Performance of Fe-(Al, B) MFI catalysts in benzene hydroxylation with N2O. The role of zeolite defects as host sites for highly active iron species

The catalytic performances of Fe-[Al, B]MFI zeolites in benzene hydroxylation with N2O are studied as a function of the nature of the T atom (Al, B) and compared with the characterization of the nature of iron species determined by UV–vis diffuse reflectance spectroscopy. The results indicate that the mechanism for generation of iron species highly active in this reaction is the creation of hydroxyl nests in the zeolite framework due to framework to extra-framework migration of the Tatom. The process is favoured when B is present as Tatom, but occurs also in the case of Fe and Al as T atoms. Iron atoms migrate to these zeolite defect sites forming highly active Fe3+ isolated species in square pyramidal or highly distorted octahedral coordination. The process occurs during the catalyst pre-treatment as well as during the initial half an hour of time on stream, leading to an increase in the phenol productivity during this step. A model of these active sites is presented. The effect of the silica source in the preparation of these catalysts is also reported.