Elucidation of structure and physical properties of pyrolytic sugar oligomers derived from cellulose depolymerization/dehydration reactions: A density functional theory study

Authors: DENSON, MELBA - Washington State University; Terrell, Evan; KOSTETSKYY, PAVLO - Northwestern University; 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: 4/24/2023
Publication Date: 5/10/2023
Citation: Denson, M.D., Terrell, E., Kostetskyy, P., Olarte, M., Broadbelt, L., Garcia-Perez, M. 2023. Elucidation of structure and physical properties of pyrolytic sugar oligomers derived from cellulose depolymerization/dehydration reactions: A density functional theory study. Energy and Fuels. 37(11):7834-7847. https://doi.org/10.1021/acs.energyfuels.3c00641.
DOI: https://doi.org/10.1021/acs.energyfuels.3c00641

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 cellulose (a fraction of biomass) during pyrolysis is modeled using advanced computational techniques (i.e., density functional theory). Cellulose is the most abundant renewable compound available globally, and has great promise for further development toward green chemistry applications. The studied computational methods provide a rigorous understanding of the underlying structures of molecules in bio-oil from pyrolysis of cellulose and biomass in general. 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: Fast pyrolysis of lignocellulosic materials is a promising research area to produce renewable fuels and chemicals. Dehydration is known to be among the most important reaction families during cellulose pyrolysis; water is the most important product. Together with water, dehydration reactions also form a range of poorly known oligomer species of varying molecular sizes, often collected as part of bio-oil water-soluble (WS) fraction. In this work, we used electronic structure calculations to evaluate the relative thermodynamic stabilities of several oligomer species resulting from up to three consecutive dehydration events from cellulose depolymerization intermediates. A library of the thermodynamically favored candidate molecular structures was compiled. Results revealed that most of the water molecules are eliminated from the non-reducing end, forming thermodynamically more stable conjugated compounds. This is consistent with results reported by other researchers in literature where dehydration reactions occur preferably at the non-reducing ends of oligomers. The physical-chemical properties of the proposed structures were estimated using quantitative structure-property relationships (QSPRs) and quantitative property-property relationships (QPPRs). The anhydro-sugars derived from cellulose are often blamed for coke formation during bio-oil hydrotreatment. Understanding their chemical structure could help to develop rational strategies to mitigate coke formation. The thermo-physical properties reported (boiling point, melting point, Gibb’s free energy of formation, enthalpy of formation, and solubility parameters among others) are also fundamental to conducting first principle engineering calculations to design and analyze new pyrolysis reactors and bio-oil up-grading units.