Eco-Friendly Synthesis of Proton-Exchanged Lignin-Based Carbon Solid Acid for Efficient Conversion of High-Concentration Fructose to HMF

Conventional sulfonation-derived carbon solid acids suffer from high energy consumption, equipment corrosion, and excessive acid usage. Here, a green proton-exchanged solid acid catalyst was developed to efficiently convert high-concentration fructose (15.6 wt %) into 5-hydroxymethylfurfural (HMF). The catalyst was synthesized by carbonizing a precursor formed from ammonium sulfate and sodium lignosulfonate (LS), followed by proton exchange. Carbonization at 350 °C preserved the −SO3Na functional group in the LS precursor, while ammonium sulfate promoted pore formation. The retained −SO3Na groups were subsequently converted to the −SO3H active sites of the catalyst by a proton-exchange reaction. The total acid density of the proton-exchanged solid acid catalyst reached 5.26 mmol/g. In the DMSO system, the proton-exchanged catalyst achieved 99% fructose conversion and 82% HMF yield under high fructose concentration. In addition, it maintained stable activity over 6 cycles. The strategy of incomplete carbonization and proton exchange of LS not only maintains the catalytic performance but is also a green and sustainable alternative to the sulfonation method.