Ethylene Epoxidation Research Articles

Structure Sensitivity of Silver-Catalyzed Ethylene Epoxidation

The influence of particle size (20–200 nm) of Ag/α-Al2O3 catalysts for epoxidation of ethylene to ethylene oxide (EO) under industrial conditions was investigated. Small silver particles up to ∼40 ...

Molecular characteristics governing chlorine deposition and removal on promoted Ag catalysts during ethylene epoxidation

The selectivity of Ag ethylene epoxidation catalysts correlates with surface coverage of the promoter chlorine. Cl coverage, in turn, is a function of co-fed organic chloride and alkane moderator pressures. Organic molecules can be classified based on their deprotonation enthalpy, as those that are inert, those capable of selective oxidation, or those that act as gas-phase acids. The influence of alkane moderator and alkyl chloride promoter identities on Cl coverage was assessed by titration of Cl by ethane in a recirculating gas-phase batch reactor immediately following steady-state reactions with various alkane and alkyl chlorides included in ethylene oxidation feedstreams. The faculty of organic molecules for Cl removal correlates with the weakest CH bond dissociation enthalpy, such that molecules with CH bonds susceptible to homolysis remove Cl more effectively than those with stronger CH bonds. Cl deposition likely occurs via heterolytic cleavage of CCl bonds in alkyl chlorides, such that molecules with weak CCl bonds deposit multiple monolayers of Cl. Cl coverage affects ethylene epoxidation rates identically regardless of the organic chloride or the aliphatic hydrocarbon employed, demonstrating that these molecules only impact catalysis by moderating Cl coverage. These findings implicate the concurrent prevalence of homolytic and heterolytic pathways during ethylene epoxidation on promoted Ag/α-Al2O3 catalysts and provide context to examine additional roles of surface oxygen beyond involvement in oxidation pathways.

Gas-Phase Epoxidation of Propylene to Propylene Oxide on a Supported Catalyst Modified with Various Dopants

In the present study, the production of propylene oxide (PO) from propylene via gas-phase epoxidation was investigated using various catalysts. Although Ag is known to be a highly active catalyst for the epoxidation of ethylene, it was not active in the present reaction. Both Al and Ti showed high levels of activity, however, which resulted in confusion. The present study was conducted to solve such confusion. Although the employment of MCM-41 modified with Ti and/or Al was reported as an active catalyst for epoxidation, the combination resulted in the formation of PO at a less than 0.1% yield. Since this research revealed that the acidic catalyst seemed favorable for the formation of PO, versions of ZSM-5 that were both undoped and doped with Na, Ti, and Ag were used as catalysts. In these cases, small improvements of 0.67% and 0.57% were achieved in the PO yield on H-ZSM-5 and Ti-ZSM-5, respectively. Based on the results of the Ti-dopant and acidic catalysts, Ag metal doped on carbonate species with a smaller surface area was used as a catalyst. As reported, Ag‒Na/CaCO3 showed a greater yield of PO at 1.29%. Furthermore, the use of SrCO3 for CaCO3 resulted in a further improvement in the PO yield to 2.17%. An experiment using CO2 and NH3 pulse together with SEM and TEM examinations for Ag‒Na/CaCO3 revealed that the greatest activity was the result of the greater particle size of metallic Ag rather than the acid‒base properties of the catalysts.

Ethylene epoxidation on Ag/YSZ electrochemical catalysts: Understanding of oxygen electrode reactions

In this study, we have investigated, for the very first time, the oxygen electrode reactions on Ag/YSZ electrochemical catalysts both in oxygen and under reaction conditions compatible with the ethylene epoxidation reaction. Electrochemical Impedance Spectroscopy (EIS) in combination with in-situ Raman spectroscopy and catalytic activity measurements were used to identify and understand the main oxygen reaction pathways. The results obtained suggested that the rate limiting step under an O2 reaction atmosphere (at 300 °C) is the O2 adsorption/dissociation process on the Ag catalyst-electrode. In addition, the polarization resistance increased with time under the presence of O2. This was attributed to the formation of Ag2O on the catalyst surface or near surface, which limits the oxygen electrode reactions. Finally, we observed that the addition of ethylene in the feed stream hinders the electrode reaction, due to its competitive chemisorption with oxygen on Ag. These results give new insights into the design of selective Ag/YSZ catalyst for ethylene epoxidation.

Selectivity-Driven Design of the Ag–Cu Alloys for the Ethylene Epoxidation

Recent experiential and theoretical studies have shown that the Ag–Cu bimetallic catalysts possess a high selectivity toward ethylene oxide (EO) in ethylene epoxidation. However, the dependence of selectivity toward EO on Ag–Cu composition remains unclear. In this work, a volcano-like selectivity trend with the increasing surface Cu content (x) of Ag1–xCux/Ag (111) surface alloy has been identified by density functional theory (DFT) calculations. The computational screening demonstrates that the Ag1–xCux/Ag (111) surface alloy would achieve the highest selectivity with around 50% surface Cu composition at 1/9 and 2/9 ML oxygen coverage. We find that the relative bond strength between the C1–O and O–metal bonds in the oxametallacycle (OMC) intermediate would affect the selectivity to EO. Moreover, the transition state, the charge distribution of the Ag–Cu surfaces, and the changes in work function of catalysts upon adsorption of Os are studied, which also to some extent affect the selectivity.

Multiscale modelling from quantum level to reactor scale: An example of ethylene epoxidation on silver catalysts

Abstract Ethylene epoxidation is one of the most important selective chemical oxidations in industry. For a controlled transformation of ethylene (ethene) into epoxide, silver is the only commercially suitable catalyst. Although it is usually doped, even with pristine silver activity, selectivity, and stability vary strongly with facets. In this work, we use this reaction on Ag( 111 ) and Ag( 100 ) as a classical formation model to demonstrate the capabilities of physical multiscale modelling, to show why Ag( 100 ) nanocubes offer superior catalysis, and to optimise reactivity. First, we describe the elementary reactions on pristine surfaces with the quantum chemistry calculations, using density functional theory (DFT). The free energies of all intermediates, kinetic rates from the transition state theory and adsorption/desorption equilibria are calculated from first principles. These results are applied to kinetic Monte Carlo (kMC) simulations, where the spatio-temporal evolution of the system on a meso-scale can be followed. The differences in activity, concentration, selectivity, and apparent activation energy are observed, investigated, and analysed. Lastly, mean-field concepts – micro-kinetics and computational fluid dynamics (CFD) – are used to simulate how the synthesis proceeds in a reactor. Mechanism, catalytic coverage and the effects of pressure, temperature, and particle composition, size and shape on the performance are evaluated. We show that multiscale modelling is a powerful instrumental approach for real unit engineering, while the level of detail required is dictated by the purpose of a representation and available resources.

From qualitative to quantitative understanding of support effects on the selectivity in silver catalyzed ethylene epoxidation

Abstract In the epoxidation of ethylene, catalyzed by supported Ag particles, the support plays not only a beneficial role, but can also negatively impact the selectivity to ethylene oxide. Especially high surface area supports, and supports containing acid groups, seem detrimental for the selectivity, which is attributed to secondary reactions on the support surface. We prepared Ag nanoparticles on supports with a wide range of specific surface areas and different chemical nature, but with similar Ag metal loading and particle size. Indeed, a strong dependence of selectivity on both the specific surface area, and the chemical nature of the oxide support was found, with α-Al2O3 giving much more selective catalysts than SiO2. Furthermore, the conversion was an important factor in determining the selectivity, while on the other hand the support had no influence at all on the ethylene conversion. We described our experimental selectivities with a reaction model that used the rate constant for the secondary reaction of oxidation of ethylene oxide to carbon dioxide and water as the only variable parameter. The model was able to adequately describe the experimental results. It gave insight into the dependence of selectivity on conversion, how the secondary reaction depended on the chemical nature of the support, and how its rate scaled linearly with the support’s specific surface area. Building on this insight we modified a SiO2 support to passivate its OH-groups, which indeed yielded a 94% decrease in the rate of the secondary reaction, performing even better than α-Al2O3 with similar specific surface area would. We have shown that the selectivity towards ethylene oxide is dependent on a metal specific, intrinsic selectivity, and the decrease in selectivity with conversion to be support dependent. This decrease is caused by the isomerization of ethylene oxide, which is found to be first order in ethylene oxide and the active sites on the catalyst support.

Formation of Carbonate on Ag(111) under Exposure to Ethylene and Oxygen Gases Evidenced by Near Ambient Pressure XPS and NEXAFS

The active oxygen species for ethylene epoxidation on Ag surfaces has been a long-standing issue. We conducted in situ observations of Ag(111) surfaces under exposure to ethylene and oxygen gases w...

The synergistic effect between crystal planes and promoters on Ag-catalyzed ethylene epoxidation

It is well-known that the promoter can enhance the selectivity to ethylene oxide (EO) of Ag catalyzed ethylene epoxidation. However, the selectivity enhancement on different crystal planes of Ag catalysts derived from various promoters is still indistinct. In this work, in order to get an atomistic insight into the synergistic effect between crystal planes and promoters on catalytic performance of Ag catalysts, we study ethylene epoxidation on Ag (1 1 1), (1 1 0) and (1 0 0) surfaces with the promoters of Re, Cs and Cu by density functional theory calculations. It is found that selectivity enhancement introduced by the promoters on Ag (1 1 1) surfaces is more obvious compared with that on Ag (1 1 0) and (1 0 0) surfaces. The sequence of EO selectivity on Ag (1 1 0) surfaces is Re < no promoter < Cs < Cu while it is Re < no promoter < Cu < Cs on Ag (1 1 1) and (1 0 0) surface. The adsorption strength of oxametallacycle intermediate works as a selectivity descriptor and has a negative correlation with valence electron depletion of surface Ag atoms. Therefore, the promoters can cause charge redistribution of Ag crystal surfaces and lead to the corresponding effect on the selectivity toward EO. In addition, the synergistic effect between promoter and crystal surface would affect the eletrophilicity of the surface atomic oxygen. We believe that this works would provide guidance to enhance the EO selectivity of Ag catalyzed ethylene epoxidation by rationally choosing suitable promoter for specific Ag crystal surfaces.

Study on the Influence of Parameter Fluctuation on Ethylene Epoxidation Reaction Process

Ethylene epoxidation reactor is a kind of critical equipment in an oxidation unit. Due to the complex coupling effect of the extreme operating environment of high temperature, high pressure and potential risk factors, the reactor tends to work abnormally under the impact of parameter fluctuation, and temperature runaway incident may occur and cause severe loss. For studying the influence of parameter fluctuation on ethylene epoxidation reaction process and preventing temperature runaway accidents, physical modeling is performed firstly in this paper to acquire the chemical reaction mechanism therein and confirm the operation parameters which should be focused. The unsteady simulation of the operation process under normal situation has been performed. On such basis, a test of single-factor effect is performed to analyze the impact of parameter fluctuation on the temperature balance and operation efficiency of the reactor. Comparison analysis has been carried out between the simulation data acquired according to the established model and real production data, and it was found that the result is consistent. The analysis result indicates: higher ethene concentration, oxygen concentration, feed flux, feed temperature, drum temperature, operating pressure or smaller porosity may improve bed temperature and accelerate reaction rate; therein, ethene concentration, oxygen concentration, drum temperature, and porosity are operation parameters that need special monitoring. Based on the risk analysis of the ethylene epoxidation reactor and the simulation of the impact of parameter fluctuation, this paper studies the cause of the reactor's temperature runaway to provide a technical basis for the forecast of reactor's temperature runaway, early warning, and high-efficient operation of equipment. Model established based on abnormal condition parameters fluctuation, can contribute to early warning reactor temperature runaway, revealing the influence mechanism of parameter fluctuation for the reaction system, monitoring reaction process, eliminating temperature runaway in the bud, and providing theoretical and technical basis for petrochemical equipment fault diagnosis, accident prevention and control and safely long-time running.

Re- and Cs-copromoted silver catalysts for ethylene epoxidation: A theoretical study

The effect of Re and Cs co-promotion of Ag catalysts for ethylene epoxidation is studied computationally using density functional theory approach. The models reflecting Re involvement in the form of oxyanion and as a component of Ag—Re alloys are considered. The synergistic effect of Cs and Re is caused by the formation of the CsReOx species, with Cs stabilizing the Re oxyanion to promote ethylene epoxidation over Ag catalysts. The role of Re is proposed to be related to altering the electron density distribution, compensating oxygen vacancies, co­vering non-selective sites, and providing the optimal retention time for initial compounds, pro­ducts, and oxametallacycles on the catalyst surface.

Ethylene Epoxidation on Ag(100), Ag(110), and Ag(111): A Joint Ab Initio and Kinetic Monte Carlo Study and Comparison with Experiments

Ethylene epoxidation is commercially one of the most important selective oxidation reactions. Despite extensive research into different catalytic materials, silver catalysts remain unrivaled in industrial applications, albeit with significant doping. Experiments have already shown that different silver facets exhibit different catalytic performance in terms of turnover frequencies as well as selectivity. In this work, we present extensive first-principles simulations and kinetic Monte Carlo (KMC) modeling of the reaction on the three pristine silver surfaces Ag(100), Ag(110), and Ag(111) and on the missing-row reconstructed Ag(110). To better understand the kinetics on different surfaces and veraciously describe the surface coverages, we explicitly take into account the lateral interactions between the adsorbates and their effect on the activation barriers. We show that the most stable Ag(111) surface maintains very low oxygen coverage while being the least active surface and only moderately selective. Ag...

Reversible Restructuring of Silver Particles during Ethylene Epoxidation

The restructuring of a silver catalyst during ethylene epoxidation under industrially relevant conditions was investigated without and with vinyl chloride (VC) promotion. During non-VC-promoted ethylene epoxidation, the silver particles grow and voids are formed at the surface and in the bulk. Electron tomography highlighted the presence of voids below the Ag surface. A mechanism is proposed involving reconstruction of the silver lattice and defect sites induced by oxygen adsorption on the external surface and grain boundaries, which finally create pores. Promotion of the catalytic reaction by VC suppresses to a significant extent void formation. The use of VC also redisperses silver particles, initially grown during ethylene epoxidation without VC. This process is rapid as the average size decreased from 172 to 136 nm within 2 h. These insights emphasize the dynamic nature of the silver particles during the ongoing ethylene epoxidation reaction and indicate that particle size and morphology strongly depend on reaction conditions.

LEED-I(V) Structure Analysis of the (7 × √3)rect SO4 Phase on Ag(111): Precursor to the Active Species of the Ag-Catalyzed Ethylene Epoxidation

According to a recently proposed mechanism, the silver-catalyzed industrial synthesis of ethylene oxide (EO) involves adsorbed SO4. The O atoms that are added to the ethylene molecules to give EO originate from SO4, which may solve the long-standing question about the active oxygen species in this reaction. Here, we report a low-energy electron diffraction structure analysis of an ordered phase of SO4 on the Ag(111) surface, forming a (7 × √3)rect structure and containing the oxygen species that before had been spectroscopically identified on the active catalyst. Using I(V) data from a low-energy electron microscope and an input model from density functional theory, the complex structure could be solved. It contains SO4 moieties on a reconstructed Ag(111) surface in which all four O atoms bind to Ag atoms. In the proposed ethylene epoxide reaction model, the structure represents the parent phase from which the active SO4 phase is formed by a lifting of the reconstruction.

Epoxidation of ethylene over carbon and silicon-doped boron nitride sheets: A comparative DFT study

Using first-principle calculations, we compare the catalytic activity of the experimentally available C- or Si-doped boron nitride nanosheet (C-/Si-BNNS) towards the epoxidation of ethylene by N2O molecule. The epoxidation reaction of ethylene over these surfaces includes the decomposing of N2O into N2 and an oxygen atom (Oads), formation of the ethyleneoxy intermediate, and the creation of ethylene oxide. Our results show that the catalytic activity of C-BNNS for epoxidation of ethylene is better than Si-BNNS, due to more favorable charge-transfer effects. The results presented here suggest a green, low-cost, and metal-free approach for low-temperature epoxidation of ethylene using C-BNNS.

Moderation of chlorine coverage and ethylene epoxidation kinetics via ethane oxychlorination over promoted Ag/α-Al2O3

Optimal ethylene oxide selectivity during steady-state ethylene oxidation over Cs and Re-promoted Ag/α-Al2O3 catalysts is attained only if co-fed alkyl chlorides deposit chlorine adatoms and co-fed alkanes moderate surface chlorine coverages. Surface chlorine coverage under reaction conditions was measured in chemical transient studies immediately after reaching steady-state by removing reactants in flowing helium and recirculating ethane and oxygen in a gas-phase batch reactor; this resulted in removal of chlorine from the catalyst as ethyl chloride and enabled assessment of the total number of moles of chlorine desorbed per surface silver atom, which varied from 0.10 to 0.45 depending on the process conditions employed. Batch recirculation experiments demonstrated that oxygen is required to remove chlorine from silver catalysts, suggesting molecular events akin to those involved in oxychlorination of alkanes moderate chlorine coverage during steady-state catalysis. The chlorine content was largely unperturbed by changes in ethylene (20–40 mol%), carbon dioxide (1–5%), or oxygen (2–7%) concentrations during steady-state catalysis. Increasing temperature resulted in a monotonic decrease in chlorine coverage. A power law model accurately describes trends in chlorine coverage as a function of steady-state ethane (0.1–15 mol%) or ethyl chloride (0.5–6.3 ppmv) concentrations. The carbon selectivity for ethylene oxide increases with chlorine coverage, in agreement with expectations from steady-state measurement of rates and selectivity as a function of co-fed alkyl chloride promoters. These results explicate the critical role of oxygen-containing species in moderating Cl coverage during ethylene epoxidation catalysis, include a model that describes chlorine coverage across a broad concentration range of alkyl chloride promoter and alkane moderators, and enable correlation between kinetic parameters measured as a function of process conditions and the predicted chlorine coverage.

Chemical Behaviour of CaAg2 under Ethylene Epoxidation Conditions

The binary compound CaAg2 is examined as a catalyst for the ethylene epoxidation reaction. During the induction phase, conversion and selectivity increase and then remain stable for several hundred hours. The presence of ethyl chloride as a promoter is crucial. The pristine CaAg2 reacts with the gaseous reactants and forms a porous microstructure of calcium‐containing oxidation products on the surface, in which particles of elemental silver are embedded. The microstructure is remarkably stable, and in particular, prevents further sintering of the silver particles.

Facet-dependent diffusion of atomic oxygen on Ag surfaces

It has been proposed that the introduction of surface and subsurface atomic oxygen into Ag catalysts is crucial for selectivity enhancement of ethylene epoxidation towards ethylene oxide. However, the diffusion of atomic oxygen on various Ag crystal surfaces, which is an essential process to form surface and subsurface atomic oxygen, has not been systematically investigated. In this work, we have employed density functional theory (DFT) to study the horizontal diffusion as well as downward diffusion of oxygen atom on different crystal facets of Ag, including (1 1 1), (1 1 0), (1 0 0) and (2 1 1) surfaces. Among all studied crystal facets, the horizontal diffusion of surface atomic oxygen is most favorable on Ag (1 1 1) surface, while Ag (1 1 0) surface is the most beneficial to horizontal diffusion of subsurface atomic oxygen. It is also found that the atomic oxygen prefers to downward diffuse on Ag (1 1 0) surface rather than downward diffuse on other surfaces. The deep insight revealed here may guide to engineer Ag surface to promote the formation of surface and subsurface atomic oxygen, and accordingly pave the avenue for the rational design of Ag catalyst for ethylene epoxidation.

Anisotropic Reactivity of CaAg under Ethylene Epoxidation Conditions.

The chemical behavior of CaAg as catalyst for ethylene epoxidation was studied using a combination of experimental (X-ray powder diffraction, scanning electron microscopy, thermal analysis and infrared spectroscopy), and quantum chemical techniques as well as real-space chemical bonding analysis. Under oxidative ethylene epoxidation conditions, the CaAg (010) surface possesses an outstanding stability during long-term experiments. It is caused by the formation of an ordered, stable and dense CaO passivation layer with a small amount of embedded Ag atoms.On this way, the(010) surface constitutes a kinetic barrier for further incorporation of oxygen into the subsurface region and thereby prevents further oxidative decomposition of CaAg. The calculated adsorption energies of the reaction species show strong adsorption of the reaction products that may explain the observed low conversion of ethylene toward ethylene oxide using CaAg as catalyst.

In\xa0situ formation of catalytically active graphene in ethylene photo-epoxidation

Ethylene epoxidation is used to produce 2 × 107 ton per year of ethylene oxide, a major feedstock for commodity chemicals and plastics. While high pressures and temperatures are required for the reaction, plasmonic photoexcitation of the Ag catalyst enables epoxidation at near-ambient conditions. Here, we use surface-enhanced Raman scattering to monitor the plasmon excitation-assisted reaction on individual sites of a Ag nanoparticle catalyst. We uncover an unconventional mechanism, wherein the primary step is the photosynthesis of graphene on the Ag surface. Epoxidation of ethylene is then promoted by this photogenerated graphene. Density functional theory simulations point to edge defects on the graphene as the sites for epoxidation. Guided by this insight, we synthesize a composite graphene/Ag/α-Al2O3 catalyst, which accomplishes ethylene photo-epoxidation under ambient conditions at which the conventional Ag/α-Al2O3 catalyst shows negligible activity. Our finding of in situ photogeneration of catalytically active graphene may apply to other photocatalytic hydrocarbon transformations.