Carbon nanotubes as ethanol factories

A group of Chinese researchers claim that carbon nanotubes loaded with rhodium particles can be used as reactors to convert a mixture of carbon monoxide and hydrogen into ethanol. This could then conceivably be used as a fossil fuel replacement.

Schematic of ethanol production from syngas inside a rhodium-loaded carbon nanotube. The coloured masses inside the tube are the rhodium-based catalyst, and the streams of light cyan represent ethanol (© Nature Publishing Group).

Following previous work with iron nanoparticles, Xiulian Pan, Xinhe Bao and their colleagues at the Dalian Institute suspected that the activity of a metal-catalysed gas-phase reaction may in general benefit from proceeding inside carbon nanotube reaction chambers.

In a paper published in Nature Materials, the researchers report a striking enhancement in the catalytic activity of rhodium nanoparticles, with the formation rate for ethanol with the metal particles inside the nanotubes exceeding that outside by more than an order of magnitude.

Even if the enhanced ethanol formation is significant (see below), the technology is nowhere near ready for scaling up. “First of all, a cost-efficient, mass production of carbon nanotubes with precisely controlled diameter and chirality is still a challenge,” says Pan. “Another challenge is the homogeneous dispersion of metal nanoparticles within the nanotube channels, since this can strongly influence the activity of these catalysts.”

Carter Kittrell, a chemist at Rice University in Houston, US, points out that ethanol yield must be calculated by the reader from data presented only in a supplement to the paper. It turns out that the ethanol yield is up a modest 50%, whereas the selectivity falls significantly, and waste carbon dioxide exceeds ethanol by mass.

“There are two different catalysts, four different substrates, several different products, different temperatures and times of evolution, and assorted measures of ‘goodness’, which are cherry-picked to mix and match,” says Kittrell. “The authors have in effect compiled a composite fantasy catalyst highlighting the best benefits from three catalyst/support systems while ignoring and/or suppressing detrimental information from each, and used the supplement as a repository for adverse data ignored in the paper.”

Figure: Schematic of ethanol production from syngas inside a rhodium-loaded carbon nanotube. The coloured masses inside the tube are the rhodium-based catalyst, and the streams of light cyan represent ethanol (© Nature Publishing Group).

Further reading: Enhanced ethanol production inside carbon-nanotube reactors containing catalytic particles, Pan et al., Nature Mat. 6, 507 (2007).

Article first published in Nanomaterials News.