Ethylene Epoxidation Catalysts Research Articles

The role of chlorine promoters in mediating particle size effects in silver-catalyzed ethylene epoxidation

Measured chlorine coverages over Ag/α-Al2O3 catalysts with varying Ag weight loadings (1.3–35 wt%) and different Ag particle size distributions when parsed in terms of Cl coverages over particles of varying size constituted in the distribution reveal that Ag particles of size below ∼30 nm are covered in more than 1 monolayer (ML) of chlorine in presence of 3.5 ppm C2H5Cl. These assessments of Cl coverages were made possible by correlating the cumulative Cl coverage to the lognormal surface area distribution of 22 Ag/α-Al2O3 samples using a single exponential decay function. Fully chlorided small Ag particles exhibit low rates of epoxidation and only large Ag particles which exhibit sub-monolayer Cl coverages catalyze ethylene epoxidation with high ethylene oxide (EO) rates and selectivity. The Ag particle size dependence of Cl coverages plausibly explains why ≥ 100 nm Ag particles are typical in promoted Ag/α-Al2O3 ethylene epoxidation catalysts.

Insight into the influence of Re and Cl on Ag catalysts in ethylene epoxidation.

Commercial ethylene epoxidation catalysts consist of α-alumina supported Ag particles and usually contain a mixture of promoters. High selectivity catalysts typically include a small amount of rhenium species. We studied a series of Ag catalysts promoted with Re loadings up to 4 at% (Re/(Re + Ag)), which is intentionally higher than in optimized commercial catalysts to facilitate characterization and to amplify the influence on catalysis. Sequential impregnation brought Re and Ag in such close contact that they formed a new characterized phase of AgReO4. Chemisorption experiments showed that both ReO x and AgReO4 species act as a reversible reservoir for O2. Ethylene epoxidation was performed without and with the industrially crucial ethyl chloride promoter in the feed. Without the chloride (Cl), the ethylene oxide selectivity increased when Re was present, whereas the combination of Re and Cl decreased the ethylene oxide selectivity at higher Re loadings. Systematic ethylene oxide isomerization experiments revealed that Re and Cl individually inhibit the isomerization on the Ag surface. However, Re and Cl combined increased the isomerization, which can be explained by the surface becoming overly electrophilic. This hence shows the importance of studying promoters both individually and combined.

Ethylene Oxide Catalysis Under Commercial Conditions – A Guide for Researchers

AbstractSelective oxidation of ethylene to ethylene oxide (EO) is performed industrially over catalysts comprised of silver particles supported on α‐Al2O3. In addition to various oxy‐anion and alkali promotors, alkyl chloride species (“chloride moderators”) are continuously introduced into commercial reactors. The effect and usage of chloride moderators is a key requirement for industrial ethylene epoxidation catalysts to reach optimal selectivity. While not an exhaustive review, the aim of this perspective is to provide guidance to academic researchers on the operation of EO catalysts under commercially relevant conditions with particular focus on the importance of performance optimization.

Chemical Titration of Promoted Ag/α‐Al2O3 Ethylene Epoxidation Catalysts

AbstractWe enumerate the active site density for a promoted Ag/α‐Al2O3 ethylene epoxidation catalyst (35 wt.% Ag) by using trifluoroethanol as a chemical titrant. Trifluoroethanol, when introduced into the reactant stream, deprotonates to deposit stable trifluoroethoxy or trifluoromethyl groups on the Ag surface and inhibits ethylene oxide (EO) rates, thus enabling us to assess catalytic site densities. Active site densities vary between 52 and 69 μmolTFE gcat−1 across different chlorine coverages (0–0.4 monolayers) and different oxygen concentrations (2.5–7.7 mol%) in the reactor feed. Site densities measured by in situ trifluoroethanol chemical titration are distinct compared to those evaluated using ex‐situ N2O decomposition (12.3 μmolN2O gcat−1) suggesting that dynamic restructuring of the Ag surface under reaction conditions may render pre‐reaction chemisorption measurements insufficient to accurately enumerate the active sites relevant for ethylene epoxidation.

Particle size dependence of ethylene epoxidation rates on Ag/α-Al2O3 catalysts: Why particle size distributions matter

The post-reaction Ag surface area distribution of 16 Ag/α-Al2O3 ethylene epoxidation catalysts with varying Ag weight loadings (6.9 – 35 wt%) containing different distributions of Ag particle sizes was correlated with measured ethylene oxide (EO) rates in presence of 3.5 ppm ethyl chloride via a particle size dependent Gaussian rate function. The Gaussian rate function model differs from a description based on average particle size as it accounts for the effects of the breadth of particle size distribution as well as the population of different particle sizes that constitute the Ag particle distribution on measured EO rates. This description of particle size effects in ethylene epoxidation catalysis facilitates accurate prediction of EO rates over a wide range of particle sizes (5–400 nm) and suggests that very small (5–50 nm) and very large Ag particles (>250 nm) have low EO rates (<3 μmol gAg-1 s−1). A narrow Ag particle size distribution centered around ∼130–150 nm would enable operation of ethylene epoxidation with high EO rates and EO selectivity.

In situ discovery on the formation of supported silver catalysts for ethylene epoxidation

Silver particles supported on alumina are the main industrial catalysts for ethylene epoxidation. Silver catalysts are usually produced by an impregnation method followed by thermal decomposition. Although the decomposition process determines the structures and properties of silver catalysts, we are short of information on this high temperature process. Herein we employed a confocal laser scanning microscope (CLSM) equipped with a heating stage to disclose the thermal decomposition process of silver precursor towards the preparation of high selective silver catalysts. It is found that the formation of silver catalysts experiences two steps: crystallization and decomposition. The crystallization occurs at the temperature below 80℃, resulting in the formation of silver complex. Further heating the sample to 140℃, leads to the formation of silver particles at alumina wafers. The atmosphere has a significant effect on the formation and performance of catalysts. When the samples are heated in the nitrogen, small silver particles with similar size are formed, which leads to a high selectivity of ethylene epoxidation. The role of thermal atmospheres on the size and property of silver catalysts is discussed in this paper.

Design of CuCs-doped Ag-based catalyst for ethylene epoxidation

Our recent theoretical studies have screened out CuCs-doped Ag-based promising catalysts for ethylene epoxidation [ACS Catal. 11, 3371 (2021)]. The theoretical results were based on surface modeling, while in the actual reaction process Ag catalysts are particle shaped. In this work, we combine density functional theory (DFT), Wulff construction theory, and micro kinetic analysis to study the catalytic performance of Ag catalysts at the particle model. It demonstrates that the CuCs-doped Ag catalysts are superior to pure Ag catalysts in terms of selectivity and activity, which is further proved by experimental validation. The characterization analysis finds that both Cu and Cs dopant promote particle growth as well as particle dispersion, resulting in a grain boundary-rich Ag particle. Besides, CuCs also facilitate electrophilic atomic oxygen formation on catalyst surface, which is benefitial for ethylene oxide formation and desorption. Our work provides a case study for catalyst design by combining theory and experiment.

Design of CuCs-doped Ag-based catalyst for ethylene epoxidation

Our recent theoretical studies have screened out CuCs-doped Ag-based as promising catalysts for ethylene epoxidation (ACS Catalysis, 2021, 3371-3383). The theoretical results were based on surface modeling, while in the actual reaction process Ag catalysts are particle shaped. In this work, we combine Density functional theory (DFT), Wulff construction theory, and microkinetic analysis to study the catalytic performance of Ag catalysts at the particle model. It demonstrates that the CuCs-doped Ag catalysts are superior to pure Ag catalysts in terms of selectivity and activity in ethylene oxide production, which is further proved by our experimental results. The characterization analysis finds that both Cu and Cs dopants promote particle growth as well as particle dispersion, resulting in a grain boundary-rich Ag particle. Besides, CuCs also facilitates electrophilic atomic oxygen formation on catalyst surface, which is benefit for ethylene oxide formation and desorption. Our work provides a case study of Ag-based catalyst design for ethylene epoxidation by combining theory and experiment.

Epoxidation of ethylene over an Ag atom embedded B-vacancy defective boron-nitride nanosheet via a trimolecular Langmuir–Hinshelwood mechanism: A DFT investigation

• An Ag atom can be stably anchored over defective h -BN nanosheet. • Ethylene epoxidation over Ag/ h -BN is highly selective. • Ethylene epoxidation proceeds via a trimolecular Langmuir-Hinshelwood mechanism. • The activation energy for ethylene epoxidation is 0.58 eV. The oxidation of ethylene (C 2 H 4 ) to produce value-added chemicals such as ethylene oxide is an important chemical process in industry. We investigated the oxidation of C 2 H 4 by O 2 catalyzed by an Ag-atom embedded boron-nitride nanosheet (Ag/ h -BN) using density functional theory calculations. The adsorption energies of O 2 and C 2 H 4 molecules on Ag/ h -BN are -0.85 and -1.21 eV, respectively, demonstrating that C 2 H 4 has a stronger tendency to interact with this surface. A trimolecular Langmuir-Hinshelwood mechanism involving the -CH 2 CH 2 OOCH 2 CH 2 - intermediate was found as the most efficient pathway for C 2 H 4 epoxidation. This intermediate is converted into two ethylene oxide molecules after passing over an activation barrier of 0.44 eV. The competing pathway to produce an acetaldehyde molecule has a very high activation barrier, indicating that C 2 H 4 epoxidation can occur selectively over Ag/ h -BN. These findings may be useful for modeling and designing high performance and selective single-metal catalysts for ethylene epoxidation.

High selective catalyst for ethylene epoxidation to ethylene oxide: A DFT investigation

Ethylene epoxidation to produce ethylene oxide is crucial in both fundamental knowledge and industrial chemical process. The reaction mechanism of ethylene epoxidation with N2O catalyzed by Mn-coordinated porphyrin-like graphene (Mn-N4GP) was investigated using dispersion-corrected density functional theory calculations. The reaction proceeds through two consecutive steps, (1) MnO active site (MnO-N4GP) formation, followed by (2) ethylene epoxidation on the MnO. The first step is a feasible process with the energy barrier of 0.77 eV and the MnO-N4GP is thermodynamically stable. The ethylene epoxidation in the second step competitively can undergo three possible intermediate-pathways; carboradical, alkoxide radical, and manganaoxetane intermediates. By systematic study of all possible pathways, we found that the pathway for alkoxide radical intermediate shows the most feasibility which it subsequently converts to three competitive products with the energy barriers of 0.25 eV, 0.56 eV, and 0.46 eV for the formation of ethylene oxide, acetaldehyde, and 5-membered ring (5MR) species, respectively. This catalyst is remarkably selective to ethylene oxide by 105 and 104 times compared with acetaldehyde and 5MR side products, respectively. Thus, the Mn-N4GP catalyst is suggested as a promising catalyst in terms of activity and selectivity for ethylene epoxidation using N2O as an oxidizing agent in mild condition.

Re- and Cs-Copromoted Silver Catalysts for Ethylene Epoxidation: A Theoretical Study

The effect of Re- and Cs-copromotion 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 the 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.

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.

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.

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.

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.

Ethylene Epoxidation Catalyzed by Ag Nanoparticles on Ag-LSX Zeolites formed by Pressure- and Temperature-Induced Auto-Reduction.

Ag+ -Exchanged LSX (Ag-LSX: Ag96 Al96 Si96 O384 ⋅n H2 O), a large pore low silica analogue (Si/Al=1.0) of faujasite, was prepared and post-synthetically modified using pressure and temperature in the presence of various pore-penetrating fluids. Using high-resolution synchrotron X-ray powder and single crystal diffraction we derive structural models of the as-prepared and post-synthetically modified Ag-LSX materials. In the as-prepared Ag-LSX model, we located 96 silver cations and 245 H2 O molecules distributed over seven and five distinctive sites, respectively. At 1.4(1) GPa pressure and 150 °C in ethanol the number of silver cations within the pores of Ag-LSX is reduced by ca. 47.4 %, whereas the number of H2 O molecules is increased by ca. 40.8 %. The formation of zero-valent silver nanoparticles deposited on Ag-LSX crystallites depends on the fluid present during pressurization. Ag-nanoparticle-Ag-zeolite hybrid materials are recovered after pressure release and shown to have different chemical reactivity when used as catalysts for ethylene epoxidation.

Preparation and particle size effects of Ag/α-Al2O3 catalysts for ethylene epoxidation

Currently, for the industrial ethylene epoxidation α-alumina supported silver catalysts are the only catalyst of choice. We demonstrate a novel method to produce these catalysts with different silver particle sizes, but without changing other key parameters that may affect the catalytic performance such as support specific surface area or metal precursor. α-Alumina was impregnated with a silver oxalate solution, and was subsequently dried and treated in different gas atmospheres and at different temperatures to tune the silver particle sizes in the range of 20–500 nm. Particles of 20 nm exhibited a lower turnover frequency than particles of 70 nm and larger, which exhibit a constant turnover frequency, in accordance with results in literature. However, the selectivity, when measured at constant conversion, was particle size independent. This is the first time that the effect of the particle size on the selectivity of ethylene epoxidation is reported at constant conversion. This was made possible by a new method of producing supported silver catalysts, which we expect that is also applicable for silver catalysts with other supports and for the preparation of other supported metal catalysts.

Effect of Rhenium Loading Sequence on Selectivity of Ag–Cs Catalyst for Ethylene Epoxidation

Rhenium is an important promotor for silver catalyst, and very little work has been focused on the effect of rhenium loading sequence on the selectivity of ethylene oxide. Re was introduced to the Ag–Cs catalyst supported on α-Al2O3 in three different ways: co-impregnation (Ag–Cs–Re), step-impregnation with Re loaded first (Re/Ag–Cs), and step-impregnation with Re loaded last (Ag–Cs/Re). The selectivity of Re/Ag–Cs and Ag–Cs–Re are about 20% higher than that of Ag–Cs/Re, which indicates that the performance of the catalyst is influenced by the synergistic effect of Re and Cs. Rhenium oxides with the valence of + 4 to + 7 are distributed around the Ag particles. The binding energy of Re is increased by about 0.2 eV by the addition of Cs in the Ag–Cs–Re catalyst, and the presence of Re with high valence around Ag will cause the electrons to shift toward the Re atom, facilitating the formation of weakly adsorbed oxygen and improving the selectivity of the Ag–Cs catalyst. On the Ag–Cs/Re catalyst, Re is mostly loaded on the external Ag surface, and the binding energies of Ag 3d5/2 and Cs 3d5/2 increase by 0.2 and 0.4 eV, respectively, from the Ag–Cs/Re catalyst to the Ag–Cs–Re and Re/Ag–Cs catalysts. The higher binding energy of Ag leads to the stronger oxygen adsorption, which contributes to the complete oxidation reaction, thereby resulting in lower selectivity.

Easy Preparation of Tannin‐Based Ag Catalysts for Ethylene Epoxidation

AbstractUltramicroporous carbons loaded with silver (Ag) nanoparticles were synthesised by submitting tannins to hydrothermal treatment in aqueous solutions of silver nitrate at different concentrations, followed by a pyrolysis step. The resultant materials were oxidised with dioxygen diluted in argon for removing the carbon partly coating the Ag nanoparticles and thus increasing the active metal surface area. Morphological, chemical, structural and textural characterisations were carried out in order to establish relationships between the physicochemical properties of the materials and their performances as catalysts for ethylene epoxidation. The latter reaction was increasingly promoted at higher silver content, and the efficiency of the oxidative post‐treatment was evidenced. The relation between selectivity and Ag content was nonlinear, in favour of the improved availability of silver nanoparticles after the removal of the carbon coating, at constant surface area and micropore size distribution of the carbon support. A favourable effect of Ag nanoparticles agglomeration on the selectivity was suggested.