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Showcases of efficient lignin uses

Lignin has the reputation of being a very complicated raw material. The saying goes “you can make anything with lignin but money”. This situation has been changing during the last decades, multiple developments have been carried out that allow to realise new applications also besides the traditional markets that were mostly based on lignosulphonates. It is the goal of this section to highlight these new developments. Some are already commercial, others very close to it, still others need still some time. But common to all is that lignin can be used in simple and controllable ways which gives rise to new EFFICIENT applications.

Showcase 1: Hollow Lignin nanospheres used for drug delivery

Organic nanotechnology can be based on all types of carbon materials. Lignin being an aromatic material having multiple phenolic rings in its structure, it was in the beginning supposed that its structure was a main hindrance for efficient use of lignin in such technology. Nowadays these difficulties have been overcome and all types of nanosized bodies can be easily and efficiently produced also with lignin: irregular nanoparticles, but also nanofibers, hollow nanotubes, nanospheres and hollow nanospheres.

This showcase exhibits a research during which hollow lignin-based nanospheres of a diameter around 200 nm were produced, then charged with an anticancer drug which was successfully released under controlled conditions. Lignin has better biocompatibility than many other possible carrier substances. In the work, it was shown that lignin has no cell-toxicity (the so-called cytotoxicity).

However, it was not pure lignin that was used in this research. In order to attract and fix the anti-cancer drug inside the lignin spheres it was necessary to graft another molecule, β-cyclodextrin, on the lignin prior to the formation of the nanospheres. There grafted molecules improved also the properties, like surface area and pore volume, of the so produced nanoparticles by about 25%.

The principle for producing such nanospheres is very simple. Lignin is first dissolved in THF (Tetrahydrofuran) a very current organic solvent. The desired nanoparticles form when water is added under very specific conditions and creates a precipitate that has the wanted nanostructure, in this case a hollow spherical particle. It is all very simple technology.

The nanospheres could be charged with up to 25% of the anticancer drug. The release was triggered by the pH of the tumor or intracellular microenvironment (pH: 5.5) as well as a physiological (pH: 7.4) environment. Release rates of up to 62% over 60 hours were obtained.

By this research, not only the feasibility of production of hollow nanospheres was demonstrated but in the same time an application of those for controlled release of an anticancer drug under body typical conditions was validated.

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Showcase 2: Lignin-based organic flow batteries, a key technology for the energy transition

The energy system is subdivided into generation, distribution and consumption sections. But the crucial forth section is energy storage. Production and consumption are most of the time out of phase. Therefore, an efficient storage of huge quantities of energy is the key point for the energy transition.

A diverse range of stationary applications are needed for onshore and offshore wind parks, for large-scale PV plants, for conventional power station locations, for electromobility providers and for operators of residential districts. Battery systems for ships featuring electric propulsion are developed and sold in the maritime segment. As far as usage is concerned, huge quantity storage is the basis for self-consumption enhancement, demand peak capping, interruption-free power supply, electromobility charging infrastructure and standalone operations.

Close to everyone’s future is the problem of how to charge electrical vehicles. The adoption of electromobility requires the comprehensive provision of charging points. A particular challenge is being able to fast-charge several electric vehicles in parallel, a process that is frequently hindered by existing delivery rates. An organic flow battery makes simultaneous fast-charging feasible.

In a battery, during the reduction process the electrolytes gain electrons, while during oxidation they lose electrons. Both chemical reactions have to occur separately from one another in the anode and cathode spaces of the energy converter. During the charging and discharging processes, the electrolytes – posolyte and negolyte – are pumped continuously through the stack’s cells, hence the term “flow”. ”Organic” refers to the fact that the electrolyte is an organic liquid. The most promising technology for that is the one based on lignin which is developed by the German company CMBlu.

Everybody knows lithium batteries which cannot be replaced by organic flow batteries at small scale applications. However, for the storage of huge and very huge quantities of energy which is the most important aspect for the use of renewable energies, (lignin based) organic flow batteries are less costly and in addition more sustainable regarding the use of resources. Their operation is more secure and stable. No problematic raw materials are used. Supply bottlenecks and cost explosions are not expected. The disposal is ecological, as most of the components are easily recycled and as no problematic waste products are generated.

Showcase 3: Lignin-based fungicide carrier based on nanotechnology goes commercial

For grapevines and other woody plants sicknesses caused by fungi, bacteria and viruses are a global challenge that leads increasingly to important economic losses. An example is the Esca-sickness in grapevines which is caused by a complex of wood-destroying fungi. Esca-fungi enter the plant through wounds created by the cutting of the vines. The fungi penetrate the vascular system of the plants and destroy the vines from the inside. Presently there is no efficient fungicide with systemic action against Esca. Treatment from the outside by spraying is not effective. Worldwide damage by Esca is estimated to be 1.5 billion USD and is expected to further increase in the coming years because of climate change and the connected increasing stress on plants.

Lignilabs GmbH in Mainz (Germany) has developed a platform technology that can encapsulate various substances in lignin nano- and microparticles. Among other applications like polymer additives and 3D printing, the use of this technology on vine stocks allows treating Esca sickness. In this process, an aqueous suspension of lignin carriers (hollow nanospheres) filled with an adapted fungicide is directly injected into the plant which allows for treatment of the sickness at the place of its origin. The product with the commercial name ESCApe acts instantaneously against Esca-fungi and acts preventively on healthy vines to avoid falling sick of the treated vines. ESCApe is active against a large spectrum of wood-destroying fungi and can potentially also be used against pathogens on fruit trees, shrubs, ornamental plants, or in the forest.

The application of ESCApe takes place by a technology specially developed for this purpose (see picture) in the course of which the ESCApe suspension is injected in minimal dosage (0.8 ml) into a drill hole (Ø 6mm, 35mm long) oblique to the vine trunk. Once injected the product distributes in the plant’s vessels and the lignin is degraded by the enzymes produced by the fungus which liberates the pesticides. The fungus destroys, therefore, itself through its enzymes. In addition to the instantaneous treatment effect, there is also a preventive action of 3 to 5 years. Thanks to lignin and its unique properties applied in a skillful way, the production and the application of ESCApe are therefore completely sustainable, user and environmentally friendly.

Showcase 4: Breakthrough for sustainable aviation fuel from lignin

Sustainable aviation fuel (SAF) is essential to decrease the carbon footprint of the aviation industry. Although many strategies have been developed to provide the linear and branched aliphatic components of SAF, few viable strategies have been demonstrated to supply the aromatic and cycloalkane fraction of the SAF at the necessary scale from bio-based feedstocks. A team in the U.S.A. has successfully addressed this challenge by developing a process that is based on a commonly occurring catalyst.

The team involving the National Renewable Energy Laboratory (NREL), the Massachusetts Institute of Technology (MIT), and Washington State University uses lignin for the production of the SAF. The two main challenges are finding an effective catalyst and to remove the oxygen from the lignin. The lignin oils in existing research projects had an oxygen content of 27 to 34 %. For aviation fuel, however, this value has to be lower than 0.5 %. Until now catalysts were used that contained expensive precious metals which in addition showed only low efficiency.

The successful new approach consists of a continuous, two-stage catalytic process using molybdenum carbide to deoxygenate lignin into aromatic hydrocarbons with 87.5% selectivity toward aromatic hydrocarbons at 86% of the theoretical carbon recovery. Tier α fuel property testing indicates that the SAF-range lignin-derived aromatic compounds are likely performance-advantaged across multiple properties relative to conventional jet fuel aromatic compounds. This work demonstrates an effective approach to converting lignin into aromatic SAF blendstocks.

The team has published its process in the journal “Joule” (Continuous hydrodeoxygenation of lignin to jet-range aromatic hydrocarbons - ScienceDirect). In the paper, the necessity of using SAFs is highlighted, as the aviation industry has engaged in a drastic reduction of carbon dioxide emissions. At the same time, the consumption of aviation fuels is strongly growing. It is said that the reduction of carbon dioxide emissions can only be achieved with the massive use of SAFs.

Showcase 5: Up to 70% polyol replacement in polyurethanes

Polyurethanes (PU) are polymers traditionally synthesized through reactions between polyols and diisocyanates, typically derived from petroleum. However, in recent decades, there has been significant interest in developing biobased PUs as part of efforts towards sustainable development. Lignin, among other renewable precursors, has emerged as a promising candidate due to its unique properties that complement polyurethane versatility. Lignin-based polyurethanes have been proposed for various applications, including elastomers, thermoplastics, adhesives, coatings, and foam formulations.

One area where lignin shows particular promise is in the production of rigid polyurethane foam (RPUF), which finds widespread use in building construction, automobiles, acoustics, machinery, and more. RPUF is especially valued as a lightweight insulation material due to its high insulation performance at minimal thickness and low density.

RPUF is typically synthesized by reacting two liquid components: (a) monomeric 4,4’ diphenylmethane diisocyanate (MDI) or polymeric diphenylmethane diisocyanate (pMDI) containing multiple isocyanate (–NCO) groups, with (b) a polyol blend containing polyols with multiple hydroxyl (–OH) groups. By carefully formulating these constituents along with flame retardants, crosslinkers, catalysts, blowing agents, and surfactants, optimizing the processing method, and controlling the cell microstructure, the resulting mechanical and thermal properties can be tailored to specific applications.

However, the main challenge in utilizing lignin in RPUF is its limited solubility in various polyols. Traditional research efforts have thus been limited to replacing only 20-30% of polyols, even with modified lignins. Bern University of Applied Sciences (BFH) in Biel, Switzerland, has now developed a new process for using lignin that allows up to 70% polyol replacement in RPUF while maintaining acceptable mechanical and thermal properties.

Showcase 6: High-performance concrete dispersants

Dispersants find wide application in suspending colloidal particles in various industries, including cosmetics, paints, pharmaceuticals, oil drilling mud, concrete, and ceramics. Recently developed by VTT, LigniOx technology offers a simple and cost-efficient alkali-O2 oxidation process to convert technical lignins into ready-to-use products for concrete plasticization. The LigniOx process can be integrated into biorefineries or operated as standalone units by the chemical industry. During oxidation, phenolic hydroxyl groups in the lignin polymer are activated, and carboxylic acidic groups are introduced while preserving the polymeric structure. Depending on oxidation conditions, particularly pH, lignin's negative charge and molar mass can be controlled. The dispersion effect likely occurs through adsorption on cement particles, leading to electrostatic repulsion.

To carry out the LigniOx process, lignin is dissolved in NaOH with a lignin content of 15 wt%. The solution is heated to 70 °C, and the reactor is pressurized with oxygen, with a reaction time of 30 minutes under efficient mixing. NaOH is continuously added to the reactor during the reaction to counteract the pH drop caused by acidic reaction products and to expedite the formation of anionic charges in lignin.

Comparing the performance of LigniOx to commercial superplasticizers, the results are as follows: for PCE 1, a commercial polycarboxylate ether, the mortar flow reaches 220 mm. Two other acrylic superplasticizers, PCE 2 and PCE 3, are less effective, allowing the mortar to flow only to 200 mm and 195 mm, respectively. The sulfonated naphthalene-based superplasticizer (SNF) increases mortar spread to only 180 mm. Two oxidized lignin dispersants produced by the LigniOx process demonstrate a strong dispersion effect, nearly matching the performance of the PCE 1 formulation.

Showcase 7: 50-100% phenol replacement in phenolic resins

The incorporation of lignin into phenolic resins has gained significant attention in recent years as a sustainable and eco-friendly alternative. Lignin possesses a phenolic structure that can be exploited for the production of phenol-formaldehyde resins. Lignin offers several advantages that render it an attractive option for enhancing phenolic resins. Integrating lignin into the binder components not only replaces a portion of the fossil-derived phenols but can also reduce formaldehyde emissions in certain applications during manufacturing and throughout the lifespan of wood panels. Moreover, the inclusion of lignin can enhance the thermal stability, flame retardancy, and mechanical properties of phenolic resins. Furthermore, economically, the utilization of lignin reduces production costs and diminishes fluctuations therein due to significant variations in the price of phenol.

The most common applications are in wood adhesives, particularly in plywood and OSB panels, as well as in high-pressure laminates. However, compared to pure phenols, lignin exhibits lower reactivity toward formaldehyde due to its high molecular weight and steric hindrance. To address this limitation, lignin currently only substitutes part of the resin’s phenol content, and it is typically treated to increase the phenol functionalities. Without modification of the lignin, phenol replacement levels of 20-50% can be achieved, depending on the resin types and applications.

However, several processes now enable higher replacement rates, achieving 50-100% phenol replacement. One such approach is the Catlignin process developed by VTT. This technology involves the thermal treatment of black liquor (at temperatures of 200-250°C), yielding highly reactive lignin. During the CatLignin process, significant demethylation and demethoxylation occur. The superior reactivity of CatLignin enables high phenol replacement levels, regardless of the wood species (50-70%), and facilitates the utilization of hardwood lignin in significant quantities in phenolic resins. Additionally, UPM, in collaboration with Chimar Hellas, has developed several lignin-based phenolic resins allowing for 80-100% phenol replacement. One approach involves the alkalization of lignin, wherein the lignin is heated at 75-95°C under alkaline conditions. This commercially available process has already been implemented in over seven countries, achieving a 60% replacement of phenol on an industrial scale. Prefere Resins has also implemented an industrial process allowing for 90% phenol replacement.

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