N. Ballarini

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.

Rutile-type Cr/Sb mixed oxides as heterogeneous catalysts for the ammoxidation of propane to acrylonitrile

Cr/Sb mixed oxides with a rutile-type structure were synthesized by calcining (700 °C) a mixture of oxohydrates obtained by coprecipitation from an alcohol solution containing the required amount of the components. The samples were characterized using X-ray powder diffraction (XRD), FT-IR spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). In the Cr/Sb/O compound prepared with Cr/Sb 1/0.8 (atomic ratio), an almost stoichiometric CrSbO4develops (no antimony or chromium oxides are found), which however is characterized by enrichment in Sb in outmost rutile atomic layers, and thus by a non-homogeneous intracrystalline distribution of the two components. For Cr/Sb ratios lower than 1, the amount of Sb in the rutile-type structure exceeds the stoichiometric value. The reactivity of Cr/Sb/O catalysts with increasing Sb contents (from Cr/Sb 1/0.8 to 1/2.8) is slightly affected by the Cr/Sb atomic ratio. In particular, the selectivity increases with increasing Sb content, while the catalytic activity is higher for the samples having higher Cr/Sb ratios. The Cr/Sb/O system presents considerable differences with respect to the V/Sb/O rutile system; these differences are discussed in reference to the properties of the transition metal components of the rutile mixed oxides.

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

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.

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.

Low-organics method to synthesize silver nanoparticles in an aqueous medium

Abstract In this report we describe the preparation of Ag nanoparticles (AgNP) in water by means of a modified (low-organics) Turkevich method, with the aim of verifying the feasibility of this procedure for the development of stable colloidal sols. The latter may be used for the deposition of AgNP over supports.

A comparison of the reaction mechanism in acid-and in basic-catalysed gas-phase methylation of phenol

This paper compares the reaction of gas-phase methylation of phenol with methanol in basic and in acid catalysis, with the aim of investigating how the transformations occurring on methanol affect the catalytic performance and the reaction mechanism. It was found that with the basic catalyst Mg/Fe/O, the true alkylating agent is formaldehyde, obtained by dehydrogenation of methanol. Formaldehyde reacted with phenol to yield salicylic alcohol, which rapidly dehydrogenated to salicylic aldehyde. The latter was isolated in tests made by variation of the residence time, and in tests made by feeding a formalin/phenol aqueous solution. Salicylic aldehyde then transformed to o-cresol, the main product of the basic-catalyzed methylation of phenol, by means of an intermolecular H-transfer. With an H-mordenite catalyst, instead, the activated methanol reacted with phenol to generate anisole, cresols and polyalkylated phenols.