One-step oxidation of benzene to phenol with nitrous oxide over Fe/MFI catalysts

The one-step oxidation of benzene to phenol with nitrous oxide has been studied over a variety of Fe/MFI catalysts at 400 and 450 °C. The turnover frequency (TOF) is found to strongly increase with decreasing Fe loading in accordance with the model of mononuclear Fe sites. The TOF is highest for steamed H-MFI catalysts with an iron impurity of Fe/Al =0.014. A large increase in phenol yield upon steaming H-MFI at 650 °C indicates that Fe ions are transferred from lattice to extralattice positions. Treating such catalysts with H2S results in a dramatic decrease of the selectivity to phenol, proving the crucial role of Fe ions. A small residual activity for phenol formation after sulfidation could indicate a different reaction path of low rate or regeneration of Fe sites by oxidation of sulfide groups. In general, three types of Fe sites have been identified in Fe/MFI catalysts: (1) mononuclear Fe, catalyzing phenol formation, (2) dinuclear, oxygen-bridged Fe oxo-ions, catalyzing NOx reduction, and (3) Fe oxide nanoparticles, catalyzing deep oxidation.