Preparation and Formation Mechanism of Renewable Lignin Hollow Nanospheres with a Single Hole by Self-Assembly

Lignin hollow nanospheres with a single hole were prepared through a straightforward self-assembly method, which included dissolving enzymatic hydrolysis lignin, a byproduct derived from biorefinery, in tetrahydrofuran and afterward dropping deionized water to the lignin/tetrahydrofuran solution. The formation mechanism and structural characteristics of the lignin hollow nanospheres were explored. The results indicated that the nanospheres exhibited hollow structure due to the effect of tetrahydrofuran on the self-assembly behavior. Hydrophobic outside surface and hydrophilic internal surface were formed via layer-by-layer self-assembly method from outside to inside based on π–π interactions. The chemical structure of lignin did not produce a significant change in the preparation process of lignin hollow nanospheres. With increasing of initial lignin concentration, the diameter of the nanospheres and the thickness of shell wall increased, while the diameter of the single hole, the surface area, and the pore volume of the nanospheres decreased. The surface area reached the maximum value (25.4 m2 g–1) at an initial lignin concentration of 0.5 mg/mL in setting concentration range. Increasing the stirring speed or dropping speed of water resulted in a decrease of the diameter of the hollow nanospheres. Moreover, an apparent change of the average diameter of the nanospheres was not observed after 15 days, and the nanosphere dispersions were stable at pH values between 3.5 and 12. The lignin hollow nanospheres with a single hole offer a novel route for a value-added utilization of lignin and would improve the biorefinery viability.