Sustainable catalytic conversion of ethanol: catalyst design and modification strategies for dehydration and petrochemical applications

Abstract

Ethanol has emerged as a promising renewable feedstock for chemicals and sustainable fuel production. Its conversion into ethylene, diethyl ether, and higher-value chemicals through dehydration over catalysts relies on carefully designed catalysts to enhance activity, selectivity, and stability. This review critically examines the catalytic transformation pathways leading to key petrochemical intermediates such as ethylene, propylene, and 1,3-butadiene. It places special emphasis on solid acid catalysts, modified oxides, zeolites, and heteropoly acids, highlighting recent advances in catalyst design, including surface modification, morphology control, and nanostructuring. The review also discusses mechanistic insights, reactor engineering, and strategies to overcome coke formation and catalyst deactivation. Finally, it explores future perspectives on integrating bioethanol into petrochemical supply chains, focusing on challenges related to catalyst performance, process scalability, and sustainable chemical production.

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Rasha S. Mohamed

Dr Rasha El-Sayed Mohamed is an Assistant Professor of Applied Physical Chemistry at the Egyptian Petroleum Research Institute (EPRI), currently seconded to the Central Laboratory for Liquid Chromatographic and Water Analysis. She received her PhD in Applied Physical Chemistry from Ain Shams University in 2015 and her MSc in Inorganic Chemistry from Helwan University in 2008. Her research focuses on the synthesis of advanced catalysts, including nano metal oxides and metal–organic frameworks, for petroleum refining, photocatalysis, hydrogen production and storage, water treatment, and diesel desulfurization. She holds two Egyptian national patents (no. 31340 and 31186) and received the Knowledge Medal from the American International Academy for Higher Education.

Heba M. El Sharkawy

Dr Heba Magdy El Sharkawy is a researcher at the Egyptian Petroleum Research Institute and a research assistant at the Energy Materials Lab, American University in Cairo. She earned her Ph.D. in Physical Chemistry from Al-Azhar University (2021), focusing on carbon quantum dots for energy storage and photocatalytic pollutant degradation. Her research spans supercapacitor development, sustainable energy applications, water treatment, materials science and green fuel production. Dr El Sharkawy has authored numerous peer-reviewed publications in energy and environmental materials and received the 2024 Dr Ferdous Abdel-Majeed Award for her contributions to new and smart materials.

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1. Introduction

The global use of ethanol as an alternative engine fuel has steadily risen due to its potential to decrease dependence on fossil fuels, enhance energy security, generate rural employment, and reduce air pollution and greenhouse gas emissions.^1–5 Over the past two centuries, industrial development has significantly contributed to environmental pollution, leading to contamination of air, water, and soil.^6–8 Notably, the concentration of atmospheric CO₂ has surged, with an estimated 38 000 million metric tons added over the last 260 years, primarily due to fossil fuel consumption.^9–11 This trend not only jeopardizes environmental sustainability but also restricts access to these non-renewable resources for future generations. Therefore, transitioning to renewable and alternative fuels is crucial for minimizing environmental impact and meeting global energy demands.^12–14 Ethanol stands out as one of the most significant biofuels, thanks to its oxygen content (∼35%), which helps reduce particulate and NO_x emissions compared to gasoline.^15–18 As illustrated in Fig. 1, ethanol can be produced through two primary pathways: chemical routes from fossil fuels and biological routes from biomass.¹⁹ Ethanol can be produced not only from petroleum and biomass but also from coal through a two-step process: gasification followed by catalytic conversion of syngas (CO + H₂) into ethanol. This thermochemical pathway is a significant non-biological method, especially in coal-rich areas, and has garnered growing interest as a component of integrated carbon utilization strategies.¹⁹ The chemical (petrochemical) route involves producing ethanol from petroleum-based feedstocks, while the biological route utilizes renewable biomass, making bioethanol production more sustainable and eco-friendly.^20,21

Fig. 1 Overview of the ethanol production pathways.

Beyond its role as a fuel, ethanol also serves as an important platform molecule for catalytic conversion into value-added chemicals, including ethylene, higher alcohols, and other petrochemical intermediates.^22–24 Many studies focus on individual catalysts or reaction pathways; however, a comprehensive evaluation is needed to critically compare catalyst design, modification strategies, and reaction mechanisms.²⁵ The development of efficient and selective catalysts is therefore essential for improving ethanol conversion processes while ensuring sustainability and economic viability.^26,27 This review aims to provide a critical assessment that highlights recent advancements, as well as current limitations, challenges, and research gaps, thereby offering guidance for future development in the field.

2. Bioethanol production

Bioethanol is poised to be a leading fuel of the future, as it is derived from renewable sources, specifically biomass, and is environmentally friendly.^28,29 Ethanol enhances the octane rating of gasoline, acts as an anti-knock agent, and has the potential to replace current fuel additives,³⁰ as illustrated in Fig. 2. With the rising cost of petroleum, the production of ethanol can be doubled, while petrochemical routes decline.^19,31 Currently, the production of ethanol from biomass is becoming more efficient and competitive, resulting in a lower-cost end product. At present, 90 percent of the ethanol market is generated from biomass.^32–34 The global ethanol market, depicted in Fig. 2, shows production trends across major producing countries. Projections for 2025 indicate that the global ethanol market value will exceed USD 114 billion, reflecting strong industry growth and increasing integration of ethanol into the energy and transport sectors.^35,36 By 2028, it is expected that more than half of ethanol production will be consumed by high-speed flexible fuel vehicles, indicating anticipated fleet expansion.^37,38

Fig. 2 Global ethanol production and market trends (million m³). Adapted from ref. 39.

Ethanol can be synthetically produced from biomass resources through fermentation, either using oil or microbial processes. This fermentation of sugar typically involves three phases: (1) developing a fermented sugar solution; (2) fermenting the ethanol; and (3) separating and purifying the ethanol, usually through distillation.^29,40 As an attractive alternative to traditional fossil fuel-derived raw materials in petrochemical production, ethanol's conversion into higher-value chemicals, such as ethylene, relies heavily on optimizing the catalyst and process to accommodate ethanol's unique properties.⁴¹

3. Ethanol as a substrate for bulk chemicals

Ethanol can be transformed into commercial chemicals through chemical or fermentation processes. However, the COVID-19 pandemic led to a decrease in ethanol production, dropping to 26 billion gallons in 2020.⁴² Despite this decline, the United States remained the world's largest producer, responsible for over half of global output⁴³ (Fig. 3).

Fig. 3 Major industrial chemicals derived from ethanol as a platform molecule (reprinted with permission,⁴⁴ copyright 2013 John Wiley and Sons).

Recent advancements in new technologies and strategic partnerships have enhanced the potential of ethanol as a platform molecule for chemical synthesis. Ethanol can be converted into a variety of significant bulk compounds,²² as illustrated in Fig. 4. It's crucial to highlight that ethanol can also function as a chemical platform molecule, allowing for the catalytic transformation into essential industrial chemical components using multifunctional catalysts in single-stage catalytic processes.⁴⁵ Chemicals such as acetic acid, ethyl chloride, acetic anhydride, cellulose acetates, ethyl ether, acetaldehyde, acetone, butanol, ethylene, and propylene all contribute to the production of acetone, which is already being manufactured in Brazil.⁴⁶ Additionally, the synthesis of 1,3-butadiene rubber monomer from ethanol was a viable business technology in Brazil in the past. However, the ethanol chemical industry has not received the same level of investment and technological development as the petrochemical sector; it was largely neglected in the 1990s and has not fully recovered.⁴⁷ In 2010, Braskem emerged as the leading producer of bio-based polyethylene from ethanol in Brazil.⁴⁸

Fig. 4 Global ethanol production by country (country, million gallons).

Given the potential for increased ethanol production and the ongoing shift towards electric vehicles, there is significant opportunity for revitalizing the ethanol chemical sector in Brazil.⁴⁹ The manufacturing of ethylene, which is the most advanced process for producing plastics, can be achieved through the dehydration of ethanol. While ethanol can also be converted into heavier fuels like jet fuel and used in the steam reforming of ethanol for strategic hydrogen production, its potential as a platform molecule remains substantial.⁵⁰

4. Dehydration of ethanol

The catalytic dehydration of ethanol is a crucial reaction for producing ethylene using solid acid catalysts. The primary process involves the intramolecular dehydration of ethanol, resulting in the formation of ethylene and water.⁵¹ Additionally, ethanol can undergo intermolecular dehydration to produce diethyl ether, which can decompose into ethylene at higher temperatures. The selectivity between ethylene and diethyl ether is strongly influenced by the reaction temperature and the acidity of the catalyst.⁵² Typically, diethyl ether formation is favored at lower temperatures, while ethylene becomes the predominant product at higher temperatures.⁵³ Side reactions may also occur, depending on the catalyst properties and operating conditions, leading to the formation of minor amounts of acetaldehyde and light hydrocarbons. Therefore, factors such as catalyst acidity, reaction temperature, pressure, and liquid hourly space velocity (LHSV) are critical in controlling product distribution and minimizing by-product formation.⁵⁴

In industrial applications, solid acid catalysts like alumina are commonly used, usually operating at moderate temperatures between 200 and 300 °C. The performance of the catalyst and the purity of the ethanol feed significantly affect conversion efficiency and product selectivity.⁵⁵

4.1. Ethylene

4.1.1. Properties of ethylene. Ethylene (C₂H₄) is the simplest alkene and a highly reactive olefin due to its carbon–carbon double bond (C=C).⁵⁶ This π-bond creates a high electron density, making ethylene prone to electrophilic addition and polymerization reactions. These chemical properties highlight its significant industrial importance and its role as a key target product in ethanol dehydration processes.⁵⁷ In catalytic systems, ethylene interacts with transition metal surfaces through temporary coordination with its π and π* orbitals, which is crucial for subsequent transformations. Therefore, understanding ethylene's reactivity is essential for evaluating routes for converting ethanol to ethylene.⁵⁸

4.1.2. Importance and applications of ethylene. Ethylene ranks among the highest-volume petrochemicals in the world, serving as a key building block for various products including polyethylene, ethylene oxide, ethylene glycol, and polyvinyl chloride (PVC).^59–61 Notably, polyethylene constitutes nearly 60% of global ethylene consumption. The persistent global demand for these derivatives underscores the strategic importance of developing sustainable ethylene production methods.^62,63 Traditionally, ethylene has been produced from ethanol through catalytic dehydration, and modern industrial processes have demonstrated the viability of generating high-purity ethylene from bioethanol, highlighting the potential of renewable feedstocks.⁶⁴

4.1.3. Ethylene production. The manufacture of ethylene can be achieved through several well-known methods, including: (i) cracking of higher hydrocarbons, (ii) plant-based production, (iii) microbial synthesis and (iv) catalytic dehydration of ethanol.

Fig. 5 illustrates the various industrial methods for ethylene production.⁶⁵ Although the cracking of higher hydrocarbons is still the primary petrochemical route, ethanol dehydration has become a significant renewable alternative, enabling bioethanol to function as a platform molecule for sustainable ethylene production.⁶⁶ While plant-based and microbial processes are not yet widely adopted in industry, they offer promising avenues for future bio-based ethylene production.⁶⁷

Fig. 5 Schematic diagram for several ethylene manufacturing processes (reprinted with permission,⁶⁸ copyright 2017 John Wiley and Sons).

4.1.3.1. Steam cracking of hydrocarbons. Steam cracking of liquefied petroleum gas (LPG) or heavier hydrocarbons is the primary industrial method for producing ethylene.^69,70 This process operates at high temperatures (750–850 °C) in the absence of oxygen and requires substantial energy input. Despite being an established technique, it encounters challenges, including catalyst deactivation from carbon deposition, by-product formation, and high operational costs due to the extreme temperatures involved.⁷¹ Additionally, fluctuations in petroleum prices and growing environmental concerns have spurred the search for alternative, renewable methods of ethylene production.⁷²
4.1.3.2. Ethylene production by plants. Ethylene is a phytohormone naturally produced in plants, playing a crucial role in regulating processes such as fruit ripening.⁷³ Its biosynthetic pathway involves the enzymes ACC synthase (ACS) and ACC oxidase (ACO), which convert S-adenosyl methionine (SAM) into ethylene. However, the production scale is currently very limited and not viable for industrial applications.⁷⁴
4.1.3.3. Ethylene production by microorganisms. Certain bacteria and fungi can produce ethylene. Despite the scientific interest in microbial ethylene biosynthesis, the production yields are insufficient for large-scale industrial use.⁶⁵
4.1.3.4. Ethylene production by catalytic dehydration of ethanol. Catalytic dehydration of ethanol presents a promising alternative to steam cracking due to its lower operating temperature and compatibility with renewable feedstocks.^75,76 Unlike hydrocarbon cracking, ethanol can be sourced from biomass, making this process independent of fossil resources.⁷⁷

The overall dehydration reactions are as follows:

C₂H₅OH ⇒ H₂O + C₂H₄, +44.9 kJ mol⁻¹ (1)

C₂H₅OH ⇒ 0.5C₂H₅OC₂H₅ + 0.5H₂O, −12 kJ mol (2)

C₂H₅OC₂H₅ ⇒ 2C₂H₄ + H₂O, +115 kJ mol⁻¹ (3)

At elevated temperatures, the reaction is endothermic and may occur through both direct ethanol dehydration and sequential dehydration via diethyl ether as an intermediate. Additionally, side reactions can produce acetaldehyde, methane, ethane, propylene, CO, CO₂, and H₂.⁵⁵ Compared to steam cracking plants, catalytic ethanol dehydration requires significantly lower capital investment and provides environmental benefits. Therefore, this method represents a sustainable and economically attractive alternative for future ethylene production.⁷⁸

4.1.4. Global market for ethylene. Ethylene is one of the highest-volume petrochemicals globally and serves as a fundamental building block in the chemical industry.⁷⁹ The largest share of ethylene consumption is attributed to polyethylene, followed by ethylene oxide and polyvinyl chloride, as shown in Fig. 6. The ongoing global demand for these derivatives highlights the strategic importance of securing reliable and diverse production routes.⁶⁴ Recent market fluctuations, particularly during the COVID-19 pandemic, have revealed the vulnerability of ethylene supply chains to global economic and energy disruptions. This volatility underscores the necessity of complementing traditional steam cracking with alternative and renewable production methods.⁸⁰ In this context, the catalytic dehydration of bioethanol presents a promising strategy to reduce reliance on fossil resources while ensuring a steady supply of essential ethylene-derived products.⁸¹

Fig. 6 World ethylene demand (A) and demand distribution by application (B).

4.2. Diethyl ether

4.2.1. Properties of diethyl ether. Diethyl ether, with the chemical formula (C₂H₅)₂O, belongs to the ether class. It is highly volatile, has an octane number greater than 125 or 110, is extremely colorless, possesses a suitable energy density for onboard storage, has a high concentration of oxygen, a low autoignition temperature, broad flammability limits, and high miscibility with diesel and ethanol fuels.⁸²

4.2.2. Importance and applications of diethyl ether. Diethyl ether is commonly used as a solvent in various fine chemistry, fragrance, and pharmaceutical processes, as well as a starting fluid in some engines. It was previously utilized as a general anesthetic until non-flammable agents like halothane were introduced. Additionally, it has been misused as a recreational drug. The production of diethyl ether is significant due to its role as a major product in the chemical industry.⁸³ It has numerous applications in the fuel chemical industry, enhancing combustion in both diesel and gasoline engines. Research has explored the partial conversion of ethanol to diethyl ether to improve ethanol volatility during cold starts, similar to gasoline.⁸² However, its high volatility, poor lubricity, and anesthetic effects present notable drawbacks.⁸⁴

5. Reactor types for the catalytic dehydration of ethanol

Ethanol dehydration occurs during the vapor phase in reactors, either fixed bed or fluidized bed, that contain a catalyst. In fluidized bed reactors, the process can be isothermal or adiabatic, although most cases tend to be adiabatic.⁸⁵

5.1. Fixed-bed reactors

Fixed-bed reactors consist of several thousand long, thin tubes housed within a vessel. Each tube, approximately 2–3 mm in diameter, contains catalyst particles. These reactors also function as heat exchangers, generating steam on the outer side of the tubes during the Fischer–Tropsch reaction.⁸⁵ A schematic diagram of the fixed-bed reactor is presented in Fig. 7. While fixed-bed reactors are simple, adaptable, and scalable, they have several drawbacks. These include high costs and the requirement for multiple reactors in tandem for large-scale operations. Additionally, large catalyst particles are necessary to avoid significant pressure drops within the reactor tubes, which can lead to reduced effectiveness. This is due to low catalyst activity per unit mass, resulting from the difficulty of reactants diffusing into the core of the catalyst particles.^86,87

Fig. 7 Schematic diagram of fixed bed reactor. Adapted from ref. 88.

5.1.1. Fixed-bed isothermal reactor. The isothermal technique is commonly used for the dehydration of ethanol. This process typically employs a multi-tubular fixed bed system, where the catalyst is contained within the tubes.⁶⁷ For the endothermic reaction, a thermal stream circulates on the shell side, operating at temperatures between 330 and 380 °C, with a liquid hourly space velocity (LHSV) of approximately 0.2 to 0.4 h⁻¹. This setup achieves ethanol conversion rates of 98–99%, with molar selectivity for ethylene reaching 95–99%. Regular catalytic regeneration is necessary due to coke deposition, which involves using air diluted with steam every 1 to 6 months, depending on the catalyst and the specific procedure. Managing the temperature in larger tubes can be challenging, which limits the scalability of the process.⁸⁷

5.1.2. Adiabatic fixed-bed reactor. In this procedure, a series of fixed-bed reactors filled with catalyst are used. An inert heat-carrying fluid supplies the necessary heat for the process. The feed stream, entering each subsequent reactor, is reheated by intermediate heating units. The typical steam-to-ethanol ratio in the input stream ranges from 2 : 1 to 3 : 1, with ethanol make-up streams added between each stage. The LHSV is around 0.15 to 0.5 h⁻¹, and the inlet temperature is maintained at 450–500 °C. This setup achieves conversion rates exceeding 99%, with ethylene molar selectivity between 97% and 99%. The catalyst regeneration cycle occurs every 6 to 12 months due to steam usage and coke production.^69,89

5.2. Fluidized-bed reactor

A fluidized bed reactor (FBR) is a versatile device designed for various multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a solid catalyst at high speeds, suspending the solid and enabling it to behave like a fluid. This process, known as fluidization, offers several significant advantages, overcoming various operational challenges.⁹⁰ The FBR demonstrates greatly improved mass and heat transfer and reduced temperature gradients. The effective mixing of the catalyst with the fluid increases the surface area available for reactions, enhancing mass and heat transfer. Fig. 8 showed a schematic diagram of a fluidized-bed reactor.⁹¹ The fluidized bed reactor facilitates near-complete ethanol conversion with exceptionally high selectivity for ethylene. Optimal temperature control minimizes by-product formation and coke accumulation. At 400 °C, ethanol is converted to ethylene with a conversion rate of 99.5% and a molar selectivity of 99.9%. The endothermic heat required for the reaction is supplied by the hot fluid and the heated catalyst, which is recycled from the catalyst regenerator.⁶⁵

Fig. 8 Basic diagram of fluidized bed reactor. Adapted from ref. 92.

In conclusion, fixed-bed reactors achieve high conversion rates of ethanol (approximately 98–99%) and high selectivity for ethylene (95–99%). Their mechanical simplicity and low cost make them suitable for low to medium production volumes.⁹³ However, challenges such as temperature gradients within the reactors, carbon deposit accumulation on catalyst surfaces, and the necessity for periodic catalyst regeneration limit their long-term performance and flexibility for large-scale applications. Additionally, adiabatic configurations require extra energy to heat the catalysts to the reaction temperature and to ensure adequate steam dilution.⁹⁴ In contrast, fluidized bed reactors offer enhanced heat and mass transfer characteristics, ensuring uniform temperature distribution and minimizing hot spot formation.⁹⁵ They also allow for continuous catalyst regeneration, which improves overall reactor operation and achieves a very high selectivity (>99%) for ethylene production. As a result, fluidized systems are generally more appealing than fixed systems for large-scale continuous production processes. However, they require a higher capital investment and more complex operational control compared to fixed beds.⁹⁶

6. Process for catalytic dehydration of ethanol

An industrial facility has been approved by the SINOPEC Shanghai Research Institute of Petrochemical Technology (SRIPT) for the dehydration of ethanol to ethylene (ETE).⁹⁷ In the ethanol-to-ethylene (ETE) industrial process, reactors facilitate the reaction between ethanol and catalysts, resulting in the formation of ethylene gas.⁹⁸ This occurs through both intermolecular and intramolecular dehydration of ethanol molecules. Lewis acid sites on the catalyst surface promote intermolecular dehydration, while Brønsted acid sites facilitate intramolecular dehydration. Typically, in a fixed bed reactor, Brønsted acid sites drive reactions within a temperature range of 350° to 450 °C. Below 350°, intermolecular dehydration between two ethanol molecules produces diethyl ether. Consequently, catalyst properties significantly influence catalytic selectivity, coke generation, and the operational lifespan of the catalyst.⁹⁹

To optimize the reaction, the fixed bed reactor operates in conjunction with an isothermally-operated reboiler, ensuring efficient heat transfer during the endothermic reaction and promoting uniform catalytic activity throughout the catalyst bed. Although downstream equipment such as a water scrubber, alkaline wash column, dryer, or stripper does not directly influence the catalytic process, it can improve overall catalyst performance by removing acidic gases, water, and heavier hydrocarbons that might disrupt the reaction equilibrium and lead to catalyst poisoning. Thus, the overall process engineering integrates both catalysis and separation operations, ensuring sustained high selectivity for ethylene production from the ethanol feed while prolonging catalyst life.¹⁰⁰

This process employs an isothermal fixed-bed reactor along with two low-temperature separation columns, a water scrubber, and an alkaline wash column, as depicted in Fig. 9. Ethanol conversion achieves 99%, with ethylene selectivity exceeding 96% at reaction temperatures of 350–450 °C and a liquid hourly space velocity (LHSV) of 0.5–1.0 h⁻¹. The catalyst has an operational lifetime of approximately one year.⁶⁸ This method is not only economical and practical but also environmentally friendly. The ethanol feed is vaporized and preheated using the reactor effluent before entering the reactor. Gases exiting the scrubber are directed to the alkaline wash column, which consists of two sections: the lower alkaline section removes acidic gases (e.g., CO₂) using a recycled diluted alkaline solution, while the upper water-scrubbing section further eliminates residual acidic gases. The rough ethylene stream is then compressed, cooled, and dried before entering the ethylene column, where ethane, C₃ compounds, and other heavier components are separated. Finally, the stream passes through a stripper to eliminate light gases such as H₂, CH₄, and CO, yielding high-purity ethylene collected at the base of the stripper for storage.¹⁰¹

Fig. 9 Schematic diagram of the ethylene production process via ethanol dehydration. Adapted from ref. 102.

7. Reaction mechanism of ethanol dehydration

The reaction mechanism for the dehydration of ethanol is depicted in Fig. 10. In this process, ethylene is primarily formed at Brønsted acid sites, while diethyl ether is predominantly produced at Lewis acid sites.^103,104

Fig. 10 Reaction network of ethanol dehydration over catalyst.

Fig. 11 illustrates the mechanism of ethanol dehydration over acid catalysts to produce ethylene. The process begins with the protonation of the hydroxyl group oxygen atom in ethanol, which enhances its ability to leave. Next, the loss of a water molecule occurs, creating carbocation intermediate. Finally, the deprotonation of the carbon atom adjacent to the carbocation results in the formation of a carbon–carbon double bond, producing ethylene.^105,106

Fig. 11 Dehydration mechanism of ethanol to ethylene. Adapted from ref. 107.

The reaction to form diethyl ether proceeds via a unimolecular nucleophilic substitution pathway. Initially, ethanol is treated with an acid catalyst that generates protons, enabling the elimination of a water molecule from one ethanol molecule, which results in the formation of a carbocation. Next, a second ethanol molecule acts as a nucleophile and attacks the electrophilic carbon of the carbocation.¹⁰⁸ This unimolecular nucleophilic substitution ultimately produces diethyl ether, as shown in Fig. 12.

Fig. 12 Formation of diethyl ether from ethanol. Adapted from ref. 107.

8. Catalysts for dehydration of ethanol

Acid catalysts are typically used in the dehydration of ethanol. Sulfuric acid and phosphoric acid are the standard homogeneous catalysts employed in nearly every industrial process.¹⁰⁹ This reaction generally requires high temperatures, leading to reduced ethylene selectivity and coke deposition, which deactivates the catalyst.⁵⁹ As a result, these acids fell out of favor as early as 1995.¹¹⁰ Solid acid heterogeneous catalysts, on the other hand, are more environmentally friendly, less corrosive, and generate less waste than their homogeneous counterparts, which often suffer from poor yields. These solid catalysts are also more stable and resilient to sulfuric and phosphoric acids, and they are easier to separate from the reaction mixture.¹¹¹

8.1. Design strategies for catalysts used in ethanol dehydration

Metal oxides are frequently used as solid acid catalysts for ethanol dehydration due to their Lewis acidic (metal atoms) and Brønsted acid sites (oxygen atoms), which influence both ethylene selectivity and catalyst stability. The dehydration reaction requires the production of 390 calories of ethylene per gram.⁷⁵ To improve catalytic performance, various preparation techniques have been developed to precisely control particle size, morphology, crystallinity, and chemical composition, all of which directly impact ethanol conversion efficiency and catalyst longevity.^112,113 These techniques are generally categorized into bottom-up approaches, where nanoparticles are assembled from atoms or molecules using methods such as chemical synthesis, sol–gel processes, colloidal assembly, and self-assembly, and top-down approaches, in which bulk materials are reduced to nanoscale through processes like deposition, etching, nano-lithography and mechanical treatments modify surface-to-volume ratios, enhancing catalytic activity. Both gas–solid methods, such as chemical vapor deposition and pulsed laser deposition, and liquid–solid methods, including co-precipitation, solvothermal synthesis, sonication, and microwave-assisted preparation, are used to optimize metal oxide catalysts for ethanol-to-ethylene conversion.^114,115 These techniques aim to maximize selectivity while minimizing coke formation. Fig. 13 illustrates the general concepts of bottom-up and top-down preparation.¹¹⁴

Fig. 13 Nanoparticular oxide production methods¹¹⁶ (DOI: https://doi.org/10.3390/nano12193465, Open-source: MDPI).

8.1.1. Catalyst preparation strategies and their impact on ethanol dehydration performance. The methods used for catalyst preparation significantly affect their physicochemical properties and, in turn, the performance of ethanol dehydration.¹¹⁷ Co-precipitation techniques result in moderate surface area and uniform metal distribution, which provide balanced Lewis acidity, but they offer limited control over pore structure and moderate resistance to coke formation.¹¹⁸ Sol–gel methods improve homogeneity and allow for tunable acidity, enhancing the accessibility of acid sites and increasing ethylene selectivity, although scaling up these processes can be challenging.¹¹⁹ Hydrothermal synthesis enhances crystallinity and structural stability, which leads to improved diffusion, hydrothermal resistance, and a longer catalyst lifetime, though at a higher cost.¹²⁰ Techniques such as microemulsion, microwave-assisted synthesis, and sonochemical methods generally produce smaller particles and better dispersion, which enhance the availability of active sites and catalytic activity, but they face limitations in industrial scalability.¹²¹ Overall, synthesis strategies that optimize acidity, porosity, and metal–support interactions tend to promote higher ethylene selectivity and improved catalyst durability, as will be discussed further.¹²²
8.1.1.1. Method of co-precipitation. The co-precipitation technique entails the controlled precipitation of metal hydroxides from salt precursors, illustrated in Fig. 14. By adjusting parameters like pH, ion concentration, and temperature, researchers can tailor particle size, morphology, and metal dispersion. These adjustments directly influence acidity, porosity, and metal–support interactions, all of which are essential for optimizing ethanol conversion efficiency, ethylene selectivity, and catalyst stability in dehydration reactions.^123,124

Fig. 14 Schematic of co-precipitation for ethanol dehydration catalysts¹²⁵ (DOI: https://doi.org/10.32628/IJSRST, Open-source: ICRTMS-2024 International).

8.1.1.2. Microemulsion technique. The microemulsion technique enables the synthesis of nanoparticles with controlled sizes and uniform structures, which are essential for effective catalyst performance in ethanol dehydration.¹²⁶ In this process, metal precursors precipitate as oxo-hydroxides within aqueous droplets, resulting in monodispersed particles. The morphology and dispersion of these particles are influenced by surfactant interactions.¹²⁷ Key parameters affecting the reverse micellar system include: (i) the water-to-surfactant molar ratio (Wx), (ii) the intermuscular exchange rate, (iii) the nature of the organic solvent, (iv) the characteristics of the surfactant and co-surfactant, and (v) packaging parameters.¹²⁸ These factors directly impact acidity, porosity, and catalytic activity, which in turn affect ethanol conversion and ethylene selectivity. Fig. 15 illustrates the microemulsion approach for preparing nanoparticles specifically designed for ethanol dehydration catalysts.¹²⁹

Fig. 15 The overall procedure involves a renowned method of microemulsion. (a) Reverse micellar system (b) one technique of microemulsion and (c) two methods of microemulsion (reprinted with permission,¹²⁹ copyright 2015 Royal Society of Chemistry).

8.1.1.3. Hydrothermal (solvothermal) technique. The hydrothermal or solvothermal technique involves treating metal precursors in a sealed autoclave under high pressure and temperature, often using surfactants to manage particle growth and prevent agglomeration.^130–132 This method facilitates the formation of well-crystallized, multi-element oxide nanoparticles with controlled size, morphology, and surface properties, which are essential for optimizing ethanol conversion efficiency, ethylene selectivity, and catalyst stability in dehydration reactions.¹¹⁵ Fig. 16 depicts the general hydrothermal synthesis process employed to prepare catalysts specifically designed for ethanol-to-ethylene conversion.¹²⁶

Fig. 16 Hydrothermal synthesis schematic for ethanol dehydration catalysts (reprinted with permission,¹²⁶ copyright 2018 Royal Society of Chemistry).

8.1.1.4. Sol–gel method. The sol–gel method allows for the preparation of metal oxide catalysts with controlled porosity, particle size, and surface properties, all of which are crucial for achieving high ethanol conversion and ethylene selectivity.¹³³ In the alkoxide route, metal alkoxides undergo hydrolysis and condensation in alcoholic solutions to create a porous gel network. Alternatively, the non-alkoxide route employs inorganic salts, such as metal chlorides, which necessitates careful removal of anions to yield pure oxides.¹²⁵ Fig. 17 illustrates the sol–gel process for preparing nanomaterials specifically designed for ethanol-to-ethylene catalysts.¹¹⁵

Fig. 17 Sol–gel synthesis schematic for ethanol dehydration catalysts¹¹⁵ (DOI: https://doi.org/10.1063/1.4915472, Open-source: AIP Conference Proceedings).

8.1.1.5. Microwave irradiation method. The microwave irradiation (MWI) technique is employed to synthesize mesoporous catalysts that exhibit enhanced crystallinity and well-controlled particle sizes ranging from 1 to 5 nm, which are essential for improving ethanol conversion efficiency and ethylene selectivity.^134,135 By directly heating the precursor molecules, microwaves provide faster and more uniform heating compared to traditional methods.¹³⁶ This results in improved metal dispersion, increased surface area, and greater accessibility of active sites, all of which are vital for effective ethanol dehydration.^137,138 Fig. 18 illustrates the MWI synthesis process for nanoparticles utilized in ethanol-to-ethylene catalysts. Microwaves generate heat directly within the molecules of the material being heated, with electromagnetic fields divided into electrical and magnetic components that interact with materials through various mechanisms.¹³⁸

Fig. 18 Microwave irradiation schematic for ethanol dehydration catalysts. Adapted from ref. 139.

8.1.1.6. Sonication method. Sonication is a highly effective technique for producing catalysts that possess unique structural properties, small particle sizes, and high purity.^140,141 It works by generating ultrasonic waves in a liquid, which create bubbles that grow and collapse.¹⁴² This process breaks chemical bonds and facilitates the formation of nanoparticles with increased surface area, acidity, and active site exposure.¹⁴³ As a result, these characteristics significantly enhance ethanol conversion and ethylene selectivity.¹⁴² Fig. 19 illustrates the sonication mechanism used to prepare ethanol dehydration catalysts.

Fig. 19 Sonication schematic for ethanol dehydration catalysts. Adapted from ref. 144.

Table 1 provides a critical comparison of synthesis strategies by correlating preparation methods with their structural effects, catalytic behavior in ethanol dehydration, and practical limitations.

Table 1 Influence of preparation methods on catalyst properties and ethanol dehydration performance

Preparation method	Structural effect	Catalytic consequence in ethanol dehydration	Main limitation
Co-precipitation	Moderate surface area with relatively uniform metal distribution and limited pore control	Balanced Lewis acidity enables good ethylene formation at high temperatures, though stability is moderate	Potential limitations in pore diffusion may lead to coke formation at high weight hourly space velocities (WHSV)
Sol–gel	High homogeneity, tunable composition, and improved surface area	Enhanced accessibility of acid sites, improved selectivity for ethylene, and better dispersion of active phases	Risk of pore collapse during drying and calcination, along with complexities in scaling up
Hydrothermal/solvothermal	High crystallinity, well-defined pore structure, and enhanced framework stability	Improved diffusion and hydrothermal stability, enhanced catalyst lifetime, and stable selectivity	Increased synthesis costs and extended preparation time
Microemulsion	Small particle size with a narrow size distribution	Increased exposure of active sites may allow for lower reaction temperatures	Limited scalability and difficulties in achieving reproducibility
Microwave-assisted	Rapid nucleation resulting in small crystallites and improved dispersion	Faster synthesis and enhanced catalytic activity resulting from smaller particle sizes	Equipment costs and constraints related to scaling up
Sonochemical	Enhanced metal–support interaction; surface defect formation	Modified Lewis acidity with tunable selectivity for ethylene and DEE	Challenges in industrial implementation

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8.1.2. Role of catalyst carriers and metal–support interaction. The catalytic activity of supported catalysts during the dehydration of ethanol is significantly influenced by the physical and chemical characteristics of the support and its interaction with the active metal phase.¹⁴⁵ Beyond providing surface area, the catalyst support affects properties such as metal dispersion, electronic structure, acid site distribution, and resistance to deactivation.¹⁴⁶ Stronger metal–support interactions (MSI) enhance the stability of the active site, modify the strength of Lewis acid sites, and influence ethanol adsorption strength, all of which impact the selectivity between ethylene and diethyl ether.¹⁴⁷ Acidic supports, such as γ-Al₂O₃ and zeolites, with abundant Lewis and Brønsted acid sites, enhance the dehydration process through their multiple acidic sites.¹⁴⁸ In contrast, neutral supports like silicones (SiO₂) facilitate dispersion but do not increase the overall acidity of the catalyst.¹⁴⁹ Reducible oxides, such as TiO₂ and ZrO₂, can alter electronic characteristics and the concentration of oxygen vacancies, affecting the stability of intermediates and resistance to coking.¹⁵⁰ Additionally, porous structures and high surface areas improve mass transfer and reduce concentration gradients, which are essential for achieving high ethylene selectivity on an industrial scale. Thus, optimizing the carrier composition and metal–support interaction is crucial for maximizing activity, selectivity, and long-term stability.^151,152 Table 2 offers a comparative overview that correlates carrier properties with the effects of metal–support interactions and their catalytic consequences in ethanol dehydration.

Table 2 Effect of catalyst carriers on ethanol dehydration performance

Carrier	Key properties	Metal–Support interaction effect	Catalytic performance characteristics	Main limitation
γ-Al2O3	Moderate to high surface area; strong Lewis acidity	Enhances metal dispersion; provides intrinsic acid sites	High conversion; good ethylene selectivity at elevated temperatures	Coke formation during prolonged operation
SiO2	High surface area; weak acidity	Improves metal dispersion without significant electronic modification	Stable but exhibits lower intrinsic dehydration activity	Limited acidity; may require promoters
Zeolites (HZSM-5)	Strong Brønsted acidity; microporous structure	Confinement effect enhances adsorption and selectivity	Very high ethylene selectivity; tunable acidity	Diffusion limitations; prone to coke formation
TiO2	Reducible oxide; oxygen vacancies	Alters electronic structure; improves metal–support interaction	Enhanced stability; moderate control over selectivity	Lower intrinsic acidity
ZrO2	Amphoteric character; thermal stability	Modifies acid–base balance; enhances coke resistance	Improved durability; tunable selectivity	Moderate activity without modification
Composite oxides	Adjustable acidity and porosity	Synergistic tuning of metal–support interaction and acid sites	Enhanced activity and stability	Synthesis complexity

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8.1.2.1. γ-Al₂O₃ catalysts: properties, modifications, and performance. γ-Al₂O₃, commonly known as alumina, is a heterogeneous Lewis acid catalyst known for its high chemical and thermal stability, large surface area, and favorable mechanical properties.^153,154 These characteristics make it a popular choice for ethanol dehydration, which produces ethylene.^155,156 The alumina catalyst derived from sodium aluminate exhibits only weak acid sites, whereas the alumina synthesized from aluminum isopropoxide possesses strong acid sites.^157–159 However, the hydrophilic surface of alumina can become saturated with water, which occupies active sites and diminishes catalytic activity. Additionally, the relatively high reaction temperature (approximately 450 °C) can lead to coke formation, accelerating catalyst deactivation.^155,156

To address these challenges, recent studies have focused on modifying γ-Al₂O₃ to improve its activity and selectivity while lowering operational temperatures. Various modifications have been explored, including metal loadings such as TiO₂,¹⁶⁰ Ni, and CaO,¹⁶¹ as well as phosphorus modification (H₃PO₄) and the addition of noble metals like Pd. For instance, the incorporation of 0.5 wt% Pd onto phosphorus-modified γ-Al₂O₃ (Pd/Al₂O₃-P, Fig. 20(a)) introduces additional Brønsted acid sites, enhancing ethylene selectivity and reducing coke deposition. This modification achieves high ethanol conversion (99.96%) and ethylene selectivity (99.4%) at 460 °C.⁷⁵ In contrast, phosphorus-modified γ-Al₂O₃ without Pd (5P/Al₂O₃, Fig. 20(b)) optimizes weaker surface acidity, making it more suitable for the selective production of diethyl ether (DEE) at moderate temperatures (∼300 °C), resulting in approximately 34% DEE yield and 70% selectivity.⁷⁶ This analysis underscores the trade-off between product selectivity and catalyst stability: while Pd/Al₂O₃-P is advantageous for ethylene production at high temperatures, 5P/Al₂O₃ is preferable for DEE synthesis at lower temperatures. Overall, these comparisons illustrate that tuning γ-Al₂O₃ through metal loading or phosphorus modification enables selective control over ethanol dehydration products by adjusting acidity, metal–support interactions, and porosity, thus providing flexibility based on the desired industrial outcome.^162,163

Fig. 20 γ-Al₂O₃ modification for ethanol dehydration. (a) 0.5 Pd/Al₂O₃-P for ethylene production, (b) 5P/Al₂O₃ for diethyl ether production⁷⁶ (reprinted with permission,⁷⁵ copyright 2021 Elsevier), (b) (DOI: https://doi.org/10.9767/bcrec.14.1.2436.1-8, Open-source publisher: BCREC Group).

8.1.3. Zeolites for ethanol dehydration. Zeolites are hydrated microporous aluminosilicates consisting of interconnected aluminum (AlO₄) and silica (SiO₄) tetrahedra. This structure forms three-dimensional frameworks with molecular-sized channels measuring 0.3 to 1.5 nm.^164,165 Their uniform pore systems, high surface area, strong Brønsted and Lewis acidity, and excellent hydrothermal stability make zeolites highly effective solid-acid catalysts for ethanol dehydration.¹⁶⁶ They exist in various crystalline structures characterized by large, regularly arranged open pores that are comparable in size to small molecules. Notable examples include zeolite A,¹⁶⁷ zeolite X,¹⁶⁷ zeolite Y (used as a catalyst for catalytic cracking),¹⁶⁸ and ZSM-5.⁵¹

Among zeolite catalysts, HZSM-5 is regarded as one of the most promising options for industrial ethanol dehydration. It operates efficiently at temperatures below 300 °C, achieving ethanol conversion rates of up to 98% with approximately 95% selectivity for ethylene at 300 °C.¹⁶⁹ A significant advantage of zeolites over alumina is their capability to process diluted ethanol feeds, which reduces the need for expensive water dehydration.¹⁷⁰ However, the inherent microporous structure of conventional ZSM-5 can restrict mass transfer and limit access to active sites, resulting in diffusion constraints and potential catalyst deactivation. To address these challenges, recent research has focused on structural and compositional engineering. Alkaline treatment has been employed to introduce mesoporosity, which enhances surface area and reactant diffusion. This modification allows for operation at lower temperatures, yielding improved ethylene selectivity (up to 99.8%) and better resistance to coking.¹⁷¹

Overall, recent advances in zeolite-based catalysts can be grouped into three main strategies: (1) generating hierarchical pores to enhance mass transport, (2) incorporating metals to modify acid–metal interactions and improve low-temperature activity, and (3) forming composite acid phases to increase strong acidity and catalytic stability.^172,173

These strategies signify a shift from merely controlling acidity to designing multifunctional catalysts that integrate optimized porosity, enhanced active-site accessibility, and improved durability.¹⁷⁴ The performance improvements documented in various studies align with these three approaches: hierarchical pore generation, metal incorporation, and composite acid-phase formation.¹⁷⁵ Table 3 provides a comparative overview of modified zeolite catalysts, highlighting the relationship between framework engineering, modification strategies, and their catalytic performance in ethanol dehydration.

Table 3 Comparative performance of zeolite catalysts in ethanol dehydration

Catalyst type	Modification strategy	Temperature (°C)	Ethanol conversion (%)	Ethylene selectivity (%)	Reference
Hierarchical MFI (Si/Al = 40)	Ultrasonic desilication (mesopore formation)	210	40–68	DEE ≥ 95	176
		270–290	100
Cu-SSZ-13	Metal incorporation (Cu complex)	212	∼100	>99	64
Dealuminated ZSM-5	Acid density optimization	220	98.5	100	51
IM-5	Topology engineering	260	100	Highest DEE	171
				Ethylene yield balance
TNU-9	Topology variation	260	100	Selective to ethylene + C3^+	171
Ti-deZSM-5	Metal-doped framework	280	96	88	177
Rho zeolite	High surface area framework	250–350	100	>98	178
Zeolite β/AgPW	Composite acid phase	—	81–86	High	179

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8.1.4. Silica–alumina. Amorphous silica–alumina (aluminosilicate) catalysts are commonly used for ethanol dehydration due to their strong Brønsted and Lewis acid sites.¹⁸⁰ The Brønsted sites can reversibly convert to Lewis sites when dehydrated at elevated temperatures, which directly impacts catalytic performance. The high density of strong acid sites in silica–alumina typically favors the conversion of ethanol to ethylene over diethyl ether (DEE). Therefore, controlling the strength and distribution of acid sites is essential for tuning selectivity.^181,182

Several strategies have been employed to regulate acidity, including the selective poisoning of strong acid sites with weak bases and modifying the Al/Si molar ratio.¹⁸³ The Al/Si ratio significantly influences acid density, acid strength, and catalytic stability.¹⁶⁷ While strong acidity promotes ethylene production, excessive acid strength can lead to side reactions and coke formation. Thus, achieving an optimized acid balance is crucial for stable and selective ethanol dehydration.¹⁶⁸

8.1.4.1. Mesoporous silica materials. To address the diffusion limitations seen in conventional amorphous silica–alumina, researchers developed mesoporous silica materials.¹⁸⁴ Notable families such as MCM (Mobil Composition of Matter), SBA (Santa Barbara Amorphous), and KIT (Korean Advanced Institute of Science and Technology) have gained significant attention due to their highly ordered mesostructures.¹⁸⁵ These materials offer large surface areas (over 900 m² g⁻¹ in some cases), tunable pore diameters (ranging from 2 to 30 nm), and narrow pore size distributions with a well-defined hexagonal layout.¹⁸⁶ These structural features increase the accessibility of ethanol molecules to active sites, thereby enhancing catalytic efficiency in dehydration reactions.

In 1992, scientists at Mobil Corporation discovered a new class of silicate materials known as the M41S phase, which features ordered mesoporosity, tunable pore diameters (2–30 nm), and high specific surface areas (greater than 900 m² g⁻¹), as illustrated in Fig. 21. The most well-known members of this class include MCM-41 (characterized by hexagonal mesopores),¹⁸⁷ MCM-48 (with a cubic mesopore design), and MCM-50 (featuring a lamellar structure).¹⁸⁸

Fig. 21 Schematic of the phase structure of M41S mesoporous materials¹⁸⁸ (DOI: https://doi.org/10.1007/978-3-319-48281-1_43-1, Open-source: Springer Nature).

8.1.4.2. MCM-41. MCM-41 is part of the M41S family and features a one-dimensional hexagonally arranged cylindrical mesopore system. It has a high surface area and large pore volume, with pore diameters that can be adjusted independently. The pores are larger than those found in zeolites, allowing for easier porosity exchange.^189,190 The large one-dimensional pores facilitate the easier diffusion of reactants to active sites, thereby increasing reaction rates.

The synthesis of MCM-41 involves surfactants that form micelles in the synthesis solution. These micelles serve as templates, facilitating the creation of the mesoporous framework.¹⁹¹ Typically, surfactants such as cetyltrimethylammonium bromide (CTAB) are used, which form rod-like micelles that act as templates for silica condensation.^192,193 Following calcination, the organic template is removed, resulting in uniform mesopores. Fig. 22 illustrates the formation mechanism of the M41S mesostructure, where surfactant micelles arrange into a hexagonal array and are subsequently coated with silica species before template removal.^190,194 Microwave-assisted synthesis enhances pore uniformity, which helps ensure consistent catalytic performance. Additionally, post-synthetic modifications, like the introduction of Brønsted acid sites or metal incorporation, further enhance catalytic selectivity and activity.

Fig. 22 Schematic illustration of the micelle-templated formation mechanism of hexagonal MCM-41 mesostructured.¹⁹⁶ (DOI: https://doi.org/10.22153/kej.2019.11.003, Open source: Al-Khwarizmi College of Engineering).

MCM-41 is characterized by a high surface area, large pore volume, and tunable pore diameter. Its larger pores significantly enhance reactant diffusion and accessibility to active sites when compared to zeolites. However, the one-dimensional pore structure may still create diffusion limitations under high space velocity conditions.¹⁹⁵ Additionally, microwave-assisted synthesis improves homogeneity and increases the nucleation rate, leading to better structural control.¹⁹⁶

8.1.4.3. SBA-15. Santa Barbara Amorphous mesoporous (SBA) material was synthesized in 1998, featuring a hexagonal array of pores.¹⁹⁷ Its well-defined pore structure, inert framework, thick walls, and excellent thermal and hydrothermal stability have attracted significant interest from researchers. SBA-15 boasts several appealing characteristics, including a large surface area (600–1000 m² g⁻¹),¹⁹⁷ two-dimensional molecular sieves with P6mm hexagonal symmetry, and thick walls measuring 3–6 nm.¹⁹⁸ The thick walls improve thermal and hydrothermal stability, which is crucial for the dehydration of ethanol at high temperatures. The synthesis process employs Pluronic triblock copolymers (PEO–PPO–PEO) as structure-directing agents under strongly acidic conditions, along with tetraethyl orthosilicate (TEOS) as the silica source.^199–202 The formation of micelles occurs due to the orientation of hydrophilic groups towards the outside and hydrophobic groups within the micelles. The interactions between water molecules and alkylene oxides create hydrogen bonds, illustrating the SBA-15 synthesis mechanism shown in Fig. 23.

Fig. 23 Schematic representation of the synthesis procedure for SBA-15.²⁰⁴ (DOI: https://doi.org/10.3390/catal9120984, Open-source: MDPI).

The large surface area facilitates efficient access of reactants to active sites, enhancing catalytic performance. Due to the weak intrinsic acidity of pristine SBA-15, the post-synthetic incorporation of aluminum or metal species creates Brønsted acid sites or bifunctional centers, which significantly boosts catalytic efficiency and selectivity.

This structural difference is crucial for its catalytic behavior. Compared to MCM-41, the thicker pore walls of SBA-15 enhance its structural stability under reaction conditions, particularly during ethanol dehydration at elevated temperatures.²⁰³ However, like pure silica MCM-41, pristine SBA-15 exhibits weak intrinsic acidity. Consequently, its catalytic performance heavily relies on post-synthetic modifications, such as aluminum incorporation to generate Brønsted acid sites or metal loading to introduce bifunctional activity.²⁰⁴

8.1.4.4. Aluminum-incorporated MCM-41 and SBA-15. Pure mesoporous silica does not possess sufficient intrinsic acidity for effective ethanol dehydration. To address this, incorporating aluminum into MCM-41 or SBA-15 frameworks introduces both Brønsted and Lewis acid sites while enhancing hydrothermal stability.^205,206

Aluminum incorporation can be achieved through two primary methods:

• Direct (one-pot) synthesis, where aluminum precursors are added during the formation of the mesostructure, ensuring strong metal–support interactions.²⁰⁷

• Post-synthesis methods, such as incipient wetness impregnation or wet impregnation, which allow for controlled loading of aluminum species.²⁰⁸

Fig. 24 depicts the wall structure of Al-incorporated MCM-41 and SBA-15 materials.²⁰⁹ The figure demonstrates that aluminum species are integrated within the silica framework rather than simply deposited on the external surface, resulting in the formation of framework-associated Brønsted and Lewis acid sites. Direct (one-pot) synthesis typically ensures better structural integration and a homogeneous distribution of aluminum within the mesoporous walls, while post-synthesis techniques offer greater flexibility in adjusting acid density and surface composition. However, aluminum incorporation may slightly alter pore diameter and surface area due to partial framework distortion.²¹⁰

Fig. 24 Wall structure of Al-incorporated MCM-41 (A) and SBA-15 (B) materials (reprinted with permission,²¹¹ copyright 2010 Elsevier).

Silica–alumina-based catalysts exhibit catalytic behavior that is heavily influenced by their acidity and structural properties. Specifically, alumina–silica composites with optimized aluminum content show enhanced performance in dehydration. As shown in Fig. 25, the dehydration pathway for ethanol and bioethanol over the Al–SSP catalyst occurs through acid-site activation, primarily resulting in ethylene formation at higher temperatures. The catalyst's performance is significantly affected by the type of acidity, porosity, and preparation method.²¹²

Fig. 25 Reaction scheme for ethanol and bioethanol dehydration over alumina–silica composite (Al–SSP) catalyst (reprinted with permission,²¹² copyright 2020 Elsevier).

The 60Al-SSP sample shows the highest acidity, resulting in 98% ethanol conversion and 99% ethylene selectivity at 400 °C, with good stability over 10 h of operation. Compared to NHSG-prepared mesoporous catalysts and conventional silica–alumina, the Al–SSP system offers a favorable balance between acid strength, conversion efficiency, and operational stability, although slight deactivation due to coke formation may occur during prolonged bioethanol dehydration. The comparative analysis in Table 4 summarizes the relative strengths, weaknesses, and industrial potential of these catalysts.

Table 4 Comparative analysis of silica–alumina-based catalysts for ethanol dehydration

	Preparation/method	Ethanol conversion (%)	Ethylene selectivity (%)	DEE selectivity (%)	Reference
NHSG mesoporous organosilica	Non-hydrolytic sol–gel with Al, Nb, or Sn	64.7–95.5	64.7–95.5	2–18	209
Commercial silica–alumina	Conventional preparation	100	96–100	∼30	213
Zeolites (HZSM-5, MFI, etc.)	Dealumination, desilication, metal/La/P impregnation catalyst type	91–100	88–100	Low	214
Alumina-silica composite (Al-SSP)	Sol–gel, varying Al content (20–80 mol%)	98	99	Low	212

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The comparative analysis of alumina–silica catalysts clearly shows their effectiveness in dehydrating ethanol to produce ethylene. This process underscores the importance of acid site density, strength, and distribution in influencing catalytic performance.²¹⁵

While dehydration is the primary method for valorizing bioethanol, ethanol can also undergo additional transformations beyond merely removing water. When using multifunctional catalysts with a balanced mix of acidic, basic, and/or metallic sites, ethanol-derived intermediates like ethylene or acetaldehyde can engage in C–C bond-forming reactions, such as oligomerization, aldol condensation, metathesis, and Guerbet coupling²¹⁶ These sequential upgrading pathways broaden the range of products to include higher hydrocarbons and oxygenates, such as propylene, 1,3-butadiene, and higher alcohols, thus enhancing the potential for sustainable catalytic conversion of ethanol for petrochemical applications.²¹⁷

9. Beyond dehydration: ethanol upgrading via C–C coupling reactions

Ethanol upgrading through C–C bond formation occurs through several key pathways, including the conversion to propylene, 1,3-butadiene, and higher alcohols.²¹⁸ These processes involve tandem catalytic sequences that integrate dehydration with dehydrogenation, condensation, and hydrogen-transfer reactions using multifunctional catalysts.²¹⁹ Unlike simple dehydration, these upgrading routes necessitate a careful balance of acidic, basic, and metallic active sites, making catalyst design a crucial factor that influences activity, selectivity, and stability.²²⁰

9.1. Ethanol to propylene

The ethanol-to-propylene (ETP) process follows integrated dehydration–coupling pathways rather than a single elementary reaction.²²¹ Initially, ethanol is dehydrated over acidic sites to produce ethylene. This ethylene then undergoes oligomerization, isomerization, and metathesis reactions with the help of bifunctional catalysts to yield propylene.^222,223 The efficiency of this cascade process heavily relies on the strength and distribution of acid sites, along with the presence of metallic or shape-selective functions that facilitate controlled C–C bond formation and minimize coke formation.²²⁴

Propylene is one of the most widely produced petrochemicals after ethylene and has extensive applications in the synthesis of polypropylene, acrylonitrile, acrylic acid, acrolein, propylene oxide, glycols, oxo alcohols, cumene, isopropyl alcohol, and acetone.²²⁵ Global propylene consumption exceeds 100 million tons and is projected to reach 135 million tons by 2025, with an annual growth rate of approximately 4%.²²⁶ Traditionally, propylene is obtained from non-renewable fossil resources, primarily through steam cracking of light naphtha at high temperatures (750–850 °C) and very short residence times (0.08–0.25 s), or through fluid catalytic cracking using Y-zeolite catalysts derived from natural gas and coal.²²⁷

Ethanol conversion to propylene encompasses a series of reactions, including dehydration, dehydrogenation, and C–C bond formation, which go beyond mere ethanol dehydration.

The general reaction sequence includes:

(1) Ethanol dehydration to ethylene:

Ethanol is first dehydrated to ethylene as described in eqn (1).

(2) Ethylene dimerization and rearrangement to higher hydrocarbons:

2CH₂=CH₂ → CH₂=CHCH₂CH₃ → CH₃CH=CHCH₃ (4)

CH₃CH=CHCH₃ + CH₂=CH₂ → 2CH₂=CHCH₃ (5)

A mechanistic proposal suggests that ethylene is activated over ZSM-5 zeolite, resulting in the formation of a carbene species through π-complexation. This species then reacts with ethylene to produce propylene:²²⁸

C₂H₄ → (CH₂) (6)

CH₂ + C₂H₄ → C₃H₆ (7)

The confinement effect in ZSM-5 micropores is essential for stabilizing carbene intermediates and promoting selective propylene formation, highlighting the significance of pore architecture in catalytic performance.²²⁹

Iwamoto et al.,²²⁵ reported a one-step conversion of ethanol to propylene using nickel ion-loaded mesoporous silica at 400 °C. This process achieved 68% ethanol conversion and 48% selectivity for propylene. The reaction mechanism involves the dimerization of ethylene to form 1-butene, followed by isomerization to 2-butene, and then metathesis with ethylene to produce propylene, as illustrated in Fig. 26. Additionally, diethyl ether (DEE) is formed as a side product at 250 °C, which can revert to ethanol and ethylene. At higher temperatures (300–400 °C), ethylene and butane intermediates are efficiently converted to propylene. These results demonstrate that metal–support interactions in mesoporous silica greatly affect propylene selectivity by adjusting the balance between oligomerization and metathesis pathways.²³⁰

Fig. 26 Proposed reaction mechanism for the conversion of ethylene to propylene on nickel ion-loaded MCM-41.²²⁵ (DOI: https://doi.org/10.3390/molecules16097844, Open-source: MDPI).

9.2. Ethanol to 1,3-butadiene

1,3-Butadiene is a crucial bulk chemical in the petrochemical industry, primarily used in the production of synthetic rubber (such as styrene–butadiene rubber and polybutadiene) and other applications, including chloroprene, nitrile rubber, acrylonitrile, elastomers, and resin.^231–233 Global production was approximately 10 million metric tons in 2012, with projections indicating growth to 15 million tons by 2023.²³⁴ Industrially, butadiene is obtained either from naphtha steam cracker fractions or through catalytic or oxidative dehydrogenation of n-butane and n-butene.^233,235 Projections indicate that butadiene consumption may increase from approximately 526 thousand tons in 2010 to over 1.2 million tons by 2030.²¹⁹

Ethanol can be converted to 1,3-butadiene through tandem catalytic pathways involving sequential dehydrogenation, aldol condensation, and dehydration reactions.²³⁶ This transformation is especially sensitive to the catalyst's acid–base balance. Excessive acidity promotes unwanted dehydration to ethylene, while insufficient redox functionality restricts the formation of acetaldehyde.²³²

The choice of catalysts significantly impacts the rate, selectivity, and yield. Zirconium-based catalysts, along with various metal oxides (such as MgO/SiO₂, TiO₂, Al₂O₃, Nb₂O₅, ZrO₂) and metals (including Ag, Cu, Ni) supported on mesoporous silica, have been extensively studied.^237,238 For example, Ag–ZrO₂/SiO₂ demonstrated 88% ethanol conversion with 73.9% selectivity to 1,3-butadiene at 320 °C, 73.9% selectivity dehydration.²³⁶ The high selectivity observed is due to the synergistic interaction between the dispersed ZrO₂ acid–base sites and the metallic Ag centers. This interaction facilitates hydrogen-transfer steps and minimizes side reactions.²³⁹

The transformation of ethanol to butadiene generally follows five key steps, as shown in Fig. 27: (i) the conversion of ethanol to acetaldehyde; (ii) aldol condensation to form 3-hydroxybutanal; (iii) dehydration to produce crotonaldehyde; (iv) a hydrogen-transfer reaction yielding crotyl alcohol; and (v) the final dehydration to generate 1,3-butadiene.²⁴⁰

Fig. 27 Reaction mechanism of ethanol conversion to 1,3-butadiene (reprinted with permission,²⁴⁰ copyright 2012 Elsevier).

This multistep mechanism clearly shows that upgrading ethanol necessitates cooperative catalytic functions instead of isolated acidic sites, setting it apart from traditional dehydration chemistry.²⁴¹

This framework highlights that upgrading ethanol to higher hydrocarbons involves more than just simple dehydration; it also includes both acid- and base-catalyzed C–C coupling reactions. These pathways showcase the versatility of modified catalysts in the sustainable valorization of bioethanol.²⁴²

9.3. Ethanol to higher alcohols (Guerbet reaction)

The upgrading of ethanol to higher alcohols (C₂⁺), particularly n-butanol, has become a significant area of research, moving beyond the simple dehydration to ethylene. Higher alcohols offer enhanced fuel properties compared to ethanol and are regarded as promising renewable fuel additives and chemical intermediates. However, developing efficient catalytic systems for selective ethanol coupling remains a challenge.²⁴³

The conversion process typically follows the Guerbet reaction mechanism, which consists of three main stages:

(i) Dehydrogenation of ethanol to acetaldehyde, (ii) aldol condensation to form β-hydroxy aldehydes, followed by dehydration, and (iii) hydrogenation to produce higher alcohols.²⁴⁴

The aldol condensation stage is often the rate-determining step, requiring well-tuned basic or amphoteric surfaces to stabilize enolate intermediates.²⁴⁵ Unlike direct dehydration, the Guerbet process necessitates multifunctional catalysts that can simultaneously promote both redox and acid–base reactions. Catalyst optimization requires a careful balance between metallic dehydrogenation sites and acid–base centers to enhance C–C coupling while minimizing parallel dehydration to ethylene.²⁴⁶

Achieving high selectivity toward C₄⁺ alcohols relies on the controlled interplay between these catalytic functions. Ethanol molecules first adsorb onto metal sites (e.g., Ni, Cu, or Ag), where they dehydrogenate into intermediates such as CH₃CH₂O*, CH₃CHO*, CH₃CO*, and CH₂CH₂OH*. These intermediates are then catalyzed by supports with aldol condensation activity on basic or amphoteric surfaces, leading to the formation of C–C coupled intermediates.²⁴⁷ These intermediates are subsequently dehydrated and hydrogenated to produce C₄⁺ alcohols (e.g., CH₃CH(OH)CH₂CH₂O*).²⁴⁸ The delicate balance between metallic dehydrogenation sites and acid–base functionalities is crucial for achieving high selectivity toward higher alcohols while minimizing competing dehydration pathways.²⁴⁹

Recent advancements underscore the importance of rational catalyst design, which includes optimizing metal–support interactions, tuning acid–base properties, and refining reaction conditions to enhance both activity and stability.²⁵⁰ This approach illustrates that ethanol valorization goes beyond simple dehydration, integrating C–C coupling strategies made possible by carefully engineered catalytic systems.

These findings confirm that valorizing ethanol through C–C coupling reactions is a sophisticated catalytic strategy. It goes beyond simple dehydration and necessitates careful engineering of both the structure and composition of the catalysts.²⁵¹

10. Conclusion and remarks

The abundance and affordability of bioethanol offer a promising opportunity for its use as a renewable platform molecule in the production of value-added chemicals. Catalytic ethanol dehydration is the most established and primary transformation route, facilitating the sustainable production of ethylene and diethyl ether, which are important petrochemical intermediates. The catalytic dehydration of ethanol serves as a sustainable alternative to traditional petrochemical methods, reducing greenhouse gas emissions and dependence on finite fossil fuels.

This review goes beyond dehydration to critically assess the upgrading of ethanol into higher-value chemicals like propylene and 1,3-butadiene using multifunctional catalytic systems. By comparing catalyst performance, reaction mechanisms, and identifying research gaps, we establish a framework for understanding the current state of the art and guiding future research efforts. Although significant progress has been made in enhancing catalyst performance, stability, and selectivity, challenges persist. These include the need for lower reaction temperatures, longer catalyst lifespans, and more economical integration of ethanol-to-ethylene and ethanol-to-propylene processes. Future research should prioritize the design of catalysts that balance acidity and basicity to optimize product distribution, investigate tandem reaction pathways for higher alcohols and hydrocarbons, and develop scalable, cost–effective processes for upgrading bioethanol. Moreover, incorporating these catalytic processes into existing biorefineries and assessing the economic and environmental impacts of each step will be crucial for the industrial implementation of ethanol valorization technologies.

In summary, although this review does not introduce any new catalysts, it offers a thorough and critical evaluation of existing catalytic systems, insights into their mechanisms, and highlights research gaps. This makes it a valuable resource for researchers and practitioners in the field, underscoring the importance of an analytical perspective in ethanol valorization studies.

Author contributions

Rasha S. Mohamed: conceptualization of the review, investigation, validation, writing – original draft preparation, writing – review and editing, visualization, supervision and resources; Heba M. El Sharkawy: resources, validation, conceptualization, visualization, draft preparation, writing – review and editing. All authors contributed to discussion of the content and have read and agreed to the published version of the manuscript.

Conflicts of interest

The authors confirm this article has no conflicts of interest.

Data availability

This manuscript is a review article and does not report new experimental, computational, or original datasets. All data discussed and presented (including those in figures and tables) are derived from previously published studies, which have been appropriately cited throughout the manuscript. Therefore, no new data were generated or analysed as part of this study.

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