Direct oxidation of benzene to phenol over hierarchical ZSM-5 zeolites prepared by sequential post synthesis modification

Hierarchical zeolites were prepared by post synthesis treatments of commercial ZSM-5 zeolite (Si/Al = 13) involving alkali treatment, acid treatment, steam treatment, combination of acid-alkali treatments, and combination of steam-acid-alkali treatments. Various characterization techniques were employed to investigate the influence of each post synthesis treatment step on the structure, composition, porosity, acidity and iron species of the samples. The catalytic activity and selectivity of the post modified samples were also measured in the direct oxidation of benzene to phenol using nitrous oxide as an oxidising agent. The combination of steam-acid-alkali treatments of ZSM-5 zeolite exhibiting low Si/Al ratio lead to significant high level of mesoporosity with considerable preserved amount of microporosity that was not possible to achieve by the other treatment methods outlined above. ESR results reveal the homogeneous distribution of iron species when commercial MFI zeolite containing low amount of iron was steam treated. The modified iron species were not significantly altered by subsequent acid and alkali treatments, while the alkali treatment heals the destructed part of the zeolite that was produced by steam and acid treatments. These modifications in the pore structure and iron species led to excellent catalytic performance in the direct oxidation of benzene to phenol using nitrous oxide.