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Posted: Oct 06, 2008
Nanotechnology catalyst improves important reaction in the chemical industry
(Nanowerk Spotlight) Back in the early 1800's it was observed that certain chemicals can speed up a chemical reaction – a process that became known as catalysis and that has become the foundation of the modern chemical industry. By some estimates 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture. Catalysis is the acceleration of a chemical reaction by means of a substance, called a catalyst, which is itself not consumed by the overall reaction. The most effective catalysts are usually transition metals or transition metal complexes.
New nanotechnology research with carbon nanotubes coming out of Germany contains some implications for catalysis in general. Researchers at the Fritz Haber Institute of the Max Planck Society in Berlin have been working for some time at metal-free catalysis using nanocarbons. While their focus initially has been on ethylbenzene, an aromatic hydrocarbon that plays an important role as an intermediate in the production of various plastic materials, they now have, for the first time, used carbon nanotubes (CNTs) to activate butane. The results indicate that the use of CNTs can make certain important chemical reactions more effective, more energy efficient, and safer.
The chemical industry produces butadiene and other alkenes by catalytic dehydrogenation of normal butane. Butadiene is an important basic chemical with a global demand of approximately 9 million metric tons. Over half of that amount is used to prepare synthetic rubbers such as the ones used in tire production.
"Carbon nanotubes were used as catalysts before, for instance in converting ethylbenzene into styrene, a precursor to polystyrene, an important synthetic material" Dr. Dangsheng Su explains to Nanowerk. "But butane is much less reactive than ethylbenzene and we were surprised how well it worked – the high selectivities we observed were unexpected."
Conventional synthesis technologies are using complex metal oxide catalysts to produce butadiene and other alkenes from butane through oxidative dehydrogenation. These catalysts require high temperatures and lots of oxygen to maintain the catalytic activity but this leads to unwanted product oxidation, which in turn leads to a low selectivity for butadiene.
What the Max Planck scientists found is that the selectivity to butadiene in the catalytic dehydrogenation of n-butane can be improved by using modified CNTs as catalyst.
"We demonstrated that carbon nanotubes can compete with the best metal oxide-based catalysts on the market for converting butane into butenes – and they were nearly twice as selective for butadiene" says Su. He and his team functionalized surfaces of pristine CNTs with oxygen-containing groups and then additionally modified them by passivating defects with phosphorus (the scientists hypothesize that the phosphorous covers up nanotube defects, preventing them reacting with oxygen and producing electrophilic oxygen species that would destroy the alkene products). With this catalyst they were able to achieve an alkene yield of 13.8%.
Performance of various carbon nanotube samples for oxidative dehydrogenation of butane. oCNT = CNTs functionalized with oxygen-containing groups. P-oCNTs = oCNTs modified with phosphorus. (Image copyright: Science)
According to Su, their CNT catalyst is as selective as the best vanadium/magnesium oxide complexes developed during the past 20 years.
Another important aspect of using CNT catalysts is improved efficiency. The new process uses less energy because it can run at much lower temperatures – 400°C - 450°C as compared to over 900°C for existing processes; and not only does it require far less oxygen (oxygen/butane ratio of 2) but ordinary air could be used in place of pure oxygen, making the reaction safer. Even after reducing the oxygen/butane ratio from 2 to 0.5 the CNT sample still exhibited outstanding stability.
In order to assure that the reactivity originated exclusively from metal-free active sites on CNTs and that the residual metals played no positive role, Su and his colleagues took great care in preparing their CNT catalysts.
"One of the key concerns in identifying and describing metal-free catalysis is the suspected influence of metal impurities in the catalyst" says Su. "Through X-ray fluorescence spectrometry and high-resolution transmission electron microscopy we made sure that the residual metals in our tested CNTs were very low and that those that remained were embedded in the carbon and not exposed to the reactants."
The next challenge for Su's team will be to demonstrate that this process also could work on an industrial scale. If it does, it could have a far-reaching impact on the use of carbon nanotubes in catalytic processes in the chemical industry.