Integrated fractionation and templating of lignin using magnesium for sustainable and tailored porous carbon

Abstract

The heterogeneity of lignin presents challenges in fabricating lignin-derived carbon materials with well-defined structures. This study investigated the use of magnesium as a fractionating agent via precipitation to modify the heterogeneity of lignin, which was then integrated with the MgO templating technique to produce functionalized porous carbon with tailored porosity. First, alkaline-dissolved spruce kraft lignin was precipitated under varied pH conditions, resulting in magnesium–lignin (MgL) complexes with different degrees of higher molecular weight lignin fractions. These complexes underwent a pre-heating treatment at 700 °C, facilitating the formation of nano-sized MgO domains within the carbon matrix. Acid leaching was subsequently carried out, followed by a high-temperature post-heating treatment at 1400 °C to remove the magnesium oxide (MgO), leaving behind nano-sized porous structures. The variation in fractionation pH affected the form of magnesium in the MgL complex, influencing MgO crystal formation during pyrolysis and allowing for tunable pore sizes in the resulting carbon during templating. Additionally, the presence of magnesium enhanced carbon yields after heat treatment at 1400 °C, with stoichiometric optimizations involving magnesium and pH increasing yields from 35% to 49%. These findings establish the groundwork for precise and more sustainable design of lignin-derived porous carbon materials.