Impact of Restricted Mass Transport on Pyrolysis Pathways for Aryl Ether Containing Lignin Model Compounds

Abstract

Pyrolysis studies have been conducted at 375 °C on several silica-immobilized phenethyl phenyl ether (PPE) model compounds, representative of related β-O-4 aryl ether linkages in lignin, to explore the impact of restricted mass transport on reaction pathways. As found previously for fluid-phase PPE, two competitive free-radical decay pathways are operative including a significant rearrangement pathway involving an O,C-phenyl shift for surface-attached PhCH2CH•OPh radicals. The selectivity for the rearrangement pathway is found to be sensitive to substituents and, in particular, to the structure of neighboring spacer molecules on the surface. In contrast to solution-phase behavior, dilution of PPE molecules on the surface with rigid aromatic spacers such as biphenyl or naphthalene hinder the rearrangement path. This phenomenon, attributed to steric constraints that decrease the rate of the 1,2-phenyl shift, is not observed when a more flexible spacer molecule (diphenylmethane) is employed. An improved knowledge of the pathways involved is important since this rearrangement pathway, which was also observed in the pyrolysis of α-aryl ether models, can result in the formation of valuable chemicals (aryl aldehydes and ketones) or undesirable refractory compounds (biphenyls and diphenylmethanes) during the thermochemical processing of lignin.

Turnover of carbon in the free light fraction with and without charcoal as determined using the 13C natural abundance method

“…The dominance of aromatic C was due to the fact that tar (mainly levoglucosan) produced during thermal degradation of cellulose undergoes condensation reactions to form aromatic C structures (Shafizadeh and Sekiguchi, 1983;Elias et al, 2001). In addition, while lignin mainly undergoes cleavage of the β-O-C4 aryl ether linkage (Britt et al, 2000), the aromatic ring resists thermal degradation and acts as a precursor of black carbon (Oren et al, 1984). The alkyl C signal arises from uncarbonized cores within the charred particles, and is consistent with the signal obtained for reference spectra of herbaceous char (Skjemstad et al, 1999b, c).…”

Effects of charring on mass, organic carbon, and stable carbon isotope composition of wood

“…The lignin samples did not decompose in a single main temperature range, but showed a very low mass loss rate over the entire range between 200 and 400 C. During pyrolysis, lignin mainly undergoes cleavage of the b-O-C4 aryl ether linkage (Britt et al, 2000), while the aromatic ring of lignin (methoxy phenyl groups) mainly resists thermal degradation under pyrolytic conditions Gonza´lez-Vila et al, 2001) and acts as a precursor for BC (Oren et al, 1984). The lower rate of mass loss for lignin 1 at all temperatures may be explained by its higher lignin content compared to lignin 2 which was chemically less decomposed.…”

Oxidative Addition of the CαCβ Bond in β‐O‐4 Linkage of Lignin to Transition Metals Using a Relativistic Pseudopotential‐Based ccCA‐ONIOM Method

“…[7] Using isotopic labeling and thermochemical kinetic analysis of the pyrolysis products obtained, they showed that PPE decomposition is a free-radical chain process following an initial C b ÀO homolytic step. [8] Similarly, Drage et al concluded…”