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David Bradley ISSUE #75
October 2008
Carbon nanotubes can convert simple butane into more complex useful chemicals (Credit J. Zhang)
Tubular Reactions

Reactive Reports previously reported how Dang Sheng Su and his colleagues at the Fritz Haber Institute of the Max Planck Society, in Berlin, Germany, have developed a way to convert tiny particles of iron oxide found in igneous rocks from volcanoes into templates for making carbon nanotubes and fibers. The process works by directly reacting a condensing organic (carbon-compound containing) gas on to the surfaces of the volcanic particles.

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Now, the team has used their surface-modified carbon nanotubes to activate an important industrial chemical, butane, without the need for an expensive metal catalyst. This process offers a cheaper alternative to the current industrial process for butane activation.

"We show that when carbon nanotubes are used as the catalyst, we can produce olefins with four carbons under very selective, mild (low reaction temperature) and safe (low oxygen concentration) conditions," Su told Reactive Reports.

Carbon nanotubes under the microscope (Credit J. Zhang)

"During the past ten years, we have continuously worked metal-free catalysis using nanocarbons, but all previous works have been focused on activation of ethylbenzene, one particular aromatic molecule," says Su. "The activation of ethylbenzene to styrene is relatively facile and the system is activated by the aromatic moiety. Butane is much less reactive and the high selectivities we have observed are unexpected."

The enormous savings in energy could help label this alternative industrial process as more environment friendly than the old method. "The oxidative dehydrogenation is exothermic and reduces the energy requirements to a great extent," Su explains. "For example, butadiene has been industrially produced via steam cracking, which is a highly endothermic process (needing a lot of heat energy)." The approach taken by Su and colleagues means the reaction produces rather than absorbs heat. "From the industrial viewpoint, this process has great potential," he adds.

The advantage of nanotubes relative to more conventional forms of carbon, such as carbon black and activated carbon, which are commonly used in industry, is that the nanotubes are much stronger, have incredibly high surface areas per unit volume and also have better thermal conductivity, properties beneficial to the chemical reactions.

The team has begun to explore the use of their carbon nanotubes as so-called heterogeneous catalysts, which work as solid particles rather than in solution, which could make them key materials in nano science, he says.

"Carbon nanotubes have a lower density than inert dilution agents, which allows us to easily separate them by mechanical screening once the reaction is complete," adds Su. Moreover, the carbon nanotubes are left unchanged after reaction and so can be used again and again, which means their initial cost can be ignored after they have been used for many reactions.

Science, 322: 73-77, 2008
RR62: Take the Volcanic Fast-track to Nanotube Production