B. Solsona

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.

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.

The selective oxidation of propane on Mo-V-Te-Nb-O catalysts: The influence of Te-precursor

Abstract Mo-V-Te-Nb-O catalysts have been prepared by hydrothermal synthesis and have been tested in the selective oxidation of propane to acrylic acid. The catalysts have been prepared by using different Te- and Mo-starting compounds. Both the catalyst characterization results (XRD, SEM–EDX, FTIR and XPS) and the catalytic tests show important differences depending on the Te- and Mo-starting compounds used in the hydrothermal synthesis. The most active and selective catalysts for the oxidation of propane to acrylic acid were those prepared from Te(VI)-compounds, i.e. telluric acid or an Anderson-type telluromolybdate. However, catalysts prepared from Te(IV)-compounds, i.e. TeO 2 or (NH 4 ) 4 TeMo 6 O 22 ·2H 2 O, presented both low activity and low selectivity to acrylic acid. In the best catalysts, several crystalline phases were observed: Mo 5 TeO 16 , (Mo 0.93 V 0.07 ) 5 O 14 , and/or Nb 0.09 Mo 0.91 O 2.80 and new Te-V-Nb-Mo-oxide crystalline phases. The presence of small crystals of MoO 3 in active and selective catalysts can also be proposed. The different catalytic performance of these catalysts could be related to the different incorporation of Te, V and Nb ions, which depends strongly on the Te-compound introduced in the synthesis gel.

Effect of potassium doping on the catalytic behavior of Mo-V-Sb mixed oxide catalysts in the oxidation of propane to acrylic acid

Small addition of potassium to a Mo–V–Sb mixed oxide catalyst (previously prepared by hydrothermal synthesis) strongly modifies its catalytic behavior. Thus, while acetic acid is mainly observed in the K-free catalysts, acrylic acid is selectively obtained in K-doped catalysts. In this case, a selectivity to acrylic acid of about 30% is achieved at propane conversions of 30%. This catalytic behavior is apparently not due to modification of the crystalline phases in the K-doped catalysts but to the elimination of the acid sites of the undoped Mo–V–Sb mixed oxide catalyst.

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.

Effect of potassium on the structure and reactivity of vanadium species in VOx/Al2O3 catalysts

Undoped and potassium doped (K/V atomic ratio of 0.7) alumina-supported vanadia catalysts (3.5 wt% of V-atoms) have been characterized by laser Raman spectroscopy, FTIR spectroscopy of adsorbed CO at low temperature, temperature-programmed reduction (TPR) and electron paramegnetic resonance (EPR). All results suggest a higher dispersion of the vanadium species on the Al 2 O 3 support by addition of potassium. This, together with the possibility of the presence of small amounts of K 2 O, leads to a decrease in the amount of Al 3+ Lewis acid sites and a lower reducibility of the vanadium species. Moreover, the reoxidation ability of the vanadium species seems to be reduced by the presence of potassium. All these results can explain the higher selectivity to oxydehydrogenation products observed over K-doped catalysts in the oxidation of C 4 hydrocarbons.

The hydrothermal synthesis of tetragonal tungsten bronze-based catalysts for the selective oxidation of hydrocarbons

Mixed metal oxides with tetragonal tungsten bronze (TTB) structure, showing high activity and selectivity for the gas phase partial oxidation of olefins, have been prepared by hydrothermal synthesis from Keggin-type heteropolyacids.

On the nature and structure of new MoVTeO and MoVTeNbO crystalline phases

New TeMoV and TeMo(V,Nb) bronze type phases have been obtained during the heat-treatment (600° C in N 2 ) of the corresponding precursor mixture. Both mixed oxides seem to be structurally related to one of the crystalline phases proposed in Mo-V-Te-Nb-O catalysts. Chemical and microstructural characterization revealed the formation of an orthorhombically distorted cell deriving from a bronze type HTB structure, which does not behave as a bronze type structure in the chemical sense. Microstructural details responsible for the new unit cell are discussed in terms of tellurium location inside the hexagonal tunnels and based on the structural model proposed.