A. Dejoz

The selective oxidative dehydrogenation of ethane over hydrothermally synthesised MoVTeNb catalysts

Mo-V-Te-Nb metal oxide catalysts prepared by hydrothermal synthesis and heat-treated in N2 at high temperatures (600-700 degrees C) show high activity and selectivity for the oxidative dehydrogenation of ethane to ethene. Yields of ethene of 75% have been obtained at 400 degrees C on the best catalysts.

Preparation, Characterization and Catalytic Properties of Vanadium-Oxides Supported on Calcined Mg/Al-Hydrotalcite

Abstract Vanadium oxide supported on calcined hydrotalcite has been investigated for the oxidehydrogenation of n-butane in the 500–550°C temperature interval. Hydrotalcite (Mg/Al atomic ratio of 2.77), consisting of a single phase only, has been employed as a support precursor. The vanadium catalysts (0–50, referred as wt.-% V 2 O 5 ) were prepared by impregnation of calcined hydrotalcite (450°C) with ammonium metavanadate (in an aqueous solution) or vanadyl acetylacetonate (in a methanolic solution), and then calcined at 600°C for 4 h. During the impregnation step, the support is transformed into hydrotalcite if aqueous solutions are used. However, it is not modified if methanolic solutions are used. After calcination, diffuse X-ray diffraction patterns of MgO, in addition of a broad peak at 2θ=35° which intensity increases with the vanadium loading, were observed. Magnesium vanadates, i.e. ortho- and pyro-magnesium vanadates, were observed by FT-IR, FT-Raman and DR UV-vis. Similar activities for n-butane conversion are observed on all the catalysts studied, although the specific activity increases with the vanadium loading. However, independently of the catalysts preparation procedure, the selectivity to each oxydehydrogenation products (1-, cis -2-, trans -2-butenes and butadiene) initially increases with the vanadium loading, showing a maximum on catalysts with 30 wt.-% V 2 O 5 . The nature of active and selective sites is also discussed.

The effect of potassium on the selective oxidation ofn-butane and ethane over Al2O3-supported vanadia catalysts

The catalytic properties of undoped and K-doped (K/V atomic ratio of 0.5) Al2O3-supported vanadia catalysts (∼4.5 wt% of V2O5) for the oxidation ofn-butane and ethane were studied. Isolated tetrahedral V5+ species are mainly observed in both undoped and K-doped samples. The incorporation of potassium decreases both the reducibility of surface vanadium species and the number of surface acid sites. Potassium-free vanadium catalysts show a high selectivity during the oxidative dehydrogenation (ODH) of ethane but a low selectivity during the ODH ofn-butane. However, the presence of potassium on the vanadium catalysts strongly influences their catalytic properties, increasing the selectivity to C4-olefins fromn-butane and decreasing the selectivity to ethene from ethane. The role of the acid-base characteristics of catalysts on selectivity to ODH reactions is proposed.

On the influence of the acid-base character of catalysts on the oxidative dehydrogenation of alkanes

Vanadium oxides supported on metal oxide, i.e. Al2O3, MgO and Mg-Al mixed oxide, and V-containing microporous materials (VAPO-5 and MgVAPO-5) have been tested in the oxidative dehydrogenation of C2-C4 alkanes. In all cases, tetrahedral vanadium species (isolated and/or associated) were mainly observed from51V-NMR and diffuse reflectance spectroscopies. The reducibility of V5+-species, determined from the onset-reduction temperature, decreases as follows: VOx/AL > VAPO-5 > MgVAPO-5 =VOx/MG > VOx/MG + AL. The acid character of catalysts, determined from the FTIR spectra of pyridine adsorbed, decreases as: MgVAPO-5 > VOx/AL > VAPO-5 > VOx/MG + AL > VOx/MG. A similar trend between V-reducibility of the catalyst and its catalytic activity for the alkane conversion was observed. However, the selectivity to olefins depends on the acid-base character of catalyst and the alkane fed. In the ODH ofn-butane, the higher the acid character of the catalyst the lower the selectivity to C4-olefins, while in the ODH of ethane an opposite trend between the catalyst acidity and the selectivity to ethene was observed.

Highly dispersed encapsulated AuPd nanoparticles on ordered mesoporous carbons for the direct synthesis of H2O2 from molecular oxygen and hydrogen.

AuPd nanoparticles (<3 nm) have been encapsulated on the pores of a nanostructured CMK-3 carbon prepared by a nanocasting procedure. This material has been shown to be an excellent catalyst for the direct synthesis of hydrogen peroxide from molecular hydrogen and oxygen.

SiO2-supported vanadium magnesium mixed oxides as selective catalysts for the oxydehydrogenation of short chain alkanes

Abstract Unsupported and SiO 2 -supported VMgO mixed oxides catalysts have been prepared, characterized and tested in the oxidation of propane, n -butane and propylene. Mg-vanadates are observed by XRD, IR and Raman spectroscopy in both pure and supported VMgO catalysts, although the effective Mg/V ratio, in the VMgO phases formed, depends on the silica content. Since SiO 2 reacts partially with MgO during the calcination step, forming Mg 2 SiO 4 , the effective Mg/V ratio depends on the silica content. In this sense, pyro -Mg 2 V 2 O 7 is mainly formed on supported catalysts with Mg/V atomic ratios lower than 4, while ortho -Mg 3 V 2 O 8 is observed on supported catalysts with Mg/V atomic ratios higher than 4. For this reason, it can be concluded that the Mg/V atomic ratio of crystalline VMgO phases observed on supported catalysts is lower than those observed on unsupported ones. The catalytic tests for the oxidation of propane and n -butane indicate that the catalytic behavior of supported catalysts depends on both the SiO 2 content and the Mg/V atomic ratio, although a different behavior during the oxidation of n -butane or propane is observed: the most selective catalysts for the oxydehydrogenation (OXDH) of propane were those with ortho -Mg 3 V 2 O 8 / pyro -Mg 2 V 2 O 7 /MgO or ortho -Mg 3 V 2 O 8 /MgO mixed phases while the most selective catalysts for the OXDH of n -butane are formed by ortho -Mg 3 V 2 O 8 /MgO mixed phases. On the other hand, V-containing compounds with VO double bonds are required in order to obtain selective catalysts for the oxidation of propylene to acrolein. In the selective oxidation of propylene, acrolein is not formed on catalysts in which ortho -Mg 3 V 2 O 8 /MgO mixed phases are mainly detected. However, acrolein was obtained on catalysts in which VO double bonds are present.

Oxidative dehydrogenation of n-butane and 1-butene on undoped and K-doped VOx/Al2O3 catalysts

The oxidative dehydrogenation (OXDH) of n-butane and 1-butene on undoped and K-doped alumina-supported vanadia catalysts has been studied. The low selectivity to OXDH products on alumina-supported vanadia catalysts is a consequence of the isomerization of olefins (low temperatures) and the formation of carbon oxides (high temperatures) on acid sites. The presence of potassium results in a decrease of the number of acid sites and a higher selectivity to OXDH products from both n-butane and 1-butene. Infrared spectroscopy data of 1-butene adsorbed on the catalysts suggest the presence of different adsorbed species: (i) O-containing species on the undoped catalyst, or (ii) adsorbed butadiene on K-doped catalyst. A reaction network including parallel and consecutive reactions is proposed.

Total oxidation of naphthalene with high selectivity using a ceria catalyst prepared by a combustion method employing ethylene glycol.

During the catalytic combustion of naphthalene, compounds other than CO(2) are often obtained. These products, as polymerized polycyclic aromatic hydrocarbons, oxygenated aromatic compounds and benzene derivate compounds, are usually more toxic than naphthalene. At the present work it is shown a nanocrystalline cerium oxide prepared by a combustion method employing a proper ethylene glycol concentration that exhibits very high activity in the decomposition of naphthalene in the presence of air and, most importantly, a selectivity value towards CO(2) of 100% for any range of conversions and/or temperatures used. In addition, it has been demonstrated that the amount of ethylene glycol employed in the synthesis of the catalyst is determinant to achieve the optimal catalytic performance. The catalytic results have been explained in terms of the amount of cerium oxide defects.

Stable anchoring of dispersed gold nanoparticles on hierarchic porous silica-based materials

The nanometric organization of MOx (M = Co, Zn, Ni) domains partially embedded inside the mesoporous silica walls but accessible to the pore voids, which is achieved through a simple one-pot surfactant-assisted procedure, define optimal anchors for the nucleation and growth of gold nanoparticles, which in turn favours an exceptional thermal stability for the final Au-supported materials. As silica support we have selected a UVM-7 silica having a highly accessible architecture defined by two hierarchic pore systems. The combination of nanometric pore length, tortuous mesopores and MOx inorganic anchors favours the stability of the final Au/CoOx-UVM-7 nanocomposites.

The role of metal oxides as promoters of V2O5/γ-Al2O3 catalysts in the oxidative dehydrogenation of propane

Summary The physicochemical properties of potassium-, bismuth-, phosphorous- and molybdenum-doped (Me/V atomic ratios of 0 to 1) V 2 O 5 /γ-Al 2 O 3 catalysts and their catalytic behavior in the oxidative dehydrogenation of propane have been compared. The incorporation of metal oxides modifies the catalytic behavior of alumina-supported vanadia catalysts by changing both their redox and their acid-base properties. In this way, the addition of potassium leads to the best increase in the selectivity to propylene. This performance can be related to the modification of the acid character of the surface of the catalysts. The possible role of both redox and acid-base properties of catalysts on the selectivity to propylene during the oxidation of propane is also discussed.