Science
Related: About this forumYet another cool allotrope of carbon.
The paper I'll discuss in this post is this one: Ultramicroporous Carbon Synthesis Using Lithium-Ion Effect in ZSM-5 Zeolite Template (Ko et al. Chem. Mater., 2018, 30 (18), pp 65136520)
When I was a kid people were taught that carbon had two allotropes, graphite and diamond. Perhaps some specialists were aware of Lonsdaleite, which may be thought of as a structural hybrid, part graphite and part diamond, an allotrope formed in meteorites on impact with the earth.
Today, many hundreds of allotropes of carbon are known, some of the more famous examples being buckminsterfullerene, C60, which as it turned out has always been present in lampblack but was not identified as such - the identification lead to a Nobel Prize - graphene, which is a graphite structure that is exactly one atom thick, and has been the subject of a huge amount of research and interest in possible applications, and single and multiple walled nanotubes, also the subject of much interest and some environmental concern.
The paper cited above reports a new one, and since my kid has been interested in templated polymeric materials, I was drawn to it.
It's "Ultramicroporous carbon."
Zeolites are well known nanostructured materials which are aluminosilicates. They occur naturally and are mined for their many useful properties, particularly where large surface areas are required, as in catalysis, carbon capture from flue gas, as well as in areas requiring separations by physics that is chromatographic in nature, depending on the diffusion into (and out of) the pores.
The authors here have used zeolites as templates for the construction of porous carbon, by utilizing the diffusion of lithium to guide acetylene into zeolite pores.
This introductory cartoon gives the general idea:
From the written introduction:
Similar to mesoporous silica, microporous zeolites have also been used as carbon templates. However, carbon synthesis using zeolite suffers from diffusion limitations for organic carbon precursors. Most zeolites have pore apertures of less than 0.9 nm in diameter, and this often leads to carbon deposition on the external surfaces of the zeolite particles.(14?21) The external carbon prevents the diffusion of the carbon source into the internal pores of the zeolite, resulting in a failure to replicate the entire pore system faithfully. To prevent external carbon deposition, the carbon precursor was fed as highly diluted in an inert gas flow, and the height of the zeolite bed in the carbon deposition reactor was minimized, preferably to a few millimeters. Nevertheless, when the carbon synthesis was scaled up to produce a zeolite bed greater than 1 cm in height, inhomogeneous carbon formation occurred between the upper and lower parts of the zeolite bed. As a strategy to solve this problem, Kim et al. incorporated La3+, Y3+, or Ca2+ ions into the zeolite template pores through a simple ion-exchange process.(22) These cations promoted the carbonization of ethylene or acetylene selectively inside the zeolite pores...
...We conjectured that ZTC synthesis using acetylene in Ca2+-ion-exchanged ZSM-5 zeolite might be improved if the Ca2+ ion (0.23 nm in diameter) is replaced by a smaller catalytic cation. We thereupon tested H+-ion-exchanged ZSM-5, but the result was poor. As a second attempt, we chose Li+ ion (0.18 nm in diameter), considering the high chemical affinity of Li+ ions with carbon to form graphite intercalation compounds, acetylides, and carbides.(30,31) With this assumption, we investigated the Li+ effect on ZSM-5-based ZTC synthesis via thermogravimetric analysis (TGA). The synthesized carbon product was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), Ar gas adsorption, powder X-ray diffraction (XRD) analysis, and electrical double-layer capacitance (EDLC) measurements. In particular, the EDLC characteristics were investigated to assess the recently reported effect of ultramicroporosity on anomalously increasing the capacitance.(32)...
The authors utilize a commercial zeolite, ZSM-11, the "11" referring to the ratio of silicon to aluminum. The ammonium in the commercial product was exchanged for lithium by treatment with a solution of lithium chloride.
They also investigated a number of other ions. This graphic shows the effects:
The caption:
Another graphic touches on these effects:
The caption:
They investigated several different gases as carbon sources, propylene, ethylene, acetylene and "bulky organic compounds" (unspecified). Only acetylene gave sufficient carbon deposition. The authors speculate that the low hydrogen to carbon ratio is responsible for this difference.
The resultant materials have controlled pore sizes:
The caption:
The resultant material shows remarkable electronic properties useful for capacitors and for electrodes. The authors map the electron density of the material.
This map is obtained from X-Ray Diffraction (XRD) and the application of software based calculations.
Here is the graphic reflecting the map:
The caption:
The materials show unusual "EDLC" (Electrical Double Layer Capacitance). The morphology of the material, allowing for the transport of metal and electrolyte ions is responsible for this effect, according to the authors and other workers. The relative drop in capacitance associated with higher current density is much smaller than for other materials.
Figure 7, in which "ZTC" stands for "Zeolite Templated Carbon":
The caption:
The summary:
...Furthermore, we confirmed that the ZSM-5 zeolite-templated carbon exhibited an anomalously high EDLC capacitance because of the presence of the ultramicropores.
Cool, I think. The electronic properties of carbon allotropes already play a huge role in technology, as I've discussed elsewhere in other posts, and to the extent that we are able to fix carbon under circumstances which have economic and technological importance - as opposed to the use of "carbon dumps" including but not limited to the planetary atmosphere - the more hope we have of slowing the atmosphere's destruction.
Have a pleasant Sunday afternoon.