Getting Closer to Absolute Molar Masses of Technical Lignins

Determination of molecular weight parameters of native and, in particular, technical lignins are based on size exclusion chromatography (SEC) approaches. However, no matter which approach is used, either conventional SEC with a refractive index detector and calibration with standards or multi-angle light scattering (MALS) detection at 488nm, 633nm, 658nm, or 690nm, all variants can be severely erroneous. The lack of calibration standards with high structural similarity to lignin impairs the quality of the molar masses determined by conventional SEC, and the typical fluorescence of (technical) lignins renders the corresponding MALS data rather questionable. Application of MALS detection at 785nm by using an infrared laser largely overcomes those problems and allows for a reliable and reproducible determination of the molar mass distributions of all types of lignins, which has been demonstrated in this study for various and structurally different analytes, such as kraft lignins, milled-wood lignin, lignosulfonates, and biorefinery lignins. The topics of calibration, lignin fluorescence, and lignin UV absorption in connection with MALS detection are critically discussed in detail, and a reliable protocol is presented. Correction factors based on MALS measurements have been determined for commercially available calibration standards, such as pullulan and polystyrene sulfonate, so that now more reliable mass data can be obtained also if no MALS system is available and these conventional calibration standards have to be resorted to.Peer reviewe

Ball milling’s effect on pine milled wood lignin’s structure and molar mass

The effect of ball milling expressed as the yield of milled wood lignin (MWL) on the structure and molar mass of crude milled wood lignin (MWLc) preparation is studied to better understand the process’ fundamentals and find optimal conditions for MWL isolation (i.e., to obtain the most representative sample with minimal degradation). Softwood (loblolly pine) MWLc preparations with yields of 20–75% have been isolated and characterized based on their molar mass distribution (by Size Exclusion Chromatography (SEC)), hydroxyl groups of different types (31P NMR), methoxyl groups (HS-ID GC-MS), and sugar composition (based on methanolysis). Classical MWL purification is not used to access the whole extracted lignin. The results indicate that lignin degradation during ball milling occurs predominantly in the high molar mass fraction and is less pronounced in the low molar mass fraction. This results in a significant decrease in the Mz and Mw of the extracted MWLc with an increase in the yield of MWLc, but has only avery subtle effect on the lignin structure if the yield of MWLc is kept below about 55%. Therefore, no tedious optimization of process variables is necessary to achieve the required MWLc yield in this range for structural studies of softwood MWL. The sugar composition shows higher amounts of pectin components in MWLs of low yields and higher amounts of glucan and mannan in high-yield MWLs, confirming that lignin extraction starts from the middle lamella in the earlier stages of MWL isolation, followed by lignin extraction from the secondary wall region.Peer reviewe

Heat Treatment of Spent Liquors to Recover Chemically Bound Xylose and Alcohol

SO₂–ethanol–water (SEW) and SO₂–isopropanol–water (SPW) spent liquors are obtained by fractionating sugarcane straw. The SEW and SPW liquors contain significant amounts of chemically bound xylose as ethyl xylosides (EX) and isopropyl xylosides (PX) respectively. The liquors are subjected to a constant temperature heat treatment to hydrolyze the alkyl xylosides to allow full recovery of xylose and alcohol. Complete hydrolysis of EX and PX is achieved at 121 °C in 70 and 30 min, respectively. The first-order kinetics of EX and PX hydrolysis are determined at temperatures from 100 to 121 °C. At full hydrolysis of the alkyl xylosides, the quantity of alcohol produced is greater than stoichiometric. Other sources of covalently bound alcohols in the spent liquors are identified to explain the excess alcohol produced

Structural and Thermal Analysis of Softwood Lignins from a Pressurized Hot Water Extraction Biorefinery Process and Modified Derivatives

In this work we have analyzed the pine and spruce softwood lignin fraction recovered from a novel pressurized hot water extraction pilot process. The lignin structure was characterized using multiple NMR techniques and the thermal properties were analyzed using thermal gravimetric analysis. Acetylated and selectively methylated derivatives were prepared, and their structure and properties were analyzed and compared to the unmodified lignin. The lignin had relatively high molar weight and low PDI values and even less polydisperse fractions could be obtained by fractionation based on solubility in i-PrOH. Condensation, especially at the 5-position, was detected in this sulphur-free technical lignin, which had been enriched with carbon compared to the milled wood lignin (MWL) sample of the same wood chips. An increase in phenolic and carboxylic groups was also detected, which makes the lignin accessible to chemical modification. The lignin was determined to be thermally stable up to (273–302 °C) based on its Tdst 95% value. Due to the thermal stability, low polydispersity, and possibility to tailor its chemical properties by modification of its hydroxyl groups, possible application areas for the lignin could be in polymeric blends, composites or in resins

Upgrading AquaSolv Omni (AqSO) biorefinery: access to highly ethoxylated lignins in high yields through reactive extraction (REx)

Publisher Copyright: © 2024 The Royal Society of Chemistry.Chemical modification of lignin (i.e., ethoxylation) improves its properties for specific applications. Reactive extraction (REx)—the simultaneous functionalization and extraction of lignin from biomass—is a green, simple, and powerful solution to minimize subsequent steps in biorefinery operations, while upgrading the isolated products (i.e., lignin or lignin-carbohydrate hybrids). In this work, we successfully introduced REx into our recently reported AquaSolv Omni (AqSO) integrated biorefinery. Here, hydrothermally treated wood solids were refluxed with various EtOH : H2O mixtures (70-99 v/v%) in the presence of catalytic amounts of H2SO4 (c = 0.15-1.2 M). The effects of the process variables on the structures and properties of the obtained lignins and residual solids were elucidated by comprehensive NMR analyses (HSQC, quantitative 13C and 31P), differential scanning calorimetry (DSC), and gel permeation chromatography (GPC). In addition, we discuss different analytical approaches—NMR vs. chromatographic methods for the quantification of ethoxy groups in lignin. Implementing REx allowed the isolation of ethoxylated lignins in 27-52% yields (based on the initial lignin content) and to tune the degree of substitution (DS) up to 40.8 EtO-groups/100 Ar (based on quantitative 13C NMR)—which is approximately five times higher compared to other established organosolv processes (i.e., Alcell). Moreover, solution state NMR analysis of residual solids after REx showed that ethoxylation also occurs in the cellulose-rich fraction. REx highly ethoxylated lignins produced through a simple and green process enhanced the performance of polyurethane (PU) adhesive formulations compared to formulations using non-ethoxylated lignins.Peer reviewe