Lignin Structure and Reactivity in the Organosolv Process Studied by NMR Spectroscopy, Mass Spectrometry, and Density Functional Theory

Abstract

There is need for well-defined lignin macromolecules for research related to their use in biomaterial and biochemical applications. Lignin biorefining efforts are therefore under investigation to meet these needs. The detailed knowledge of the molecular structure of the native lignin and of the biorefinery lignins is essential for understanding the extraction mechanisms as well as chemical properties of the molecules. The objective of this work was to study the reactivity of lignin during a cyclic organosolv extraction process adopting physical protection strategies. As references, synthetic lignins obtained by mimicking the chemistry of lignin polymerization were used. State-of-the-art nuclear magnetic resonance (NMR) analysis, a powerful tool for the elucidation of lignin inter-unit linkages and functionalities, is complemented with matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry (MALDI-TOF MS), to gain insights into linkage sequences and structural populations. The study unraveled interesting fundamental aspects on lignin polymerization processes, such as identifications of molecular populations with high degrees of structural homogeneity and the emergence of branching points in lignin structure. Furthermore, a previously proposed intramolecular condensation reaction is substantiated and new insights into the selectivity of this reaction are introduced and supported by density functional theory (DFT) calculations, where the important role of intramolecular π−π stacking is emphasized. The combined NMR and MALDI-TOF MS analytical approach, together with computational modeling, is important for deeper fundamental lignin studies and will be further exploited.

Lignin Carboxymethylation: Probing Fundamental Insights into Structure–Reactivity Relationships

“…To examine this, DFT modeling experiments were initiated on β-O–4′ hexamers of hardwood lignin (comprising four S units and two G units) and softwood lignin (six G units) (Figure ). We recently verified the presence of structures consisting only of β-O–4′ linkages in native spruce lignin by combining HSQC with MALDI-ToF analysis . Thus, the modeled structural motifs represent structural populations in cyclic extracted organosolv lignin.…”

“…This reduction in β-O–4′ linkages, ranging from approximately 14 to 32%, can be attributed to the carboxymethylation conditions, which may lead to the cleavage of sensitive linkages such as β-O–4′. Despite this decrease, the total β-O–4′ content for all carboxymethylated lignin samples remained at or above 20 per 100 aromatic units (Ar). , Such findings underscore the mildness of the carboxymethylation reaction conditions to retain the native structure of lignin. Lignin’s intrinsic structure, exemplified by its β-O–4′ linkages, is a critical aspect when assessing modification techniques because these structural components dictate its functionality, properties, and potential applications.…”

“…Such accessibility is influenced by the molecular conformation of lignin in specific media. Lignin molecules exhibit rigid and flexible bonds; notably, the β-O–4′ bond is flexible, permitting rotation and folding and facilitating intermolecular π–π stacking. , Therefore, these molecular conformations could possibly play an important role in the carboxymethylation reaction. To examine this, DFT modeling experiments were initiated on β-O–4′ hexamers of hardwood lignin (comprising four S units and two G units) and softwood lignin (six G units) (Figure ).…”

“…Parts b and d of Figure provide detailed insights into the role of β-O–4′ in the intramolecular π–π stacking between phenolic rings . Two distinct conformations are evident: one with sandwiched stacked phenolic rings and the other with T-shaped stacking phenolic rings.…”

Thermogravimetric Analysis as a High-Throughput Lignocellulosic Biomass Characterization Method

“…That work indicated that more recalcitrant lignins had both lower deconstruction yields and higher degradation temperatures; although, that approach was not extended to more complex, whole LCB feedstocks . Deconstruction yields of whole biomass have previously been predicted by the fraction of β-O-4 bonds, which can be measured via 31 P-, 19 F-, and heteronuclear single-quantum coherence nuclear magnetic resonance spectroscopy techniques. ,,− However, these spectroscopic approaches either cannot provide absolute quantifications due to differences in backbone and pendant/end-group relaxation times or require fractionation and chemical functionalization prior to analysis, thus limiting their throughput and ultimate biorefinery applicability. ,,,, …”