Tapping the Full Potential of Infrared Spectroscopy for the Analysis of Technical Lignins.

We present an approach to overcome the challenges associated with the increasing demand of high-throughput characterization of technical lignins, a key resource in emerging bioeconomies. Our approach offers a resort from the lack of direct, simple, and low-cost analytical techniques for lignin characterization by employing multivariate calibration models based on infrared (IR) spectroscopy to predict structural properties of lignins (i. e., functionality, molar mass). By leveraging a comprehensive database of over 500 well-characterized technical lignin samples - a factor of 10 larger than previously used sets - our chemometric models achieved high levels of quality and statistical confidence for the determination of different functional group contents (RMSEPs of 4-16 %). However, the statistical moments of the molar mass distribution are still best determined by size-exclusion chromatography. Analyses of over 500 technical lignins offered also a great opportunity to provide information on the general variability in kraft lignins and lignosulfonates (from different origins). Overall, the effected savings in analysis time (>7 h), resources, and required sample mass combined with non-destructiveness of the measurement satisfy key demands for efficient high-throughput lignin analyses. Finally, we discuss the advantages, disadvantages, and limitations of our approach, along with critical insights into the associated chemical-analytical and spectroscopic challenges.

From liquid to solid-state, solvent-free oxidative ammonolysis of lignins - an easy, alternative approach to generate "N-lignins".

A new chemical modification protocol to generate N-lignins is presented, based on Indulin AT and Mg2+-lignosulfonate. The already known ammonoxidation reaction in liquid phase was used as a starting point and stepwise optimised towards a full solid-state approach. The "classical" liquid ammonoxidation products, the transition products from the optimization trials, as well as the "solid-state" products were comprehensively analysed and compared to the literature. The N-lignins obtained with the conventional ammonoxidation protocol showed the same properties as reported. Their molar mass distributions and the hydroxy group contents, hitherto not accessible due to solubility problems, were measured according to a recently reported protocol. N-Indulin showed an N-content up to 11 wt% and N-lignosulfonate up to 16 wt%. The transition experiments from liquid to solid-state gave insights into the influence of chemical components and reaction conditions. The use of a single chemical, the urea-hydrogen peroxide complex (UHP, "carbamide peroxide"), was sufficient to generate N-lignins with satisfying N-content. This chemical acts both as an N-source and as the oxidant. Following the optimization, a series of solid-state ammonoxidation tests were carried out. High N-contents of 10% in the case of Indulin and 11% in the case of lignosulfonate were obtained. By varying the ratio of UHP to lignin, the N-content can be controlled. Structural analysis showed that the N is organically bound to the lignin, similar to the "classical" ammonoxidation products obtained under homogeneous conditions. Overall, a new ammonoxidation protocol was developed which does not require an external gas supply nor liquids or dissolved reactants. This opens the possibility for carrying out the lignin modification in closed continuous reactor systems, such as extruders. The new, facile solid-state protocol will hopefully help N-lignins to find more consideration as a fertilizing material and in soil-improving materials.

High-Resolution Profiling of the Functional Heterogeneity of Technical Lignins.

In technical lignins, functionality is strongly related to molar mass. Hence, any technical lignin exhibits concurrent functionality-type distribution (FTD) along its molar mass distribution (MMD). This study combined preparative size-exclusion chromatography with offline characterizations to acquire highly resolved profiles of the functional heterogeneity of technical lignins, which represent crucial information for their material use. The shape of these profiles showed considerable dissimilarity between different technical lignins and followed sigmoid trends. Determining the dispersity in functionality (DF) of lignins via their FTD revealed a rather homogeneous distribution of their functionalities (DF of 1.00-1.21). The high resolution of the acquired profiles of functional heterogeneity facilitated the development of a robust calculation method for the estimation of functional group contents of lignin fractions based simply on their MMD, an invaluable tool to simulate the effects of intended purification processes. Moreover, a more thorough evaluation of separations based on functionality becomes accessible.

A general solvent system for the analysis of lignosulfonates by 31P NMR.

31P nuclear magnetic resonance (NMR) spectroscopy is the most common and most accurate analytical method to quantitatively determine the hydroxy group contents of technical lignins. However, for lignosulfonates, liquid-state NMR analysis is often limited due to solubility problems in commonly used solvent systems, which may arise from the broad range of lignosulfonates from different wood sources, pulping conditions, and purification procedures used in biorefineries. Finding a suitable solvent system is even more difficult for chemically modified or fractionated lignosulfonates. In this study, a novel and fast approach for the solubilization of genuine, modified, and fractionated lignosulfonates and subsequent quantitative analysis of hydroxy groups by 31P NMR after derivatization with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane is presented. The implementation of the ionic liquid 1-ethyl-3-methylimidazolium chloride [emim]Cl to the already validated and commonly used DMF/pyridine solvent system caused complete solubility of previously insoluble samples, especially in the case of hard-to-dissolve ammonoxidized lignosulfonates. The applicability, accuracy, and robustness of the novel solvent system for 31P NMR analysis were comprehensively investigated with lignin model compounds and commercial lignosulfonates, including otherwise insoluble, real-world lignosulfonate specimens. The results were compared to the conventional DMF/pyridine solvent system. With the novel solvent system in hand, a much larger number of different lignosulfonates can be analyzed. In particular, the hydroxy group contents of ammonoxidized lignosulfonates were determined for the first time directly by 31P liquid-state NMR.

Hydrophobic Interaction Chromatography in 2 D Liquid Chromatography Characterization of Lignosulfonates.

Lignosulfonates are bulk-scale byproducts of industrial sulfite pulping. Their amphiphilic character plays a central role in their successful application in large-scale materials production. As an inherent feature of the chemical structure, this amphiphilic character poses a major analytical challenge. In this study, the amphiphilic behavior of an industrial lignosulfonate was investigated by hydrophobic interaction chromatography (HIC). This technique exploits hydrophobic regions present on the surface of lignosulfonates. Extensive characterization of the obtained fractions from preparative HIC, in terms of elemental composition, functional-group content, chemical structure, and molecular weight distribution, revealed a detailed picture of the chemical composition distribution. The charge-to-size ratio, that is, differences in the degree of sulfonation, was the dominant factor governing separation in HIC. A combination of HIC with size exclusion chromatography showed good orthogonality of separation and demonstrated the power of this 2 D liquid chromatography approach for an in-depth characterization, in general, and amphiphilicity, in particular.