Date of Award
9-5-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Growing consumption of fossil resources for energy and materials production has significantly increased greenhouse gas emissions, which will have an increasingly negative, and potentially catastrophic, environmental impact in the future. Lignocellulosic biomass is one of the few sustainable carbon resources that has the abundance and geographical distribution to displace fossil-based fuels and materials. However, the under-utilization of by-products such as lignin fraction remains a key challenge in expanding the utilization of lignocellulosic materials, necessitating the development of technologies for valorization of lignin into high-value products. Lignin is a promising precursor for the synthesis of a diverse range of high-value nanomaterials. However, the conventional solvothermal techniques to obtain lignin-based nanomaterials often involve multistep batch processes and large volumes of solvents/activating agents that are not cost-effective and environmentally benign, thereby limiting their scalability. Therefore, to advance scalable (i.e., environmentally benign and economically viable) valorization of lignin to lignin-based nanomaterials, it is essential to develop alternative techniques. This thesis demonstrates the development of gas-phase aerosol techniques for the valorization of lignin to lignin-based nanoparticles (NPs). Key features of the aerosol technique include ultrafast processing (seconds), continuous and single-step operation, minimal use of solvents, and precise control over material properties. This thesis is divided into three sections concerning the: (1) demonstration of a furnace aerosol reactor (FuAR) for the controlled synthesis of lignin-based NPs; (2) design and development of a novel aerosol reactor for the in-situ synthesis of lignin-based nanocomposites; and (3) scalability assessment of FuAR for the synthesis of carbon nanoparticles (CNPs) from lignin. The first section demonstrates the application of FuAR for the controlled synthesis of lignin-based NPs, specifically lignin nanoparticles (LNPs) and CNPs, from bulk lignin. The key operating parameters of FuAR, lignin concentration, temperature, and residence time, are systematically investigated for their roles in determining the nanoparticle size, morphology, surface area, and chemical structure. Furthermore, the impacts of various inter/intra particle processes (such as nucleation, reactions, collision, and sintering) on nanoparticle properties are illustrated to enable controlled synthesis of NPs. To determine the kinetics of simultaneous reaction and sintering, a generalized geometric model (GM) predicting the size and shape evolution of multiparticle aggregates is presented. Using the experimental results on nanoparticle properties and knowledge of reaction and sintering kinetics of lignin obtained from the GM, a detailed mechanism for the evolution of bulk lignin to LNPs and CNPs in FuAR is proposed. The as-synthesized LNPs and CNPs are evaluated for their performance in UV protection and supercapacitor applications, respectively. CNPs exhibited a specific capacitance of 247 F/g at 0.5 A/g with excellent capacity retainment of over 98% after 10,000 cycles, which is a clear demonstration of their superior performance compared with supercapacitors synthesized earlier from lignin. Moreover, the FuAR approach requires significantly less time for CNPs synthesis: on the order of seconds in comparison to hours for conventional solvothermal methods, highlighting significant promise for rapid valorization of lignin. The second section focuses on the design and development of a novel aerosol-based reactor to advance the application of the FuAR system for in-situ synthesis of lignin-based carbon nanocomposites, specifically Carbon-Silicon (C-Si) and Carbon-Sulfur (C-S). The fundamentals involved in the aerosol reactor design and the impact of processing parameters on composite properties are systematically investigated. Based on the analysis results, detailed mechanisms for the formation of C-S and C-Si composites in the respective FuAR-based systems are proposed. The impact of the distribution and loading of Si in C-Si and S in C-S on their performance as electrodes in Li-ion batteries is investigated to establish property function relationships. The third section assesses the scalability of the FuAR system for the synthesis of CNPs using a comprehensive life cycle assessment (LCA). Initially, the assessment is focused on lab-scale FuAR to investigate the significance of various processing parameters on the environmental impact of CNPs synthesis. Based on the LCA of lab-scale FuAR, an optimized scaled-up FuAR design, that incorporates recirculation of exhaust gas, heat recovery, and H2O recovery, is proposed. The scaled-up FuAR exhibited a global warming potential of 28.31 kg CO2 eq./kg of carbon material and 0.117 kg CO2 eq./F of capacitance based on mass and energy storage performance in supercapacitors, respectively. The superior sustainability along with the simple (single-step, continuous, and rapid) operation indicates the significant promise of using FuAR for the synthesis of CNPs from lignin. Finally, this dissertation also includes the demonstration of electrospray for lignin valorization. In summary, this dissertation comprehensively covers all aspects, from fundamentals to scale-up, for advancing scalable valorization of lignin to high-value nanomaterials using single-step aerosol techniques.
Language
English (en)
Chair
Pratim Biswas
Committee Members
Amit Naskar; Marcus Foston; Peng Bai; Ramesh Raliya; Vijay Ramani