(359a) Scalable and Sustainable Valorization of Lignin Using Single-Step Aerosol Method

Growing demand for energy and materials has led to increased greenhouse gas emissions from the use of fossil resources. Lignocellulose biomass is one of the few renewable resources of carbon that has the abundance and geographical distribution to displace fossil resources. However, the under-utilization of by-products (i.e., lignin fraction) remains one of the challenges to the rapid growth of biorefinery. Thus, developing technologies that can valorize lignin to high-value products is crucial. Lignin could be used as a precursor for the synthesis of a diverse range of high-value nanomaterials, including lignin nanoparticles (NPs), carbon NPs, and functionalized carbons [1, 2]. However, conventional synthesis methods of these nanomaterials involve multistep and batch processes or large volumes of solvents/activating agents that are not cost-effective and environmentally benign, limiting the potential for their large-scale synthesis.

To overcome these challenges of conventional synthesis methods, the present work explores the synthesis of high-value nanomaterials from lignin using a novel and simple furnace aerosol reactor (FuAR) technique. The as-synthesized nanomaterials are tested for their performance in various modern applications (e.g., efficient energy storage and carbon capture). Key features of FuAR are (i) ultra-fast processing (order of seconds); (ii) continuous and single-step operation; (iii) minimal use of solvents; and (iv) simple and precise control over material properties; making it scalable and environmentally benign for lignin valorization [3, 4].

First, the FuAR is demonstrated for controlled synthesis of lignin NPs of mean sizes between 50 and 68 nm [5]. The as-synthesized lignin NPs are analyzed for size and functional groups. The mean size of lignin NPs showed an increasing trend with lignin solution concentration. Furthermore, the bulk and as-synthesized LNPs were tested for UV protection applications. The observed improvement in UV protection with a decrease in lignin particle size is systematically investigated using the optical absorption parameter [5].

Secondly, with the systematic understanding of the role of temperature and residence time in the FuAR on the pyrolysis product of lignin, high surface area carbon nanoparticles (up to 925 m²/g) are synthesized without the use of activating/templating chemicals [6]. Furthermore, the as-obtained carbon nanoparticles are tested for specific capacitance which showed a linear trend with surface area. The best-performing material with a surface area of 925 m²/g exhibited a specific capacitance of 247 F/g at 0.5 A/g with excellent capacity retainment of over 98 % after 10,000 cycles [6]. This is a clear demonstration of their superior performance compared to supercapacitors synthesized earlier from lignin. Moreover, the size and morphology of carbon particles, which are impacted by interparticle collision and sintering, are known to play a crucial role in their stability as well as energy storage performance. Consequently, the kinetics of reaction, sintering, and collisions in the FuAR is systematically studied using a novel and generalizable numerical model based on a geometric modeling approach [7].

Finally, the FuAR is used for in-situ nitrogen functionalization to synthesize nitrogen-functionalized porous carbons. Urea is added to the lignin solution as a precursor for nitrogen functional groups. The effects of temperature, residence time, and the urea-to-lignin ratio on carbon nanoparticle functional groups, pore size distribution, surface area, and morphology are systematically investigated. Our findings indicate that surface area increases with temperature and residence time in the FuAR. Furthermore, the as-obtained carbon nanoparticles are tested for CO₂ adsorption and the material with a maximum surface area of 1051 m²/g exhibited a CO₂ adsorption capacity of 62 mg/(g of carbon) and excellent cyclability.

Overall, these studies demonstrate the unexplored potential of FuAR for the scalable valorization of lignin as well as the excellent performance of obtained nanomaterials in respective applications, which will help advance displacing the fossils with renewable resources.

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] 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.

[3] 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.

[4] 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.

[5] 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).

[6] 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).

[7] 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.