Reaction Analysis of Diaryl Ether Decomposition under Hydrothermal Conditions

Abstract

The reactivity and decomposition pathway of models for α-O-4 and β-O-4 linkages, found within lignin, have been examined using methoxy-substituted (−OCH3) and -unsubstituted (−H) aryl groups under hydrothermal conditions. α-O-4 model compounds readily underwent conversion at comparatively mild temperatures (140–300 °C) and short reaction times (5–80 min), in contrast with the β-O-4 containing model compounds which required temperatures up to 340 °C and longer reaction times up to 240 min. Pseudo-first-order rate constants and apparent activation energies were calculated for hydrothermal conversion of the model compounds based on experimental data. The cleavage of these linkages proceeded via hydrolysis and direct elimination pathways, with the resulting products prone to undergoing further reactions including condensation, and dehydration. The presence of methoxy functionalities on the aromatic rings was found to destabilize both the α-O-4 and β-O-4 ether linkages, decreasing the temperature and reaction times required to decompose them under hydrothermal conditions. In addition, the methoxy substituents were partially hydrolyzed under hydrothermal conditions at temperatures exceeding 280 °C, resulting in a number of substituted guaiacol products.

A Review on Lignin Liquefaction: Advanced Characterization of Structure and Microkinetic Modeling

“…However, further detailed reading can be found elsewhere. 129−133 NMR is capable of probing the nature of 1 H and 13 C nuclei in a given molecular structure, with 2D techniques, like heteronuclear single/multiple quantum coherence (HSQC/HMQC) NMR, allowing for the assessment of heteronuclear (e.g., C−H bonds), multiple-bond, or throughspace correlations that may exist in a structure. NMR techniques are able to resolve lignin structural characteristics because different relevant lignin subunitswhether they be dimers containing specific interunit linkages (e.g., phenylcoumaran, resinol, spirodienone) or monomers themselves (i.e., H, S, G derivatives)have different and identifiable chemical shifts within the NMR spectra (Supporting Information).…”

“…The first major component of a microkinetic model is a well-defined lignin structure or library, which can be created from a comprehensive structural model. 3,69−71 In the case of the CPD model 65 to describe coal pyrolysis, the chemical structure of characterized coals was derived from the solid-state 13 C NMR measurements. In the CPD model, coal was described as a blend of small and large clusters.…”

“…A more complete review of qualitative and quantitative assessment of lignin can be found in the excellent, in depth article from Lupoi et al 108 Because lignin structure is always changing (in the plant, during extraction, and processing) a conceptual framework to describe the chemical structure of modified lignins should be developed. Coal structure based on 13 C NMR analyses used in the CPD model 60 is a good reference. In this model, coal was described as blend of small molecules extractable with solvents and macromolecules formed by clusters of fused aromatic rings with attached side chains connected by weak aliphatic bridges.…”

Combined lignin defunctionalisation and synthesis gas formation by acceptorless dehydrogenative decarbonylation

“…S18 †). Furthermore, in order to confirm the presence of the LC structure, the reported NMR data of veratrylglycol-β-guaiacyl ether (SP1) 25 were compared with the 2D-HSQC NMR spectra of the defunctionalized lignin. The comparison indicated that the β signal in the LC structure actually corresponded to H α -C β (δ H /δ C 3.99-4.29/ 73.19-74.62 ppm) in the 2D-HSQC NMR spectra (Fig.…”

“…1a). [24][25][26][27] The application of the B-type linkage motif was reported to avoid side reactions such as the retro-aldol reaction, which is a typical side reaction for the A-type linkage motif, especially under oxidative depolymerisation conditions. 21,[28][29][30] Thus, the use of B-type linkage motif allows for some innovative elegant approaches to valorise lignin that are not accessible for the polymeric lignin that contains A-type β-O-4 motifs.…”