Computational Study on the Thermal Decomposition of Phenol‐Type Monolignols

Abstract

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).

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

“…3,4′-Dihydroxypropiophenone HOPh­(CO)­CH 2 CH 2 OH ( A15 ) is another species which chemical activation has predicted to be consistent across all temperatures and pressures in Tables and . A15 is also a key product in pyrolysis of lignin and other models., ,, known as 3-hydroxy-1-(4-hydroxyphenyl)­propan-1-one, HHPP . This product formation shows rate-constant trends similar to other H atom elimination products, A5 , A16 , and A17 , where increasing temperature correlates to increasing rate constants and importance for RA1* chemical activation.…”

“…Monolignols are the most important primary products of lignin pyrolysis. ,,,− Their primary formation and decomposition are currently the dominating view on the decomposition mechanisms and depolymerization of lignin during fast pyrolysis. ,,− This makes the pyrolysis of monolignols a very challenging process to be studied kinetically. A fundamentally based pyrolysis mechanism of these key intermediates is necessary to develop detailed kinetic models of lignin pyrolysis so as to accomprehend the kinetic models of biomass pyrolysis because the pyrolysis kinetics of other two biomass components, viz., cellulose and hemicellulose is much more developed (see, e.g., refs − ).…”

“…Note that primarily considered sets of products formed at temperatures near 600 °C did not include more intricate low temperature intermediatesoxygenates. On the other hand, the addition of monolignols, containing a large reactive side chain attached to the phenol moiety, to this list, advocated by Norinaga and co-workers, , is an important step toward the development of a fundamentally based kinetic model of biomass. These partial improvements already included some of the recently emerged fundamental mechanistic results on pyrolysis of monolignols and low-energy pathways.…”

“…The pyrolysis of monolignols as aromatic nuclei, mimicking the primary products of the lignin pyrolysis as well as the structural motifs of the lignin side groups, is actively explored only in recent years. ,,− ,− , In our previous studies, we have shown that the pyrolysis of p -CMA and its bare model CnA generates a large variety of phenolic compounds and benzene derivatives such as phenol, p -cresol, 4-vinylphenol, indene, styrene, benzaldehyde, 1-propynyl benzene, and 2-propenyl benzene and corresponding para -hydroxy-derivatives depending on pyrolysis conditions by utilizing both conventional and fractional pyrolysis approaches. ,− Remarkably, the yields of a small set of phenolics in low-temperature fractional pyrolysis of p -CMA constitute as high as 70% of the total gas-phase products …”

OH-Initiated Reactions of p-Coumaryl Alcohol Relevant to the Lignin Pyrolysis. Part I. Potential Energy Surface Analysis

“…Three primary decomposition pathways of p-CMA have been studied by Furutani from DFT-analysis and rate constants evaluation. 61 They involved C(8)−C (9) and O(9)-H bond cleavages, and H-addition to C β -atom followed by C(8)−C (9) bond fission in the formed adduct. Two H-abstraction reactions from terminal OH-group by H and CH 3 -radicals have also been evaluated.…”