Giovanna Ruoppolo

Oxidative dehydrogenation of propane over vanadium and niobium oxides supported catalysts

Publisher Summary This chapter discusses the activity and selectivity of catalysts based on niobium and vanadium oxides supported on high surface area anatase TiO2 in ethane oxidative dehydrogenation (ODH). Specifically, the influence of the cooperation of vanadium and niobium oxides supported phases as components, inducing redox and acid properties, respectively, together with the effect of the preparation conditions on the catalytic performances have been studied. The vanadia–itania catalysts are very active, but with low selectivity, because of their high reducibility. Catalytic performances of VOx/TiO2 systems in ethane ODH are improved by the addition of niobium. When TiO2 is coimpregnated by vanadium and niobium oxides, the presence of niobium enhances the selectivity to ethylene at low vanadium content, whereas it slightly depresses the activity without enhancing the selectivity at high vanadium content. This should be because of the effect of niobium on vanadium reducibility, especially affected at low vanadium content. By changing the order of addition of vanadia and niobia to the support, catalysts with slightly different redox and acid properties are obtained. At low vanadium loading, supporting the two oxides at the same time results in the best catalytic performances, while at high loading a two steps impregnation gives the best results.

TiO2 supported vanadyl phosphate as catalyst for oxidative dehydrogenation of ethane to ethylene

Abstract Bulk and TiO 2 supported VOPO 4 has been investigated for the oxidative dehydrogenation of ethane. XRD, SEM, TG analyses and BET surface area measurements indicated that vanadyl phosphate is highly dispersed on the support up to mono-layer coverage. A fraction of vanadium is present as V(IV) in the calcined samples as evaluated by EPR and TPR techniques. Both reducibility and acidity of vanadium phosphate is strongly enhanced by deposition on TiO 2 with respect to the bulk phase, as shown by TPR and NH 3 TPD technique, respectively. The supported catalysts are active and selective in the oxidative dehydrogenation of ethane to ethylene in the temperature range 450–550°C, the mono-layer catalyst giving the best performances. Ethylene selectivity decreases with the contact time but increases with the temperature. The former effect indicates that ethylene is further oxidized to CO x at high contact times. The effect of the temperature was attributed to the formation of V(IV), favoured at increasing temperature. This hypothesis was supported by TPR experiments carried out after catalytic tests at 550°C that indicated a significant increase of the fraction of V(IV) after the reaction.

Oxidative dehydrogenation of ethane on γ-Al2O3 supported vanadyl and iron vanadyl phosphates: Physico-chemical characterisation and catalytic activity

Abstract γ-Al2O3 supported vanadyl and iron vanadyl phosphates (VOPO4 and Fe0.23(VO)0.77PO4) calcined at 550 or 650xa0°C have been investigated as catalysts for the oxidative dehydrogenation (ODH) of ethane to ethylene in the temperature range 450–650xa0°C in a fixed bed reactor operating under atmospheric pressure. Catalysts have been characterised by X-ray diffraction (XRD), BET surface area measurements, X-ray photoelectron spectroscopy (XPS), NH3 temperature programmed desorption (TPD) and temperature programmed reduction (TPR). A good dispersion of the active phase has been obtained. The presence of various vanadium species (vanadyl phosphate, V4+ and V3+ ions, V5+ and V4+ oxides) have been detected on the catalysts surface with a significantly different distribution between supported vanadyl and iron vanadyl phosphate, V4+ ion being the prevailing species on VOPO4/Al2O3 (VOP/Al). The different surface species show different acidity and reducibility. Supported vanadyl phosphate catalysts are more active than iron modified samples and catalysts calcined at 650xa0°C give better catalytic performances than those calcined at 550xa0°C. Catalytic tests promote the formation of V3+, whose fraction increases with the reaction temperature. The better catalytic properties of supported vanadyl phosphate have been attributed to the presence of surface V4+ ions.

COMPARISON OF BEHAVIOUR OF RARE EARTH CONTAINING CATALYSTS IN THE OXIDATIVE DEHYDROGENATION OF ETHANE

Abstract Catalyst promotion by addition of either La and Sm to MgO or Na aluminate to Sm2O3 and La2O3 has been investigated for the oxidative dehydrogenation of ethane in the temperature range 550–700°C. With all unpromoted and promoted catalysts, the selectivity to ethylene is strongly enhanced by the temperature, the highest values being obtained at 700°C. Sm2O3 is the most active among the bulk oxides, while samarium addition to MgO results in higher surface area, but does not enhance the catalytic activity. Ethylene productivity on La2O3 promoted MgO samples is higher than with pure La2O3, Sm2O3 and MgO, not only due to the stabilising effect of La on MgO surface area, but also due to a higher intrinsic activity. With both bulk oxides and rare earth promoted MgO, the selectivity to ethylene strongly increases by decreasing the O2/C2H6 feed ratio, while it is quite unaffected by ethane conversion and catalyst composition, in agreement with the hypothesis that the main role of catalyst in the experimental conditions investigated is to produce ethyl radicals which are converted in the gas phase to CO and C2H4. When La2O3 is modified by the addition of sodium aluminate the catalytic behaviour significantly changes, likely due to a different, mostly heterogeneous reaction mechanism. On aluminate promoted lanthana, ethane is converted to ethylene with higher yields which do not depend on the feed ratio. Moreover, only CO2 is produced as by-product, the formation of CO being quite negligible.

XPS characterisation of iron-modified vanadyl phosphate catalysts

Iron-modified vanadyl phosphate is an interesting material with potential applications in catalysis due to the oxidising and dehydrogenating properties of the trivalent metal. Alumina-supported samples have been characterised by X-ray photoelectron spectroscopy (XPS) and compared with respect to the temperature of calcination and their catalytic behaviour in the oxidative dehydrogenation of ethane. XPS has been used to analyse the surface chemical composition of the samples and the modifications induced by different temperatures of calcination and catalysis. The oxidation state, amount, distribution and evolution of vanadium species have been investigated both after calcination at T=450 and 550°C and after catalytic tests at T=450, 550 and 650°C. Quantitative XPS analysis has been used to determine the surface concentration of different vanadium species.

Comparative study of catalytic behaviour of bulk-like and highly dispersed supported vanadyl orthophosphate catalysts in the oxidative dehydrogenation of ethane

The catalytic behaviour in the oxidative dehydrogenation of ethane of TiO2- and SiO2-supported catalysts containing bulk-like VOPO4 particles has been compared to that of TiO2-supported highly dispersed VOPO4. The catalysts have been characterised by XRD analysis and BET measurements. Redox properties have been studied by TPR experiments. A correlation of catalytic activity and C2H4 selectivity with redox properties has been proposed.

VOPO4⋅2H2O and Fe(H2O)x(VO)1-xPO4⋅2H2O Supported on TiO2 as Catalysts for Oxidative Dehydrogenation of Ethane

The effect of iron substitution on bulk and TiO2-supported VOPO4⋅2H2O on the catalytic performances in oxidative dehydrogenation of ethane has been studied in a fixed-bed reactor at 550 °C. The samples have been characterized by XRD, BET measurements, TPR and ammonia TPD experiments. A uniform dispersion of the active phase on the support has been obtained. Both iron substitution and dispersion on TiO2 significantly modify redox and acid properties of vanadyl orthophosphate. A good correlation between the acid and redox properties of the catalysts and their catalytic performances has been observed.

Redox behavior of high Si/Al ratio Cu-ZSM5 in NO decomposition

Commercial H-ZSM5 zeolites with a Si/Al ratio equal to 25 and 75 have been exchanged using copper acetate aqueous solutions with different concentrations. Copper saturation is reached at the 130 and 230% level of Cu exchange for Si/Al equal to 25 and 75, respectively, although FTIR spectra showed that a fraction of Al-OH exchange positions is still available. Catalytic activity experiments of NO decomposition have been carried out at 450°C in a fixed bed reactor. Catalysts have been characterized with H2 TPR and NO adsorption experiments at 120°C. All samples are partially reduced upon thermal treatment under inert flow (He) leading to the formation of Cu+-containing sites in addition to a fraction of differently reduced copper species. The Cu+-containing sites, also responsible for NO adsorption and subsequent production of N2O at 120°C, have been proposed to be the active centers. A quantitative estimation of these species, likely having multi-ionic structure, has been provided.