John R. Monnier

The direct epoxidation of higher olefins using molecular oxygen

Recent developments in olefin epoxidation have shown promising results indicating that higher olefins can be directly epoxidized using molecular oxygen, or indirectly, by using molecular oxygen to generate an active and selective oxidant in situ during reaction. Indirect approaches have utilized bifunctional catalysts that combine new catalyst components that generate such oxidants as H2O2 in situ with the functional component that activates H2O2 for olefin epoxidation. This approach is currently limited by the low rates of in situ generation of H 2O2 and subsequent low rates of olefin epoxide formation. Heavily-modified, silver catalysts have also shown promise as catalysts for propylene epoxidation. These catalysts contain much higher silver and alkali and alkaline earth metal loadings than their analogs used for ethylene epoxidation and are quite different in terms of their chemical and physical properties. Currently, these compositions exhibit activities and selectivities to propylene oxide that are too low for commercial application, although further research and development may further improve catalyst performance. Silver-based catalysts have also been used to epoxidize a wide variety of higher olefins, such as 1,3-butadiene, that do not contain allylic hydrogen atoms, or higher olefins that contain non-reactive allylic hydrogen atoms, such as norbornene. Silver-based catalysts used for selective epoxidation of non-allylic, or kinetically-hindered, olefins require promoters, typically cesium, rubidium, or thallium salts, to assist in the desorption of the olefin epoxide. Such catalysts are extremely active, selective, and stable under extended reaction conditions.

Synthesis, characterization, and catalytic activity of LaRhO3

Abstract Lanthanum rhodate, LaRhO3, has been prepared by high-temperature reaction of the nitrates, prepared in situ from the corresponding oxides. Use of a slight excess of the rare-earth oxide, and removal of this component from the product with a warm, dilute acetic acid leach, provided the pure perovskite. The presence of pure monophasic material was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Comparison of the material made with a slight excess of the rare-earth oxide to a sample prepared using a slight excess of Rh2O3 showed that the excess Rh2O3 cannot be removed by leaching. These studies demonstrate the need for careful attention to the synthetic method used for the preparation of solid-state materials which are to be evaluated as catalysts, and the utility of XPS for monitoring the purity of such materials. Temperature-programmed reduction and XPS studies of LaRhO3 have shown that, contrary to claims in the literature, no stable Rh+ species are observed on heating this material in pure H2 or 1:1 H2:CO, although in the latter case some stabilization of the original perovskite is observed vs reduction in pure H2. The activity of LaRhO3 as a catalyst for the formation of linear alcohols from syn gas (i.e., 1:1 H2:CO), as well as for ethylene hydroformylation, is also reported. The results imply that oxygenate formation occurs on Rh0 centers and that the temperature-dependent competition between associative and dissociative CO adsorption on Rh0 is the major factor in selectivity distributions.

A study of the catalytically active copper species in the synthesis of methanol over CuCr oxide

Abstract CuCr oxide catalysts formulated with various Cu Cr ratios were prepared and evaluated as CH 3 OH synthesis catalysts. From the physical characterization of these catalysts by X-ray diffraction, X-ray photoelectron spectroscopy, and temperature-programmed desorption of CO, Cu + was identified as the active site responsible for CO chemisorption and CH 3 OH formation. The Cu + is stable under reaction conditions and exists as a crystalline CuCrO 2 phase. The concentration of surface Cu + (or CuCrO 2 ) is dependent upon the Cu Cr ratio, the calcination temperature, and the nature of the catalyst pretreatment.

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: 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 selective epoxidation of non-allylic olefins over supported silver catalysts

Abstract The epoxidation of non-allylic, or kinetically-hindered, olefins can be carried out using supported silver catalysts. While epoxidation does occur for unpromoted catalysts, the strength of olefin epoxide adsorption leads to low activity and selectivity, as well as irreversible catalyst fouling. The addition of certain alkali metal salts, such as CsCl, lowers the desorption energy of the olefin epoxide, permitting dramatic increases in activity, selectivity, and catalyst lifetime. In the case of butadiene, the addition of an optimum level of CsCl increases activity and selectivity from approximately 1% butadiene conversion and 50% selectivity for epoxybutene to 15% conversion and 95% selectivity, respectively. Epoxidation of butadiene occurs by addition of dissociatively-adsorbed oxygen to one of the localized C=C bonds to form epoxybutene. The addition of oxygen across the terminal carbon atoms does not occur to any measurable extent. The direct participation of molecular oxygen can be ruled out based both on selectivity arguments as well as the kinetic model for the reaction. The kinetics imply a dual site mechanism. One site, which is unpromoted, serves as the site for butadiene adsorption, while the second site, which is promoted, functions as the site for dissociative oxygen adsorption and epoxybutene formation. Epoxybutene and derivatives represent the beginning of several new families of chemicals that were either not available, or were too expensive, to be considered for large-scale, or even fine chemical, production. More than one hundred chemicals have been prepared so far; several of these are in commercial production at the semiworks scale.

Pd–Ag/SiO2 bimetallic catalysts prepared by galvanic displacement for selective hydrogenation of acetylene in excess ethylene

A series of bimetallic Pd–Ag/SiO2 catalysts were prepared by galvanic displacement with increasing loadings of Pd on Ag. The catalysts were characterized by atomic absorption spectroscopy, Fourier-transform infrared spectroscopy of CO adsorption and X-ray photoelectron spectroscopy. An actual Pd deposition beyond the theoretical limit for galvanic displacement suggested that the large difference in surface free energy for Pd and Ag resulted in Pd diffusion into the bulk of Ag particles, or Ag diffusion to the surface to provide fresh Ag atoms for further galvanic displacement. Characterization results indicated that on this series of catalysts the Pd atoms are distributed in very small ensembles or possibly even atomically on the Ag surface, and there was a transfer of electrons from Pd to Ag at all Pd loadings. For comparison, the catalysts were also evaluated for the selective hydrogenation of acetylene in excess ethylene at the conditions used in our previous study of the reverse Ag–Pd/SiO2 catalysts. The selectivities for C2H4 formation remained high and constant due to the geometric effects that Pd atom existed as small ensembles. However, the electronic effects resulted in lower selectivities for C2H4 formation than those from the catalysts with high coverage of Ag on Pd.

Relationship between stable monovalent copper in copper–chromia catalysts and activity of methanol formation

The activity for methanol formation of copper-chromia catalysts is sensitive to the temperature of calcination pretreatment; X-rays photoelectron spectroscopic studies of the reduced catalysts showed a correlation between the amount of stable surface Cu+ and the activity for methanol formation.

Evidence for the stabilization of copper(I) in Cu-Cr oxide methanol catalysts

Abstract Cu-Cr oxide catalysts were studied to determine the nature of the active copper species in the synthesis of methanol from syngas. The surface electronic structure of the copper component in particular was investigated by X-ray photoelectron spectroscopy (XPS). A correlation was observed between the rate of methanol formation and the surface concentration of Cu+. Bulk stabilization of CuCrO2 (Cu+) appears to be correlated with surface-stabilized Cu+ species. High-temperature heating of Cu-Cr oxide catalysts enhances the formation of CuCrO2 at the catalyst surface, which is associated with increased methanol activity based upon normalized surface areas.

The catalytic oxidation of propylene: III. Additional evidence for surface-initiated, homogeneous reactions

Abstract The catalytic oxidation of propylene was studied in flow reactors having different postcatalytic volumes. Several different metal oxide catalysts were studied, including bismuth molybdate and cuprous oxide. Additional evidence is presented for a surface-initiated, homogeneous reaction that can occur in the postcatalytic volume. The reaction initiator is apparently an allyl peroxide or allyl hydroperoxide species which is formed on the surface by a mechanism involving the reaction of an adsorbed allyl species with molecular oxygen. Moreover, the results suggest that this allyl peroxide or allyl hydroperoxide species may also undergo a decomposition on the surface to produce acrolein in addition to the initiation of the homogeneous reaction.