Richard J.A. Gosselink

New insights into the structure and composition of technical lignins: a comparative characterisation study

Detailed insight into the structure and composition of industrial (technical) lignins is needed to devise efficient thermal, bio- or chemocatalytic valorisation strategies. Six such technical lignins covering three main industrial pulping methods (Indulin AT Kraft, Protobind 1000 soda lignin and Alcell, poplar, spruce and wheat straw organosolv lignins) were comprehensively characterised by lignin composition analysis, FT-IR, pyrolysis-GC-MS, quantitative 31P and 2D HSQC NMR analysis and molar mass distribution by Size Exclusion Chromatography (SEC). A comparison of nine SEC methods, including the first analysis of lignins with commercial alkaline SEC columns, showed molar masses to vary considerably, allowing some recommendations to be made. The lignin molar mass decreased in the order: Indulin Kraft > soda P1000 > Alcell > OS-W ∼ OS-P ∼ OS-S, regardless of the SEC method chosen. Structural identification and quantification of aromatic units and inter-unit linkages indicated that all technical lignins, including the organosolv ones, have considerably been degraded and condensed by the pulping process. Importantly, low amounts of β- ether linkages were found compared to literature values for protolignin and lignins obtained by other, milder isolation processes. Stilbenes and ether furfural units could also be identified in some of the lignins. Taken together, the insights gained in the structure of the technical lignins, in particular, the low β-O-4 contents, carry implications for the design of lignin valorisation strategies including (catalytic) depolymerisation and material applications.

Molar mass determination of lignins by size-exclusion chromatography: towards standardisation of the method

Abstract The reactivity and physicochemical properties of lignins are partly governed by their molar mass distribution. The development of reliable standard methods for determination of the molar mass distribution is not only relevant for designing technical lignins for specific applications, but also for monitoring and elucidating delignification and pulping processes. Size-exclusion chromatography (SEC) offers many advantages, such as wide availability, short analysis time, low sample demand, and determination of molar mass distribution over a wide range. A collaborative study has been undertaken within the “Eurolignin” European thematic network to standardise SEC analysis of technical lignins. The high-molar-mass fraction of polydisperse lignins was shown to be the main source of intra- and interlaboratory variations, depending on the gel type, elution solvent, detection mode, and calculation strategy. The reliability of two widespread systems have been tested: one based on alkali and a hydrophilic gel (e.g., TSK Toyopearl gel) and the other based on THF as solvent and polystyrene-based gels (e.g., Styragel). A set of practical recommendations has been deduced.

Lignin depolymerisation in supercritical carbon dioxide/acetone/water fluid for the production of aromatic chemicals

Valorisation of lignin plays a key role in further development of lignocellulosic biorefinery processes the production of biofuels and bio-based materials. In the present study, organosolv hardwood and wheat straw lignins were converted in a supercritical fluid consisting of carbon dioxide/acetone/water (300-370°C, 100bar) to a phenolic oil consisting of oligomeric fragments and monomeric aromatic compounds with a total yield of 10-12% based on lignin. These yields are similar to the state-of-the-art technologies such as base-catalysed thermal processes applied for lignin depolymerisation. Addition of formic acid increases the yield of monomeric aromatic species by stabilizing aromatic radicals. Supercritical depolymerisation of wheat straw and hardwood lignin yielded monomeric compounds in different compositions with a maximum yield of 2.0% for syringic acid and 3.6% for syringol, respectively. The results of the present study showed that under the applied conditions competition occurred between lignin depolymerisation and recondensation of fragments.

Fractionation, analysis, and PCA modeling of properties of four technical lignins for prediction of their application potential in binders.

Abstract Functional properties of technical lignins need to be characterized in more detail to become a higher added value renewable raw material for the chemical industry. The suitability of a lignin from different plants or trees obtained by different technical processes can only be predicted for selected applications, such as binders, if reliable analytical data are available. In the present paper, structure dependent properties of four industrial lignins were analyzed before and after successive organic solvent extractions. The lignins have been fractionated according to their molar mass by these solvents extractions. Kraft and soda lignins were shown to have different molar mass distributions and chemical compositions. Lignin carbohydrate complexes are most recalcitrant for extraction with organic solvents. These poorly soluble complexes can consist of up to 34% of carbohydrates in soda lignins. Modeling by principle component analysis (PCA) was performed aiming at prediction of the application potential of different lignins for binder production. The lignins and their fractions could be classified in different clusters based on their properties, which are structure dependent. Kraft softwood lignins show the highest potential for plywood binder application followed by hardwood soda lignin and the fractions of Sarkanda grass soda lignin with medium molar mass. Expectedly, the softwood lignins contain the highest number of reactive sites in ortho positions to the phenolic OH group. Moreover, these lignins have a low level of impurities and medium molar mass.

Biorefining of wheat straw using an acetic and formic acid based organosolv fractionation process

To assess the potential of acetic and formic acid organosolv fractionation of wheat straw as basis of an integral biorefinery concept, detailed knowledge on yield, composition and purity of the obtained streams is needed. Therefore, the process was performed, all fractions extensively characterized and the mass balance studied. Cellulose pulp yield was 48% of straw dry matter, while it was 21% and 27% for the lignin and hemicellulose-rich fractions. Composition analysis showed that 67% of wheat straw xylan and 96% of lignin were solubilized during the process, resulting in cellulose pulp of 63% purity, containing 93% of wheat straw cellulose. The isolated lignin fraction contained 84% of initial lignin and had a purity of 78%. A good part of wheat straw xylan (58%) ended up in the hemicellulose-rich fraction, half of it as monomeric xylose, together with proteins (44%), minerals (69%) and noticeable amounts of acids used during processing.

Wood preservation by low-temperature carbonisation

Pine (Pinus sylvestris L.) wood with dimensions (100 x 10 x 10mm) was thermally treated at 275degreesC in a muffle oven to impart resistance to microbial degradation. Low-temperature carbonised pine resulted in a visually homogeneously treated product with a substantial (about 70% w/w) reduced non-cellulosic carbohydrates content, as compared with untreated pine. These components have been removed or have been partly converted into hydrophobic polymerisation products, which results in reduced water absorption in air of different relative humidity (RH) for carbonised pine. The flexural strength at high RH and after immersion in water of carbonised wood is comparable with untreated and creosote impregnated wood. However, at lower RH the carbonised wood is less strong. Only trace amounts of polycyclic aromatic hydrocarbons (PAHs) and other phenolic compounds could be detected both in water leached from carbonised wood and untreated wood. Furans, mainly 5-hydroxymethylfurfural, have been demonstrated to leach in substantial amounts in the water from carbonised pine. An accelerated decay test in soil showed that low-temperature carbonisation of pine resulted in a product with considerably enhanced durability, as compared with untreated wood. Resistance of carbonised wood against environmental decay was similar or even better than creosote impregnated wood

Lignocellulose-Based Chemical Products

The use of plant biomass as starting material is one of the alternatives to reduce the dependency on fossil oil for transportation fuels and is the main alternative to replace petrochemicals. The biomass can be transformed into energy, transportation fuels, various chemical compounds and materials such as natural fibers by biochemical, chemical, physical and thermal processes. The fermentation and the chemical conversion of carbohydrates into value-added compounds has received increasing interest in the last decade, and in a biorefinery different advantages may be taken from both processes. However, the potential competition with food and feed applications and the consequent rise in feedstock prices is an important aspect to take into consideration. Therefore the use of lignocellulosic feedstocks (often referred to as second-generation feedstocks) is strongly advocated. In addition to carbohydrates also substantial amounts of lignin is produced when using lignocellulosic feedstocks. In this chapter the composition of lignocellulosic biomass is discussed followed by an overview of the most important pretreatment and fractionation technologies. Especially the effect of the different technologies on the subsequent fermentative/chemocatalytic conversions is addressed. The importance is illustrated by an overview of the most important commercial as well as anticipated chemical building blocks from carbohydrates and lignin with a special emphasis on the production of furan-based building blocks from carbohydrates and aromatic building blocks originating from lignin.

Toxic hazard and chemical analysis of leachates from furfurylated wood

The furfurylation process is an extensively investigated wood modification process. Furfuryl alcohol molecules penetrate into the wood cell wall and polymerize in situ. This results in a permanent swelling of the wood cell walls. It is unclear whether or not chemical bonds exist between the furfuryl alcohol polymer and the wood. In the present study, five different wood species were used, both hardwoods and softwoods. They were treated with three different furfurylation procedures and leached according to three different leaching methods. The present study shows that, in general, the leachates from furfurylated wood have low toxicity. It also shows that the choice of leaching method is decisive for the outcome of the toxicity results. Earlier studies have shown that leachates from wood treated with furfuryl alcohol prepolymers have higher toxicity to Vibrio fischeri than leachates from wood treated with furfuryl alcohol monomers. This is probably attributable to differences in leaching of chemical compounds. The present study shows that this difference in the toxicity most likely cannot be attributed to maleic acid, furan, furfural, furfuryl alcohol, or 2-furoic acid. However, the difference might be caused by the two substances 5-hydroxymethylfurfural and 2,5-furandimethanol. The present study found no difference in the amount of leached furfuryl alcohol between leachates from furfurylated softwood and furfurylated hardwood species. Earlier studies have indicated differences in grafting of furfuryl alcohol to lignin. However, nothing was found in the present study that could support this. The leachates of furfurylated wood still need to be

Functionality and physico-chemical characteristics of wheat straw lignin, BioligninTM, derivatives formed in the oxypropylation process.

Abstract The new organosolv process of wheat straw fractionation elaborated by Compagnie Industrielle de la matiere Vegetale (France) corresponds to the biorefinery approach, which allows separating cellulose, hemicelluloses, and lignin. The straw lignin (Biolignin™) is an attractive product, for which new applications are sought. In the present work, straw lignin (L) was converted into liquid lignopolyols via a batch reaction with propylene oxide (PO). The effects of the lignin content (L%) in the initial reaction mixture (L/(L+PO) on the oxypropylation process and the properties of whole lignopolyols and L/PO copolymers were studied. Almost complete disappearance of L-OHphen and L-COOH groups in copolymers was accompanied by an increase in the L-OHaliph groups’ content therein. The polydispersity (Mw/Mn) of all copolymers obtained decreased essentially compared to the reference. The extent of PO grafting onto OH groups increased with decreasing lignin content in the initial reaction mixture. At a lignin content of 15–30% in the initial reaction mixture, the lignopolyols fulfil the requirements of polyol polyethers for rigid polyurethane foam production. The further increase in the lignin content leads to the appearance of the non-liquefied fraction and the undesirable increase in the viscosity of the liquefied part.