Chemistry of lignin and hemicellulose structures interacts with hydrothermal pretreatment severity and affects cellulose conversion

Abstract

Understanding of how the plant cell walls of different plant species respond to pretreatment can help improve saccharification in bioconversion processes. Here, we studied the chemical and structural modifications in lignin and hemicellulose in hydrothermally pretreated poplar and wheat straw using wet chemistry and 2D heteronuclear single quantum coherence nuclear magnetic resonance (NMR) and their effects on cellulose conversion. Increased pretreatment severity reduced the levels of β─O─4 linkages with concomitant relatively increased levels of β─5 and β─β structures in the NMR spectra. β─5 structures appeared at medium and high severities for wheat straw while only β─β structures were observed at all pretreatment severities for poplar. These structural differences accounted for the differences in cellulose conversion for these biomasses at different severities. Changes in the hemicellulose component include a complete removal of arabinosyl and 4‐O‐methyl glucuronosyl substituents at low and medium pretreatment severities while acetyl groups were found to be relatively resistant toward hydrothermal pretreatment. This illustrates the importance of these groups, rather than xylan content, in the detrimental role of xylan in cellulose saccharification and helps explain the higher poplar recalcitrance compared to wheat straw. The results point toward the need for both enzyme preparation development and pretreatment technologies to target specific plant species.

Poplar lignin structural changes during extraction in γ-valerolactone (GVL)

“…, β–β and β-5 bonds) as well as the formation of new bonds by electrophilic aromatic substitution by benzylic carbocation intermediates on the electron-rich aromatic rings. 18,69,79 Benzylic carbocation intermediates are believed to be generated from dehydration of side-chain hydroxyl groups, and the electron-rich benzene ring is the resonance form of regular lignin units ( e.g. , guaiacyl unit).…”

“…† Prior studies indicated that the condensation of lignin molecules may be attributed to the stability of native C-C bonds (e.g., β-β and β-5 bonds) as well as the formation of new bonds by electrophilic aromatic substitution by benzylic carbocation intermediates on the electron-rich aromatic rings. 18,69,79 Benzylic carbocation intermediates are believed to be generated from dehydration of side-chain hydroxyl groups, and the electron-rich benzene ring is the resonance form of regular lignin units (e.g., guaiacyl unit). 79 As shown in Table S1, † the relative content of β-β-coupled units slightly increases with reaction time.…”

“…, aldol condensation, Michael addition, and electrophilic aromatic substitution), 10–13 as lignin is removed efficiently from biomass at higher temperatures (above 170 °C) using delignification technologies, such as alkaline 14 and sulfite pulping, 15 ammonia fiber expansion, dilute acid, 16 and hydrothermal pretreatments. 17–19 The severe conditions of such techniques degrade the hemicellulose and lignin. 20…”

“…9 During fractionation, the β-aryl ether bonds can cleave, and new C-C bonds can form through multiple reaction pathways (e.g., aldol condensation, Michael addition, and electrophilic aromatic substitution), [10][11][12][13] as lignin is removed efficiently from biomass at higher temperatures (above 170 °C) using delignification technologies, such as alkaline 14 and sulfite pulping, 15 ammonia fiber expansion, dilute acid, 16 and hydrothermal pretreatments. [17][18][19] The severe conditions of such techniques degrade the hemicellulose and lignin. 20 Lignin derives primarily from three monolignols, the hydroxycinnamyl alcohols p-coumaryl, coniferyl, and sinapyl alcohols that produce the p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units in the polymer.…”