Polyimides as metal-free catalysts for organic dye degradation in the presence peroxymonosulfate under visible light irradiation

Polymer based materials, polyimides (PI) were synthesized by direct thermal condensation of melamine and pyromellitic dianhydride and characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transformed infrared spectroscopy, N2 adsorption/desorption isotherms and X-ray photoelectron spectroscopy. The materials were then applied as catalysts for organic dye degradation under visible light irradiation in the presence of peroxymonosulfate (PMS). The results revealed that the metal-free catalysts can activate PMS to generate reactive radicals under visible light irradiation, leading to the efficient degradation of acid orange 7 (AO7) and other organic dyes in aqueous solution. Recycling of the catalyst indicated no obvious deactivation during the entire catalytic reaction, suggesting the good stability of metal-free polymeric photocatalysts. The intermediate products after AO7 degradation were analyzed by UV-vis spectra, ESI-MS and GC/MS. The effect of operation parameters such as the concentration of the catalyst, PMS and organic dye, and initial solution pH on the catalytic activity was also investigated. This study demonstrated a promising polymer material for organic dye degradation with PMS under visible light irradiation.