Formation of high carbohydrate and acylation condensed lignin from formic acid-acetic acid-H₂O biorefinery of corn stalk rind

Corn stalk rind was used for biorefinery using the formic acid-acetic acid-H2O (FA-AA-H2O) system, which is sustainable and can be applied industrially. Fiber, cellulose, lignin, and hemicellulose can be obtained simul-taneously from biorefineries in an environment-friendly manner. To provide a theoretical basis for the utilization of organic acid biorefinery lignin (OABL) in developing high-value products, the lignin reaction mechanism, structure and properties were studied in detail. The OABL was investigated using wet chemistry, Fourier-transform infrared spectroscopy, C-13- and 2D-nuclear magnetic resonance spectroscopy, and gel permeation chromatography. OABL was found to be composed of guaiacyl (G), syringyl (S), and p-hydroxyphenyl units with molar contents of 25.3 %, 52.2 %, and 22.5 %, respectively, along with 21.9 % p-coumarate (pCA) and 4.80 % ferulate (FAL). During the biorefinery process, the lignin was highly depolymerized and the beta-O-4 bonds decreased from 52.2 % to 5.07 %. Condensation reaction between lignin C-alpha-C-6 to form diphenylmethane structures took place. The ratio of the condensed G and S units (G(cond.)/G and S-cond./S) increased to 68.1 % and 59.5 %, respectively. OABL contains a large number of lignin-carbohydrate complexes (LCC), with phenyl glycoside (PhGlc) LCC of 4.86/100Ar, and benzyl ethers (BE) LCC of 5.22/100Ar. Furthermore, OABL was highly acylated: the acetylation, formylation, and total acylation degree by molar were 33.1 %, 10.3 %, and 43.4 %, respectively. Acylation mainly occurred at Cy of the side chain and C4 of the aromatic ring. High carbohydrate, high acyl, and high condensed structures in OABL make it have broad application prospects.