Elucidating biomass-derived pyrolytic lignin structures from demethylation reactions through density functional theory calculations

Authors: MANRIQUE, RAIZA - Washington State University; Terrell, Evan; KOSTETSKYY, PAVLO - Northwestern University; CHEJNE, FARID - National University Of Colombia; OLARTE, MARIEFEL - Pacific Northwest National Laboratory; BROADBELT, LINDA - Northwestern University; GARCIA-PEREZ, MANUEL - Washington State University

Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2023
Publication Date: 3/13/2023
Citation: Manrique, R., Terrell, E., Kostetskyy, P., Chejne, F., Olarte, M., Broadbelt, L., Garcia-Perez, M. 2023. Elucidating biomass-derived pyrolytic lignin structures from demethylation reactions through density functional theory calculations. Energy and Fuels. 37(7):5189-5205. https://doi.org/10.1021/acs.energyfuels.2c04292.
DOI: https://doi.org/10.1021/acs.energyfuels.2c04292

Interpretive Summary: One promising way to utilize biomass resources more effectively is through a thermochemical conversion process known as pyrolysis. In this process, biomass (e.g., wood, grass, agriculture residues) is heated in an inert environment and subsequently broken down into a bio-oil product. This bio-oil is comparable to petroleum and can be utilized for renewable energy and fuels; however, there is still significant research necessary to understand the key differences between biomass-derived and fossil-derived resources. In this work, the conversion of lignin (a fraction of biomass) during pyrolysis is modeled using advanced computational techniques (i.e., density functional theory). These computational methods provide a rigorous understanding of the underlying structures of molecules in bio-oil from pyrolysis. Once the structures are understood, their fuel properties and other chemical characteristics can be accurately estimated. This allows for more precise implementation of engineering processes to use biomass resources efficiently for energy and fuels applications.

Technical Abstract: Pyrolytic lignin is a fraction of the pyrolysis bio-oil that contains a wide variety of phenolic compounds that can be used as platform chemicals in the energy and fuels, pharmaceutical and cosmetic industries. However, the characteristics of the lignin polymer structure makes it difficult to establish a pyrolysis mechanism and to determine the characteristics of the pyrolytic lignin structure. This study proposes dimer, trimer, and tetramer structures based on their relative thermodynamic stability for a hardwood model lignin in the pyrolysis process. Different configurations of oligomers were evaluated by varying the positions of the guaiacyl (G) and syringyl (S) units in the hardwood model lignin through electronic structure calculations. The homolytic cleavage of 'O4 bonds is assumed to occur and generate two free radical fragments. These can stabilize taking hydrogen radicals that may be in solution during the formation of the intermediate liquid (pathway 1) before the thermal ejection. An alternative pathway (pathway 2) could happen when the radicals use intramolecular hydrogen, turning themselves into stable products. Subsequently, a demethylation reaction can take place through the homolysis of the O-CH3 bond by temperature increasing, thus generating a methane molecule and a range of structures of varying stability and characteristic spectral signatures. The most probable resulting structures were characterized through simulated FTIR and NMR spectra, and the thermophysical properties were calculated using group contribution methods. The results give insights about the lignin oligomers structures and the mechanism through which some of these molecules are formed. They also provide useful information for the design of pyrolysis bio-oil separation and upgrading equipment.