Model Lignin Oligomer Pyrolysis: Coupled Conformational and Thermodynamic Analysis of β-O-4′ Bond Cleavage

Abstract

The abundance, carbon content, and functionalized nature of lignin make it a promising candidate for targeted valorization to fuels and polymer composites. While lignin modeling by the application of computational chemistry is an active area of research, electronic structure methods have been limited mainly to structures in the dimeric or trimeric range. In this study, we have modeled a lignin structure composed of 10 β-O-4′ linked guaiacyl (G) units, such that this work represents, to the best of our knowledge, the largest structure that has been examined to date using quantum mechanical calculations. As such, this work can provide information on a model, the size of which is more representative of the lignin polymer than has been previously reported. We have calculated bond dissociation enthalpy (BDE) for the homolytic cleavage reaction between each G unit in our model lignin oligomer, which occurs as one of the initial reactions during lignin pyrolysis. The objective of the current work was to determine how or if reactivity within the oligomer changes as a function of bond cleaving position within the chain. The methods used were classical molecular mechanics for conformational sampling and quantum mechanically based density functional theory (DFT) calculations. We have developed a novel and robust method for conformational sampling, which maps the conformational energy landscape efficiently and provides multiple low-energy structures that are then used to determine the BDE values by DFT. Our results for BDE calculations of lignin exhibit significant position dependence along the oligomer chain. To the best of our knowledge, we have reported for the first time the calculated standard thermodynamic properties including enthalpy of formation, heat capacity, entropy, and Gibbs free energy. Despite using a simplified model lignin oligomer structure, our calculated values for standard thermodynamic properties have a remarkable agreement with the experimental values.

Vacuum Pyrolysis of Hybrid Poplar Milled Wood Lignin with Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry Analysis of Feedstock and Products for the Elucidation of Reaction Mechanisms

“…The various interunit bonds connecting lignin monomers have a range of bond dissociation enthalpies. Ether linkages, such as β-O-4, tend to be ∼60–70 kcal/mol, while C–C bonds tend to be ∼100 kcal/mol. − The location of a given bond in a lignin oligomeric unit can also have an impact on its dissociation enthalpy for β-O-4 homolytic cleavage . Because of bond dissociation enthalpy disparities between C–O and C–C bonds, during the early stages of pyrolysis, there should be preferential cleavage of ether linkages. , During these primary reactions (e.g., ether cleavage, demethoxylation, decarboxylation, and deacylation) that result in deoxygenation and molecular weight decreases for initial MWL, oligomers with a sufficiently low vaporization temperature are formed and subsequently devolatilized directly from the reaction front.…”

Synthesis and Characterization of Lignin-graft-poly(ethylene brassylate): a Biomass-Based Polyester with High Mechanical Properties

“…The main repeating units, which are coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, form the three types of segments within lignin: p -hydroxyphenyl, guaiacyl, and syringyl. , These materials contain numerous functional groups: aliphatic and phenolic hydroxyl, carboxylic, carbonyl, methoxy groups, and covalent linkages. The important carbon–oxygen links in lignin are β-O-4, α-O-4, and 4-O-5, and carbon–carbon linkages are β-5, 5–5, β-1, and β–β linkages. , …”

“…The important carbon−oxygen links in lignin are β-O-4, α-O-4, and 4-O-5, and carbon−carbon linkages are β-5, 5−5, β-1, and β−β linkages. 6,8 Although lignin possesses important properties as a raw material of polymers, 2 it has poor solubility in organic solvents when lignin is produced on a large scale due to its irregularly networked chemical structure and high concentration of aromaticity. The poor solubility and not well-defined chemical structure of lignin are the most significant challenges for ligninbased polymer synthesis and characterization.…”

On the Use of Molecular Dynamics Simulations for Elucidating Fine Structural, Physico-Chemical and Thermomechanical Properties of Lignocellulosic Systems: Historical and Future Perspectives

“…The interactions between bonded atoms were subsequently optimized with respect to quantum chemical vibrational data using the Automated Frequency Matching Method developed by Vaiana et al [24]. It was found that dihedral rotations around the β-O-4 linkage within the lignin structure played a significant role in determining the configuration of the lignin macromolecule [25,26]. Petridis et al studied two equivalent dihedrals in lignin model system for showing the dihedral rotations [23].…”