Selective Oxidation of Ethylene to Ethylene Oxide on Silver Catalysts at Industrial Conditions: Reactor Profiles, Kinetics, and Chlorine Inhibition

Selectivity is the key parameter in industrial ethylene oxide (EO) production by oxidation of ethylene with oxygen on Ag/α–Al2O3 catalysts. Accurate temperature control in wall-cooled multitubular fixed-bed reactors and chlorination of the silver surface by feeding small chlorinated hydrocarbons such as 1,2-dichloroethane (DCE) are required to fine-tune electrophilicity and surface oxygen coverage for maximum EO selectivity at economic ethylene conversion. Temperature and molar flow rate profiles of C2H4, O2, EO, CO2, H2O, DCE, and chlorine-containing reaction products vinyl chloride (VC) and ethyl chloride (EC) were measured in a compact profile reactor (CPR) and in a pilot-scale profile reactor (PSPR) to explore the spatial interplay between DCE concentration, temperature, inlet flow rate, and O2 conversion. Chlorine and oxygen compete for the same active silver sites despite more than 4 orders of magnitude different concentrations (ppm vs vol %). Chlorine coverage increases from inlet to outlet due to the decreasing partial pressure of O2 along the bed, leading to shutdown of all reactions if all active Ag sites are blocked by chlorine. A kinetic model is derived from a dual-site mechanism taken from the literature. Kinetic parameters are determined from differential initial rate measurements, Arrhenius plots, and by fitting the rate expressions implemented in a plug flow model to the species and temperature profiles in the CPR. A very good agreement is reached. PSPR profiles are modeled by implementing the derived kinetic model into a 2D pseudohomogeneous reactor model. At conversions <10%, the experimental profiles are well captured, but the model fails to accurately reproduce the point of thermal runaway in the catalyst bed of the PSPR caused by a too low reactor temperature and resulting insufficient chlorine coverage of the silver surface

Ambient-Pressure Soft X‑ray Absorption Spectroscopy of a Catalyst Surface in Action: Closing the Pressure Gap in the Selective <i>n</i>‑Butane Oxidation over Vanadyl Pyrophosphate

In order to close the pressure gap in the investigation of catalyst surfaces under real operation conditions we have developed a variable-pressure soft X-ray (<i>h</i>ν ≤1.5 keV) absorption cell coupled to a gas analysis system to study the pressure dependency of the electronic and catalytic properties of catalyst surfaces in reactive atmospheres at elevated temperatures. With this setup we investigated the vanadium L₃-edge and catalytic performance of polycrystalline vanadyl pyrophosphate in the selective oxidation of <i>n</i>-butane to maleic anhydride between 10 and 1000 mbar at 400 °C. As a result, major gas phase and pressure dependent spectral changes are observed at energies attributed to V 2p-3d_<i>z</i>² excitations assigned to vanadium atoms square-pyramidally coordinated to oxygen atoms. This can be interpreted in terms of a shortened vanadyl bond (VO) and an increased vanadium oxidation state with higher pressures. Since this is accompanied by an increasing catalytic activity and selectivity, it indicates that vanadyl oxygen is actively involved in the selective oxidation of the alkane

Statistical Analysis of Coordination Environments in Oxides

Coordination or local environments (e.g., tetrahedra and octahedra) are powerful descriptors of the crystalline structure of materials. These structural descriptors are essential to the understanding of crystal chemistry and the design of new materials. However, extensive statistics on the occurrence of local environment are not available even on common chemistries such as oxides. Here, we present the first large-scale statistical analysis of the coordination environments of cations in oxides using a large set of experimentally observed compounds (about 8000). Using a newly developed method, we provide the distribution of local environment for each cation in oxides. We discuss our results highlighting previously known trends and unexpected coordination environments, as well as compounds presenting very rare coordinations. Our work complements the know-how of the solid state chemist with a statistically sound analysis and paves the way for further data mining efforts linking, for instance, coordination environments to materials properties

Platinum Group Metal Phosphides as Heterogeneous Catalysts for the Gas-Phase Hydroformylation of Small Olefins

A method for the synthesis of highly crystalline Rh₂P nanoparticles on SiO₂ support materials and their use as truly heterogeneous single-site catalysts for the hydroformylation of ethylene and propylene is presented. The supported Rh₂P nanoparticles were investigated by transmission electron microscopy and by infrared analysis of adsorbed CO. The influence of feed gas composition and reaction temperature on the activity and selectivity in the hydroformylation reaction was evaluated by using high throughput experimentation as an enabling element; core findings were that beneficial effects on the selectivity were observed at high CO partial pressures and after addition of water to the feed gas. The analytical and performance data of the materials gave evidence that high temperature reduction leading to highly crystalline Rh₂P nanoparticles is key to achieving active, selective, and long-term stable catalysts