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Posted: March 29, 2008
Researchers achieve scalable fullerene production and separation
(Nanowerk News) They showed promise in everything from drug delivery to the development of superconductive compounds, but so far fullerenes have been difficult to produce and purify in sufficiently large quantities for useful applications.
Now Russian researchers claim to have found a way to produce and purify high yields of fullerenes for commercial use by carefully tweaking existing techniques.
Fullerenes are complex molecules, composed entirely of carbon, that take the shape of hollow spheres (the famous bukyball), ellipsoids or tubes, providing them with unique chemical and electrical properties. Scientists hope that they could be attached to antibodies to deliver drugs to tumours, and they could form new materials with interesting thermal and electrical properties. They also act as efficient catalysts for certain processes, and they can be used to alter the structure of polymer materials, even when present in only very small quantities.
However, previous processes, such as the Krätschmer method which uses a carbon-helium plasma to erode graphite cores, are difficult to scale to produce the large quantities necessary for artificial use. In addition, these traditional methods typically produce a mixture of many different types of fullerenes, and the separation of the different allotropes is even more difficult to scale.
"Separation of individual fullerenes is carried out now mostly with chromatographic method," said Nikolay Charykov, from the Innovations of Leningrad Institutes and Enterprises in St Petersburg, who worked on the new technique. "It is badly scaled and in classical variant is characterized by low productivity owing to very high similarity of fullerenes: the molecules consist from the same atoms of carbon with rather close linear sizes."
Charykov believes he has overcome these difficulties with a new process that consists of three separate units for the creation, purification and separation of the fullerenes. The first unit creates the fullerenes using a variation on the Krätschmer technique. To increase the yield, the system carefully controls multiple streams of the plasma using an electromagnetic field, which allows the plasma to erode a greater volume of graphite, increasing the possible amount of fullerenes produced.
Fullerenes compose about 14 per cent of the products made during the process - the rest is soot that needs to be separated from the useful carbon allotropes. The second unit creates supersaturated solutions of the fullerenes and other carbon allotropes present in the soot, which can then be filtered from the mixture.
The third unit then separates and purifies the different fullerenes. "We carry out preliminary rough (up to 95 per cent mass purity) separation of fullerenes using the method of polythermal recrystallization. Then, with three series-parallel columns (one sorptional and two chromatographic) we receive individual fullerenes using different aromatic solvents and different sorbents," explains Charykov.