A Review on Production of Ethylene Oxide from Epoxidation of Ethylene: Catalysis, Mechanism and Kinetics

Abstract

This review describes the different developments in the production of ethylene oxide (EO) by epoxidation of ethylene. EO is an important chemical intermediate for the manufacture of a variety of industrial and consumer products, such as ethylene glycol, plastics, and pharmaceuticals. The conventional gas-phase epoxidation process using silver-based catalysts suffers from major drawbacks, including low selectivity and high carbon dioxide emissions. This review underlines emerging solutions for efficiency and sustainability improvement in EO production. Major developments in catalyst design, including novel silver-based hybrid nanostructures, Mn-N₄GP catalysts, and chemical looping epoxidation processes, are presented. It also discusses developments in reaction kinetics, including catalyst surface optimization and the use of dopants. The article also outlines catalyst deactivation challenges, cost, and scalability and describes future research directions on renewable feedstocks, reducing energy consumption and most importantly environmental impact. These innovations are oriented toward a more sustainable and economical route for large-scale manufacturing of ethylene oxide.

1. Introduction

Ethylene oxide (EO) ranks as the third-most important ethylene derivative. Its energy content should be emphasized as a value addition to its other applications. It is extensively used in several industries, including producing glycols and polymers, essential chemicals for several consumer products. EO is also an intermediate in the petrochemical industry, with around 70 percent of its output used in the production of mono-, di– and triethylene glycols [1]. EO is extensively employed as an essential feedstock in various industries, such as plastics, solvents, detergents, and textiles. It has a critical role in the production of consumer goods like glycol derivatives, ethoxylates, and polymers, apart from ethylene glycol and several key fine petroleum and chemical intermediates [2,3,4]. In addition, it is an essential feedstock for the manufacture of various key commodity chemicals, including ethylene glycol, glycol ethers, and epoxy resins [5]. This also serves as an important intermediate needed by many commercially manufactured products, which include coolants, polyesters, detergents, surfactants, lubricants, paints, cosmetics, and pharmaceuticals [6]. It is an inflammable, colorless gas that has a smell that is sweet in nature. Ethylene oxide (EO) is one of the most widely produced chemicals, with an annual market size exceeding 34 million tons, also the level of global demand for EO and its derivatives that had been reached by the end of 2021 [6,7,8]. Increasing EO demand is underpinned by increasing consumption in various industries, including pharmaceuticals, personal care, detergents, automotive, agrochemicals, food and beverages, textiles, and other end-user industries [9]. Growing academic interest mirrors EO’s industrial importance, as shown by the rising number of ethylene epoxidation publications over the last two decades (Figure 1).

Ethylene oxide (EO) has traditionally been manufactured by the direct oxidation of ethylene in the presence of molecular oxygen (O₂) via silver catalysis. The reaction of the conventional process proceeds at high temperatures (392 to 500 °F) and high-pressure conditions (10 to 30 bar). Although this process is well established, it strongly depends on fossil fuels, leading to a high environmental impact, as it emits approximately 1.9 tons of CO₂ for every ton of EO produced [1,10,11]. Very recently, Yang et al. [12] surveyed the latest advances in heterogeneous catalyst architectures for ethylene epoxidation—covering metal nanoparticles, single-atom sites, bimetals, and the role of novel promoters—and highlighted emerging electrocatalytic and photocatalytic routes to EO with near-atom efficiency [12]. However, EO has poor selectivity to the epoxidation reaction, obtaining only ca. 85%, since it gives rise to very high amounts of CO₂ as by-products, this process being the second-largest contributor to CO₂ emissions after ammonia synthesis in chemical processes [1].

Various researchers have tried alternatives to replace the widely practiced conventional process of epoxidation in the gaseous phase, which is associated with CO₂ emissions, in order to produce a clean liquid-phase epoxidation (LPE) process. This method uses a homogeneous methyltrioxorhenium (MTO) catalyst with H₂O₂ as the oxidant. The LPE process demonstrated excellent catalytic performance, with nearly 100% selectivity toward ethylene oxide (EO) without emitting CO₂ [13]. However, the application of MTO to this process faces two main problems. Firstly, it tends to deactivate the catalyst due to the deposition of water as a by-product in the reaction. The second problem arises because rhenium (Re) is an expensive and scarce metal, making its use economically and practically difficult. Consequently, the substitution of Re with easily available, cheap catalytic materials with high H₂O₂ utilization efficiency should improve the economic feasibility as well as the practical ability of the process [14].

Another disadvantage of ethylene oxide, EO, arises from the inherently high reactivity of the compound, which always tends to result in disagreeable routes, inclusive of direct burning or the isomerization reactions of acetaldehyde [15]. Products from acetaldehyde burning can be CO₂ and water [16]. Moreover, ethylene itself can undergo direct combustion to further contribute to the formation of CO₂ and water, as illustrated in Figure 2 [9]. The mechanism of ethylene epoxidation reaction and the nature of the active species remain under debate, despite several decades of research [17,18,19].

Despite these challenges, ethylene oxide (EO) is an essential constituent of many industrial processes, and demand for this compound continues to grow. To this end, research into epoxidation has turned its focus to the investigation of the effectiveness of various materials as catalysts in ethylene oxide production [20,21]. Catalysis, reactor design, and reaction kinetics are promising approaches to enhance efficiency and support sustainability. This review provides a comprehensive overview of recent developments regarding the creation of novel catalysts, advanced kinetic models, and improved reactor designs for addressing the environmental and economic challenges facing ethylene oxide production.

2. Role of Catalysts in the Process of Epoxidation of Ethylene

This reaction has been at the center of chemical synthesis: the catalytic conversion of ethylene to ethylene oxide, sustained by continuous efforts in developing highly effective and selective catalysts. These developments have introduced different types of catalyst systems over time, each presenting different ways to improve performance. Starting from the electronic structure of Mn-N₄GP (Mn-N4 porphyrin-like graphene) to the stability of Ag–h-BN (silver supported on hexagonal boron nitride) (Table 1, Entry 2) up to synergistic effects in Ag–CuO (Ag supported on CuO) and AgNiO₂ (perovskite-type silver nickel oxide) systems, catalyst design advancements have considerably improved EO production. Perovskites, hybrid nanostructures, and silver-based catalysts have been explored over time for maximum EO production through a balancing act between selectivity and reactivity. The structural, electrical, and surface properties of each catalyst affect its performance and control the creation of products and intermediate stabilization. This review of the literature will be focused on the developments made so far in ethylene epoxidation catalyst design, emphasizing their special contributions to the enhancement of EO selectivity and reduction of undesirable by-products.

According to [22], the Mn-N₄GP catalyst has the best ethylene epoxidation properties due to its unique electronic structure and high selectivity (Table 1, Entry 1). Its active site, Mn=O, can efficiently transfer oxygen to ethylene. It forms the target molecule ethylene oxide as the main product, but minimizes the production of by-products such as acetaldehyde and five-membered ring species, with energy barriers of 0.56 and 0.46 eV, respectively, according to kinetic studies, with a formation rate for ethylene oxide 10⁵ and 10⁴ times faster. DOS (density of states) surface oxide reconstruction analysis confirmed the Mn=O site’s efficient electron transfer to activate ethylene’s π-bond, making Mn-N₄GP a selective and efficient catalyst for green ethylene oxide production under mild conditions [23,24].

The Ag–h-BN catalyst manifests high activity and selectivity for ethylene epoxidation, owing to the unique interaction of Ag atoms with reactants. Anchored at the B-vacancy defect in the boron nitride nanosheet (as shown in Figure 3), the Ag atom is the active site that favors ethylene adsorption (−1.21 eV) over oxygen [25]. A low energy barrier for ethylene oxide formation (0.58 eV) and resistance to isomerization to acetaldehyde ensure selective partial oxidation [26]. Molecular dynamic simulations also confirm that the catalyst is structurally stable at high temperatures and thus suitable for large-scale applications [27,28]. Silver-based catalysts remain the cornerstone of ethylene oxide production, with continued improvements in selectivity through the use of promoters such as chlorine and cesium significantly enhancing catalytic performance [29].

Silver’s catalytic efficiency arises from stabilizing oxygen species and providing selective sites. However, pristine Ag surfaces show low EO selectivity under industrial conditions due to the dominance of the OMC–DH (oxometallacycle–dehydrogenation) pathway, leading to combustion products. High oxygen pressures enhance EO selectivity via Ag surface oxides (AgO_x) (Table 1, Entry 5), where positively charged Ag atoms suppress dehydrogenation. While facets like Ag(100) (Table 1, Entry 3) are more active, they favor OMC–DH and reduce selectivity. The surface oxidation and the structures of Ag, including nanoparticles and a partially oxidized surface, should be optimized to improve ethylene epoxidation performance [30,31].

According to a study by [32], silver’s remarkable activity in ethylene epoxidation is a consequence of the moderate strength of its oxygen bonding, which allows O₂ to dissociate effectively, but prevents overoxidation. Ag(100) surfaces are particularly efficient in stabilizing the OMC intermediate (as shown in Figure 4), lowering the activation energy for ethylene oxide (EO) formation. In contrast, Cu binds oxygen too strongly to form oxide phases, while Au exhibits poor O₂ activation and favors acetaldehyde (AA) formation. Scaling relations between oxygen-binding energy and selectivity reveal that Ag has an optimal balance of kinetics and thermodynamics, which together make it the best catalyst for EO production [33,34].

Silver catalyst systems are highly selective towards ethylene oxide (EO) because of their intermediate oxygen-binding affinity, which enables the stabilization of key reaction intermediates like the oxometallacycle (OMC). Oxygen-rich surfaces like Ag₁.₈₃O(111) and AgO_p(4 × 4) (4 × 4 gene of silver under O₂-rich conditions) exhibit especially high selectivity, and the strong adsorption of oxygen is important in the stabilization of intermediates and the reduction of activation energy barriers for EO synthesis [35,36]. Electronic structure analyses, including Bader charge analysis, show that enhanced charge transfer and more stable Ag–O bonding are directly linked to improved EO selectivity. AgO_p(4 × 4) has the highest charge transfer (Q₀ = −0.98 |e|), which is in line with its better performance [37]. Also, thermodynamic modeling indicates that various silver oxide phases may coexist in industrial conditions, and both oxygen coverage and surface oxygen species character are significant parameters in determining product selectivity. In addition, the cooperation of Langmuir–Hinshelwood and Eley–Rideal mechanisms over these surfaces enhances both selectivity and reactivity, which further upholds the excellent standing of silver-based catalysts in industrial ethylene epoxidation [38].

Metal dopants in Ag-catalyzed ethylene epoxidation optimize EO selectivity by engineering oxophilicity. Dopants like Cs, Cu, and Co recreate catalytic sites where oxygen binds to the dopant and ethylene to Ag, reducing EO formation energy barriers while limiting by-product stabilization [39]. DFT (density functional theory) calculations show dopants tune oxygen adsorption energy (ΔG_o*) to balance reactivity and selectivity, breaking pristine Ag’s scaling relations and placing doped systems at the selectivity volcano’s peak. Dual dopants, such as Co–Cu or Cs–Re, further enhance selectivity through synergistic stabilization of intermediates and efficient EO desorption [40,41,42].

Silver supported on α-alumina is the benchmark catalyst for ethylene epoxidation due to its unique oxidative properties [43]. The low surface area of α-alumina (around 1–8 m²/g) minimizes the density of hydroxyl groups, which could otherwise catalyze the overoxidation of EO to CO₂ and H₂O, improving EO selectivity. Additionally, silver particles supported on high-surface-area α-alumina (around 30 m²/g) exhibit enhanced stability against sintering under reaction conditions, reducing particle growth via mechanisms like Ostwald ripening (see Figure 5) [43]. This stability is attributed to the larger interparticle distances afforded by higher-surface-area supports, which limit silver particle coalescence and maintain catalytic performance over extended operation periods [16,44].

Electrocatalysts like RuO₂ and BaOx/IrO₂ excel in halide-mediated ethylene epoxidation by creating active oxygen sites for selective EO production. Halide-modified RuO₂ favors epoxide-like ethylene adsorption, minimizing CO₂ formation, while BaOx-modified IrO₂ boosts faradaic efficiency above 85% by inhibiting hypochlorous acid cleavage. Their stability and ability to modulate oxygen species make these catalysts highly promising for efficient, large-scale EO production, highlighting the importance of tailored structures and surface properties [45].

A study by [46] revealed that strontium ferrite perovskites facilitate chemical looping epoxidation by balancing oxygen capacity and release (see Figure 6). The cubic SrFeO₃ phase provides high oxygen capacity, while the layered Sr₃Fe₂O₇ phase enables faster oxygen release and uptake. Together, they supply lattice oxygen to Ag catalysts, which selectively convert ethylene to EO while minimizing CO₂ formation. A 1:1 SrFeO₃:Sr₃Fe₂O₇ ratio optimizes oxygen release kinetics, quadrupling EO yield (1.6%) compared to SrFeO₃ alone. This highlights the importance of tailored catalyst design in enhancing chemical looping epoxidation efficiency [46,47].

CeO₂-modified SrFeO₃ acts as the oxygen carrier, while Ag constitutes the active catalytic site. Silver prevents total combustion because it stabilizes the crucial OMC intermediate and allows only the selective oxidation of ethylene to EO. CeO₂ enhances the rate of oxygen release and uptake during reduction and reoxidation; thus, it significantly promotes the performance of the catalyst. Such cooperation by CeO₂ is effective in providing an oxygen bridge: transferring oxygen rapidly between the gas phase and the SrFeO₃ lattice [48]. Consequently, μO₂ (chemical potential of oxygen) increases with enhanced phase cooperation, effectively resulting in greater EO selectivity and maintaining constant performance after numerous redox cycles of catalysts. The prepared catalyst design achieves the maximum limit of desired performance above and beyond state-of-the-art traditional direct gas-phase epoxidation processes and gives improved selectivity than unmodified SrFeO3 [48,49]. In Figure 7, the SEM–EDS analysis illustrates the distribution of silver and cerium in both (CeO₂)impr(SrFeO₃) (CeO₂ impregnated into SrFeO₃) and (CeO₂)ss(SrFeO₃) (CeO₂ incorporated via solid-state synthesis with SrFeO₃) samples, revealing a uniform spread of strontium, iron, and oxygen across the material surfaces.

The Ag–SrFeO₃ catalyst is highly efficient for ethylene oxidation to ethylene oxide (EO) in the CLE (chemical looping epoxidation) process. SrFeO₃ acts as a lattice oxygen reservoir, while silver catalyzes the selective reaction. The Ag–SrFeO₃ interface facilitates oxygen transfer, and higher calcination temperatures (e.g., 650 °C) improve EO selectivity by forming larger Ag particles that minimize overoxidation. Ceria (CeO₂) doping enhances oxygen transport and catalyst regeneration, achieving 60% EO selectivity and 15% ethylene conversion [15].

The Ag–CuO hybrid nanocatalyst demonstrates enhanced performance due to the synergistic effect between silver and copper oxide domains. The unique structural configuration, where Ag emerges on the CuO surface, provides active interfaces for selective epoxidation. Chlorine acts as a promoter by influencing the adsorption–desorption energies of intermediates and products [50]. It reduces the interaction strength of EO with the catalyst surface, decreasing the probability of its overoxidation to CO₂. Additionally, chlorine promotes the pseudo-direct pathway, increasing EO selectivity while driving AA formation through the less favorable OMC pathway. These combined effects explain the experimentally observed higher EO/CO₂ ratio and maximum EO selectivity of 60% under optimized conditions [51,52].

A study by [53] revealed that under sun radiation, plasmonic silver (Ag) nanoparticles are essential for increasing ethylene epoxidation. LSPR (localized surface plasmon resonance) action of Ag nanoparticles generates hot, energetic electrons that enhance the reactivity of molecular oxygen while allowing efficient visible light absorption. The generated hot electrons can stabilize reactive intermediates, while O₂ is reduced to promote the production of EO [54]. Plasmonic Ag reduces the activation temperature compared with conventional thermal catalysts, saving energy consumption and improving selectivity, with higher EO yields because the Ag catalyst surface is inhibiting all the oxidation pathways. Solar-powered technology provides EO without the use of high pressures or heat from fossil fuels; hence, it is energy-efficient and environmentally friendly [55,56].

The epoxidation reaction is aided by AgNiO₂ nanoparticles in two ways. While the nickel oxide component (Ni²⁺) stabilizes the oxygen species and stops overoxidation to CO₂, the silver component serves as the active site for the selective oxidation of ethylene to EO. The AgNiO₂ structure partially breaks down into metallic silver (Ag) and nickel oxide (NiO) at high temperatures (over 150 °C), which lowers the selectivity of EO. Nevertheless, the delafossite structure offers a stable interface for ethylene and oxygen activation at ambient temperature, resulting in effective EO synthesis. Lattice oxygen is essential for facilitating the catalytic process without causing appreciable structural degradation, and the synergistic interaction between Ag and Ni guarantees a balance between reactivity and selectivity [57].

The oxidation state and kind of oxygen species on the Ag surface determine how well the Ag–α-Al₂O₃ catalyst performs in ethylene epoxidation. Silver particles produce surface and subsurface oxygen species that actively contribute to the process when exposed to severe oxidative conditions. Atomic oxygen encourages non-selective pathways that result in combustion products, whereas dioxygen species (Ag₄-O₂) are found to be extremely selective for EO formation [58]. Furthermore, dynamic oxidation-reduction cycles on the Ag catalyst surface control the ratio of selective to non-selective oxygen species. High catalytic activity is maintained by the α-AlO₃ support, which also minimizes unwanted interactions and guarantees the Ag nanoparticles’ thermal stability. Controlled surface oxidation is necessary for optimal EO selectivity in order to stabilize Ag₄-O₂ species without causing an excessive amount of strongly bound oxygen species to develop [59,60].

Titanium silicalite 1 (TS-1) contains two different active sites, so it has great activity and selectivity in oxidative hydration. By creating and maintaining TiOOH intermediates, isolated and dinuclear titanium sites lower the activation energy barriers for the synthesis of EO and promote the epoxidation of ethylene [61]. Because they stabilize the OMC intermediate and allow for the catalytic conversion to ethylene glycol, hydrolyzed titanium sites with Ti−OH groups are crucial for the hydration of EO. These hydrolyzed sites are necessary for efficient tandem catalysis, because according to structural optimizations, they have lower energy barriers for hydration than ideal titanium sites. This dual-purpose mechanism brings to the fore the importance of structural tailoring in TS-1 to maximize epoxidation and hydration for industrial applications [62,63]. The comparative data presented in Table 1 summarize various catalysts investigated for ethylene epoxidation, highlighting their mechanisms, structural types, operating conditions, stability, and selectivity toward ethylene oxide. The table also outlines the impact of promoters and dopants, offering insights into how each catalyst’s design influences its performance.

Based on these observations, we can say that the selectivity of ethylene epoxidation towards ethylene oxide (EO) is influenced by a number of factors. Active species like Mn=O in Mn–N₄GP and Ag₄-O₂ in Ag-based catalysts significantly enhance EO selectivity. Dioxygen species favor EO formation, but atomic oxygen favors combustion. Structural stability provided by support materials such as α-Al₂O₃ and h-BN restricts the formation of by-products, while promoters such as chlorine and dopants such as Cs and Cu facilitate the reaction towards more selective pathways. All these together highlight the significance of optimizing catalyst design in order to obtain maximum EO yield.

Table 1. Comparative summary of catalysts for ethylene epoxidation: mechanisms, structures, and performance parameters.

Entry No.	Catalyst	Mechanisms	Key Feature	EO Selectivity	Catalyst Structure/Type	Operating Conditions	Stability	Promoters/Dopant	References
1	Mn-N4 Porphyrin-like graphene (Mn-N4GP)	Two-step: (1) N2O reduction to form Mn=O site, (2) ethylene epoxidation via alkoxide radical pathway	High selectivity and stability; thermodynamically favorable Mn=O site	105× higher than acetaldehyde, 104× higher than 5MR species	Mn single-atom on porphyrin-like graphene	DFT modelled, mild	High (DFT shows stability)	None	[22]
2	Ag on B-vacancy h-BN (Ag–h-BN)	Trimolecular Langmuir–Hinshelwood mechanism with -CH2CH2OOCH2CH2- intermediate	Single-atom catalyst; strong selectivity; thermally stable	High, dominant over acetaldehyde (activation barrier: EO 0.44 eV vs. AC much higher)	Ag single-atom on B-vacancy h-BN	DFT modeled, ambient	High (embedded single atom)	None	[25]
3	Ag metal (Ag(111), Ag(100))	OMC (oxometallacycle) mechanism and dominant OMC-dehydrogenation path	Low EO selectivity under low oxygen pressure; selectivity improves with Ag(100) facet	<30% on Ag(111), ~65% on Ag(100)	Metallic Ag surfaces	High temp (500 K), 20 bar	Moderate (metal surface oxidation sensitive)	Cs, Cl, alkali metals	[32]
4	Group IB metals (Ag, Cu, Au)	Three stages: (i) O2 dissociation, (ii) Oxometallacycle formation, (iii) EO or AA formation	Ag offers best balance of O- and C-binding; Cu oxidizes easily; Au poor O2 dissociation	Ag(100) > Ag(111) > Au/Cu (EO formation barrier higher on Au/Cu)	Bulk Group IB metals (Ag, Cu, Au)	Various surfaces and O2 pressures	Low for Cu/Au due to poor O2 handling	None	[34]
5	AgOx (surface oxide Ag structures)	Multiple: Langmuir–Hinshelwood, Eley–Rideal; involves OMC intermediate	Diverse mechanisms; phase-dependent activity; AgO_p(4 × 4) surface enables ER	Varies with surface; LH dominant in most AgOx, ER in AgO p(4 × 4)	Ag surface oxides	Varied O2 pressure & surface coverage	Phase-dependent	None	[64]
6	Doped Ag catalysts (with metal dopants)	Cocatalytic mechanism optimizing oxophilicity; modifies O-affinity via dopants	Improved selectivity beyond pristine Ag by tuning O-binding without altering C-binding	Higher than pristine Ag (specific numbers not stated)	Ag with metal dopants (Co, Cu, etc.)	DFT modeled, optimized O-binding	Enhanced by dopants	Co, Cu, Cs, Cl	[39]
7	Iridium single-atom on α-MnO2 (Ir1–α-MnO2)	π-coordination enabled OMC intermediate formation	Molecular-like catalysis; π-interaction between Ir and ethylene enhances selectivity	~99%	Ir single-atom on α-MnO2	~200–250 °C, ambient pressure	High	None	[65]
8	Ag on α-alumina	Standard Ag-catalyzed epoxidation; affected by particle size and support structure	Stability via particle size and interparticle distance; Ostwald ripening dominant	~80% (industrial with Cl/alkali promoters)	Ag on α-Al2O₃	Industrial ~230–270 °C	Good if particle sintering avoided	Cs, Cl	[43]
9	Electrochemical systems (e.g., RuO2 mediated with Cl−)	Electrochemical halide-mediated pathway; forms hypobromite intermediates	Green method; improved selectivity via isolated OMC sites	High with halide mediation	RuO2 electrode with halide mediation	~Room temp, electrochemical	Good (electrolyte optimized)	Cl−, Br−	[66]
10	Ag on strontium ferrite perovskite (SrFeO3/Sr3Fe2O7 mix)	Chemical looping with lattice oxygen donation	High oxygen capacity; safe oxygen delivery; phase composition influences selectivity	Up to 25%, it improved to 60% with ceria doping	Ag on SrFeO3/Sr3Fe2O7 perovskite	Chemical looping, cyclic oxidation/reduction	Improved with RP-phase mixing	None	[46]
11	Ru single-atom on beta zeolite	Heterogeneous oxidation via isolated Ru active centers	Highly dispersed Ru atoms on zeolite provide high activity	High	Ru single-atom on Beta Zeolite	~200–300 °C (lab-scale)	High (zeolite structure stabilizes)	None	[65]
12	CoCu Co-doped Ag(111) with oxygen reconstruction	OMC mechanism with enhanced desorption and reactivity via dual dopants	Dual metal dopants (Co, Cu) optimize oxygen affinity, enhancing EO formation and desorption	Up to 89.5%	CoCu-doped Ag(111)	~300–400 °C	Improved by dual dopants	Co, Cu	[67]
13	Pt/ZSM-5 with fluorine promotion	Low temperature oxidation mechanism enhanced by F-modified acid sites	F-doping enhances low temperature activity and selectivity	Improved vs. unmodified Pt/ZSM-5 (quantitative data not provided)	Pt/ZSM-5 with F	~Low temp	Improved with F	F	[65]
14	Ag–ZSM-5	Room temperature catalytic oxidation of ethylene	Supports selective EO formation at mild conditions	30–40% depending on support acidity and Ag loading	Ag–ZSM-5	Room temperature	Stable under mild	None	[65]
15	Electrochemical RuO2 with Cl−	Halide-mediated pathway via Cl-inhibited sites leading to EO production	Inhibits combustion path; enables EO pathway	High selectivity	RuO2 with Cl−	Electrochemical	Good with halide control	Cl−	[68]

Table 1. Comparative summary of catalysts for ethylene epoxidation: mechanisms, structures, and performance parameters.

Entry No.	Catalyst	Mechanisms	Key Feature	EO Selectivity	Catalyst Structure/Type	Operating Conditions	Stability	Promoters/Dopant	References
1	Mn-N4 Porphyrin-like graphene (Mn-N4GP)	Two-step: (1) N2O reduction to form Mn=O site, (2) ethylene epoxidation via alkoxide radical pathway	High selectivity and stability; thermodynamically favorable Mn=O site	105× higher than acetaldehyde, 104× higher than 5MR species	Mn single-atom on porphyrin-like graphene	DFT modelled, mild	High (DFT shows stability)	None	[22]
2	Ag on B-vacancy h-BN (Ag–h-BN)	Trimolecular Langmuir–Hinshelwood mechanism with -CH2CH2OOCH2CH2- intermediate	Single-atom catalyst; strong selectivity; thermally stable	High, dominant over acetaldehyde (activation barrier: EO 0.44 eV vs. AC much higher)	Ag single-atom on B-vacancy h-BN	DFT modeled, ambient	High (embedded single atom)	None	[25]
3	Ag metal (Ag(111), Ag(100))	OMC (oxometallacycle) mechanism and dominant OMC-dehydrogenation path	Low EO selectivity under low oxygen pressure; selectivity improves with Ag(100) facet	<30% on Ag(111), ~65% on Ag(100)	Metallic Ag surfaces	High temp (500 K), 20 bar	Moderate (metal surface oxidation sensitive)	Cs, Cl, alkali metals	[32]
4	Group IB metals (Ag, Cu, Au)	Three stages: (i) O2 dissociation, (ii) Oxometallacycle formation, (iii) EO or AA formation	Ag offers best balance of O- and C-binding; Cu oxidizes easily; Au poor O2 dissociation	Ag(100) > Ag(111) > Au/Cu (EO formation barrier higher on Au/Cu)	Bulk Group IB metals (Ag, Cu, Au)	Various surfaces and O2 pressures	Low for Cu/Au due to poor O2 handling	None	[34]
5	AgOx (surface oxide Ag structures)	Multiple: Langmuir–Hinshelwood, Eley–Rideal; involves OMC intermediate	Diverse mechanisms; phase-dependent activity; AgO_p(4 × 4) surface enables ER	Varies with surface; LH dominant in most AgOx, ER in AgO p(4 × 4)	Ag surface oxides	Varied O2 pressure & surface coverage	Phase-dependent	None	[64]
6	Doped Ag catalysts (with metal dopants)	Cocatalytic mechanism optimizing oxophilicity; modifies O-affinity via dopants	Improved selectivity beyond pristine Ag by tuning O-binding without altering C-binding	Higher than pristine Ag (specific numbers not stated)	Ag with metal dopants (Co, Cu, etc.)	DFT modeled, optimized O-binding	Enhanced by dopants	Co, Cu, Cs, Cl	[39]
7	Iridium single-atom on α-MnO2 (Ir1–α-MnO2)	π-coordination enabled OMC intermediate formation	Molecular-like catalysis; π-interaction between Ir and ethylene enhances selectivity	~99%	Ir single-atom on α-MnO2	~200–250 °C, ambient pressure	High	None	[65]
8	Ag on α-alumina	Standard Ag-catalyzed epoxidation; affected by particle size and support structure	Stability via particle size and interparticle distance; Ostwald ripening dominant	~80% (industrial with Cl/alkali promoters)	Ag on α-Al2O₃	Industrial ~230–270 °C	Good if particle sintering avoided	Cs, Cl	[43]
9	Electrochemical systems (e.g., RuO2 mediated with Cl−)	Electrochemical halide-mediated pathway; forms hypobromite intermediates	Green method; improved selectivity via isolated OMC sites	High with halide mediation	RuO2 electrode with halide mediation	~Room temp, electrochemical	Good (electrolyte optimized)	Cl−, Br−	[66]
10	Ag on strontium ferrite perovskite (SrFeO3/Sr3Fe2O7 mix)	Chemical looping with lattice oxygen donation	High oxygen capacity; safe oxygen delivery; phase composition influences selectivity	Up to 25%, it improved to 60% with ceria doping	Ag on SrFeO3/Sr3Fe2O7 perovskite	Chemical looping, cyclic oxidation/reduction	Improved with RP-phase mixing	None	[46]
11	Ru single-atom on beta zeolite	Heterogeneous oxidation via isolated Ru active centers	Highly dispersed Ru atoms on zeolite provide high activity	High	Ru single-atom on Beta Zeolite	~200–300 °C (lab-scale)	High (zeolite structure stabilizes)	None	[65]
12	CoCu Co-doped Ag(111) with oxygen reconstruction	OMC mechanism with enhanced desorption and reactivity via dual dopants	Dual metal dopants (Co, Cu) optimize oxygen affinity, enhancing EO formation and desorption	Up to 89.5%	CoCu-doped Ag(111)	~300–400 °C	Improved by dual dopants	Co, Cu	[67]
13	Pt/ZSM-5 with fluorine promotion	Low temperature oxidation mechanism enhanced by F-modified acid sites	F-doping enhances low temperature activity and selectivity	Improved vs. unmodified Pt/ZSM-5 (quantitative data not provided)	Pt/ZSM-5 with F	~Low temp	Improved with F	F	[65]
14	Ag–ZSM-5	Room temperature catalytic oxidation of ethylene	Supports selective EO formation at mild conditions	30–40% depending on support acidity and Ag loading	Ag–ZSM-5	Room temperature	Stable under mild	None	[65]
15	Electrochemical RuO2 with Cl−	Halide-mediated pathway via Cl-inhibited sites leading to EO production	Inhibits combustion path; enables EO pathway	High selectivity	RuO2 with Cl−	Electrochemical	Good with halide control	Cl−	[68]

In conclusion, ethylene epoxidation requires catalysts to achieve high EO selectivity and process efficiency. The significance of adjusting surface characteristics, oxygen dynamics, and electronic structures to improve catalytic performance is highlighted by advancements in silver-based catalysts, hybrid nanostructures, and perovskite systems. New techniques, including chemical looping, plasmonic nanostructures, and doped systems, show promise for increasing EO yields while lessening their negative effects on the environment. More sustainable and scalable EO production methods will surely be fueled by ongoing research in catalyst development and mechanism elucidation.

3. Mechanisms of the Process

Epoxidation is the major process for the manufacture of ethylene oxide, an industrially important chemical. The process has undergone extensive studies, resulting in the proposal of several catalytic routes and mechanisms that are intended to provide maximum efficiency and selectivity with minimum by-products. While new catalysts such as Mn–N₄ graphene, Ag-based alloy systems, and Ir single-atom catalysts have been developed through progress in materials science, each containing different mechanisms involving such intermediates as OMCs and alkoxide radicals, conventionally used silver-based catalysts prevail in industrial processes. By underlining the interaction between catalyst composition, reaction intermediates, and activation energy barriers, this literature review summarizes current knowledge of these mechanisms and gives insight into the development and improvement of ethylene epoxidation techniques.

A two-step epoxidation of ethylene into ethylene oxide has been proposed by [22] based on the Mn-coordinated porphyrin-like graphene catalyst Mn–N₄GP (see Figure 8) The end-on adsorption of N₂O to the Mn center is coupled with the lengthening of the N-O bond and hence charge transfer. The subsequent steps are N-O bond breaking to form a stable Mn=O species via overcoming the energy barrier of 0.77 eV, acting as an active site of ethylene epoxidation. In the second step, ethylene interacts with the Mn=O site via one of three intermediates: carbon radical, alkoxide radical, or manganaoxetane. The alkoxide radical pathway, with a 0.89 eV barrier, is the most favorable, resulting in ethylene oxide formation [69,70].

According to [9], ethylene epoxidation on the Ag atom-embedded B-vacancy defective boron nitride nanosheet follows the trimolecular Langmuir–Hinshelwood mechanism. Two ethylene molecules co-adsorb at the Ag defect site, while further physisorption of an O₂ molecule creates a -CH₂CH₂OOCH₂CH₂- intermediate. This promptly cyclizes and decomposes into two ethylene oxide molecules via a low activation barrier of 0.44 eV; hence, this three-molecule path should be energetically more favorable compared to the bimolecular route because of higher and more complicated activation barriers [25,71]. Figure 9 illustrates the catalytic behavior of unpromoted silver foil during ethylene oxidation, showing time-dependent trends in product formation and EO selectivity, changes in surface oxygen species via O1s XPS spectra, and the evolving Oelec/Onucl ratio over time.

A study by [13] states that the silver-catalyzed epoxidation of ethylene occurs through a well-defined oxometallacycle (OMC) intermediate thath forms following the dissociative adsorption of O₂ molecules on the silver surface, resulting in the formation of reactive oxygen species, which then react with ethylene [32]. Following the formation of the OMC intermediate, three different reaction channels are possible: (1) cyclization to form ethylene oxide (EO), (2) hydrogen transfer to form acetaldehyde (AA), and (3) a new OMC–dehydrogenation (OMC–DH) pathway [72,73,74]. The OMC–DH pathway proposed by Diao et al. has the lowest energy barrier among the three and leads to dehydrogenation of an OxoE (surface-bound oxoethylene) intermediate, which is accountable for low EO selectivity, particularly at low oxygen partial pressures [36]. These pathways are highly surface-sensitive; for instance, the Ag(100) face favors the OMC–DH pathway due to its relatively low activation energy, but even then, EO formation competes favorably owing to its good kinetics on this surface [75]. The Cu and Au surfaces, on the other hand, exhibit poor activity towards O₂ dissociation and limited EO formation, further consolidating the unique applicability of Ag to selective epoxidation of ethylene [76].

This general trend for ethylene epoxidation in TM (transition metal)-doped Ag catalysts follows a co-catalytic mechanism related to the different sites for oxygen adsorption and the activation of ethylene [39]. On clean Ag surfaces, this reaction proceeds through an OMC intermediate, which can further either ring-close to EO or isomerize into AA. All these paths possess energy barriers dependent on the adsorption affinities of both oxygen and carbon. Metal dopants enhance EO selectivity by stabilizing the OMC intermediate and lowering energy barriers for EO formation, acting as cocatalysts to promote EO production while minimizing AA formation [39,77,78,79].

These Ir1–α-MnO₂ (Table 1, Entry 7) single-atom catalysts catalyze the epoxidation of ethylene through a unique mechanism, as shown in Figure 10. The π-coordination among iridium, ethylene, and molecular oxygen lowers the activation energy for oxygen dissociation in such a way that atomic oxygen can directly react with ethylene to the five-membered-ring oxometallacycle intermediate, producing ethylene oxide selectively [65]. The Ir1–α-MnO₂ system showed near-unity EO selectivity (∼99%) with much higher catalytic activity compared to Ag-based catalysts. The iridium center stabilizes the intermediates, and the α-MnO₂ support increases catalytic efficiency because of the stabilization of oxygen species. Structural analysis indicates that iridium atoms are present in substitutional and adsorbed states, where the latter are more catalytically active. This synergy ensures exceptional performance for EO production [65,80,81,82].

Ethylene epoxidation over silver catalysts supported on α-alumina (Ag–α-Al₂O₃) is predominantly via the Langmuir–Hinshelwood (LH) mechanism, in which both ethylene and oxygen species adsorb on the catalyst surface before they react [60]. Selectivity for ethylene oxide (EO) formation arises from adsorbed ethylene reacting selectively with active oxygen species, such as the Ag₄–O₂ dioxygen species on the reconstructed p(4 × 4)-O-Ag(111) surface, to produce the oxometallacycle (OMC) intermediate and subsequently EO. Competition from side reactions with atomic oxygen can lead to combustion products (CO₂ and H₂O) [83]. Steady-state isotopic transient kinetic analysis (SSITKA) confirms that EO formation is predominantly via the LH pathway with gas-phase oxygen, whereas CO₂ formation is primarily via a minor Mars–van Krevelen (MvK) pathway [84,85]. Furthermore, catalyst activity is very sensitive to silver particle size with an optimum at 60 nm: sintered particle sizes that are larger cause negative effects on both catalytic activity and selectivity through a reduction in surface area and alteration of active site distribution [43,86]. The gentle interaction between silver and oxygen at optimum particle size creates mild oxidation conditions favorable for selective EO formation [76].

Electrochemical ethylene epoxidation proceeds via a multistep mechanism. The most important steps concerning halide-mediated reactions have been taken into consideration. Generally, the forms of active halides such as Cl⁻ or Br⁻ facilitate the formation of halohydrin intermediates that undergo further dehydrohalogenation in order to afford ethylene oxide (EO) [68]. In this respect, oxidized halides at the anode would yield the active halide species reacting with ethylene to form halohydrins, further converted into EO at the cathode. Studies on RuO₂-based catalysts (Table 1, Entry 9) show that halides selectively block diol-like ethylene configurations, favoring epoxide formation and preventing overoxidation to CO₂, thereby enhancing EO selectivity [66,87]. Figure 11 illustrates the preferred adsorption configurations and corresponding reaction pathways involved in the electrocatalytic epoxidation of ethylene to ethylene oxide (EO). It compares the surface behavior of pure RuO₂ with that of RuO₂ modified by adsorbed chloride ions, highlighting the role of Cl⁻ in altering the reaction mechanism and potentially enhancing EO selectivity [45].

In the chemical looping epoxidation (CLE) of ethylene, silver (Ag) functions as the active catalyst, while strontium ferrite-based perovskites, such as SrFeO₃ and Sr₃Fe₂O₇, serve as oxygen carriers, enabling the selective oxidation of ethylene without the use of gaseous oxygen. The CLE process operates via two sequential steps: (1) a reduction step, in which lattice oxygen from the perovskite is released and transferred to the Ag surface, where it reacts with ethylene to form ethylene oxide (EO) through a key oxometallacycle (OMC) intermediate; and (2) a reoxidation step, wherein the oxygen-depleted perovskite is regenerated in air to restore its lattice oxygen content [46,48]. The performance of this process is highly dependent on the oxygen release rate and the redox stability of the oxygen carriers. A synergistic 1:1 combination of SrFeO₃ and Sr₃Fe₂O₇ enhances oxygen transport and EO yield due to improved lattice oxygen mobility. Moreover, supporting Ag on CeO₂-modified SrFeO₃ perovskites significantly boosts redox kinetics and oxygen release capacity, achieving up to 60% EO selectivity at 10% ethylene conversion over 15 cycles [88,89]. The size of Ag particles also plays a critical role, with larger particles formed at higher calcination temperatures exhibiting better EO selectivity by stabilizing the OMC intermediate and promoting efficient oxygen transfer [15,90]. This integrated CLE strategy offers a promising pathway for selective ethylene epoxidation under milder and more sustainable conditions [91].

Chlorine-promoted ethylene epoxidation over a Ag–CuO nanocatalyst goes by a parallel pathway: a so-called oxometallacycle path and a direct path. The ethylene forms an OMC intermediate that further cyclizes to form EO through the OMC path, whereas the direct path goes via a direct reaction with chemisorbed oxygen. Chlorine lowers the activation energy for both pathways, hence facilitating the formation of EO. It lowers the activation barriers by 20% for the OMC route and 56% for the direct pathway; hence, it improves the selectivity of EO. Chlorine also stabilizes ethylene oxide and prohibits further oxidation to acetaldehyde and CO₂ [92,93].

In the case of solar-driven epoxidation of ethylene, the active sites include the plasmonic Ag nanoparticles, whereby LSPR is used to enhance the activation toward oxygen. The photoinduced excitation of Ag NPs into hot electrons is transferred first to the adsorbed molecular oxygen (O₂). This step is the one that involves the transient superoxide species, O₂⁻, and is important in the activation and dissociation of O₂ to reactive oxygen atoms. Further reaction with ethylene, C₂H₄, absorbed on the surface of Ag through an intermediate known as oxometallacycle (OMC) is an important form that undergoes cyclization, forming ethylene oxide, EO [53]. The plasmonic enhancement effect lowers the activation energy for O₂ dissociation and provides the possibility of performing epoxidation at a lower temperature and thus preventing overoxidation to CO₂ [94,95].

The O₅ phase formed on the Ag(100) surface under industrial conditions promotes the epoxidation of ethylene over silver catalysts through the stabilization of the oxometallacycle intermediate by the square-pyramidal subsurface oxygen atoms (O₅β), yielding ethylene oxide [96]. The O₅ phase reduces the activation energy for ethylene adsorption and EO production to 0.86 eV and hence energetically is preferreds over competing pathways, such as full oxidation to CO₂ or acetaldehyde [97,98]. In contrast to the Ag(111) and Ag(110) surfaces, which are known to favor combustion products, the Ag(100) surface promotes selective EO production because of its unique O₅ phase and electronic properties [90,99].

Epoxidation of ethylene over delafossite-based AgNiO₂ nanoparticles is a reaction where the lattice oxygen and ethylene are activated at the silver–nickel interface. Ethylene reacts with adsorbed oxygen species on the AgNiO₂ surface to form the OMC intermediate, which cyclizes to afford EO without overoxidation. Oxygen species with binding energies of 528–530 eV are responsible for selective epoxidation, and the nickel component stabilizes oxygen vacancies, thus enhancing catalytic performance. The mixed oxide structure is stable at temperatures below 150 °C, ensuring a consistent supply of reactive oxygen [98].

A study by [64] found that the two major mechanisms for ethylene epoxidation over silver-based catalysts were the Langmuir–Hinshelwood (LH) mechanism and the Eley–Rideal (ER) pathway, as illustrated in Figure 12. The LH mechanism proceeds with molecular oxygen dissociated on the silver surface to yield atomic oxygen, which reacts with adsorbed ethylene to yield the oxometallacycle (OMC) intermediate—a key precursor to both ethylene oxide (EO) and acetaldehyde (AA) [35]. This pathway is a prevailing mechanism on most Ag and AgOx surfaces, such as oxygen-covered Ag(111) where microkinetic calculations confirm EO formation as a kinetically controlled process [100,101]. In contrast, the ER mechanism prevails on very oxygen-rich surfaces such as AgO_p(4 × 4) where gas-phase ethylene directly reacts with pre-adsorbed atomic oxygen in the gas phase [67]. This direct interaction significantly lowers the activation energy and enhances EO selectivity. Among the tested surfaces, Ag₁.₈₃O(111) presents the lowest energy barrier for EO formation, showing its high efficiency and highlighting the significance of surface composition and oxygen coverage in controlling the prevailing reaction mechanism and product selectivity [72,102].

The oxidation hydration of ethylene into ethylene glycol on TS-1 catalysts proceeds through the two-step tandem mechanism of ethylene epoxidation followed by hydration of the so-formed ethylene oxide [103]. Epoxidation includes the reaction of ethylene with H₂O₂ at isolated or dinuclear titanium sites to form ethylene oxide through Ti-hydroperoxy (TiOOH) intermediates [104]. In the hydration step, hydrolyzed titanium sites generate reactive hydroxyl groups that stabilize the oxometallacycle (OMC) intermediate, which undergoes ring opening to produce ethylene glycol. This mechanism emphasizes the need for dual-site cooperation: perfect titanium sites for epoxidation and hydrolyzed sites for EO hydration [105,106].

4. Kinetics of Epoxidation of Ethylene

Knowledge of reaction kinetics is helpful in optimization toward good yield and high selectivity for the desired product in ethylene oxide production. For partial ethylene oxidation to ethylene oxide on silver-based catalysts, a number of kinetic models have been used in order to describe epoxidation [107]. The Langmuir–Hinshelwood mechanism remains the most frequently applied to competitive adsorption of ethylene, oxygen, and the selectivity promoter dichloroethane onto catalyst surfaces. These models take into account the interaction of partial and total oxidation reactions of ethylene and ethylene oxide, in which DCE (dichloroethane) is an important modifier for promoting EO selectivity through the inhibition of undesirable side reactions, such as complete oxidation [108]. Kinetic parameters, including reaction rate constants, activation energies, and adsorption constants, are essential in the design of the reactor and process optimization. Notably, models will differ in their inclusion of reverse reactions and DCE effects, leading to variations in predictive accuracy considering different operating conditions [109].

The reaction kinetics in the continuous liquid-phase epoxidation of ethylene with hydrogen peroxide and a titanium–silicate catalyst, TS-1, were studied under a wide range of experimental conditions, such as temperature, pressure, hydrogen peroxide concentration, and liquid flow rates [110]. The catalyst exhibited excellent stability and high selectivity for ethylene oxide for a long period of 150 h. The results indicated that ethylene oxide selectivity strongly depends on temperature and pressure. With increasing temperature, the conversion of ethylene increased, but the selectivity to ethylene oxide decreased because of the enhanced decomposition of hydrogen peroxide [111]. In addition, increasing the hydrogen peroxide concentration showed positive effects on both the conversion of ethylene and selectivity. Higher water concentrations, on the other hand, reduced selectivity by the promotion of secondary reactions [112]. These results indicate that the reaction conditions, particularly the balance between hydrogen peroxide and water, should be optimized in order to maximize ethylene oxide while minimizing by-products formation [113,114].

In the study of ethylene epoxidation with hydrogen peroxide in a trickle bed reactor (TBR), modeling of transient kinetics was conducted in order to enhance the understanding of dynamic behavior. Epoxidation involves the reaction between ethylene (C₂H₄) and hydrogen peroxide (H₂O₂) on the surface of a titanium–silicate (TS-1) catalyst. The key reaction mechanism involves adsorption of H₂O₂ on the catalyst surface followed by reaction with dissolved ethylene to form ethylene oxide (C₂H₄O) and water [113]. Side reactions include the methanolysis of ethylene oxide. A dynamic model, including mass balances for gas, liquid, and surface phases, was used to simulate both transient and steady-state conditions [115]. The model included axial dispersion for both the gas and liquid phases, where adsorption–desorption and reaction steps took place on the catalyst surface. Estimation of the parameters was conducted by nonlinear regression of experimental data, and it was found that some kinetic parameters—activation energy and rate constants—were very important in the prediction of the behavior of the system [116].

The kinetics of ethylene epoxidation over chloride-promoted silver catalysts is greatly influenced by chlorine coverage in a manner influencing reaction rates and product selectivity to a large extent. Kinetic studies on 35 wt% Ag–α-Al₂O₃ catalysts reveal that the mechanism is greatly dependent on the chlorinating effectiveness factor of Z* for the level of promotion by surface chlorides [107]. With rising Z*, the reaction order with respect to oxygen increases from approximately 0.7 to close to 1, reflecting higher dependence on oxygen activation [107]. In contrast, the reaction order with respect to ethylene drops because of suppressed ethylene adsorption, owing to higher chloride coverage. Chloride influences catalytic activity through ensemble and electronic effects, and ensemble disruption selectively poisons overall oxidation pathways, resulting in decreased CO₂ production and thus enhancing EO selectivity [13,75]. In addition, electronic effects induced by chloride alter the adsorption energies of oxygen and ethylene species and impact reaction kinetic orders and product distribution. A maximum rate of ethylene oxide production is at intermediate chlorine coverage (0–15% monolayer) with a corresponding steady reduction in CO₂ production [117,118]. Ethyl chloride and CO₂ co-feeding studies also provide this selectivity enhancement as reversibly inhibited EO and CO₂ production by co-fed ethyl chloride and preferential inhibition of combustion paths by co-fed CO₂ [119]. Thus, ensemble–electronic models give a complete and correct explanation of the observed kinetics, emphasizing the pivotal and intricate role of chlorine in inducing ethylene oxide selectivity in the silver-catalyzed ethylene epoxidation reaction [120].

The kinetics of ethylene epoxidation over silver catalysts has been the focus of extensive studies and modeling to understand the mechanism of this reaction and to optimize reactor performance. In a two-site model, the reaction involves adsorption of ethylene and oxygen species onto active sites on the catalyst surface with the formation of ethylene oxide, EO, via an intermediate oxometallacycle, OME [9]. The most important findings clearly show that models such as Linic–Barteau (LB), Huš–Hellman (HH), and Stegelmann–Stoltze (SS) differ considerably in the reproduction of experimental data. Single-site models are sufficient to reproduce broader tendencies of the system, including the temperature dependence of EO production and the effect of oxygen concentration on selectivity [60]. More detailed models have to be used in order to describe the subtleties of surface oxygen interactions and EO selectivity, especially during transient states, such as multi-pulse oxidation-titration experiments [69]. Notably, the inclusion of a subsurface oxygen reservoir in the SS model has been shown to significantly enhance its ability to simulate EO production and selectivity; however, further refinements are needed to capture the rate of oxygen species decomposition accurately [97,121].

5. Conclusions

One of the more prominent roots of industrial chemical synthesis is ethylene oxide production through ethylene epoxidation, as ethylene oxide is a significant intermediary in a great many consumer goods, petrochemical intermediates, and fine chemicals. Despite the maturity of this process, there are some inherent defects to traditional gas-phase epoxidation using silver catalysts: poor selectivity, high energy use, and significant emissions of CO₂.

This literature review highlights such important developments in the catalyst design and reaction process to overcome these limitations. Improved hybrid nanostructures, doped systems, and silver-based catalysts have shown less by-product formation and more selectivity. The emerging catalytic materials, through electronic structure optimization and lattice oxygen dynamics—such as Mn–N₄GP and AgNiO₂ nanoparticles, respectively—promise potentially viable alternatives for enhancing the reaction pathways while reducing the ecological footprint.

This literature review sheds light on how many important improvements have been made at the level of catalyst design and reaction process in surmounting the challenges cited above. Subsequent enhancements in hybrid nanostructures, doped systems, and silver-based catalysts, on the other hand, have shown lower production of by-products, resulting in increased selectivity. Designing electronic structures for optimization and lattice oxygen dynamics—linked to new materials such as Mn-N₄GP and AgNiO₂ nanoparticles, respectively—bring encouraging alternatives with more promising pathways and less harm to nature.

With these developments, challenges still abound. In this respect, more research is needed when factors such as catalyst deactivation, the economic feasibility of using different materials, and how to scale up new processes come into play. Further revolutionizing of the industry, therefore, may be gained with the incorporation of renewable feedstocks and deployment of even more predictive computational models for the guidance of EO production toward better sustainability and efficiency.

In the final analysis, this confluence of sophisticated catalyst design, thorough mechanistic research, and sustainable technology ushers in a new era in the synthesis of ethylene oxide. Its continuous innovation and focus on environmental stewardship have the industry well placed to meet the growing demand for EO globally while minimizing its ecological footprint.