Extraction of Lignin with High β-O-4 Content by Mild Ethanol Extraction and Its Effect on the Depolymerization Yield

This method answers some key questions in the renewal resources utilization field about lignin, a key component of lignocellulosic biomass. The main advantage of this technique is that it allows you to obtain lignin of good structural quality from different biomass sources. This noted methodology specifically targets the beta 04 linkage.

At the same time, it prevents recondensation processes from happening. This is very important as it prevents char formation. Using this method, relationships can be established between the lignin extraction procedure and the de-polymerization efficiency.

Generally, any of us new to these methods, we struggle with the workout procedures as well as the analysis of the four lignins, and the de-polymerization product mixture. To produce cut walnut shells, equip a hammer cutter with a five millimeter siv at the outlet, and feed the walnut shells into the hammer cutter, collecting the fractured shells in a one liter glass beaker. To produce the small walnut shell fragments required for milling, feed the fractured shells into a micro hammer cutter equipped with a 2 millimeter siv at the outlet, collecting the ground shells in a new one liter glass beaker.

To remove extractives from the ground walnut shells, add 150 grams of the shell sample into a 500 millimeter round bottom flask containing a stir bar, and add 200 milliliters of toluene to the flask. Attach a reflex condenser to the flask, and heat the mixture to a reflux temperature of 111 degrees celsius in an oil bath with vigorous stirring. After two hours, remove the flask from the bath to allow the mixture to cool down to room temperature, and strain the mixture through a 185 millimeter diameter, ten micrometer pore sized filter to remove the toluene.

After overnight heating in a vacuum oven at 80 degrees celsius and 50 millibars of pressure, add 40 grams of toluene treated walnut shell particles to a 250 milliliter zirconium dioxide grinding bowl containing seven 20 millimeter diameter zirconium dioxide grinding balls. 60 millimeters of isopropanol is added to the grinding bowl. Place the grinding bowl in the rotary bowl mill.

Grind the shell particles in four cycles of two minutes grinding at 27 times g followed by four minutes of rest per cycle. After the fourth grinding cycle, transfer the finely ground walnut shells into a 500 milliliter round bottom flask and remove the isopropanol by rotary evaporation at 40 degrees celsius and 125 millibars of pressure. Then dry the walnut shells in a vacuum oven overnight at 50 degrees celsius and 50 millibars of pressure.

To obtain ethanosolv lignin with the highest beta 04 content, add 25 grams of feedstock into a new 500 milliliter round bottom flask and add 200 milliliters of an 80 to 20 ethanol to water solution, four milliliters of 37%hydrochloride solution, and a magnetic stir bar to the flask. Attach a reflux condenser to the flask and heat the mixture in an oil bath at 80 degrees celsius for five hours with vigorous stirring. After the mixture has cooled to room temperature, filter the solution through a 185 millimeter diameter ten micrometer pore sized strainer into a new 500 milliliter round bottom flask, washing the residue four times with 25 milliliters of ethanol per wash.

Concentrate the collected liquor by rotary evaporation at 40 degrees celsius and 150 millibars of pressure, followed by dissolution of the obtained solid in 30 milliliters of acetone. To precipitate the lignin, add the mixture to 600 milliliters of water and collect the precipitated lignin in a 185 millimeter diameter, ten micrometer pore sized strainer, washing the lignin four times with 25 milliliters of water per wash. Airdry the lignin overnight at room temperature in the filter, followed by overnight drying in a vacuum oven at 50 degrees celsius and 50 millibars of pressure in a glass vial.

Then determine the yield of extracted lignin on a balance. For analysis of the lignin structure by two-dimensional nuclear magnetic resonance or NMR analysis, dissolve 60 milligrams of dried lignin in 0.7 milliliters of deuterated acetone, and add this mixture to an NMR tube. Add this NMR tube to an NMR spectrophotometer and set the desired parameters to obtain a suitable 2D proton hetero-nuclear single quantum coherence spectrum.

For lignin de-polymerization, add 50 milligrams of dried lignin into a 20 milliliter microwaveable reaction vial equipped with a magnetic stirrer and add 0.85 milliliters of 1, 4 dioxane, 50 microliters of ethylene glycol in 1, 4 dioxane, and 50 microliters of octadecane in 1, 4 dioxane to the vial. After closing the vessel, heat the solution to 140 degrees celsius with stirring. When the reaction vessel reaches the target temperature, add 50 microliters of iron three trifluoromethanesulfate in 1, 4 dioxane to the vessel, and stir the reaction for an additional 15 minutes.

After cooling the reaction to room temperature, filter the liquid over celite into a two milliliter centrifuge tube. Then concentrate the collected liquid overnight at 35 degrees celsius in a rotational vacuum concentrator. To extract the low molecular weight products, suspend and swell the residue in 0.15 milliliters of dichloromethane by vortexing, 15 minutes of sonication, and 30 minutes in an automatic wheel.

To perform an efficient monomer extraction, it is critical to first properly swell the obtained oil in the chloromethane. Then it is important to add the right amount of toluene in order to selectively precipitate the underside oligomers. Precipitate the oligomers with 0.75 milliliters of toluene followed by vortexing and ten minutes of sonication.

Centrifugate the samples for ten seconds at 5000 RPM and separate the light organic liquid from the solid thick oily residue and filter the liquid over a plug of celite into a glass vial. Then concentrate the combines organic phases by rotary evaporation at 40 degrees celsius and 20 millibars of pressure. And dissolve the oily residue in one milliliter of dichloromethane for analysis by gas chromatography.

The lignins obtained after each extraction exhibit a wide range of colors and particle sizes, with the polymers obtained from mild treatments demonstrating a typically red-pink color and small flakes of material. When harsher conditions are applied, the obtained lignins exhibit a brown to brownish-yellow color, with an overall increase in yield of material, an effect that was much more profound for walnut, beech, and cedar woods, compared to pine wood. NMR analysis of the different lignins determines the SGH ratio and the amount of linkages of the extracted polymers, specifically the amount of beta 04 linkages.

For example, lignins extracted at milder conditions typically give higher amounts of linkages. Acetolysis reactions with iron three trifluoromethanesulfate in the presence of ethylene glycol yields three different phenolic acetals that relate to the S, G and H units present within the lignins, with higher combined phenolic acetal yields observed for lignins extracted under mild conditions. A clear trend is visible when the total beta 04 content is considered wherein the higher beta 04 content generally results in a higher acetal yield.

When the lignin extraction yield and subsequent de-polymerization yield are combined, an overall acetal yield is obtained from the biomass source showing clear differences between the different biomass sources. While performing lignin extractions, it's important to balance the extraction yield with the structural quality of the obtained lignin. So if you follow the reported de-polymerization method, you can actually obtain value-added aromatics in high selectivity.

These techniques will help other researchers gain access to value-added products from the lignin fraction of lignin cellulosic biomass in good yields.