Molecular Products and Fundamentally Based Reaction Pathways in the Gas-Phase Pyrolysis of the Lignin Model Compound p-Coumaryl Alcohol

Abstract

The fractional pyrolysis of lignin model compound para-coumaryl alcohol (p-CMA) containing a propanoid side chain and a phenolic OH group was studied using the System for Thermal Diagnostic Studies at temperatures from 200 to 900 °C, in order to gain mechanistic insight into the role of large substituents in high-lignin feedstocks pyrolysis. Phenol and its simple derivatives p-cresol, ethyl-, propenyl-, and propyl-phenols were found to be the major products predominantly formed at low pyrolysis temperatures (<500 °C). A cryogenic trapping technique was employed combined with EPR spectroscopy to identify the open-shell intermediates registered at pyrolysis temperatures above 500 °C. These were characterized as radical mixtures primarily consisting of oxygen-linked conjugated radicals. A comprehensive potential energy surface analysis of p-CMA and p-CMA + H atom systems was performed using various DFT protocols to examine the possible role of concerted molecular eliminations and free-radical mechanisms in the formation of major products. Other significant unimolecular concerted reactions along with formation and decomposition of primary radicals are also described and evaluated. The calculations suggest that a set of the chemically activated secondary radical channels is relevant to the low temperature product formation under fractional pyrolysis conditions.

Computational Study on the Thermal Decomposition of Phenol‐Type Monolignols

“…Asatryan et al have detected large amounts of phenol from the pyrolysis experiment of p ‐coumaryl alcohol containing a propanoid side chain, presumably because of the side‐chain conversion including cracking and radical chain reactions. Thus, the thermal decomposition pathways are proposed, which assume that all phenol‐type monolignol compounds containing a side chain (i.e., HPP, p ‐coumaryl alcohol, HHPP, and HPPD) finally decompose into phenol via side‐chain conversion.…”

“…Figure shows the side‐chain conversion processes of (a) p ‐coumaryl alcohol, (b) HHPP, and (c) HPPD, involving bond cleavage ( channels 43–45, 47, 50–52, 54, and 57), H‐addition ( channels 46 and 53), abstraction by an H atom ( channels 48, 55, and 58), and by a CH 3 radical ( channels 49, 56, and 59), and the activation energies for each proposed elementary reaction channel calculated at the M06–2X/6–311++G(d,p) level. Asatryan et al have theoretically investigated the unimolecular decomposition of p ‐coumaryl alcohol at the M06–2X/6–31G(d,p) level of theory. Following this theoretical study , the thermal decomposition pathways of p ‐coumaryl alcohol leading to the formation of A6 and A8 have been proposed by our group (Fig.…”

“…Asatryan et al have theoretically investigated the unimolecular decomposition of p ‐coumaryl alcohol at the M06–2X/6–31G(d,p) level of theory. Following this theoretical study , the thermal decomposition pathways of p ‐coumaryl alcohol leading to the formation of A6 and A8 have been proposed by our group (Fig. a).…”

Measuring biomass fast pyrolysis kinetics: State of the art

“…Thermal decomposition of lignin monomer: p‐coumaryl alcohol via free‐radical mechanism (Adapted with permission Asatryan et al, . Copyright 2017 ACS Publications)…”

“…Alternatively, model compounds of the three biomass constituents have been used to derive the details of intermediate species generated during their thermal decomposition. Pyrolysis of model compounds also provides insights on the influence of the substituent functional groups or bond linkages on the product distribution (Asatryan et al, ; Asmadi, Kawamoto, & Saka, ; Buckingham et al, ; Choi et al, ; Custodis et al, ; Jarvis et al, ; Kawamoto & Saka, ; Khachatryan et al, ; Lou et al, ; Nowakowska et al, ; Patwardhan et al, ; Robichaud, Nimlos, & Ellison, ; Scheer et al, ; Scheer et al, ; Scheer, Mukarakate, Robichaud, Ellison, & Nimlos, ; Vasiliou et al, ; Wang, McDonald, et al, ; Xu et al, ; Zhou, Nolte, et al, ).…”