Effect of the structure of coal-related model ethers on the rate and mechanism of their thermolysis

Korobkov, V. Yu.; Grigorieva, E.N.; Bykov, V. I.; Kalechitz, Igor V.

doi:10.1016/0016-2361(88)90295-5

Cited by 14 publications

(10 citation statements)

References 10 publications

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“…It is also interesting to compare the present resulta with those of Korobkov et al (1988), who studied the thermolysis to phenols and toluene of a series of substituted benzyl phenyl ethers (X-C6H4-oCH2C6H5) in the presence of tetralin and hydrogen. They showed that the apparent first-order rate constants correlate with the Hammett a-constants and found the reaction constant to be pH = -2.75 at 325 "C. It seems reasonable, although yet speculative, to assume that thermolysis of substituted anisoles under similar conditions would also lead to a negative pH, i.e., one of opposite sign to the solvent-effect-free reaction constant for hydrolysis in supercritical water.…”

Section: Discussionsupporting

confidence: 52%

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Decoupling substituent and solvent effects during hydrolysis of substituted anisoles in supercritical water

Klein¹,

Mentha²,

Torry³

1992

Ind. Eng. Chem. Res.

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The kinetics of hydrolysis in supercritical water of a series of substituted anisoles to methanol and the corresponding substituted phenol probed the coupling of solvent and substituent effects. For supercritical water concentrations [H20] 5 3 mol/L, the substituent effects exhibited compelling correlation according to the classic Hammett formalism. For [H20] > 5 mol/L, however, the Hammett correlation had an internal minimum at cr = 0, indicative of rate acceleration by both electronwithdrawing and electron-donating substituents. The solvent and substituent effects were thus defined to be "decoupled" for [H20] 5 3 mol/L, where the dielectric constant t I 1.28. The hydrolysisHammett reaction constant increased with decreasing water density. The reaction constant was also of opposite sign to reported values for pyrolysis of substituted benzyl phenyl ethers, suggesting that the practical significance of the present work is the ability to design a substituent-specific solvent effect for selective hydrolysis or pyrolysis.

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Section: Discussionsupporting

confidence: 52%

“…The free-radical pyrolysis of anisole and substituted anisoles has been studied in detail (Schlosberg et al, 1983;Vuori, 1986;Vuori and Bredenberg, 1987;Korobkov et al, 1988). The pyrolysis reaction kinetics can be represented by a first-order rate law without mechanistic (e.g., unimolecular) inference (Korobkov et al, 1988;Vuori, 1986).…”

Section: Kineticsmentioning

confidence: 99%

Decoupling substituent and solvent effects during hydrolysis of substituted anisoles in supercritical water

Klein¹,

Mentha²,

Torry³

1992

Ind. Eng. Chem. Res.

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“…Much less is known about retrogressive reactions for ether-containing model compounds. The pyrolysis of benzyl phenyl ether (BPE) has been extensively investigated as a model for labile ether bridges. − BPE pyrolysis is initiated by homolytic scission of the weak (ca. 52 kcal mol -1 ) C−O bond, which occurs at T ≥ 275 °C to produce benzyl and phenoxy radicals that hydrogen abstract from BPE or from an added hydrogen donor to produce toluene and phenol as dominant products (eq 1).…”

Section: Introductionmentioning

confidence: 99%

Pyrolysis of Silica-Immobilized Benzyl Phenyl Ether: Competing Radical Rearrangement Pathways under Restricted Diffusion

et al. 1998

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Pyrolysis studies of silica-immobilized benzyl phenyl ether (≈PhOCH2Ph or ≈BPE), a model for related ether structures in fuel resources, have been conducted at 275−325 °C to examine the impact of restricted mass transport on the pyrolysis mechanism compared with previous studies in fluid phases. Significant rearrangement chemistry is observed for ≈BPE occurring through two competitive free-radical pathways that are both promoted by the diffusional constraints. One path involves recombination of incipient benzyl and surface-bound phenoxy radicals to form benzylphenol isomers, 10. The second, previously unreported rearrangement path for ≈BPE involves a 1,2-phenyl shift in an intermediate radical, ≈PhOCH·Ph, leading to formation of benzhydrol (8) and benzophenone (9) as principal products. The rearrangement products 8−10 typically account for ca. 50% of the pyrolysis products. However, the path selectivity is a sensitive function of ≈BPE surface coverage and the presence of spacer molecules that either facilitate or hinder hydrogen atom transfer steps on the surface.

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“…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

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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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Effect of the structure of coal-related model ethers on the rate and mechanism of their thermolysis

Cited by 14 publications

References 10 publications

Decoupling substituent and solvent effects during hydrolysis of substituted anisoles in supercritical water

Decoupling substituent and solvent effects during hydrolysis of substituted anisoles in supercritical water

Pyrolysis of Silica-Immobilized Benzyl Phenyl Ether: Competing Radical Rearrangement Pathways under Restricted Diffusion

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

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