BiOx clusters occluded in a ZSM-5 matrix: preparation, characterization, and catalytic behavior in liquid-phase oxidation of hydrocarbons

A series of Bi-ZSM-5 containing 4.26, 5.54, and 6.64 wt% bismuth oxide was prepared by introducing Bi during ZSM-5 synthesis. For comparison, 5 wt% Bi (expressed as bismuth oxide)-impregnated ZSM-5 samples were also prepared. These samples were characterized by N2 adsorption–desorption isotherms at 77 K, XRD, XPS, IR spectroscopy in the lattice vibration range and in the presence of chemisorbed pyridine and CO2 and EXAFS. The characterization data confirmed the occlusion of bismuth oxide clusters in the zeolite pores. An intrinsic acidity due to bismuth oxide clusters was evidenced. This consisted, on one hand, in weak Lewis acid sites, ascribed to coordinatively unsaturated bismuth cations, and, on the other hand, in strong Brönsted acid sites, attributed to boundary Bi⋯OSi linkages. These catalysts were tested in the liquid-phase oxidation of benzene, toluene, and cyclohexane using hydrogen peroxide as oxidizing agent. The results showed that Bi-ZSM-5 catalysts are more active than the reported titanium silicalites, but the efficiency of H2O2 use was quite small, an important part of this being nonselectively decomposed. A good selectivity was observed in cyclohexane oxidation.