Golden cat

A more environmentally friendly way to make ethylene (a primary feedstock for the chemical industry, which also goes by the name of ethene) would use natural gas as the raw material rather than cracking crude oil. Now, a golden opportunity in the form of a two-centred gold complex has come to light.

Ethylene is one of the chemical industry’s primary feedstock materials from which a whole range of different compounds are made, among them the coolant and antifreeze compound ethylene glycol and plastics such as polyethylene. Ethylene is currently produced from oil by steam cracking. However, direct conversion of natural gas, methane, to ethylene could also be viable on an industrial scale if a suitable catalyst and reaction setup were developed. One of the big advantages of such an approach is that methane supplies are likely to provide us with raw material long after oil has dwindled.

Scientists working with Thorsten Bernhardt at the University of Ulm, Germany and Uzi Landman at the Georgia Institute of Technology, in Atlanta, Georgia, USA, explain that the problem with the conversion of methane to ethylene is one of bond strength. The bonds between carbon and hydrogen atoms in the methane molecule, are very strong and so difficult to break. To compel the carbon in methane to form bonds with other carbon atoms requires a lot of energy. Additionally, any efforts to do so, usually produce lots of side-products containing more than the requisite two carbon atoms found in ethylene.

Activating methane to allow it to form bigger molecules, such as ethylene, is a complex process. The team explains that in order to find a suitable catalyst a clearer understanding of the process at the molecular level is needed. As such Bernhardt and colleagues investigated different catalytic metal clusters, aggregates of a few metal atoms, as model systems for the process. They found that particles containing two charged gold atoms (ions) in pairs could convert methane to ethylene at low temperatures and pressures in the gas phase without forming significant quantities of side-products.

methane to ethane using a golden cat (Credit: Bernhardt et al/Wiley-VCH)
methane to ethane using a golden cat (Credit: Bernhardt et al/Wiley-VCH)

In order to understand how this process works, the team “trapped” the reaction intermediates and used computational techniques to model the energy levels of starting materials, catalyst, intermediates, and products. They found that each gold ion in the dimer binds to a single methane molecule, this severs hydrogen from the methane and the two carbon atoms can then form a single bond to each other. This is a precursor to ethylene itself which has a strong double bond between its carbon atoms. The precursor initially remains bound to one of the gold ions, while the second gold ion, suddenly finds itself able to bond to a new methane molecule. In the final step, this second methane molecule nudges away the ethylene precursor, the carbon single bond, which can then form a carbon double bond. The newly trapped methane then pairs up with yet another methane and the cycle begins again.

“Both the activation of the carbon-hydrogen bonds of the methane and the subsequent splitting off of the ethylene molecule require cooperative action of several atoms bound to the gold dimer,” Berhnardt explains. “Our insights are not only of fundamental interest, but may also be of practical use.”

LINKS

Angew Chem Int Edn 2010, 49, 980-983
http://dx.doi.org/10.1002/anie.200905643

Bernhardt
http://www.uni-ulm.de/iok/bernhardt/

Non-carbon nanotubes

Carbon nanotubes rose to prominence on the back of the buckyball chemistry revolution in the 1990s and are now emerging from prototype applications across academic and industrial laboratories. They have potential in microelectronic circuits, novel sensor devices, special light conductors, and light-emitting nanotubes for display technology.

Indeed, applications as diverse as medical technology, for fibres with ultrahigh tensile strength, in hydrogen storage, for rechargeable batteries, in catalysis, and in nanotechnology are being developed. There are even applications for antifouling coatings for ships.

With this in mind, chemists in Germany who work with inorganic materials have now developed an approach to synthesising tin sulfide nanotubes which could expand the nanotube concept much further still and open up yet more avenues for applications. After all, carbon does not have a monopoly on nanotubes. Early in the development of tubular fullerene structures, inorganic chemists opted to make their nanotubes from metals and non-carbon atoms: tungsten sulfide, nickel chloride, vanadium sulfide, titanium sulfide, and indium sulfide. Many others have been produced.

Wolfgang Tremel and colleagues, Aswani Yella, Martin Panthoefer, Helen Annal Therese, Enrico Mugnaioli, Ute Kolb, of the Johannes Gutenberg-Universitaet in Mainz, Germany, have now developed a new process for the production of tin sulfide nanotubes, which they report in the Wiley journal Angewandte Chemie, the researchers found they could “grow” tin sulfide nanotubes from a drop of metal using a bismuth catalyst.

The team were faced with one of the fundamental problems of synthesising sulfidic nanotubes in that they require a high temperature to force the planar layers of material to bend and fuse into tubular structures. For tin sulfide, the situation is complicated still further by an unstable intermediate that is almost impossible to trap because it decomposes at a lower temperature.

The researchers used a different approach. First, they employed a vapour-liquid-solid (VLS) process, a technique borrowed from semiconductor scientists for producing nanowires as opposed to nanotubes. The process involved mixing bismuth metal powder with minute flakes of tin sulfide and heating this mixture in a tube furnace under a stream of the relatively unreactive noble gas argon. The product of the reaction forms a deposit at the cooler end of the tube.

The team explains that tiny droplets on the nanometre scale are form within the oven. These nanodroplets act as local points of contact for the tin so that the reactants become concentrated within the metal droplet and nanotubes can then grow from these seeds.

“In this process, the metal drop is obtained as a sphere at the end of the tube, and the nanotubes grow out of the sphere like a hair out of a follicle,” explains Tremel. “Catalysis by the metal droplet makes growth possible at low temperatures.”

The team has successfully grown nanotubes comprising multiple layers of tin sulfide with few defects. The nanotubes have diameters of between 30 and 40 nanometres and are 100 to 500 nm in length.

Tin sulfide nanotubes grow from droplets (Credit: Tremel et al/Wiley-VCH)

Angewandte Chemie, in press

Group of Prof. Dr. Wolfgang Tremel

Tiny acid drops

The world’s smallest droplet of acid has been produced by scientists working at very low temperatures. The droplet of hydrochloric acid comprises just a single hydrogen chloride (HCl) molecule and four water molecules. This seemingly esoteric piece of ultracold chemistry could have significant implications for our understanding of how chlorine-containing compounds such as chlorofluorocarbons (CFCs) damage the ozone layer in the upper atmosphere.

Writing in the 19th June issue of the journal Science, the team based at Ruhr-Universität Bochum, in Germany, explain that how acids dissociate into charged fragments, and are then surrounded by water molecules, or are solvated, at ultracold temperatures in nanoenvironments is very different from their behaviour in bulk volumes of water at room temperature. Such behaviour occurs outside our everyday experience in the upper atmosphere and in interstellar chemistry and is so far poorly understood, the researchers say.

Chemists Anna Gutberlet, Gerhard Schwaab, Özgür Birer, Marco Masia, Anna Kaczmarek, Harald Forbert, Martina Havenith, and Dominik Marx, formed their acid drop in superfluid helium at a fraction of a degree above absolute zero, 0.37 Kelvin.

In order to study this minimalist acid, the team used the powerful analytical technique of high-resolution mass-selective infrared laser spectroscopy. This tool revealed how the successive accumulation of water molecules around the central HCl molecule to form HCl(H2O)n, quickly leads to the formation of the hydronium ion (H3O+) when n = 4. Hydronium is essentially a water molecule with an added hydrogen ion, or proton, and it is the fundamental acidic species in solutions of hydrogen chloride and other acidic compounds.

To augment their studies, the team carried out a simulation of the process from first principles. This ab initio simulation uses the structures of the molecules and the various energies involved and predicts the same step-by-step assembly of undissociated clusters that the team observed. It also points to electrostatic “steering” of the water molecules around the HCl up to n = 3 which form an intermediate ring-shaped system.

The simulation explains how the addition of the fourth water molecule to this undissociated ring HCl(H2O) 3 spontaneously causes the structure to dissociate into an ion pair H3O+ with (H2O)3Cl–. Somehow, the aggregation mechanism bypasses the deep local energy minima that exists when four water molecules attempt to surround the HCl. “This offers a general paradigm for reactivity at ultracold temperatures,” the researchers explain in their Science paper.

Tiniest droplets of hydrochloric acid (Credit: Image courtesy of Ruhr-Universitaet-Bochum)

Science, 2009, 324, 1545-1548