Atomistic modeling and simulation derived novel insights for biomass valorization

Abstract

Due to the depletion of finite fossil resources and increasing concerns for mitigating the environmental damages causing global warming, the global scientific community is emphasizing the valorization of alternative resources. Among the alternative resources, lignocellulosic biomass is a renewable source with the valorization potential to produce fuel, materials, and a spectrum of chemicals from it. Lignocellulosic biomass has three structural fractions: cellulose, hemicellulose, and lignin. Lignin is the structural component of lignocellulosic biomass that contributes the most recalcitrance to its overall valorization because of its complex structure and physicochemical properties.

Unlike cellulose, a complete understanding of lignin’s structure at the molecular level is yet to be established. Moreover, lignin can be categorized into various forms, including native and technical forms. Significant structural differences exist among these different types of lignin, which makes improving the existing processes and developing novel processes of biomass and lignin valorization challenging. The atomistic modeling methods, such as the electronic structure method, can be particularly helpful for lignin research as they allow one to model lignin biopolymer and extract quantum-level insights about its structure, property, and reactivity during different valorization processes. Because of its cost-effectiveness, pyrolysis is one of the most promising and extensively investigated processes that thermochemically convert lignin into several types of value-added products. Despite an extensive research effort to improve and develop this process for lignin valorization, this is still far from being an established process for biorefineries, which warrants further research. In this dissertation, the lignin pyrolysis process has been studied by employing molecular modeling and simulations to provide novel insights in terms of kinetics and thermodynamics.

Lignin pyrolysis is a complex and multistep process in which the initial step involves the homolytic cleavage of the β-O-4 present in lignin. This dissertation presents reaction energetics studies for this initial step in lignin pyrolysis using model lignin oligomers. To my best knowledge, the largest molecular models for lignin were used in these studies to investigate reaction energetics for pyrolysis using the electronic structure method. A molecular mechanics-based conformational sampling method was developed to identify the lowest energy conformers, later used in Density Functional Theory (DFT) and statistical mechanics calculations. In addition to checking a hypothesis regarding the bond dissociation enthalpy calculation using these large molecular models for lignin, the computed range of reaction enthalpies and standard thermodynamic properties were reported in these studies.

To shed light on the reaction mechanism for lignin pyrolysis, automated reaction network generation and microkinetic modeling approaches were employed to construct comprehensive reaction networks using model lignin compounds and dimers for this process. Detailed reaction mechanisms were proposed under this research domain for fast pyrolysis of anisole, guaiacol, and phenethyl phenyl ether as the model compounds and dimer representing lignin. Extensive reaction pathway analysis was performed to identify the precursor for major product formation involved in these processes. Simulation results from the computational works presented in this dissertation were compared with the available experimental studies. This allowed us to identify the areas of further improvement for applying automated microkinetic modeling for reaction mechanisms in lignin pyrolysis. Overall, this dissertation attempts to use computational chemistry, lignin chemistry, and reaction engineering to provide novel insights into reaction energetics and mechanism for lignin pyrolysis.