Flexible Synthetic Strategies for Lignin-Derived Hierarchically Porous Carbon Materials

Abstract

Conversion of lignin to higher value-added materials, such as hierarchically porous carbon, is very important for improving economic feasibility of lignocellulosic biomass-based refinery processes. However, previous research on lignin-derived hierarchically porous carbon materials was conducted with kraft alkaline lignin (lignin produced from kraft pulping processes) or bioethanol lignin (lignin produced from industrial cellulosic bioethanol plants) separately, because organic and inorganic components and content of kraft alkaline lignin are significantly different from those of bioethanol lignin. In this study, we report a synthetic strategy of lignin-derived hierarchically porous carbon materials which is flexible toward types of lignin. We found that when KOH as a pore forming agent was added into kraft alkaline lignin, a NaOH/KOH eutectic mixture was formed by interaction between added KOH and intrinsic NaOH contained in kraft alkaline lignin, and mesopores as well as micropores were simultaneously formed by the NaOH/KOH eutectic mixture. However, when KOH was added into bioethanol lignin, only micropores were formed because of the absence of intrinsic NaOH in bioethanol lignin. Inspired by kraft alkaline lignin-derived carbon materials, NaOH as well as KOH were added into bioethanol lignin, which resulted in successful preparation of hierarchically porous carbon materials from bioethanol lignin. The hierarchically porous carbon prepared with kraft alkaline lignin (KIE-9) and bioethanol lignin (KIE-10) showed high surface area (1825, 2345 m2/g) and pore volume (1.72, 1.74 cm3/g). Such a synthetic strategy based on the NaOH/KOH eutectic mixture can be used commonly for both cases of using kraft alkaline lignin and bioethanol lignin as a carbon source, and is expected to improve the economic feasibility of the synthetic processes for lignin-based carbonaceous materials.