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Sun Sep 30, 2018, 11:20 AM

Yet 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 6513–6520)

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:

Ordered nanoporous carbons with controlled pore diameters have attracted increasing attention in recent years because of their pore-size-specific effects in catalysis,(1−3) adsorption,(4,5) water and air purification,(6,7) and electrical energy storage.(8−10) One of the most interesting methods for the synthesis of nanoporous carbons is to use mesoporous silica, such as MCM-48, SBA-15, or KIT-6, as a template.(11−13) The silica mesopores are wide enough (2–50 nm) for the infiltration of organic compounds without diffusion limitations. Pyrolytic carbonization of the organic compounds can be performed inside the template pores to produce rigidly interconnected carbon frameworks through the pores. After the dissolution of the silica template with HF or NaOH, the resultant carbon exhibits an ordered mesoporous structure, which corresponds to an inverse replica of the silica template.

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:

Figure 1. Quantities of carbonaceous deposition onto ZSM-5 zeolite with exchanged cations are plotted as a function of time under a N2–acetylene–H2O mixed gas flow at 500 °C. Elemental symbols indicate cations that were exchanged into the zeolite.

Another graphic touches on these effects:

The caption:

Figure 2. SEM images of (a) ZSM-5 zeolite and (b)–(e) carbon products from different cation-exchanged ZSM-5 zeolites

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:

Figure 3. NLDFT pore size distribution of (a) LiZSM-5-templated carbon and (b) CaZSM-5-templated carbon. The insets show the corresponding Ar adsorption–desorption isotherm for each carbon.

The resultant material shows remarkable electronic properties useful for capacitors and for electrodes. The authors map the electron density of the material.

To further investigate the framework connectivity, a powder XRD analysis by Rietveld refinement was performed after acetylene carbonization in LiZSM-5 zeolite at 500 °C. In particular, we obtained an electron density map for the carbon atoms using the difference Fourier method (see Experimental Section for details). The electron density in Figure 6 shows that the carbon atoms formed a rodlike shape inside the narrow 10MR channels in the ZSM-5 zeolite. This was markedly different from the hollow nanotube-like shape of carbon that was formed in the case of carbon synthesis using 12MR FAU zeolites.(22) It is reasonable that the ZSM-5 zeolite channels would be too narrow to build a nanotube-like hollow carbon structure. The electron density map shows that the rodlike carbon frameworks were rigidly built inside the straight channels along the b-axis of the ZSM-5 zeolite. However, in the sinusoidal channels running perpendicular to the b-axis (i.e., in the ac plane), the carbon frameworks had many empty defects. The channel tortuosity in the sinusoidal channels appeared to hinder continuous growth of the carbon framework,(28,29) causing the formation of defects. Because of the defects, the carbon framework could undergo conspicuous contraction when the zeolite template was dissolved. More contraction could occur in the c-direction among a- and c-axes as the zeolite channels were connected more tortuously in this direction. This may explain why the carbon morphologies in Figure 5 show noticeable anisotropic shrinkage in the c-direction.

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:

Figure 6. Electron density map of ZSM-5 zeolite after carbonization of acetylene. Electron density map of the carbon framework (cyan) is visualized from different viewpoints: along the (a) [010] and (b) [100] axis. Yellow lines represent the ZSM-5 zeolite framework. The iso-surface level of the electron density is set to 0.37 electrons Å–3. Blue areas represent cross-sectional cuts. Red arrows represent disconnected parts of the carbon framework. Compared to the left panel, the right panel shows the corresponding electron density maps without the zeolite

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.

As shown in Figure 7, the capacitance value decreases as the current density is increased. This is due to an increase in the electric resistance under high current densities. Carbons with similar surface properties are expected to exhibit capacitance values that are approximately proportional to their specific surface areas, if all measurements are performed under the same conditions except for the current density.(39,40) However, according to a recent work by Gogotsi et al.,(32) ultramicroporous carbons exhibited an anomalously high EDLC capacitance value when compared to microporous carbons with the same specific surface area. The high EDLC capacitance of the ultramicroporous carbon was attributed to increasing concentration of the electrolyte ions when the pores were too small to form electrical double layers and solvation shells.

Figure 7, in which "ZTC" stands for "Zeolite Templated Carbon":

The caption:

Figure 7. (a) Specific capacitance and (b) specific capacitance normalized by BET surface area of various carbons, as a function of discharge current density in an aqueous solution of 6 M KOH.

The summary:

ZSM-5 zeolite-templated carbon synthesis has remained a challenge because of the diffusion limitations in the extremely narrow 10MR pores. Regarding this issue, we discovered that Li+-ion exchange into ZSM-5 is an effective means to promote acetylene carbonization in the zeolite pores. The use of Li+ ions improved the connectivity and microporosity of the resultant carbon product by enabling a 20% greater amount of carbon deposition, compared to the previously reported Ca2+-ion-exchanged ZSM-5...

...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.

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