R.X Valenzuela

Nanostructured ceria-based catalysts for oxydehydrogenation of ethane with CO2

Nanostructured cerium oxides, pure or doped with CaO, prepared from amorphous acetate precursors obtained by the freeze drying method, exhibit high surface areas and are active and selective catalysts for the oxydehydrogenation of ethane (ODE) using CO2as an oxidant. The incorporation of Ca into a solid solution in the ceria framework reduces the activity but improves markedly the selectivity to ethene and the efficiency of CO2. Under reaction conditions, the pure ceria was stable even at 1023 K, while the surface area of the Ca-containing sample decreased markedly. This change was accompanied by a drastic increase in the selectivity to ethene, allowing to obtain yields of ethene up to 22% with 91% selectivity.

Oxidative dehydrogenation of isobutane over LaBaSm oxide catalyst: Influence of the addition of CO2 in the feed

Abstract Oxidative dehydrogenation (ODH) of isobutane over LaBaSm oxide catalyst in the temperature range 450–600°C, and the influence of the addition of CO2 into the feed were investigated. It was found that LaBaSm oxide is an active and stable catalyst for ODH of isobutane. Upon reaction conditions the specific surface area decreases and a new phase, La2O(CO3)2, is formed, which causes an increase in the surface specific conversion. The selectivity to isobutene as well as isobutane conversion can be improved by adding CO2 into the feed. These effects may be explained as due to the combined effects of improvement of the active phase formation and the competition between molecular O2 and CO2 adsorption on the sites which are responsible for total oxidation.

Oxidative dehydrogenation of isobutane on MCM-41 mesoporous molecular sieves

Abstract MCM-41 mesoporous molecular sieves containing silicon and vanadium have been prepared by a direct hydrothermal route. Aluminium-containing [Si,Al]-MCM-41 and purely siliceous [Si]-MCM-41 solids were synthesised for comparison purposes. 51V static and magic-angle-spinning NMR studies of [Si,V]-MCM-41 showed one type of VO4 in the as-prepared samples (site I) and two types of monodispersed and distorted to various extent VO4 tetrahedra, chemically bound to the walls of MCM-41 (sites IVa and IVb), in the calcined and rehydrated samples. NMR parameters of vanadium units are given. The catalytic performance of MCM-41 materials was probed in the oxidative dehydrogenation (ODH) of isobutane. The highest selectivities to isobutene (48–59%) were observed for the [Si,V]-MCM-41 materials with Si/V=85 and 30, respectively. On the contrary, a sample containing V2O5 bulk-like species exhibited much lower selectivity to isobutene, especially at the isobutane conversion higher than 5%. The presence of isolated vanadium species, as found by 51V NMR, was responsible for the high olefin selectivity in the ODH of isobutane.

Effect of different additives on the performance of V−Mg−O catalysts in the oxidative dehydrogenation of propane

The addition of K2O, Cr2O3, Sm2O3 or Nb2O5 to V−Mg−O catalyst decreased both activity and selectivity in propane oxydehydrogenation. By comparing these results with the characterization data, a relationship between propylene selectivity and catalyst reducibility was revealed. The most easily reduced catalyst was the most selective.

Oxidative dehydrogenation of propane on zeolite catalysts

Abstract Oxidative dehydrogenation of propane has been studied on zeolite Na-Y, stabilized zeolite Y (USY) and ZSM-5. The supports with the faujasite structure were modified with boron, gallium and indium oxides and Ca, Mg, Sn, and Sb cations. The samples containing simultaneously In2O3 and Ga2O3 in the USY support were compared. A detailed analysis of the zeolite catalysts performance in the title reaction has been carried out.

Propane partial oxidation to acrolein over combined catalysts

A novel approach for the partial oxidation of propane to acrolein, based on the use of layers of combined catalysts in a single reactor, provides good yields of acrolein with selectivity above 62%. The results depend strongly on the layer configuration, and reveal new mechanistic features for the process.

On the intrinsic activity of vanadium centres in the oxidative dehydrogenation of propane over V–Ca–O and V–Mg–O catalysts

The performance of V–Ca–O catalysts for the oxidative dehydrogenation of propane has been studied for the first time, being compared with that of similar V–Mg–O catalysts, and their differences are interpreted in terms of their physico-chemical properties. The VCaO catalyst showed the same initial selectivity to propene as the most selective VMgO composition, but it decreased faster with increasing propane conversion. Vanadium species present in the catalyst surface were different for the two supports: isolated V5+ tetrahedra on CaO and magnesium orthovanadate and pyrovanadate on MgO. The intrinsic activity of isolated vanadium centres in the surface of the V–Ca–O catalyst was more than one order of magnitude higher than that of dimeric or polymeric tetahedral species in the magnesium-containing catalysts.

Gallium oxide promoted zeolite catalysts for oxidehydrogenation of propane

Abstract Novel gallium-containing catalysts for oxidehydrogenation of propane, based on zeolite Beta, ZSM-5 and ferrierite, have been prepared and characterised by scanning electron microscopy, IR, MAS NMR and Raman spectroscopies. The catalytic properties of zeolitic matrixes with B, Al, and both ions at tetrahedral sites have been studied. Transformation of propane on pure zeolites and promoted with gallium (III) oxide depended on the structure of the matrix, its morphology and the type of cations occupying zeolite framework sites. Formation of new hydroxyl groups has been evidenced for some MFI zeolites promoted with Ga 2 O 3 .

Ethane oxidehydrogenation selectivity and reductibility of mixed NiVSb oxides

Alumina-supported NiV, SbV and NiVSb oxides were characterized and tested as catalysts for the oxidehydrogenation of ethane. The results of their surface and bulk reduction by hydrogen revealed that the most easily reduced NiVSb catalyst was the most selective. The relationship between catalyst reducibility and selectivity was interpreted on the assumption that the reaction occurred through a redox mechanism.

High performance of V–Ga–O catalysts for oxidehydrogenation of propane

Abstract The catalytic performance in the oxidehydrogenation (ODH) of propane of vanadium oxide catalysts supported on gallium oxide, VO x /Ga 2 O 3 , with vanadium coverages lower or near the theoretical monolayer has been studied as a function of the vanadium content and compared with those of other known effective V–M–O (M=Mg, Ca) catalysts. Catalyst activity was very high and increased with the increase of vanadium loading in the range studied, while the selectivity trend was similar for the studied catalysts, excepting that with the lower V content. FT-Raman and 51 V solid state NMR spectroscopies show that for coverages below the theoretical monolayer vanadium atoms are in tetrahedral co-ordination either in isolated or polymeric species, while the onset of vanadia formation is detected above that coverage. Interestingly, these catalysts show an one order of magnitude higher area-specific rate, similar initial olefin selectivity and slightly higher selectivity decrease with the increase of conversion than the best VMgO catalyst. This is due to the high intrinsic activity of isolated tetrahedral vanadium species. The combination of these factors produces an enhanced olefin productivity of V–Ga–O catalysts.