Changes in the phase composition of a multicomponent Co-Mo-Bi-Fe-Sb-K-O catalyst under conditions of the partial oxidation of isobutylene to methacrolein: A comparison with the oxidation of propylene to acrolein

The partial oxidation of isobutylene to methacrolein on a multicomponent multiphase Co-Mo-Bi-Fe-Sb-K-O catalyst and on catalysts from which some components were absent was studied. Activity and selectivity changes in the case of isobutylene oxidation were the same as in the oxidation of propylene; however, the rate of propylene oxidation was higher than that of isobutylene oxidation. The X-ray diffraction analysis of the catalysts before and after the reaction indicated the occurrence of a number of phases in the samples: α-CoMoO₄, β-CoMoO₄, Fe₂(MoO₄)₃, reduced bismuth molybdate species, MoO₃, and reduced MoO _x species. Under catalytic reaction conditions, redox phase transformations occurred. Iron molybdate and molybdenum oxide phases underwent the largest transformations. Of two molybdenum oxide phases, a β-Mo₄O₁₁ phase with a structure of a crystallographic shift is formed in the course of catalysis, whereas the second phase (MoO₃) almost does not participate in catalysis and occurs in an excess.     

Action of Co-Mo-Bi-Fe-Sb-K Catalysts in the Partial Oxidation of Propylene to Acrolein: 1. The Composition Dependence of Activity and Selectivity

The role of various components of a multiphase oxide catalytic system in the partial oxidation of propylene to acrolein is investigated. Catalytic activity is studied for the Co_6–8Mo₁₂Fe_2–3Bi_0.5–0.75Sb_0.1K_0.1O_x catalyst, which is taken to be the reference, and for catalysts in which the amount of some component is progressively reduced down to zero. The results obtained provide insights into the role of the components of the catalyst.CoMoO₄ forms the structural framework of the catalyst. Iron molybdate can be stabilized on CoMoO₄ as β-phase. As its content is increased, the catalyst gains activity but its selectivity declines. Bismuth molybdate is responsible for the selectivity of the process. When present in small amounts, MoO₃ raises the selectivity, binds free oxides, and converts reduced molybdates into their oxidized forms. Excess molybdenum trioxide causes a dramatic fall in the catalytic activity. Potassium and antimony decrease the catalytic activity, but even small amounts of these elements raise the selectivity of the catalyst. Chromium can substitute for iron atoms in the multicomponent catalyst. Ni, Mn, and Mg substitute for Fe in iron molybdate to decrease the catalytic activity.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 569–579.Original Russian Text Copyright © 2005 by Udalova, Shashkin, Shibanova, Krylov.     

Recovery and Reduction of Spent Alumina-Supported Cobalt–Molybdenum Oxide Catalyst via Plasma Sintering Technique

A thermal plasma process for the recovery and reduction of the spent alumina-supported cobalt–molybdenum oxide catalyst (Co₃O₄–MoO₂/Al₂O₃) was developed. The spent catalyst was sintered at >1,500 K under plasma condition and the cobalt–molybdenum oxide therein was reduced to cobalt–molybdenum, which was proven by XRD and EDX. By application of SEM and GC technique, the organic tar on the surface of the spent catalyst was found to decomposed and converted to syngas (CO, CO₂, and H₂), which might be the reducing agents in the process. A gasification mechanism for the generation of syngas and the reduction of cobalt–molybdenum oxide under plasma conditions was proposed.     

Phosphorus modified MoO<Subscript>3</Subscript>–Bi<Subscript>2</Subscript>SiO<Subscript>5</Subscript>/SiO<Subscript>2</Subscript> catalyst for gas-phase epoxidation of propylene by molecular oxygen

The phosphorus (P) modified MoO₃–Bi₂SiO₅/SiO₂ catalyst was prepared by a simple co-impregnation method and investigated in the epoxidation of propylene by molecular oxygen. The catalyst was characterized by X-ray diffraction (XRD), N₂ adsorption–desorption analysis, NH₃-temperature-programmed desorption (NH₃-TPD), transmission electron microscopy, and Raman spectroscopy. It was found that the P-modified MoO₃–Bi₂SiO₅/SiO₂ catalyst with a P/Mo molar ratio of 0.5 exhibits the best catalytic performance for epoxidation of propylene by O₂, the TOFs for propylene oxide (PO) formation was four times higher than that of the unmodified one at 633 K. The modification by P could promote the dispersion of MoO₃ nanoparticles and increase the number of weak and moderate acid sites with respect to the phosphorus-free MoO₃–Bi₂SiO₅/SiO₂ catalyst, which were beneficial to the formation of PO. Moreover, the introduction of P also could protect the mesoporous structure by inhibiting the formation of Bi₂Mo₃O₁₂, which was beneficial to the dispersion of active species. We suppose that the phosphorus, bismuth and molybdenum species of P-modified MoO₃–Bi₂SiO₅/SiO₂ catalyst play important roles for propylene epoxidation by molecular oxygen.     

Reactions of Neutral and Charged Molybdenum Oxide Clusters with Low-Molecular-Weight Alcohols in a Gas Phase Studied by Ion Cyclotron Resonance

A set of oxygen-containing molybdenum oxide clusters Mo _x O _y (x = 1–3; y = 1–9) was obtained with the use of a combination of a Knudsen cell and an ion trap cell. The reactions of positively charged clusters with C₁–C₄ alcohols were studied using ion cyclotron resonance. The formation of a number of organometallic ions, the products of initial insertion of molybdenum oxide ions into the C–O and C–H bonds of alcohols, and polycondensation products of methanol and ethanol were found. The reactions of neutral molybdenum oxide clusters Mo _x O _y (x = 1–3; y = 1–9) with protonated C₁–C₄ alcohols and an ammonium ion were studied. The following limits of proton affinity (PA) were found for neutral oxygen-containing molybdenum clusters: (MoO) < 180, (Mo₂O₄, Mo₂O₅, and Mo₃O₈) = 188 ± 8, PA(MoO₂) = 202 ± 5, PA(MoO₃, Mo₂O₆, and Mo₃O₉) > 207 kcal/mol.     

Phase relations in the system K2MoO4–KPO3–MoO3–Bi2O3: A new phosphate K3Bi5(PO4)6

Phase equilibrium in the pseudo-quaternary system K₂O–MoO₃–P₂O₅–Bi₂O₃ was studied as three-component solvent K₂MoO₄–KPO₃–MoO₃ containing 15 mol% Bi₂O₃ during slow cooling and spontaneous crystallization. The results of the investigation were shown on a composition diagram, which indicates the crystallization fields of K₂Bi(PO₄)(MoO₄), K₅Bi(MoO₄)₄, BiPO₄ and K₃Bi₅(PO₄)₆. New phosphate K₃Bi₅(PO₄)₆ was characterized by single-crystal X-ray diffraction (space group C2/c, a=17.680(4), b=6.9370(14), c=18.700(4) Å, β=113.79(3)°) and FTIR spectroscopy. The possibility of lone electron pair stereoactivity of bismuth was suggested using the calculations of characteristics of the Voronoi–Dirichlet polyhedra for K₃Bi₅(PO₄)₆ and K₂Bi(PO₄)(MoO₄).     

Catalyst and technology for production of carbon nanotubes

Modifying the iron-aluminum catalyst with molybdenum oxide affords a marked increase in the yield of carbon nanotubes from 1,3-butadiene diluted with hydrogen. The optimum catalyst composition is 6.5% MoO₃-52% Fe₂O₃-Al₂O₃. With this catalyst, it is possible to obtain over 100 g of carbon nanotubes per gram of catalyst in a reactor fitted with a McBain balance. Replacing expensive 1,3-butadiene with the cheaper commercial propane-butane mixture (80 mol % propane + 20 mol % butane) leads to a sharp decrease in the nanotube yield because of the lower reactivity of the latter. Based on the concept of the catalytic decomposition of hydrocarbons and the formation of nanosized carbon materials via the carbide cycle mechanism, a new, efficient, CoO-MoO₃-Fe₂O₃-Al₂O₃ catalyst has been developed. The enhancement of the activity of the MoO₃-Fe₂O₃-Al₂O₃ catalyst by promoting it with cobalt oxide is achieved without a change in the rate-limiting step of the process. The design of a continuous reactor for carbon nanotube synthesis is suggested. The characteristics of the resulting nanotubes are presented.     

Growth of carbon nanotubes from butadiene on a Fe-Mo-Al2O3 catalyst

The MoO₃-Fe₂O₃-Al₂O₃ catalysts were prepared from metal nitrates using a coprecipitation method. It was found that the modification of an alumina-iron catalyst with molybdenum oxide resulted in the formation of a solid solution based on hematite, in which a portion of iron ions was replaced by aluminum and molybdenum ions. The MoO₃-Fe₂O₃-Al₂O₃ catalyst was reduced with a reaction mixture at 700°C. Under the action of 1,3-butadiene diluted with hydrogen, the solid solution based on hematite was initially converted into magnetite and then into an Fe-Mo alloy. The modification of an alumina-iron catalyst with molybdenum oxide considerably changed its properties in the course of carbon nanotube formation. As the Mo content was increased, the yield of carbon nanotubes passed through a maximum. The optimum catalyst was 6.5% MoO₃–55% Fe₂O₃-Al₂O₃. The addition of small amounts of MoO₃ (to 6.5 wt %) to the aluminairon catalyst increased the dispersity and modified the properties of active metal particles: because of the formation of an Fe-Mo alloy, the rate of growth decreased but the stability of carbon nanotube growth and the yield of the nanotubes increased. A further increase in the molybdenum content decreased the yield because molybdenum is inactive in the test process.     

The determination of surface polarity of bismuth-molybdenum oxidation catalysts by gas chromatography

Summary The adsorption of aromatic and aliphatic hydrocarbons was investigated using gas chromatography on Bi₂O₃, MoO₃ and mixed Bi–Mo oxidation catalysts. As a measure of polarity of a catalyst, the difference between the chemical potential of aromatic and aliphatic hydrocarbons at the same surface concentration was used. The chemical potentials were estimated from elution chromatographic data. The data for C₆–C₉ methylbenzenes and C₆–C₁₂ n-alkanes were obtained in the temperature range 60–300°C in nitrogen as a carrier gas. Using air as carrier gas, introduction of water pulses on a catalyst does not change the elution characteristics. The elution of alkenes, alkynes, dienes and carbonyl compounds was disturbed by reaction of these compounds on the surface. The polarity of catalysts decreased in the order mixed Bi–Mo catalysts, MoO₃, Bi₂O₃. The polarities observed are compared with polarities of some other solids and liquids and the role of polarity of the surface in catalytic oxidation reactions is briefly discussed.     

Research on the electrochemistry of oxygen ion conductors in the former Soviet Union

Following previous reviews of research results on oxygen ion-conducting materials obtained in the former USSR, this article addresses the case of Bi₂O₃-based compositions. Phase formation in oxide systems with Bi₂O₃, thermal expansion, stability, bulk transport properties and oxygen exchange of bismuth oxide solid electrolytes are briefly discussed. Primary attention is focused on oxides with high ionic and mixed conductivity, including stabilized fluorite-type (δ) and sillenite (γ) phases of Bi₂O₃, γ-Bi₄V₂O₁₁ and other compounds of the aurivillius series. Another major point being addressed is on the applicability of these materials in high-temperature electrochemical cells, which is limited by numerous specific disadvantages of Bi₂O₃-based ceramics. The electrochemical properties of various electrode systems with bismuth oxide electrolytes are also briefly analyzed. Electronic Publication     

Selective oxidation using catalyst electrodes supported on solid electrolyte

The major oxygenated products of the selective oxidation of 1-butene by using a catalyst electrode were maleic anhydride on V₂O₅/YSZ/Ag and butadiene on MoO₃–Bi₂O₃–Ag/YSZ/Ag. Their selectivities were enhanced as compared with the non-electrochemical system.     

Microstructure and Conduction of Composites Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> Near the Eutectic Melting Point

Microstructure and conduction of ceramic composites Bi₂CuO₄ + xBi₂O₃ (x = 5, 10, 15, 20 wt %) near the eutectic melting point (770°C) are studied. Bismuth oxide, initially randomly distributed over the ceramics bulk, after quenching from temperatures exceeding the eutectic melting point, becomes localized at triple junctions and grain boundaries in Bi₂CuO₄, which is caused by wetting grain boundaries and forming a liquid-channel structure. The jumpwise change in the composites’ conductivity near 730 and 770°C caused by polymorphic transformation of Bi₂O₃ and the eutectic melting with simultaneous formation of a liquid-channel structure. Transport numbers of the oxygen ion are measured at 770°C by coulomb-volumetric method. The conduction by oxygen ions increases in the composites with decreasing average size of Bi₂CuO₄ crystallites.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 596–601.Original Russian Text Copyright © 2005 by Lyskov, Metlin, Belousov, Tret’yakov.     

Subsolidus phase relations in the Na2MoO4-CoMoO4-Cr2(MoO4)3 system

Triple molybdate NaCoCr(MoO₄)₃, a phase of variable composition Na₂MoO₄-CoMoO₄-Cr₂(MoO₄)₃ (0 ≤ x ≤ 0.5) having nasicon structure (space group R $ \bar 3 $ \bar 3 c), and triple molybdate NaCo₃Cr(MoO₄)₅ crystallizing in triclinic space group P $ \bar 1 $ \bar 1 were synthesized in the subsolidus region of the Na₂MoO₄-CoMoO₄-Cr₂(MoO₄)₃ ternary salt system. Crystal parameters were calculated for the newly synthesized molybdates and phases. The vibration spectra of Na_{1 − x} Co_{1 − x} Cr_{1 + x} (MoO₄)₃ and electrophysical properties were studied. Upon Na + Co → Cr(III) substitution, chromium cations are distributed to cobalt sites and additional vacancies are generated in the sodium sublattice.     

Effect of the Conditions of Thermal Treatment of Molybdenum–Titanium and Vanadium–Molybdenum–Titanium Oxide Catalysts on Pore Structure Formation

The effect of the conditions of thermal treatment on the texture formation in molybdenum–titanium oxide (Mo–Ti–O) and vanadium–molybdenum–titanium oxide (V–Mo–Ti–O) catalysts was studied. It was found that the presence of MoO₃ in the Mo–Ti–O catalyst resulted in the stabilization of the surface area of anatase and in the retention of the fine pore structure upon thermal treatment because of the insertion of highly dispersed molybdenum crystallites into the aggregates of anatase crystallites, preventing from their agglomeration over a wide range of temperatures. In the presence of MoO₃ and V₂O₅ in the catalyst, anatase particles underwent agglomeration as the temperature was increased. This resulted in a more drastic decrease in the specific surface area and an increase in the pore size, as compared with binary samples, because of the formation of a thermally labile vanadium–molybdenum compound at the surface of anatase.     

Phase Composition and Charge Transport in Bismuth Molybdates

The study of structural and electrical properties of three pure bismuth molybdate phases, α-Bi₂Mo₃O₁₂, β-Bi₂Mo₂O₉ and γ-Bi₂MoO₆, prepared by the spray drying technique, is described and discussed. The structure of polycrystalline, layered samples investigated by means of XRD and Raman spectroscopy was found to be monoclinic for the α- and β-phases and orthorhombic for the γ-phase. The microstructure of the as-prepared samples was not sufficiently developed under the given conditions of preparation, however, the thermal treatment can improve it. The high polarizability of Bi³⁺ cations with their lone-pair electrons influences the stability of the disordered oxygen sublattice. All as-prepared phases undergo a slight structural change in the temperature region of 280–430°C resulting in a decrease of the electrical conductivity probably due to an order ⇄ disorder transition in the oxygen arrangement during the sample heating. The change of electrical conductivity observed was found to be reversible in the high-temperature region and irreversible in the low-temperature one. The blocking of oxygen transport by the bismuth lone-pair electrons results in an increase of the activation energy and a decrease of the electrical conductivity in the high-temperature region. Relatively high relative dielectric permittivities ɛ_r, 36–52, were observed in dependence on the investigated phase.__________From Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 523–528.Original English Text Copyright © 2005 by Hartmanova, Le, Driessche, Hoste, Kundracik.This article was submitted by the authors in English.     

The support and characterization of C60 and MoO3 on Al2O3

A new type of catalyst from supporting C₆₀ on MoO₃ and Al₂O₃ has been prepared. The effect of different order of impregnation and calcination atmosphere on catalyst are investigated by the solution test in toluene, UV-VIS spectroscopy and temperature programmed reduction (TPR). The results show that when the catalyst was prepared by supporting MoO₃ on C₆₀/Al₂O₃ and calcined in N₂, there is a stronger interaction between C₆₀, MoO₃ and Al₂O₃, but when supporting C₆₀ on MoO₃/Al₂O₃, the interaction is relatively weak. We consider that in the former method a new complex, Mo–C₆₀–O–Al, is formed.     

Bi7−xAs1+xMo3O21 (0⩽x⩽1) a new pillar structure type: synthesis,crystal structure and conductivity

An original structure of chemical formula Bi₁₃As₃Mo₆O₄₂ has been obtained in the system Bi₂O₃:MoO₃:As₂O₃ by chemical transport reaction in presence of As₂O₃. It crystallizes in the monoclinic system, space group P2₁/n with a=12.7770(11) Å, b=5.5890(4) Å, c=27.971(2) Å and β=101.009(7)°. The structure exhibits infinite [Bi₁₃As₃Mo₆O₄₂]_n complex pillars with a quite different organization compared with original [Bi₁₂O₁₄]_n⁸ⁿ⁺ columns surrounded by (MoO₄) tetrahedra in the Bi_2/3[Bi₁₂O₁₄](MoO₄)₅ prototype structure. Nevertheless, the heavy atoms design almost perfect fluorite subnetwork—a common structural feature of these pillar structures. The conditions of synthesis via solid-state chemistry using basic oxides Bi₂O₃, As₂O₃ and MoO₃ have been established and the phase identified by X-ray powder pattern. The indexing fits single crystal data as well as the values of volumic mass, ρ_exp=7.04(4) g cm⁻³ for ρ_X=7.096 g cm⁻³ for Z=4. This Bi₁₃As₃Mo₆O₄₂ phase shows also an interesting anionic conductivity around σ=7.98×10⁻⁴ S cm⁻¹ at 980 K and is compared with related phases.     

Investigation par analyse thermique differentielle du systeme Gd2(MoO4)3-Bi2(MoO4)3

The phase diagram of the system Gd₂(MoO₄)₃-Bi(MoO₄)₃ has been studied by differential thermal analysis (DTA). Sealed platinum tubes were used as sample holders, in order to prevent the loss of Bi₂O₃ and MoO₃ through volatilization at high temperature. Various solid solutions and new phases are reported: α-Gd_2-x-Bi_x(MoO₄)₃, β -Gd_2-x-Bi_x(MoO₄)₃, α-Bi_2-xGd_x(MoO₄)₃, 3Gd₂(MoO₄)₃·2Bi₂(MoO₄)₃, etc.     

Microstructure and microanalysis of bismuth molybdates

The microstructures of commercially important bismuth molybdate catalysts in relation to olefin oxidation reactions are examined by electron microscopy (EM) techniques. The microstructural characterization has been carried out using dynamic (in situ) EM, high resolution EM, and microanalysis. The coprecipitated catalyst system Bi₂MoO₆ or γ, together with the γ phase, contains small amounts of tetragonal Bi₂MoO₆ phase, Bi₂Mo₃O₁₂ (α phase), Bi₂O₃, and MoO₃. In reduction with propylene, at catalyst operating temperatures of 400–500°C, in the dynamic experiments conducted on α- and γ-phase crystallites under reaction conditions no evidence for extended defects such as crystallographic shear planes has been obtained, instead an ordered intermediate phase similar to (101) Bi₂Mo₂O₉ (β phase) is observed which is found to be unstable. Observations by electron microscopy have been confirmed with parallel measurements made in a reactor connected to a gas chromatograph and mass spectrometer system. The possible influence of the microstructural changes on the catalytic behavior of the system is examined.     

Possibility of Mo<Subscript>2</Subscript>O<Subscript>6</Subscript> formation at high temperatures in the range of pressures from 1 to 1 × 10<Superscript>−5</Superscript> bar

Gibbs free energy minimization was used to consider the formation of complex molybdenum oxide (Mo₂O₆) at 2400 K in the range of pressures from 1 to 1 to 1 × 10⁻⁵ bar for the basic component ratio Mo: O₂ = 1: 1. Several ways are shown to lead to Mo₂O₆ formation: when P = 1 bar, a synthesis reaction involving simple molybdenum oxides (MoO, MoO₂, MoO₃) is the main way; when P = 1 × 10⁻³ bar or lower, reactions of (MoO₃) _n (n = 3−5) complex oxides with metallic molybdenum and molybdenum monoxide (MoO) are.