Hydroxylation of Benzene and Phenol on Zeolite Catalysts

Phenol and dihydroxybenzenes (DHB) as industrially important fundamental chemicals are generally prepared through oxidative hydroxylation of benzene/phenol on zeolite-based catalysts. Fe-MFI zeolite shows high selectivity to phenol in gas-phase oxidation of benzene with dinitrogen monoxide. The different types of Fe species, such as Fe(II) sites in mononuclear species, oligonuclear species with at least two oxygen-bridged Fe(II) sites, and Fe(II) sites with Fe 2 O 3 nanoparticles, play vital roles in determining the catalytic performances. Furthermore, oxidized products of DHB are generally generated through liquid-phase reactions using Fe- or Ti-containing zeolites. The reaction mechanism is highly dependent on the types of catalysts: (1) on Fe-containing zeolite, the Fenton reaction proceeds, producing the hydroxyl radicals, which subsequently react with phenol to form the DHB with relatively equal amounts of catechol (CAT) and hydroquinone (HQ); (2) titanium in the framework of Ti-zeolite interacts with protonic solvent molecules to lengthen the TiO bond. The CAT is formed on the external surface, whereas the HQ is mainly produced inside the Ti-zeolite channels due to the steric restriction. It is deduced that the ratio of CAT/HQ can be adjusted through controlling the location of Ti active sites. Another pathway is proposed in non-protic solvent, where the Ti sites would interact with OH from phenol molecules. The six-membered transition state is formed between the Ti peroxo species and phenol, further resulting in the formation of CAT. In addition, the high selectivity to CAT can also be obtained using Y zeolite supported with transition metal.