OH-Initiated Reactions of para-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part II. Kinetic Analysis
- 20 May 2020
- journal article
- research article
- Published by American Chemical Society (ACS) in The Journal of Physical Chemistry A
- Vol. 124 (24), 4875-4904
- https://doi.org/10.1021/acs.jpca.9b11894
Abstract
Monolignols are precursor units and primary products of lignin pyrolysis. The currently available global (lumped) and semi-detailed 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-CH2OH, 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 (Asatryan et al., 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 this Part II of our study, 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 QRRK method and the master equation analysis for falloff and stabilization. Enthalpies of formation, entropies, and heat capacities are calculated using DFT 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 α- and 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 efficiency due to 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 preliminary experiments involving pyrolysis of the vaporized monomers support this basic statement. A novel mechanism for formation of benzofuran, identified in experimentation, is also provided based on the potential conversions of hydroxyphenylactaldehyde and corresponding isomers, which are kinetically favored products.Keywords
Funding Information
- University at Buffalo
- National Science Foundation (1805677, CBET 1330311)
This publication has 135 references indexed in Scilit:
- Selective Production of 4-Vinylphenol by Fast Pyrolysis of Herbaceous BiomassIndustrial & Engineering Chemistry Research, 2013
- Dihydrogen Catalysis: A Degradation Mechanism for N2-Fixation IntermediatesThe Journal of Physical Chemistry A, 2012
- Radical Coupling Reactions in Lignin Synthesis: A Density Functional Theory StudyThe Journal of Physical Chemistry B, 2012
- Understanding the Fast Pyrolysis of LigninChemSusChem, 2011
- Compositional Analysis of Lignocellulosic Feedstocks. 1. Review and Description of MethodsJournal of Agricultural and Food Chemistry, 2010
- Modeling Lignin Polymerization. I. Simulation Model of Dehydrogenation PolymersPlant Physiology, 2010
- Thermochemistry of ligninRussian Journal of Applied Chemistry, 2010
- Solvent effects on the electronic state of monolignol radicals as predicted by molecular orbital calculationsJournal of Wood Science, 2008
- Pyrolysis reactions of various lignin model dimersJournal of Wood Science, 2007
- Temperature dependence of the reaction of oxygen atoms with olefinsJournal of the American Chemical Society, 1976