George W. Keulks

The catalytic oxidation of propylene: VI. Mechanistic studies utilizing isotopic tracers

Abstract The mechanism of propylene oxidation has been investigated over well-characterized selective oxidation catalysts: Bi 2 Mo 3 O 12 , Bi 2 MoO 6 , and Bi 3 FeMo 2 O 12 . Oxygen-18 and deuterated propylene were used as tracers under steady-state reaction conditions over the temperature range 350 to 450 °C. These data indicate that acrolein is formed exclusively via the redox mechanism over the molybdate catalysts and that numerous sublayers of lattice oxygen are involved. Carbon dioxide is formed by the consecutive oxidation of acrolein over Bi 2 Mo 3 O 12 and Bi 2 MoO 6 and by the consecutive and parallel pathways over Bi 3 FeMo 2 O 12 . The rate-limiting step of acrolein formation is the abstraction of an allylic hydrogen from propylene.

SELECTIVE OXIDATION OF PROPYLENE

Publisher Summary This chapter focuses on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the questions in mind, such as “How is propylene activated? What is the nature of the reactive oxygen species?” The selective oxidation of propylene is an important model reaction for studying oxidation reactions over oxide catalysts. Much information has been gathered over the past three decades that helps to answer these questions. Considerable evidence exists that indicates the selective oxidation of propylene proceeds via the formation of a symmetrical ally species. Subsequent steps may vary as a function of the catalyst. Some catalyst systems may abstract a second hydrogen atom before the insertion of oxygen. Others may add molecular oxygen, forming a hydroperoxide intermediate, which may then subsequently decompose into acrolein and water. While a number of oxygen species can exist on the surface of oxides, the reactive oxygen for the selective oxidation of propylene is lattice oxygen. Under reaction conditions, solid-state reactions occur that are a function of the temperature and the composition of the reacting gas mixture.

The investigation of the type of active oxygen for the oxidation of propylene over bismuth molybdate catalysts using infrared and Raman spectroscopy

The investigation of the type of active oxygen for the oxidation of propylene over bismuth molybdate catalysts using infrared and Raman spectroscopy showed that in the ..cap alpha..-phase (Bi/sub 2/Mo/sub 3/O/sub 12/) only a few types of lattice oxygens, possibly bridged oxide ions, were involved in the reaction; in the ..beta..-phase (Bi/sub 2/Mo/sub 2/O/sub 9/) most, but not all types of lattice oxygen were involved; and in the ..gamma..-phase (Bi/sub 2/MoO/sub 6/) all lattice oxygens were involved. The tests were conducted with oxygen-18 labeled oxygen.

The catalytic oxidation of propylene: VIII. An investigation of the kinetics over Bi2Mo3O12, Bi2MoO6, and Bi3FeMo2O12

Abstract The kinetics of propylene oxidation have been investigated over the temperature range of 325 to 475 °C. The apparent activation energies changed from 15 to 18 kcal/mole at high temperature (> 400 °C) to 43–53 kcal/mole at lower temperatures. The reaction orders for both oxygen and propylene also changed with temperature. It was found that these changes in the kinetic parameters could be completely explained in terms of the coupled kinetics of catalyst reduction and reoxidation.

The catalytic oxidation of propylene: IX. The kinetics and mechanism over β-Bi2Mo2O9

Abstract The kinetics and mechanism of propylene oxidation over β-Bi2Mo2O9 from 300 to 470 °C have been investigated. By using oxygen-18 and deuterated propylenes under steady-state reaction conditions and temperatures ranging from 350 to 450 °C, it was determined that the selective oxidation of propylene to acrolein over the β-phase occurs via the redox mechanism through the involvement of numerous sublayers of lattice oxygen. From the kinetic and isotopic data it was learned that the kinetics and energetics of propylene oxidation over the β-phase can be completely described in terms of the coupled kinetics of catalyst reduction and reoxidation. At the higher temperatures in which the rate of acrolein formation is limited by catalyst reduction, the apparent activation energy is approximately 20 kcal/mole and is indicative of allyl formation from adsorbed propylene. At lower temperatures the rate of acrolein formation is limited by catalyst reoxidation and the apparent activation energy is approximately 43 kcal/mole. The kinetic dependencies of oxygen and propylene also reflect the changes in the rate-determining step of the reaction. Carbon dioxide is produced from both the consecutive oxidation of acrolein and the oxidation of a hydrocarbon residue which is present on the surface of the catalyst at steady-state conditions; the former pathway predominates at low temperatures (below 400 °C), while the latter pathway contributes significantly at high temperatures to carbon dioxide formation. Both pathways utilize only lattice oxygen; the extent of lattice oxygen participation is approximately the same as acrolein formation.

The catalytic oxidation of propylene: I. Evidence for surface initiated homogeneous reactions

Abstract The catalytic oxidation of propylene over bismuth molybdate was studied in a flow reactor at atmospheric pressure and at 425 °C. Several reactor designs were utilized in an effort to define more clearly the nature and the importance of homogeneous reactions which may be occurring in the postcatalytic volume. Evidence is presented for a surface initiated homogeneous reaction which results in an enhanced conversion of propylene and the formation of propylene oxide. Allyl peroxide or allyl hydroperoxide species, formed on the surface, are suggested as possible initiators for this homogeneous reaction. A modified reaction mechanism is proposed which incorporates the surface initiated homogeneous reaction, as well as a surface reaction of the allyl peroxide or allyl hydroperoxide species. Four additional oxide catalysts, copper-molybdenum, iron-molybdenum, manganesemolybdenum, and zinc, are offered as other examples on which the surface initiated homogeneous reaction occurs.

The physicochemical properties of the bismuth iron molybdate system

Abstract A number of spectroscopic techniques were used to obtain further information about the scheelite structure of compounds found in the ternary oxide system of bismuth, molybdenum, and iron. Infrared data indicated the presence of discrete tetrahedral and corner sharing octahedral molybdenum. Ultraviolet-visible spectroscopy provided evidence for the presence of an additional electronic level in the lattice, resulting in a shift of the charge transfer energy to longer wavelengths. The combination of ir and uv-visible spectroscopy indicated that at high temperatures distortion of the metal-oxygen bonds occurs. Iron was found to be present as Fe 3+ and in two nonequivalent environments by Mossbauer spectroscopy. ESCA revealed that the surface layers contained bismuth, iron, molybdenum, and oxygen. ESCA measurements also revealed that Bi, Mo, and Fe were present in the surface layers as Bi 3+ , Mo 6+ , and Fe 3+ , respectively.

The catalytic oxidation of propylene: IV. Preparation and characterization of α-bismuth molybdate

Abstract A coprecipitation method is described which results in the preparation of the α-phass bismuth molybdate catalyst in high purity and which is reproducible. This preparation is based on the known principles of isopolymolybdate and molybdate heteropoly chemistry. The existence of a bismuth-molybdate heteropoly precursor having a stoichiometric ratio of bismuth to molybdenum of 2 to 3 is postulated using both the solution chemistry of the molybdates as well as X-ray diffraction measurements on several air-dried and oven-dried samples of precipitates obtained from the coprecipitation procedure.

The catalytic oxidation of propylene: VII. The use of temperature programmed reoxidation to characterize γ-bismuth molybdate

Temperature programmed reoxidation (TPR) was used to characterize the redox properties of γ-Bi2MoO6. Reoxidation of a prereduced catalyst by this technique yielded two well-defined temperature ranges for reoxidation. When the catalyst was prereduced by propylene, the maximum of a low-temperature peak occurred at 158 °C and the maximum of a high-temperature peak occurred at 340 °C. The physicochemical changes associated with these reoxidation temperatures were further investigated by X-ray diffraction, Auger spectroscopy, and kinetic measurements. The low-temperature reoxidation peak was found to be a result of the reoxidation of Mo4+ to Mo6+ and Bi0 to Bim+, where 0 < m < 3. The high-temperature reoxidation peak was found to be a result of the reoxidation of Bim+ to Bi3+. The high-temperature reoxidation may involve the direct oxidation of bismuth by gas phase oxidation or may involve shear structures. The high-temperature reoxidation process also appears to be related to the rate-limiting step for propylene oxidation to acrolein at temperatures below 400 °C.

The catalytic oxidation of propylene: V. X-ray characterization of iron-containing bismuth molybdate catalysts

Abstract The effects on phase composition and structure of adding iron to bismuth molybdate catalysts were investigated by X-ray diffraction methods. Structural and phase transformations were observed to be functions of the calcination temperature and composition. In addition, ternary compounds containing bismuth, molybdenum, and iron were identified. These ternary compounds were formed when the calcination temperature was greater than 550 °C and were characterized by a monoclinic unit cell which was derived from and closely related to a scheelite unit cell. At calcination temperatures of 450 °C or less, a poorly crystallized compound with a scheelite structure was identified.