Structure, acidity, and redox aspects of VOx/ZrO2/SiO2 catalysts for the N-butane oxidative dehydrogenation

ZrOx/SiO2 and VOx/ZrOx/SiO2 catalysts (5 wt %25–25 wt %25 Zr, 4 wt %25 V) were prepared by grafting zirconium and vanadium alkoxides on Aerosil 380. All samples were characterized by temperature programmed reduction, N2 physisorption, X-ray diffraction, Raman spectroscopy, and ammonia adsorption microcalorimetry. Tetragonal ZrO2 and zircon (ZrSiO4) were present at 25 wt %25 Zr, but only amorphous zirconia overlayer existed for lower loadings. At lower Zr loadings (5 wt %25–10 wt %25 Zr), exposed silica surface leads to V2O5 crystallites and isolated VO4 species, although V reducibility behavior changes, from being similar to VOx/SiO2 (5 wt %25 Zr) to showing VOx/ZrO2 behavior at 10 wt %25 Zr, and a diminished total amount of reducible V. Highly acidic ZrO2 sites are covered by the vanadium grafting, forming weaker sites (60–100 kJ/mol NH3 adsorption strength). Catalytic conversion and selectivity for the oxidative dehydrogenation of n-butane (673 K, n-C4/O2 = 2.2) over VOx/ZrOx/SiO2 show that 1,3-butadiene is favored over cis-2-butene and trans-2-butene, although there is some selectivity to the 2-butenes when VOx/ZrO2 behavior is evident. At low Zr loadings, butadiene formed during reaction acts as the diene species in a Diels–Alder reaction and gives rise to a cyclic compound that undergoes further dehydrogenation to produce benzaldehyde. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

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