V. A. Zazhigalov

Mechanochemistry : the activation method of VPO catalysts for n-butane partial oxidation

Abstract The paper describes the influence of the introduction of alkali and alkaline-earth metals into the VPO precursor and the effect of bismuth additive introduced into the VPO precursor by the mechanochemical treatment. XRD, XPS, DTA, TPD of NH3 and determination of catalytic activity and selectivity in n-butane oxidation have been carried out. The preparations studied after mechanochemical treatment in a planetary mill were: (a) VPO, composed of VOHPO4 · 0.5H2O, (b) VPBiO, containing the bismuth additive introduced during synthesis, (c) VPO/Bi2O3 and VPO/BiPO4, mechanical mixtures of the VPO precursor and Bi2O3 or BiPO4, respectively. Introduction of alkali and alkaline-earth metals additives increases the basicity of surface oxygen in vanadium pyrophosphate, which entails increase in the rate of n-butane oxidation. The mechanochemical treatment of the preparations changed their morphology, favouring the formation of the vanadyl (001) plane in the phosphate precursor. The XRD peaks of bismuth phosphate appeared instead of those of bismuth oxide and the +0.8 eV chemical shift in the XPS Bi4f band was observed. The change in the chemical state of VPO/Bi2O3 are an evidence of the occurrence of chemical reaction. Mechanochemical treatment of VPO/BiPO4 and VPO/Bi2O3 catalysts increases their activity in n-butane oxidation and the selectivity to maleic anhydride. Thus, mechanochemistry may be a promising method for introduction of promoting additives into the basic VPO composition.

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.

Tribomechanical modification of Bi promoted vanadyl phosphate systems 1: An improved catalyst and insight into structure-function relationship

Bi promoted VOHPO4·0.5H2O (Bi-VHP) was mechanically treated in ethanol and air for several times and the morphology was studied by several characterisation techniques such as SEM, TEM, EELS, BET and XRD. The data were compared to those from the reference sample (Bi-VHP unmilled) and to those from a sample thermally treated in vacuum. The equilibrated product (VO)2P2O7 (VPP), that was obtained from Bi promoted VOHPO4·0.5H2O was included in the comparison. SEM shows that the reference sample consists mainly of flat smooth needles, that form a blossom secondary morphology. Under mechanical treatment the secondary morphology is lost and packages form with coin-shape particles. XRD revealed that milling in ethanol for 5 min reduced the particle size but did not change the phase. Milling in air for 28 min resulted in an amorphous phase, whereas treatment above 30 min led to crystalline VPP. Thermal activation induced a phase change to VPP with the initial material still being present as a minority phase. The ball mill method is effective for catalysts activation as it increases the specific surface area and hence activity and selectivity per volume. On the other hand ball milling destroys the crystallinity and reduces the conversion and yield per unit surface area.

The mechanism of n-pentane partial oxidation on VPO and VPBiO catalysts

Abstract The paper addresses the mechanism of maleic (MA), citraconic (CA) and phthalic (PhA) anhydride formation in the oxidation of n -pentane on VPO catalysts. In the key experiments oxidation of n -butane and n -pentane and reaction of its products with 1,3-butadiene were studied in the two reactor system working on-line. A large quantity of PhA appeared in the products when 1,3-butadiene was introduced into the n -butane–air reaction mixture at the inlet of the second reactor. This study clearly shows that the basic route of the PhA formation in n -pentane oxidation on VPMeO catalyst is the Diels–Alder reaction between diolefin C 4 and MA.

Mechanochemical synthesis of BaTiO3 from barium titanyl oxalate

The effect of mechanochemical processing in air and water on the physicochemical transformations of barium titanyl oxalate has been studied using X-ray diffraction, thermal analysis, FTIR spectroscopy, temperature-programmed argon desorption, and particle size measurements. The results demonstrate that mechanochemical processing of barium titanyl oxalate in air leads to the formation of structurally imperfect barium titanate. During subsequent air calcination at 550°C, this material transforms into well-crystallized cubic BaTiO3, whereas thermal decomposition of barium titanyl oxalate only yields cubic BaTiO3 starting at 800°C. Mechanochemical processing in water leads to partial amorphization of barium titanyl oxalate, and conversion of the product to BaTiO3 requires heat treatment at 700°C. All of the BaTiO3 samples obtained via mechanochemical processing have a larger specific surface in comparison with samples prepared by conventional calcination of barium titanyl oxalate or other known processes.

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.

Synthesis, physical–chemical, and catalytic properties of mixed compositions Ag/H3PMo12O40/SiO2

Deposited catalysts composition H3PMo12O40/SiO2 and Ag/H3PMo12O40/SiO2 have been synthesized on the basis of fumed silica, including milling technique. Physical–chemical characteristics of prepared catalysts have been studied by means of XRD, DTA-TG, FTIR, UV–Vis spectroscopy, and adsorption of nitrogen. Catalysts possess meso- or meso-macroporous structure and contain deposited Keggin heteropolycompounds. Deposition of heteropolycompounds on support with high specific surface area results in increase of selectivity to epoxide in epoxidation reactions. The use of milling during catalyst synthesis leads to further growth of selectivity of epoxides formation.

Effect of Bismuth Additives on the Properties of Vanadium–Phosphorus Oxide Catalyst in the Partial Oxidation of n-Pentane

The selective oxidation of n-pentane on vanadium–phosphorus oxide (VPO) catalysts with bismuth additives (Bi/V = 0–0.30) is studied. The catalysts are characterized by XRD, XPS, and specific surface area measurements using nitrogen adsorption. Their acidic properties are studied (using ammonia TPD and the 2-methyl-3-butyn-2-ol reaction). It was found that the introduction of bismuth insignificantly affects the specific surface area but increases the surface concentration of phosphorus and changes the acidic properties of the catalysts. The specific catalytic activity of samples in n-pentane oxidation correlates with the effective charge of surface oxygen (Eb of O1s electrons). The selectivity to citraconic anhydride increases with an increase in the general surface acidity. The selectivity to maleic anhydride increases with an increase in the Brønsted acidity of the surface. The selectivity to phthalic anhydride increases with an increase in the Lewis acidity. The pathways of product formation in the partial oxidation of n-pentane are proposed.