Computational Study on the Thermal Decomposition of Phenol-Type Monolignols

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). (C) 2018 Wiley Periodicals, Inc.