Metal-Modified Zeolites for Catalytic Dehydration of Bioethanol to Ethylene: Mechanisms, Preparation, and Performance

Abstract

With increasing demands for sustainable chemical production, bioethanol-derived ethylene offers a promising alternative to petroleum-based routes. This review examines recent advances in metal-modified zeolites for the catalytic conversion of bioethanol to ethylene. The fundamental reaction mechanisms and preparation methodologies are systematically analysed. Various metal modification strategies are discussed alongside their effects on catalyst properties. The influence of zeolite framework characteristics, metal species selection, and reaction parameters on catalytic performance are evaluated. Detailed attention is given to deactivation mechanisms and strategies for catalyst regeneration and lifetime extension. The analysis provides insights into rational catalyst design for sustainable ethylene production, highlighting opportunities for future research in enhancing catalyst stability and efficiency.

1. Introduction

With the depletion of fossil fuels and growing environmental concerns, the development of sustainable alternatives for chemical production has become increasingly important [1,2]. The need for renewable resources has driven research towards bio-based feedstocks that can effectively replace fossil-based raw materials while reducing environmental impact [3,4]. Among various sustainable alternatives, bioethanol stands out as a promising platform molecule, which can be produced through the fermentation of different agricultural sources such as sugar cane and corn [5,6,7].

Initially, bioethanol production primarily relied on edible biomass through conventional fermentation processes. However, to address concerns about competition with food supplies, research focus has shifted towards utilizing non-food biomass feedstocks such as lignocellulose materials [8,9]. The transition towards renewable energy, coupled with technological advancements, has led to substantial growth in global bioethanol production. As illustrated in Figure 1, the United States and Brazil collectively produce 80% of global ethanol, with the current worldwide production at approximately 31,100 million gallons [10,11]. The increased availability and decreased cost of bioethanol have opened new opportunities for its utilization as a renewable feedstock for producing various value-added chemicals such as light olefins [12,13], hydrocarbons and oxygenated chemicals [14,15,16], hydrogen [17,18], acetic acid [19], etc.

Among the various bioethanol conversion processes, catalytic dehydration to ethylene has garnered significant attention due to its potential to replace conventional petrochemical routes [20,21,22]. As of recent estimates, global ethylene production exceeds 150 million tons annually, with projections indicating it could surpass 255 million tons by 2035 [23,24]. Currently, ethylene production relies heavily on steam cracking of fossil feedstocks, which operates at temperatures above 750 °C [25,26]. The traditional ethylene production process presents significant environmental and energy challenges, consuming 15–27 MJ/kg of ethylene produced and generating approximately 2 tons of CO₂ equivalent per ton of ethylene [25,27]. With approximately 8% of the sector’s primary energy consumption, this process stands as the most energy-consuming process in the chemical industry [28]. In contrast, bioethanol dehydration offers a more sustainable alternative, operating at moderate temperatures (250–450 °C) and potentially reducing both energy consumption and carbon emissions [20,29,30]. However, the successful implementation of ethanol dehydration to ethylene is hindered by challenges such as catalyst deactivation, process complexity, and the need for precise operational conditions.

To improve the efficiency of the ethanol dehydration process, various solid catalysts have been developed, including γ-Al₂O₃, noble and non-noble metal, oxide, heteropoly acids, phosphoric acid, amorphous silica-alumina, and zeolites [31,32,33,34,35,36]. While γ-Al₂O₃ is a widely used industrial catalyst with high activity and low cost, its catalytic performance remains limited and is challenging to modulate across diverse reaction conditions [37]. Among various studied catalytic materials, zeolite catalysts have attracted considerable attention for bioethanol conversion owing to their unique properties such as well-defined porous structures, tuneable acidic characteristics, and shape selectivity [38,39,40]. These crystalline aluminosilicates possess three-dimensional frameworks with precisely controlled pore dimensions and channel systems, enabling molecular-level control over reactant access and product formation [41,42,43]. The framework topology and aluminium distribution can be systematically modified during synthesis, allowing fine-tuning of properties such as thermal stability, acid site strength and distribution, and catalytic selectivity [37,44,45,46].

In contrast to traditional industrial γ-Al₂O₃ catalysts, zeolites significantly lower catalytic temperatures and improve product selectivity. They have attracted significant attention in bioethanol dehydration catalyst research, with investigations spanning multiple framework types including MFI, FAU, BEA, FER, RTH, and others [47,48,49]. The growing research interest reflects the recognition of zeolites’ unique advantages in this process, particularly their ability to enable dehydration at lower temperatures with enhanced selectivity compared with conventional catalysts such as alumina [50]. As shown in Figure 2, the number of research publications on ethanol conversion catalysts as well as zeolites demonstrated a notable increase from 2000 to 2024. The variety of zeolites being studied reflects efforts to understand structure–property relationships for optimized catalytic performance in dehydration reactions.

Despite their numerous advantages, conventional zeolites may encounter deactivation challenges during ethanol conversion due to carbon deposition and formation of higher hydrocarbons, which can progressively block active sites and reduce catalytic activity over time [47,51]. Therefore, the incorporation of metal species into zeolite frameworks has motivated extensive research as a promising strategy to enhance catalyst stability, modify acid site distribution, and improve catalytic performance during bioethanol conversion processes [52].

Metal modification of zeolites used in ethanol dehydration is crucial for enhancing their catalytic performance, particularly in terms of activity, selectivity, and stability. This process involves incorporating metals into the zeolite structure to improve the interaction between the metal and the zeolite, thereby enhancing the overall catalytic properties. The incorporation of various metallic elements has demonstrated remarkable improvements in both acidic properties and catalytic performance [52,53,54,55]. Metal modification strategies have been extensively studied using different types of metal species. Noble metals such as Pd, Pt, and Ru exhibit excellent catalytic properties [56,57]. Non-noble metals including Fe, Ni, and Cu have also attracted considerable attention owing to their cost-effectiveness and comparable catalytic performance [55,58,59]. Systematic studies have been conducted to investigate the effects of alkaline earth metals on zeolite properties [60]. Other metals such as Mn, W, and La have also been explored as potential modifiers [50,61,62,63]. These metal-modified zeolites demonstrate superior performance not only in lowering reaction temperature but also in improving ethanol conversion and product selectivity. The synergistic effect between metal species and zeolite framework contributes to the suppression of side reactions and enhanced catalyst stability, making metal-modified zeolites particularly promising for applications. However, achieving precise control over metal incorporation remains challenging, particularly in maintaining high dispersion and preventing metal aggregation during reactions. This has motivated research into developing advanced synthesis strategies and understanding the fundamental aspects of metal–zeolite interactions.

This review specifically examines metal-modified zeolites for catalytic ethanol dehydration, focusing on the literature from the past decade addressing ethanol dehydration mechanisms, zeolite synthesis, catalytic performance, and catalyst stability. The review is structured into three main sections following a logical progression from fundamental mechanisms to practical applications. The first section elucidates reaction mechanisms, analysing dehydration pathways on zeolite catalysts and preliminary insights into metal modification’s role. The second section comprehensively details preparation methodologies, encompassing zeolite support synthesis and metal modification strategies. The third section critically examines catalytic performance relationships, investigating zeolite framework characteristics, metal species impacts, reaction parameters, and catalyst stability. The review concludes with an assessment of current achievements and identification of challenges and future opportunities in this field. Through this comprehensive analysis of structure–property–performance relationships in metal-modified zeolites for bioethanol dehydration, this review aims to guide future research towards the rational design of more efficient and stable catalysts for sustainable ethanol utilization.

2. Mechanism

2.1. Dehydration Pathways on Zeolite Catalysts

Ethanol dehydration to ethylene stands as a pivotal reaction in the realm of sustainable chemistry, offering a bio-renewable route to a vital building block chemical. The fundamental mechanisms governing this transformation, particularly when catalysed by zeolite materials, have been the subject of extensive investigation. Understanding these pathways is crucial for the rational design of more efficient and selective catalysts.

At a basic level, ethanol dehydration can proceed via several established mechanisms, including intramolecular and intermolecular dehydration pathways [32]. For intramolecular dehydration, shown in Figure 3a, E1, E2, and E1cB mechanisms are proposed. The E1 reaction, a unimolecular elimination, initiates with the protonation of the hydroxyl group, followed by a rate-determining step involving the loss of water to generate a carbocation intermediate. Subsequent deprotonation of this carbocation then yields ethylene. In contrast, the E2 mechanism is a bimolecular, concerted process in which proton abstraction and water elimination occur simultaneously. The reaction rate in E2 is influenced by the concentration of the two compounds, which is a second-order reaction. The E1cB mechanism, a unimolecular conjugate base elimination, involves the initial abstraction of a proton to form a carbanion (conjugate base) followed by the departure of the hydroxyl group to generate the olefin [32]. Alternatively, ethanol can undergo intermolecular dehydration to form diethyl ether, as shown in Figure 3b,c. This reaction proceeds via two possible mechanisms: S_N1 (single-molecule nucleophilic substitution reaction) or S_N2 (bimolecular nucleophilic substitution reaction). In the S_N1 reaction, the reactants dissociate to carbocations and the negatively charged leaving group, which is the rate-controlling step. In the S_N2 reaction, lone pair electrons of the nucleophile attack the electrophilic electron-deficient central atom, forming the intermediate and losing the leaving base group at the same time. The generation of ether essentially follows the reaction mechanism of S_N1 or S_N2. Some mechanistic studies have revealed that ethanol dehydration proceeds through surface ethoxy intermediates, which can either decompose directly to ethylene at high temperatures or react with undissociated ethanol to form diethyl ether at lower temperatures. The reaction order with respect to ethanol has been found to be higher for DEE formation compared with ethylene formation, providing a kinetic explanation for the temperature-dependent product distribution [64].

The pore architecture of zeolite catalysts plays a decisive role in directing the reaction pathways and product distribution in ethanol dehydration. Zeolites with different pore dimensions exhibit distinct catalytic behaviours and product selectivity patterns. Small-pore zeolites, such as Rho, containing 8-membered rings, demonstrate remarkable catalytic efficiency and stability for ethanol-to-ethylene conversion under mild conditions, with their performance strongly dependent on the strength and concentration of solid-acid sites [65]. In medium-pore zeolite structures, such as ZSM-5, optimized textural properties can mitigate diffusion limitations, achieving high ethylene yields. Incorporating hierarchical mesoporous structures further enhances performance by reducing side reactions and coke formation [47]. In contrast, large-pore zeolites like FAU and H-Beta, characterized by 12-membered rings, tend to promote the formation of various hydrocarbons through a hydrocarbon pool mechanism [66,67]. Their larger pore dimensions, while beneficial for molecular transport, can lead to the increased formation of heavier hydrocarbon products and accelerated coke deposition.

Another investigation has shed light on the role of triethyloxonium ions (TEO) as key intermediates in the zeolite-catalysed dehydration of ethanol. Zhou et al. proposed a comprehensive reaction network encompassing direct ethanol dehydration, diethyl ether decomposition, and TEO-mediated ethylene formation, as shown in Figure 4. The in situ and ex situ NMR studies revealed that TEO is a stable and reactive surface species, which ethylates the zeolite to form ethoxy species that then yield ethylene. This TEO–ethoxide pathway appears energetically favourable in the initial stages, suggesting that TEO formation is a crucial step in understanding and optimizing zeolite-catalysed ethanol dehydration [68].

The interplay between zeolite acidity, pore structure, and reaction conditions dictates the intricate network of pathways involved in ethanol dehydration. While ethylene is the primary product, the formation of other olefins, such as propylene, can also occur under specific catalytic conditions. Xia et al. demonstrated that H-ZSM-5 zeolite could catalyse the conversion of ethanol to propylene and indicated that the reaction pathway involves the initial dehydration of ethanol to ethylene followed by conversion to propylene, butene, and C₅₊ olefins [69]. This highlights the potential for tuning zeolite catalysts to selectively produce a range of valuable olefins from bioethanol.

2.2. Role of Metal Sites in the Reaction Mechanism

The incorporation of metal species into the zeolite structure introduces a significant dimension to the ethanol dehydration reaction mechanism. As illustrated in Figure 5, various metal categories, including alkali metals, alkaline earth metals, transition metals, and noble metals, can be strategically incorporated into zeolite frameworks to achieve diverse catalytic enhancements. Metal sites can effectively modify the electronic properties of the zeolite support, thereby modifying both the acidity type and overall catalytic activity. These modifications manifest in several key areas: selectivity control through targeted product formation, acid site modification by altering Brønsted and Lewis acid distributions, textural property changes including pore structure and surface characteristics, stability improvement through enhanced resistance to deactivation, and synergistic effects between metal sites and zeolite frameworks. Certain metals can function as acid sites, potentially playing a direct role in the activation of ethanol molecules during the reaction [70]. For instance, a study carried out by Saini et al. shows that Sr modification reduces strong acid sites, suppressing ethylene oligomerization and coke formation, thereby enhancing catalyst stability [55]. Similarly, Chen et al. also find that iron exchange in HZSM-5 diminishes strong acid sites, promoting ethylene selectivity [59]. While weak acid sites favour ethylene formation, strong acid sites tend to promote undesirable side reactions [55]. Furthermore, in the case of bimetallic catalysts, the presence of two different metals can lead to synergistic effects, where the combined catalytic performance exceeds the sum of the individual contributions of each metal [71]. Conversely, the presence of zinc can suppress ethanol dehydration to ethylene and promote dehydrogenation to acetaldehyde, depending on the specific zeolite support [72]. The incorporation of metals can also affect the activation energy required for different reaction steps, thereby shifting the preference towards a particular reaction mechanism [50]. The diverse effects exhibited by different metals on product selectivity and reaction pathways clearly indicate that the choice of metal is a critical factor that must be carefully considered based on the desired catalytic outcomes.

3. Preparation Methods

3.1. Zeolite Support Preparation

Hydrothermal synthesis represents the most widely adopted and well-established methodology for zeolite preparation and is typically conducted under autogenous pressure in sealed vessels. The fundamental principle underlying zeolite formation involves the systematic assembly of tetrahedral precursors into increasingly complex structural hierarchies, as illustrated in Figure 6. The synthesis process begins with primary building units (PBUs), typically [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedra, which serve as the fundamental building blocks for zeolite framework construction [73]. These primary units organize into diverse secondary building units (SBUs) through corner-sharing oxygen atoms, creating various polyhedral configurations including 4-rings, 6-rings, 8-rings, and more complex geometries. The strategic arrangement and connectivity of these SBUs, guided by structure-directing agents (SDAs), determine the specific zeolite framework topology [74]. The synthesis process involves the transformation of silicon and aluminium precursors into crystalline zeolite frameworks through carefully controlled temperature, pressure, and temporal parameters [75,76,77]. The selection of precursor materials significantly influences nucleation kinetics. For example, molecular-scale tetraethyl orthosilicate (TEOS) demonstrates an accelerated nanocrystal formation rate compared with colloidal silica sources [78]. The synthesis medium’s composition and physical state, ranging from clear solutions to dense gels, can be strategically manipulated to control crystal size and morphology [79].

As illustrated in Figure 6c, the crucial nucleation stage occurs following gel formation and is characterized by the emergence of small, ordered regions within the gel matrix, resulting in the formation of abundant nuclei that are fundamental for subsequent crystal development [82,83]. The nuclei then undergo crystal growth, facilitated by the continuous supply of silicate and aluminate species from the solution, which are systematically incorporated into the expanding crystal lattice [84,85]. Throughout the synthesis process, phase transformations may occur, involving the conversion of intermediate phases into the desired zeolite structure [85]. The crystallization process ultimately reaches completion when the relative crystallinity of the zeolite attains its maximum value, although it should be noted that prolonged crystallization periods may lead to the dissolution of the formed zeolite and the potential formation of alternative phases [75].

Several zeolite frameworks are commonly employed for ethanol dehydration, including ZSM-5, Zeolite-Y, H-mordenite, and Beta zeolite [50,86]. ZSM-5, characterized by a high silica-to-alumina ratio, exhibits strong acidity and shape selectivity due to its unique channel structure [50]. Zeolite-Y, with its large pore size and high surface area, generally possesses weaker acidity compared with ZSM-5 [87]. H-mordenite (H-MOR) has shown high activity and selectivity for ethanol dehydration, particularly at low temperatures [86]. Beta zeolite (BEA) is another promising catalyst known for its high surface area, high thermal stability, and larger pore size compared with H-ZSM-5 [88]. The distinct properties of each zeolite framework significantly influence their catalytic behaviour, making the choice of framework a critical factor before considering metal modification to achieve the desired performance characteristics.

3.2. Metal Modification Strategies

The modification of zeolites with metal species has emerged as a crucial strategy for tailoring catalyst properties. As shown in Figure 7, metal incorporation can be achieved through two primary synthetic approaches: in situ synthesis and post-synthesis modification. In situ synthesis involves the simultaneous formation of zeolite frameworks and metal incorporation during the crystallization process, encompassing strategies such as gel composition modulation, organic ligand stabilization, and kinetic regulation. Post-synthesis modification, conversely, introduces metal species into pre-formed zeolite structures through various techniques including atomic layer deposition, impregnation methods, and ion-exchange processes. Among these diverse methodologies, commonly employed approaches include ion exchange, wet impregnation, and in situ encapsulation, each presenting distinct advantages and technical considerations that significantly influence the final catalyst structure, metal dispersion, and catalytic properties.

Ion exchange is a widely employed method for introducing metal cations into the zeolite framework and is particularly effective for introducing cationic metal species, which can act as different acid sites within the zeolite [89]. The typical procedure involves contacting the zeolite with a solution containing the desired metal salt. The zeolite material is soaked in this solution for a specific duration, allowing the metal ions to diffuse into the zeolite channels and exchange with the existing cations [90]. Several factors can influence the efficiency and extent of ion exchange, including the concentration of the metal salt solution, the temperature at which the exchange is performed, the contact time between the zeolite and the solution, and the charge and size of the metal ion being introduced [91]. Higher concentrations of the metal salt solution and elevated temperatures typically lead to a greater degree of metal incorporation [92].

Impregnation is another common and relatively simple method for preparing metal-modified zeolites, during which metal precursors are dissolved in a solvent volume that matches the pore volume of the support [50]. The technique provides a straightforward approach to control metal loading through careful management of solution concentration and support pore volume. Factors such as the concentration of the precursor solution, the chemical nature of the metal precursor, the surface properties of the zeolite, and the conditions of the drying and calcination steps can significantly influence the final dispersion and particle size of the metal species on the zeolite [93]. Impregnation is a versatile method that allows for the introduction of a wide range of metal species, including those that may not be easily incorporated through ion exchange [91].

In situ encapsulation represents an advanced approach for achieving highly dispersed metal species within zeolitic frameworks. As illustrated in Figure 8, this method involves introducing metal precursors during zeolite synthesis, often employing complexing agents to enhance the stability of the precursor’s interaction with the zeolite framework. Subsequent post-treatment steps, such as reduction, yield highly dispersed metal nanoparticles within the zeolite framework [58].

While conventional wet impregnation shows procedural simplicity and requires no specialized equipment, it often leads to uneven metal distribution due to slow metal ion diffusion at ambient temperature and pressure [61]. Compared with impregnation methods, ion exchange typically yields catalysts with higher dispersion, though it requires precise control of exchange conditions and multiple iterations to ensure adequate loading [55]. The in situ encapsulation method can be employed for the preparation of zeolite catalysts loaded with metal nanoparticles. While this approach involves a more intricate procedure, when combined with reduction methods, it enables the production of ultra-small nanoparticles that exhibit enhanced catalytic performance in reactions [58].

4. Catalytic Performance

4.1. Effect of Zeolite Support

The catalytic performance of zeolites in ethanol dehydration to ethylene is fundamentally governed by their structural and chemical properties, particularly pore architecture, acid site distribution, and morphological features [52,94,95]. Quantitative studies reveal that pore size significantly influences product selectivity. Small-pore zeolites, such as Rho zeolites with 8-membered ring channels (3.6 × 3.6 Å), demonstrate superior ethylene selectivity of 83% at low temperatures (200 °C) compared with medium-pore frameworks like ZSM-5, which exhibit less than 20% selectivity under identical conditions. This selectivity advantage becomes more pronounced at elevated temperatures, with small-pore zeolites maintaining > 98% ethylene selectivity at 250 °C, while medium-pore zeolites show approximately 89% selectivity [65]. Additionally, the confined porous structure of small-pore frameworks (3.5 × 4.8 Å for FER zeolite) effectively inhibits the formation of larger hydrocarbon intermediates, achieving ~97% light olefin selectivity. In contrast, large-pore zeolites (12-membered ring FAU) produce diverse hydrocarbon products with only ~67% ethylene selectivity due to competing reactions via hydrocarbon pool mechanisms [66].

Moreover, the introduction of hierarchical structures like mesopores, nanosheets, and nanotubes significantly enhances mass transport properties and catalyst stability. For example, nanosheet-like SAPO-34 modified with Sr demonstrates remarkable performance improvements: the specific surface area increases from 573 m²/g for conventional cube-like SAPO-34 to 644 m²/g with external surface area enhancement from 12 m²/g to 75 m²/g. This structural modification results in substantially improved ethylene yield (94.5% vs. 82.4%), prolonged catalyst lifetime (913 min vs. 160 min), and reduced coke deposition (5.6% vs. 11.8%) compared with conventional cube-like SAPO-34. The nanosheet morphology, with thicknesses of 25–50 nm, provides shortened diffusion pathways and optimized medium-strong acidity [96]. Similarly, hierarchical FER demonstrates superior catalytic stability, maintaining 100% bioethanol conversion and ~95% ethylene yield over 24 h, while commercial FER suffers significant deactivation, with conversion and yield dropping to ~75% and ~50%, respectively, after only 4 h [66]. This enhanced performance results from facilitating faster product desorption and reactant diffusion through its interconnected porous network.

The catalytic performance of zeolites in ethanol dehydration is fundamentally governed by their acidic properties, particularly the distribution and strength of acid sites. The framework aluminium distribution achieved through isomorphous substitution of Al for Si plays a pivotal role in creating these catalytically active sites, with isolated Al atoms generating stronger acid sites than atoms with neighbouring Al in close proximity and the framework topology further influencing acid strength through T-O-T bond angle constraints [97]. The strategic positioning of aluminium atoms within the zeolite framework can be precisely controlled during synthesis through careful manipulation of gel composition and the incorporation of structure-directing agents [98]. Zeolites exhibit dual acidic functionality through their Brønsted and Lewis acid sites, with their proportions systematically modified through both synthetic strategies and post-synthetic treatments [99,100]. A synergistic effect between Brønsted and Lewis acid sites has been observed to enhance ethylene formation at low temperatures, where Brønsted sites primarily drive ethanol dehydration while Lewis sites stabilize reaction intermediates [101].

In summary, optimal catalytic behaviour emerges from integrating the shape selectivity of small-pore frameworks with the enhanced mass transport of hierarchical structures. The former suppresses byproduct formation and ensures high ethylene selectivity, while the latter improves stability by mitigating diffusion limitations and coke deposition. Moreover, a synergistic balance between Brønsted and Lewis acid sites promotes low-temperature dehydration pathways, thereby enhancing overall process efficiency.

4.2. Influence of Metal Modification

Metal modification of zeolite catalysts has emerged as a promising strategy for enhancing catalytic performance in ethanol dehydration reactions. The modification approach fundamentally alters catalyst properties through multiple mechanisms, particularly by tuning acid site characteristics and stability [56,62]. As shown in Table 1 and Figure 9, the incorporation of various metal species into different zeolite frameworks showcases the versatility and effectiveness of metal modification across diverse zeolite frameworks.

Figure 9. Performance evaluation of metal-modified zeolite catalysts in ethanol-to-ethylene conversion: (a) HZSM-5 catalyst modified with Ni and W showing morphology and catalytic performance [62]; (b) ZSM-5 catalyst modified with Ni and Sr demonstrating metal distribution and long-term stability with high ethylene selectivity [55]; (c) Cu-modified SSZ-13 catalyst exhibiting low-temperature activity and reaction mechanism [54]; (d) Ba-modified zeolite Y catalyst for bioethanol conversion to green ethylene [102]. Copyright Elsevier, 2022. Copyright Elsevier, 2023. Copyright American Chemical Society, 2020. Copyright Elsevier, 2024.

Figure 9. Performance evaluation of metal-modified zeolite catalysts in ethanol-to-ethylene conversion: (a) HZSM-5 catalyst modified with Ni and W showing morphology and catalytic performance [62]; (b) ZSM-5 catalyst modified with Ni and Sr demonstrating metal distribution and long-term stability with high ethylene selectivity [55]; (c) Cu-modified SSZ-13 catalyst exhibiting low-temperature activity and reaction mechanism [54]; (d) Ba-modified zeolite Y catalyst for bioethanol conversion to green ethylene [102]. Copyright Elsevier, 2022. Copyright Elsevier, 2023. Copyright American Chemical Society, 2020. Copyright Elsevier, 2024.

Table 1. Performance comparison of metal-modified zeolites for ethanol dehydration to ethylene.

Catalyst	Modification Strategy	Metal Loading	Reaction Conditions	Conversion	Ethylene Selectivity/Yield	Stability	Reference
Ni-ZSM-5	Impregnation	5 wt%	230 °C 50 vol% ethanol	91.90%	37.20%	Reusable for 4 cycles	[103]
Ni/Sr-ZSM-5	Ion exchange + impregnation	0.5 wt% Sr + 3.3 wt% Ni	250 °C 6% ethanol WHSV = 9 h−1	~96%	~98%	Stable for 30 h	[55]
NiW-HZSM-5	Impregnation	5 wt% Ni + 1 wt% W	230 °C 6 wt% catalyst dose	91.29%	21.98%	96.25% efficiency after 5 cycles	[62]
Fe-ZSM-5	Ion exchange	0.46 wt%	260 °C LHSV = 0.81 h−1	98%~99%	97%~99%	Stable for 60 d	[59]
Fe-ZSM-5	Impregnation	5.0 wt%	220 °C WHSV = 2.4 h−1	~90%	73%	N/A	[104]
Cu-ZSM-5	Impregnation	2.5 wt%	220 °C WHSV = 2.4 h−1	~82%	35.50%	N/A	[104]
FeCu-ZSM-5	Impregnation	2.5 wt% Fe + 2.5 wt% Cu	220 °C WHSV = 2.4 h−1	~65%	34.70%	N/A	[104]
Sr-ZSM-5	Impregnation	Sr/Al molar ratio = 1.0	400–600 °C W/F = 0.0125 g/(mL/min)	~100%	41.8%~99%	Stable for 25 h	[60]
Ce-ZSM-5	Ion exchange	5 wt%	350 °C WHSV = 5 h−1	~100%	~100%	Stable for 12 h	[52]
Ce-HZSM-5	Impregnation	2.5 wt%	400 °C 20 vol% ethanol WHSV = 3 h−1	99.10%	47.60%	Stable for 120 h	[105]
La-ZSM-5	Impregnation	N/A	240 °C 20% bioethanol WHSV = 1.5 h−1	97.40%	98.10%	Stable for 75 h	[50]
La-HZSM-5	Impregnation	3 wt%	260 °C 50 vol% ethanol LHSV 1.1 h−1	98.50%	99.50%	Stable for 950 h	[63]
Ru-HBZ	Impregnation	0.33 wt%	350 °C WHSV = 22.9 h−1	~90%	98.30%	N/A	[53]
Pt-HBZ	Impregnation	0.33 wt%	350 °C WHSV = 22.9 h−1	~95%	99.80%	N/A	[53]
Pd-HBZ	Impregnation	1.57 wt%	350 °C WHSV = 22.9 h−1	~95%	96%	N/A	[53]
Ba-Y	Ion exchange	4.5 wt%	280 °C 95% bioethanol contact time = 10 g/mL/min	99%	99.50%	Stable for 15 h with 98.5% yield	[102]
Cu-SSZ-13	One-pot synthesis	12.9 wt%	212 °C WHSV = 1.63 h−1	>99%	>99%	95.2% conversion after 12 h	[54]
Sr-SAPO-34	Ion exchange	3.2 at%	350 °C 15 vol% bioethanol WHSV = 1.5 h−1	98.40%	94.50%	Stable for 913 min	[96]
Mn-SAPO-34	Hydrothermal	5 wt%	340 °C 20% ethanol WHSV = 2 h−1	99.35%	98.44%	Stable for 10 h	[61]
Zn-SAPO-34	Hydrothermal	5 wt%	340 °C 20% ethanol WHSV = 2 h−1	~97%	~96%	N/A	[61]
Ni-APSO-34	Hydrothermal	Ni/Al molar ratio = 0.0084	350 °C 70 vol% ethanol LHSV 3 h−1	~95%	92.30%	Stable for 100 h	[37]
HZSM-5	-	-	300 °C 70 vol% ethanol LHSV 3 h−1	~95%	93.70%	Stable for 60 h	[37]
H-Beta	-	-	350 °C WHSV = 22.9 h−1	~95%	98.70%	N/A	[53]
γ-Al2O3	-	-	450 °C 70 vol% ethanol LHSV 3 h−1	~86%	78.70%	Stable for 80 h	[37]

Table 1. Performance comparison of metal-modified zeolites for ethanol dehydration to ethylene.

Catalyst	Modification Strategy	Metal Loading	Reaction Conditions	Conversion	Ethylene Selectivity/Yield	Stability	Reference
Ni-ZSM-5	Impregnation	5 wt%	230 °C 50 vol% ethanol	91.90%	37.20%	Reusable for 4 cycles	[103]
Ni/Sr-ZSM-5	Ion exchange + impregnation	0.5 wt% Sr + 3.3 wt% Ni	250 °C 6% ethanol WHSV = 9 h−1	~96%	~98%	Stable for 30 h	[55]
NiW-HZSM-5	Impregnation	5 wt% Ni + 1 wt% W	230 °C 6 wt% catalyst dose	91.29%	21.98%	96.25% efficiency after 5 cycles	[62]
Fe-ZSM-5	Ion exchange	0.46 wt%	260 °C LHSV = 0.81 h−1	98%~99%	97%~99%	Stable for 60 d	[59]
Fe-ZSM-5	Impregnation	5.0 wt%	220 °C WHSV = 2.4 h−1	~90%	73%	N/A	[104]
Cu-ZSM-5	Impregnation	2.5 wt%	220 °C WHSV = 2.4 h−1	~82%	35.50%	N/A	[104]
FeCu-ZSM-5	Impregnation	2.5 wt% Fe + 2.5 wt% Cu	220 °C WHSV = 2.4 h−1	~65%	34.70%	N/A	[104]
Sr-ZSM-5	Impregnation	Sr/Al molar ratio = 1.0	400–600 °C W/F = 0.0125 g/(mL/min)	~100%	41.8%~99%	Stable for 25 h	[60]
Ce-ZSM-5	Ion exchange	5 wt%	350 °C WHSV = 5 h−1	~100%	~100%	Stable for 12 h	[52]
Ce-HZSM-5	Impregnation	2.5 wt%	400 °C 20 vol% ethanol WHSV = 3 h−1	99.10%	47.60%	Stable for 120 h	[105]
La-ZSM-5	Impregnation	N/A	240 °C 20% bioethanol WHSV = 1.5 h−1	97.40%	98.10%	Stable for 75 h	[50]
La-HZSM-5	Impregnation	3 wt%	260 °C 50 vol% ethanol LHSV 1.1 h−1	98.50%	99.50%	Stable for 950 h	[63]
Ru-HBZ	Impregnation	0.33 wt%	350 °C WHSV = 22.9 h−1	~90%	98.30%	N/A	[53]
Pt-HBZ	Impregnation	0.33 wt%	350 °C WHSV = 22.9 h−1	~95%	99.80%	N/A	[53]
Pd-HBZ	Impregnation	1.57 wt%	350 °C WHSV = 22.9 h−1	~95%	96%	N/A	[53]
Ba-Y	Ion exchange	4.5 wt%	280 °C 95% bioethanol contact time = 10 g/mL/min	99%	99.50%	Stable for 15 h with 98.5% yield	[102]
Cu-SSZ-13	One-pot synthesis	12.9 wt%	212 °C WHSV = 1.63 h−1	>99%	>99%	95.2% conversion after 12 h	[54]
Sr-SAPO-34	Ion exchange	3.2 at%	350 °C 15 vol% bioethanol WHSV = 1.5 h−1	98.40%	94.50%	Stable for 913 min	[96]
Mn-SAPO-34	Hydrothermal	5 wt%	340 °C 20% ethanol WHSV = 2 h−1	99.35%	98.44%	Stable for 10 h	[61]
Zn-SAPO-34	Hydrothermal	5 wt%	340 °C 20% ethanol WHSV = 2 h−1	~97%	~96%	N/A	[61]
Ni-APSO-34	Hydrothermal	Ni/Al molar ratio = 0.0084	350 °C 70 vol% ethanol LHSV 3 h−1	~95%	92.30%	Stable for 100 h	[37]
HZSM-5	-	-	300 °C 70 vol% ethanol LHSV 3 h−1	~95%	93.70%	Stable for 60 h	[37]
H-Beta	-	-	350 °C WHSV = 22.9 h−1	~95%	98.70%	N/A	[53]
γ-Al2O3	-	-	450 °C 70 vol% ethanol LHSV 3 h−1	~86%	78.70%	Stable for 80 h	[37]

Various alkali metals, including lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and caesium (Cs), have been investigated as zeolite catalyst modifiers for dehydration. The modification mechanism involves selectively eliminating strong Brønsted acid sites while generating weak Lewis acid centres. Alkali metal cations function as Lewis acid sites with framework oxygens serving as Lewis base centres. This transformation creates a synergistic acid–base environment that preferentially promotes dehydration pathways and suppresses undesirable side reactions such as decarbonylation and decarboxylation typically associated with strong acid sites [90].

Copper modification of zeolites has shown promise for the low-temperature dehydration of ethanol to ethylene. Synthesis methods include incipient wetness impregnation and one-pot synthesis using a copper-tetraethylenepentamine complex [106]. A notable example is the Cu-SSZ-13 zeolite, which achieved an ethylene yield higher than 99% at a relatively low temperature of 212 °C. The introduction of copper species is believed to increase the number of medium acid sites, which are considered active for ethanol dehydration to ethylene, while simultaneously suppressing the side reaction of ethanol conversion to acetaldehyde [54]. Sukkatorn et al. demonstrated that modification of the SUZ-4 zeolite with Cu and Fe enhanced diethyl ether formation during low-temperature ethanol dehydration, with characterization studies revealing an increase in both strong and medium acid sites, while Cu specifically enlarged pore size and Fe reduced crystallinity without affecting mean particle diameter [106].

Zinc modification of zeolite has demonstrated promising results for the direct conversion of ethanol. The type of zinc precursor used during the modification process was found to significantly influence the catalyst’s performance by allowing for the tuning of the amount and strength of acid sites in Beta zeolite, which is crucial for controlling the reaction pathway and product selectivity [107]. Furthermore, SAPO-34 and SAPO-11 modified with Zn²⁺ and Mn²⁺ displayed increased intensity of both weak and strong acid sites, resulting in enhanced catalytic activity for ethanol dehydration in the following order: Mn-SAPO-34 > Zn-SAPO-34 > SAPO-11 > Mn-SAPO-11 > Zn-SAPO-11 > SAPO-34 [61]. Costa et al. demonstrated that Zn significantly altered the acid distribution of β-zeolite, reducing Brønsted acid sites while increasing Lewis acid sites. This modulation of acid–base balance effectively suppressed coke formation while maintaining sufficient active sites for reaction progression [108].

The incorporation of nickel into zeolite catalysts has been investigated for its impact on bioethanol conversion. Studies have demonstrated that nickel-modified HZSM-5 catalysts achieve ethanol conversion rates exceeding 97%, with nickel effectively modulating acid sites to promote ethoxy intermediate formation and suppress side reactions [109]. The proposed mechanism for nickel’s beneficial effect involves modulating acid site equilibrium within the zeolite. Nickel doping reduces strong acid sites and promotes weak acid synergistic effects through electronic interactions, thereby regulating the local environment. Additionally, nickel can form stable tetrahedral coordination with the zeolite framework, which notably improves acid site characteristics under high-temperature conditions [103,109]. Furthermore, other studies highlight the role of nickel in improving the hydrothermal stability and reducing dealumination of the zeolite support [110].

Noble metals such as palladium, platinum, and ruthenium are critical catalysts. Investigations on H-Beta zeolite modified with these metals reveal their highly dispersed presence, minimally affecting zeolite crystallinity. Metal incorporation primarily modifies acid site distribution and introduces promoting effects, with mechanisms varying by metal type. Modification with Ru and Pt reduces total acid sites, particularly weak acid sites, while they act as chemical promoters that enhance intrinsic catalytic activity. This results in improved ethanol conversion and increased ethylene yield at moderate temperatures. Conversely, Pd modification does not significantly alter total acidity, but it increases surface hydroxyl group concentration, enhancing diethyl ether (DEE) formation at low temperatures. Consequently, while Ru and Pt substantially boost ethylene yield at moderate temperatures, Pd modification demonstrates limited impact on ethylene yield at high temperatures, but it markedly improves DEE production [53,56].

Beyond traditional metal modification, the integration of metal oxides with zeolite frameworks represents an emerging strategy for enhancing ethanol dehydration performance, demonstrating significant improvements over individual components. The TiO₂ nanotube-supported ZSM-5 composite exhibits enhanced ethanol conversion and ethylene selectivity, attributed to the electron-accepting properties of TiO₂ nanotubes and their unique one-dimensional structure [111]. Ketkaew et al. demonstrated that incorporating ceria into hierarchical ZSM-5 zeolite nanosheets, particularly through an ion-exchange method, resulted in an unprecedented ethylene yield approaching 100% [52]. This enhanced catalytic performance was attributed to a synergistic combination of improved metal–support interactions, optimized acidity derived from both the zeolite framework and the ceria species, a higher proportion of active sites, and enhanced reducibility of the ceria [52,105].

Metal modifications of zeolites reveal complex interactions that distinctly influence catalytic performance. For example, iron exchange in ZSM-5 selectively eliminates strong acid sites while preserving weak to moderate sites, benefiting ethanol dehydration to ethylene by minimizing side reactions. Isolated Fe³⁺ species serve as active sites for ethylene formation, and reduced Brønsted acid density extends the catalyst lifetime by mitigating carbonaceous species formation [59]. Conversely, copper modification enhances acid sites, introducing Cu species that increase the medium and strong acid sites critical for low-temperature ethanol dehydration. These Lewis acid centres promote ethanol-to-ethylene conversion while suppressing ethanol dehydrogenation to acetaldehyde, demonstrating how metal valence states can direct reaction selectivity [54]. These contrasting examples illustrate that understanding metal modification effects relies heavily on empirical observations and case-by-case analysis, as universal predictive rules for metal–zeolite interactions remain elusive in catalyst design.

Recent advances in metal modification techniques have demonstrated remarkable improvements in catalyst stability and performance under practical conditions. Notably, the incorporation of alkaline earth metals such as strontium and barium enhances hydrothermal stability, while transition metals like nickel effectively suppress undesired side reactions [55,102]. The synergistic effects of multiple metal modifications have shown particular promise—for example, combined nickel–strontium modification of ZSM-5 achieves both improved selectivity and enhanced stability under high water content conditions [55].

The method of metal incorporation significantly impacts catalyst performance. Ion exchange typically yields well-dispersed metal species and selectively modifies acid sites [59], whereas impregnation methods often result in surface-deposited metal oxides that can offer additional catalytic functions [62,104]. Innovative approaches like metal encapsulation within zeolite frameworks have shown promising outcomes in preventing metal sintering and maintaining long-term stability [58].

Metal modifications enhance zeolite performance in ethanol dehydration by transforming acidic supports into multifunctional catalytic systems. These modifications simultaneously optimize reaction selectivity through intrinsic acidity tuning and introduce novel catalytic functionalities. Furthermore, metal modifications improve the catalyst’s structural and chemical stability, mitigating deactivation risks. Such synergistic systems inherently link enhanced operational stability with optimized catalytic activity, addressing critical requirements for industrial applications. No universal metal modifier exists for ethanol dehydration; the optimal choice depends on the desired product, specific reaction conditions, and zeolite type. This necessitates a tailored approach to catalyst design considering the interplay between metal modifiers and zeolite supports. The growing interest in bimetallic and multimetallic modifications underscores the potential for achieving further catalytic performance enhancements through strategic metal combinations.

4.3. Impact of Reaction Conditions

The catalytic conversion of bioethanol to ethylene is profoundly influenced by several key operational parameters, shown in Figure 10, with reaction conditions directly determining the reaction pathway selectivity and overall process efficiency. Systematic investigation of these parameters reveals their crucial roles in optimizing the dehydration process.

Temperature regulation plays a crucial role in determining reaction pathways, product distribution, and overall efficiency of ethanol dehydration. While uncatalysed dehydration typically requires temperatures up to 400 °C, the introduction of zeolite catalysts can effectively lower the required reaction temperature [20]. Studies by Knozinger et al. demonstrated that catalytic systems could reduce the temperature threshold for product formation by approximately 60 °C compared with uncatalysed conditions [112,113]. Mechanistic studies have revealed distinct temperature-dependent reaction pathways. At lower temperatures (200–250 °C), the reaction predominantly proceeds through a bimolecular mechanism wherein ethoxy groups react with undissociated ethanol to form diethyl ether as the major product [64,114,115]. As temperatures increase to 300–400 °C, the reaction mechanism shifts significantly. The decomposition of ethoxy groups becomes thermodynamically favourable, leading to ethylene as the predominant product [53,88]. This temperature-dependent product distribution aligns with the endothermic nature of ethanol dehydration to ethylene [1]. However, elevated temperatures also present challenges, particularly regarding catalyst stability. Higher temperatures accelerate the transformation of coke precursors into hard coke, potentially compromising catalyst performance [116].

Weight hourly space velocity (WHSV) is a crucial parameter in heterogeneous catalysis; it is defined as the weight of the feed processed per unit weight of catalyst per unit time, typically expressed in h⁻¹. It is inversely related to the contact time between the reactants and the catalyst. Generally, a lower WHSV (longer contact time) tends to result in higher ethanol conversion because the reactant molecules have more time to interact with the active sites on the catalyst surface. However, it is important to note that excessively low WHSV values can sometimes lead to an increased formation of undesired byproducts due to over-reaction, where the primary products undergo further transformations [55,62]. WHSV, along with other parameters, allows for the control of the extent to which the dehydration reaction proceeds and the resulting distribution of products. For example, at higher reaction temperatures, operating at a lower WHSV can sometimes lead to secondary reactions of the initially formed ethylene, resulting in the production of higher hydrocarbons [117].

The composition of the feedstock can impact catalytic performance and reaction mechanisms. In a study by Chen et al., the influence of varying water contents in bioethanol on ethanol dehydration was investigated using Mn-modified SAPO-34 zeolites. It was observed that the byproduct ethyl ether transformed into propylene, with the highest ethanol conversion rate and ethylene selectivity achieved at a water content of 20% [61]. Moreover, carrier gas-induced variations in ethanol concentration may significantly influence experimental outcomes. A study demonstrated that higher ethanol concentrations can enhance ethylene yield and selectivity [50]. However, this relationship is not consistent, as another study revealed that elevated ethanol concentrations may negatively impact conversion rates [35]. Ethanol concentration also impacts the formation of surface intermediates such as ethanol monomers and dimers, which, in turn, determine the dominant reaction mechanism. High ethanol pressures favour dimer-assisted etherification, while increased temperatures enhance ethoxide-mediated routes [118].

The catalytic dehydration of ethanol over metal-modified zeolites exhibits catalyst-specific performance characteristics. Optimal reaction conditions vary significantly depending on the metal modifier and zeolite framework, influencing ethanol conversion and product selectivity. The interdependence of reaction parameters precludes a universal optimal setup, necessitating systematic optimization for each catalyst system. Achieving desired catalytic performance requires a comprehensive evaluation of the intricate interactions between catalyst properties and reaction conditions.

4.4. Catalyst Stability and Regeneration

The industrial viability of bioethanol-to-ethylene conversion processes is heavily dependent on catalyst stability and effective regeneration strategies. Understanding deactivation mechanisms and developing appropriate regeneration protocols are crucial for maintaining long-term catalytic performance. The systematic investigation of zeolite stability and deactivation mechanisms requires comprehensive characterization and evaluation methods.

Catalyst deactivation during ethanol dehydration over metal-modified zeolites primarily occurs through coke formation, a process involving carbonaceous byproduct accumulation on the catalyst surface [119]. Systematic characterization of deactivated catalysts provides a direct insight into carbon deposition effects (as shown in Figure 11). Macroscopically, the deactivated catalyst exhibits a darker colour, serving as evident physical evidence of carbon deposition. Thermogravimetric analysis curves quantitatively confirm this observation, with significant weight loss peaks at high temperatures corresponding to carbon species’ oxidative combustion. Coke deactivation manifests through multiple pathways, including chemisorption or physisorption that hinders access to active sites, total encapsulation of active sites, and plugging of micropores and mesopores [120]. Studies have identified three main coke morphologies: encapsulating coke formed at lower temperatures (< 500 °C) through adsorption and subsequent condensation or polymerization of reaction intermediates over metal particle surfaces; filamentous coke generated at higher temperatures (> 450 °C) via adsorption of precursors on metal sites, carbon diffusion through metal particles, and precipitation as carbon nanotubes or nanofibers; and pyrolytic coke formed by thermal cracking at severe conditions (> 600 °C) with non-selective deposition across the entire catalyst surface [119]. In ethanol dehydration, ethylene produced as an intermediate is particularly active in oligomerization reactions leading to amorphous coke formation [121]. Metal modification profoundly influences catalyst stability through multiple mechanisms. At low temperatures, metal modification reduces strong acid site activity, mitigating excessive ethylene polymerization and enabling prompt oxidation of nascent carbon species [105]. Under moderate temperatures, metal ions with oxidative capabilities effectively convert carbon precursors to CO₂ [60,109]. At higher temperatures, metal-modified zeolites preserve diffusion channels and significantly lower the carbonaceous deposit combustion temperature, facilitating efficient gasification and removal [122].

The stability of metal-modified zeolite catalysts during ethanol dehydration is influenced by several key factors, including the reaction temperature, the duration of the reaction (time on stream), the presence of impurities in the ethanol feedstock, the weight hourly space velocity (WHSV), and the loading of the metal modifier on the zeolite. Elevated reaction temperatures can favour the formation of ethylene, a desired product in many applications, but they can also accelerate the deposition of coke on the catalyst surface. Lower reaction temperatures might enhance the production of diethyl ether, but catalyst deactivation can still occur over extended periods of operation [106]. For instance, a study involving Ni/HZSM-5 catalyst demonstrated high stability in ethanol conversion for the initial 48 h of operation at 623 K, after which the conversion rate began to decline [123]. This observation underscores the complex relationships between reaction temperature, product selectivity, and catalyst stability.

The composition of the ethanol feedstock, particularly the presence of impurities such as water, can also significantly influence catalyst stability. While direct bioethanol use is economically advantageous, its water content may compromise zeolite catalyst structural integrity and acidity. However, water also plays a crucial mitigating role in coke formation by chemisorbing onto acid catalyst surfaces, forming strong bonds that occupy active sites and protect them from deactivation, thereby reducing overall coke deposition rates [124]. Interestingly, a heterobimetallic catalyst, InV-ZSM-5, demonstrated robust performance in hydrocarbon production even with a wide range of water content (5–95%) in the ethanol feed [71]. These findings highlight the complex effects of feedstock impurities on catalyst stability, emphasizing the need for careful consideration when utilizing different ethanol sources.

The amount of metal loaded onto the zeolite catalyst also plays a significant role in its stability and performance. Studies on Ni-, Ga-, and Fe-modified HZSM-5 catalysts found that low metal loadings (below 1 wt%) had a positive impact on the production of light olefins, paraffins, aromatics, and higher hydrocarbons. However, increasing the metal content led to a decrease in the yield of these products, which was attributed to the formation of bulky metal clusters that could block the zeolite pores, reducing the accessibility of the active sites [117]. Furthermore, high metal doping in ZSM-5 has been reported to increase coke deposition as a result of the high concentration of strong acid sites, leading to catalyst deactivation [125]. These observations underscore the importance of determining the optimal metal loading to enhance catalyst stability and activity without causing detrimental effects such as pore blockage or increased coke formation.

Upon deactivation, metal-modified zeolite catalysts can often be regenerated to restore their catalytic activity. Several methods are commonly employed for this purpose, with thermal treatment being a widely used approach [123]. Thermal treatment, also known as oxidative regeneration, involves burning off the accumulated coke deposits from the catalyst surface using air or an oxygen-containing gas at elevated temperatures. The colour restoration of the regenerated catalyst and the reported activity recovery in Figure 11b provide strong evidence that catalyst deactivation by carbon deposition is largely reversible [123]. However, it is crucial to precisely control the regeneration conditions, including temperature, time, and the concentration of oxygen, to avoid potentially damaging the zeolite structure or causing sintering of the metal nanoparticles dispersed on the catalyst [126].

The long-term performance of metal-modified zeolites in ethanol dehydration is governed by complex deactivation mechanisms, primarily involving coke formation and pore blockage. These mechanisms are influenced by interconnected operational factors. Strategic catalyst design approaches that balance metal incorporation with pore architecture optimization can enhance resistance to deactivation, while appropriate regeneration protocols restore catalytic activity. These findings underscore the importance of integrated considerations of catalyst formulation, operational parameters, and regeneration strategies to achieve extended catalyst lifetimes and economic viability in bioethanol-to-ethylene processes.

5. Conclusions and Perspectives

5.1. Current Achievements

Metal-modified zeolites have significantly advanced the field of bioethanol conversion, contributing to the development of more sustainable chemical production pathways. Substantial progress in mechanistic understanding has clarified the relationships between metal species, acid sites, and zeolite frameworks, providing insights into how these components influence reaction pathways and product distributions. This foundational knowledge has facilitated more systematic approaches to catalyst design that address specific performance limitations.

Considerable advancements have been made in synthesis methodologies for controlling metal incorporation within zeolite structures. These developments have enabled the creation of catalysts with properties that effectively address several constraints of conventional systems. Through the modification of acidity profiles, pore architectures, and metal–support interactions, researchers have enhanced catalytic performance while improving selectivity towards target products. The development of hierarchical structures has addressed diffusion limitations, resulting in improved mass transport properties that benefit catalyst effectiveness and stability.

Notable improvements in operational performance under relevant conditions have been documented in the literature. Metal-modified zeolites demonstrate effective bioethanol conversion at moderate temperatures compared with conventional processes, suggesting a potential for reduced energy requirements. Improved resistance to deactivation when processing feedstocks containing water and other impurities has been achieved through appropriate metal selection and incorporation strategies. These advancements address important considerations for potential applications in bioethanol conversion processes.

Progress in understanding deactivation mechanisms has contributed to the development of appropriate regeneration protocols. These approaches aim to restore catalytic activity while maintaining structural characteristics and metal distribution. Such regeneration strategies may extend the operational lifetime of catalysts, which represents an important factor in process viability considerations.

5.2. Challenges and Future Opportunities

Despite substantial progress in metal-modified zeolites for bioethanol conversion, several significant challenges remain that offer opportunities for future research. These challenges span multiple aspects of green synthesis, catalyst development, characterization, and implementation.

Optimizing synthesis methods and implementing green chemistry principles are essential for the development of metal-modified zeolites. Current synthesis approaches pose significant environmental challenges, such as the use of organic solvents, high-energy calcination processes (500–800 °C), generation of metal-contaminated wastewater, and lengthy heating durations (1–20 days). Green alternatives, including microwave-assisted synthesis, solvent-free methods, and ionothermal synthesis using ionic liquids, offer solutions to these issues [78]. These approaches reduce energy consumption, synthesis time, and pollutant generation while operating at ambient pressure to enhance safety [127]. Considering the lifecycle of metal-modified zeolites, the scarcity of noble metals highlights the importance of shifting focus towards sustainable alternatives like iron, nickel, and copper. Strategies such as template recycling and end-of-life metal recovery further support the principles of a circular economy in zeolite synthesis [128]. Despite these advances, green synthesis methods remain significantly less developed compared with conventional hydrothermal approaches. Significant challenges remain in industrial-scale implementation, including scalability, economic feasibility, and technical optimization.

Advanced characterization methodologies are essential to address the fundamental knowledge gaps regarding metal–zeolite interactions. Current techniques provide limited information about the dynamic behaviour of metal species under reaction conditions. Development of sophisticated in situ and operando spectroscopic methods would enable real-time monitoring of structural changes, metal oxidation states, and acid site transformations during catalysis. Spatially resolved techniques capable of mapping metal distribution at the nanoscale would further enhance the understanding of structure–performance relationships. These advanced characterization capabilities are crucial for elucidating deactivation mechanisms and designing more stable catalysts.

Computational approaches for mechanism elucidation offer powerful tools for catalyst design and optimization. The integration of theoretical models with experimental data can provide atomic-level insights into reaction pathways and active site behaviour that are challenging to obtain experimentally. Density functional theory calculations, microkinetic modelling, and molecular dynamics simulations can guide rational catalyst design by predicting how specific metal modifications will influence catalytic performance. Machine learning approaches applied to comprehensive catalytic datasets hold potential for identifying optimal catalyst compositions and preparation conditions with reduced experimental effort.

Coke formation resistance and regeneration strategies remain critical challenges for practical implementation. While various metal modifications have been studied for their effects on catalytic activity and selectivity, the systematic relationship between specific metal types and coke resistance remains poorly understood. Future research should focus on fundamental understanding of the deactivation mechanisms specific to metal-modified zeolites, as these may differ substantially from conventional zeolite catalysts. This understanding would enable the design of inherently coke-resistant catalysts through strategic metal incorporation and pore architecture modification. The development of efficient regeneration protocols that preserve metal dispersion and prevent sintering is equally important for maintaining long-term performance through multiple reaction–regeneration cycles.

Furthermore, bridging the gap between laboratory-scale success and industrial viability requires research efforts on catalyst performance under realistic feedstock conditions. While the existing literature predominantly focuses on idealized, pure material feeds, future research needs to systematically examine the effects of water content and common bioethanol impurities on catalyst performance including activity, selectivity, and longevity. Establishing robust design principles for catalysts capable of withstanding thermal cycling during continuous operation and regeneration is critical. A shift towards studies correlating specific catalyst properties, such as promoter type and metal particle confinement, with stability under industrially relevant conditions will be essential for the rational design of practical catalysts.

The advancement of metal-modified zeolites for bioethanol conversion requires collaborative research across various scientific and engineering fields. Future progress depends on addressing fundamental questions about metal–zeolite interactions while simultaneously developing practical solutions for catalyst preparation, testing, and implementation. By integrating insights from materials science, catalysis, and reaction engineering, researchers can overcome the current limitations and develop more efficient, stable, and sustainable catalytic systems for biomass conversion. This interdisciplinary approach is essential for transitioning metal-modified zeolite catalysts from promising laboratory materials to commercially viable technologies for renewable chemical production.