Beyond Yield Optimization: The Impact of Organosolv Process Parameters on Lignin Structure

Abstract

When optimizing the process parameters of the acidic ethanolic organosolv process, the aim is usually to maximize the delignification and/or lignin purity. However, process parameters such as temperature, time, ethanol and catalyst concentration, respectively, can also be used to vary the structural properties of the obtained organosolv lignin, including the molecular weight and the ratio of aliphatic versus phenolic hydroxyl groups, among others. This review particularly focuses on these influencing factors and establishes a trend analysis between the variation of the process parameters and the effect on lignin structure. Especially when larger data sets are available, as for process temperature and time, correlations between the distribution of depolymerization and condensation reactions are found, which allow direct conclusions on the proportion of lignin's structural features, independent of the diversity of the biomass used. The newfound insights gained from this review can be used to tailor organosolv lignins isolated for a specific application.

Semicontinuous Aqueous Acetone Organosolv Fractionation of Lignocellulosic Biomass: Improved Biorefinery Processing and Output

“…The molar mass of lignin is affected by depolymerization and recondensation reactions, generally depending on temperature, time and liquor acidity. 56 As detailed in previous work, lignin aggregates can cause an overestimation of lignin molar mass when alkaline Size Exclusion Chromatography is used for the analysis. Therefore, the weight average molecular weight (M w ) in Figure 7 shows the lignin M w analyzed after a deaggregation treatment at 150 °C in methanol (see also Figure S19 and Tables S6 and S7).…”

“…Lignin solubilization occurs via (acid-catalyzed) cleavage of lignin β-O-4 linkages which starts with elimination of the α-hydroxyl group of the linkage generating a reactive carbenium ion and, depending on the pathway, a Hibbert ketone end group . Both of which are involved in lignin repolymerization/condensation reactions and formation of stabile C–C bonds …”

“…55 Both of which are involved in lignin repolymerization/condensation reactions and formation of stabile C−C bonds. 56 To provide better insight into the gradual changes that occur in the (solvated) lignin structures during fractionation, additional experiments with varying fractionation times were conducted using beech wood. For the B15 experiment, the solvent was replaced with fresh reaction liquid every 15 min to a total reaction time of 90 min, after which the lignin was isolated from the combined liquor from the 6 cycles.…”

A systematic study on the processes of lignin extraction and nanodispersion to control properties and functionality

“…Although it can help with understanding the various species present within lignin, the intensity of the signals is not inherently quantitative. 26…”

“…The primary mechanism of lignin extraction, even a milder extraction process such as the organosolv process, is the cleavage of C β –O bonds from β-O-4′ structures and the cleavage of C α –O bonds from α-O-4′ structures. 26 The aryl–aryl–ether bonds of lignin are shown to be labile under the given extraction conditions, and hydrolysis creates two open phenols from one C–O–C. Electron-rich phenols such as those from monolignoles, are strong antioxidants and care should be taken in the lignin isolation protocol to avoid the presence of dioxygen and other oxidants.…”