OH-Initiated Reactions of <i>para</i>-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part III. Kinetics of H-Abstraction by H, OH, and CH<sub>3</sub> Radicals

Hudzik, Jason M.; Bozzelli, Joseph W.; Asatryan, Rubik; Ruckenstein, Eli

doi:10.1021/acs.jpca.9b11898

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“…A second interaction also exists between the proton of the OH radical and π-electrons of the double bond between C7 and C8. In Part III, our analysis will focus on chemical kinetics of H-abstractions from p -CMA where we will detail more H-abstraction prereaction complexes …”

Section: Resultsmentioning

confidence: 99%

“…In Part III, our analysis will focus on chemical kinetics of H-abstractions from p-CMA where we will detail more H-abstraction prereaction complexes. 160 Bond length relaxed scans are performed, Figure 5a,b, from the more stable and directional CR1 complex to both RA1 and RB1 adducts over distances of 1.52−3.32 and 1.58−3.38 Å, respectively. These scans located corresponding transition states, denoted as TSa0 for OH addition to C7(α) and TSb0 for OH addition to C8(β), to the adduct using ωB97XD with the moderate 6-31G(d,p) basis set.…”

Section: Resultsmentioning

confidence: 99%

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

et al. 2020

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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...

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confidence: 99%

Section: Resultsmentioning

confidence: 99%

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

et al. 2020

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Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds: Coniferyl alcohol, theory and experiment

Barekati-Goudarzi

Khachatryan

Xu³

et al. 2022

Journal of Analytical and Applied Pyrolysis

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

Cited by 2 publications

References 82 publications

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 II. Kinetic Analysis

Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds: Coniferyl alcohol, theory and experiment

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

Cited by 2 publications

References 82 publications

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 II. Kinetic Analysis

Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds: Coniferyl alcohol, theory and experiment

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