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Title: Radical Nature of C-Lignin

Journal Article · · ACS Sustainable Chemistry & Engineering
 [1];  [2];  [1];  [1]
  1. National Renewable Energy Lab. (NREL), Golden, CO (United States)
  2. USDA-Forest Service, Auburn, AL (United States). Southern Research Station

The recently discovered lignin composed of caffeoyl alcohol monolignols or C-lignin is particularly intriguing given its homogeneous, linear polymeric structure and exclusive benzodioxane linkage between monomers. By virtue of this simplified chemistry, the potential emerges for improved valorization strategies with C-lignin relative to other natural heterogeneous lignins. To better understand caffeoyl alcohol polymers, we characterize the thermodynamics of the radical recombination dimerization reactions forming the benzodioxane linkage and the bond dissociation into radical monolignol products. These properties are also predicted for the cross-coupling of caffeoyl alcohol with the natural monolignols, coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol, in anticipation of polymers potentially enabled by genetic modification. The average BDEs for the C-lignin benzodioxane $$\alpha$$- and β-bonds are 56.5 and 63.4 kcal/mol, respectively, with similar enthalpies for heterodimers. The BDE of the $$\alpha$$-bond within the benzodioxane linkage is consistently greater than that of the β-bond in all dimers of each stereochemical arrangement, explained by the ability the $$\alpha$$-carbon radical generated to delocalize onto the adjacent phenyl ring. Relative thermodynamics of the heterodimers demonstrates that the substituents on the phenyl ring directly neighboring the bond coupling the monolignols more strongly impact the dimer bond strengths and product stability, compared to the substituents present on the terminal phenyl ring. Enthalpy comparisons furthermore demonstrate that the erythro stereochemical configurations of the benzodioxane bond are slightly less thermodynamically stable than the threo configurations. The overall differences in strength of bonds and reaction enthalpies between stereoisomers are generally found to be insignificant, supporting that postcoupling rearomatization is under kinetic control. Projecting the lowest-energy stereoisomer internal coordinates to longer polymer C-lignin strands highlights how significantly the stereochemical outcomes in polymerization may impact the macromolecular structure and in turn material and chemical properties. Lastly, through these comparisons of geometry, bond strengths, and reaction enthalpies, we shed light on the distinctive properties of C-lignin's radical recombination and decomposition chemistry, and its potential as a natural lignin solution for biorefinery feedstocks and unique materials science applications.

Research Organization:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Organization:
USDOE Office of Energy Efficiency and Renewable Energy (EERE), Bioenergy Technologies Office (EE-3B)
Grant/Contract Number:
AC36-08GO28308
OSTI ID:
1328643
Report Number(s):
NREL/JA--5100-66620
Journal Information:
ACS Sustainable Chemistry & Engineering, Journal Name: ACS Sustainable Chemistry & Engineering Journal Issue: 10 Vol. 4; ISSN 2168-0485
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English

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Cited By (4)

Clicking Biobased Polyphenols: A Sustainable Platform for Aromatic Polymeric Materials journal July 2018
Systematic parameterization of lignin for the CHARMM force field journal January 2019
Enzymatic basis for C‐lignin monomer biosynthesis in the seed coat of Cleome hassleriana journal May 2019
An “ideal lignin” facilitates full biomass utilization journal September 2018

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