An article published in the International Journal of Biological Macromolecules reports an easy method to convert lignin directly into carbon foam materials.
Study: Simple, additive-free, extra pressure-free process to direct convert lignin into carbon foams. Image Credit: Rattiya Thongdumhyu/Shutterstock.com
Valuable Applications of Lignin
While being one of nature's most richly available aromatic polymers, lignin remains a waste product with few uses in modern industry. Lignin has a global output yield of about 70 million tons per annum, the majority of which is derived as a byproduct of paper and pulp industries. The rest of it comes from bio-renewable refineries, and most of it is merely burnt as a low-grade fuel.
Since lignin comprises heterogeneous and cross-linked aromatic structures, carbon atoms can be preserved rather than converted into gases containing carbon when heated to high temperatures. As a result, lignin is a suitable precursor for producing high-value carbon products.
Products of carbon nanofiber, activated carbon, carbon fiber, porous graphene and carbon film, etc., have all been created successfully from lignin, and their uses have been thoroughly investigated.
To promote lignin optimization, relevant aspects such as fabrication cost and scalability should also be considered. This inspires researchers to continue looking for a more convenient and environmentally friendly way to use lignin.
Characteristics and Production of Carbon Foam
Carbon foam is a prominent material in the carbon family because of its 3D porous structures. It has distinct qualities, including self-standing, high strength, structural integrity, porous architecture and low density.
Much work has been done with respect to applications, property characterization and synthesis of carbon foam since the 1960's. Carbon foam can be produced from various precursors, including pitches, biomass and polymers. Irrespective of the precursor utilized, molding, pressing, heating, blowing, and precursor pretreatment are commonly implemented to achieve the required results. Attempts have been undertaken to develop simpler methods for carbon foam manufacturing.
Production of Carbon Foam using Lignin
This work describes the development of a more facile technique for directly obtaining carbon foam from lignin powder, in addition to the usual methods. This result was obtained during the production of lignin-based activated carbon, where the treated lignin powder was directly heated into block materials having porous architecture.
To verify that carbon foam and lignin can be directly produced using these simple methods, four industrially accessible lignins were evaluated under the same heating conditions as initially implemented. Interestingly, all of the lignins were observed to be transformed into monolithic structures.
According to this phenomenon, lignin may inherently have prerequisite characteristics for directly transforming into carbon foam. Firstly, due to its cross-linked structure and strong aromaticity, lignin can retain the majority of carbon at high temperatures in a static atmosphere. Secondly, it has a vast temperature range of fusing, making it a natural lignin molecular binder. Moreover, lignin is thermally unstable and includes volatiles, making it a potential foaming agent.
Lastly, at high temperatures, lignin is prone to crosslinking, which can serve as the framework of carbon foam. Therefore, two carbon foams with various properties were successfully obtained by adjusting the heating profiles of fractionating lignin.
This work claims the finding that carbon foam can be directly obtained from lignin even without the use of multiple methods or additives that are needed by other precursors. It was discovered that different lignins underwent chemical and physical changes at various priorities based on their examination during thermal treatments.
At a particular temperature range, the ability of lignin to create appropriate lignin foam, which is a precondition for carbon foam, was sensitively tied to its crosslinking, decomposition-governed foaming, and order of fusing.
The important guidelines for making lignin foam are to avoid early fusing or lignin with high glass transition temperatures (Tg). Lignin monoliths having foam skeletons were effectively produced using the proposed fractionated or temperature-controlled procedures. FC (carbon foam derived from HL260) and TC (carbon foam derived from HLH350) exhibit densities that are similar to carbon foams made from other precursors or through more complex procedures.
FC and TC have a wide range of applications due to their conductivity, lightweight, porosity, high stability and strength. The methods described in this work represent an advancement in the production of carbon forms based on lignin, as they are simple but innovative approaches to lignin valorization and manufacturing enhanced carbon foam products.
Qu, W., Zhao, Z., Liang, C., Hu, P., & Ma, Z. (2022). Simple, additive-free, extra pressure-free process to direct convert lignin into carbon foams. International Journal of Biological Macromolecules. Available at: https://www.sciencedirect.com/science/article/pii/S0141813022007693?via%3Dihub