Sulfonated carbon catalysts, as a type of solid protonic acid, have been widely recommended for various applications. However, syntheses of them typically require strict conditions, posing challenges in efficiency and environmental impact. Herein, we report a rapid, acid-free method to introduce sulfonic and sulfate ester groups onto carbon nanotubes (CNTs) by simply stirring them in an aqueous sodium persulfate solution (Na₂S₂O₈) at room temperature. Within 45 min, the treated CNTs reached sulfur-containing acid densities up to 0.34 mmol g⁻¹ without thermal treatment and hazardous reagents. The resulting catalyst demonstrated effective performance in terms of cellulose hydrolysis, attaining 31.6% conversion and 23.2% glucose yield. The process requires only the energy input of magnetic stirring, underscoring its environmental and practical advantages. This simple approach provides a sustainable and cost-effective alternative for the preparation of carbon-based catalysts, offering significant potential for biomass conversion and other green-chemistry applications.