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

• Hybrid catalyst of 4,4′-bipyridine-molybdovanadophosphoric acid is prepared.
• The catalyst exhibits high phenol yield from hydroxylation of benzene with O2.
• The catalyst is heterogeneous with high potential for reusability.
• Two N atoms of bipyridine play a key role in heterogeneous catalytic performance.

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: 22.5 mmol benzene, 0.2 g catalyst, 4.55 mmol ascorbic acid, 25 mL aqueous acetic acid solution (60 vol%, solvent), 2.0 MPa O2, 373 K and 10 h. 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.

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