Quantitative Extraction of p-Coumaric Acid and Ferulic Acid in Different Gramineous Materials and Structural Changes of Residual Alkali Lignin

Abstract

Ferulic acid (FA) and p-coumaric acid (pCA) in bagasse, wheat straw, corn straw, and corncob were extracted by alkaline hydrolysis and characterized by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). It was found that the FA and most of the pCA in gramineous biomass could be dissociated and released after being treated with 1 M NaOH at 100°C for 4 h. The yields of pCA/FA in bagasse, wheat straw, corn straw, and corncob determined by GC-FID are 39.8/11.5, 13.7/11.0, 28.0/11.0, and 35.1/14.5 mg/g, respectively. The raw materials and the treated solid residues were characterized by gel-state 2D Heteronuclear Single Quantum Coherence Nuclear Magnetic Resonance (2D HSQC NMR). It was found that only a small amount of lignin was detected in the residue after alkali treatment, indicating that the alkali treatment conditions can effectively cleave the FA and pCA. Additionally, the lignin in the alkali solution was recovered and characterized by 2D HSQC NMR. The FA was not able to be detected by NMR, whereas a small amount of pCA remained in the alkali lignin. This study reveals the structural change of residual lignins during the quantitative isolation of FA and pCA, which is essential for the selective isolation of pCA/FA and valorization of residual alkali lignin.

Structural Changes of Lignin during OrganoCat Processing

“…While the S/G ratio determined by Py-GC-MS for beech and eucalyptus decreases in the extracted lignin, it increases when measured by HSCQ-NMR. The observed differences between both methods are consistent with other studies . According to Happs et al, HSQC-NMR is determined to be the most suitable method for comparing the lignin monomer ratio between different species, although it exhibits some limitations .…”

“…H units can be overestimated due to the 2/6 signal of p -hydroxyphenyl overlapping with the 3/5 correlation peak of phenylalanine, although the expected amount in the whole cell wall is very low. More prominently, the p -coumarate signal that is especially found in grasses also presents a possible overlap with the 2/6 signal of H-units, as exemplarily pointed out in the spectra shown in Figure D–F. − Therefore, in the following, the linkages refer to S and G units only.…”

“…The observed differences between both methods are consistent with other studies. 32 According to Happs et al, HSQC-NMR is determined to be the most suitable method for comparing the lignin monomer ratio between different species, although it exhibits some limitations. 35 It underestimates the amount of S units, which we can observe in our data, and the S/G ratio measured by Py-GC-MS is higher for most plants except for the S dominant lignin of beech and eucalyptus.…”