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, one of the few renewable resources of carbon, has an 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. However, conventional synthesis methods of these nanomaterials are limited by multistep and batch processes or large volumes of solvents/activating agents. 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.<br/><br/>First, the FuAR is demonstrated for controlled synthesis of lignin NPs of mean sizes between 50 and 68 nm. The as-synthesized lignin NPs are analyzed for their size and functional groups. 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). 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 [1].<br/><br/>Next, temperature and residence time are expected to have a key impact on products obtained from the pyrolysis of lignin. With the systematic understanding of the role of these parameters, carbon nanoparticles with high surface area (up to 925 m2/g) are synthesized in FuAR without the use of activating/templating chemicals. This one-step approach requires significantly less time for synthesis: an order of seconds in comparison to hours for conventional methods. Furthermore, 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 [2,3].<br/><br/>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. Furthermore, the as-obtained carbon nanoparticles are tested for CO₂adsorption 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.<br/>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.<br/><br/><b>References:</b><br/><br/>[1] 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, 3(5), 671–681<br/>[2] 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).<br/>[3] 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.