Green synthesis and applications of macroporous lignin carbon-based solid acids

Carbon-based solid acid catalysts via traditional sulfonation suffer from high energy consumption, severe equipment corrosion, and large acid dosage, which can be solved by proton exchange. In this study, a green macroporous lignin-based carbon solid acid catalyst LS a SS b H was developed. The catalyst's macroporous framework was co-constructed by sodium lignosulfonate (LS), sodium p-aminobenzenesulfonate, and ammonium sulfate via ice-templating, retained after 300 °C low-temperature carbonization. Incompletely decomposed -SO 3 Na groups in the precursor were converted to -SO 3 H active sites via proton exchange, replacing high-temperature sulfonation. This catalyst not only possesses a total acid density of 5.46 mmol/g and a sulfonic acid density of 0.36 mmol/g, but also features a macroporous structure ranging from 60 to 1000 nm. It exhibits excellent performance in catalyzing the conversion of high-concentration fructose (15.4 wt%) to 5-hydroxymethylfurfural (HMF) in DMSO system, achieving a maximum HMF yield of 86.9% under reaction conditions of 150 °C for 120 min, and maintaining a yield of over 70% even after six cycles. Combined with quantum mechanics calculations and molecular dynamics simulations, the formation mechanism of the macroporous carbon framework was clarified. Meanwhile, it was confirmed that the S and N heteroatom doping introduced by sodium p-aminobenzenesulfonate and ammonium sulfate had a positive regulatory effect on the catalytic reaction. Compared to the sulfonation method, solid acids obtained through proton exchange not only retain excellent catalytic activity but are also more environmentally friendly.