Polyimide-TiO2 hybrid photocatalysis: Visible light-promoted selective aerobic oxidation of amines

Polyimides are a class of high-performance polymers with good thermal and chemical stability. One specific polyimide can be formed via thermal polymerization of melamine (MA) and pyromellitic dianhydride (PMDA) precursors. Herein, polyimide-TiO2 hybrids were straightforwardly synthesized with a mixture of MA, PMDA and TiO2 by annealing at 325 °C. The crystal phase of anatase TiO2 can be preserved and polyimide can be formed on the surface of TiO2 to enable visible light absorption and high specific surface areas for the polyimide-TiO2 hybrid. Next, this polyimide-TiO2 hybrid as a photocatalyst were successfully applied for visible light-promoted selective aerobic oxidation of amines into imines under irradiation of 460 nm blue light-emitting diodes (LEDs). With 4 mol% of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as the co-catalyst, polyimide-TiO2 hybrid photocatalyst reached 8 times of that of pristine TiO2 for the selective aerobic oxidation of benzylamine. Overall, the cooperative photocatalysis of polyimide-TiO2 hybrid and TEMPO was demonstrably efficient for visible light-promoted selective oxidation of amines with aerial O2 as the oxidant. Moreover, by means of reactive oxygen species (ROS) quenching experiments and in-situ electron paramagnetic resonance (EPR) spectroscopy, superoxide anion radical (O2−) was certified as the crucial ROS in coupling polyimide-TiO2 hybrid photocatalysis and TEMPO catalysis. This work suggests that hybridization of metal oxide semiconductor with polymer like polyimides is viable to forge visible light photocatalysts.