Doctor of Philosophy
Chemical and Biochemical Engineering
Xu, Chunbao (Charles)
In this doctoral study, high-value bio-based materials, including a light-weight and high-strength wood-based composite material and a bio-based adsorbent material for removal of heavy metals in water, were developed from natural wood by a novel reengineering approach. In this approach, natural wood was first fractionated into a porous de-lignified wood and lignin. The porous de-lignified wood was then composited by impregnation of an epoxy resin synthesized with the lignin extracted from wood.
The wood-based composite preparation consisted of three main steps: (i) de-lignification of natural wood, (ii) lignin modification and epoxidation, and (iii) impregnation of the de-lignified wood with lignin-based epoxy resin. An ultrasound-assisted organosolv fractionation was employed, resulting in a de-lignified pine wood with significantly low lignin content (approx. 4 wt.%) and 24 wt.% yield of lignin (85% purity) at the best fractionation condition (formic acid (88%) as the solvent at a solid-to-solvent mass ratio of 1:10 (w/w), 80 °C, 6 h). The extracted lignin was modified through phenolation to increase its aromatic content and reduce molecular weight. The most significant parameters in the phenolation of lignin (reaction time, temperature, and phenol/lignin ratio) were optimized through a central composite design. Under the optimal conditions, the phenolated lignin has an aromatic content of 5.3 mmol/g, four times lower molecular weight, and hence markedly increased reactivity toward the synthesis of lignin-based epoxy resins. Different organic solvents were investigated, of which ethanol proved to be the best solvent for the epoxidation process. The produced lignin-based epoxy resins at a yield up to 112 wt.% have an epoxy content up to 13 g eq./100 g and curing activation energy similar to that of a petroleum-based epoxy resin. Finally, the de-lignified wood was impregnated with the lignin-based epoxy resin and hot-pressed to obtain a light weight and high-strength wood-based composite material. The produced wood-based composite have a tensile strength of 225 MPa with a Young’s modulus of 25 GPa and a flexural strength and modulus of 375 MPa and 18 GPa, respectively. Moreover, the low density of the material (around 1050 kg/m3) results in specific mechanical properties superior to many metal alloys, hence with promise as a renewable functional material for structural applications.
In addition, the de-lignified wood was functionalized into a bio-based adsorbent material with high adsorption capacity for removal of heavy metals in water. For a simulated wastewater containing 500 mg/L Cu2+, the adsorption capacity of the bio-based adsorbent attained 0.26 mg/g.
The key academic contribution of this study is the development of a novel reengineering approach to producing wood-based composite materials for structural application and a bio-based adsorbent material for environmental application. This approach avoids drawbacks of conventional methods for wood functionalization, involving typically a lengthy and costly wood de-lignification process and lignin as a waste stream. The wood-based composite has elevated mechanical properties, superior to many metal alloys, and is thus a promising renewable functional material for structural applications. Moreover, a lignin valorization route has been demonstrated in this study, which is of great industrial significance too.
Summary for Lay Audience
This doctoral study aimed to investigate novel applications to wood-based materials, such as bio-composites and adsorbent materials. Although wood has been used by humankind for millennia, its relatively low mechanical strength limits its application to furniture and low-rise buildings. Alternatively, innovative wood-based materials are becoming increasingly significant in advanced engineering applications as our society moves to a more bio-based and sustainable economy and less dependent on petroleum-based materials.
The bio-based composite preparation consisted of three steps: (i) ultrasound-assisted de-lignification, (ii) lignin modification and epoxidation, and (iii) de-lignified wood impregnation with lignin-based epoxy resin. Wood is comprised of three main components: cellulose, hemicellulose, and lignin. By extracting lignin, wood becomes a porous material with a unique honeycomb-like structure. Lignin, as an aromatic polymer, can be further utilized in the synthesis of epoxy resins. However, the lignin complex molecule limits its reactivity towards epoxidation. A phenolation reaction was employed, breaking down lignin’s molecule and increasing its aromatic content, thus increasing its reactivity. Phenolated lignin was used in the synthesis of lignin-based epoxy resins. Different organic solvents were investigated, of which ethanol promoted the epoxidation process. The produced lignin-based epoxy resins showed curing process characteristics close to conventional (petroleum-based) epoxy resin. Finally, de-lignified wood was impregnated with the lignin-based epoxy resin and hot-pressed to obtain a high-strength and light-weight bio-composite material.
Additionally, the multi-functionality of de-lignified wood was demonstrated in the preparation of adsorbent materials for the removal of heavy metals from water. Natural wood and de-lignified wood were functionalized to increase the number of carboxyl functional groups, which have strong affinity towards heavy metal ions. The renewable adsorbent material was tested targeting the adsorption of copper ions in water. Functionalized de-lignified showed promising results as a renewable adsorbent material with higher adsorption capacity towards copper than natural wood.
The main academic contribution of this study is to propose alternative methods to produce advanced wood-based materials. The reengineering approach taken in the wood-based composite preparation avoids typical drawbacks of functionalized wood preparation, such as a lengthy de-lignification process and waste of lignin extracted from wood. The wood-based composite has elevated mechanical properties, superior to many metal alloys, and is thus a promising renewable alternative for structural applications. Moreover, a lignin valorization route is presented, which is of industrial significance.
Beims, Ramon Filipe, "Highly efficient de-lignification and reengineering approach to modify wood into a bio-composite and an adsorbent material" (2022). Electronic Thesis and Dissertation Repository. 8835.
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