Doping of Nanostructured Co₃O₄ with Cr, Mn, Fe, Ni, and Cu for the Selective Oxidation of 2-Propanol

A series of transition-metal-substituted (M = Cr, Mn, Fe, Ni, Cu) ordered mesoporous cobalt oxide catalysts were synthesized via nanocasting method using KIT-6 silica as a hard template. While the pristine Co3O4 formed as a perfect replication of KIT-6, metal substitution resulted in less ordered and smaller domains of the replica oxides. The catalysts were applied in the selective oxidation of 2-propanol in the gas phase to reveal the role of the systematic metal substitution. Cu and Ni substitutions were found to be beneficial for the catalytic activity, while Cr, Mn, and Fe substitutions were detrimental. Cofeeding water vapor shifted the onset temperature of 2-propanol conversion to higher temperatures (ΔT = 10–20 K), while a beneficial effect was observed at high temperatures (>260 °C) decreasing deactivation by slowing the reduction of active Co3+ and/or reducing coke deposition. The activity scaled with the reducibility of the catalysts probed by H2 temperature-programmed reduction with the positive effect of a higher reducibility, indicating the crucial role of oxygen activation during 2-propanol oxidation at the gas–solid interface. 2-Propanol activation probed by adsorption/desorption experiments monitored by diffuse reflectance infrared Fourier transform spectroscopy showed a weakening of the interaction and changing of the adsorption mode from dissociative to molecular adsorption following the periodic table from Cr to Cu, suggesting that the activation of 2-propanol plays a minor role compared with oxygen activation. Fe-substituted Co3O4 was the least active catalyst due to the decrease of the number of active Co3+ sites

Selective cyclohexene oxidation with O-2, H(2)O(2)andtert-butyl hydroperoxide over spray-flame synthesized LaCo(1-x)Fe(x)O(3)nanoparticles

The elimination of waste and by-product generation and reduced dependence on hazardous chemicals are the key steps towards environmentally sustainable chemical transformations. Heterogeneously catalysed oxidation of cyclohexene with environmentally friendly oxidizing agents such as O-2, H(2)O(2)andtert-butyl hydroperoxide (TBHP) has great potential to replace existing processes using stoichiometric oxidants. A series of spray-flame synthesised nanoparticulate LaCo(1-x)Fe(x)O(3)catalysts was employed for cyclohexene oxidation, and the comparative results showed that TBHP led to the highest initial activity and allylic selectivity, but O(2)resulted in higher conversion for longer reaction times. Furthermore, the influence of Fe substitution was studied, which did not show any beneficial synergistic effects. LaCoO(3)was found to be the optimum catalyst for cyclohexene oxidation with O-2, following first-order reaction kinetics with an apparent activation energy of 57 kJ mol(-1). The catalyst showed good reusability due to its highly stable particle size, morphology and perovskite structure. 7-Oxabicyclo[4.1.0]heptan-2-one was identified to be formed by the oxidation of 2-cyclohexene-1-one with 2-cyclohexene-1-hydroperoxide

Investigation of Synergistic Effects between Co and Fe in Co_3-xFe_xO₄ Spinel Catalysts for the Liquid-Phase Oxidation of Aromatic Alcohols and Styrene

Transition metal oxides are attractive catalyst alternatives in liquid-phase oxidation reactions due to their lower cost and higher abundance compared with conventional noble metal catalysts. We investigated the catalytic properties of a systematic series of Co3-xFexO4 spinel catalysts synthesized by a hard-templating method, which were applied in the liquid-phase oxidation of styrene, benzyl alcohol and cinnamyl alcohol. O2 and tert-butyl hydroperoxide (TBHP) were used as the oxidants in a comparative manner. For alcohol oxidation, TBHP leads to similar or slightly higher selectivity to the corresponding aldehydes compared with O2. For the activation of C=C bonds, TBHP favors the oxidative cleavage pathway, while O2 favors the epoxidation pathway. The comparison of the catalytic performance revealed that the activity of Co3O4 does not benefit from Fe doping using O2 as the oxidant, while the substitution of Fe ≤ 10 % in the spinel structure is beneficial when TBHP is used. This is attributed to the different activation mechanisms of the oxidizing agents, being spin transfer in case of O2 and partial decomposition in case of TBHP. Heterogeneity tests and reusability studies demonstrated the stability of the spinel catalysts