The cleaving of technical lignin by different technologies is a promising but complex approach for the production of value-added compounds from biomass. A rigorous description of the underlying mechanisms and influencing parameters is complicated by lignin's heterogeneity, hampering comparability and reproducibility. This frequently leads to contradicting data and theories in the scientific community. By applying a statistical design of experiment approach to the electro-oxidation of Kraft lignin, the statistical variation of the experimental data can be separated from the systematic effects of reaction parameters on lignin properties. The data allow a clear quantification of the effect of temperature, alkalinity, catalyst, lignin concentration and current density on the molecular weight, monomer production, UV absorbance as well as acid-solubility of the treated lignin. The method described can be applied to reliably investigate the whole range of lignin cleavage technologies in spite of the pronounced heterogeneity of lignin. † Electronic supplementary information (ESI) available. See

Facing the challenge of lignin valorization is one of the unsolved key steps for a sustainable and economically feasible biorefinery. Several processes were developed with the aim of producing value-added compounds from lignin. Thermal, enzymatic, and catalytic processes represent common techniques for lignin valorization. However, expensive catalysts or enzymes and harsh conditions hampered the implementation of these methodologies on an industrial scale. Here, we propose the utilization of a simple "swiss-roll" electrochemical reactor for the production of valuable carboxylic acids. We showcase that production of phenolic compounds, such as vanillin, is hindered by the electrochemical mechanism. Additionally, electrochemical stability experiments of possible products showed high reactivity of vanillin against the low reactivity of mono-and dicarboxylic acids. Simultaneously, the electrochemical process leads to stable carboxylic acids with high yields of 6.4, 26.8, and 4.2 % for oxalic, formic, and acetic acids, respectively, thus representing a competitive alternative to the catalytic and hydrothermal degradation process for the production of carboxylic acids.

BackgroundThe valorization of biomass for chemicals and fuels requires efficient pretreatment. One effective strategy involves the pretreatment with ionic liquids which enables enzymatic saccharification of wood within a few hours under mild conditions. This pretreatment strategy is, however, limited by water and the ionic liquids are rather expensive. The scarce understanding of the involved effects, however, challenges the design of alternative pretreatment concepts. This work investigates the multi length-scale effects of pretreatment of wood in 1-ethyl-3-methylimidazolium acetate (EMIMAc) in mixtures with water using spectroscopy, X-ray and neutron scattering.ResultsThe structure of beech wood is disintegrated in EMIMAc/water mixtures with a water content up to 8.6 wt%. Above 10.7 wt%, the pretreated wood is not disintegrated, but still much better digested enzymatically compared to native wood. In both regimes, component analysis of the solid after pretreatment shows an extraction of few percent of lignin and hemicellulose. In concentrated EMIMAc, xylan is extracted more efficiently and lignin is defunctionalized. Corresponding to the disintegration at macroscopic scale, SANS and XRD show isotropy and a loss of crystallinity in the pretreated wood, but without distinct reflections of type II cellulose. Hence, the microfibril assembly is decrystallized into rather amorphous cellulose within the cell wall.ConclusionsThe molecular and structural changes elucidate the processes of wood pretreatment in EMIMAc/water mixtures. In the aqueous regime with >10.7 wt% water in EMIMAc, xyloglucan and lignin moieties are extracted, which leads to coalescence of fibrillary cellulose structures. Dilute EMIMAc/water mixtures thus resemble established aqueous pretreatment concepts. In concentrated EMIMAc, the swelling due to decrystallinization of cellulose, dissolution of cross-linking xylan, and defunctionalization of lignin releases the mechanical stress to result in macroscopic disintegration of cells. The remaining cell wall constituents of lignin and hemicellulose, however, limit a recrystallization of the solvated cellulose. These pretreatment mechanisms are beyond common pretreatment concepts and pave the way for a formulation of mechanistic requirements of pretreatment with simpler pretreatment liquors.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0422-9) contains supplementary material, which is available to authorized users.

We propose an approach for monitoring the concentration of dissociated carboxylic acid species in dilute aqueous solution. The dissociated acid species are quantified employing inline Raman spectroscopy in combination with Indirect Hard Modeling (IHM) and Multivariate Curve Resolution (MCR). We introduce two different titration-based Hard Model (HM) calibration procedures for a single mono- or polyprotic acid in water with well-known (method A) or unknown (method B) acid dissociation constants p<i>K</i>_a. In both methods, spectra of only one acid species in water are prepared for each acid species. These spectra are used for the construction of HMs. For method A, the HMs are calibrated with calculated ideal dissociation equilibria. For method B, we estimate p<i>K</i>_a values by fitting ideal acid dissociation equilibria to acid peak areas that are obtained from a spectral HM. The HM in turn is constructed on the basis of MCR data. Thus, method B on the basis of IHM is independent of a priori known p<i>K</i>_a values, but instead provides them as part of the calibration procedure. As a detailed example, we analyze itaconic acid in aqueous solution. For all acid species and water, we obtain low HM errors of less than 2.87×10^-4 mol mol^-1 in the cases of both method A and B. With only four calibration samples, IHM yields more accurate results than partial least squares regression. Furthermore, we apply our approach to formic, acetic, and citric acid in water, thereby verifying its generalizability as a process analytical technology for quantitative monitoring of processes containing carboxylic acids.