Carbon aerogels/monoliths from sugars

The classical synthetic pathway towards carbon aerogels is via the formation of organic gels from the polymerization-induced phase separation. The commonly used precursors are resorcinol/formaldehyde (RF gels; Figure 2A) or melamine/formaldehyde (MF gels). The organic gels are supercritically dried and carbonized later on. RF polymers usually give resins and consist of micrometer-sized spherical particles. However, nanostructured gels can be obtained by means of acid or base catalysis.

A novel, sustainable approach towards carbonaceous materials is the so-called hydrothermal carbonization (HTC). The chemical mimic of natural coalification uses carbohydrates as carbon precursors. The typical product e.g. from the hydrothermal carbonization of glucose, like in the RF system, are micron-sized spherical particles (Figure 2B).^1,2

The use of borax, as buffer and novel catalyst of the HTC process opens the way to respective sol-gel chemistry.³ Borates catalyse the isomerization of glucose to fructose and therefore accelerate the dehydration under hydrothermal conditions. Additionally, the condensation reaction is accelerated, most likely due to the reactivity-increasing formation of acetals. The catalytic effect shifts the kinetics of dehydration and condensation to allow the formation of hydrothermal carbon aerogels/monoliths (Figure 3).

Similar to the classical RF system the ratio of carbon precursor to catalyst can be set to adjust the final particle size down to the 10 nm range.

Melamine-Formaldehyde gels after carbonization lead to nitrogen-doped carbon aerogels. By using appropriate nitrogen-containing precursors consistent with the hydrothermal condensation scheme of sugars it is also possible to obtain nitrogen-doped hydrothermal carbon aerogels/monoliths (Figure 4).⁴

Worth to mention is the possibility to independently control particle size and nitrogen content by using different ratio sugar/borax and sugar/nitrogen source (Figure 5).

References:

1. Titirici, M. M.; Thomas, A.; Antonietti, M. New Journal of Chemistry 2007, 31, 787.
2. Titirici, M. A. a. M. M. Comptes Rendus Chimie 2010, 13, 167.
3. Fellinger, T. P.; White, R. J.; Titirici, M. M.; Antonietti, M. Advanced Functional Materials 2012, 22, 3254.
4. Wohlgemuth, S. A.; Fellinger, T. P.; Jaker, P.; Antonietti, M. Journal of Materials Chemistry A 2013, 1, 4002.