Carbon Aerogels as a possible Alternative to Activated Carbon in Gas Filter Applications

Abstract

In many sectors, such as the chemical and pharmaceutical industries as well as the fire and rescue services, respiratory protection is an essential means of protecting workers against harmful substances. For such filter applications, like respiratory masks today almost exclusively activated carbons are used which are produced by carbonizing and activating of coal, coconut shells and coniferous and deciduous wood. Here the large inner surface of the activated carbon plays a decisive role, as the pollutants adsorb on it, whereby they are extracted from the gas flow. The drawbacks of activated carbons are that they usually have long transport routes and that in case of the renewable resources like coconut shells and wood the pore structure is significantly influenced by the grown structure which can vary greatly depending on local and temporal climatic conditions. One possible alternative to activated carbon are carbon aerogels, highly porous materials that consist of up to 99 % air and have a very large surface area of up to 3,000 m²/g. In general, such aerogels are synthesized by a sol-gel transformation of molecules, polymers or microparticles into a liquid containing gel in which the building blocks are bound by chemical or physical interactions. In a subsequent step, the solvent within the gel is exchanged by a gas, usually air, by maintaining the pore structure. This can best be done by applying supercritical drying using carbon dioxide. However, in case of aqueous gels the water has to be exchanged against an organic solvent (liquid with a high solubility in carbon dioxide like ethanol, isopropanol or acetone) in order to use the supercritical drying with carbon dioxide. For the synthesis of carbon aerogels organic based aerogels like e.g. cellulose, lignin and alginate are carbonized under inert condition i.e. exclusion of oxygen whereas an aerogel with a carbon framework remains. In a second step the inner surface of the aerogel can be further improved by an activation with carbon dioxide which reacts at high temperature with carbon from the carbon network forming carbon monoxide. By this reaction micropores (pores smaller than 2 nm) are formed which significantly increase the inner surface of the carbon aerogel. As describes, for the synthesis of carbon aerogels organic precursors are needed. A very promising candidate for this is lignin which is one of the most abundant biopolymers on earth and which is in varying proportions part of every plant cell. In that way wood and more specifically waste wood, which has reached the end of its life cycle, is especially interesting since it is available in large quantities in Germany and Europe. Hence, in the current project lignin-based carbon aerogels are synthesised and tested for the application in gas respiratory filters. For the synthesis the chemical crosslinking of lignin with formaldehyde and the subsequent supercritical drying with carbon dioxide described by Aufischer is used [1]. The variation of the reaction/gelling temperature, the lignin concentration, the ratio of lignin to formaldehyde as well as the overall lignin concentration will be described. Furthermore, the results of the porosity, the inner surface by nitrogen adsorption and the pore size distributions are going to be presented. For selected compositions the microstructure and the characteristic data before and after the carbonisation activation will be shown. [1] Aufischer, G.; Kamm, B.; Paulik, C., International Journal of Biobased Plastics, 2021, Vol. 3, 1, 19-28.

This work was performed within the project AltHolzAerogel (funding number 2221HV038B) supported by the German Federal Ministry of Food and Agriculture.