J.M. López Nieto

Oxidative dyhydrogenation of short chain alkanes on supported vanadium oxide catalysts

Abstract This paper summarizes the main data appeared in the last years on the oxidative dehydrogenation (ODH) of short chain alkanes on supported vanadium oxide catalysts. The acid-base character of metal oxide support influences the dispersion of vanadium on the surface of the support, as well as the nature of the vanadium species. The reducibility and structure of surface vanadium oxide species and the acid-base character of catalysts, in addition to their catalytic properties in the ODH of C2–C4 alkanes, strongly depend on the metal oxide used as support and the vanadium loading. In this way, it appears that tetrahedral V5+-species are active and selective sites in the ODH of C2–C4 alkanes. The effect of the coordination number and aggregation state of surface vanadium oxide species, and the presence of acid/base sites on the catalytic behavior of supported vanadium oxide catalysts are discussed. It is concluded that these are important factors that must be considered to develop selective catalysts in ODH reactions.

Influence of preparation conditions on the structure and catalytic properties of SO42−/ZrO2 superacid catalysts

Abstract SO42−/ZrO2 samples were prepared by impregnation of hydrous zirconium oxides with 0.5M H2SO4 solutions. It was found that the pH of precipitation of the precursor Zr(OH)4 has a strong influence on the structure of crystalline ZrO2 and on the catalytic properties of the SO42−/ZrO2 for the isomerization of n-butane. The nature and strength of acid sites in SO42−/ZrO2; samples were studied by infrared spectra of absorbed pyridine and by temperature-programmed desorption (TPD) of ammonia. Acidity and catalytic results have been compared with those obtained on an acid zeolite as H-Mordenite. It has been found that infrared spectra of pyridine can not be used for the determination of the superacid character of SO42−/ZrO2. However, the NH3-TPD technique shown that in the case of SO42−/ZrO2 samples there are superacid sites in which the NH3-desorption peak appears at about 815 K. A correlation between the number of those superacid sites and the activity for the isomerization of n-butane has been observed.

Selective Oxidation of Propane to Acrylic Acid on MoVNbTe Mixed Oxides Catalysts Prepared by Hydrothermal Synthesis

MoVNbTe mixed oxides have been prepared by both hydrothermal synthesis and slurry methods and have been tested in the selective oxidation of propane to acrylic acid. For comparative purpose, ternary metal oxides have also been prepared and tested. Characterisation results (X-ray diffraction and EPR) show important differences between the catalysts prepared hydrothermally and one prepared by a slurry method. The catalysts prepared hydrothermally show a higher activity and selectivity to acrylic acid than those prepared by slurry method. A reaction network for the partial oxidation reaction is tentatively proposed from the catalytic results obtained during the oxidation of propane and propylene on these catalysts.

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, characterisation and catalytic behaviour of a new TeVMoO crystalline phase

TeMxMo1.7O mixed oxides (M = V and/or Nb; x = 0-1.7) have been prepared by calcination of the corresponding salts at 600 °C in an atmosphere of N2. A new crystalline phase, with a Te/V/Mo atomic ratio of 1/0.2-1.5/1.7, has been isolated and characterised by XRD and IR spectroscopy. This phase is observed in the TeVMo or TeVNbMo mixed oxide but not in the TeNbMo mixed oxide. The new crystalline phase shows an XRD pattern similar to Sb4Mo10O31 and probably corresponds to the M1 phase recently proposed by Aouine et al. (Chem. Commun. 1180, 2001) to be present in the active and selective MoVTeNbO catalysts. Although these catalysts present a very low activity in the propane oxidation, they are active and selective in the oxidation of propene to acrolein and/or acrylic acid. However, the product distribution depends on the catalyst composition. Acrolein or acrylic acid can be selectively obtained from propene on Nb-free or Nb-containing TeVMo catalysts, respectively. The presence of both V and Nb, in addition to Mo and Te, appears to be important in the formation of acrylic acid from propene.

Selective oxidation of n-pentane on 12-molybdovanadophosphoric acids

Abstract The selective oxidation of n-pentane on 12-molybdovanadophosporic acids with 0, 1, 2, and 3 vanadium atoms was studied and compared with the behaviour of vanadyl pyrophosphate. Catalysts were characterized before and after the catalytic test by electron spin resonance, Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, krypton BET surface area and visible diffuse reflectance spectroscopy. The activity in the n-pentane conversion process increases linearly with increasing number of vanadium atoms, except for the H6PMo9V3O40 sample, for which physico-chemical characterization suggests partial surface degradation during the catalytic tests. A turnover frequency of about 5·10−4 molecules of n-pentane transformed per atom of vanadium and per second for both 12-molybdovanadophosporic acids and vanadyl pyrophosphate was calculated. Oxidation of n-pentane on heteropoly compounds formed only maleic anhydride and the selectivity increased linearly with increasing number of vanadium atoms present in the Keggin unit. In contrast, under the same reaction conditions both maleic and phthalic anhydride are formed on vanadyl pyrophosphate.

Surface properties of Co-precipitated VTiO catalysts and their relation to the selectiveoxidation of isobutene

Abstract The interaction between V 2 O 5 and TiO 2 (anatase) in a series of co-precipitated V Ti O catalysts was studied by X-ray diffraction, X-ray photoelectron spectroscopy, temperature-programmed reduction, laser-Raman spetroscopy, and catalytic performance for isobutene oxidation. It was found that the catalytic activity of these preparations depended on the V/(V+Ti) ratio, reaching a maximum for V/(V+Ti) = 0.11. For this catalyst the X-ray photoelectron spectra showed surface enrichment in a vanadium oxide phase, whereas the temperature-programmed reduction profiles revealed the lowest onset temperature for hydrogen reduction. Moreover, while X-ray diffraction data and Raman spectra showed that the oxidic vanadium species were highly dispersed over TiO 2 in V/(V+Ti)≤0.11 catalyst, the appearance of Raman bands at 998 and 697 cm −1 in vanadium-rich [V/(V+Ti)=0.2 and 0.5] catalysts were clear proof of the presence of tridimensional V 2 O 5 crystallites. The same conclusion was also reached by line broadening analysis of the most intense diffraction V 2 O 5 peaks in the X-ray diffraction patterns of the latter preparations, for which an average crystal size of 30.2 nm was calculated. From these data it seems that activity is directly related to a highly dispersed oxidic vanadium phase.

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.

Oxidative dehydrogenation of propane on vanadium supported on magnesium silicates

Abstract Sepiolite supported vanadium catalysts have been studied in the oxidative dehydrogenation of propane. Different vanadium species are present on the catalysts and the proportion changes with the preparation methods, support modifications, and vanadium content. At low vanadium content, isolated tetrahedral species are formed and both activity and selectivity to propene increase when increasing the vanadium content. At medium or high vanadium content the appearance of associated vanadium species can be observed and, while the activity still increases when increasing total vanadium, the selectivity to propene remains constant. MgV 2 O 6 and V 2 O 5 crystallites are formed at vanadium content higher than 30 wt.% of V 2 O 5 , and, depending on the propane conversion level, the selectivity to propene remains constant or decreases when increasing the vanadium content. On a comparative basis, the oxidative dehydrogenation of propane has also been carried out on a V 2 O 5 /MgO catalyst. From the comparison between catalytic properties in the oxidative dehydrogenation of propane on V/sepiolite and V/MgO catalysts a reaction network has been established and both type of catalysts show the same conversion-selectivity behaviour. The nature of the sites responsible for selective and non-selective oxidation on V/sepiolite catalysts has been proposed.