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№ 76

 

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  • XII Европейский конгресс по катализу (EuropaCat XII)
    “Катализ: сбалансированное использование ископаемых
    и возобновляемых ресурсов”
  • За рубежом
  • Импакт-факторы журналов по катализу и физической химии за 2014 год
  • Приглашения на конференции



XII Европейский конгресс по катализу (EuropaCat XII)

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За рубежом

How Researchers Formed A Pure, Chiral Crystal From Mixed Ingredients

Stereochemistry: Team grows homochiral crystals despite expectations

Scientists have long thought that chiral molecules could only form pure, orderly chiral crystals if those molecules all have the same chirality. Virgil Percec of the University of Pennsylvania and coworkers have now defied that long-established notion about the need for homochiral building blocks by growing high-purity supramolecular crystals from racemic and diastereomeric mixtures of perylene bisimides (PBIs) (Nat. Chem. 2015, DOI: 10.1038/nchem.2397).

Racemic and diastereomeric mixtures of PBIs (left, with chiral center in each R group) self-assemble into columnar supramolecular helices (right), which interlock with others by a cogwheel-like mechanism to form homochiral crystals.

 

In a few past studies, research groups had shown that enantiomerically mixed polymers or supramolecular assemblies could separate into or form enantioenriched domains or crystals, but the products were not chirally pure or highly ordered. The new crystals, on the other hand, form when mixtures of PBIs stack on top of one another, self-assembling into chiral supramolecular helices. These columnar structures then combine to form pure homochiral crystals.

“This is a very significant discovery challenging the accepted view that homochirality is an essential prerequisite for self-assembly to form high-quality homochiral supramolecular crystals,” says Boris Rybtchinski, an expert on organic self-assembly at the Weizmann Institute of Science. The findings suggest that advanced homochiral materials for organic light-emitting diodes, spintronics, and other applications “could be achieved in a cost-effective way, starting from readily available nonhomochiral mixtures instead of very expensive, pure enantiomers.”

Percec and his team propose that their PBIs form homochiral crystals because the columnar helices that initially form by self-assembly have alkyl chains around their outsides that interlock in a cogwheel-like manner with chains on neighboring helices, which go on to interlock with other helices. Each PBI has six stereocenters, which end up facing inward toward the central axes of the columnar helices. Those stereocenters therefore don’t interfere with the interlocking process, making their configurations inconsequential to crystal formation.

Although the researchers observed the phenomenon on just one series of PBIs, the proposed mechanism suggests that it might be possible to extend the concept to other nonhomochiral building blocks with similar molecular features, says Ricardo Riguera of the University of Santiago de Compostela, whose interests include controlled chirality.

Supramolecular systems specialist E.W. (Bert) Meijer of Eindhoven University of Technology calls the study “remarkable.” It shows “that mixtures of molecules with even six stereocenters and all possible chiralities always give one type of supramolecular product,” he says. “This is unexpected for many in the field. The results give new insights into the amplification of chirality in large supramolecular structures and could be important for understanding the origins of homochirality in nature.”

 

 

CO2 Levels Approaching 400 PPM, Global Scientific Group Reports

Climate Change: Pledges for new treaty aren’t enough to restrain temperature rise to 2 °C

Concentrations of greenhouse gases in the atmosphere continue to break records, with global average carbon dioxide concentrations reaching 397.7 parts per million in 2014, the World Meteorological Organization (WMO) says.

The annual average carbon dioxide concentration worldwide is approaching 400 parts per million. SOURCE: World Meteorological Organization


“We will soon be living with globally averaged CO2 levels above 400 ppm as a permanent reality,” says WMO Secretary-General Michel Jarraud.

Two other key greenhouse gases also set records in 2014. Methane levels were 1,833 parts per billion and concentrations of nitrous oxide reached 327 ppb, says WMO, the United Nations agency considered the world’s scientific authority on Earth’s atmosphere.

Radiative forcing—a warming effect—from greenhouse gases increased 36% between 1990 and 2014, WMO says. The organization points out that warming caused by climbing CO2 concentrations has led to an increase in the level of water vapor, which is also a greenhouse gas. Warmer air holds more moisture, WMO explains.

“We have to act now to slash greenhouse gas emissions if we are to have a chance to keep the increase in temperatures to manageable levels,” Jarraud warns.

Global diplomatic talks to cut those emissions are under way and are expected to produce a new climate change treaty in Paris next month. Thus far, 160 countries have individually pledged to carry out specific actions to control emissions.

But a scientific assessment of those pledges, conducted by the United Nations Environment Programme (UNEP), says they fall short of what’s needed to hold the global temperature rise to 2 °C by 2100 when compared with preindustrial levels.

Annual global greenhouse gas emissions need to be equivalent to 42 billion metric tons of CO2 in 2030 for at least a 66% chance of meeting that policy goal, UNEP says. Full implementation of all pledges made thus far would lead to emissions equivalent to 52 to 57 billion metric tons of CO2 in 2030, the agency estimates. This level would put the world on track for a temperature rise of about 3 °C by the end of the century, UNEP adds.

 

 

This Liquid Has Holes In It, Thanks To Chemistry

Materials Science: Liquid made with cage compounds has permanent pores and could one day help with gas separations

To generate a holey liquid (bottle), researchers dissolved a porous core structure (space-filling) with crown-ether decorations (ball and stick) in a crown-ether solvent.

Stuart James of Queen’s University Belfast and coworkers have devised a liquid with permanent porosity (Nature 2015, DOI: 10.1038/nature16072). Such holey fluids could be useful in gas separation, process chemistry, and other applications, if they can be made economically. Their persistent porosity comes from hollow organic cage molecules coated with solvent-soluble surface groups. The cage openings are too small to be clogged by the surface groups or by large solvent molecules in the surrounding solvent.

Liquids contain spaces between their molecules, but they are tiny. Bubbles can be blown into liquids, but they will quickly float to the surface and dissipate—air bubbles in glass being a rare exception.

On the other hand, solids such as zeolites and metal-organic frameworks have permanent pores. They can be used to separate molecules by size and can host added catalysts to drive chemical reactions. But unlike liquids, these materials can’t flow through channels or be smoothed onto surfaces.

James got the idea for holey liquids a decade ago, when a colleague, Queen’s University chemical engineer David W. Rooney, wondered whether mixtures of porous solids and liquids might be pumped through pipes and used in continuous processes more easily than is generally possible with porous solids alone.

Porous solids expert Russell Morris of the University of St. Andrews comments that the concept of liquids with permanent porosity has actually been around for decades, but “the practicalities of designing such a system take skill and perseverance. This work delivers real liquids that show that property, which is a great achievement.”

The researchers initially achieved liquid porosity by dissolving a hollow organic core structure coated with crown-ether surface groups in a crown-ether solvent. To optimize gas absorption capacity, the team packed the fluid with as many cage molecules as possible—one for every 12 solvent molecules. When exposed to methane, the liquid absorbs eight times as much gas as the solvent alone.

But the crown ethers were hard to synthesize and the liquid thick and slow-flowing. So collaborators Andrew I. Cooper and Rebecca L. Greenaway at the University of Liverpool developed another porous liquid by coating a hollow organic cage with a mixture of diamines and dissolving it in the solvent hexachloropropene. The resulting porous liquid flows ten times as readily as the crown-ether-based material. The researchers synthesize the coated cages in a single step, and the solvent is available commercially.

In earlier work, Shannon M. Mahurin and Sheng Dai at Oak Ridge National Laboratory and coworkers created hollow colloidal silica nanoparticles with a fluidlike polymer surface coating (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201409420), but the new porous liquids are easier to molecularly modify. Dai comments that “the liquids will open up new frontiers in how we think about porosity.”

The surface area and gas uptake of the new materials are too modest for them to compete with porous solids immediately, says cage compound specialist Michael Mastalerz of Germnay’s Heidelberg University in a Nature commentary. “They should instead be seen as a prototype of a new class of material [that] will undoubtedly find technological applications” if these properties can be improved.

 

 

Tiny Boxes Hold Catalytic Surprise

Materials: Cages made from nanoparticles could challenge dominance of ELISA enzyme

Placing copper hydroxide nanoparticles in a copper-ammonia solution causes the particles to transform into nanoribbons, which become the bars of a cage with an empty center.

Researchers have been exploring ways to build tiny containers out of nanoparticles; such superstructures can help deliver drugs to specific tissues, hold molecules that act as biological sensors or catalyze reactions, and make it easier for microscopes or MRI machines to image tiny targets. Now, one group has come up with an easy way to build nanoparticle cages, and to their surprise, the boxes are much more efficient catalysts than an enzyme commonly used in ELISA, the popular immunoassay for detecting many types of biomolecules (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b09337).

Weihong Tan, a chemist at the University of Florida and Hunan University, in China, and his group were working to create large-surface-area nanostructures that could hold and deliver high doses of drugs. Existing processes for making such superstructures are time consuming and complex, and the researchers were hoping their approach would be simpler.

The team started by making copper hydroxide nanoparticles, which they dispersed in water with polyvinylpyrrolidone. Next, they added a copper-ammonia complex to the solution, triggering a series of reactions that led to migration of the copper hydroxide into one-dimensional “nanoribbons” that formed evenly spaced bars surrounding the nanoparticles. Eventually the 200- to 250-nm particles completely disappeared, leaving behind a self-assembled three-dimensional cage of the same size with a hollow center. “The formation is very regular, and it gives you a uniform structure,” Tan says. He does not yet have an explanation for the mechanism leading to that structure.

Transmission electron micrographs show cages formed from spontaneous reshaping of copper hydroxide nanoparticles.

The group tested the material’s catalytic activity and found that it had high activity for the colorimetric reaction typically catalyzed by the horseradish peroxidase enzyme. This reaction is central to many biological assays, especially the widely used ELISA, or enzyme-linked immunosorbent assay, used to detect the presence of antigens in samples. In the presence of the group’s cages, a solution containing the reagents tetramethylbenzidine and hydrogen peroxide changed from clear to deep blue within five minutes at room temperature. The same reaction with horseradish peroxidase took longer, even though the concentration of the enzyme was 2,000 times that of the copper hydroxide cages. Tan says he needs to do more work to figure out how the system works. “I don’t really know why we have all this catalytic activity,” he says.

Jianfang Wang, a professor of physics who studies nanoparticles at the Chinese University of Hong Kong, is impressed with the work. “I am very surprised by the fact that such complex nanoarchitectures can be synthesized by such a simple wet-chemistry method in aqueous solutions,” he says. “This is probably the most complex structure that I have ever seen among those made by wet-chemistry methods.”

He would like to know more about the mechanism that causes the cages to form, as well what causes the high catalytic activity. He’s also interested in learning how selective the material is. “Selectivity is probably more important than activity for enzymes,” Wang says.

Tan says his next step is to see whether he can make a complete assay that might be able to replace ELISA.

Chemical & Engineering News

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