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[en] Full text: The Mn-Na₂WO₄/SiO₂ is the state-of-the-art catalyst for the oxidative coupling of methane (OCM) because it presents high stability (~500 h) and C2 hydrocarbon yield (14-27%). Several works have indicated the synergistic interactions between its components (i.e., WOx, NaOx, and MnOx) and proposed distorted WO₄ tetrahedra as the active sites. However, these conclusions were drawn from characterizations obtained at room temperature, which is so far from typical high OCM temperatures (>750 ºC). In this sense, this work aims to establish a structure-activity relationship of the Mn-Na₂WO₄/SiO₂ catalyst for the OCM from experimental evidence collected at relevant reaction conditions. The mean oxidation state and the distortion degree of WOx sites present on trimetallic Mn-Na₂WO₄/SiO₂, bimetallic Na₂WO₄/SiO₂, and monometallic WO₃/SiO₂ catalysts were studied using X-ray diffraction (XRD) and in situ W LIII-edge X-ray absorption near edge structure (XANES) analysis. At room temperature, the XANES results indicate that the mean W oxidation state for all samples is near 6+. Furthermore, the white line shape of the XANES spectra shows that WOx sites are octahedrally coordinated on the monometallic catalyst and tetrahedrally coordinated on both tri- and bimetallic catalysts. These results are consistent with the crystalline phases identified by XRD (i.e., WO₃ for monometallic and Na₂WO₄ for bi and trimetallic catalysts). As increasing temperature, the change in the white line shape suggests a variation of the distortion degree of the WOx sites. For mono- and bimetallic catalysts, the distortion degree increased, while for trimetallic catalyst it decreased presumably due to the interaction with Mn atoms. Therefore, the superior OCM behavior of conventional trimetallic catalyst when compared with bi- and monometallic catalysts is associated with the presence of tetrahedrally coordinated and lowly distorted WOx sites. (author)

[en] Nb₂O₅/ZrO₂, Cu–Nb₂O₅/ZrO₂ and Ag–Nb₂O₅/ZrO₂ were prepared by incipient wetness impregnation and evaluated in the ethanol conversion into higher value-added products. The catalysts were characterized by X-ray fluorescence, X-ray diffraction, N₂-adsorption–desorption, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction, temperature-programmed desorption of CO₂ and NH₃, and FTIR pyridine adsorption. The catalytic activity and selectivities were evaluated at different temperatures and space velocities. ZrO₂ exhibited strong acid sites and led to ethylene as the main product. The addition of niobium yielded a surface Nb₂O₅ overlayer and also crystalline nanoparticles, and also increased the acid site density of the catalyst. The impregnation procedure yielded Ag and CuO nanoparticles highly dispersed over the Nb₂O₅/ZrO₂ surface, promoting ethanol dehydrogenation and consequently increasing acetaldehyde selectivity. Besides, results revealed that a decrease in temperature favors ethanol dehydrogenation to acetaldehyde, while an increase in temperature favors ethanol dehydration to ethylene. Moreover, 1,3-butadiene and ethyl acetate selectivities were increased by higher contact times. Nevertheless, the higher acid sites density and the presumable deactivation of Ag and Cu dehydrogenation sites led to high ethylene formation, which reinforces the importance of a suitable balance of acid and basic sites. (author)