4,4'-Bipyridine-modified molybdovanadophosphoric acid: A reusable heterogeneous catalyst for direct hydroxylation of benzene with O₂

An organic-inorganic hybrid heterogeneous catalyst Bipy₂PMoV₁ was prepared by combining 4,4'-bipyridine (Bipy) with Keggin-structured molybdovanadophosphoric acid (PMoV₁). The catalyst was characterized by elemental analysis, fourier transform infrared spectroscopy, X-ray diffractometer, electron spin-resonance spectroscopy, scanning electron microscope and energy dispersive X-ray spectrometry. Catalytic performances of the powdered catalyst Bipy₂PMoV₁, as well as various control samples, were assessed in direct hydroxylation of benzene to phenol by O₂ with a pressured titanium batch reactor. Bipy₂PMoV₁ shows a relatively high phenol yield of 7.8% with 100% selectivity based on gas chromatography analysis at the following optimized reaction conditions: 22.5mmol benzene, 0.2g catalyst, 4.55mmol ascorbic acid, 25mL aqueous acetic acid solution (60vol%, solvent), 2.0MPa O₂, 373K and 10h. Moreover, catalytic recycling tests demonstrate that the heterogeneous catalyst exhibited high potential for reusability. It is revealed that the organic modifier 4,4'-bipyridine not only plays an important role in the formation of heterogeneous system but also is crucial for the good structural stability in recycling of the heterogeneous catalyst. The results indicate that the modification of heteropoly acid with suitable organic group is one efficient way to prepare new heterogeneous catalyst for the direct hydroxylation of benzene.