I.V. Bacherikova

Influence of the mechanochemical treatment on the reactivity of V-containing oxide systems

Abstract Mechanical treatment of V 2 O 5 or V-P-O catalysts causes a substantial increase of both catalytic activity in n -butane oxidation and the selectivity to maleic anyhydride. Changes in specific surface area and anisotropic deformation take place. Mechanochemical treatment of the initial reagent impairs such properties that they remarkably influence the catalytic properties of the final catalyst.

n-Butane oxidation on VPO catalysts. Influence of alkali and alkaline-earth metal ions as additions

Abstract The mechanism by which the introduction of some elements modifies principal properties of the VPO system in oxidation of n-butane to maleic anhydride has been studied. The incorporation of Li, Na, K, Cs, Be, Mg, Ca, Ba at different concentrations which can easily donate electrons to the framework of vanadyl phosphate with P V ratio = 1.07 and 1.20, leads to an increase of the effective negative charge on oxygen atom and of the rate of butane oxidation. The presence of additives causes an increase of the surface P V ratio and corresponding changes of acidic properties of the catalysts. Selectivity towards maleic anhydride passes through a maximum when plotted as a function of the amount of acidic centres at the surface. The preparation of a catalyst characterised by high activity in butane oxidation and high selectivity to maleic anhydride requires a fine tuning of the basicity of surface oxygen atoms to accelerate the activation of butane and of the acidity of the surface to secure the appropriate residence time of the reaction intermediates.

Effect of the mechanochemical treatment of a V2O5/MoO3 oxide mixture on its properties

The mechanochemical treatment of a V2O5/MoO3 oxide mixture (V/Mo = 70/30 at %) was performed in planetary and vibratory mills under varying treatment times and media. The resulting samples were characterized using XRD analysis, micro-Raman spectroscopy, and XPS; their specific surface areas and catalytic activities in n-butane and benzene oxidation reactions were determined. It was found that the treatment of the oxide mixture in water resulted in chaotic degradation of the parent oxides, a decrease in crystallite sizes, and an increase in the specific surface area at a sufficiently uniform oxide distribution over the sample. The treatment in ethanol was accompanied by an anisotropic deformation of the V2O5 crystal by layer sliding in parallel to the vanadyl plane (010) and a chaotic degradation of MoO3 crystals. This process was accompanied by the partial nonuniform supporting of vanadium oxide crystals onto the surface of molybdenum oxide to increase the V/Mo ratio on the sample surface. In this case, the particle size of oxides decreased and the specific surface areas of samples increased. It was found that the treatment of the oxide mixture in air (dry treatment) resulted in the most significant decrease in the sizes of V2O5 and MoO3 crystals and a growth in the specific surface area. The amorphization of the parent oxides and the formation of MoV2O8 were observed as the treatment time was increased; in this case, an excess of amorphous vanadium oxide was supported onto the surface of this compound. It was found that, in all types of mechanochemical treatment, the binding energies of the core electrons of vanadium and molybdenum remained almost unchanged to indicate the constancy of the oxidation states of these elements. Mechanochemical treatment resulted in an increase in the activity of the samples in n-butane and benzene oxidation reactions and in an increase in the selectivity of maleic anhydride formation. In this case, an increase in the specific catalytic activity of the samples correlated with a decrease in the crystallite size of vanadium oxide, whereas selectivity correlated with an increase in the relative concentration of the V2O5 plane (010). In these reactions, samples after dry treatment exhibited a maximum activity, which can be related to the formation of MoV2O8.

A novel route in partial oxidation ofn-pentane over the VPO catalysts: formation of citraconic anhydride

Oxidation ofn-pentane on bismuth-doped vanadyl pyrophosphate was shown to give three anhydrides: maleic, phthalic and citraconic. The presence of a bismuth additive (Bi/V=0.1) increases the selectivity to citraconic anhydride which is formed via skeletal isomerization ofn-pentane to isopentane, followed by its oxidation. Phthalic anhydride seems to be formed via condensation of maleic anhydride with intermediate C4 olefins.

Oxidative dehydrogenation of ethane on vanadium-phosphorus oxide catalysts

Bi-doped VPO catalysts have been prepared, characterized and studied in the oxidation of ethane. The catalysts activation strongly influence both the characteristics of catalysts and their catalytic behavior. (VO) 2 P 2 O 7 and BiPO 4 were mainly observed in catalysts calcined in ethane/oxygen mixtures. These catalysts are highly selective in the oxydehydrogenation of ethane. In addtion, the incorporation of a big ionic radius cation as Bi, favors the increase of yield of ethene as was previously observed in the oxidation of n-butane. On the other hand, β- IVOPO 4 and BiPO 4 were observed in catalysts calcined in air, which present a poor selectivity to selective oxidation products. The presence of Bi, could stabilize both overstoichiometric phosphorus at the surface and the presence of V 4+ on the surface of catalysts.

Alternative methods to prepare and modify vanadium-phosphorus catalysts for selective oxidation of hydrocarbons

Publisher Summary This chapter discusses the possibilities of mechanochemical and barothermal treatments applied at different stages of the vanadium-phosphorus catalyst synthesis: the initial reactants, the precursor, and the final catalyst. All the catalysts synthesized, because of the mechanochemically treated V 2 O 5 , show an increased selectivity towards maleic anhydride and higher specific rate of n-butane and n-pentane oxidation as compared to those obtained in traditional synthesis. It has been demonstrated that mechanochemistry is obviously a promising method for the pretreatment of initial reagents to synthesize efficient vanadium phosphorus oxide (VPO) catalysts of paraffins oxidation. It has been established that mechanochemical treatment increases the specific surface area of V 2 O 5 and produces V 4+ ions. The change of V 2 O 5 texture during its mechanochemical treatment in ethanol leads to the synthesis of VPO-E1 precursor with low specific surface area and unchanged texture. In the synthesis, using V 2 O 5 in solvent-free conditions, β -VOPO 4 has been found to be the only product, but quite a high temperature and long time are needed for the reaction to be completed. In case of the use of VO 2 , a new compound has been formed. It has been indicated that the barothermal treatment favors an increase of the selectivity in paraffins oxidation. Moreover, the treatment in n-butanol also leads to the growth of catalytic activity of the samples. In case of the treatment with phosphoric acid vapors, the catalytic activity remains almost unchanged, because of the effect of water steam as described in the chapter. Thus, it should be emphasized that mechanochemical and barothermal methods have been shown to be advantageous as alternative technologies for the preparation and modification of VPO catalysts for partial oxidation of saturated hydrocarbons.

Mechanochemistry in preparation and modification of vanadium catalysts

V 2 O 5 , V 2 O 5 /TiO 2 and VPO/Bi systems were subjected to mechanochemical treatment and their structure and texture was examined by XRD, DTA, SEM, XPS and STM. Catalytic activity and selectivity in oxidation of butane and benzene to maleic and o-xylene to phthalic an hydride were determined as function of time of mechanochemical treatment. This treatment caused increase of surface area and exposure of the basal (010) plane of V 2 O 5 in V 2 O 5 and V 2 O 5 /TiO 2 resulting in considerable increase in activity and selectivity. In case of VPO catalyst and its mixture with Bi 2 O 3 and BiPO 4 both changes of morphology and surface chemical transformations were observed leading to an increase of activity and selectivity of n-butane oxidation to maleic anhydride

New reaction: n-Butane direct catalytic oxidation to tetrahydrofuran

Abstract The posibility of direct catalytic n-butane-to-tetrahydrofurane (THF) oxidation has been demonstrated. Influence of the catalyst composition on the selectivity of the reaction was investigated and its mechanism suggested.

Mechanochemical synthesis of nanodispersed compounds in the ZnO-MoO3 system

Mechanochemical treatment of a 1 : 1 mixture of ZnO and MoO3in a planetary ball mill in the air leads to the formation of nanodimensional MoO3·0.5H2O with subsequent formation of β -ZnMoO4, partially as nanorods.

Mechanochemistry in preparation of VPO catalysts for paraffins oxidation

It is shown that mechanochemical treatment can be a progressive method to activate both VPO precursor and starting compounds for VPO or MeVPO catalysts synthesis to get a noticeable improvement in VPO catalytic properties.