Touch wood: Conversion of lignin to dicarboxylic acids using biochar-based solid-acid photo-Fenton catalysts

The depolymerisation of lignin into dicarboxylic acids (DCA) by the Fenton oxidation process is a promising approach to valorising lignin. However, Fenton oxidation is challenging due to the demand for acidic reaction conditions and homogeneous Fe2+ catalysts, which cannot be recovered. Herein, we fabricated a biochar-based solid-acid catalyst (SBC) from the pyrolysis of rice straw biomass followed by H2SO4 treatment. This novel catalyst achieved an enhanced catalytic performance in the activation of H2O2 under circumneutral pH, visible-light irradiation and Fe-free conditions. Under optimum conditions, a cumulative yield of 40.1 μ mol. gcat−1. h−1 of malonic, maleic, fumaric, succinic and gallic acid was obtained using 40 mM H2O2, 1 g/L catalyst loading and 4 h reaction time, which is comparable to yields reported in the literature under elevated temperatures and pressures. The activity of SBC stems from the synergy of oxygenated surface functional groups and persistent free radicals (PFRs), which can chemisorb and subsequently activate H2O2 and dissolved O2 to yield •OH and 1O2. Addition of NaN3 scavenger for 1O2 enhanced the cumulative DCA yields to 282.4 μ mol. gcat−1.h−1, suggesting that 1O2 is the main reactive oxygen species in the reaction. Finally, the SBC catalysts remained stable at the said reaction conditions and could be recycled without significant deactivation. After five cycles, the lignin conversion and cumulative DCA yields decreased by 19.7% and 23% respectively. These results show that Fe-free, visible-light Fenton-like catalytic oxidation of lignin with acid-modified biochars is a promising way to produce high-value platform chemicals.