Ugo Cornaro

FT-IR characterization of silicated aluminas, active olefin skeletal isomerization catalysts

Abstract The surface structure of silicated aluminas, very good as catalysts for n-butene isomerization to isobutene, has been investigated by FT-IR spectroscopy of the silicate species, of the surface hydroxy-groups and of adsorbed pyridine. It has been concluded that silication results in the formation of a surface spinel phase where silicon substitutes for aluminum in tetrahedral sites. This phase exposes surface silanol groups that are not Bronsted acidic enough to protonate pyridine at room temperature. The increase in activity without loss in selectivity to isobutene is mainly attributed to the increased surface hydroxy-group concentration arising from silication, occurring without an increase in Bronsted acidity that would result in loss of selectivity and fast deactivation.

Oxydehydrogenation of propane catalyzed by VSiO cogels: enhancement of the selectivity to propylene by operation under cyclic conditions

Abstract The selectivity to propylene in the oxydehydrogenation of propane is improved when the reaction is carried out under cyclic, redox-decoupled conditions rather than under stationary, co-feeding conditions, and using vanadium oxide dispersed in silica as the catalyst. However, the improvement in selectivity is possible only in samples having a V content which is lower than 10 wt% V2O5. This points out the importance of having dispersed vanadium oxide to limit the consecutive combustion of propylene.

Reactivity of V/Nb mixed oxides in the oxidehydrogenation of propane under co-feed and under redox-decoupling conditions

Abstract V/Nb mixed oxides were prepared, characterized and tested as catalysts for the oxidehydrogenation of propane to propylene. The reactivity tests were carried out under co-feed and under redox-decoupling conditions, with alternate-feeds of propane and air (cyclic operation). The comparison between co-feed and alternate-feed operations evidenced that in the latter case a higher selectivity to propylene is achieved at low propane conversion. This was attributed to a reaction mechanism mainly involving propane dehydrogenation rather than oxidehydrogenation, occurring on the reduced catalysts. The selectivity to propylene, however, under both operating conditions decreased with increasing propane conversion. During reaction under redox-decoupling conditions VNbO5 decomposed yielding vanadium oxide and V/Nb mixed oxides having V/Nb ratio lower than 1. The former compound played the major role in the reaction cycle.

Cr/V/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile: Part II. Effect of catalyst composition on catalytic performance

Abstract Rutile-type, Cr/V/Sb mixed oxides having different atomic ratio between components were studied as catalysts for propane and propylene ammoxidation to acrylonitrile. Catalysts were more active than Cr/Sb and V/Sb mixed oxides; this was attributed to (i) the higher specific surface area of Cr/Sb/O and Cr/V/Sb/O with respect to V/Sb/O, and (ii) the preferential formation of V 4+ in Cr/V/Sb/O. The nature of the V species and the catalytic performance of Cr/V/Sb/O samples were functions of the (Cr+V)/Sb atomic ratio. When the latter was higher than ≈1, the prevailing species was V 4+ ; the catalysts were very active but poorly selective to acrylonitrile (selectivity lower than 20%) because of the prevailing formation of carbon oxides and propylene. This was due to the absence of sites able to transform the unsaturated intermediate to acrylonitrile. When the (Cr+V)/Sb ratio was between ≈1 and ≈0.5, catalysts reached a selectivity to acrylonitrile between 20 and 30%, and to propylene lower than 10%. In these samples, the presence of intra-crystalline Sb gradients in the rutile lattice provided a Sb surface enrichment and the development of allylic ammoxidation sites, able to transform the unsaturated intermediate to acrylonitrile. When the (Cr+V)/Sb ratio was lower than ≈0.5, i.e. in systems having excess Sb, the prevailing species was V 3+ ; the selectivity to acrylonitrile was higher than 30%, with low formation of carbon oxides and of propylene. In this case additional sites for allylic ammoxidation were provided by excess antimony oxide dispersed over the rutile surface.

Cr/V/Sb mixed oxide catalysts for the ammoxidation of propane to acrylonitrile. Part I: Nature of the V species

Abstract Rutile-type Cr/V/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile, were prepared and characterized. For atomic ratios between components equal to Cr/V/Sb 1/x/1 and 1/x/2 the systems were monophasic, but different types of compounds formed depending on the ratio between the three metals. The compositional parameter which most affected the nature of the compound formed was the (Cr+V)/Sb atomic ratio. When this ratio was between 2 and ≈1, a rutile Cr3+/V4+/Sb5+ mixed oxide of composition Cr1VxSb1O4+2x developed (0

In situ simultaneous synchrotron powder diffraction and mass spectrometry study of methane anaerobic combustion on iron-oxide-based oxygen carrier

Reduction of iron oxide/CeO2 solid due to methane anaerobic combustion has been studied using in situ time-resolved synchrotron X-ray powder diffraction and mass spectrometry measurements. All the chemical reactions involved in the process have been mapped. Rietveld profile fitting analysis has given a quantitative estimation of the different phases at every step of the reaction. The reaction temperature has been estimated using thermal expansion of both Fe2O3 and CeO2. From the structural parameters of CeO2 it has been possible to claim that CeO2 participates in the reduction process by transforming into a reduced CeOx form. The number, x, of O atoms per formula unit has been estimated by measuring the percentage variation of the unit-cell parameters.

The synthesis, characterization and use of metal niobates as catalysts for propane oxidehydrogenation

Abstract Metal (Cr, Fe and V) / Nb mixed oxides were prepared with the co-precipitation technique, and then characterized and used as catalysts for the oxidehydrogenation (ODH) of propane to propylene, under both co-feed and cyclic conditions. VNbOs formed after thermal treatment in air at 500-550°C, but decomposed if treated at higher temperature, while CrNbO 4 (a rutile-type compound) and FeNbO 4 were stable up to 700°C. In the case of the Ga/Nb sample, instead, the formation of the mixed oxide GaNbO 4 occurred only at a minor extent. The most active and selective catalyst in propane ODH under cyclic conditions was VNbO 5 , which however decomposed under reaction conditions yielding Nb-rich V/Nb mixed oxides and VO x ; the latter was the active phase in the reaction. However, no advantage was found in terms of selectivity to propylene with respect to the co-feed conditions. Fe/Nb and Cr/Nb samples acted as oxidehydrogenation catalysts, but gave very poor performance, while the Ga/Nb sample rather dehydrogenated propane, giving high selectivity to propylene under anaerobic conditions in the cyclic operation, but was quite unselective in the co-feed mode.

One-Step-Hydrogen: a new direct route by water splitting to hydrogen with intrinsic CO2 sequestration

Publisher Summary The One-Step-Hydrogen process is aimed to produce hydrogen from water splitting and also to tackle the carbon management issue targeting a zero carbon emission at lower costs than allowed by the Best Available Technologies (BAT). Well known water possesses an oxidizing potential which can be utilized in a simple water splitting application where it reacts with selected metals to give pure hydrogen. So combining in a cycle the water oxidative potential and the reverse action by a reducing agent like hydrocarbons, it allows the hydrogen production and as well as the intrinsic carbon dioxide sequestration. This chapter proposes this new process. In the first step, a suitable oxide takes up the oxygen from water splitting producing hydrogen. The solid acts as an oxygen storage medium. Such “lattice” oxygen is in turn released through one or more elemental steps. The overall reaction is endothermic and an energy supply is needed to close the energy balance. The decoupling of the overall reaction into three separated reactions allows the process to overcome the thermodynamics constraints of the overall equilibrium. The major breakthrough is however the exit of the two main products—hydrogen and carbon dioxide—from two different vessels, so that the carbon dioxide, once water is condensed, is pure and ready to be buried.

Improvement of the selectivity to propylene by the use of cyclic, redox-decoupling conditions in propane oxidehydrogenation

Abstract Catalysts based on silica-supported vanadium oxide were tested in the oxidative dehydrogenation of propane,under both co-feed and redox-decoupling conditions. It was found that it is possible to achieve higher selectivity to the olefin by separation of the two steps of the redox mechanism. The performance was affected by the amount of vanadia loaded on silica.