Elsevier

Surface Science

Volume 708, June 2021, 121834
Surface Science

Ethylene oxidation on unpromoted silver catalysts: Reaction pathway and selectivity analysis using DFT calculations

https://doi.org/10.1016/j.susc.2021.121834Get rights and content

Highlights

  • Periodic plane-wave Density Functional Theory (DFT) was implemented to study the reaction pathways in ethylene oxidation on the unpromoted Ag(111) surface facet.

  • Binding modes with the lowest energies were determined for all reactants, products, and intermediates at low surface coverages.

  • Activation barrier energies (Ea) were calculated for the pathways of partial and complete ethylene oxidation and product selectivity followed experimental trends.

  • Potential energy surfaces (PES) of reaction were constructed, and key intermediates impacting selectivity were identified.

Abstract

Chemical conversions in catalytic partial oxidation processes of light hydrocarbons are responsible for the production of numerous industrial chemicals, plastics, and intermediates. These processes are relatively expensive to perform, and are typically operated at high thermodynamic inefficiency, so the development of novel, highly efficient catalysts would prove to be very cost effective. Herein, our study focused on surface catalytic mechanisms of the ethylene oxide (EO) formation process. Periodic plane-wave Density Functional Theory (DFT) methods were used to analyze related reaction mechanisms on the Ag(111) surface facet with low coverage. Energetic changes of related species and pathways were calculated. Key surface species are identified to suggest the factors for the observed selectivity during EO formation. Our results are consistent with previous kinetic modeling efforts in the literature which did not employ DFT analysis. Lastly, our study demonstrates how fundamental theoretical investigations and multi-scale modeling techniques are currently impacting the advancement of rational catalyst design and microkinetic modeling techniques in the light hydrocarbon processing industry.

Introduction

Production of ethylene oxide (EO) through partial oxidation of ethylene using heterogeneous catalysis is an important industrial process for its direct application as a sterilizer or disinfectant, and as a precursor in the production of other bulk and specialty chemicals [1]. EO is one of the largest-scale products of the chemical industry, which takes up approximately half of the total amount of organic chemicals from heterogeneous catalytic oxidation [2]. The demand of global EO market achieved approximately USD 46 billion in 2018 and is expected to reach USD 69 billion by 2025. The annual growing rate is around 5% from 2019 to 2025 [3]. Accounting for the magnitude of production along with the potential decrease of reactant waste and slowing of catalyst deactivation, the improvement of selectivity within the ethylene epoxidation process is predicted to result in a tremendous economic upturn. As a result, the research in experimental and computational areas of the surface catalytic mechanism of selective partial oxidation of ethylene to EO has attracted a great deal of interest in recent years [4].

EO is commonly produced with the chlorohydrin method designed by Wurtz at the lab scale [5]. Industrially, it is commonly produced through the selective partial oxidation (epoxidation) of gas-phase ethylene with specially formulated industrial catalysts in fixed-bed tubular reactors at temperatures and pressures ranging 200-300 °C and 1-3 MPa, respectively [6, 7]. The concentrations of gas-phase ethylene and oxygen used in the initial operation phase, which are calculated from the total reaction mixture, are usually in range of 30-45 mol% and 6-12 mol%, respectively [8]. The partial oxidation process has two highly thermodynamically favored side reactions, the undesired complete oxidation or combustion of ethylene and EO (see Fig. 1), thus making the selectivity of the target product, EO, one of the most important indicators for the performance of industrial EO catalysts.

The most common and outstanding industrial catalysts for partial oxidation of ethylene to EO used are silver-based catalysts. The selectivity of EO under the effect of an unpromoted silver catalyst is about 50% experimentally, and an excellent promoted catalyst with proper feed ratios of gas-phase raw materials can achieve nearly 90% for EO selectivity [9]. Both computational and experimental studies on the mechanism of selective ethylene oxidation and novel EO catalyst design have received continued attention from researchers globally, as incremental changes in selectivity and yield of EO can result in considerable cost effectiveness due to the scale of production [10].

The feed ratio of ethylene to oxygen for ethylene oxidation process can directly determine the final selectivity of the target product, EO. The selection of appropriate feed ratios is one of the most important pre-operations for EO process. For industrial production, the concentrations of ethylene and oxygen used in the initial operation phase, which are calculated from the total reaction mixture, are usually in range of 30-45 mol% and 6-12 mol%, respectively. The low oxide coverage in our models are supported by these oxygen starved conditions for commercial operation. The highest selectivity of EO that can be achieved is about 87.6-89% with proper feed ratios and specific industrial catalysts [11].

To improve the performance of EO process, the gas-phase mixture of feed always contains not only ethylene and oxygen, but also one or more components, such as gas-phase diluents or inert gases and additives. A gas-phase additive is always included to minimize the formation of by-products, CO2 and water, and improve the selectivity of the target product, EO. Organic halides are the preferred additives that show their advantages and high efficiency in this respect without suppressing the main reaction [9, 12]. Organometallic compounds and aromatic hydrocarbons can also be used as additives, but their performances are not as outstanding as that of organic halides [13]. The inert gases or diluents, usually nitrogen, argon or saturated hydrocarbons like methane, may be used as a carrier gas in an effort to decrease the partial pressure of the reactive gases and reaction space velocity to better control the reaction rate and guarantee production safety [14]. However, the accumulation of inert gas or diluent caused by continuous recycling of unconverted reactive gases needs to be minimized to reduce separation costs and to significantly improve single pass conversion rates of ethylene [8].

Fundamental surface science on the reaction mechanism shows its benefit on understanding the electronic and geometric factors which control the formation of desired/undesired products. This knowledge can be leveraged to depress side reactions by decreasing the corresponding reaction rates thus improving the desired product selectivity. In the recent years, most of the computational and experimental studies on the selective oxidation of ethylene are based on the oxametallacycle (OMC) mechanism at low to moderate oxygen coverage [15], [16], [17], [18], [19] In this mechanism, there exists the reaction between ethylene and atomic oxygen adsorbed on the naked metal surface leading to the formation of a common surface intermediate, OMC, through a Langmuir–Hinshelwood (L–H) mechanism. The further transformation of the OMC into EO (epoxidation, desired partial oxidation) or acetaldehyde (AA) (undesired partial oxidation) in the EO reaction network with similar energy barriers contributes to the observed ~50% EO selectivity trends according to the reaction rate ratio.Meanwhile, another OMC mechanism was raised by Stegelmann and Campbell. In this mechanism, a surface oxygen layer with electrophilic oxygen (O/Os) is formed on the silver surface [20, 21]. O/Os is formed by the adsorption of one oxygen atom binding on the nucleophilic surface oxide sites (/Os). Ethylene is adsorbed on oxide sites and reacts with O/Os to form OMC. The isomerization of OMC to EO and AA on oxide sites is similar as that in the first mechanism. AA combusts rapidly and forms the final products, carbon dioxide and water. A parallel pathway of ethylene combustion (undesired total oxidation) is also mentioned in the experimental studies [16]. In this pathway, ethylene oxidizes to vinyl alcohol, and then hydrogen is eliminated thus forming CH2CHO similar as AA [18, 19]. This pathway provides an additional explanation of selectivity trends at low pressures, but it is always accompanied with other reactions and very difficult to remove the interference, thus our analysis will shed light on the formation and consumption of difficult to measure surface intermediates.

Spanning several decades, the experimental studies with surface science methods on the mechanism of catalytic ethylene epoxidation under low surface coverages are very mature but ongoing. [13, [15], [16], [17], [18], [19], [20], [22], [23], [24]] For instance, it has been reported using near-edge X-ray fine structure (NEXAFS) and high-resolution electron energy loss spectroscopy (HREELS), that H2C=CH2 adsorbs molecularly to oxygen on Ag(111) with the C=C bond parallel to the catalytic surface. For the intermediate reaction analysis, several experimental studies have reported the adsorption and decomposition of EO and AA using several techniques such TPD and temperature-programmed reaction spectroscopy (TPRS) studies and electron energy loss spectroscopy (EELS) on Ag. Meanwhile, catalytic ethylene epoxidation and its corresponding potential energy surfaces of reaction have been computationally studied with several factors, including the structure of metallic catalysts[1, 25, 26], the coverage of surface facets[27, 28], varying reaction conditions[29], and the effect of promoters and inhibitors [4, 20, 30].

Industrially, silver-based catalysts are the most active and selective catalyst for the process of ethylene epoxidation [7]. After addition of solid promoters to the silver-based catalysts, the selectivity of EO can improve dramatically from 40-50% on unpromoted silver to 90% on promoted silver [31], [32], [33], [34], [35] In recent studies, alkali metals and chloride species are shown to be able to increase activity of reaction catalysts through dipole or polarization effects [12, [36], [37], [38]] The relative amount or ratio of the surface facets on Ag crystallites can also influence the mechanisms of ethylene oxidation. As shown in previous works [[18], [26], [39], [40]], the increased surface oxygen coverage can weaken the adsorption of ethylene molecules at the metal sites and strengthen the adsorption on oxide sites. The adsorption on exposed naked metal surfaces of Ag(111) was observed as not probable to occur when the surface coverage is beyond 33% [28]. Also, the increase of the number of metal and metal oxide active sites for ethylene adsorption by the decrease of Ag crystallite size in nanoparticles or other forms can also affect the observed selectivity due to overall increase in observed reaction rates, which is reported in the literature [41], [42], [43], [44] The factors, which cause the disparities of reactions on several different silver catalytic surfaces, have also been studied. To maintain high EO selectivity, current industrial catalysts have deposited Ag on supports in concentrations of 7-20 wt% with dispersed silver particles of diameters ranging from 0.1-1 μm with relatively low specific surface area [7]. Nonetheless, the commercial EO process involves many surface intermediates and their contributions to the desired partial and undesired full oxidation of ethylene have yet to be fully clarified. Thus, further theoretical research on the role of electronic and geometric factors on surface reaction phenomena and observed EO selectivity is still needed for rational catalyst design and reactor optimization efforts.

In this work, periodic plane-wave DFT was used to theoretically study the ethylene epoxidation and full oxidation on silver catalysts. A 30-step reaction network of ethylene oxidation process was built as an extension of the conventional mechanisms mentioned above to deliver a self-consistent and detailed study of all possible elementary steps for partial and complete oxidation of ethylene (shown in Table 1). This study provides useful information on the selectivity trends of ethylene epoxidation. The reaction network combines the existing and new mechanisms for self-consistency and sheds more details on key intermediate structures. The reactions from ethylene and atomic oxygen to OMC were studied on both naked metal sites and oxide sites. A representative surface oxygen coverage for the above two mechanisms was selected to provide a proper environment in which all the desired and undesired reactions are possible to occur. Two new pathways of EO combustion instead of the pathway through EO isomerization and AA combustion were designed and tested in this project to provide new insights. The combustion of AA which is usually approximated as overall reactions were expanded into elementary reaction steps with sub-reactions and systematically studied. The unpromoted Ag(111) surface facet was chosen as the model catalytic surface for our studies herein. The Ag(111) surface facet is the most thermodynamically stable surface for silver, and it is the most abundant facet for industrial Ag catalysts for EO production which are comprised of relatively large Ag crystallites up to a micron in diameter [26, 45]. The adsorption or binding modes of all gas-phase reactants and products, as well as key surface intermediate species, with the lowest energy conformation were included in the reaction network of ethylene partial and full oxidation. The reaction energy and activation energy of each step were obtained, such that the transition states in the reaction steps represent the lowest energy pathway of reaction. The reaction pathways for partial and total oxidation of ethylene with minimum energy were then assembled to form a self-consistent potential energy surface. Lastly, our thermodynamic and reaction kinetics results were discussed in the context of rational catalyst design and reaction conditions to improve both selectivity and single-pass conversion of commercial EO processes.

Section snippets

Computational methodology

As reported, the periodic plane-wave DFT methods have a relatively high accuracy when used to calculate binding and adsorption energies, which can be controlled within 0.25–0.35 eV of the experimental values [46], [47], [48], [49] Similar accuracy can be achieved for the prediction of reaction activation energies. These calculations, or predictions, are reasonably accurate and useful for the relative comparison of binding energies, and overall reaction energies and activation barriers for

Adsorption (binding) modes of reactants, intermediates, and products

The Ag(111) catalytic surface is comprised of multiple active adsorption sites which allow initial reactants, ethylene and oxygen, to adsorb. In principle, the reactions between ethylene and oxygen adsorbed at all possible combinations of adsorption sites would have to be considered. However, to simplify the plane-wave DFT calculations and effectively improve research efficiency, we only considered the situations when the species existing in the reaction network are adsorbed only at the most

Summary

In summary, a theoretical investigation on all probable reactions in both the OMCs and OMC/Os mechanism of ethylene oxidation on the unpromoted Ag(111) surface facet has been presented. The related energies were calculated using plane-wave DFT methods and compared with previous experimental and computational literature. In line with literature data, the weaker adsorption of ethylene on the Ag(111) surface oxide sites in our model at higher surface coverage provides new microscopic insights into

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Acknowledgements

The authors are grateful for the support of this work by the Alabama Supercomputing Center, Auburn University Hopper high-performance compute cluster resources, Auburn University new faculty start-up funding, and the Department of Energy (DOE) Rapid Advancement in Process Intensification Deployment (RAPID) institute (Project DE-EE0007888-8.9). The authors wish to thank Dr. John R. Monnier and Dr. Madan Bhasin for fruitful discussions.

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