G. Mazzoni

Relationship between structural/surface characteristics and reactivity in n-butane oxidation to maleic anhydride: The role of V3+ species

Abstract V/P/O-based catalysts were prepared by thermal treatments of VOHPO 4 0·5H 2 O precursors prepared with the organic procedure. Different methods for precursor dehydration led to compounds which were characterized by the prevailing presence of crystalline (VO) 2 P 2 O 7 , but which also contained either V 5+ species or V 3+ species. The catalytic performance of these compounds in n -butane oxidation under almost-equilibrated conditions was compared. It was found that the presence of either V 3+ or V 5+ enhances the specific activity in n -butane oxidation, while the selectivity to maleic anhydride at low n -butane conversion (30%) remains substantially unaffected. A fully equilibrated, well-crystallized (VO) 2 P 2 O 7 was reduced with H 2 . The reduced compound was more active than the fully equilibrated vanadyl pyrophosphate, while exhibiting comparable selectivity to maleic anhydride.

The catalytic performance of Cs-doped V/Ti/O catalysts in the oxidation of o-xylene to phthalic anhydride: a TPR/TPO and reactivity study

Abstract Titania-supported vanadia (V/Ti/O) systems were modified by addition of cesium oxide for application as catalysts in the selective oxidation of o-xylene to phthalic anhydride (PA). Catalytic tests demonstrated that cesium is a strong promoter of the activity and selectivity to PA, but this effect is evident only under well-defined reaction conditions. Samples with a Cs content lower than 0.35xa0wt.% Cs2O exhibited a considerable increase in conversion as compared with the undoped V/Ti/O system. Catalytic tests made with varying o-xylene and oxygen concentrations in the feed demonstrated that in Cs-doped V/Ti/O catalysts the rate-determining step is the re-oxidation of vanadium by molecular oxygen. Thermal-programmed reduction (TPR) and thermal-programmed re-oxidation (TPO) tests evidenced that the addition of Cs decreases the vanadium reducibility and increases the re-oxidizability of the reduced vanadium sites. The positive effect of Cs on selectivity to PA was evident only for o-xylene concentrations in feed lower than 1.5xa0mol%.

A new approach to the characterization of V species in doped-V/Ti/O catalysts by means of TPR and TPO measurements: a study of the effect of promoters in the oxidation of o-xylene

Abstact Titania-supported vanadium oxide systems, catalysts for the oxidation of o-xylene to phthalic anhydride, were characterized by means of Raman spectroscopy, Thermal-Programmed Reduction and Oxidation, and adsorption/TP Desorption of methanol, with the aim of defining a method for the quantification of the different V species. It was found that vanadium oxide, either as polyvanadate dispersed over titania, or in the form of bulk vanadia, spontaneously releases molecular oxygen at 600-650°C, whereas isolated V species, chemically interacting with the support, is not susceptible of self-reduction. The latter species is that predominant in samples having low vanadium oxide loading ( 2 surface area 22.5 m2/g), and possesses the highest intrinsic activity in o-xylene conversion. In samples having higher vanadia loading, instead, the activity is determined by the amount of dispersed polyvanadate and of bulk vanadia. The effect of Sb, promoter of activity for V/Ti/O catalysts, was explained in terms of an increase of the dispersion of the most active species, and of stabilization of the latter towards segregation. These promotional effects are more pronounced in the co-presence of Cs and Sb.

Production of Maleic and Phthalic Anhydrides by Selective Vapor Phase Oxidation with Vanadium Oxide Based Catalysts

A study of the oxidation of o-xylene, n-butane, benzene, n-pentane, 1-pentene and a mixture of C4-C5 hydrocarbons on three commercial catalysts for the synthesis of anydrides was carried out. Oxidation of o-xylene was the easiest reaction to achieve, obtaining in all cases phthalic anhydride in high amount, while oxidation of n-paraffins to selective oxidation products was the most difficult reaction to achieve, obtaining high amount of maleic anhydride only using vanadia-phosporous based catalyst. V-P mixed oxide is the most polyfunctional catalyst because it gave high anhydride yields in the oxidation of all the feedstocks, excepted olefins. Vanadia-molybdenum based catalyst gave high yield in anhydrides only in oxidation of o-xylene and benzene and some interesting selectivity in oxidation of olefins. Vanadia-titania based catalyst gave selective oxidation products only in oxidation of o-xylene. It is proposed that differences in selectivity in oxidation of o-xylene and benzene are due to different type and/or reactivity of intermediates. In particular it is suggested that in o-xylene transformation to phthalic anhydride phthalide is an intermediate for a less selective pathway while a dialdehyde species adsorbed on the catalytic surface is the intermediate in selective oxidation of o-xylene. Besides in n-paraffins oxidation V-P gave higher selectivity in anhydrides than V-Mo. This is attributed to different capabilities of the two catalysts to promote dehydrogenation and oxygen insertion reactions.

125 The effect of cesium dopant on catalytic and chemical-physical properties of V/Ti/O, catalysts for the selective oxidation of o-xylene to phthalic anhydride

Abstract Titania-supported vanadia (V/Ti/O) systems were modified by addition of cesium oxide for application as catalysts in the selective oxidation of o-xylene to phthalic anhydride. Catalytic tests demonstrated that cesium is a strong promoter of activity and of selectivity to PA, but that this effect is evident only under well-defined reaction conditions. Samples with a Cs content lower than 0.35 wt.% Cs 2 O exhibited a strong increase of conversion as compared with the undoped V/Ti/O system. Catalytic tests made by varying the o-xylene and the oxygen concentrations in feed demonstrated that in Cs-doped V/Ti/O catalysts the rate-determining step is the re-oxidation of vanadium by molecular oxygen. Thermal-programmed reduction and re-oxidation tests evidenced that the addition of Cs decreases the vanadium reducibility and increases the re-oxidizability of reduced vanadium sites.