Nanoporous carbon materials obtained by alkaline activation of wood waste

The production of porous carbon materials can be considered as one of the most promising ways of utilizing logging waste and processing wood. An important class of porous carbon materials are microporous sorbents, for which a high volume of micropores with dimensions <1.0 nm is characteristic. The presence of such pores provides the carbon material with the ability to efficiently sorb low-molecular gases, and also gives it molecular sieve properties, which makes it possible to use it in gas separation processes.

However, conventional methods for producing porous carbon materials using steam-gas activation are ineffective to produce microporous carbon materials. For their preparation, alkaline activation can be used to promote the development of a microporous structure.

Porous carbon materials are widely used in industry and environmental protection. The fields of their application are constantly expanding due to the development of methods to produce porous carbon materials with fundamentally new properties: carbon composite materials, molecular sieves, fibers, fullerenes, hollow nanotubes. Large-scale use of carbon sorbents for environmental protection (treatment of effluents, gas emissions, contaminated soils) requires expansion of the production of porous carbon materials from cheap types of organic raw materials: fossil solid fuels, various natural and technogenic organic wastes. Based on the existing theoretical ideas on the mechanism of formation of the structure of a porous carbon material in the pyrolysis of solid and gaseous organic materials, the activation of carbon materials, effective methods for obtaining carbon sorbents with the required complex of properties are developed. In particular, high-speed methods of pyrolysis and activation in fluidized bed apparatuses make it possible to obtain cheap carbon sorbents from available raw materials, which can be successfully used in wastewater treatment and gas emission processes instead of expensive sorbents obtained from unusual raw materials (anthracites, cellulose, peaks).

Perspective directions of the use of porous carbon materials are associated with the catalytic synthesis of carbon-mineral and carbon-carbon composites with unique properties. Since porous carbon materials are derived from any kind of carbonaceous raw materials, including waste, and are themselves used for environmental protection, it can be confidently predicted that porous carbon materials will make an important contribution to addressing the urgent problems of sustainable human development in the 21st century. The scale of the use of carbon molecular sieves in processes of the so-called short-cycle, non-heated adsorption, in which the adsorption of the components is carried out at a higher pressure than their desorption, is rapidly increasing. Now carbon molecular sieves are used for drying gases, separating air into oxygen and nitrogen, cleaning molecular hydrogen from impurities of carbon monoxide and hydrocarbons.

Unlike zeolites (inorganic molecular sieves), carbon molecular sieves are weakly sensitive to the adsorption of water vapor, which reduces the adsorption capacity of zeolites during their use. Prospectively, the use of carbon sorbents to store gaseous and liquid substances, as well as to store energy. In balloons with adsorbent, the adsorption capacity of a unit of volume increases and it becomes possible to reduce the pressure without reducing the cylinder capacity.The use of adsorbents in thermal machines and batteries will save energy and create compact and environmentally friendly heating and cooling devices.

For various applications, carbon sorbents with a specific set of properties are required: a certain porous structure, a specific composition of the surface functional groups of the desired shape, strength, purity. For adsorption purification of liquids and recovery of precious metals from solutions, an indicator such as the hydrophilicity of the surface of the carbon sorbent is important. The purity of the sorbent (for example, the amount of ashen part) does not play a significant role in the processes of cleaning up contaminated sewage and gas emissions. However, for sorption extraction of precious metals, for example gold, the amount of mineral impurities in the sorbent is regulated. The pore size, strength and shape of the carbon sorbent are critical when used as a porous membrane. For the sorbents used in Chromatography, the composition of surface functional groups is important.

The cost of carbon sorbents is a limiting factor in their large-scale use for environmental protection. For purification of gas emissions and effluents, they try to use cheap enough powdered sorbents obtained from affordable and inexpensive raw materials (minerals of solid fuels, wood waste) using intensive technologies of combined pyrolysis and activation in a reactor with a fluidized mass.

The properties of higher-quality activated carbonaceous in many cases depend on the structure of the initial porous carbon materials, which is determined by the conditions for their production. As follows from the research data, the sorption capacity obtained from brown lignite in conditions of practically no oxygen in the gas phase is the maximum sorption capacity. These include porous carbon materials obtained by pyrolysis in the catalytic fluidized mass.

Control questions:

1. Describe, briefly, the structure of carbon nanotubes. Mention existing types.

2. Explain some characteristics of carbon nanotubes.

3. Mention three applications of carbon nanotubes.

4. Mention some properties of graphene.

5. Explain the importance of porous carbon materials.

6. Give some examples of sources that can be used for production of nanoporous materials.

7. Explain one method for production of nanoporous materials.