In this study, dihydroxybenzene isomers such as catechol, resorcinol, and hydroquinone were used as models of volatiles from solid fuels to better elucidate the mechanism of the thermochemical conversion of solid fuels. These isomers were pyrolyzed in a two-stage tubular reactor with a residence time up to 3.6 s and temperature ranging from 650 to 950 °C. In total, 51 products were identified by online gas chromatography. The product distribution from the pyrolysis of the three isomers was quite different. p-Benzoquinone was the primary product obtained from the pyrolysis of hydroquinone, while o-benzoquinone and m-benzoquinone were not detected from the pyrolysis of catechol and resorcinol, respectively. CO was the major final product formed from catechol, resorcinol, and hydroquinone, with maximum yields of 27.3 wt%, 19.4 wt%, and 20.0 wt%, respectively, whereas CO 2 (15.5 wt%) generated from resorcinol was significantly higher than those generated from hydroquinone (1.0 wt%) and catechol (0.8 wt%) at 950 °C and 0.3 s. Considering the mass selectivity of C 1-C 5 light hydrocarbons, the possible reaction pathways leading to CO and CO 2 were analyzed to describe the pyrolysis of catechol, resorcinol, and hydroquinone. Catechol mainly afforded CO and C 4 hydrocarbons by H-migration and ring-opening reactions. Hydroquinone mainly generated CO, C 2 , and C 4 hydrocarbons, with the initial step being the formation of p-benzoquinone. During the pyrolysis of resorcinol, competition reactions occurred for the formation of CO and CO 2 with the production of C 1-C 5 hydrocarbons at a nearly equal mass selectivity. Compared with catechol and hydroquinone, resorcinol produced significantly more C 5 hydrocarbons, which suggested the intramolecular combination of aldehyde radical and ketone radical for explaining the large amount of CO 2 formed from the pyrolysis of resorcinol.
A detailed chemical kinetic model has been developed to theoretically predict the pyrolysis behavior of phenol‐type monolignol compounds (1‐(4‐hydroxyphenyl)prop‐2‐en‐1‐one, HPP; p‐coumaryl alcohol, 3‐hydroxy‐1‐(4‐hydroxyphenyl)propan‐1‐one, HHPP; 1‐(4‐hydroxyphenyl)propane‐1,3‐diol, HPPD) released from the primary heterogeneous pyrolysis of lignin. The possible thermal decomposition pathways involving unimolecular decomposition, H‐addition, and H‐abstraction by H and CH3 radicals were investigated by comparing the activation energies calculated at the M06–2X/6–311++G(d,p) level of theory. The results indicated that all phenol‐type monolignol compounds convert to phenol by side‐chain cleavage. p‐Coumaryl alcohol decomposes into phenol via the formation of 4‐vinylphenol, whereas HPP, HHPP, and HPPD decompose into phenol via the formation of 4‐hydroxybenzaldehyde. The pyrolytic pathways focusing on the reactivity of the hydroxyl group in HPP and producing cyclopentadiene (cyc‐C5H6) were also investigated. The transition state theory (TST) rate constants for all the proposed elementary reaction channels were calculated at the high‐pressure limit in the temperature range of 300–1500 K. The kinetic analysis predicted the two favorable unimolecular decomposition pathways in HPP: the one is the dominant channel below 1000 K to produce cyc‐C5H6, and the other is above 1000 K to yield phenol (C6H5OH).
The theoretical aspects of the development of a chemical kinetic model for guaiacol and catechol pyrolysis are presented to describe the pyrolysis behaviors of the individual lignin-derived components. The possible pyrolysis pathways involving both unimolecular and bimolecular decomposition were investigated by the potential energy surfaces (PES) calculated at CBS-QB3 level. The high-pressure limiting rate constants of each elementary reaction step were evaluated based on the transition state theory (TST) to determine the dominant pyrolysis pathways. The kinetic analysis results predicted the most favorable catechol unimolecular decomposition pathways, where catechol isomerization to 2-hydroxycyclohexa-2,4-dien-1-one occurred via migration of the hydroxyl H atom, followed by decomposition into 1,3-cyclobutadiene, acetylene, and CO. In the case of the bimolecular reaction of catechol, a hydrogen radical is coupled to the carbon atom in the benzene ring, leading to the formation of phenol and a hydroxyl radical through dehydroxylation. On the other hand, guaiacol is likely to form catechol and phenol via the O-CH homolysis and coupling of a hydrogen radical to the carbon atom with the methoxyl group, respectively.
Possible pathways for the pyrolysis of resorcinol with the formation of CO and CO as final products were proposed and evaluated using ab initio calculations. Our experimental study revealed that large quantities of CO are generated in the pyrolysis of 1,3-dihydroxybenzene (resorcinol), while the pyrolysis of the dihydroxybenzene isomers 1,2-dihydroxybenzene (catechol) and 1,4-dihydroxybenzene (hydroquinone) produces little CO. The fate of oxygen atoms in catechol and hydroquinone was essentially the formation of CO. In the proposed pathways, the triplet ground state m-benzoquinone was generated initially from simultaneous cleavage of the two O-H bonds in resorcinol. Subsequently, the direct cleavage of a C-C bond of the m-benzoquinone diradical yields 2-oxidanylcyclopenta-2,4-dien-1-yl-methanone, which can be converted via two channels: release of CO from the aldehyde radical group and combination of the ketone radical and carbon atom in the aldehyde radical group to form the 6-oxabicyclo[3.2.0]hepta-2,4-dien-7-one, resulting in the release of CO. Potential energy surfaces along the proposed reaction pathways were calculated employing the CBS-QB3 method, and the rate constants at the high-pressure limit were also evaluated based on transition-state theory to assess the feasibility of the proposed reaction pathways.
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