Highly efficient de-lignification and reengineering approach to modify wood into a bio-composite and an adsorbent material

Ramon Filipe Beims, The University of Western Ontario

Abstract

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.