Making a porous three-dimensional version of the 2D carbon monolayer material, graphene, using a zeolite as a template is now possible thanks to scientists at KAIST in Daejeon, South Korea. The periodic minipores possible in such materials could be exploited in batteries and catalysts. [Kim et al., Nature (2016), DOI: 10.1038/nature18284]
The nanoscopic pores within a zeolite can accommodate carbon nanotubes , according to KAIST chemist Ryong Ryoo and colleagues. They explain that using zeolites to template carbon-compound formation using ethylene and acetylene molecules as a carbon source for a carbonization synthesis usually requires high temperatures and leads to coking of the zeolite exterior rather than producing structured carbon materials. Now, the team has embedded lanthanum(III) ions (displacing the sodium ions) in the zeolite pores to lower the temperature at which carbonization of ethylene or acetylene occurs.
The result is that graphene-like sp2 carbon structures can be selectively formed within the zeolite template, without coking of the external surfaces. Once the zeolite template is removed with hydrofluoric and hydrochloric acids, the carbon framework left behind exhibits electrical conductivity some two orders of magnitude higher than amorphous mesoporous carbon. The team suggests that the approach represents a general method for synthesizing carbon nanostructures with various topologies corresponding to the template zeolite pore topologies, whether FAU, EMT, beta, LTL, MFI, or LTA type.
"Lanthanum(III) ions are unreducible under carbonization process condition, so they can stay inside the zeolite pores instead of moving to the outer zeolite surface in the form of reduced metal particles," explains team member Kim Kyoungsoo. "Within the pores, they can stabilize ethylene and the pyrocondensation intermediates to form a carbon framework in zeolites."
In a proof of principle experiment, the team added lanthanum(III) ions to a Y zeolite (LaY) sample as well as other samples such as NaY and HY. Carbon deposition occurs rapidly at 800 degrees Celsius, the team found. At lower temperature (just 600 degrees Celsius there is a dramatic shift in response. With only LaY zeolite remaining active at this lower temperature and NaY and HY failing to act as templates for an internal structured carbon deposit.
The scalable approach could be used to manufacture a wide variety of 3D graphene foams that might be tested for a range of applications in electrochemical science and catalytic applications as well as for fuel storage and replacements for zeolite-type materials themselves in some areas.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".