Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure. Part 2: Influence of autohydrolysis intensity

Abstract

Three different levels of autohydrolysis intensity, expressed as the prehydrolysis (P)-factor, were applied to Eucalyptus globulus wood at a liquor/wood ratio of 5:1. Lignin fractions were isolated from the wood residue as milled wood lignin (MWL), from the hydrolysate by centrifugation (insoluble fraction) and by ethyl acetate extraction (soluble fraction), and from the reactor wall as precipitate. With increasing autohydrolysis duration, a decrease in the content of aliphatic hydroxyl groups and of β-O-4 structures was detected in all lignin fractions, whereas the content of phenolic hydroxyl groups increased in the same order. MWL isolated from wood residue after autohydrolysis at the highest P-factor contained only half the β-O-4 structures contained in native lignin. Molecular weight distribution measurements revealed that fragmentation reactions dominated over condensation reactions in all lignin samples investigated. However, low-molecular-weight lignin dissolved in autohydrolysate exhibited extremely high reactivity towards acid-catalysed condensation reaction, which inevitably leads to the formation of sticky precipitates during storage at elevated temperature under the acid conditions prevailing.

Sulfur-free pulping of hot-water-extracted spruce sawdust

“…On the other hand, the lowest lignin content was determined from the black liquors, which were cooked from severely extracted wood (PHWE 2). As discussed above, enhanced delignification took place within a very narrow range of pre-treatment time, and significant decreases in lignin solubility and reactivity after harsh autohydrolysis conditions have been reported (Leschinsky et al 2008(Leschinsky et al , 2009Li, Gellerstedt 2008;Timilsena et al 2013;Zhu et al 2015). These observations were also in agreement with this study, as highly prolonged PHWE seemed to alter the structure of lignin in such a way that its dissolution was clearly hindered during the subsequent alkaline pulping.…”

“…Possible explanation could include, for example, various condensation reactions taking place during pretreatments, which rendered lignin structure insoluble and very resistant towards alkaline (but sulfur-free) cooking. This explanation was supported by previous studies (Leschinsky et al 2008;Li, Gellerstedt 2008;Timilsena et al 2013;Zhu et al 2015), in which a decreased delignification after harsh auto-hydrolysis conditions was observed. In these studies, it was also reported that the maximum extractability of lignin could be achieved within a very narrow range of reaction time, and a significant decrease in lignin solubility and reactivity was reported after harsh auto-hydrolysis conditions.…”

“…In addition, as strong electrophiles carbonium ions have the ability to participate in substitution reactions resulting in the formation of carbon-carbon bonds, and thus, in the repolymerization of dissolved lignin. On the other hand, it has been suggested that under mild acidic conditions at elevated temperatures homolytic cleavage of the β-O-4 structures may occur leading into the formation of intermediate quinone methides (Leschinsky et al 2008(Leschinsky et al , 2009Li, Gellerstedt 2008;Santos et al 2013). These intermediates can then further undergo radical-exchange and radical-coupling reactions, the latter being able to form β-β-, β-1-, and β-5-type linkages.…”

Comparative Analysis of Milled Wood Lignins (MWLs) Isolated from Sugar Maple (SM) and Hot-Water Extracted Sugar Maple (ESM)

“…Recently some important studies in this area have been reported [7,9,13]. Our attempts in this direction include isolation of MWL (in accordance with the Björkman procedure [14] with minor modifications) from AW pre-extracted SM (SM_AW) and ESM [pre-extracted prior to ball-milling with AW, ESM_AW or dichloromethane (DCM), ESM_DCM].…”

Otimização de parâmetros operacionais do pré-tratamento da palha de cana-de-açúcar

“…Esse efeito caracteriza-se por aumentar a reatividade da lignina remanescente e assim dificultar o aproveitamento dos açúcares pela enzima aplicada posteriormente na etapa de sacarificação [31,32,33]. Em face dos resultados obtidos na hidrólise enzimática, sugere-se que essa consequência negativa foi detectada nos ensaios que sofreram deslignificação.…”