Oak Wood Pretreatment Strips Lignin to Build Stronger Green Fibers

Scientists in South Korea have found a way to take ordinary oak wood and strip away nearly all of its lignin, the tough natural polymer that gives trees their rigidity, in order to produce fine lignocellulosic fibers that could one day replace energy-intensive synthetic materials in everything from car panels to packaging. The study, published in the journal Advances in Industrial and Engineering Chemistry, describes a two-step process that combines chemical pretreatment with mechanical milling, and it reports lignin removal rates of up to 94.7 percent from oak biomass.

The research, led by Fei Wang and Tae Hyun Kim of Hanyang University together with colleagues at Namu BioChem Inc., addresses one of the central bottlenecks in the bioeconomy. Lignocellulose, the structural material that makes up the bulk of wood, straw, and other plant matter, is the most abundant renewable biomass resource on Earth. Yet its three main components, cellulose, hemicellulose, and lignin, are interlocked in a dense, chemically resistant architecture that has frustrated engineers for decades. Breaking that architecture apart efficiently, and without destroying the valuable components, is the key to turning wood waste into fuels, chemicals, and advanced materials.

The team chose oak wood for practical reasons. Oak contains roughly 25 to 30 percent lignin by dry weight, along with high cellulose and hemicellulose contents, making it a rich feedstock. It also absorbs relatively little water, which means it reacts predictably even when processed at high solid loadings, and it is commercially available in more than 44 countries, giving any resulting process a secure supply chain. Oak powder with a particle size between 35 and 60 mesh was used as the starting material for all experiments.

The researchers tested two very different chemical strategies. The first was an organosolv approach, in which the oak powder was treated with a solution of 60 percent ethanol and 0.25 percent sulfuric acid. This combination is attractive because ethanol is cheap, relatively non-toxic, miscible with water, and easy to recover, while dilute sulfuric acid is effective at removing hemicellulose. The second strategy used alkaline pretreatment with sodium hydroxide solutions at concentrations of 2.0 and 5.0 percent by weight. Alkaline methods are prized for their selectivity: sodium hydroxide attacks the ester and ether bonds that tie lignin to hemicellulose within the lignin-carbohydrate complex, cleaving benzyl ether linkages and ferulic acid ester bonds while leaving much of the cellulose intact.

Reactions were carried out in small stainless steel batch reactors immersed in an oil bath, with temperatures ranging from 120 to 180 degrees Celsius and reaction times of 30, 60, and 90 minutes. The results showed that temperature, not time, was the dominant variable. With the ethanol-sulfuric acid system, raising the temperature from 140 to 180 degrees Celsius increased the relative glucan content of the remaining solid from 51.3 to 82.6 percent, while xylan, a key hemicellulose sugar, fell from 17.6 to 3.7 percent and lignin dropped from 28.8 to 15.5 percent. Extending the reaction time beyond 60 minutes brought little additional benefit, likely because dissolved lignin began to re-deposit onto the cellulose surfaces, a well-known phenomenon that limits delignification efficiency.

The alkaline route proved far more powerful. At 2.0 percent sodium hydroxide, lignin removal barely responded to temperature until the reaction reached 180 degrees Celsius, where the lignin content of the treated solid fell from 27.8 percent at 30 minutes to 20.3 percent at 90 minutes. But at 5.0 percent sodium hydroxide, the critical temperature dropped to 140 degrees Celsius, and the lignin content of the solid residue plunged from 31.3 percent at 120 degrees Celsius to just 5.4 percent at 180 degrees Celsius. In the best conditions, the team removed up to 94.7 percent of the lignin originally present in the wood, while preserving a cellulose-rich solid that retained a tunable fraction of residual lignin.

That tunability is the study’s most intriguing feature. Rather than aiming for complete delignification, the researchers deliberately produced fibers with three different residual lignin contents: 9.7, 7.6, and 5.4 percent by weight. The pretreated oak was then subjected to colloidal milling, a mechanical fibrillation process in which the material was passed ten times through a precision ceramic grinding stone at 100-micrometer intervals, running at 1200 revolutions per minute. The resulting aqueous suspensions were centrifuged, freeze-dried, and stored as fine fiber powders. Because lignin content can be dialed in through the pretreatment conditions, the process offers a way to adjust the properties of the final material to match specific applications, from flexible films to reinforcing agents in composites.

The suspensions displayed a striking visual signature of their colloidal nature. When a laser beam was shone through them, a distinct Tyndall effect appeared, the same light-scattering phenomenon that makes sunbeams visible in misty air, confirming that nanoscale particles were uniformly dispersed in the water. Scanning electron microscopy revealed a hybrid morphology: fibrous cellulose networks interspersed with small spherical particles, which the authors interpret as lignin particles generated during milling and deposited around the fibers. Dynamic light scattering measured a major particle population centered near 462.5 nanometers, with an average diameter of 824.7 nanometers and more than 80 percent of particles falling between 300 and 600 nanometers. The polydispersity index of 0.492 indicated a relatively broad size distribution rather than a uniform one.

Spectroscopic and thermal analyses filled in the chemical picture. Fourier transform infrared spectroscopy showed a sharp decline in the peak near 1730 inverse centimeters, which corresponds to the ester bonds in hemicellulose, confirming that pretreatment removed and degraded that component. Changes around 1590 inverse centimeters, associated with lignin’s aromatic rings, indicated structural rearrangement of the remaining lignin, while variations in the broad hydroxyl band near 3320 inverse centimeters pointed to increased cellulose crystallinity and greater exposure of surface hydroxyl groups. Thermogravimetric analysis showed that pretreated samples decomposed rapidly between 300 and 400 degrees Celsius, consistent with their smaller particle size and higher surface area, but left more residual mass near 600 degrees Celsius than untreated wood, suggesting that the pretreatment thermally stabilized the lignin and promoted char formation.

The authors are careful to frame the work as a foundation rather than a finished technology. They note that the dispersion results should be read as preliminary evidence of property modulation, not conclusive proof of long-term colloidal stability, and they call for further study of the chemical and functional role of residual lignin within the fibril network. Still, the implications are considerable. Lignocellulosic fibers are lightweight, low-cost, recyclable, carbon-dioxide neutral, and gentle on processing equipment, and fine fibers can achieve tensile strengths comparable to or higher than high-strength low-alloy steel at a fraction of the density. Fiber-reinforced composites built from such materials are already used in aircraft, automobiles, and construction. If the alkaline pretreatment chemicals can be recovered and reused at scale, as the authors suggest, oak-derived lignocellulose fibers could become a sustainable ingredient for composites, coatings, and packaging, turning one of forestry’s most common byproducts into a precision-engineered material whose composition can be tuned almost as easily as the temperature of an oil bath.

Subject of Research: Chemical and mechanical pretreatment of oak wood to remove lignin and produce fine lignocellulosic fibers

Article Title: Chemical and mechanical pretreatment of oak wood for lignocellulosic fiber production

Article References: Wang, F., Kim, T. Y., Jin, S. B., Yoon, C., & Kim, T. H. (2025). Chemical and mechanical pretreatment of oak wood for lignocellulosic fiber production. Advances in Industrial and Engineering Chemistry, 1(1), Article 22. https://doi.org/10.1007/s44405-025-00025-w

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00025-w

Keywords: oak wood, lignocellulose, lignin removal, pretreatment, sodium hydroxide, organosolv, colloidal milling, nanocellulose, biomass, Tyndall effect, composites, renewable materials