(2ik) Single-Step Aerosol Method for Scalable and Sustainable Valorization of Lignin

The utilization of fossil resources, such as coal and petroleum products, for the synthesis of materials used across different sectors (e.g., energy storage, healthcare, and agriculture) is contributing to the alarming rise in greenhouse gas emissions. Therefore, the exploration of environmentally benign and cost-effective alternatives, such as lignocellulosic materials, holds immense promise for a more sustainable future. My research centers around the development of novel techniques for scalable and sustainable synthesis of high-performance nanomaterials from lignin, a waste byproduct generated from biorefineries (~ 100 mt/year) [1, 2].

Previous studies have successfully demonstrated the synthesis of a wide range of lignin-based nanomaterials employing various solvothermal methods. However, the scalability of these methods is hindered by their reliance on multistep and batch processes, as well as the requirement for large volumes of solvents or activating agents, which can be both costly and environmentally unfavorable [1, 3]. To overcome these limitations, we are exploring an alternative gas phase synthesis technique known as the furnace aerosol reactor (FuAR). The FuAR offers several key advantages such as ultra-fast processing, continuous and single-step operation, and precise control over material properties, ensuring consistency and facilitating scalability [4, 5]. Therefore, my research primarily focuses on three aspects: (i) engineering the FuAR technique for the synthesis of lignin-based nanomaterials with controlled properties (size, shape, porosity, and composition), (ii) developing models for kinetic studies of inter/intra particle interactions, and (iii) investigating the relationship between properties of synthesized nanomaterials and its performance in the respective applications.

First, the FuAR is demonstrated for controlled synthesis of lignin NPs of mean sizes between 50 and 68 nm [6]. The mean size of lignin NPs showed an increasing trend with lignin solution concentration. Based on the changes in functional groups, the maximum temperature in FuAR to obtain lignin NPs without significant chemical degradation was found to be around 300 ^oC (at a residence time of 5.8 s) [6]. Furthermore, with a systematic understanding of the effect of temperature and residence time on lignin particles, porous carbon nanoparticles (NPs) with high surface area (up to 925 m²/g) are synthesized in FuAR without the use of activating/templating chemicals [7]. This one-step approach requires significantly less time for synthesis: an order of seconds in comparison to hours for conventional methods. The FuAR is then engineered for the synthesis of nitrogen-functionalized porous carbon NPs using urea as a nitrogen source.

The size and morphology of synthesized carbon NPs, which are impacted by interparticle collision and sintering, are known to play a crucial role in their performance such as energy storage capacity as well as stability. Therefore, next, we systematically investigate the kinetics of reaction, sintering, and collisions in lignin particles using a novel geometric modeling approach [8]. The kinetic parameters for lignin sintering, which are the pre-exponential factor and activation energy, were estimated as 6.6x10^-8 s/nm and 116.4 kJ/mol, respectively. Knowledge of kinetics will provide better control over the size and morphology for efficient utilization of carbon nanoparticles in energy storage [8].

Finally, as-synthesized lignin-based nanomaterials are tested for respective applications to investigate property-performance relationships. The bulk lignin and as-synthesized LNPs were tested for UV protection application and UV protection improved with a decrease in lignin particle size [6]. The as-obtained carbon nanoparticles are tested for specific capacitance which showed a linear trend with surface area. The best-performing material (with the highest surface area) exhibited a specific capacitance of 247 F/g at 0.5 A/g with excellent capacity retainment of over 98 % after 10,000 cycles [7]. Furthermore, the as-obtained nitrogen-functionalized carbon nanoparticles are tested for CO₂ adsorption (adsorption capacity of 62 mg/g of carbon).

In summary, by employing this aerosol-based method, we aim to overcome the limitations of previous approaches, paving the way for a more efficient, cost-effective, and environmentally benign synthesis of lignin-based nanomaterials.

In the future, as a post-doc candidate, I would like to utilize my current experience in material synthesis, in-situ characterizations, and modeling the kinetics of inter/intra particle processes to advance sustainable nanomaterial synthesis. Further, I would like to expand my knowledge in investigating the sustainability and economic viability of different materials and synthesis techniques using life cycle assessment and techno-economic analysis.

References:

[1] W. Zhang, X. Qiu, C. Wang, L. Zhong, F. Fu, J. Zhu, Z. Zhang, Y. Qin, D. Yang, C.C. Xu, Lignin derived carbon materials: current status and future trends, Carbon Research 1(1) (2022) 1-39.

[2] D. Bajwa, G. Pourhashem, A. Ullah, S. Bajwa, A concise review of current lignin production, applications, products and their environmental impact, Industrial Crops and Products 139 (2019) 111526.

[3] M. Kienberger, Potential Applications of Lignin, in: Y. Krozer, M. Narodoslawsky (Eds.), Economics of Bioresources: Concepts, Tools, Experiences, Springer International Publishing, Cham, 2019, pp. 183-193. https://doi.org/10.1007/978-3-030-14618-4_12.

[4] M. Ago, S. Huan, M. Borghei, J. Raula, E.I. Kauppinen, O.J. Rojas, High-throughput synthesis of lignin particles (∼ 30 nm to∼ 2 μm) via aerosol flow reactor: size fractionation and utilization in pickering emulsions, ACS applied materials & interfaces 8(35) (2016) 23302-23310.

[5] H. Zhou, M. Kouhnavard, S. Jung, R. Mishra, P. Biswas, One-step aerosol synthesis of a double perovskite oxide (KBaTeBiO 6) as a potential catalyst for CO 2 photoreduction, Nanoscale 13(27) (2021) 11963-11975.

[6] S. Modi, M.B. Foston, P. Biswas, Controlled Synthesis of Smaller than 100 nm Lignin Nanoparticles in a Furnace Aerosol Reactor, ACS ES&T Engineering (2023).

[7] S. Modi, O. Okonkwo, S. Saha, M. Foston, P. Biswas, Reuse of Lignin to Synthesize High Surface Area Carbon Nanoparticles Using a Continuous and Single-step Aerosol Method, (under review).

[8] S. Modi, O. Okonkwo, H. Zhou, S. Kavadiya, M. Foston, P. Biswas, Geometric Model for Predicting the Size and Morphology Evolution of Multiparticle Aggregates during Simultaneous Reaction and Sintering, Chem. Eng. J. (2023) 141423.