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Title: Mechanistic investigation into the formation of humins in acid-catalyzed biomass reactions
Authors: Calderón, José Carlos Velasco
Arora, Jyotsna Sudhir
Mushrif, Samir H.
Keywords: Engineering::Chemical engineering
Issue Date: 2022
Source: Calderón, J. C. V., Arora, J. S. & Mushrif, S. H. (2022). Mechanistic investigation into the formation of humins in acid-catalyzed biomass reactions. ACS Omega, 7(49), 44786-44795.
Journal: ACS Omega 
Abstract: Humins are carbonaceous, polymeric byproducts formed during the acid-catalyzed condensed phase transformation of biomass-derived moieties and are responsible for significant carbon loss and catalyst deactivation. There exists very limited knowledge about their formation chemistry and composition. Infrared spectra of humins formed during the dehydration of glucose/fructose to 5-HMF show that the furan ring and the hydroxy methyl group of 5-HMF are present in humins, but the carbonyl group is not. Based on this, aldol addition and condensation between 5-HMF and other derived species are proposed as the main reactions that initiate humin formation. Hence, in this work, density functional theory (DFT)-based calculations are performed to compute the reaction pathways, activation barriers, and reaction free energies associated with all elementary reaction steps in the 5HMF-initiated, acid-catalyzed reactions leading to humin formation. The humin formation is initiated with the rehydration of HMF to form 2,5-dioxo-6-hydroxy-hexanal or DHH (key promoter of humin formation), followed by its keto-enol tautomerization and aldol addition and condensation with HMF. The rate-determining step in this pathway is the aldol-addition reaction between the DHH-derived enols with 5-HMF. Within the implicit solvation approximation, the formation of the 5-HMF-DHH dimer is slightly endergonic, whereas the 5-HMF rehydration leading to DHH is thermodynamically downhill. This mechanistic understanding of initiation reactions for humins could pave the way to screen and design solvent and catalyst systems to deter their formation.
ISSN: 2470-1343
DOI: 10.1021/acsomega.2c04783
Schools: School of Chemical and Biomedical Engineering 
Rights: © 2022 The Authors. Published by American Chemical Society. This is an open-access article distributed under the terms of the Creative Commons Attribution License.
Fulltext Permission: open
Fulltext Availability: With Fulltext
Appears in Collections:SCBE Journal Articles

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