Phosphotungstic anion-paired quinoline salt for heterogeneous photocatalytic hydroxylation of benzene to phenol with air

Heterogeneous catalysts are preferred in production of chemicals due to the facile production separation and catalyst reusage. However, it is still one challenge to construct efficient heterogeneous catalyst for target reaction. Herein, functional polyoxometalates paired ionic salts (IL-POMs) were prepared through pairing quinoline cations with Keggin-type phosphotungstic (PW) anions. Long alkyl chain in the organic cations enabled the solid nature of these hybrids and endowed them as the recyclable heterogeneous photocatalysts. They exhibited high yield (20.9%) and selectivity (> 99%) in the benzene hydroxylation to phenol by using atmospheric air as the sole oxidant. The activity of the IL-POM hybrid is even higher than the quinoline salt precursor, thanks to the unique redox property of PW anions that promotes the generation, separation and transport of photoinduced carriers. After reaction, the catalyst was facilely reused with stable activity. The catalyst was also active and recoverable in the oxidation of benzyl alcohol to benzaldehyde with atmospheric air.