Carlotta Cortelli

Surface Dynamics of A Vanadyl Pyrophosphate Catalyst for n‐Butane Oxidation to Maleic Anhydride: An In Situ Raman and Reactivity Study of the Effect of the P/V Atomic Ratio

This work focused on investigating the effect of the P/V atomic ratio in vanadyl pyrophosphate, catalyst for n-butane oxidation to maleic anhydride, on the nature of the catalytically active phase. Structural transformations occurring on the catalyst surface were investigated by means of in situ Raman spectroscopy in a non-reactive atmosphere, as well as by means of steady-state and non-steady-state reactivity tests, in response to changes in the reaction temperature. It was found that the nature of the catalyst surface is affected by the P/V atomic ratio even in the case of small changes in this parameter. With the catalyst having P/V equal to the stoichiometric value, a surface layer made of alpha(Iota)-VOPO(4) developed in the temperature interval 340-400 degrees C in the presence of air; this catalyst gave a very low selectivity to maleic anhydride in the intermediate T range (340-400 degrees C). However, at 400-440 degrees C delta-VOPO(4) overlayers formed; at these conditions, the catalyst was moderately active but selective to maleic anhydride. With the catalyst containing a slight excess of P, the ratio offering the optimal catalytic performance, delta-VOPO(4) was the prevailing species over the entire temperature range investigated (340-440 degrees C). Analogies and differences between the two samples were also confirmed by reactivity tests carried out after in situ removal and reintegration of P. These facts explain why the industrial catalyst for n-butane oxidation holds a slight excess of P; they also explain discrepancies registered in the literature about the nature of the active layer in vanadyl pyrophosphate.

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.

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.

Selective oxidation of o-xylene to phthalic anhydride: from conventional catalysts and technologies toward innovative approaches

This paper discusses the gas-phase selective oxidation of o-xylene to phthalic anhydride, one of the chemical processes most studied in the 1980s and 90s. Specifically, it is our aim to examine some aspects which may offer ideas for the development of more innovative reactor technologies or catalyst formulations, and to discuss some peculiarities of this process and of the V/Ti/O catalysts that have been overlooked in the past.

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.

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.