B. Grzybowska

Interaction of allyl iodide with molybdate catalysts for the selective oxidation of hydrocarbons

Abstract The interaction of allyl iodide with MoO 3 , Bi 2 O 3 , and molybdates of Bi, Co, and Mg has been studied in the temperature range of 270–480 °C. Although practically inactive in the allylic oxidation of propylene, MoO 3 yielded 100% acrolein at 320 °C, when in contact with allyl iodide. On Bi 2 O 3 , only 1,5-hexadiene and benzene were formed, whereas bismuth molybdate produced both oxygenated and nonoxygenated products. This confirms the multicenter model of allylic oxydation, the low-valent cation being responsible for activation of the hydrocarbon molecule and Mo-O polyhedra for insertion of oxygen. Stoichiometric CoMoO 4 and MgMoO 4 were inactive in the formation of acrolein from allyl iodide because the acrolein-like intermediate formed on the surface was unable to desorb. This indicates that the third condition of an active catalyst is the ability to desorb the oxygenated product by releasing the oxygen ion of the lattice sufficiently readily. When an excess of MoO 3 is present in these molybdates, they become active in the formation of oxygenated products.

Effect of alkaline promoters on catalytic activity of V2O5/TiO2 and MoO3/TiO2 catalysts in oxidative dehydrogenation of propane and in isopropanol decomposition

Oxidative dehydrogenation of propane has been studied on V2O5/TiO2 and MoO3/TiO2 catalysts, consisting of 1 and 5 monolayers of the deposited oxides, promoted with Li, K and Rb cations. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS) and by isopropanol decomposition, a probe for acid-base properties. For both types of catalysts, irrespective of the vanadium or molybdenum content, the total activity in oxidative dehydrogenation of propane decreased for the promoted catalysts in the sequence: non-promoted ≥ Li > K > Rb-promoted catalyst. The propene yields and selectivities at equal conversion increased in the same order. The same sequence was also observed for the increase in the rate of acetone formation and the decrease in the rate of propene formation in the isopropanol decomposition on the promoted catalysts. It is proposed that the alkaline promoters decrease the acidity and increase the basicity of the catalysts, and hence facilitate desorption of propene from the catalyst surface, preventing it from further oxidation to carbon oxides. The XPS data suggest that the alkaline promoters increase the dispersion of the vanadia or molybdena phase in the high vanadium or molybdenum content catalysts.

O-xylene oxidation on the V2O5-TiO2 oxide system: I. Dependence of catalytic properties on the modification of TiO2

Abstract O-xylene oxidation and isopropanol decomposition have been studied on V2O5-TiO2 oxides prepared by decomposition of a vanadia phase on anatase and rutile modifications of titania under conditions in which the solid state reactions involving formation of solid solutions of vanadium in rutile are excluded. It has been shown that anatase-containing catalysts show higher activity and selectivity in o-xylene oxidation, lower reduction degree of the catalyst in the stationary state of catalytic reaction, and lower dehydration/dehydrogenation ratio as compared with rutile-containing samples. Properties of the rutile-containing catalysts resemble those of pure vanadia: the modifying action of anatase is observed even at low vanadia concentration (XXX 1.0 mole %) and it is ascribed to the formation of a special structure of vanadia of possibly a monolayer type on the anatase surface which exhibits the isopropanol dehydrogenation centres and has low acidity.

Silica supported molybdena catalysts: Characterization and methane oxidation

Abstract The Mo/SiO2 catalysts (1–15 wt.-% Mo), prepared by impregnation of aerosil silica with ammonium heptamolybdate, were characterized by physical techniques and tested in methane oxidation using nitrous oxide and oxygen as the oxidizing agents. Characterization by X-ray photoelectron spectroscopy (XPS), laser Raman spectroscopy, surface potential techniques and acidimetric titration resulted in three ranges of molybdenum loading being distinguished in which the nature of the Mo species and the mode of its distribution were different. The present data principally confirmed the results previously reported in the literature on the nature of Mo species dispersed on silica, but in addition they demonstrated the mode of distribution of different forms of molybdenum. For a Mo content between 1 and 5 wt.-% Mo, silicomolybdic acid (SMA) which covered the silica uniformly, was the main Mo species. At Mo loadings between 5 and 10 wt.-% the presence of polymolybate species on top of the SMA layer was observed, the coverage of the silica surface being constant. At still higher Mo loading the amount of SMA decreased to zero (at 15 wt.-% Mo) and crystalline MoO3 appeared, covering the surface of both silica and the polymolybdates. The oxidation of methane by nitrous oxide on samples in the first range yielded formaldehyde as a partial oxidation product, carbon monoxide being the other principal product. The yield and selectivity to formaldehyde increased with the amount of SMA in the catalysts. At higher Mo content total oxidation was the predominant reaction, although small amounts of C2 products of the methane oxidative coupling were also observed. In the presence of oxygen as the oxidizing agent methane conversion was higher, the main reaction in the whole range of the Mo content being total oxidation.

X-ray and ultraviolet-photoelectron spectra of bismuth molybdate catalysts

Changes of the surface structure of bismuth molybdate catalysts Bi2MoO6, Bi2Mo2O9, and Bi2(MoO4)3 after outgassing, after interaction with hydrogen and after interaction with propylene-oxygen mixture have been studied by XPS and UPS. Outgassing of MoO3 and Bi2MiO6 results in surface reduction of Mo6+ to Mo4+ions, Mo4+ constituting two kinds of donor center above the valence band. This process is enhanced on interaction with H2, but it is only in later stages of the reduction that Bi3+ ions become reduced. In the conditions of the catalytic oxidation of propylene the surface composition of the three bismuth molybdate phases reflects that of the bulk, whereas after vacuum or reducing treatment the three compounds show similar values of the BiMo ratio.

The chemistry of Bi-Mo oxide catalysts: I. Phase composition of catalysts and its relation to the structure of precursors

The effect of the conditions of preparation and thermal treatment on the phase composition of Bi-Mo oxide catalysts at the atomic ratio BiMo = 11 has been studied. It has been shown that bismuth molybdate of the composition Bi2O3·2MoO3 (β-phase) is formed on crystallization of the precipitate prepared from appropriate salts at pH = 2 without additional reactions in the solid state. In the case of precipitation at pH = 7 and subsequent filtering, the calcined catalysts contained Bi2O3·MoO3 (γ-phase). The precursor formed in these conditions was identical to the precipitate obtained by treating the precursor of the β-phase with concentrated aqueous ammonia, which led to dissolution of half of the molybdenum present in the sample. Comparison of ir spectra leads to the conclusion that the β-phase precursor is composed of layers of [MoO6] octahedra and [MoO4] tetrahedra, the latter being dissolved on ammonia treatment leaving [MoO6] octahedra as the precursor of the γ-phase (koechlinite). Catalysts obtained by precipitation at pH = 7 and subsequent evaporation are composed of a mixture of γ- and α-phase (Bi2O3·3MoO3). The β-phase is unstable in the temperature range 400–550 °C, disproportionating on prolonged heating into α- and γ-molybdates. The region of stability of the β-molybdate is from 550–670 °C.

Oxidative dehydrogenation of isobutane on supported chromia catalysts

Abstract Oxidative dehydrogenation, ODH, of isobutane has been performed on chromia supported on SiO 2 , Al 2 O 3 , TiO 2 , ZrO 2 and MgO. The potassium promoted (K/Cr= 0.1) catalysts have also been examined. The supported chromia has been found active in the ODH of isobutane at relatively low temperatures 200–400°C, the total activity and selectivity to isobutene depending on the nature of the support and the potassium presence. The highest selectivities to isobutene (ca. 70% at 5% conversion) and the isobutene yields of ca. 9% have been obtained for Cr/O x TiO 2 and K-promoted CrO x /Al 2 O 3 preparations.

Effect of Potassium Addition to V2O5/TiO2 and MoO3/TiO2 Catalysts on Their Physicochemical and Catalytic Properties in Oxidative Dehydrogenation of Propane

Addition of potassium to V2O5 / TiO2 and MoO3/TiO2 catalysts leads to the increase in selectivity in oxidative dehydrogenation of propane to propene, the effect being particularly distinct for catalysts with high specific surface area TiO2. The amelioration of the selectivity can be related to the modification of some physicochemical properties of the catalysts in the presence of K. It has been shown, that the potassium addition brings about the decrease in acidity, as inferred from the decrease in isopropanol dehydration to propene, lowers surface potential (work function) and hinders the formation of electrophilic O− species. The coverage of TiO2 with VOx and MoOx species is higher in the presence of K, as suggested by XPS measurements.

Oxidative dehydrogenation of isobutane on Cr-Ce-O oxide II. Physical characterizations and determination of the chromium active species

Abstract Cr–Ce–O catalysts with different Cr contents and prepared by different methods have been characterized by X-ray diffraction (XRD), XPS, ISS and UV–Vis spectroscopies, and the surface potential technique. On the basis of these studies, a model of catalysts has been proposed and the correlations between the structure of the catalysts and their performance in the oxidative dehydrogenation (ODH) of isobutane have been discussed. At low Cr content corresponding to the coverage of the ceria surface with chromium θ 6+ O x species are the only Cr species, irrespective of the preparation method. In the θ range 35–61%, well dispersed Cr 3+ species appear as Cr 2 O 3 . The maximum coverage of 75% is obtained for the samples prepared by impregnation at 20 Cr atoms per nm 2 . From this Cr content, agglomeration of Cr 2 O 3 is observed. The specific rate of isobutene formation increases with the increase in Cr content up to the coverage of about 61% and then remains constant. It is concluded that the activity is due to the well dispersed Cr 6+ O x species on the ceria surface, whereas agglomerates of Cr 2 O 3 do not contribute to the reaction.

O-xylene oxidation on V2O5 - TiO2 oxide system: II. ESR studies of the catalysts

Abstract The ESR spectra of V2O5 deposited in different quantities on the surface of anatase or rutile modification of TiO2 have been registered. The reduced vanadia on both TiO2 modifications is present in the form of (V=O)2+ groups dispersed on the TiO2 surface and V4+ ions in V2O5 phase. (V=O)2+ groups on anatase containing samples are densely placed, and resistant to oxidation in an oxygen atmosphere. Those on rutile are more scarcely dispersed and are, in contrast to anatase samples, easily oxidized. Comparison of total reduction degree with that determined by the ESR method suggests, moreover, the presence of lower vanadium oxides on rutile containing samples.