Molybdate Catalysts Research Articles

Partial oxidation of butadiene to furan at bismuth molybdate catalysts

The partial oxidation of 1,3-butadiene to furan was investigated on pure phases of α- and γ-bismuth molybdate (BiMo) as well as on a mixture of 50% α-BiMo and γ-BiMo. The kinetic measurements were carried out in an isothermal tube reactor with plug flow assumed. Along the length of the catalyst bed the gas phase composition was analysed. The set-up of parallel and consecutive reactions with first-order kinetics made a mathematical modelling possible. Within a temperature range of 380 to 440°C butadiene conversions of up to 75% were achieved. Main organic product was furan with a maximum yield of 11%.

Selective oxidation of hydrogen sulfide over Bi [sbnd]Mo catalysts

The catalytic performances of bismuth-molybdenum binary oxides for the selective oxidation of hydrogen sulfide to elemental sulfur were studied in a fixed-bed glass reactor in the temperature range 200–280°C. A synergistic phenomenon in catalytic activity was observed for the bismuth molybdate catalysts. Under identical reaction conditions, the hydrogen sulfide conversions of the binary oxide catalysts were superior to those of the corresponding single oxide catalysts. The most active bismuth/molybdenum composition for the binary oxide catalysts was found to be 2:3 atomic ratio, which suggests that the compound Bi 2Mo 3O 12 formed in the binary oxides is much more active than the corresponding single oxide catalysts for the selective oxidation of hydrogen sulfide. The reaction rates of hydrogen sulfide oxidation over the catalysts were found to exhibit first-order dependence on the hydrogen sulfide concentration.

Oscillatory Behavior during Methanol Oxidation on Mo–Ca–O/SiO2Catalyst

The authors have recently reported on the observation of an oscillatory behavior during mild methanol oxidation on a silica-supported molybdenum catalyst (1). This unusual behavior in mild oxidation on oxide catalysts is nevertheless well known for CO total oxidation on platinum or palladium catalysts (2-5). The oscillatory behavior appears as a periodic or nonperiodic (chaotic) variation in the reaction rate and selectivities, although the external conditions of the reaction (oven reactor temperature, flow rate, and feed composition) are constant. This phenomenon was extensively studied for total oxidation reactions occurring on metallic catalysts (5) and comparatively, only a few number of examples of mild oxidation reaction on oxide catalysts are reported in the literature (6-9). Nevertheless, the diversity of the catalytic reactions showing an oscillatory behavior suggests that this phenomena is rather common in the complex system formed by catalyst, products, and reactants. The authors here on the study of an oscillatory behavior observed during methanol oxidation on a calcium molybdate catalyst supported on silica. 21 refs., 7 figs., 4 tabs.

Characteristic Features of Raman Band Shifts of Scheelite-Type Molybdate Catalysts Exchanged with the18O Tracer via Redox Reactions

The oxide oxygen of scheelite-type α-MnMoO4, β-CoMoO4, and α-Bi2Mo3O12were exchanged with the18O tracer using a reduction–oxidation method. The Raman band shifts in the spectra of these catalysts were examined. Normal coordination analysis and diatomic approximation reported in previous literature have shown that the Raman bands are correlated to the Mo–O stretchings of these molybdate catalysts. With α-MnMoO4, the band at 945 cm−1corresponding to the shortest Mo=O of a Mo tetrahedron was exchanged preferentially while with β-CoMoO4, which is isotypic with α-MnMoO4, all the oxygen of Mo tetrahedra were exchanged with an increase in the18O exchange. With α-Bi2Mo3O12which has two twin Mo tetrahedra, the bands at 865 and 845 cm−1were shifted preferentially. Oxygen insertion seems to take place more selectively on the Mo tetrahedron sites where Bi ions are present. A comparative study and discussions on the preferential exchange in these molybdates and reoxidation sites have been reported here.

Preparation of nickel-tungstate catalysts by a novel impregnation method

Nickel-tungstate/γ-alumina (NiW) catalysts were prepared by an incipient wetness impregnation method using citric acid as a complexing agent. Citric acid has been used by our research group in preparing cobalt-molybdate and nickel -molybdate catalysts. The extended X-ray absorption fine structure (EXAFS) data of the impregnating solutions indicated that citric acid contributes to the formation of polytungstate anions that are smaller than the dodecatungstate ions formed when conventional ammoniacal solutions are used. Sulfided NiW catalysts prepared by using citric acid showed higher hydrogenation activity and hydrogenation selectivity than NiW catalysts prepared using the conventional ammoniacal solutions.

On the Structures of Mo2O5(OCH3)2 and Mo2O5(OCH3)2 ·2CH3OH

AbstractThe reaction of molybdenum trioxide dihydrate, MoO3·2H2O, with methanol produces the title compounds. That these molybdenum oxy-methoxides decompose with liberation of CH2O suggests that they represent exquisite models for selective oxidation of methanol to formaldehyde over molybdate catalysts. While a number of physical techniques have been employed to elucidate certain of their structural aspects, unfortunately, the detailed structures of these materials remain unknown. Problems, both physical and crystallographic, encountered in attempts to determine these structures are discussed. Also discussed is the recent progress made by employing the complimentary nature of electron and powder diffraction techniques.

Bismuth and mixed bismuth-lanthanide carboxylates as precursors for pure and Ln-promoted bismuth molybdate catalysts

Several bismuth(III) carboxylates (formate, acetate, propionate, oxalate, lactate) and the analogous tervalent lanthanum, samarium and praseodymium formates and acetates were synthesized and characterized in view of their use as precursors for the formation of bismuth- or lanthanide-based molybdate catalysts upon controlled thermal decomposition on MoO3 support. Mixed Bi-Ln carboxylates were prepared and used in parallel as precursors for Ln-promoted bismuth molybdate catalysts. Both the nature of the lanthanide element and the calcination temperature were found to determine the formation of specific molybdate-type phases on the support. Detailed examination of these catalysts by X-ray diffraction and FTIR spectroscopy identified α-M2Mo3O12 phases in Bi- and Pr-containing samples calcined at 823 K, and hexamolybdate phases M2Mo6O21 in La- or Sm-containing catalysts calcined at 973 K.

Bismuth and mixed bismuth-lanthanide carboxylates as precursors for pure and Ln-promoted bismuth molybdate catalysts

Several bismuth(III) carboxylates (formate, acetate, propionate, oxalate, lactate) and the analogous tervalent lanthanum, samarium and praseodymium formates and acetates were synthesized and characterized in view of their use as precursors for the formation of bismuth- or lanthanide-based molybdate catalysts upon controlled thermal decomposition on MoO 3 support. Mixed Bi-Ln carboxylates were prepared and used in parallel as precursors for Ln-promoted bismuth molybdate catalysts. Both the nature of the lanthanide element and the calcination temperature were found to determine the formation of specific molybdate-type phases on the support. Detailed examination of these catalysts by X-ray diffraction and FTIR spectroscopy identified α-M 2Mo 3O 12 phases in Bi- and Pr-containing samples calcined at 823 K, and hexamolybdate phases M 2Mo 6O 21 in La- or Sm-containing catalysts calcined at 973 K.

Effect of alkali metal promoters on nickel molybdate catalysts and its relevance to the selective oxidation of butane

The role of alkali metal promoters (Li, Na, K, Cs) in the α and β phases of NiMoO 4 for butane oxidative dehydrogenation (ODH) was studied. The catalysts were characterized by BET, ICP, X-ray fluorescence, AA, XRD, TG, DSC, IR, and XPS techniques. It was evidenced that the added alkali metals are deposited only on the catalyst surface and practically do not affect the bulk. An influence of the concentration and basicity of the promoters on the decrease of the catalyst surface area was observed. Reaction studies show that addition of promoters generally reduces the catalytic activity but the promoters induce a high selectivity to butenes and butadiene. The greater the basicity and the amount of promoter ion, the higher is the selectivity and this improvement arises largely by suppression of total oxidation. The combination of activity and selectivity effects leads to improvement of yields of C 4 hydrocarbons by addition of small amounts of alkali promoters up to an optimum loading. Overdoping deteriorates yields, specially for β phases. The observed changes are understood by considering the physical and chemical effects related with the interaction of the alkali promoters with the catalyst. Basicity is shown to be fundamental to promote the selective ODH of butane in the NiO MoO 3 system.

Propane oxidation over various metal molybdate catalysts

Catalytic activities of various metal molybdates were tested for the gas-phase partial oxidation of propane with molecular oxygen under an atmospheric pressure in the temperature range 325–500°C. Metal molybdate catalysts were found to mostly promote the oxidative dehydrogenation of propane to propene. More than 80% selectivity to propene was attained on each catalyst but their catalytic activities differed greatly. Of the catalysts, cobalt molybdate showed the highest catalytic performance for the oxidative dehydrogenation and the catalytic property strongly depended on the catalyst composition. Co 0.95MoO x catalyst gave 60% selectivity to propene at 20% conversion of propane at 450°C. The catalytic activity and selectivity of each catalyst in the propane oxidation is discussed in terms of surface acidity and reactivity of lattice oxide ions.

Transient Kinetics from the Tap Reactor System: Application to the Oxidation of Propylene to Acrolein

Transient kinetic data have been extracted by use of the Temporal Analysis of Products (TAP) reactor system. In the TAP reactor high resolution transient pulse experiments are performed under vacuum conditions. A rigorous model is developed and applied to the determination of Knudsen diffusivities from experiments with inert pulses. The model was applied to the catalytic oxidation of propylene over an industrial multicomponent bismuth molybdate catalyst. By fitting simulated curves to the experimentally observed response curves, kinetic parameters for the different elementary steps of the reaction were determined. It was shown that the adsorption of propylene is practically irreversible at reaction temperature and that CO 2 and acrylic acid are formed in consecutive reactions from propylene via acrolein. From the values for the kinetic parameters it is proven that under industrial conditions the adsorption of propylene is rate determining. The surface coverage under typical conditions is shown to be quite low. The kinetic model is successfully used for the simulation of a steady state reactor operating under industrial conditions.

Hydrogenation of Model Compounds in Syngas-D2O Systems

Coal model compounds were hydrogenated and desulfurized in the presence of solvent and catalyst by using syngas with steam in place of hydrogen. Nickel molybdate catalyst exhibited good activities for hydrogenation and desulfurization of model compounds at high pressure and elevated temperature with the use of syngas and steam. Extensive water-gas shift conversion also took place simultaneously. Hydrogenation of phenanthrene and dibenzothiophene was carried out in syngas-water systems, and the results were compared with those obtained in H 2 system. The extent of hydrogenation increased with the increase of H 2 /CO mole ratio in syngas. Hydrogen consumed during the hydrogenation was replenished by the accompanying water-gas shift reaction in syngas-H 2 O systems. Hydrodesulfurization of dibenzothiophene carried out in H 2 -C0-D 2 O and D 2 -C0-H 2O systems yielded deuterated products of cyclohexylbenzene and biphenyl in both systems, indicating that both active deuterium produced via water-gas shift reaction of CO with D 2O as well as deuterium in gas phase participated in the hydrodesulfurization along with the hydrogen/deuterium exchange reaction with substrates

ChemInform Abstract: Oxidative Dehydrogenation of Propane to Propene over Cobalt Molybdate Catalysts.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Oxidative dehydrogenation of propane over magnesium molybdate catalysts

Catalytic activities of magnesium molybdates were investigated for the oxidative dehydrogenation of propane with and without molecular oxygen under atmospheric pressure. Catalytic properties drastically changed with the catalyst composition, and it turned out that Mg0.95MoOx catalysts having slight excess molybdenum showed the highest activity in the oxidative dehydrogenation of propane, which gave 61% selectivity to propene at 22% conversion of propane at 515°C. The catalytic activities strongly depended on the acidic properties of the catalysts. It was also revealed that the lattice oxide ions of the catalysts participated as an active oxygen in the oxidative dehydrogenation of propane.

Oxidative Dehydrogenation of Propane to Propene over Cobalt Molybdate Catalysts

Abstract Cobalt molybdates showed high catalytic activities for the oxidative dehydrogenation of propane to propene. Composition of the catalyst strongly affects surface acidic properties, resulting in a drastic activity change. Co0.95 MoO4 calcined at 550 °C showed the highest catalytic performance, giving 60% selectivity to propene at 19% conversion of propane.

Investigations of the mechanisms and kinetics leading to a loss of molybdenum from bismuth molybdate catalysts

The mechanisms and kinetics leading to a loss of molybdenum from bismuth molybdate catalysts with SiO 2 and Al 2O 3 as supports, respectively, were studied as the basis of research into the reasons for the loss of molybdenum from single MoO 3 catalysts. It was found that the loss of molybdenum was due to the sublimation of MoO 2(OH) 2, formed by the reaction of active species with water vapor. The kinetics leading to the loss of molybdenum in the presence of water vapor are expressed as r=kVC 3.5 s. The difference in the rate of molybdenum loss from single MoO 3 catalysts and bismuth molybdate catalysts are discussed.

Surface vs. bulk composition of an oxidic FeMo multicomponent catalyst

Multicomponent molybdate catalysts are active and selective contacts for the ammoxidation of propene. They exhibit a finely grained consistency. Mössbauer surface (CEMS) and transmission spectra show equivalent patterns. Consequently, the catalytically active material taking part in the reaction is not localized on the surface, but the whole of the grains is involved in the catalytic process.

Characterization, Activity, and Adsorption/Desorption Behavior of Alkali-Promoted Molybdate Catalysts for the Oxidative Coupling of Methane

MnMoO 4 and alkali-promoted (Li, Na, or K) MnMoO 4 catalysts have been examined for the methane oxidative coupling reaction. The catalysts were characterized by XRD, XPS, LRS, TPR, and TPD. The molybdate structure appeared to remain intact, although the presence of the promoter ions was found to change the TPR and TPD profiles. Increased selectivity to C 2 hydrocarbons has been observed for similar conversion levels (Driscoll, Zhang, and Ozkan, in "Catalytic Selective Oxidation" (S. T. Oyama and J. W. Hightower, Eds.), Vol. 523, p. 340. ACS, Washington, DC, 1993) through the addition of the alkali-promoter ions, especially potassium. Further examination of the catalysts at equal residence times revealed an increased activity due to the addition of the alkali-promoter ions, with no change in activity or selectivity when excess carbon dioxide was added to the feed. While the TPD experiments provided clues as to the nature and function of surface oxygen, laser Raman spectroscopy studies provided evidence of the existence of carbonyl groups on the surface.

Laboratory evaluation of four coal liquefaction catalysts prepared from modified alumina supports

A blank alumina catalyst support was modified by deposition of TiO 2, ZrO 2 or carbon, and cobalt molybdate catalysts prepared from the modified supports were evaluated in a laboratory coal liquefaction microreactor unit. Performance was referenced to a commercial CoMo/alumina catalyst, Amocat 1A, which was derived from the same blank support. All catalysts possessed very similar pore structures, and thus the effect of pore structure was practically eliminated from the comparison. The effects of support modification on catalyst performance were found to be minimal.

Coupling of Oxidative Dehydrogenation and Aromatization Reactions of Butane

Coupling of oxidative dehydrogenation and aromatization of butane by using a dual function catalyst has led to a significant enhancement of the yields (from 25 to 40%) and selectivities to aromatics (from 39 to 64%). Butane is converted to aromatics by using either zinc-promoted [Ga]-ZSM-5 or zinc and gallium copromoted [Fe]-ZSM-5 zeolite as a catalyst. However, the formation of aromatics is severely limited by hydrocracking of butane to methane, ethane, and propane due to the hydrogen formed during aromatization reactions. On the other hand, the oxidative dehydrogenation of butane to butene over molybdate catalysts is found to be accompanied by a concurrent undesirable reaction, i.e., total oxidation. When two of these reactions (oxidative dehydrogenation and aromatization of butane) are coupled by using a dual function catalyst they have shown to complement each other. It is believed that the rate-limiting step for aromatization (butane to butene) is increased by adding an oxidative dehydrogenation catalyst (Ga-Zn-Mg-Mo-O). The formation of methane, ethane, and propane was suppressed due to the removal of hydrogen initially formed as water. Studies of ammonia TPD show that the acidities of [Fe]-ZSM-5 are greatly affected by the existence of metal oxides such as Ga 2O 3, MgO, ZnO, and MoO 3.