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Posted: Sep 02, 2008

Nanotechnology production materials come flowing out of volcanoes

(Nanowerk Spotlight) The demand for the raw materials of the nanotechnology revolution – nanoparticles, carbon nanotubes, fullerenes, quantum dots, etc – is rising explosively and large chemical companies keep expanding their production capacities. As production moves from a workshop model to an industrial production process, prices for these materials are coming down fast. Nowhere has this trend been more noticeable than with carbon nanotubes (CNTs), where prices have more or less collapsed from their astronomical levels: a kilogram of multi-walled carbon nanotubes (MWCNTs) sold for tens of thousands of dollars just a few years ago; by now, the price for some types of MWCNTs has fallen to only hundreds of dollars per kg.
As this trend continues and nanomaterials become simple commodities, sooner or later mundane production issues such as the limitation of available resources (just as an example, the Earth is running out of indium, which makes indium tin oxide coatings, used in nearly all flat panel displays and microdisplays, a dead-end option going forward), the cost of production materials, and the amount and cost of energy used in nanomaterial synthesis will become the key cost drivers and bottlenecks.
The recent interest in development of fuels from biomass ('biofuels') is one example of how 'natural resources' constitute much more than just valuable, and often limited, supplies such as fossil fuels or precious minerals. Of course, in the case of biofuels, the fuel production competes with food production, thereby solving one problem by creating a different one. But there are vast amounts of 'useless' natural materials literally lying around – rocks and stones – which could find their way into nanotechnology.
Researchers in Germany have made a proof of concept demonstration that using natural nanostructures found in lava rocks is suitable for nanomaterial synthesis and for use in catalysis for production of butadiene and styrene.
Scanning electron microscopy image of a lava–CNF composite, showing CNT/CNFs
grown on igneous rocks
Scanning electron microscopy image of a lava–CNF composite, showing CNT/CNFs grown on igneous rocks. (Reprinted with permission from Wiley)
"The key message of our work is the use of iron oxide particles existing abundantly in minerals as natural catalysts for carbon nanotube growth without any prior treatment of the lavas and the use of lava–carbon nanofiber composites as catalyst, also without any prior treatment" Dr. Dangsheng Su tells Nanowerk. "We have demonstrated a good and stable performance of the synthesized composite in catalytic reactions. Because iron exists in a large number of minerals, clays, and soils and even in plants, we may open a new era for the cheaper production of immobilized CNTs and extend the application potential of CNTs, owing to the low costs, into other areas such as catalysis."
Su, a scientist at the Fritz Haber Institute of the Max Planck Society in Berlin, Germany, together with colleagues from his institute and the Molecular Physics Laboratory at the Rudjer Boskovic Institute in Zagreb, Croatia, have published their findings in the August 27, 2008 online edition of Advanced Materials ("Mount-Etna-Lava-Supported Nanocarbons for Oxidative Dehydrogenation Reactions").
In a previous Nanowerk Spotlight ("Lava as nanotechnology catalyst") we have already covered an earlier report by Su and colleagues that describes their first steps in using lava as support and catalyst for CNT and CNF synthesis.
In their latest experiments, the researchers found the CNT production to be highly efficient: On the laboratory scale, without any chemical pretreatment of the crushed lava stone, 1.05 grams of nanocarbons can be immobilized on 0.2 grams lava.
"The immobilization of CNTs/CNFs on minerals without preparation of the catalyst is highly motivating, because owing to their loose form as 'soot' CNTs/CNFs are unsuitable for many applications, for example, in catalysis or drinking water purification" says Su. "CNTs and CNFs are reported to be highly active catalysts for chemical reactions. However, technical problems such as hot spots or pressure drops in a reactor induce the loss of catalytic performance. This can be avoided by immobilization of the nanocarbons onto the support."
With the ability to synthesize CNTs and catalysts for chemical reactions in one step and by utilizing an abundant natural catalysts, production costs of CNTs could be brought down further.
As test reactions, the Max Planck scientists choose the oxidative dehydrogenation of hydrocarbons (ODH), that is, the production of butadiene from 1-butene, and styrene from ethylbenzene. The latter reaction is one of the ten largest chemical industrial processes in the world.
Su says that for the 1-butene to butadiene reaction, a conversion of 1-butene as high as 65% is observed at the beginning of the reaction. "This conversion increases and stabilizes at about 80% after about 10 hours A stable butadiene yield higher than 50% is obtained after the activation period. For the styrene production from ethylbenzene, the conversion rate of ethylbenzene stabilizes at about 30% after a short deactivation period at the beginning of the reaction. A high selectivity to styrene of more than 85% is achieved, giving a styrene yield of about 25%."
The pure lava does not exhibit any significant reactivity in the tested reaction. The scientists found that the reaction rate is much faster when CNTs supported on lava are used to catalyze the reaction in comparison to when loose CNTs are used, indicating the advantage of immobilized CNTs for catalysis. Similarly, the reaction rate for ethylbenzene to styrene over immobilized CNTs is also higher than when only loose CNTs are used. Su points out that, in addition, the obtained reaction rate for this reaction is also higher than those of the catalysts reported in the literature.
While this has been just a proof-of-concept demonstration, it appears that carbon hybrid catalysts such as those used by the Max Planck scientists have potential to compete with industrially optimized systems for the tested reactions; and more generally, that waste materials and abundant, common natural resources could prove to be useful materials for chemistry, catalysis and nanotechnology.
By . Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology (RSC Nanoscience & Nanotechnology) and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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