Isolation and Structural Characterization of Lignin from Cardoon (Cynara cardunculus L.) Stalks

Abstract

The lignin from Cynara cardunculus stalks was isolated by the classical Björkman method and characterized by pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC/MS), two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR), and derivatization followed by reductive cleavage (DFRC). The milled Cynara lignin (MCyL) was constituted mainly by guaiacyl (G) and syringyl-units(S) (S/G molar ratio of 0.7), with the complete absence of p-hydroxyphenyl (H) units. The 2D-NMR analysis indicated a predominance of alkyl-aryl ether linkages (70 % of all inter unit linkages are β-O-4′) and significant amounts of condensed structures such as phenylcoumarans (β-5′, 14 %), resinols (β-β′, 7 %), spirodienones (β-1′, 5 %), and dibenzodioxocins (5-5′, 4 %). Furthermore, the analyses indicated that the lignin is partially acylated at the γ-OH (12 % acylation) by acetate groups and that acetylation occurs preferentially on syringyl-units. As in other plants, acetylation occurs at the monomer stage, and sinapyl acetate behaves as a real lignin monomer participating in lignification in cardoon stalks. The detailed structural characterization of cardoon lignin reported here will foster the industrial use of this biomass for the production of biofuels and other bio-based chemicals under the lignocellulosic biorefinery.

Characterization of Lignin Extracted from Willow by Deep Eutectic Solvent Treatments

“…To investigate the structural characteristics of DES-lignin, 2D-HSQC NMR for DES-lignin sample (D 12h ) extracted from willow by DES-treatment at 120 °C for 12 h and the EMAL of willow were obtained (Figure 4). Assignment of the peaks in the 2D-HSQC NMR spectra was conducted based on previous reports [32,43,44,45]. The peaks corresponding to methoxyl (3.81/56.7 parts per million (ppm)) and β-aryl ether are shown for A α (4.91/70.3 ppm) and A γ (4.05/87.5 ppm), β–β are shown for C α (4.71/86.1 ppm), C β (3.08/53.9 ppm) and C γ (4.11/70.3 ppm), guaiacyl (G 5/6 , 6.79/115.1 ppm), syringyl (S 2/6 , 6.61/102.9 and 7.19/116.2 ppm), and p -hydroxybenzoate (P 2/6 , 7.61/131.2 ppm) appeared in the DES-lignin (D 12h ) NMR spectrum indicating functional groups associated with lignin were present and not severely damaged during DES treatment.…”

Compositional Variability of Lignin in Biomass

“…Lignin content in the stalks varies from 16.4 to 19.2% [131,134,135]. The lignin monomeric composition of cardoon stalks separated in depithed and pith regions shows a H:G:S relation of 1:6:8 and 1:4:9, respectively, determined by Py-GC/MS (FID), corresponding to a S/G ratio of 1.3 and 2.2, respectively [136]. Whole stalks lignin was studied after isolation by the Bjӧrkman method and characterized by Py-GC/MS, 2D NMR and DFRC' [136].…”

“…The lignin monomeric composition of cardoon stalks separated in depithed and pith regions shows a H:G:S relation of 1:6:8 and 1:4:9, respectively, determined by Py-GC/MS (FID), corresponding to a S/G ratio of 1.3 and 2.2, respectively [136]. Whole stalks lignin was studied after isolation by the Bjӧrkman method and characterized by Py-GC/MS, 2D NMR and DFRC' [136]. The 2D NMR analysis showed that the isolated lignin has a H:G:S relation of 0:58:42, i.e., a S/G ratio of 0.7 and a predominance of β-O-4′ alkyl-aryl ether structures (70% of all inter-unit linkages), followed by a considerable amount of condensed structures: phenylcoumarans (β-5′, 14%), resinols (β-β′, 7%), spirodienones (β-1′, 5%) and dibenzodioxocins (5-5′, 4%).…”

“…The lignin was partially acylated (12%) at the carbon γ-OH by acetate groups that were preferentially attached over S units (32% of S units and only 1% of G units). Lourenço et al [136] showed that sinapyl acetate acts as a real monolignol involved in the lignification of cardoon stalks.…”

Chemical Characterization of Lignocellulosic Materials by Analytical Pyrolysis

“…The lignin present in the whole stalks was studied after isolation by the Bjӧrkman method (MCyL) and characterized by Py-GC/MS [72]. In general, there was a similar distribution of the lignin-derived compounds formed during pyrolysis of MCyL comparatively to depithed and pith samples pyrolysis, and the G units were released in higher abundances than the respective S units, attaining an S/G molar ratio 0.3.…”