarkable,” Manthiram says.

One likely reason for the performance loss over time is buildup of ammonia gas in the system, explained MacFarlane. The researchers opted for a fixed-volume cell configuration for this proof-of-concept study. The researchers are now working to test the process on a larger scale using a flow setup, which would continuously remove ammonia and avoid this problem. Another “elephant-in-the-room” problem that plagues the field more broadly is the use of tetrahydrofuran as a solvent, said Suryanto. Tetrahydrofuran is electrochemically unstable and polymerizes over time, slowing diffusion during long-term experiments.


Reduction carves path to chiral compounds

Asymmetrical Zn is key to compounds with 4 different carbon groups

Many pharmaceuticals need to be a certain shape to fit into the binding pocket of the enzyme they target. Synthesizing molecules with the desired chirality requires controlled addition of functional groups, which is challenging. Now Pengwei Xu and Zhongxing Huang from the University of Hong Kong have made this process a little easier. They found a way to transform malonic esters into chiral compounds with four different carbon substituents (Nat. Chem. 2021, DOI: 10.1038/s41557-021-00715-0). The chemists created six classes of molecules containing a variety of functional groups, which wasn’t possible with prior methods.

Previously, researchers used pig liver esterases to transform malonic esters into chiral compounds. But these enzymes don’t work well when the esters have bulky side groups or multiple side groups similar in size, which limits the enzymes’ utility. The team used cheap starting materials to make a library of malonic esters of different sizes. The researchers used tetradentate prolinol ligands and a dinuclear diethylzinc catalyst to reduce one ester group to make over 70 compounds. These include amino and hydroxyl esters, alcohols, and diols, all in moderate to good yields and as high as 98% enantioselectivity. This catalyst works in the presence of multiple functional groups, including amines, olefins, and thioethers, Huang says.

The diethylzinc catalyst spontaneously ignites in air, but Huang says the team is researching alternatives. This tool will allow researchers to make a wider variety of complex natural products more easily and is a good companion to the existing enzyme method, Huang says.


Spontaneously arising electric fields affect reactions on catalyst particles in solution

Finding could provide handle for controlling reactions and sorting out mechanisms

As gaseous reagents react on Pt particles in liquids, electric fields arise spontaneously and affect the reaction rate.

Electric fields arise spontaneously at the surface of solid catalysts im-mersed in liquids, and this common yet largely overlooked phenomenon directly affects the rates of a large class of reactions. Using a meticulous set of techniques, chemists have been able to measure this elusive effect for the first time (ACS Cent. Sci. 2021, DOI: 10.1021/acscentsci.1c00293).

Researchers have known for years that charged moieties in enzymes can cause electric fields to arise at their active sites and that the fields drive biochemical reactions. A similar process should occur at the surface of catalytic particles in solution because of the charges carried by electrons and ions that come and go during reactions. But because there’s no simple way to wirelessly monitor electrical events occurring at particles submerged in solvents, the phenomenon has remained out of sight and out of mind.

To measure these tough-to-see electric fields and study their influence on heterogeneous catalytic reactions, Thejas S. Wesley, Yuriy Román-Leshkov, and Yogesh Surendranath of the Massachusetts Institute of Technology applied techniques from spectroscopy, electrochemistry, and other areas to a test reaction—solution phase ethylene hydrogenation using a platinum catalyst.

First, the team applied infrared spectroscopy to a reporter molecule to show that the extent of spontaneous polarization at the interface between Pt and water can readily be tuned by varying the pH, a measure of ion concentration. Polarization is caused by accumulation of charge (ions) at the interface and is directly related to electric fields that arise there.

Then the group ran the reaction in a cell containing Pt catalyst particles and a Pt electrode that tracked a voltage corresponding to pH-dependent changes in polarization. They found that the reaction rate changed systematically with changes in pH.

The results show that in aqueous solution, where proton transfer causes Pt particles to become polarized and spontaneously sets up electric fields, the fields directly influence reaction rates. The group found similar results when it repeated the experiment in aprotic organic solvent. In that case, the polarization and electric fields result from electron transfer.

Controlling electric fields could provide a handle for controlling catalytic reactions, and the work provides insights into the underlying mecha-nisms of heterogeneous catalysis, Surendranath says.

“This is very creative work” that will inspire follow-up research, says Jahan Dawlaty, a catalysis specialist at the University of Southern California. It makes sense that electric fields should arise at catalyst particles in solution, he says, but the effect “has never been studied before in a beautiful and systematic way.”

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