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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2018 год > № 87

№ 87

 

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  • Александр Степанович НОСКОВ – к 65-летнему юбилею
  • III Научно-технологический симпозиум
    «Нефтепереработка: катализаторы и гидропроцессы»
  • Инвестиционный проект Института катализа СО РАН
    «Центр коллективного пользования
    «Опытное производство катализаторов»
  • За рубежом
  • Приглашения на конференции



Александр Степанович НОСКОВ
к 65-летнему юбилею

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III Научно-технологический симпозиум
«Нефтепереработка: катализаторы и гидропроцессы»

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Инвестиционный проект Института катализа СО РАН
«Центр коллективного пользования
«Опытное производство катализаторов»

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Catalyst drives carbon-coupling chemistry without making CO2

Highly pure form of iron carbide may improve efficiency and lower cost of Fischer-Tropsch process

A simple synthesis method converts iron oxide to a highly pure form of iron carbide, yielding a catalyst for industrial carbon-coupling reactions that produces exceptionally low quantities of CO2, according to a study (Sci. Adv. 2018,  DOI: 10.1126/sciadv.aau2947). The finding may improve efficiency and may also reduce the energy consumption and cost of operating Fischer-Tropsch (FT) reactors, which convert synthesis gas, or syngas (CO + H2), to liquid hydrocarbons such as diesel fuel. Iron- and cobalt-based catalysts both drive FT chemistry. Iron catalysts are less expensive, but they convert up to 30% of the CO in syngas to unwanted CO2, which lowers process efficiency and is costly to separate. So a team led by Emiel J. M. Hensen of Eindhoven University of Technology examined conventional iron FT catalysts and found that in addition to containing iron carbide, the desirable component, they also contain metallic iron and iron oxides, the CO2-forming culprits. The team evaluated preparation methods and found they could make highly pure iron carbide from low-cost iron oxide by fully reducing the starting material in hydrogen and then treating it with nitrogen-diluted syngas. Under industrial conditions, the new catalysts generated as little as 5% CO2 and remained stable for more than 150 hours.

 

Attached to MOFs, frustrated Lewis pair catalysts become recyclable

Tunable pairing opens new avenues for heterogeneous catalysis

Frustrated Lewis pairs (FLPs) stepped into the synthetic limelight just over a decade ago. Composed of a Lewis acid and a Lewis base whose blocked inclination to bond grants the molecules great powers, the main-group molecules were hailed for catalyzing hydrogenation reactions previously accomplished only with transition metals. The use of FLPs has been limited, however, by their lack of recyclability and their sensitivity to air and moisture.

Now, researchers led by Shengqian Ma at the University of South Florida have developed stable FLPs. Anchored inside a metal-organic framework (MOF), they efficiently catalyze hydrogenation and imine reduction reactions (Chem 2018,  DOI: 10.1016/j.chempr.2018.08.018). With simple filtration, the catalyst can be recycled in the latter reaction at least seven times without loss of activity.

To construct the MOF-FLP pair, the team docked an amine Lewis base at the MOF’s open metal sites and then attached a boron Lewis acid (shown). The team then compared the heterogeneous MOF-FLP’s ability to catalyze reactions with that of a homogeneous catalyst. While the researchers observed comparable yields for the hydrogenation reaction, Ma says the MOF-FLP showed “interesting” steric and size selectivity in the imine reduction. The MOF-FLP matched the homogeneous catalyst’s high yield unless the substrate contained a buried imine, suggesting that the MOF environment restricts access to certain substrates. Substrates beyond a certain size were not reduced, likely because they can’t squeeze through the MOF’s pores, the authors say.


Catalysts that combined metal-organic frameworks and Lewis pairs were recycled
seven times or imine reduction reactions such as this one, without loss of activity.

 

“The beauty of MOFs is that they’re very tunable,” Ma says. Future MOF designs could easily introduce chirality or  superhydrophobicity to expand the FLP catalysts’ reaction scope and durability, he says.

The University of Toronto’s Douglas W. Stephan, whose group pioneered the use of FLPs, calls the work a “remarkably clever and facile strategy.” The readily handled MOF-LPs avoid the air and moisture sensitivity of most homogeneous FLP catalysts, and their recyclability could also lower costs, Stephan says. The study, he adds, “foreshadows a vast potential for an array of new stable, recyclable, and selective FLP catalysts embedded in MOFs.”

 

Catalytic teamwork transforms alkenes selectively

Photocatalyst pairs with ene-reductase enzyme to obtain one enantiomer in reductions of mixtures

 

 

Double-bond-forming reactions commonly yield a mixture of alkene isomers with different geometric arrangements of functional groups. Subsequent transformations, such as reducing the double bond to a single bond, lead to mixtures that can be difficult to separate. John F. Hartwig of the University of California, Berkeley;  Huimin Zhao of the University of Illinois, Urbana-Champaign; and their colleagues think strategic pairing can solve that problem and others. Catalyst pairs—both in biology and in industrial contexts—can carry out higher-yielding and more-selective reactions than can the individual catalysts in sequence. The team paired enzymes called ene-reductases with photocatalysts, which tend to be biocompatible because they often generate catalyst intermediates that are stable in water. The photocatalyst interconverts the two alkene isomers, and the awaiting enzyme selectively reduces one of the two isomers to a chiral product (Nature 2018, DOI: 10.1038/s41586-018-0413-7). The combination process also reduces single alkene isomers that would not react with the enzyme on its own (example shown). The team has filed a provisional patent application on the technology. A commentary accompanying the report notes that so far, the reaction requires substrates with aromatic groups, and the substrate concentrations are lower than industrial processes require.

 

Single atoms catalyze Suzuki reactions

Flow chemistry process takes place on carbon nitride surface

In this artist's rendering, a palladium atom is anchored on an exfoliated graphitic carbon nitride surface.

As effective as the palladium-catalyzed Suzuki reaction is for forming carbon-carbon bonds, researchers see room for improvement. Homogeneous palladium catalysts can be tough to separate from products, and fixing the problem by attaching catalysts to solid supports can lead to problematic metal leaching. Meanwhile, heterogeneous palladium nanoparticle catalysts on solid supports rarely catalyze the reaction efficiently. Now, Javier Pérez-Ramírez of ETH Zurich and colleagues suggest another route to Suzuki chemistry: single-atom catalysis (Nat. Nanotechnol. 2018, DOI: 10.1038/s41565-018-0167-2). Multiple labs have demonstrated catalysis of over 10 reactions by isolated atoms supported on a solid. This team anchored palladium atoms to nanometers-thick sheets of exfoliated graphitic carbon nitride, which contain metal-stabilizing nitrogen-rich sites. The catalyst performed a Suzuki reaction in flow for 13 hours with negligible metal leaching. Calculations from coauthor Núria López of the Institute of Chemical Research of Catalonia suggest that the palladium atoms continuously adapt their coordination within the same site on the carbon nitride as the reaction progresses, minimizing energy at each step. Experimentally verifying that result will be challenging, the authors say, but improvements to catalyst-monitoring spectroscopy techniques could help. The authors plan to test other reactions and metals with their system as well as launch a company to market the catalysts.

 

Nucleotide construction gets new chiral tool

Reagent developed through Scripps and BMS collaboration couples nucleosides with potential applications in antisense therapeutics

 


A new phosphorus(V) reagent developed by researchers at BMS and Scripps couples nucleosides diastereoselectively.

Antisense drugs are on the rise. The U.S. FDA has approved two antisense therapies to treat genetic diseases in the past two years with more in the pipeline. These compounds often consist of nucleoside chains connected by phosphorothioates—chiral linkages that make the agents more stable in the body yet exponentially increase their complexity. For example, the 18-mer Spinraza, which is a spinal muscular atrophy drug from Ionis and Biogen that costs $125,000 per dose, incorporates these linkages and is delivered as a mixture of more than 130,000 isomers theoretically.

Controlling the stereochemistry at phosphorus, though, could increase a drug candidate’s potency by reducing the number of less active isomers in these mixtures. To gain that control, chemists have mostly turned to phosphorus(III)-based chemistry. Now, scientists at Bristol-Myers Squibb and Scripps Research Institute California report a novel P(V)-based reagent that couples nucleosides with high diastereoselectivity (Science 2018, DOI: 10.1126/science.aau3369).

P(V) reagents are generally less reactive than P(III) but they let chemists skip cumbersome protection and deprotection steps required by P(III) chemistry, the authors say. Building on work by Wojciech J. Stec and colleagues, the team synthesized a P(V) reagent in one step from limonene oxide, which served as the chiral scaffold. The researchers point out that P(III)-based reagents are air and moisture sensitive and require specialized equipment to handle, while the new reagent is bench-stable, which they say could improve medicinal chemists ability to rapidly synthesize these molecules and discover new medicines.

The collaborators used their reagent to make a variety of dinucleotides in 61–97% yield. They also synthesized several oligonucleotides and cyclic dinucleotides, a class of compounds that has attracted attention as  possible anticancer agents. The P(V) reagent produced a single diastereomer of a cyclic dinucleotide in five steps while the traditional P(III) approach produced four diastereomers in nine steps.

“It is exciting to see new developments in this challenging and important area of nucleotide chemistry,” says Jonathan Hall at ETH Zurich. “If it can be extended to pharmaceutically relevant oligonucleotide derivatives, it will represent a game-changer for oligonucleotide therapeutics.”

Chandra Vargeese, head of drug discovery at Wave Life Sciences, which develops stereopure nucleotides using a P(III)-based platform with industrial partners such as Takeda and Pfizer, says the approach is elegant and offers excellent diastereoselectivities on par with P(III) chemistry. The stepwise coupling yields, however, are on the low side compared to Wave’s platform and would need to be optimized for commercial production, she says. Another challenge for the P(V) reagent, Vargeese says, is that it doesn’t offer the same level of access as P(III) reagents do to structures with mixed phophorothioate and phosphodiester backbones that are important to the field.

The BMS-Scripps team says the P(V) reagent will soon be available for purchase through MilliporeSigma.

Chemical Engineering News


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