To the question of oxidation on the surface of oxides: temperature- programmed oxidation of cyclohexanol

Анотація

Temperature-programmed reaction (TPR) method with mass spectrometric control of the products was used to study of cyclohexanol oxidation into cyclohexanone on individual and mixed oxides supported by γ-Al₂O₃ and silica gel. In the TPR profiles the temperature of a maximum rate of cyclohexanone formation varies from 125°C for MoO₃/Al₂O₃ to 235°C for less active CuO/Al₂O₃. The catalytic activity of individual oxides decreases in the order MoO₃/Al₂O₃> V₂O₅/SiO₂ > Fe₂O₃/Al₂O₃ > Bi₂O₅/Al₂O₃ > TiO₂/SiO₂ ≈ СeO₂/Al₂O₃ > TiO₂/Al₂O₃ > SnO₂/Al₂O₃. As "reactive" oxygen in our TPR experiment was supplied only from oxide lattice, oxide activity is determined by different energy of the surface Me – O bonds. The approach to search for mixed active oxides based on decreasing coordination number of O^2- ions is proposed, that confirmed by the example of CuO-WO₃/Al₂O₃ catalyst. The mixed supported oxides, especially CuOCrO₃/Al₂O₃, CuO-MoO₃/Al₂O₃, MoO₃-SnO₂/Al₂O₃ and Bi₂O₃–SnO₂/Al₂O₃, are more active in С₆Н₁₂О + 1/2О₂ → С₆Н₁₀О + Н₂О oxidation. The synthesized CuO-CrO₃/Al₂O₃ catalyst provides cyclohexanone formation without side   cyclohexanol dehydration and can be used for the oxidation of ethylene glycol – methanol mixture into methyl glycolate. CuO-Cr₂O₃/Al₂O₃ with a spinel structure of CuCr₂O₄ ([CuO₄] ⁶⁻ tetrahedra, Cu²⁺ sp³-hybridization) is more active in cyclohexanol oxidation than CuO/Al₂O₃ with flat [CuO₄] ⁶⁻squares, Cu²⁺ dsp²-hybridization. This is explained by the lower energy of Cu-O bonds at sp³-hybridization of Cu²⁺ ions.