Challenges in the Computation of Rate Constants for Lignin Model Compounds

Beste, Ariana; Buchanan, A. C.

doi:10.1002/9781118166123.ch7

Cited by 8 publications

(22 citation statements)

References 89 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The study of elementary step models is essential to obtain a better understanding of kinetics and conversion pathways and to determine the stability of intermediate compounds formed during the lignin pyrolysis. The most extensive and impressive results have been obtained for reactivity of model dimers containing β-O-4 and α-O-4 aryl ether linkages typical for lignin macromolecules. − The pyrolysis of substituted dimers and monolignols bearing a typical C3-side chain terminated with an aliphatic OH group (Figure ) has been studied to a lesser degree, with most of the product-formation mechanisms being tentative. − A more detailed analysis of the pyrolysis products has been performed for a series of phenylpropanoid and phenylpropanol models. , In our previous studies, we also reported some mechanistic analysis of the CnA and p- CMA lignols’ pyrolyses. ,,, …”

Section: Introductionmentioning

confidence: 99%

Molecular Products and Fundamentally Based Reaction Pathways in the Gas-Phase Pyrolysis of the Lignin Model Compound p-Coumaryl Alcohol

et al. 2017

View full text Add to dashboard Cite

The fractional pyrolysis of lignin model compound para-coumaryl alcohol (p-CMA) containing a propanoid side chain and a phenolic OH group was studied using the System for Thermal Diagnostic Studies at temperatures from 200 to 900 °C, in order to gain mechanistic insight into the role of large substituents in high-lignin feedstocks pyrolysis. Phenol and its simple derivatives p-cresol, ethyl-, propenyl-, and propyl-phenols were found to be the major products predominantly formed at low pyrolysis temperatures (<500 °C). A cryogenic trapping technique was employed combined with EPR spectroscopy to identify the open-shell intermediates registered at pyrolysis temperatures above 500 °C. These were characterized as radical mixtures primarily consisting of oxygen-linked conjugated radicals. A comprehensive potential energy surface analysis of p-CMA and p-CMA + H atom systems was performed using various DFT protocols to examine the possible role of concerted molecular eliminations and free-radical mechanisms in the formation of major products. Other significant unimolecular concerted reactions along with formation and decomposition of primary radicals are also described and evaluated. The calculations suggest that a set of the chemically activated secondary radical channels is relevant to the low temperature product formation under fractional pyrolysis conditions.

show abstract

Section: Introductionmentioning

confidence: 99%

Molecular Products and Fundamentally Based Reaction Pathways in the Gas-Phase Pyrolysis of the Lignin Model Compound p-Coumaryl Alcohol

et al. 2017

View full text Add to dashboard Cite

show abstract

“…Primary pyrolysis of lignin (typically from 200 to 400 °C) produces characteristic low-molecular mass compounds, mostly phenolics. ,,− ,,− Aromatic compounds produced during the primary pyrolysis of G-lignins, for instance, are predominantly 4-substituted guaiacols (2-methoxy phenols), whereas S-lignins are represented by syringol (2,6-dimethoxiphenol) derivatives. ,,− ,,− The primary pyrolysis reactions, mainly studied on dimer models, are believed to occur via homolytic bond cleavage processes followed by secondary radical reactions, as well as concerted decomposition pathways such as Maccoll and retroene pericyclic intramolecular-type rearrangements. ,,− ,− Other alternative routes have not been ruled out, such as the one noted by Elder and Beste for dimer models containing aliphatic γ-OH groups (see also a series of works by Kawamoto reviewed in ref ). Further complications of decomposition patterns and product distributions arise from the highly reactive side groups. ,,,− …”

Section: Introductionmentioning

confidence: 99%

“…The readers are referred to Part I of our study (and preceding papers − ) for a detailed overview and discussion of p -CMA reactivity and pyrolysis reactions, as well as the experimental and theoretical analysis of the pyrolysis of other relevant models, including widely studied dimers containing primarily α-O-4 and β-O-4 linkages (see also refs ,,,,− ,,− ,,− ). A few important points are highlighted below.…”

Section: Introductionmentioning

confidence: 99%

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part II. Kinetic Analysis

et al. 2020

View full text Add to dashboard Cite

Monolignols are precursor units and primary products of lignin pyrolysis. The currently available global (lumped) and semidetailed kinetic models, however, are lacking the comprehensive decomposition kinetics of these key intermediates in order to advance toward the fundamentally based detailed chemical-kinetic models of biomass pyrolysis. para-Coumaryl alcohol (HOPh−CHCH−CH 2 OH, p-CMA) is the simplest of the three basic monolignols containing a typical side-chain double bond and both alkyl and phenolic type OH groups. The two other monomers additionally contain one and two methoxy groups, respectively, attached to the benzene ring. Previously, we developed a detailed fundamentally based mechanism for unimolecular decomposition of p-CMA (as well as its truncated allyl and cinnamyl alcohol models) and explored its reactivity toward H radicals generated during pyrolysis. The reactions of p-CMA with pyrolytic OH radicals is another set of key reactions particularly important for understanding the formation mechanisms of a wide variety of oxygenates in oxygen-deficit (anaerobic) conditions and the role of the lignin side groups in pyrolysis pathways. In Part I of the current study (J. Phys. Chem. A, 2019, 123, 2570−2585), we reported a detailed potential energy (enthalpy) surface analysis of the reaction OH + p-CMA with suggestions for a variety of chemically activated, unimolecular, and bimolecular reaction pathways. In Part II of our work, we provide a detailed kinetic analysis of the major reaction channels to evaluate their significance and possible impacts on product distributions. Temperature-and pressure-dependent rate constants are calculated using the quantum Rice−Ramsperger−Kassel method and the master equation analysis for falloff and stabilization. Enthalpies of formation, entropies, and heat capacities are calculated using density functional theory and higher-level composite methods for stable molecules, radicals, and transition-state species. A significant difference between well depths for the chemically activated adduct radicals, [p-CMA-OH]*, is found for the αand β-carbon addition reactions to generate the 1,3-and 1,2-diol radicals, respectively. This is due to the synergistic effect from conjugation of the proximal radical center with the aromatic ring and the strong H-bonding interaction between vicinal OH groups in the β-adduct (1,2-diol radical). Both adducts undergo isomerization and low-energy transformations, however, with different kinetic efficiencies because of the difference in stabilization energies. Reaction pathways include dissociation, intramolecular abstraction, atom and group transfers, and elimination. Of particular interest is a roaming-like low-energy dehydration reaction to form O-centered intermediate radicals. The kinetic analysis demonstrated the feasible formation of various products detected in pyrolysis experiments, suggesting that the gas-phase reactions of OH radicals can be a key process to form major products and complex oxygenates during lignin pyrolysis. Our prelim...

show abstract

“…Pyrolysis of lignin and its model compounds produces characteristic low-molecular-mass, primarily phenolic compounds, ,,− which are believed to form via unimolecular concerted reactions involving various intramolecular rearrangements and free-radical reactions initiated by homolytic (mostly etheric) bond cleavages followed by secondary bimolecular and isomerization reactions. − ,,,− …”

Section: Introductionmentioning

confidence: 99%

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part III. Kinetics of H-Abstraction by H, OH, and CH₃ Radicals

et al. 2020

View full text Add to dashboard Cite

Lignin is the most complex component of biomass, and development of a detailed chemical kinetic model for biomass pyrolysis mainly relies on the understanding of the lignin decomposition kinetics. para-Coumaryl alcohol (p-CMA, HOPh−CHCH−CH 2 OH), the focus of our analysis, is the simplest of the lignin monomers (monolignols) containing a typical side-chain double bond and both alkyl-and phenolic-type OH-groups. In parts I and II of our work (

show abstract

Challenges in the Computation of Rate Constants for Lignin Model Compounds

Cited by 8 publications

References 89 publications

Molecular Products and Fundamentally Based Reaction Pathways in the Gas-Phase Pyrolysis of the Lignin Model Compound p-Coumaryl Alcohol

Molecular Products and Fundamentally Based Reaction Pathways in the Gas-Phase Pyrolysis of the Lignin Model Compound p-Coumaryl Alcohol

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part II. Kinetic Analysis

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part III. Kinetics of H-Abstraction by H, OH, and CH₃ Radicals

Contact Info

Product

Resources

About

Challenges in the Computation of Rate Constants for Lignin Model Compounds

Cited by 8 publications

References 89 publications

Molecular Products and Fundamentally Based Reaction Pathways in the Gas-Phase Pyrolysis of the Lignin Model Compound p-Coumaryl Alcohol

Molecular Products and Fundamentally Based Reaction Pathways in the Gas-Phase Pyrolysis of the Lignin Model Compound p-Coumaryl Alcohol

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part II. Kinetic Analysis

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part III. Kinetics of H-Abstraction by H, OH, and CH3 Radicals

Contact Info

Product

Resources

About

OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part III. Kinetics of H-Abstraction by H, OH, and CH₃ Radicals