Characterization of dissolved lignins from acetic acid Lignofibre (LGF) organosolv pulping and discussion of its delignification mechanisms

Abstract

Birch chips were cooked by means of the Lignofibre (LGF) organosolv process in acetic acid (AA) and phosphinic acid (H 3 PO 2 ) at 150°C. The delignification rate and structure of the dissolved lignin was followed as a function of time. The degree of delignification increased steadily up to 88% during the 120 min treatment time. The dissolved lignins were precipitated from the spent liquor (SL) by water addition, washed, and purified for the analyses. Elemental analysis, 31 P nuclear magnetic resonance (NMR), heteronuclear single-quantum coherence (HSQC) NMR, pyrolysis-gas chromatography/mass spectrometry (GC/MS), and gel permeation chromatography (GPC) were applied for the structural elucidation. It was found that the cleavage of the β-aryl ether linkages is the main reaction leading to delignification, accompanied by the formation of free phenolic hydroxyl groups and reduction in the content of aliphatic hydroxyl groups. The structure of the dissolved lignin remained the same after the drastic changes at the early stages of cooking (up to 30 min cooking time), indicating that secondary reactions (e.g., condensation) do not take place to a significant extent. H 3 PO 2 probably enhances the acidolysis reaction via an ester derivative that both boosts the cleavage reaction and prevents the formation of the carbocation intermediate that induces condensation. Homolytic cleavage reactions may take place parallel to the acidolytic reactions.

Condensed lignin structures and re‐localization achieved at high severities in autohydrolysis of Eucalyptus globulus wood and their relationship with cellulose accessibility

“…The C 2 –H 2 , C 5 –H 5 , and C 6 –H 6 correlations for guaiacyl units (G, red) were observed at δ C /δ H 111/7.0 ppm, 115/6.7–7.0, and 119/6.8 ppm, respectively, in the E. globulus wood without pretreatment, but disappeared in the pulp samples, indicating a decrease in the protonated aromatic carbon and the condensation of lignin in the hydrothermal pretreatment. New signals were observed in the spectra at δ C / δ H 105.7/6.5, 112.5/6.7, and 115.05/6.8 ppm, which are similar to those reported by Kangas et al (), which positively correlated with the pretreatment severity and could be associated with the re‐condensation of lignin, and ring aromatic modification. Also a new signal was observed at δ C /δ H 55.9/4.54, which could be associated with changes in resinol subunits due to β–β linkage modification produced at higher severity of the pretreatment, as previously observed by Tran et al () in Kraft lignin experiments.…”

Optimisation of Acetic Acid Lignofibre Organosolv Process

“…Phosphinic acid is proposed to enhance the delignification, cleaving β-aryl ether bonds in lignin via the formation of phosphinic acid ester derivatives, also preventing further condensation reactions. The structural characteristics of LGF lignin indicate that reduction of lignin took place (Kangas et al 2014). In the cooking liquors of cooks performed at 125, 130, 135, and 150 °C, very little or no hypophosphite remained in the spent cooking liquors.…”

“…Depending on the process conditions, 24 to 62% of the added phosphorous was found as phosphorous oxo-anions in the spent cooking liquor, with the recovery generally decreasing with increasing temperature and cooking time. Some phosphorous was found attached to LGF cellulose (Kangas et al 2013) and lignin (Kangas et al 2014). The total phosphorous content in the spent liquor determined in earlier studies was 1.5 g/L, indicating that most of the phosphorous added to the process was found in the spent cooking liquor, but it was only partially detectable as phosphorous oxo-anions.…”

“…The total phosphorous content in the spent liquor determined in earlier studies was 1.5 g/L, indicating that most of the phosphorous added to the process was found in the spent cooking liquor, but it was only partially detectable as phosphorous oxo-anions. The phosphorous remaining unaccounted for in the CE analyses could be present as esters in lignin (Kangas et al 2014) and sugars.…”

“…It was found previously that acetic acid LGF cooking in the presence of phosphinic acid results in improved delignification as compared to other acetic acid processes in which mineral acids serve as catalysts (Kangas et al 2013). According to recent results, the phosphinic acid in the acetic acid LGF process catalyses delignification by acidolysis via phosphinic acid esterification (Kangas et al 2014). In addition, phosphinic acid can potentially protect lignin against the typical condensation reactions taking place under acidic conditions.…”