Solid protonic acids and luminescent carbon dots derived from waste expanded polystyrene

We reported a simple route to prepare strong solid protonic acids (SPAs) from waste expanded polystyrene using a small amount of sulfuric acids while stirring at 130 °C for 3 h followed by heating at 220 or 260 °C, respectively, for 2 h. Such SPAs were amorphous carbon materials. It was found that they were efficient and recyclable catalysts for acid-catalyzed esterification reaction. Meanwhile, luminescent carbon dots were found on the surface of the obtained SPAs at 220 °C. The photoluminescence emission peak position shifted from 415 to 542 nm as the excitation wavelengths progressively increased from 310 to 470 nm. The quantum yield was 13.2% at 310 nm excitation wavelengths.