A theoretical study on bond dissociation enthalpies of coal based model compounds

To help understand the pyrolysis mechanism of lignin, a non-phenolic lignin dimer model compound with b-O-4 linkage, namely 1-methoxy-2-(4-methoxyphenethoxy) benzene, was prepared. Its pyrolysis mechanism was investigated by density functional theory calculations and confirmed by the analytical pyrolysis-gas chromatography/mass spectrometry experiments. Possible pyrolytic pathways were proposed and analyzed based on three initial pyrolysis mechanisms of the model compound, including the C b -O homolytic mechanism, the C a -C b homolytic mechanism and the C b -O concerted decomposition mechanism. The results indicate that the lignin dimer model compound is thermally stable at low pyrolysis temperature (300°C), whereas at moderate pyrolysis temperature (500°C), four major pyrolytic products will be generated via the initial C b -O concerted decomposition and C b -O homolysis, including the 1-methoxy-4-vinylbenzene, 2-hydroxybenzaldehyde, 1-ethyl-4-methoxybenzene and 2-methoxyphenol. Products from the C a -C b homolysis can hardly be formed, due to the high-energy barriers and limitation of the available free hydrogen atoms. Secondary cracking of the primary pyrolytic products will take place to form some light compounds, which will be enhanced with the increase in pyrolysis temperature.

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“…The BDEs are the approximations of the activation energies for homolytic reactions [27]. The BDE is calculated as [38]:…”

Section: Experimental Computational Dft Calculationsmentioning

confidence: 99%

Pyrolysis mechanism of a β-O-4 type lignin dimer model compound

Zhang

Jiang

et al. 2015

J Therm Anal Calorim

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“…For example, phenyl (C 6 H 5 •) and benzyl (C 6 H 5 CH 2 •) radicals are important intermediates in molecular growth chemistry forming polycyclic aromatic hydrocarbons (PAHs), 12,13 coke, 14 and soot 15 . Phenoxy (C 6 H 5 O•) radicals are crucial for the coal processing, 16 antioxidant, 17,18 and lignin chemistry. 19,20 One area of great interest in chemical engineering is the conversion of lignocellulosic biomass to liquid fuels, energy, and chemicals.…”

Section: Introductionmentioning

confidence: 99%

“…For example, phenyl (C 6 H 5 •) and benzyl (C 6 H 5 CH 2 •) radicals are important intermediates in molecular growth chemistry forming polycyclic aromatic hydrocarbons (PAHs), coke, and soot. Phenoxy (C 6 H 5 O•) radicals are crucial for the coal processing, antioxidant, and lignin chemistry …”

Section: Introductionmentioning

confidence: 99%

Group additive modeling of substituent effects in monocyclic aromatic hydrocarbon radicals

et al. 2016

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The thermodynamic properties of the unsubstituted and substituted phenyl, phenoxy, anisyl, benzoyl, styryl and benzyl radicals with six substituents (hydroxy, methoxy, formyl, vinyl, methyl, and ethyl) are calculated with the bond additivity corrected (BAC) post‐Hartree‐Fock G4 method. Bond dissociation energies of monocyclic aromatic hydrocarbons are calculated and used to identify substituent interactions in these radicals. Benson's Group Additivity (GA) scheme is extended to aromatic radicals by defining 6 GAV and 29 NNI parameters through least squares regression to a database of thermodynamic properties of 369 radicals. Comparison between G4/BAC and GA calculated thermodynamic values shows that the standard enthalpies of formation generally agree within 4 kJ mol−1, whereas the entropies and the heat capacities deviate less than 4 J mol−1 K−1. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2089–2106, 2017

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“…These physical reasons contribute to the higher relative ignition tendency of toluene. Moreover, the difference in the C–H bonding energies in the side alkyl chains of toluene (88.5 ± 1.5 kcal/mol), 54 ethylbenzene (85.4 kcal/mol for the CH 2 group 54 and 98.6 kcal/mol for the CH 3 group) 54 and n -propylbenzene (86.1 kcal/mol for the CH 2 group attached to benzene-ring, 54 95.1 kcal/mol for the second CH 2 group 55 and 98.4 kcal/mol for the CH 3 group), 55 may be also a reason for that D70-T30 shows comparative reactivity as or higher reactivity than D70-EB30 and D70-NPB30. The higher C–H bond energy in the CH 3 group in toluene molecule limits the H-abstraction rate, 56 thus reducing the scavenging effects of the H-abstraction process from toluene on the active radicals produced by n -decane low-temperature reactions.…”

Section: Resultsmentioning

confidence: 99%

Effects of branch structure of alkylbenzenes on spray auto-ignition of n-decane and alkylbenzenes blends

Duan

Liu

Liang

et al. 2020

International Journal of Engine Research

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Spray auto-ignition characteristics of the blends of n-decane and several alkylbenzenes were carried out on a heated constant-volume spray combustion chamber. The derived cetane numbers of the fuel blends were determined, and the temperature-dependent ignition delay times and combustion durations were measured across a range of temperatures from 808 to 911 K. The results reveal that blending alkylbenzene to n-decane inhibits fuel spray auto-ignition propensity. For mono-alkylbenzenes, the fuel blend containing toluene has a higher derived cetane number than those with ethylbenzene and n-propylbenzene, but has a lower derived cetane number than the fuel blend containing n-butylbenzene. For those binary fuels containing ethylbenzene, n-propylbenzene and n-butylbenzene, their derived cetane numbers increase with the side alkyl chain length. The derived cetane numbers of the fuel blends with C8H10 isomers follow the trend of n-decane/ o-xylene > n-decane/ethylbenzene > n-decane/ m-xylene ∼ n-decane/ p-xylene, given the alkylbenzene blending fraction. For the blends with C9H12 isomers, those containing 1,2,3-trimethylbenzene and 1,3,5-trimethylbenzene have the highest and lowest derived cetane numbers, respectively, while the fuel blends containing 1,2,4-trimethylbenzene, n-propylbenzene and i-propylbenzene have comparatively intermediate derived cetane numbers. The blending effects of alkylbenzenes on ignition delay time are consistent with the observation on fuel derived cetane numbers. Both the number and proximity of substituted methyl groups significantly affect fuel auto-ignition propensity, and the adjacent methyl groups could increase the auto-ignition propensity. The combustion duration for the test fuels, except for n-decane and the n-decane/ n-butylbenzene blend, monotonically decreases with increased temperature. The non-monotonic dependence of combustion duration on temperature, for neat n-decane and the n-decane/ n-butylbenzene blend, may result from the increased diffusive burnt fraction. Finally, the comparison between gas-phase and spray auto-ignition reactivity of the test fuels highlights the contribution of both fuel physics and chemistry in spray auto-ignition.

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A theoretical study on bond dissociation enthalpies of coal based model compounds

Cited by 57 publications

References 50 publications

Pyrolysis mechanism of a β-O-4 type lignin dimer model compound

Pyrolysis mechanism of a β-O-4 type lignin dimer model compound

Group additive modeling of substituent effects in monocyclic aromatic hydrocarbon radicals

Effects of branch structure of alkylbenzenes on spray auto-ignition of n-decane and alkylbenzenes blends

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