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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2016 год > № 77




  • Саламбек Наибович Хаджиев
    К 75-летию со дня рождения
  • Олег Герольдович Синяшин
    К 60-летию со дня рождения
    Отчет о научно-организационной деятельности в 2015 году
  • 3-я международная конференция
    «Катализ для переработки возобновляемого сырья:
    топливо, энергия, химические продукты» (CRS-3)
  • За рубежом
  • Приглашения на конференции

Саламбек Наибович Хаджиев
К 75-летию со дня рождения

Переход к элементу


Олег Герольдович Синяшин
К 60-летию со дня рождения

Переход к элементу


Отчет о научно-организационной деятельности в 2015 году

Переход к элементу


"3-я Международная конференция
Катализ для переработки возобновляемого сырья:
энергия, химические продукты" (CRS-3)

Переход к элементу


За рубежом

Переход к разделу

ACS Meeting News: Chemists Unveil Truly Sustainable Polymers

When it comes to making an environmentally friendly polymer, chemists tend to think that starting with a renewably sourced monomer is all it takes. But according to Eugene Y.-X. Chen of Colorado State University, a biobased monomer is only one of three criteria needed to build such a material: A green synthesis and product recyclability are important too.

To emphasize that point, Chen and postdoctoral researcher Miao Hong have developed a new synthetic process to create what they believe is a truly sustainable polymer. They polymerize a biobased monomer using a metal-free organocatalyst to make a fully recyclable material. Chen and Hong described their approach in a pair of presentations during a Division of Polymer Chemistry session Sunday at the American Chemical Society national meeting in San Diego.

Chen and Hong created what they call truly sustainable polymers by using a biobased monomer in an organocatalyst polymerization process to make recyclable polymers.

The Colorado State researchers started out by designing the first practical ring-opening polymerization of γ-butyrolactone, a sugar-derived compound that is used as a solvent and building block in fine and specialty chemical applications. Although ring-opening polymerization of cyclic lactones is a common way to make biodegradable polyesters, γ-butyrolactone is an exception. Chemists had labeled the compound nonpolymerizable, Chen noted, because the ring is too stable except under extreme pressure and moderate temperature.

Nevertheless, after probing an array of thermodynamic, kinetic, and processing conditions, Chen and Hong discovered lanthanum an d yttrium catalysts that polymerize γ-butyrolactone via a coordination-insertion mechanism. The team figured out how to control the reaction equilibrium and vary the amount of catalyst to produce high-molecular-weight linear and cyclic polyesters. They also discovered that heating the bulk materials above 200°C completely recycles the polymers back to the γ-butyrolactone monomer (Nat. Chem. 2015, DOI: 10.1038/nchem.2391).

But that still wasnt good enough, Chen said. The team took the polymerization to the next level by discovering an organocatalyst to sidestep toxicity and availability issues associated with metal catalysts. The researchers again probed an array of reaction conditions to zero in on a polyaminophosphazine catalyst with alcohol initiators. The phosphazine is a superstrong base capable of deprotonating the γ-butyrolactone ring to generate the active species that propagates the polymerization (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201601092).

This is a real tour-de-force study that combines mechanistic understanding, sustainability, and practical importance, commented Cornell Universitys Geoffrey W. Coates, whose group develops catalysts for preparing biodegradable polymers. These new phosphazene initiators, coupled with polymer recycling, constitute an exciting advance in the development of new biodegradable polyesters.

Chen and Hong convincingly demonstrate that what was thought to be an impossible hurdle can be overcome by a smart selection of appropriate catalysts and reaction conditions, added Jean-Franois Carpentier, a polymerization catalysis expert at the University of Rennes 1. Although the catalytic rate and other improvements are needed to make the process cost effective, and the health and safety of the organocatalyst must be considered, the achievement will surely launch new interest in this field, Carpentier noted. The ability to start with biomass-derived cyclic lactones creates a nice playground for future investigations and developments.


Cheap catalyst converts tough plant lignin into valuable chemicals

Waste material from the energy crop elephant grass yields flavor and fragrance compounds

Fast-growing grasses and trees such as switchgrass and poplar offer a sustainable alternative to petroleum as a source of fuels and chemicals. But to make fuels from plant sugars in cellulose and hemicellulose, producers generally have to remove the surrounding lignin, a hardy phenolic polymer that helps support the plants structure. Instead of wasting this abundant resource, scientists have been seeking ways to make useful chemicals from it. Now a new study shows that an inexpensive nickel-based catalyst can transform 68% of lignin from the grass Miscanthus, or elephant grass, into valuable aromatic compounds, while preserving plant sugars for subsequent conversion to fuel (ACS Sustainable Chem. Eng.  2016, DOI: 10.1021/acssuschemeng.5b01776).

Converting lignin to useful chemicals is challenging because the polymer is highly heterogeneous and recalcitrant to degradation. Recently, however, scientists have found that some precious metals can catalyze the breakdown of lignin into valuable aromatic monomers used in flavor and fragrance chemistry. Mahdi M. Abu-Omar of Purdue University and his colleagues used a palladium-zinc catalyst to convert 50% of lignin in poplar into two phenolic products (Green Chem. 2015, DOI: 10.1039/C4GC01911C). In 2014, Abu-Omar founded the company  Spero Energy to develop the groups technology. But to make the process less expensive and more environmentally friendly, they sought a cheaper, more earth-abundant catalyst. Nickel, just above palladium on the periodic table, was the obvious choice.

In the current study, the researchers adapted their previous method to use a nickel catalyst supported with activated carbon. In a reactor, they combined 1 g of milled Miscanthus with 45 ml methanol, and introduced the catalyst in a microporous stainless steel cage to prevent solid reaction products from recombining with the catalyst. Under high pressure, they heated the reactor to 225°C, which breaks the lignin polymer into shorter, soluble units that contact the catalyst through the cage. The nickel catalyst then helps deoxygenate these oligomers into aromatic products.

Using this method, the team turned 68% of the lignin from Miscanthus into phenolic products, including dihydroeugenol, propylsyringol, and ferulic acid methyl esters, which can be used to make vanillin. The reaction also left behind a solid, carbohydrate residue, the researchers found. With an iron chloride catalyst, the team then converted this to furfural and levulinic acid with up to 86% yield. These products can be used to make biofuels or other chemicals.

R. Tom Baker, a catalysis researcher at the University of Ottawa, calls the study a landmark paper, noting that a simple two-step process converts more than half the mass of the grass feedstock into value-added chemicals. Although the biobased chemicals industry is currently facing major economic challenges from unreasonably inexpensive petroleum, he says, this exciting work shows that technical advances towards viable biorefineries are well underway.


Nanoscale system reaches perfect efficiency for solar fuel production step

Energy: Nanoparticle-based photocatalyst system evolves hydrogen gas from water with 100% efficiency

A solar-powered nanoscale system can evolve hydrogen from water with perfect efficiency. A cadmium sulfide quantum rod (yellow) absorbs photons, activating electrons (negative charges) that are transferred to a platinum catalyst (purple) to produce hydrogen gas (small purple spheres), and leaving behind holes (positive charges) that are localized to an embedded quantum dot (green).

A major goal in renewable energy research is to harvest the energy of the sun to convert water into hydrogen gas, a storable fuel. Now, with a nanoparticle-based system, researchers have set a record for part of the process, reporting 100% efficiency for the half-reaction that evolves hydrogen (Nano Lett. 2016, DOI: 10.1021/acs.nanolett.5b04813).

To make such water-splitting systems, researchers must find the right materials to absorb light and catalyze the splitting of water into hydrogen and oxygen. The two half-reactions in this processthe reduction of protons to hydrogen, and the oxidation of water to oxygenmust be isolated from each other so their products dont react and explode. Completing the cycle in an efficient, stable, safe fashion with earth-abundant elements is an ongoing challenge, says chemist Nathan S. Lewis of Caltech, who was not involved in this study.

Until recently, the efficiency of the reduction step had maxed out at 60%. One challenge is that electrons and positive charges formed in the light absorption process can rapidly recombine, reducing the number of electrons available for the hydrogen production reaction. To overcome this problem, several years ago, Lilac Amirav of TechnionIsrael Institute of Technology and her colleagues designed a nanoparticle-based system (J. Phys. Chem. Lett. 2010, DOI: 10.1021/jz100075c) that would physically separate the charges formed during photocatalysis.

The teams system comprises a light-harvesting cadmium sulfide quantum rod with a quantum dot made of cadmium selenide embedded near one end. A platinum tip on the opposite end catalyzes hydrogen production. When placed in water in a gas-tight reaction cell and exposed to visible light, the quantum rod absorbs photons, releasing electrons. The rod then transfers electrons to the platinum tip, reducing protons to form hydrogen, and leaving behind positively charged vacancies called holes at the cadmium selenide dot.

When the team initially tested the system in water, however, it had only 20% efficiency for hydrogen production. To improve performance, the researchers tested a variety of reaction conditions. In the new study, they achieved success by increasing the pH and adding isopropyl alcohol. Under these conditions, the holes oxidize hydroxide anions, which are more abundant at the high pH, to yield hydroxyl radicals. The alcohol then donates electrons to the radicals, preventing charge recombination and keeping more electrons available to evolve hydrogen.

When the team measured hydrogen production with this updated setup, they achieved up to 100% efficiency. The results shatter the previous benchmarks for all systems, Amirav says.

Lewis says the teams achievement is a step toward a stable, efficient system for solar fuels production. He notes, however, that this work addresses only one of the necessary half-reactions.

Adapting the work to a full water splitting system may be challenging, says Amirav, because cadmium sulfide corrodes under long exposure to light. However, she has recently shown that adding another catalyst, such as iridium oxide or ruthenium, could improve its photochemical stability (J. Mater. Chem. A 2015, DOI: 10.1039/C4TA06164K; Angew. Chem., Int. Ed. 2015, DOI: 10.1002/anie.201411461).


Fluorescence Method Maps Reactivity Hot Spots On A Catalysts Surface

Imaging: Technique could help improve catalyst performance and make reactions such as water splitting viable


By using a fluorescence microscopy method with nanometer resolution, researchers pinpointed catalytic hot spots on a TiO2 nanorod (white outline) where positive-charge-mediated reactions (left) and electron-mediated reactions (center) occur. Red regions are the most active. Knowing the hot-spot locations, they then selectively modified those areas with a cocatalyst (bumps in SEM image, right).

Solid chunks of matter catalyze most of todays industrial-scale chemical processes. If scientists knew exactly where reactions occur on these catalytic solids, they could customize them to improve the performance of catalysts already in use and help bring new ones to market.

By devising a fluorescence microscopy technique with single-molecule resolution, researchers at Cornell University have created a method for pinpointing the sites of catalytic reactions with nanoscale resolution. The scientists used the method to generate surface maps of microscopic catalytic hot spots and then boosted the activity of those spots by decorating them with a cocatalyst (Nature 2016, DOI: 10.1038/nature16534).

The team, which was led by Cornells Justin B. Sambur and Peng Chen, applied the method to TiO2 nanorods used in light-induced water-splitting reactions. Solar-driven water splitting has been studied intensely for decades because it could provide a nearly limitless supply of clean-burning hydrogen if a suitable catalyst can be found.

When light shines on a TiO2 anode in a photoelectrochemical cell, short-lived excited electrons and positive charges form on the catalysts surface. The positive charges, called holes, can oxidize water, evolving O2 and forming hydrogen ions (H+). Then the electrons can reduce the ions to form H2. Often, however, electrons and holes quickly recombine, quenching the excitation before the catalytic reaction can occur.

Depositing a cocatalyst on the anode could help enhance the oxygen evolution step of water splitting. But knowing exactly where to put this oxygen evolution catalyst (OEC) is a challenge: Putting it in the wrong place can hinder light absorption, weakening the anodes performance.

To find the answer, the Cornell team scanned nearly 40 TiO2 nanorods for catalytic hot spots by using two types of probe reactionsan electron-induced reduction and a hole-induced oxidation. Both reactions convert a nonfluorescent organic molecule thats been added to the water-splitting cell to a fluorescent one. The molecule lights up immediately, identifying where on the nanorod it was converted. From those measurements, the team made catalytic activity maps and used them along with the microscopy method to selectively deposit tiny quantities of a cobalt borate OEC on the nanorod hot spots. This deposition improved the nanorods water-splitting abilities.

An imaging technique that uses cleverly chosen fluorescent reporter molecules to examine the fate of photogenerated electrons and holes in a semiconductor at the nanometer scale is a breakthrough, says Bruce A. Parkinson, a photocatalysis specialist at the University of Wyoming.

Not only did the Cornell scientists devise a sensitive mapping method, they made several unexpected observations. For example, most of the nanorods contain just a couple of reactive hot spots and large, relatively unreactive areas, even though the composition and structure of the rods are fairly uniform. Oxidations and reductions also tend to occur in nearly the same spots. And even though adding a cocatalyst to a hot spot makes it hotter as expected, adding the OEC to a cold spot on the TiO2 surface causes an improvement in catalytic activity thats greater than the improvement at the hot spot.

Many of the insights obtained in this study are somewhat counterintuitive, Wyomings Parkinson says. Researchers working in this field will have to take them into account in future studies of photodriven catalytic reactions, he adds.

Chemical & Engineering News

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