Revealing Structural Differences between Alkaline and Kraft Lignins by HSQC NMR

Abstract

Technical lignins, mostly generated as byproducts from the pulping industry, are highly abundant aromatic sources. However, they are underutilized due to their complexity, as well as structural alteration during chemical pulping processes. In-depth elucidation of technical lignins is becoming essential for their valorization in view of the lack of understanding of technical lignin structures. In this work, alkaline and kraft lignins were prepared and comparatively characterized by 2D HSQC NMR. By applying authenticated reference compounds, the phenylglycerol structures, characteristic of the cleavage of nonphenolic β-aryl ether by soda pulping, in the alkaline lignin were identified and quantified by 2D HSQC NMR. Phenylglycerol structures in alkaline lignin were estimated to be 8−14%, which was much higher than that in kraft lignin. This finding was supported by the results from model studies; i.e., the yield of the phenylglycerol product was 40% that obtained from the soda pulping treatment of a nonphenolic β-aryl ether compound while the yield of that from kraft treatment was lower than 5%. In addition, styryl ether structures from phenolic β-aryl ethers of alkaline lignin were also revealed by 2D NMR. These new findings will benefit the mechanistic understanding of lignin reactions and structural elucidation of technical lignins, which will provide useful information (or new insights) for the development of lignin valorization strategies.

Recent Advances in the Catalytic Depolymerization of Lignin towards Phenolic Chemicals: A Review

“…As illustrated in Table , Zhao et al. reported severe structural modifications of the spruce and eucalyptus lignins during alkali extraction process, induced by the depletion of β‐O‐4 linkages and the appearance of aryl enol ether moieties …”

Secondary Bonds Modifying Conjugate‐Blocked Linkages of Biomass‐Derived Lignin to Form Electron Transfer 3D Networks for Efficiency Exceeding 16% Nonfullerene Organic Solar Cells

“…To elucidate the structural evolution of KL, both of KL and D Me KL were characterized by X‐ray photoelectron spectroscopy (XPS), [ 32 ] 2D heteronuclear single quantum coherence (HSQC) NMR, [ 23,33,34 ] quantitative 31 P NMR, [ 35–37 ] headspace gas chromatography (HS‐GC), [ 38 ] UV–vi spectrum, [ 39 ] gel permeation chromatography (GPC) and element analysis. The XPS C1s spectra in Figure 1c indicate that, after demethylation and solvolysis, the conjugated carbon with sp2 hybridization and carbon–oxygen double bond increases from 65.1% and 1.87% to 72.9% and 3.28%, respectively.…”

Naphthalene Structures Derived from Lignins During Phenolation

“…[15c] Therefore alkali lignin of eucalyptus can have up to 12.7 % of resinol structures. [12] In this work, alkali lignin of eucalyptus was used for phenolation study with focus on resinol structures of the lignin.…”

“…[11] Moreover, the structures of β-β are relatively stable in alkaline pulping process and remain significantly as reported that resinol structures of a alkali lignin from eucalyptus was 12.7 %. [12] Therefore, it is interesting and significant to determine how the resinol (β-β) structures of lignins are transformed during an acid-catalyzed phenolation. Reported here is an investigation using syringaresinol and pinoresinol as model compounds to understand the reaction products involving resinol structures in lignins under sulfuric acid catalyzed phenolation conditions.…”