Synthesis and characterisation of microporous bimetallic Fe–Cr–Si–O materials derived from silsesquioxane precursors

Abstract

Calcination of silsesquioxane mixtures of (c-C₅H₉)₇Si₇O₉(OH)₃, 1, (c-C₅H₉)₇Si₇O₁₂Fe(CH₃)₂N(CH₂)₂N(CH₃)₂, 2, and (c-C₅H₉)₇Si₇O₉(OSiMe₃)O₂CrO₂, 3, led to microporous amorphous bimetallic Fe–Cr–Si–O materials with different Fe∶Cr ratios. A set of complementary characterisation techniques including N₂ physisorption, XRD, XPS, RS, IR, HRTEM and Mössbauer spectroscopy were used to follow the variation of the textural properties, metal oxide dispersion and speciation with metal content. Fe–Cr–Si–O materials possess high surface areas and uniformly controlled micropores with an average pore size diameter of around 6–7 Å. Metal oxide speciation of calcined silsequioxane mixtures appears to be significantly different from that observed for these metals in the individually calcined metal silsesquioxanes. The iron oxide and monochromates are the predominant species in the calcined precursors 2 and 3 while very small particles (2–4 nm) of bimetallic mixed oxides are the major species in the Fe–Cr–Si–O materials. This suggests that the metal oxide species are highly interdispersed and can come into close contact with each other during the calcination procedure thus favoring the formation of the bimetallic mixed oxide phase. In contrast, a silica reference material containing 7% Fe and 3% Cr prepared via the impregnation method showed only chromate species and large particles (10–30 nm) of iron oxide. This suggests that metallasilsesquioxane mixtures may be used as versatile precursors for the preparation of silica-based catalysts containing very small and well dispersed particles of mixed metal oxides.