F. Frusteri

Factors controlling the selectivity of V2O5 supported catalysts in the oxidative dehydrogenation of propane

Abstract The surface properties of a series of V 2 O 5 catalysts supported on different oxides (Al 2 O 3 , H–Na/Y zeolite, MgO, SiO 2 , TiO 2 and ZrO 2 ) were investigated by transmission electron microscopy and FTIR spectroscopy augmented by CO and NH 3 adsorption. In the case of the V 2 O 5 /SiO 2 system TEM images evidenced the presence of V 2 O 5 crystallites, whereas such segregated phase was not observed for the other samples. VO x species resulted widely spread on the surface of Al 2 O 3 , H–Na/Y zeolite, MgO and SiO 2 , whereas on TiO 2 and ZrO 2 they are assembled in a layer covering almost completely the support. Furthermore, evidences for the presence in this layer of V–OH Bronsted acid sites close to the active centres were found. It is proposed that propene molecules primarily produced by oxydehydrogenation of propane can be adsorbed on this acid centres and then undergo an overoxidation by reaction with redox centres in the neighbourhood. This features could account for the low selectivity of V 2 O 5 /TiO 2 and V 2 O 5 /ZrO 2 catalysts.

Effect of vanadia loading in propane oxidative dehydrogenation on V2O5/SiO2 catalysts

The influence of V2O5 loading on the catalytic behaviour of V2O5/SiO22 catalysts in the oxidative dehydrogenation of propane to propylene (POD) has been investigated. The different activity-selectivity pattern of low (5 wt%) and highly (>10 wt%) loaded V2O5/SiO2 catalysts is explained in terms of different surface vanadia species.

A comparative study of the partial oxidation of methane to formaldehyde on bulk and silica supported MoO3 and V2O5 catalysts

Abstract The mechanism of the partial oxidation of methane to formaldehyde with O2 has been investigated on bulk and differently loaded silica supported (4–7 wt%) MoO3 and (5–50 wt%) V2O5 catalysts at 600–650°C in a pulse reactor connected to a quadrupole mass spectrometer. The reaction rate and product distribution in the presence and in the absence of gas-phase O2 have been evaluated. On bare SiO2, low and medium loaded silica supported MoO3 and V2O5 catalysts the reaction proceeds via a concerted mechanism involving the activation of gas-phase oxygen on the reduced sites of the catalyst surface as proved by the direct correlation between catalytic activity and density of reduced sites evaluated in steady-state conditions, while on highly loaded catalysts as well as on bulk MoO3 and V2O5 the reaction rate drops dramatically and the reaction pathway via redox mechanism becomes predominant. The results indicate that the surface mechanism is essentially more effective than the redox mechanism enabling also a higher selectivity to HCHO.

On the nature of active sites of silica based oxide catalysts in the partial oxidation of methane to formaldehyde

The partial oxidation of methane to formaldehyde with molecular O2 has been investigated on various commercial bare SiO2 samples and silica supported M0O3 and V2O5 catalysts at 550–650°C. Amongst the different SiO2 samples, the highest HCHO productivity (STYHCHO, g·kgcat−1·h−1) is found with ‘precipitated’ silica, while ‘fumed’ SiO2 results in the least reactive silica. Incorporation of molybdena depresses the STYHCHO value for the ‘precipitated’ silica but enhances the STYHCHO for bare ‘fumed’ silica. In contrast, addition of vanadia to either ‘precipitated’ or ‘fumed’ silicas leads to higher STYHCHO values. On the basis of a series of experiments performed by continuous scanning of the reaction mixture with a quadrupole M.S., the participation of lattice oxygen in the main reaction pathway has been ruled out. A straight correlation between the density of reduced sites (p, 1016sr·gcat−1), evaluated in steady-state conditions by O2 chemisorption, and the reaction rate has been disclosed. MoO3 and V2O5 dopants modify the catalytic properties of SiO2 by affecting the process of oxygen activation on the catalyst surface.

Selective partial oxidation of light paraffins with hydrogen peroxide on thin-layer supported Nafion-H catalysts

Thin-layer carbon supported Nafion-H catalysts were found to be active and highly selective (S>98%) for the partial oxidation of C1-C3 alkanes, in a three phase catalytic membrane reactor (3PCMR), under mild conditions and in the presence of H2O2. The influences of the catalyst teflon loading and H2O2 concentration on the reaction rate have been evaluated. A reaction pathway, based on the electrophilic hydroxylation of the C-H bond of alkanes with protonated hydrogen peroxide (H3O2+), is discussed.

High yields in the catalytic Partial Oxidation of Natural Gas to formaldehyde : Catalyst development and reactor configuration

The research strategy for the development of silica based oxide catalysts suitable for the industrial exploitation of the Partial Oxidation of Natural Gas to Formaldehyde (MPO) is presented. A precipitated commercial SiO 2 sample, selected as the more suitable catalyst system has allowed to achieve very high HCHO yields (15-25 mol %) at 650-690 °C in a mini-plant reactor device operated in continuous flow recycle mode. Several engineering and technological aspects related to the process configuration, reactor material and geometry, HCHO absorption system and catalyst form have been addressed.

Oxidative dehydrogenation of propane on supported V2O5 catalysts. The role of redox and acid-base properties

The influence of several oxide carriers on the activity of vanadia catalysts in the oxidative dehydrogenation of propane (POD) has been investigated. The effects of both redox and acid-base properties of the support on the surface structures and reactivity of vanadia have been assessed. The occurrence of competitive reaction paths on bare oxide carriers and supported vanadia catalysts has been discussed.

Active species and working mechanism of silica supported MoO3 and V2O5 catalysts in the selective oxidation of light alkanes

Publisher Summary Supported MoO 3 and V 2 O 5 based catalysts play an excellent role in the selective oxidation of light (C1–C3) alkanes. Because of this, a considerable research effort has been directed to ascertain the working mechanism of such oxide systems as well as the nature of the active sites and the origin of the oxygen involved in the formation of the reaction products. This chapter discusses the catalytic behavior of silica supported MoO 3 and V 2 O 5 systems in both the selective oxidation of methane to formaldehyde (MPO) and the oxidative dehydrogenation of propane to propylene (POD), disclosing that V 2 O 5 acts as a promoter of the reactivity of the SiO 2 surface while the action of MoO3 strictly depends on the kind of the silica support. This chapter explains the correlation between the catalytic patterns of differently loaded silica supported MoO 3 and V 2 O 5 catalysts in myeloperoxidase (MPO) and peroxi-dase (POD) reactions with their surface and redox features to highlight the nature of the active surface species in the selective oxidation of light alkanes. The study clearly indicates a direct relationship between the density of reduced sites of low-medium loaded silica based oxide catalysts and reaction rate in both MPO and POD strongly suggests the occurrence of a concerted reaction mechanism, involving the activation of gas-phase O 2 , on the reduced sites of the catalyst surface.

On the reduction of NiO forms in magnesia supported catalysts

Reducibility of Ni/MgO catalysts has been studied by the temperature-programmed reduction (TPR) technique in the temperature range of 373–1273 K. The profile of reduction reveals the presence of various “forms” of NiO. The effects of nickel loading and treatment temperature on catalyst reducibility have been evaluated. The formation of NiO−MgO solid solution controls the whole reduction of Ni/MgO system.AbstractВосстановление катализаторов Ni/MgO исследовали с помощью ТПВ в интервале температур 373–1273 К. Профиль восстановления свидетельствует о присутствии различных “форм” NiO. Определяли влияние никелевой загрузки и температуры обработки на восстановление катализаторов. Все восстановление системы Ni/MgO контролируется образованием твердых растворов NiO−MgO.

Tailoring Effective FeOx/SiO2 Catalysts in Methane to Formaldehyde Partial Oxidation

A preparation method of highly effective methane to formaldehyde partial oxidation (MPO) FeOx/SiO2 catalysts (Fe 0.09--0.43 wt%) is reported, based on “adsorption-precipitation” (ADS/PRC) of FeII ions under controlled conditions. The performance of ADS/PRC catalysts in the MPO reaction at 650 °C has been compared with that of conventional systems prepared by “incipient wetness” (INC/WET) of silica carriers with aqueous solutions of FeIII. The ADS/PRC method, likely enabling a higher dispersion of the active phase, provides very effective MPO catalysts featuring CH4 turnover frequency (TOF) and HCHO productivity (STYHCHO) values larger than those of the counterpart INC/WET systems.