Spectroscopic characterization of iron nanoparticles in Fe-mesoporous silicate catalysts

Iron-containing mesoporous silicates used as catalysts for the alkylation of benzene with benzyl chloride were characterized using several spectroscopic techniques. Electron spin resonance and Mössbauer spectroscopies, along with electron microscopy and X-ray diffraction, allowed differentiation of several iron species. These species correspond to (i) hematite (α-Fe2O3) particles, (ii) very small “isolated” or oligomeric FeIII species possibly incorporated in the mesoporous silica wall, and (iii) FeIII oxide clusters either isolated or agglomerated, forming “rafts” at the surface of the silica and exhibiting ferromagnetic ordering. Because of their agglomeration, these clusters appear with a two-peak size distribution, with one peak (ca. 2–3 nm diameter) corresponding to the isolated clusters formed in the mesopores and still embedded in them and the other (ca. 10 nm) corresponding to the agglomerates spread on the surface of the mesoporous silica particles. These species, which should be the most active species, appear similar to those recently observed at the surface of tunsgtated zirconia with iron promoter.