Preparation of a nanostructured iron chromite spinel in the pure form and its catalytic activity for the selective oxidation of benzene to phenol: experimental and DFT studies

Selective oxidation of benzene to phenol in a single step is a long-standing challenge and has been a subject of potential interest for researchers. In this study, a nanostructured iron chromite spinel in the pure form was prepared, and it was found that this catalyst selectively oxidized benzene to phenol in a single step under mild and environmentally friendly conditions. The material was thoroughly characterized by XRD, Raman spectroscopy, BET, TEM, SEM, TPR, XPS and TGA. Under optimum reaction conditions, ∼71% benzene conversion with a phenol selectivity of ∼100% was achieved. The recyclability test was conducted and it was found that there was no significant loss of catalytic activity even after 5 recycles confirming the truly heterogeneous nature of the catalyst. For comparison, pure Cr₂O₃ and Fe₂O₃ were also prepared by the same hydrothermal method and characterized, and their catalytic activity results were also tested. DFT calculations showed that sp² C-H bond activation of benzene over FeCr₂O₄ (102) and Cr₂O₃ (102) surfaces required an activation energy of 133.6 kJ mol⁻¹ and 144.2 kJ mol⁻¹, respectively, matching the experimentally calculated activation barrier of 114 kJ mol⁻¹ and 127.4 kJ mol⁻¹, obtained from kinetic studies.