The green face of coal

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FACULTY OF CHEMISTRY

 

Most of us associate coal with a raw material that is mined and then used as a solid fuel. However, it turns out that carbon materials are still more often produced on a laboratory or industrial scale with the idea of be used in various advanced applications.

The therapeutic effect of activated coal – a specific form of carbon that perfectly binds chemical compounds existing in the environment – has been known for a long time. For example, in food poisoning, activated coal eliminates microorganisms and toxins from the human body.

Unique properties

However, the scope of application of activated coal is much wider. These materials can be found mainly in environmentally- friendly technologies, where they are used as adsorbents to eliminate contaminants present in water and air. They are also used as catalyst carriers, catalysts and electrode materials.

The main advantage of activated coal, which makes these materials so useful, is their enormous surface area (which can be imagined in form of a solid that is cut through and laid flat on a surface) characterized by a specific chemical structure. Typically, the surface area of 1 gram of such material exceeds 1000 m2, i.e., 10 ares. Squeezing such a large surface area into a small portion of material is enabled only by the presence of countless, very narrow channels (of a diameter smaller than 2 nm), called "micropores". The surface area of their sidewalls adds up to the total mentioned above. In addition, it is worth noting that apart from elemental carbon, it also contains atoms of other elements, including mainly hydrogen and oxygen. The presence of a large number of superficial centers containing oxygen determines the above average adsorption capacities of activated coal.

The microporosity described above is at the same time a disadvantage of activated coal, as larger particles cannot migrate into such narrow channels because their size is greater than the diameter of the pores. This significantly limits the applications of activated coal, which has become a serious challenge for the scientific world.


Carbon replica type CMK-3, image taken with use of transmission electron microscopy (TEM)

Carbon replicas

Various methods of synthesis of carbon materials with a structure containing much wider channels, e.g., of a diameter of 2 nm to 50 nm (called mesopores), are being developed on an increasingly wide scale. One of the groups of materials proposed by scientists that are characterized by such properties is carbon replica. An innovative synthesis method of such materials has been developed at the Organic Technology Research Group at the Faculty of Chemistry.

This synthesis is based on the application of a so-called structural template, i.e., mesoporous silica (SiO2) characterized by a very homogeneous distribution of channels. If the pores of such template are filled with a substance constituting a carbon precursor (a compound from which, after further processing, carbon is formed), then after carbonization (heating at a high temperature in an oxygen-free atmosphere) a hybrid system emerges, composed of carbon located inside the silica channels. The dimensions and shape of the carbon particle are determined by the arrangement of the pores present in the SiO2.

The final stage of synthesis of the mesoporous material is the removal of the silica by washing it out in hydrofluoric acid, which selectively dissolves the SiO2 without damaging the carbon component. As a result, carbonaceous materials are obtained (e.g., CMK-3 type material), which exhibit a high structural ordering. A good visualization of the structure described is an image of a ready carbon replica, type CMK-3, taken with use of transmission electron microscopy (TEM).

The method of synthesis of the discussed carbon replicas proposed by the Organic Technology Research Group is cheap, effective and replicable, which has been verified experimentally with the use of appropriate physical and chemical methods (low-temperature nitrogen sorption, powder X-ray diffraction, transmission electron microscopy).

Adsorption and absorption. These two terms are often confused although they refer to completely different physicochemical phenomena. Adsorption consists in the "adhesion" of particles of various chemical compounds to the given surface by means of physical (van der Waals force) or chemical interactions (formation of chemical bonds). Absorption, on the other hand, refers to the ability of the given medium to take up (absorb) a given substance by its volume. Both processes can be jointly referred to as "sorption".

Applications

The synthesized carbon sieve of a specific surface area within the range of 1000 – 1100 m2/g and a total pore volume of approximately 0.9 cm3/g has proven to be a very active catalyst in oxidative dehydrogenation of ethylbenzene. In the future, this process may become an alternative, more environmentally- friendly method of producing styrene, which is one of the basic monomers used in the plastics industry. The effectiveness of the CMK-3 replica in removing volatile organic compounds from the air was also tested and enhanced adsorption properties were observed. Thus, it can be assumed that the material developed may constitute a basis for the operation of modern filters used in air purification.


Research team from the Jagiellonian University:Piotr Kuśtrowski, PhD; Paula Janus, MSc; Sebastian Jarczewski, MSc