Characterization and reactivity of Fe-[Al,B]MFI catalysts for benzene hydroxylation with N2O Original Research Article

The characteristics of Fe-[B,Al]MFI catalysts prepared by changing: (i) the gel composition during the hydrothermal synthesis (Si/Al and Si/B ratio) and (ii) other synthesis aspects (silica source, use of phosphoric acid to complex iron) were studied. The samples were investigated in terms of composition, structural (X-ray diffraction (XRD), Fourier transform-IR (FT-IR)) and morphological (scanning electron microscopy (SEM)) aspects and distribution of the iron species determined by UV–vis DR spectroscopy. The results were correlated with the reactivity of these samples (phenol productivity and selectivity during benzene hydroxylation with N2O). A linear relationship was observed between Si/B ratio and phenol maximum productivity. This indicates that the main factor determining the catalytic performances is the amount of B introduced in the zeolite which in turn depends on the gel composition during hydrothermal synthesis and the preparation method. The linear relationship could be interpreted considering that the role of B is related to its easier migration from framework to extra-framework position with respect to Al and therefore easier creation of defect sites during heat treatment. These defect sites are important, because during the initial stage of the catalytic reaction, iron ions migrate to these sites creating highly active iron sites responsible of the initial increase in the phenol productivity. The presence of Al in the gel influences negatively the catalytic performances, because both Al competes with B to be introduced in the MFI structure as T atom. Furthermore, the presence of Al as T atom determines a lowering of the selectivity to phenol with respect to both carbon oxides and carbonaceous deposits formation.