Direct CO₂ Hydrogenation over Bifunctional Catalysts to Produce Dimethyl Ether—A Review

Abstract

Hydrogenation of CO₂ represents a promising pathway for converting it into valuable hydrocarbons and clean fuels like dimethyl ether (DME). Despite significant research, several challenges persist, including a limited understanding of reaction mechanisms, thermodynamics, the necessity for catalyst design to enhance DME selectivity, and issues related to catalyst deactivation. The paper provides a comprehensive overview of recent studies from 2012 to 2023, covering various aspects of CO₂ hydrogenation to methanol and DME. This review primarily focuses on advancing the development of efficient, selective, and stable innovative catalysts for this purpose. Recent investigations that have extensively explored heterogeneous catalysts for CO₂ hydrogenation were summarized. A notable focus is on Cu-based catalysts modified with promoters such as Zn, Zr, Fe, etc. Additionally, this context delves into thermodynamic considerations, the impact of reaction variables, reaction mechanisms, reactor configurations, and recent technological advancements, such as 3D-printed catalysts. Furthermore, the paper examines the influence of different parameters on catalyst deactivation. The review offers insights into direct CO₂ hydrogenation to DME and proposes paths for future investigation, aiming to address current challenges and advance the field.

1. Introduction

In today’s world, climate change is widely recognized as a major problem, largely driven by the rapid growth of industries and cities. The release of CO₂ into the atmosphere has made climate changes a global concern. In this context, capturing and converting CO₂ into useful chemicals and fuels have become crucial aspects [1]. One potential solution involves converting CO₂ into DME as an alternative fuel with a capacity for efficient combustion [2]. DME has grown in popularity over the years, initially as (1) a propellant in aerosol cans, (2) chemical feedstock, (3) and liquid petroleum gas (LPG) blending. Today, it finds practical uses in areas such as fuel cells, transportation, power generation, and even as an environmentally friendly refrigerant [3]. It has a lower calorific value compared to LPG but is a cleaner fuel with lower pollution and emissions of nitrogen oxide (NOx) and sulfur oxide (SO_x) upon combustion. Recently, DME has been considered as a “hydrogen carrier” in the context of storing and transporting hydrogen [4]. DME can be produced from biomass, syngas, or coal, and the resulting DME is classified as an environmentally friendly fuel [5]. Recent studies work on producing DME by the process of CO₂ hydrogenation. Employing metal catalysts, methanol is initially produced as the primary product. This methanol can then be further converted into DME by processing it on an acidic support [6]. To overcome the thermodynamic limitations and equilibrium constrain of the methanol synthesis step, there is a rising interest in the concept of direct CO₂ hydrogenation using bifunctional catalysts within a single reactor [7]. This approach seeks to make the process more efficient and integrated.

This comprehensive study aims to review research conducted on how different factors affect the enhancement of the CO₂ hydrogenation process efficiency, including thermodynamic study, catalyst development, and configuration of reactors. It provides an overview of the latest advancements in the research on the direct hydrogenation of CO₂ to DME. It starts with a discussion on the potential value-added products that can be produced through CO₂ hydrogenation and gives an introduction to the thermodynamics of CO₂ hydrogenation to DME. The context then delves into the role of various promoters and examines the properties of active sites and supports. Finally, the study will discuss the factors that deactivate the catalyst and the types of reactors designed to minimize deactivation.

2. CO₂ Conversion by Hydrogenation

This section reviews the current processes used for converting CO₂ into hydrocarbons using hydrogen. The basic process of obtaining carbon-rich products from CO₂ is outlined in Figure 1.

The hydrogenation of CO₂ can be divided into two routes, one that produces C1 compounds such as methane (CH₄), methanol (CH₃OH), formaldehyde (HCOH), and formic acid (CH₂O₂) and another that forms C1₊ compounds, such as hydrocarbons and oxygenates (see Figure 2). To improve process efficiency towards target products, factors such as thermodynamics (temperature and pressure) and H₂/CO₂ ratio as well as optimizing catalyst properties like surface area, porosity, and metal dispersion are imperative. Achieving the desired conversion is difficult due to the lack of favorable thermodynamic conditions, which increases the need for catalyst development to address this limitation.

2.1. CO₂ Hydrogenation to C1 Compounds

The main products of CO₂ hydrogenation are methane, methanol, formaldehyde, and formic acid. This section will discuss their respective reactions and suitable catalysts.

Methane as a product: The primary approach for methane (CH₄) production through CO₂ hydrogenation is centered on the thermal catalytic process [8].

$${\mathrm{C}\mathrm{O}}_2+{4\mathrm{H}}_2\to {\mathrm{C}\mathrm{H}}_4+{2\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-164.7 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(1)

CO formation through the reverse water gas shift (RWGS) (Equation (2)) reaction leads to methane formation (Equation (3)) [9].

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=+41.3 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(2)

$$\mathrm{C}\mathrm{O}+{3\mathrm{H}}_2\to {\mathrm{C}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-206.2 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(3)

The Sabatier reaction (Equation (1)) is the process of reaction of CO₂ with H₂ to produce CH₄ and water. It occurs at temperatures ranging from 200 °C to 450 °C and 1 bar of pressure, resulting in almost complete CO₂ conversion and 100% selectivity of CH₄ [10,11]. The reaction is exothermic—CO₂ conversion and CH₄ yield both are increased with the pressure and decreased with temperature [11]. To overcome the kinetic barrier at low temperatures, active metal oxides are used as catalysts [12]. Noble and non-noble metal catalysts have been used for methanation, with the activity order being Ru > Fe > Ni > Pd > Pt, and the selectivity order being Pd > Pt > Ni > Fe > Ru [13]. Ni-based catalysts supported on Al₂O₃, SiO₂, MgO, CeO₂, and ZrO₂, and zeolites have been widely studied in CO₂ methanation [14]. The effect of adding La, Zr, Ce, and Pr as promoters on the catalytic activity and stability of Ni-based catalysts was studied. Among these promoters, Pr performed with higher CO₂ conversion (80%) and CH₄ selectivity (98%) over 50 h [15].

The process of CO₂ methanation operates through two distinct pathways:

• CO₂ Associative Pathway: CO₂ is adsorbed with chemisorbed hydrogen (H_ad) to form an oxygenate, which is then hydrogenated to produce methane.
• CO₂ Dissociative Pathway: CO₂ undergoes direct dissociation, and the intermediate CO is then hydrogenated to form methane.

Methanol as a product: The production of methanol, which has potential uses as both a solvent and fuel, has made it a crucial component in the chemical industry [7]. Methanol synthesis from CO₂ is achieved through the following reaction (Equation (4)). This exothermic reaction occurs at low temperature and high pressure (200 °C and 3 MPa). Water formation during the reaction limits the equilibrium CO₂ conversion, making catalyst selection a crucial factor in the process. Modifying catalysts with promoters results in the development of hydrophobic properties. Additionally, utilizing supports can establish a protective barrier, effectively obstructing direct interaction between water and the active sites of the catalyst.

$${\mathrm{C}\mathrm{O}}_2+{3\mathrm{H}}_2\leftrightarrow {\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-49.4 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(4)

Research has been conducted on both promoters and supports to enhance the activity of catalysts in methanol production [16,17]. One of the most common catalysts used for converting CO₂ to methanol is Cu/ZnO [18]. The addition of promoters like Zn, Zr, and Ce helps to reduce the sintering of Cu and increases its dispersion, leading to higher catalyst activity. Supports such as Al₂O₃, SiO₂, MgO, Ga₂O₃, La₂O₃, TiO₂, and In₂O₃ are used to control the degree of acidity and maintain the redox properties. Wang et al. [19] studied the use of In₂O₃ as a catalyst in the CO₂ hydrogenation process to produce methanol. They found that pure In₂O₃ metal oxide significantly enhances the catalytic activity by inhibiting the RWGS reaction.

Formaldehyde as a product: Formaldehyde (HCOH) serves as the primary raw material for variety of industries, including pharmaceuticals, germicides, paints, inks, cosmetics, resins, and polymers [20]. One of the important products derived from formaldehyde is Oxymethylene dimethyl ether (OME) oligomers, which play a vital role in combustion engines and can be synthesized by condensing methanol and formaldehyde [21]. Lee et al. [22] identified that formaldehyde can be obtained through the hydrogenation of CO₂ using Pt-Cu/SiO₂ catalysts under reaction conditions of 25–350 °C and 6 bar. The researchers found that Cu/SiO₂ catalysts did not produce formaldehyde, and Pt was necessary for adsorbing H₂, which was then transferred to the copper surface [23].

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\to \mathrm{H}\mathrm{C}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=+39.8 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(5)

Formic acid as a product: Formic acid (CH₂O₂) plays a crucial role in hydrogen storage, as it can be broken down into CO₂ and H₂ [24]. However, the reaction involving formic acid has limited activity due to thermodynamic constraints. To address this issue, Hao et al. [25] discovered that using Ru-based catalysts can speed up the hydrogenation of CO₂ to formic acid. Specifically, they found that Ru/$\gamma$-Al₂O₃ catalyst increases the selectivity of formic acid when used in a reactor operating at 80 °C and 13.5 MPa. In a separate study, a Cu/ZnO/Al₂O₃ catalyst was developed to improve the selectivity of formic acid production, with a selectivity rate of 59.6%. [26].

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\to \mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-31.5 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(6)

2.2. CO₂ Hydrogenation to C1₊ Hydrocarbons

C1₊ refers to a group of hydrocarbons with more than one carbon in their structure. This group includes light olefin, gasoline (C5–C11), jet fuel (C8–C16), diesel (C10–C20), and aromatics [27]. These substances can be produced through the process of CO₂ hydrogenation via two pathways namely (a) the CO₂-modified Fischer–Tropsch synthesis (CO₂-FTS) route and (b) the methanol-mediated (MeOH) route [27]. The CO₂-FTS consists of two steps of RWGS and FTS that produce a broad range of products but its selectivity is restricted to certain products such as C2–C4 hydrocarbons, gasoline, jet fuel, and diesel [28]. In contrast, the methanol-mediated (MeOH) route is a promising method to enhance product selectivity via CO₂ hydrogenation. This can be achieved through two distinct pathways, the formate and RWGS-CO routes, whereby CO₂ is first converted to MeOH and then to hydrocarbons (MTH), olefins (MTO), gasoline (MTG), and aromatics (MTA) [29]. The MTO and MTA reactions occur at high temperatures of 300–500 °C which may hinder the conversion of CO₂ to MeOH [27]. The CO₂-FTS route utilizes iron (Fe) and cobalt (Co) as catalysts for CO₂ hydrogenation to hydrocarbons while the MeOH route employs more expensive and complex catalysts like Cu and Zn [27].

• CO₂—modified Fischer–Tropsch synthesis (CO₂-FTS) route

The CO₂-modified Fischer–Tropsch synthesis (CO₂-FTS) route is a reaction route that typically produces linear alkanes and alkenes, but it can also be tailored to produce branched hydrocarbons through the careful selection of catalysts and reaction conditions [30,31]. The CO₂-FTS route is one of the two main routes for CO₂ hydrogenation to olefins and aromatics [32]. It has several advantages over other routes for CO₂ hydrogenation. For example, it affords relatively high CO₂ conversion (30–50%) with moderate aromatic selectivity (25–68%) compared to the methanol-mediated pathway [33]. The following are relevant equations for CO₂-FTS route.

RWGS:

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=+41.3 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(7)

FTS (e.g., ethylene and propylene):

$$2\mathrm{C}\mathrm{O}+4{\mathrm{H}}_2\to {\mathrm{C}}_2{\mathrm{H}}_4+{2\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-210.2 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(8)

$$3\mathrm{C}\mathrm{O}+6{\mathrm{H}}_2\to {\mathrm{C}}_3{\mathrm{H}}_6+3{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-373.6 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(9)

In the aromatization process, hydrocarbons from FTS can be converted to aromatics. The structure of the catalysts used in CO₂-FTS route promotes the formation of aromatics and increases mass-transfer efficiency, thus inhibiting carbon deposition and extending the lifetime of the catalysts [33].

$${-\left({\mathrm{C}\mathrm{H}}_2\right)}_{\mathrm{n}}\to \mathrm{a}\mathrm{r}\mathrm{o}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{s}$$

(10)

As previously stated, the products of CO₂ hydrogenation with CO₂-FTS route consist of light olefin, gasoline, jet fuel, diesel, and aromatics, which are briefly discussed in the following section [34].

Light olefins as a product: Light olefins, including ethylene (${\mathrm{C}}_2{\mathrm{H}}_4$), propylene (${\mathrm{C}}_3{\mathrm{H}}_6$), and butylene (${\mathrm{C}}_4{\mathrm{H}}_8$), are important raw materials for various industries like petrochemicals, plastics, medicines, and paints. These olefins are produced by steam cracking of hydrocarbons, particularly petroleum-based naphtha, fluid catalytic cracking (FCC), dehydrogenation of hydrocarbons, and conversion of methanol into light olefins via CO₂ hydrogenation [35]. Due to the increasing demand for light olefins, researchers are exploring the possibility of producing them through the hydrogenation of CO₂. Fe-based catalysts are widely recognized as crucial catalysts in light olefin production [36].

Gasoline as a product: Gasoline (${\mathrm{C}}_8{\mathrm{H}}_{18}$) typically consists of hydrocarbons with carbon atom numbers ranging from C5 to C11. In order to produce high gasoline selectivity and low olefin content, the use of CO₂-FTS catalysts with solid acidic catalysts is necessary for the hydrocracking and isomerization of primary products. Several studies have focused on using CO₂ hydrogenation to produce gasoline minimizing the production of light olefins [37,38].

Jet fuel as product: The air transport industry heavily relies on jet fuels, which typically contain linear and branched alkanes and cycloalkanes (${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2-2\mathrm{r}}$) with carbon atom numbers ranging from C8 to C16. CO₂ hydrogenation is a promising renewable method for producing jet fuels over Co- or Fe-based catalysts due to its potential for cost reduction and energy savings [39].

Diesel as a product: Diesel (${\mathrm{C}}_{12}{\mathrm{H}}_{23}$) fuel with a range of C10-C20 hydrocarbons is extensively used in transportation, power generation, etc. [40]. Producing diesel fuel through CO₂ hydrogenation offers the benefit of lower sulfur and aromatic content, as well as reduced emissions of NO_x. To improve the selectivity of diesel which is limited to 39%, CO₂ hydrogenation over Fe-based catalysts was investigated in several studies [41].

Aromatics as a product: A valuable chemical product that can be obtained through CO₂ hydrogenation is aromatics, such as benzene, toluene, and xylene (BTX), which are utilized as solvents, gasoline-fuel additives, and precursors. CO₂ hydrogenation produces a higher selectivity of aromatics compared to the traditional naphtha reforming process, and Fe-based catalysts over zeolites are the primary catalysts in this process. The acidity of the zeolite, particularly HZSM-5, plays a crucial role in controlling the distribution of aromatics and can be regulated by various parameters, such as Si/Al ratio, synthesis method, and pre-treatment method [42]. The following section will discuss hydrocarbons produced over the MeOH route.

• Methanol-mediated (MeOH) route

Compared to the CO₂-FTS route, the MeOH route is more selective in producing the desired products, as the restrictions of the Anderson–Schulz–Flory mechanism are avoided [43]. Both direct and indirect pathways in the MeOH route can yield value-added chemicals, but the direct CO₂ hydrogenation pathway has higher selectivity for the desired products. Figure 3 shows the two routes in the MeOH route: (1) the RWGS-CO route and (2) the formate route. Furthermore, the possible outcomes of CO₂ hydrogenation via methanol-mediated (MeOH) route including olefins, gasoline, and aromatics will be considered in the following section.

Methanol to olefin (MTO): Union Carbide Corporation introduced the MTO process in 1981, operating within the temperature range of 400–550 °C and at atmospheric pressure [44]. For the production of methanol through CO₂ hydrogenation, catalysts consisting of metals and metal oxides—such as Cu, Zn, and Zr—can be employed [45]. In contrast, the MTO reaction takes place on zeolites with weakly acidic sites, notably SAPO-34. The small pore size of SAPO-34 (3.5 Å) restricts the diffusion of larger hydrocarbons, thereby enhancing the selectivity for olefins. Along with selecting an appropriate catalyst, reaction conditions are also important for producing the desired products. For instance, temperature and space velocity can impact CO₂ hydrogenation, leading to the production of light olefins [46].

$${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}\to \left[{\mathrm{C}\mathrm{H}}_2\right]+{\mathrm{H}}_2\mathrm{O}$$

(11)

Methanol to gasoline (MTG): Mobil researchers introduced the MTG process in 1977 [44]. Initially, methanol is dehydrated over an acidic catalyst like ZSM-5, resulting in a mixture of methanol, DME, and water. Subsequently, this mixture is converted into olefins, aromatics, and hydrocarbons up to C10. The MTG process employs catalysts based on ZSM-5 zeolite, operating at temperatures ranging from 350–400 °C and atmospheric pressure. In addition to controlling the acidity of the catalyst, incorporating metals into the catalyst composition enhances the selectivity of gasoline during the MTG process. Ruddy et al. [47] utilized beta zeolite modified with copper (Cu) to convert DME into high-octane gasoline. Meanwhile, Fujiwara et al. [48] revealed that Fe-Zn and zeolite Y can directly convert CO₂ into gasoline through the MeOH route due to the higher acidity level of zeolite Y.

$${2\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}\to {\mathrm{C}\mathrm{H}}_3\mathrm{O}{\mathrm{C}\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_{2-5} \mathrm{O}\mathrm{l}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{s}\hspace{1em}\hspace{1em}\begin{array}{c}\mathrm{Aromatics}\\ \mathrm{Paraffins}\\ \mathrm{Naphtenes}\\ \mathrm{Higher} \mathrm{Olefins}\end{array}$$

(12)

Methanol to aromatic (MTA): To produce aromatics from methanol using the MeOH route, high reaction temperatures (>300 °C) and pressures of 3 MPa are required [49]. Producing methanol from CO₂ using metal oxides, mainly Fe-based catalysts, combined with H-ZSM-5 leads to aromatics formation. However, one challenge with MTA catalysts is the difficulty in the diffusion of long-chain hydrocarbons from the catalyst’s pores, which leads to coke formation and deactivates the catalyst. To improve the diffusion of products, promoters like Zn, Cr, and Na are incorporated into ZSM-5 catalysts. This approach helps control the pore size, adjust the Si/Al ratio, and optimize the operating temperature [50]. Lowering the reaction temperature causes a reduction in both the extent of aromatization and the selectivity towards aromatic compounds. However, this decrease in temperature has the benefit of prolonging the catalytic lifetime of the system. Fu et al. [51] investigated the Zn/ZSM-5 catalyst in MTA process that showed higher selectivity of aromatics up to 36.3% at reaction conditions of 460 °C and 3 MPa. Reducing the temperature to 360 °C resulted in a 26% selectivity towards aromatic compounds but with an extended lifetime from 39 to 67 h. In another study, Wang et al. [52] used a Cr-based catalyst to improve the selectivity of aromatics to 67.9%.

Among all hydrocarbons formed via CO₂ hydrogenation, DME stands out and receive significant attention from the industry due to its diverse applications which are described in the next section. As a sustainable and cost-effective fuel, it aligns with global efforts to reduce greenhouse gas emissions and decrease dependence on fossil fuels.

2.3. Overview of Reactions in CO/CO₂ Hydrogenation to DME

In case of CO₂ hydrogenation, DME production has been paid more attention due to the higher selectivity than C2₊ alcohols. Additionally, DME has a higher energetic efficiency, meaning that less energy is required to produce a given amount of DME compared to other C2₊ alcohols [53]. Researchers have been focusing on optimizing the CO₂ hydrogenation process to maximize DME production and improve efficiency. DME’s unique properties and versatility make this optimization crucial. The growing attention to CO₂ hydrogenation for DME production is evidenced by the increasing number of research papers published on the topic, as shown in Figure 4 which illustrates the percentage of articles published from 2012 to the end of 2023.

Initially, DME was created using a process called standard synthetic route (STD), which stands for syngas hydrogenation [54]. This method was first introduced by Haldor Topsøe and involves the use of a bifunctional catalyst. There are two ways to produce DME from syngas as outlined below.

• Conversion of syngas to methanol followed by conversion of methanol to DME, in two stages.
• Direct conversion of syngas to DME.

Figure 5 illustrates the simple direct and indirect CO₂ hydrogenation to DME routes.

The direct conversion of syngas into DME is a simpler process and requires less equipment investment costs, which makes it a more attractive option compared to the indirect one [55]. It is also more efficient than the indirect process, as it eliminates the need for intermediate purification and transportation units by occurring in a single reactor. This integration of reactions in the same reactor makes the process more thermodynamically favorable, leading to increased CO₂ conversion and DME selectivity. However, in the direct synthesis of DME from syngas, CO is consumed in the water–gas shift reaction to form CO₂, which is not ideal for commercial purposes [56]. To exploit the thermodynamic and economic benefits of the direct synthesis of DME, CO₂ + H₂ can be used instead of syngas (CO + H₂). However, this process requires an additional amount of hydrogen to remove an oxygen atom from CO₂ and form water as a by-product. Below are the subsequent reactions involving the direct hydrogenation of CO₂ to DME.

$$\begin{array}{l}{\mathrm{C}\mathrm{O}}_2+{3\mathrm{H}}_2\leftrightarrow {\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-49.5 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ (\text{MeOH} \text{synthesis})\end{array}$$

(13)

$$\begin{array}{l}{2\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}\leftrightarrow {\mathrm{C}\mathrm{H}}_3\mathrm{O}{\mathrm{C}\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-23.4 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ (\text{MeOH} \text{dehydration})\end{array}$$

(14)

$$\begin{array}{l}{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\leftrightarrow \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-23.4 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ \left(\text{RWGS}\right)\end{array}$$

(15)

$$\begin{array}{l}\mathrm{C}\mathrm{O}+{2\mathrm{H}}_2\leftrightarrow {\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-90.6 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ (\text{MeOH} \text{synthesis} \text{with} \text{CO})\end{array}$$

(16)

$$\begin{array}{l}2\mathrm{C}\mathrm{O}+{4\mathrm{H}}_2\leftrightarrow {\mathrm{C}\mathrm{H}}_3\mathrm{O}{\mathrm{C}\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298}^{°}=-102.4 \mathrm{K}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\\ (\text{DME} \text{synthesis} \text{with} \text{CO})\end{array}$$

(17)

3. Dimethyl Ether (DME) Applications

One of the key products of CO₂ hydrogenation process is DME, an organic molecule with a chemical formula ${\mathrm{C}\mathrm{H}}_3\mathrm{O}{\mathrm{C}\mathrm{H}}_3$, which is classified as an environmentally safe ether [57]. DME as a non-toxic and non-corrosive fuel has various advantages, including noiseless and smokeless combustion, ultra-low emissions, a high cetane number, compatibility with low-cost LPG equipment, and a high energy density. Because of its autoignition feature, it is also regarded as the cleanest option among compression ignition fuels. The DME industry is expected to be valuable not only for its role as a chemical precursor for various substances, such as olefins and petrochemicals, but also for its broader applications. Referring to the properties of DME presented in Figure 6, it can be divided into four mainusess: (1) residential cooking/heating (LPG blend), (2) diesel replacement as an ignition engine and transportation fuel, (3) gas turbine fuel in the power generation sector, and (4) aerosol propellants [58].

3.1. Residential Cooking/Heating

DME is being considered as a potential substitute for LPG in household cooking and heating applications. LPG which primarily contains propane (${\mathrm{C}}_3{\mathrm{H}}_8$) and butane (${\mathrm{C}}_4{\mathrm{H}}_{10}$) is used as a cylinder gas in households’ applications instead of natural gas (NG), including methane, in pipelines.

Table 1. Characteristics of DME and primary components in NG and LPG [59].

	Methane	Propane	Butane	DME
Chemical Formula	${\mathrm{C}\mathrm{H}}_4$	${\mathrm{C}}_3{\mathrm{H}}_8$	${\mathrm{C}}_4{\mathrm{H}}_{10}$	${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}$
Boiling Point (°C)	−161.5	−42.07	−0.6	−24.9
Explosion Limit (%)	5–15	2.1–9.5	1.9–8.5	3.4–27
Lower Heating Value (kJ/kg)	49,900	46,360	45,740	28,620
Auto-Ignition Temperature (°C)	595	450	405	235
Vapor Pressure at 20 °C (bar)	-	8.4	2.1	5.1

presents a comparison of the properties of methane, propane, butane, and DME [59]. The properties of DME exhibit similarities to LPG; therefore, it can be considered as a potential substitute for LPG. However, due to its lower heating value, a larger quantity of DME would be required to generate an equivalent amount of heat.

China is currently the largest producer of DME, with 90% of its production being used for LPG blending [60]. Other countries such as South Korea, Egypt, Indonesia, India, and Vietnam are also exploring the use of DME in this market [61]. Studies have shown that using 15–20% volume of DME in LPG/DME blends for existing household and restaurant cooking appliances would not require any modification of existing distribution and users’ appliances [60]. However, various regulations and industry standards have been or are being implemented in China for the use of DME in such applications. Additional experimental studies were investigated in the U.S. to determine the safe limits for DME in LPG/DME blends as a prerequisite to commercialization of such blends using existing appliances for residential and restaurant applications [62]. It is worth noting that LPG blending is the major segment of the DME market. Makmool et al. [63] examined how well a domestic cooking burner performed when using a combination of DME and LPG fuels, both on a traditional burner and on a porous radiant burner. The investigation utilized a 30% weight ratio of DME and determined that a 20% blend was most effective. However, increasing the concentration of DME resulted in a reduction in thermal efficiency for both the conventional and porous radiant burners.

3.2. Ignition Engines and Fuels for Transportation

The search for alternative fuels to be used in automotive vehicles started to gain popularity in the 1980s [64]. DME is a potential alternative fuel for internal combustion engines, including spark-ignition (SI) and compression-ignition (CI) engines without any significant modifications due to its high flammability [65]. CI and SI engines are widely used in different applications, including passenger and commercial vehicles, electricity generation, and industrial settings due to their high-power output and efficiency [65]. In these engines, the combustion process is crucial as it directly transforms the chemical energy of fuel into heat [66]. The primary fuel utilized in CI engines is diesel fuel, while SI engines rely on gasoline. DME combustion has several benefits over conventional liquid fossil fuels when used in both CI and SI engines. One major advantage is that it emits lower levels of NO_x and particulate matter with higher thermal efficiency [67]. DME is a clean burning fuel, non-toxic, and renewable alternative to diesel for use in specially designed compression ignition diesel engines [68]. It has several advantages over diesel fuel, such as low cost, reduced engine noise, no cold-start problems, and the potential for lightweight development [69]. It can be used in larger diesel engines, such as those found in railroad locomotives, generators, and marine equipment to significantly reduce exhaust emissions [70]. However, DME requires more energy to compress due to its higher compressibility, but it doesn’t need high injection pressures like diesel for forming fine sprays. It cannot be directly used in diesel injection systems because of poor lubrication, but the addition of biodiesel can improve its lubricity. It has a lower density and heating value, so it needs higher injection mass flow rates than diesel [71]. Nevertheless, it has the potential to produce more power than diesel in the same engine. Bio-DME, a biofuel produced through biomass gasification, has the potential to be used as a blend fuel at a level of around 15–20% volume for LPG in various countries [72]. Governments are promoting the use of biofuels to lower greenhouse gas (GHG) emissions. Bio-DME can reduce CO₂ emissions by 80–95% without requiring CO₂ capture and sequestration at the bio-DME manufacturing plant [70]. Roh et al. [73] explored the impact of injection strategy on the combustion and toxic emissions of biodiesel-DME blends. He observed that increasing the proportion of DME in the blend results in reduced soot emissions.

Table 2. Comparison of key properties of different fuels.

Property	Diesel	CNG	LNG	DME	Fischer–Tropsch Diesel	Gasoline
Lower Heating Value (MJ/kg)	43	50	50	28.6	43	43
Volumetric Heating Value (MJ/L)	36.5	8	21.1	18.2	33.1	32.2
Density (g/mL)	0.85	0.18 @ 207 bar	0.422	0.66	0.77	0.75
Cetane Number	45	-	-	55+	80	-

presents a comparison of the properties of DME with other types of fuels such as diesel, compressed natural gas (CNG), liquefied natural gas (LNG), Fischer–Tropsch, and gasoline [74].

3.3. Gas Turbine Fuel

A gas turbine is an internal combustion engine that converts the chemical energy of fuel into mechanical energy or electricity through a series of combustion and expansion processes. Gas turbines are widely used in power generation, where they provide energy for various equipment like pumps, compressors, and generators involved in the oil and gas industry, including the drilling, extraction, processing, and transportation of oil and natural gas [75]. They are also utilized in liquefied natural gas (LNG) plants to drive refrigeration compressors. In the petrochemical industry, gas turbines are employed in heating and cooling systems. They make use of waste heat generated by the turbine to produce hot water, steam, or chilled water for heating and cooling purposes in buildings and facilities.

Gas turbines can be fueled by different types of fuels based on their design and intended use [76]. The most common fuel for gas turbines is natural gas, which mainly consists of methane [77]. However, in specific markets where natural gas supply is limited, alternative fuels like DME vapor can be more attractive due to its higher calorific value, increased efficiency, and environmentally friendly nature. It serves as an ideal choice in scenarios where traditional natural gas supplies are not easily accessible. Various companies, including BP Amoco (UK), EPDC (Japan), Toyo Engineering (Japan), Chiyoda Corporation (Japan), and Haldor Topsoe (Denmark), have evaluated the potential of DME as a gas turbine fuel [60]. Research conducted by Basu et al. [78] suggests that DME could be a viable solution for utilizing associated gas when LNG is not suitable due to the size of gas reserves or target market. Zhongya et al. [79] aimed to enhance the combustion intensity of biogas, which is a renewable gas turbine fuel consisting mainly of CH₄ and CO₂ to improve flame stability of biogas in gas turbine. To achieve this, they used DME as a co-firing agent with biogas in a gas turbine model combustor. The addition of DME improved the reactivity of CH₄ by enhancing its ignition and flame characteristics.

3.4. Aerosol Propellants

DME has many advantageous properties that make it an attractive alternative to conventional propellants for space propulsion systems. In comparison to other propellant, DME presented higher thrust, specific power, and thruster efficiency at a given discharge current or specific power [80]. Numerous studies have explored the use of DME as a propellant in arcjet thrusters for space propulsion systems [81,82]. Arcjet thrusters are attractive for space propulsion applications because they offer high specific impulse (the amount of thrust generated per unit of propellant consumed), which allows spacecraft to achieve high velocities and travel further distances with less fuel. DME can be stored in its liquid phase under relatively low pressure without requiring complex temperature management systems. This is due to DME’s freezing point of −141 °C, boiling point of −24 °C, and vapor pressure of 6 bar at room temperature. In conventional space propulsion systems, a combination of hydrazine (${\mathrm{N}}_2{\mathrm{H}}_4$), a kind of monopropellant, and hydrogen with the aid of a catalyst generates high-temperature and high-pressure gas that is expelled through a nozzle to create thrust [83]. In contrast, DME does not require a catalyst for gasification. It can be easily converted into a gas or liquid state by adjusting pressure or temperature.

In addition, the combination of DME and nitrous oxide (${\mathrm{N}}_2\mathrm{O}$) as an oxidant is being considered for use in the next generation of space vehicles due to its usability and capability [84]. The DME and N₂O combination also has the advantage of having desirable vapor pressures (6 bar for DME and 32 bar for N₂O at room temperature) and low freezing points (−142 °C for DME and −102 °C for N₂O at atmospheric pressure) [84]. This eliminates the need for pressures to drive the propellants to the combustion chamber and heaters to prevent the propellant from freezing. Using DME-N₂O as a rocket propellant would enable the development of small, simple, safe, and high-performing satellite thrusters for attitude or orbital control [85].

3.5. Replacement to CFCs

Chlorofluorocarbons (CFCs) have been widely used as working fluids for the organic Rankine cycle (ORC) due to their favorable thermodynamic properties [86,87]. However, CFCs, Freon and R-134 have been found to have a destructive effect on the Earth’s ozone layer, leading to a global effort to phase them out. DME has emerged as a potential replacement for CFCs due to its low toxicity, and high energy density. Additionally, DME has a short atmospheric lifetime of about 5 to 7 days, which means that it has a lower impact on the environment compared to CFCs, which can persist in the atmosphere for years or even decades [87]. Furthermore, DME is also renewable, as it can be produced from various biomass sources, such as agricultural waste and forestry residues, making it a more sustainable alternative to CFCs. However, further research and development is needed to fully evaluate DME’s potential as a replacement for CFCs in Organic Rankine Cycles. Valentin et al. [88] studied to evaluate DME as a more environmentally friendly alternative refrigerant to CFCs in a single-stage vapor-compression refrigeration system. The thermodynamic performance of the system was compared under the same conditions and cooling load for various refrigerants, including DME. The results indicated that DME offered several advantages, such as operating at lower pressures, eliminating the risk of entering a vacuum, and higher specific mass heat load allowing for lower mass flow rates and smaller equipment. As a result, DME had lower energy consumption for compression and a higher coefficient of performance compared to CFCs.

3.6. DME as a Hydrogen Carrier

The concept of storing energy in the form of hydrogen chemically bound within organic molecules, rather than as compressed gas or cooled liquid, is gaining significant research interest [89,90]. Several molecules can serve as hydrogen carriers, including liquid organic hydrogen carriers (LOHC), hydrocarbons, methane, formic acid, carbonate, and formate. Among these, DME emerges as a particularly suitable candidate for a circular hydrogen carrier, being highly efficient for storage and transport. DME is also a key component in developing a sustainable and low-carbon hydrogen-based economy [89]. Compared to liquid hydrogen, DME has a high gravimetric hydrogen density (13 wt%) and higher volumetric hydrogen density, (DME: 86.9 kg—H₂/m³, liquid hydrogen: 70.9 kg—H₂/m³). Moreover, DME has a lower flammability range in the air (3.4–17%) compared to hydrogen (4–75%) [91].

DME is produced from CO₂ and hydrogen in an exothermic reaction, with hydrogen released through endothermic steam reforming. The steps of H₂ storage and release occur in two coupled chemical processes: the exothermic synthesis of DME via CO₂ hydrogenation and the endothermic steam reforming of DME. This two-step process makes DME a viable renewable hydrogen source, increasingly recognized for its ability to store both hydrogen and CO₂, which can then be used in fuel cells and other energy applications. This addresses key challenges in renewable energy storage, transport, and CO₂ emission reduction.

The following section will investigate a more comprehensive exploration of the production of DME through CO₂ hydrogenation. This will include the study of thermodynamic conditions, reaction mechanisms, and catalyst design involved in the process.

4. Thermodynamic Aspects of CO/CO₂ Hydrogenation to MeOH and DME

In the context of CO₂ hydrogenation, thermodynamics plays a crucial role in determining the feasibility of the reaction and predicting the outcome of the reaction under different conditions. In particular, the thermodynamic properties of the reactants, intermediates, and products, such as enthalpy, entropy, and Gibbs free energy (∆G), can provide insights into the energy requirements and the direction of the reaction [92]. By analyzing these thermodynamic properties, researchers can optimize the reaction conditions to increase the yield, selectivity, and efficiency of the reaction. The Gibbs free energy change is an important factor in understanding the thermodynamics of a process. In case of CO₂ hydrogenation, increasing the temperature leads to an increase in ∆G for most of the reactions, indicating a reduction in the desired products. Based on CO₂ hydrogenation reactions (Equations (13)–(17)), a decrease in reaction temperature or an increase in reaction pressure would be favorable for DME synthesis [93]. The high stability and inertness of CO₂ make low temperature a limitation for CO₂ hydrogenation from a thermodynamic standpoint [92]. On the other hand, from a kinetic standpoint, increasing the reaction temperature would facilitate the activation rate of CO₂, leading to the formation of DME. However, this process is also accompanied by the competitive formation of methanol and/or carbon monoxide [94].

From a thermodynamic perspective, CO₂-to-DME reaction exhibits lower enthalpy and lower Gibbs free energy, compared to the CO₂-to-methanol reaction [95]. This suggests that CO₂ hydrogenation to DME is more favorable than CO₂ hydrogenation to methanol in the temperature range of 160–320 °C and pressure range of 10–100 MPa [96]. Additionally, when the partial pressure of H₂ is decreased, CO₂ hydrogenation to DME shows higher catalytic activity than CO₂ hydrogenation to methanol.

Shen et al. [97] calculated the equilibrium yield of DME, CO, and methanol in the CO₂ hydrogenation reaction and found that under the same reaction conditions, the equilibrium yield of DME is significantly higher than that of methanol in the CO₂ hydrogenation process (see Figure 7).

The hydrogenation process using a mixture of CO and CO₂ has been shown to be practical and efficient, indicating that blending these gases may be more beneficial for industrial applications than using each gas separately [93]. The hydrogenation of CO, which often results in higher C-based equilibrium yields compared to CO₂, benefits from lower temperatures and higher pressures due to the exothermic nature of the reactions [93]. Adding CO₂ helps mitigate the WGS reaction for the CO hydrogenation, leading to a lower equilibrium selectivity to CO₂. Although CO₂ hydrogenation to alcohols and hydrocarbons like DME involves competitive RWGS reactions that can affect the yield and selectivity of DME, these effects are minimized under optimal conditions (low temperature and high pressure) [98]. Employing a CO/CO₂ mixture as feed not only simplifies gas separation but also increases the overall DME yield, enhancing process efficiency by enabling the recycling of reaction tail gases and making the hydrogenation process more cost-effective.

4.1. Temperature

The temperature at which a reaction takes place is an important factor in determining whether the process is driven by thermodynamics or kinetics. In the case of CO₂ hydrogenation, high activation energy is required for the reaction, so increasing the temperature will increase the rate of the reaction. However, CO₂ hydrogenation is an exothermic process for which increasing the temperature will reduce the thermodynamic equilibrium (see Figure 7). From a thermodynamic perspective, DME is produced at low temperatures (200–300 °C) [99]. As the reaction temperature increases gradually, the equilibrium conversion of CO₂ initially decreases and then slightly increases. The selectivity for DME decreases, while the selectivity for CO increases, and the selectivity for MeOH initially increases and then decreases. These trends can be attributed to the fact that the overall reaction of CO₂ hydrogenation to DME is exothermic. Fujitani et al. [100] found that the upper temperature limit for CO₂ hydrogenation is 220 °C.

Although high temperatures are required for the activation of CO₂ as an inert component, the RWGS reaction, occurring concurrently with methanol synthesis, results in catalyst agglomeration and sintering. This reaction is less active at lower temperatures and remains unaffected by pressure. Given its endothermic nature, the RWGS reaction becomes increasingly significant as temperature rises, thereby reducing methanol production. At lower temperatures, the formation of DME (Equation (14)) and the predominance of the RWGS reaction led to a reduction in CO₂ conversion with increasing temperature. Conversely, higher temperatures can enhance the RWGS reaction, which may subsequently increases CO₂ conversion [98]. Increasing the temperature has a negative effect on CO₂ hydrogenation in two ways. Firstly, the reaction is driven towards the production of CO and water by endothermic RWGS reaction (Equation (15)), which is a side reaction. It simultaneously reduces the equilibrium yield of methanol and DME [101]. Secondly, increasing the temperature causes another side reaction to occur, which is the decomposition of methanol to CO and H₂ (${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}\to \mathrm{C}\mathrm{O}+{2\mathrm{H}}_2$). Although MeOH selectivity increases and then decreases with increasing temperature, the amount of methanol produced is not dominant throughout the process [98].

4.2. Pressure

According to the literature, direct synthesis of DME from CO₂ is more effective at high pressure due to a reduction in the number of moles [102]. However, methanol synthesis is still the limiting step in the overall reaction, as it has a mole-number-reducing stoichiometry. High pressure can increase the conversion of CO₂ and DME productivity, as well as improve DME and methanol selectivity, while reducing the likelihood of side reactions such as RWGS and methanol decomposition. Increasing pressure does not affect RWGS and methanol dehydration reactions, which have the same number of moles on both sides of the reaction. Therefore, CO₂ hydrogenation to methanol is the reaction most influenced by pressure [103]. However, high pressure in CO₂ hydrogenation causes significant challenges, which need to be protected against on an industrial scale. Challenges include considering effectiveness and economic feasibility of reaction in larger scale and the need for cost-effective materials to construct reactors, should be considered. Moreover, enhancing DME selectivity under high-pressure conditions is crucial to minimize the formation of unwanted by-products such as coke, which can adversely effect on catalyst efficiency and potentially resulting in its deactivation [104]. Studies have shown that high-pressure conditions around 2–10 MPa are ideal for CO₂ hydrogenation [105,106,107]. Tidona et al. [108] found that methanol yield over Cu/ZnO/Al₂O₃ is independent of pressure at temperatures lower than 250 °C. Saravanan et al. [109] proposed a very high-pressure approach (36 MPa) using CZA/HZSM-5 bifunctional catalyst for direct synthesis of DME from CO₂ hydrogenation remarkably boosted DME selectivity of 89% with CO₂ conversion of 97%. From the thermo-economic aspects of a CO₂ hydrogenation plant, Bellotti et al. [110] demonstrated that using CO₂ at a pressure of 8 MPa has higher energy waste (57% higher) than applying 3 MPa pressure. Consequently, pressures exceeding 10 MPa have excessive costs and energy consumption and provide low advantage at an industrial scale.

Despite optimal conditions, the prediction of chemical equilibrium using gas-phase thermodynamics fails when experiments are conducted under reaction conditions. Both gas-phase and two-phase thermodynamic models show significant inconsistencies in CO₂ conversion at lower temperatures, as illustrated in Figure 8, where gas-phase equilibrium is depicted by dashed lines [111].

4.3. H₂/CO₂ Ratio

Utilizing CO₂ hydrogenation to DME is processed with excess H₂ to control the molar fraction of water within the reactor and minimize the negative impact of water adsorption on the catalyst [111]. Increasing the H₂/CO₂ ratio has a positive impact on the synthesis of DME from CO₂. This can be elucidated by considering the sensitivity of the methanol synthesis reaction (Equation (13)) to the partial pressure of H₂. Figure 9 illustrates the comparison of the equilibrium yield of DME, MeOH, and CO with an increasing H₂/CO₂ ratio. The MeOH and DME yields both are improved as the initial H₂/CO₂ ratio increases. However, the CO yield from DME production slightly decreases as the H₂/CO₂ ratio increases, while the CO yield from methanol formation slightly increases with a higher H₂/CO₂ ratio in the feed. Additionally, this figure indicates that the initial H₂/CO₂ ratio has a greater impact on the DME yield compared to its influence on the methanol yield. At H₂/CO₂ > 3, the conversions of CO₂ and methanol, as well as the selectivity towards DME are higher. However, the greater H₂/CO₂ ratio is beneficial to methanol selectivity and limits by products formation. A higher ratio of H₂ negatively impacts the economics of the process as it demands more H₂, thereby increasing the operational costs. Previous studies have suggested that a H₂/CO₂ ratio of 3 is an appropriate feed gas ratio for CO₂ hydrogenation [112].

4.4. Water Content Effects and Current Water Removal Strategies

In the direct synthesis of DME, water can be formed through RWGS reaction and methanol dehydration. At high temperatures, the RWGS reaction becomes favorable, consuming CO₂ and H₂ to produce water and reduce the selectivity towards DME [109]. Produced water can negatively effects on catalyst development such as the activity and stability of the catalyst. The production of water can also inhibit the production of methanol on the hydrogenation sites of catalysts due to the strong adsorption of water molecules on the surface of the catalysts, blocking the methanol production sites [113]. The high content of water resulting from the hydrogenation of CO₂ over the metallic component of the hybrid catalyst enhances the deactivation of the Lewis sites of the acid component due to strong water adsorption [114]. However, water is known to reduce the formation and deposition of coke over both functions of hybrid catalysts [115,116].

Water formation is a deactivation factor for metal oxides, and at high temperatures, it can convert the metal to metal oxide (Equation (18)) and induce the metal to lose its active valence. The production of CO, on the other hand, can transfer the metal oxide to the metal and CO₂ (Equation (19)), allowing for the recycling of CO₂ for utilization in hydrogenation.

$$\mathrm{C}\mathrm{u}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{u}\mathrm{O}+{\mathrm{H}}_2$$

(18)

$$\mathrm{C}\mathrm{u}{\mathrm{O}}_{\mathrm{x}}+\mathrm{C}\mathrm{O}\to \mathrm{C}\mathrm{u}+\mathrm{C}{\mathrm{O}}_2$$

(19)

Two strategies—(1) catalyst development and (2) reactor configuration—decrease the negative effect of water on the efficiency of CO₂ hydrogenation to DME. However, reducing the temperature of the endothermic RWGS process prevents the production of water, the optimum temperature depends on CO₂ activation.

The addition of hydrophilic promoters to the catalyst in the CO₂ hydrogenation process results in higher selectivity for DME by preventing the adsorption of water on active sites. As an example, the superior performance of the Cu-ZnO-ZrO₂ system can be attributed to the weak hydrophilic nature of zirconium oxide (ZrO), which hinders the strong adsorption of water, and increases surface basicity that facilitates CO₂ adsorption and methanol productivity [55]. Furthermore, the impact of the addition of CO to the H₂/CO₂ feed gas is demonstrated, revealing its inhibitory effect on the RWGS reaction. Instead, this addition promotes WGS reaction, leading to the consumption of water and the generation of CO₂, which serves as the feed gas for methanol synthesis [117].

From the reactor design, in situ membranes are one of the strategies to prevent water from degrading catalysts. It is still in its early stages, mostly limited to laboratory-scale investigations. Advancing to the industry scale of this technology remains a significant challenge. One of the primary obstacles to implementing water removal membranes on a large scale is low water selectivity [118]. Figure 10 depicts a schematic of a membrane reactor designed for the CO₂ hydrogenation to DME [118]. There is a scarcity of experimental studies on the DME synthesis process using a membrane reactor. Yue et al. [119] investigated the selectivity of DME in fixed bed reactor and membrane reactor and found that DME selectivity is enhanced from 53.7% to 100% by utilizing membrane reactor. Villora-Picó et al. [120] employed a membrane reactor during the CO₂ hydrogenation reaction to address water formation and establish a moisture-free setting. Their research revealed a significant enhancement in CO₂ conversion using CZA catalyst supported by HZSM-5, resulting in a 3–5-fold increase in CO₂ conversion. This boost consequently elevated the selectivity of DME from 12% to 54.5%. The DME yield increased from 8.71% to 22.8%, and CO₂ conversion rose from 21.4% to 33.7%. Rodriguez-Vega et al. [121] demonstrated the advantages of utilizing a membrane reactor equipped with a zeolite membrane, which increased DME yield from 2.8% to 6.57% and CO₂ conversion from approximately 22.5% to 35% at 325 °C using a small catalyst bed. Qiaobei Dong et al. [118] conducted research on a prototype catalytic membrane reactor for DME synthesis via CO₂ hydrogenation over Cu-ZnO-ZrO₂-Al₂O₃/HZSM-5 catalyst. In comparison to a fixed-bed catalytic reactor, the catalytic membrane reactor is equipped with notably improved DME yield and CO₂ conversion. However, experimental research on CO₂ hydrogenation into DME using a membrane reactor requires further understanding and testing for clarification.

Additionally, Figure 11 examines CO₂ conversion and DME selectivity in different types of reactors, including catalytic fixed bed reactor (CFBR), catalytic membrane reactor (CMR), packed bed membrane reactor (PBMR), and catalytic non-permselective membrane reactor (CNMR) [119]. It is shown that CMR has higher conversion of CO₂ with a 100% selectivity for producing DME under the conditions of 280 °C and 3 MPa.

In addition to considering the thermodynamic conditions, investigating the mechanism of reactions helps researchers in the development of effective catalysts. The main emphasis of the next section is on understanding the reaction mechanism of direct CO₂ hydrogenation to DME.

5. Reaction Mechanism

In the process of CO₂ hydrogenation to DME, it is important to understand the mechanism of the reaction. The mechanism refers to the sequence of steps and intermediate species involved in the transformation of CO₂ and H₂ into DME. The effective parameters for product selectivity are the binding energy of adsorbed species and the energetics of intermediate reactions between the reactant and catalyst. Numerous researchers have examined the mechanisms involved in CO₂ hydrogenation to DME, however, the mechanism at the molecular level lacks comprehensive understanding [122,123,124]. Understanding the sequence of elementary steps in this process is crucial for improving the development of new and more effective catalysts for methanol synthesis and DME production. There are two mechanism steps in direct CO₂ hydrogenation to DME—Step 1: CO₂ to MeOH and Step 2: MeOH to DME

5.1. CO/CO₂ Hydrogenation to MeOH

In Step 1, CO₂ is adsorbed via metal oxide to generate MeOH, and its activation follows two pathways, as depicted in Figure 12.

Two possible routes include:

• Mechanism of formate: When CO₂ directly combines with H atom from H₂, it produces formate molecules (*HCOO or *COOH), which then generate *OCH₃ (methoxy) and with further hydrogenation produce MeOH.
• Mechanism of RWGS: CO₂ is transformed into CO and subsequently into MeOH through hydrogenation intermediates.

It is crucial to understand whether a particular mechanism is the limiting step or if multiple mechanisms occur concurrently. Methanol formation from CO₂ does not take place in a consecutive sequence involving RWGS reaction followed by CO₂ hydrogenation. Instead, it seems that RWGS and CO₂ hydrogenation via formate occurs via parallel pathways, which means that different active surface sites are responsible for each reaction. Early studies initially supported the formate pathway, suggesting transformation to dioxymethylene (CH₂O₂*), formaldehyde (CH₂O*), and then methoxy (CH₃O*) species. However, only formate and methoxy intermediates were observed.

In this case, to understand the reaction mechanism, in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) characterization was used to provide a deep understanding of which pathway is dominant. This characterization analyses the surface adsorption and intermediate species interaction with the surface of the catalyst. Yuhao Wang et al. [125] examined DRIFTS measurements for CO₂ hydrogenation to DME over Cu–ZnO–ZrO₂ catalysts under reaction conditions (250 °C and 3 MPa). They revealed that CO₂ hydrogenation to methanol on CZZ catalysts follows the formate pathway. The results of DRIFT spectra are shown in Figure 13a,b, revealing strong bands at 1589, 1386, and 1362 cm⁻¹, corresponding to the formate species. Over the reaction period, the intensity of formate bands slightly decreases while methoxy (at 2930 and 2821 cm⁻¹, and C–O at 1145 and 1045 cm⁻¹) gradually increases. This suggests that methoxy likely originates from the hydrogenation of formate species. Furthermore, water vapor bands (1400–1800 cm⁻¹), which were observed after 10 min of reaction, are possibly produced from the hydrogenation of formate. DFT calculations validate these results, indicating that active sites for CO₂ adsorption and conversion reside at ZnO–ZrO₂ interfaces. Additionally, Sheng et al. [99] investigated the reaction mechanism using DRIFTS on a CuZnZr/ferrierite hybrid catalyst. Methanol formation was observed through bidentate-formate (b-HCOO) species undergoing sequential hydrogenation steps through a formate pathway, while DME is formed from methanol via interactions with surface methoxy species. Tabatabaei et al. [126] detected formate species in CO₂ hydrogenation experiments. They found bidentate formate as an intermediate for the RWGS reaction and monodentate formate for CH₃OH synthesis on ZnO from CO₂/H₂ feeds. In situ DRIFTS has indicated that a significant presence of formate intermediates enhances the conversion process. Additionally, formate intermediate is unstable and quickly hydrogenated into methoxy groups contributing to improved performance in DME formation. Previous research on various bifunctional Cu-based catalysts has established that the formate pathway is the predominant mechanism for CO₂ hydrogenation to produce MeOH and DME [127,128,129].

5.2. MeOH Dehydration to DME

Step 2—the MeOH-to-DME dehydration step, which is mostly conducted over the most popular support ZSM-5—involves two main mechanism pathways [130]. Both pathways are catalyzed by Brønsted acid sites within the zeolite structure. The location of Brønsted acidic sites impacts methanol dehydration, and neither the associative mechanism nor the dissociative mechanism is superior.

• MeOH associate mechanism: Two co-adsorbed methanol molecules at the Bronsted acid sites associate into DME and water.
• MeOH dissociate mechanism: One molecule of adsorbed methanol at the Bronsted acid sites is dissociated into a surface methoxy species by losing water and then reacts with another methanol molecule to form DME.

Despite extensive research on reaction kinetics [130,131], the predominant mechanism, whether associative or dissociative, remains inconclusive. Figure 14 illustrates different intermediates and transition states present in the conversion process of methanol to DME for both pathways.

Numerous experimental and computational studies in the literature have aimed to cover the favored mechanism of methanol dehydration to DME on acidic zeolites. Blaszkowski et al. [132] suggested that the associative pathway is the primary mechanism, supported by lower activation barriers estimated through density functional theory (DFT) calculations on zeolite clusters. Jones et al. [133] have used in situ IR spectra along with DFT and found that associative routes are responsible for the formation of DME on zeolitic acids. However, at higher temperatures and lower pressures, dissociative routes become more prevalent. This occurs because these dissociative routes feature smaller transition states with higher enthalpy and higher entropy compared to those found in associative routes. Ghorbanpour et al. [134] conducted a detailed study using DFT to investigate the methanol-to-DME reaction on H-ZSM-5 considering both associative and dissociative pathways. They found that at higher temperatures, the increase in entropic contributions leads to a transition temperature for each active site location. Beyond this temperature, the dominant mechanism shifts from associative to dissociative.

Figure 14. Dissociative (left) and associative (right) reaction pathways for the conversion of methanol to DME catalyzed by an H-form zeolite [134].

Figure 14. Dissociative (left) and associative (right) reaction pathways for the conversion of methanol to DME catalyzed by an H-form zeolite [134].

Figure 15 illustrates the schematic of CO₂ adsorption to form methanol and DME over Cu/Zn/Zr/Al₂O₃. In a dual-site mechanism, H₂ dissociates on Cu sites and CO₂ adsorbs dissociatively and reacts with H to form intermediate formate species (b-HCOO and m-HCOO) [135]. The spillover of hydrogen from metal oxide to activated CO₂ results in either monodentate (m-HCOO-) or bidentate (b-HCOO-) formates. The b-HCOO species react with H to form HCOOH, which is further hydrogenated to H₂COOH. Then, H₂CO and OH are formed by decomposing H₂COOH, while H₂CO undergoes consecutive hydrogenation (preceding methoxy (CH₃O-)) to form H₃COH. The desorbed methanol molecules via acid sites of zeolite produce methoxy groups, and finally, two surface methoxy groups interact to form DME and water. Moreover, the water molecule dehydrogenates to the intermediate and transfers back the H to the support to induce an acidic site on the support surface. The production of water is an important effecting parameter on the activation of reactants and making possible pathways through carboxyl intermediates. Other mechanisms, such as carbonate formation and carboxyl formation, occur concurrently with the primary mechanism, which involves MeOH dehydration to DME [136]. In carbonate formation, adsorption of CO₂* with O* via the hydrogenation process forms m-HCOO species, and then it decomposes to CO and OH [135]. A small amount of CO is formed by RWGS, leading to methanol production. The rest of the CO desorbed from the surface does not affect methanol selectivity. As a result, promoting Cu-based catalysts with other promoters will improve the binding of CO and intensify its hydrogenation to HCO-, simplifying methanol synthesis [137]. Several works have reported the enhancing effect of numerous metal oxides on the reactivity of copper [18,138,139,140]. These reports highlight the structural, chemical, and electronic impacts of these metal oxides on copper, resulting in increased exposure of the copper surface and unique reactivity. The widely studied promoter Zn has been extensively discussed for its synergistic effects with Cu. Arena et al. [138] suggested that ZnO might serve as a reservoir for atomic hydrogen, thereby accelerating the hydrogenation of intermediates. Additionally, they proposed that ZnO could potentially confer a distinctive morphology to the copper particles or create additional active sites on the copper surface. Shyam Kattel et al. [18] through experimental and theoretical investigations, have demonstrated that ZnCu undergoes surface oxidation under reaction conditions, resulting in the conversion of surface Zn to ZnO. This conversion allows ZnCu to attain comparable activity levels to ZnO/Cu, with similar Zn coverage. Fujita et al. [139] studied the mechanism of Cu/ZnO/Al₂O₃ in producing methanol. They found that the carbonate formation mechanism yields the best catalytic performance, as it facilitates the hydrogenation of surface intermediates and allows moderate CO bond formation without causing any poisoning or gas formation [141]. In another investigation, Studt et al. [140] showed that the presence or absence of Zn greatly influenced on both the activity and reaction mechanism. Using DFT calculations, they found that in CO₂ hydrogenation, intermediates attach to the surface through oxygen atoms, with Zn acting as a promoter. However, in CO hydrogenation, intermediates bind via carbon atoms, and a complete layer of Zn obstructs these sites, inhibiting CO hydrogenation.

5.3. Spillover of H Atoms

In addition to the effect of CO₂ adsorption on the reaction mechanism, the process of spillover, in which hydrogen migrates to the adsorbed CO₂, plays a critical role in the formation of intermediate species. The preceding section illustrates that the spillover of hydrogen from metal oxide to activated CO₂ leads to the formation of formates, which holds significance. Spillover is the transportation of adsorbed or formed species from one surface to another surface that does not adsorb them under the same conditions. The investigation of spillover was started in 1964 by Khoobiar and his colleagues [142], who reduced WO₃ by H₂ to WO₃₋x in contact with Pt catalyst. During the process, over the dissociative chemisorption of H₂ molecules on the Pt particles, H atoms were transferred from the Pt (active site) surface to yellow WO₃ particles (support) and produced blue WO₃₋x. Hydrogen spillover is the transportation of H atoms that are created on a metal surface to the surface of the support (from a hydrogen-rich to a hydrogen-poor surface) [143]. This occurs when the difference of free enthalpy between the initial state and the final state is negative.

From the catalytic point of view, spillover of hydrogen is beneficial to increase the efficiency of the overall catalyst. In all catalysts and bifunctional catalysts, metal is responsible for supplying H atoms to spillover to the support [144]. H atoms bonded with active sites on the catalyst are much stronger than those bonded with the support material. Spillover of H atoms from a metal surface to a non-reducible support—such as alumina, silica, silica–alumina, and zeolites—has been studied by researchers [144]. A support with defect sites is suitable for adsorbing H atoms as long as the distance between defects is not too large. However, spillover of H atoms from a metal surface to a defect-free support, such as zeolites, Al₂O₃, and SiO₂, is not possible because the migration of H atoms is highly endothermic [143]. Spillover from a metal particle to an oxygen-containing group on a carbon surface is also useful, such as in CO₂ hydrogenation, where spillover of hydrogen may react with coke and produce methane [145].

As shown in Figure 16, H atoms migrate from metal-to-metal oxide and the surface of the support to react with the metal oxide or with molecules adsorbed on the support.

• Effect of hydrogen spillover on CO₂ hydrogenation to MeOH

Numerous studies have explored the potential of spillover hydrogen as a novel method for hydrogenation reactions [146,147,148,149]. The rate of hydrogen spillover is a crucial factor that affects the speed of hydrogenation, with the decrease in free enthalpy between the initial and final states driving the thermodynamics of spillover. Studies have investigated the role of hydrogen spillover in CO₂ hydrogenation for the production of methanol, with various catalysts [150,151]. They have attempted to utilize the phenomenon of H₂ spillover to facilitate the formation of methoxy groups, which can potentially enhance the effectiveness of CO₂ hydrogenation [148]. Catalysts such as M/ZnO and M/ZrO₂ can facilitate the spillover of H atoms, which can then react to form active sites for the hydrogenation of CO₂ to methanol [152]. The hydrogenation of CO₂ and H₂ to formate species occurs on the Cu surface of Cu/ZnO or Cu/ZrO₂, with CO₂ hydrogenation primarily occurring on the Zn and Zr surface [153]. The spillover of H from Cu to Zn results in the formation of HCOO–Zn to CH₃O–Zn over Cu/ZnO, with methoxy species being released more quickly over Zn than Cu. Temperature program desorption (TPD) results, have indicated two peaks of H₂ desorption, suggesting that H atoms on the ZnO surface were formed by spillover from the Cu surface. In addition to trapping H atoms from the Cu surface to the ZnO surface to formate methoxy species, ZnO also facilitates the spillover of H atoms from one Cu surface to another Cu surface. In Cu/Zr catalysts, the m-ZrO₂ (monoclinic) phase has higher activity than the t-ZrO₂ (tetragonal) phase [113]. Cu/m-ZrO₂ promotes the formation of methoxide species due to spillover of H atoms from Cu to m-ZrO₂, which has a high concentration of anionic vacancies on its surface. These vacancies act as Lewis acid sites and enhance the Brønsted acidity of adjacent Zr–OH groups, which in turn enhances the adsorption of CO as HCOO–Zr groups, the precursors to methanol. Xaba et al. [154] studied the role of hydrogen spillover in CO₂ hydrogenation over Ga₂O₃ and ZrO₂ modified Cu-Zn catalysts. They found that Cu/Zn/ZrO₂ had higher spillover, resulting in higher methanol yield. The study also found that low dispersion of Cu reduces spillover of hydrogen, resulting in poor interaction between Cu and metal oxides and lower methanol production. In another study, Zhang et al. [150], investigated the addition of ZrO₂ to Ni/In₂O₃ to generate oxygen vacancies for CO₂ activation, while also improving the dispersion of Ni over the catalyst surface to facilitate hydrogen spillover. By reducing the distance between active sites and support, the spillover of H atoms is promoted, leading to full hydrogenation.

As the topic of utilizing hydrogen spillover in CO₂ hydrogenation is relatively new, there is limited research on the subject, and it is not yet thoroughly comprehended. Consequently, further research is necessary to fully understand its potential applications.

6. Catalyst Development

Several types of catalysts have been developed for CO₂ hydrogenation to MeOH and subsequently to DME. The first experiment on methanol synthesis used zinc chromium oxide as a catalyst [1]. However, in 1966, a Cu-based catalyst was developed to produce better methanol from CO/CO₂ hydrogenation. The challenge in producing DME was solved by using a multifunctional hybrid system that overcame obstacles such as mass transfer constraints, uniform distribution of active sites, and intimate contact between produced methanol and dehydration catalyst [105]. The ideal bifunctional catalyst for high selectivity and low coke generation consists of active phase, supporter, and promoters. Active sites convert CO/CO₂ to MeOH, while supporter, acting as an acidic group, produces DME from MeOH. A variety of transition metals, metal oxides, nitrides, sulfides, and carbides can be used as active phases [155]. In direct CO₂ hydrogenation, the bifunctional catalyst includes metal and acidic groups produce methanol and DME, respectively. DME is formed by dehydrating methanol in the presence of an acid catalyst. The appropriate bifunctional catalysts in direct DME production have a positive effect on dispersion, crystallite size, surface area of metallic sites, and nature of acid strength along with acid sites of supports. Optimizing the compositions of the two functional components is an important strategy for preparing a highly active hybrid catalyst. The ratio of metal and acid function is crucial in achieving higher DME productivity [156].

Several recent studies have investigated the use of various metal oxides and supporting materials to enhance CO₂ conversion, methanol production, and DME selectivity [157,158,159]. The first catalyst chosen by BASF in 1923 to convert syngas to methanol under high pressure was ZnO–Cr₂O₃ [160]. However, impurities like sulfur, chlorine, and heavy metals in syngas has a harmful impact on the catalyst’s activity. To address this, Imperial Chemical Industry (ICI) developed Cu/ZnO catalysts in 1966, which can operate at 5–10 MPa pressure and 200–300 °C temperature, and by incorporating ZnS, the catalyst is protected from poisoning [157]. ZnO has an active lattice electron pair that facilitates methanol production [158]. When ZnO is added to a Cu-based catalyst, it improves the surface area and dispersion of Cu and hinders the poisoning of Cu by feed impurities. However, due to Zn’s basic sites, it neutralizes the acidity of supports, which can prevent bifunctional catalysts from producing by-products like heavy-weight hydrocarbons [159]. Cu-based catalysts are frequently employed for methanol synthesis due to their superior activity among bifunctional catalysts. As a result, further investigation has been conducted on Cu-based catalysts with the addition of promoters.

6.1. Component for MeOH Synthesis

Bimetallic catalyst: To improve catalyst performance in DME production, addition of promoters to Cu-based catalysts is a promising method. Promoters increase the dispersion of the metal and surface area, lower the reduction temperature, and decrease the crystallite size [161]. Promoters enable interactions between the major components and adjust the active sites of the catalysts. Two types of promoters, structural and electronic, can be used in bifunctional catalysts. Structural promoters hinder catalyst sintering and are inert in the process, while electronic promoters intensify the active sites and their reduction.

Promoters such as Zn [162], Zr [163], In [164], and Fe [165] are widely used to develop the dispersion of Cu and control the characteristics of the catalyst in CO₂ hydrogenation. The interfacial contact between Cu and promoters improves the electronic and structural properties of bifunctional catalysts, optimizing catalyst performance [166,167]. Godini et al. [168] improved the DME yield of the bifunctional catalysts CuO over HZSM-5 for direct CO₂ hydrogenation by adding Zn to obtain a selectivity of 65% and a yield of 12.5% of DME. Adding promoters to the catalyst enhances both metal dispersion and the specific surface area of the bifunctional catalyst. Doping ZnO to improve the interaction of Cu-ZnO and increasing the defects of the Cu phase are excellent strategies to develop successful CO₂ hydrogenation catalysts. Bifunctional catalysts of CuO/ZnO/HZSM-5 were synthesized with the co-impregnation method in a fixed bed reactor under the reaction conditions of 260 °C and 2 MPa [168]. The spaces between Cu are filled by the ZnO promoter to maintain the active Cu metal in optimal dispersion in the Cu/ZnO catalyst, which reaches 65% selectivity of DME. The interaction of Cu and Zn forms active sites like Cu⁺–O–Zn, which also acts as a binding site for adsorbing hydrogen. ZrO₂ is a good promoter instead of ZnO to increase CO₂ hydrogenation. ZrO₂ has the ability to activate the adsorbed CO₂, favoring the formation of CO₂* species. These CO₂* species can react with the H₂* species to form intermediate species (formate and methoxy) during methanol synthesis [169]. ZrO₂ also favors the formation of oxygen vacancies during reduction, which facilitates Cu dispersion and increases Cu-ZnO contacts. The interaction of Cu with Zr increases the stability of the Cu sites, explaining both the improvement in Cu dispersion and the increase in the Cu-ZnO contacts [56]. The type of crystallinity of ZrO₂ depends on the pH of the catalyst preparation, where tetragonal and monoclinic ZrO₂ are formed at pH = 10 and pH = 7, respectively. Monoclinic ZrO₂ is more active than tetragonal ZrO₂. By increasing the calcination temperature, the structure of ZrO₂ becomes more monoclinic [170]. Adding In₂O₃ to CuO supported on halloysite nanotubes (HNT) at 250 °C and 4 MPa can improve the DME selectivity and CO₂ conversion to 65% and 7.6%, respectively [171]. Forming an alloy between Cu and noble metals such as Pt, Rh, and Pd can also improve the activity of Cu-ZnO catalysts in the synthesis of methanol from CO₂. Din et al. [172] have studied the Mg promotion over zeolite-based copper nanocatalysts can also be studied to improve Cu dispersion and methanol selectivity.

Noble metals like Pd [173], Ag [174], Pt [175], and Ir [176] can be used as promoters in CO₂ hydrogenation. One factor contributing to the exceptional performance of noble metal catalysts in CO₂; hydrogenation is their distinctive electronic and geometric characteristics. Noble metals have a high affinity for electrons, which allows them to activate the CO₂ molecule and facilitate its conversion into products [177]. Additionally, their high surface energy and unique crystal structure can provide a favorable reaction environment for CO₂ hydrogenation. Saravanan et al. [178] revealed that adding Pd to Cu/SiO₂ intensifies the CH₃OH formation rate, suggesting that CO₂ is the main reactant for methanol rather than CO. Pd/In catalyst has a higher methanol synthesis—about 70%—but it produces considerable CO in the hydrogenation process and methane instead of methanol [179]. Additionally, Pd/CeO₂ was investigated by Fan et al. and has higher activity with a longer lifetime [180]. However, Pd-based catalysts have lower activity than Cu-based catalysts [181]. Addition of Ag to a Cu-based catalyst in CO₂ hydrogenation to methanol exhibited high activity of the catalyst [182]. Increasing Ag concentration intensifies the methanol selectivity to 61%. Koppel et al. [183] exhibited that higher selectivity of methanol will be achieved by Ag/ZrO₂ compared to Cu/ZrO₂. Doping Ag to Cu/ZrO₂ can also increase methanol selectivity, but it should be noted that a higher concentration of Ag in dispersion may reduce methanol selectivity due to the relationship between Ag dispersion and the rate of RWGS reaction [184]. Figure 17 demonstrates the proportion of different promoters used for CO₂ hydrogenation in recent studies of catalysts, from 2012 to 2023. The most commonly used promoters in methanol synthesis over bifunctional catalysts are Zn, Zr, and Fe.

Trimetallic catalysts: The utilization of trimetallic catalysts can control synthetic and characterization reactions, as the combination of three different metallic elements can result in distinctive catalytic properties that cannot be achieved by a single-metal catalyst or a bimetallic catalyst. For instance, one metal component of the trimetallic catalyst may increase the surface area for catalysis, while another may enhance the catalytic activity, and the third metal may improve catalyst stability. Using trimetallic catalysts in CO₂ hydrogenation to DME can yield unique catalytic properties that can be harnessed to optimize reaction conditions low temperature and pressure. This approach enhances CO₂ conversion and increases the yield and selectivity of DME over a long lifetime.

The addition of various promoters—such as Zr [185], Ga [186,187] Ag [188], Cr [189], Mn [190], Pd [167], Ti [191], Fe [192,193], Ce [194], and Mo [189]—is becoming more popular for modifying catalysts used in direct DME production. Recent research has shown that adding ZrO₂ as a promoter to Cu-ZnO catalyst leads to a more active catalytic system that achieves high DME selectivity [185]. Singh et al. [195] investigated the effect of ZrO₂ promotion on the physicochemical properties of Cu/ZnO-based catalyst and achieved 60% DME selectivity and 14% CO₂ conversion at a temperature of 260 °C and a pressure of 3 MPa. Cu-ZnO-ZrO₂/HZSM-5 (CZZ/HZSM-5) catalyst, which was developed through in situ growing HZSM-5, achieved 34.1% CO₂ conversion and 67.7% DME selectivity at 250 °C and 5 MPa [196]. Frusteri et al. [197] accomplished Cu-Zn-Zr/Ferrierite under the reaction conditions of 280 °C and 5 MPa, which resulted in 28% CO₂ conversion and 70% DME/MeOH selectivity. Gallium oxide (GaO) is another promoter to Cu/Zr that can increase Cu-based catalyst activity and change Cu state to Cu⁺ while controlling the ratio of Cu⁺/Cu⁰ to keep it constant. The Ga promoter also enhances catalytic properties through the inverse spillover of hydrogen, which has a lesser promoting effect compared to the Zr promoter. The single-step microwave-assisted preparation method was used to prepare Cu/Zn/Ga, which resulted in high methanol production by increasing copper dispersion [184]. Akula et al. [198] compared three promoters of Ga, La, and Y (yttrium) in increasing the DME yield and found that yttrium-modified Cu-Zn-Al showed better activity with a DME yield of 47.7%. In a study by Tao et al. [189], the hybrid catalyst Cu–ZnO–Al₂O₃–Cr₂O₃ with ZSM-5 was found to have relatively high activity, with 90% selectivity of DME and 26% DME yield at 250 °C and 3 MPa. The study showed that the size of the copper particles was crucial in preventing side reactions, and a balance between textural, structural, and surface factors is needed to design an efficient multi-functional system for CO₂ conversion to DME [199]. Huang et al. [167] investigated the addition of Pd as a promoter on Cu-ZnO-Al₂O₃, which reduced the oxidation of active Cu by CO₂ and increased the stability of active Cu, resulting in intensified CO conversion and DME yield. Sun et al. [172] studied a Pd-modified CuO/ZnO/Al₂O₃/ZrO₂/HZSM-5 catalyst, which resulted in high DME selectivity (74%) and low CO formation.

CO₂ hydrogenation was investigated on CuO-Fe₂O₃-CeO₂ with HZSM-5 as support, and different amounts of CeO₂ were added to analyze CuO dispersion, CuO crystallite size, and the specific surface area of the catalyst [200]. By adding CeO₂, both the number and intensity of acid sites were increased, resulting in enhanced selectivity of DME and CO₂ conversion to 63.1% and 20.9%, respectively. Ren et al. [185] developed a study on the addition of La and Ce as promoters to Cu-Fe/HZSM-5, which resulted in higher performance in direct CO₂ hydrogenation compared to using Cu-Fe-Zr/HZSM-5, with CO₂ conversion at 18.1% and DME selectivity at 52.0%. The study showed that Zr, La, and Ce affect the catalytic activities of the catalyst by exchanging electrons with CuO, with Ce having the greatest effect on the outer shell electron in decreasing the density of Cu. As a result, the strength of acid sites and increase in Cu particle sizes enhances catalytic performance by reducing CO selectivity. Xiao et al. [201] synthesized CuO-ZnO with the addition of TiO₂ to improve methanol production, with optimum TiO₂ loading at 10%, resulting in CO₂ conversion and methanol selectivity of approximately 15% and 52%, respectively. Wang et al. [191] synthesized CuO-TiO₂-ZrO₂ catalysts with various Ti and Zr molar ratios mixed with HZSM-5 zeolite to synthesize DME at 200 °C and 2 MPa and showed that Cu-based catalysts with Zn and Zr as promoters had the highest selectivity of DME.

Table 3. Multifunctional catalyst in CO₂ hydrogenation to DME.

Catalyst	CO2 Conversion (%)	DME Selectivity (%)	Temperature (°C)	Pressure (MPa)	H2/CO2	Time (h)	GHSV (mL/g/h)	Ref.
Cu/Zn/Al2O3	NR	59	220	4	H2/CO=1	14	15,000	[167]
Cu/Zn/ Al2O3/SAPO-18	10	68.9	275	3	3	30	NR	[202]
C/ Cu/Zn/ Al2O3/HT-1/HZ	28	48	260	3	3	NR	2400	[203]
Cu/Zn/Ce/SAPO-34	4.8	60.8	240	1	3	20	3000	[204]
Cu/Zn/ZSM-5	20	65	260	2	3	NR	200 1/h	[56]
Cu/Zn/Al/(Al–Zr1:1)-SBA-15)	22	70	240	3	3	100	1500	[205]
Cu/Zn/Al/FER	19.8	37.8	250	3	3	36	600	[206]
Cu/Zn/Mg/γ-Al2O3	50 (XCO + CO2)	83	260	4	H2/(CO + CO2) = 3	72	2000 gcat/h	[207]
Cu/Zn/Zr/Al2O3	26.5	69.2	240	2.7	3	100	472 1/h	[185]
Cu/Zn/Zr/H-ZSM5	14.2	60	260	3	3	8	2400	[195]
Cu/Zn/Zr/HZSM-5	34	68	250	5	3	NR	2400 NL/g/h	[196]
Cu/Zn/Zr/HZSM-5	22.2	67.6	250	3	3	3	3600	[208]
Cu/Zn/Zr-WOx-ZrO2	18.5	63.3	240	3	3	10	NR	[209]
Cu/Zn/Al/Cr/HZSM-5	15	90	250	3	3	350	6150	[189]
Cu/Fe/Ce/HZSM-5	20.9	63.1	260	3	4	NR	1500	[200]
Cu/Ti/Zr/HZSM-5	15	52	250	3	3	4	2400 1/h	[201]
Cu/Ti/Zr/HZSM-5	12.93	48.1	250	3	3	4	1500 1/h	[191]
Cu/Zn/Zr/Al/Pd/HZSM-5	18.60	73.6	200	3	3.3	NR	1800 1/h	[210]
Cu/Zn/Zr/ferrierite	23.60	47	220–260	5	3	NR	8800 Nl/Kg/H	[211]
Cu/Al2O3 (Mesoporous)	72 (XCO)	69	310	5	H2/CO = 2	15	NR	[169]
Cu/Zn/Zr/MFI Zeolite	23.6	49.3	240	5	3	NR	10,000	[212]
Cu/Zn/Zr/MFI Zeolite	4.2	71	200	5	3	NR	8800 NL/Kg/h	[197]
Cu/Zn/Zr/MOR Zeolite	5.2	78	200	5	3	NR	8800 NL/Kg/h	[197]
Cu/Zn/Zr/FER Zeolite	5.6	79	200	5	3	NR	8800 NL/Kg/h	[197]
Cu/Zn/Zr/FER Zeolite	8	91	250	5	3	200	18,000	[213]
Cu/Fe/Ce/HZSM-5	18.1	52	260	3	4	15	1500	[194]
CuO-Fe2O3-CeO2/HZSM-5 MM	20.9	63.1	260	4	4	NR	1500 W	[200]
Cu/Fe/La/HZSM-5	17.2	51.3	260	3	4	15	1500	[194]
Cu/Fe/Zr/HZSM-5	28.4	64.5	260	3	5	16	1500	[192]
Cu/Zn/La/Al2O3/HZSM-5	43.8	71.2	250	3	3	8	3000 1/h	[214]
Cu/Zn/Al2O3/γ-Al2O3	62 (XCO)	67.4	225	5	H2/CO = 2	NR	NR	[215]
12Cu–6Mo/HZSM-5	12.36	77.19	240	2	3	10	1500 1/h	[216]
CuO-ZnO-Cr2O3/HY (50:50) MM	24.2	86.6	250	3	3	NR	1800 W	[217]
MK-121/Al-MCM-41	14	76	260	5	3	5	2000	[218]
Pd/Zn/ZSM-5	3.5	73.4	190	2	3	20	NR	[219]
Pd/Zn/Ti/ZSM-5	13.3	37.6	270	2	3	20	NR	[219]
CuO/TiO2/ZrO2/HZSM-5 PM	15.6	47.5	250	3	2.8	NR	1500 W	[191]
Nano Pd-In2O3/HZSM-5	9	44.1	295	3	3	NR	NR	[220]
Cu-Ga/Ce-Zr	14	30	300	5	4	NR	15,600 1/h	[220]
CuO-ZnO-Ga2O3/H-Ga-silicate PM	19.4	19.9	250	2.8	3	NR	33.33 gcat·h/mol	[187]
CuO-ZnO- Ga2O3/SAPO-34 PM	19.6	19.4	250	2.8	3	NR	33.33 gcat·h/mol	[187]

NR = not reported. MM = mechanical mixing. PM = physical mixing.

summarizes the most practical multifunctional catalysts in CO₂ hydrogenation to DME, showing that Cu-based catalysts with Zn and Zr as promoters have the highest selectivity of DME.

In addition to promoters, other factors also play a crucial role in developing high-quality bifunctional catalysts, such as method of catalyst synthesis, precipitation and calcination temperature, and pH of precipitation. Baltes et al. [221] investigated the impact of these parameters on Cu/ZnO/Al₂O₃ catalysts to achieve optimal methanol productivity and high selectivity of DME. The study revealed that catalysts synthesized from precursors precipitated at a pH range of 6–8 and a temperature of 70 °C exhibited the best catalytic performance. The optimum calcination temperature was found to be in the range from 250–300 °C. Moreover, Luyang et al. [208] investigated the effect of calcination temperature on the synthesis of CuO-ZnO-ZrO₂ with HZSM-5. Their findings showed that increasing the calcination temperature resulted in reduced DME selectivity, mainly due to a decrease in metallic copper surface area, CO₂ adsorption capacity, specific surface area, and CuO reducibility.

6.2. Component for MeOH Dehydration to DME

The main objective of support in catalyst design is to disperse active sites on the surface, maintain the thermal stability of the catalyst, and prevent the sintering of metal oxides [222]. The use of solid support has been shown to provide chemical, thermal, and mechanical stabilization to the catalytic species, resulting in higher catalytic performance. The porosity of the catalyst can also be controlled by the support, leading to high mechanical strength and a high surface-to-volume ratio.

In case of CO₂ hydrogenation to DME, the support has a significant impact on the selectivity and activity of DME [223]. In addition to addressing all the objectives of support mentioned, the measurement parameter of acidity is crucial for designing and developing effective catalyst systems for the dehydration of methanol to DME. The presence of weak and moderate acid sites (known as Lewis and Brønsted acid sites) in varying strengths and quantities determines the type of support used, such as zeolites, alumina, and silica. The surface of the support contains an acidic site responsible for converting methanol into DME. Increasing acidity can enhance DME production by strengthening the interaction between the metal and the support, as well as improving metal dispersion [224]. However, excessive acid strength can result in the formation of light olefins and high-molecular-weight hydrocarbons, which can eventually lead to catalyst degradation through coke formation [225]. Researchers can manipulate the degree of acidity by adjusting various parameters, such as: (1) calcination temperature, (2) ratio of acidic and basic sites, and (3) characteristics of metal oxides or their physical properties, such as blending different metal oxides. Methanol dehydration can be achieved using several acid catalysts, including γ-Al₂O₃; silica–alumina; zeolites such as HZSM-5, HY, HMCM-49, and HMCM-22; SAPOs; ferrierite; and Nb₂O₅ [226]. The following supports have been discussed in CO₂ hydrogenation to DME as the main supports in most research: zeolite, γ-Al₂O₃, and silica.

Zeolite: Zeolites are crystalline, microporous aluminosilicates composed of corner-sharing SiO₄⁻ and AlO₄⁻ tetrahedra that form an open cage-like “framework” structure. The complex structure of zeolites includes channels that can be classified as one-, two-, and three-dimensional [227,228]. Zeolites are highly stable solids with relatively high melting points (over 1000 °C) and resistance to environmental conditions that can challenge many other materials [229]. As a result, they have a wide range of applications as catalysts, adsorbents, and ion exchangers [182]. Studies suggest that zeolites are a viable option for solid acid catalysts in the methanol dehydration process due to their ability to withstand temperatures between 250–400 °C, pressures up to 5 MPa, and their acidic properties, which are a key factor in CO₂ hydrogenation to DME [230]. The performance of zeolite catalysts is greatly influenced by their specific characteristics, such as porosity, specific surface area, population, location, strength of acid sites, and interaction with active metals. Zeolites have higher surface areas (ranging from 100 to over 1000 m²/g) compared to other microporous materials. However, due to the size of pores in zeolites, DME may be trapped in the pores, resulting in the production of by-products that can negatively impact catalytic activity and DME efficiency [80].

Zeolites as solid acid catalysts in DME production contain various types of acid sites, including Brønsted and Lewis acid sites [80]. Generally, Brønsted acid sites are responsible for methanol dehydration to DME, while Lewis acid sites can promote the formation of by-products. The presence of both types of acid sites is required for high DME selectivity. The acid strength of these sites can be modified by the Si/Al ratio and the addition of metal oxides or other elements. HZSM-5 zeolite, which has a lower Si/Al ratio, is known for its high Brønsted acid sites and thus exhibits higher DME selectivity [231]. On the other hand, zeolites with higher Si/Al ratios can decrease the strong acid sites and result in higher DME selectivity [232]. The strength of acid sites can be modified using ions, alkali metals, transition metals, rare earth metals, and composite oxides [232]. Certain metals like MgO [233], CaO [141,234], ZnO [140,235], Sb₂O₅ [236,237], Zr [238], and Fe [239] can modify the zeolite by maintaining the acid strength toward weaker sites, resulting in an improvement in DME selectivity and stability. In pursuit of better surface distribution of metal oxides and more active sites, selecting the best catalyst synthesis has led to highly effective zeolite crystallite morphology [195].

Different types of zeolites, like ZSM-5, ferrierite (FER), MFI, and mordenite (MOR), were investigated in the process of CO₂ hydrogenation to DME. Krim et al. [5] explored that CuO/ZnO/Al₂O₃ with nano-sized hollow ZSM-5 zeolites led to a DME selectivity of 74% in a fixed-bed reactor operated at 225 °C and 3 MPa. In another study, Frusteri et al. [197] found that FER zeolite promotes better dispersion of cluster oxides, generating Lewis basic sites for CO₂ activation and easier accessibility of Brønsted acid sites for the MeOH-to-DME dehydration step, leading to higher selectivity. Bonura et al. [211] proposed a hybrid Cu/Zn/Zr/FER system for the synthesis of DME via catalytic hydrogenation of CO₂, achieving a CO₂ conversion of 23.6% and selectivity values to DME, MeOH, and CO of 47%, 15%, and 38%, respectively. A 2D ferrierite (FER) zeolite was used as a metal oxide carrier, considering that its bidimensional structure can help to better realize a more effective interaction among the neighboring catalytic functionalities responsible for primary methanol formation and consecutive dehydration to DME. In a study by Bonura et al. [240] two different zeolites—typical 2D ferrierite (FER) and 1D mordenite (MOR)—were compared. FER was found to promote more efficient mass-transferring of MeOH from Cu/Zn/Zr sites to the zeolite surface, resulting in higher selectivity and yield of Cu/Zn/Zr over FER zeolite compared to MOR and MFI zeolites. This result is due to more Lewis basic sites for CO₂ conversion to methanol and more accessible Bronsted acid sites for dehydration of methanol to DME. Cu/Zn/Zr with MFI-type zeolites were also evaluated in a fixed bed reactor under the conditions of 240 °C and 4 MPa, exhibiting high catalytic performance [212]. Moreover, the Si/Al ratio of 38 was found to be the best for tuning the acidity of the catalyst as well as MeOH conversion and selectivity to DME. Wang et al. [241] developed a ZSM-5/MCM-41 composite for the methanol dehydration catalyst, which combined the acidity of ZSM-5 with the channels of MCM-41 to create a mesoporous catalyst. As a result, the acid sites of mordenite and ZSM-5 are significantly stronger than those of FER zeolite, leading to the production of hydrocarbons with branches as the acid sites and their strength increase [242]. Y zeolite (SiO₂/Al₂O₃ = 6) having better performance than MOR (SiO₂/Al₂O₃ = 10) due to the presence of a higher number of moderate acid sites [185]. García-Trenco et al. [243] studied various zeolites, including ZSM-5, FER, IM-5, and MCM-22 and found that low external surface areas and Al concentration on the external surface are beneficial for DME selectivity, yield, stability, and minimization of by-product formation. These properties were found to strongly depend on the topology of the zeolites.

γ-Al₂O₃: γ-alumina or γ-Al₂O₃ is a type of alumina (aluminum oxide) that is commonly used as a support material in catalysts for DME production [244]. The high surface area and thermal stability of γ-Al₂O₃ make it a suitable support for various catalytic reactions, including DME synthesis. It provides a stable surface for the deposition of active metal species, which in turn promotes the desired reaction. Additionally, γ-Al₂O₃ has a high resistance to water, which is crucial for maintaining the stability of the catalyst during the reaction [244]. γ-Al₂O₃ has been found to have the highest activity for converting methanol to DME compared to other types of alumina such as α-Al₂O₃ and κ-Al₂O₃. This is because γ-Al₂O₃ has a higher surface area, approximately 150 m²/g [245].

Various research focuses on investigating the impact of using γ-Al₂O₃ as a support material on the performance of the DME production process. Sung et al. [245] investigated different crystalline phases of alumina and observed that dehydration of γ-Al₂O₃, creates metal cation and oxide anion as Lewis acid and basic sites, respectively that assist DME production in CO₂ hydrogenation process. However, it is important to note that γ-Al₂O₃ is amorphous and vulnerable to high temperatures, which can affect the stability of the catalyst. Suratno et al. [55] developed the Cu/Zn/Al over γ-Al₂O₃ as a bifunctional catalyst in CO₂ hydrogenation process which resulted in 36% yield and 36.53% selectivity of DME. This study also examined that Lewis acid-base pairs on the surface of γ-Al₂O₃ are the main acid sites responsible for methanol dehydration [246].

Unlike zeolites, the γ-Al₂O₃ support is known to have a wider pore size distribution and a lower surface area, but it has higher mechanical stability [247]. However, the presence of water molecules can block the active sites of γ-Al₂O₃, which can hinder the adsorption of methanol, leading to lower activity compared to zeolites. Huang et al. [167] compared the use of two different supports, ZSM-5 and γ-Al₂O₃, in a fixed bed reactor under the reaction conditions of 260 °C and 5 MPa. They found that CO₂ conversion was 10% higher when ZSM-5 was used as an acidic group compared to γ-Al₂O₃. Huang et al. [167] investigated the ranges of reaction temperature and H₂/CO₂ ratio in CO₂ hydrogenation over CuO/ZnO, supported by Al₂O₃ and HZSM-5, and found that the maximum DME yield was 2.3 g/g_cat·h. They also examined the acidity of three dehydration catalysts and found the order of acidity strength to be ferrierite > HZSM-5 >> γ-Al₂O₃.

While alumina is commonly used as a support material for methanol production through CO₂ hydrogenation, it exhibits lower stability over prolonged reaction periods and has a hydrophilic nature that promotes water adsorption [248]. Since the acidic sites present on the support play a crucial role in converting methanol to DME, thus maximizing the yield and selectivity of DME, it becomes necessary to modify these solid acids. As a result, significant research has been done on modifying γ-Al₂O₃ with metal oxides. The activity of γ-Al₂O₃ was enhanced by adding promoters such as silica [85]; niobia [86]; and anions like fluorine [249], sulphate [250], and phosphate [251]. The promotion of strong acid sites on the support increases the WGS reaction to produce CO₂. Takeguchi et al. [252] conducted research on the effect of addition of silica on γ-Al₂O₃ and found that a higher concentration of silica resulted in increased surface area and Brønsted acid sites, leading to better performance of CO₂ hydrogenation process. By modifying the alumina support with niobia and using a CuO-ZnO-Al₂O₃ catalyst, the acidity of the catalyst increases, and the extent and strength of CO adsorption on the surface of γ-alumina decreases [190]. The researchers found that even a small amount of niobia loading on alumina resulted in better performance than pure alumina [190]. The surface area and acidity of alumina can be changed by using NH₄F solution, which was found to be the best for both methanol dehydration and DME production [249]. Treating γ-Al₂O₃ and HZSM-5 with formaldehyde and sodium carbonate increases the number of weak acid sites, leading to an increase in DME selectivity from 13.3% to 59.7% [253].

6.3. Catalyst Preparation Technology

Conventional catalyst synthesis technology: Creating a bifunctional catalyst involves a detailed approach to designing and preparing its catalytic functions for both CO₂ to methanol and methanol to DME. The preparation of bifunctional catalysts is a critical step that can significantly impact the performance of the process by ensuring suitable kinetic characteristics of metal and acidic sites for optimal reaction conditions as well as controlling the contact properties between the metal and acidic sites [101]. Unlike the physical or mechanical mixing of metal with the solid acid catalyst, bifunctional catalysts are prepared using various methods of synthesis, including precipitation, impregnation, sol–gel, and physical mixing synthesis [254,255]. Each synthesis method has its own set of benefits and drawbacks with regards to catalyst activity, selectivity, and stability, which are demonstrated in Figure 18.

The traditional methods of catalyst synthesis do not offer accurate control over properties such as surface area, pore size, and distribution, which may result in energy inefficiencies, and reduced potential for multifunctional catalysts. These limitations can affect the catalyst’s activity, selectivity, and stability, leading to suboptimal dispersion of the catalyst material and limiting its performance. As a result, researchers are investigating alternatives that can achieve high surface area while also ensuring optimal metal loading and dispersion. One of the emerging technologies in catalyst synthesis involves the use of additive manufacturing methods to create 3D printed catalysts, which will be further discussed in the next section.

Advanced catalyst synthesis technology: Additive manufacturing, also known as 3D printing, reforms the creation of three-dimensional objects by adding material layer by layer based on digital designs. Its rapid advancements have significantly impacted various sectors, including manufacturing, architecture, and medicine. Recently, 3D printing has emerged as a novel approach in catalyst design, offering customizable heterogeneous catalyst fabrication [256]. Exploiting 3D printing technology in catalyst design presents advantages such as enhanced precision, coverage, and versatility at reduced costs. Initially introduced in the 1980s, 3D printing employs layer-by-layer material accumulation, enabling the production of diverse products with different material types [257].

The effectiveness of 3D-printed catalysts in CO₂ hydrogenation is noteworthy. By facilitating the creation of diverse structures of monolithic catalysts, 3D printing enhances mass and heat transfer within the catalyst, thereby improving reaction efficiency. The integration of metal oxide, promoters, and support materials into the printed material enables the construction of 3D-printed catalysts, such as honeycomb monoliths, which are effective in fabricating various metal/oxide heterogeneous catalytic systems. This approach supports catalyst recovery and operation in a continuous flow format, boasting advantages such as ease of preparation, high reactivity, recyclability, and minimal metal contamination [256].

The primary advantage of employing 3D printing in CO₂ hydrogenation lies in its ability to efficiently produce specialized catalysts on a large scale. Achieving high porosity and well-designed morphology of supports is crucial for enhancing the performance of CO₂ hydrogenation to DME, and 3D printing technology offers promise in controlling these factors during the structuring of 3D supports. While the fabrication of catalysts using 3D printing typically yields materials ranging from millimeter to micrometer sizes, challenges persist in printing nano- or atomic-sized catalysts [258]. Figure 19 illustrates an example of a 3D-printed catalyst utilized in CO₂ hydrogenation.

Recent studies have shown that direct 3D printing of metal oxides alongside H-ZSM-5 can modify surface properties and bulk oxide phase dispersion, resulting in enhanced metal oxide reducibility and exceptional CO₂ hydrogenation performance [259]. Similarly, 3D-printed HZSM-5 had a positive effect on the conversion of methanol to DME by controlling the density of acidity and porosity, leading to significantly increased selectivity to DME, up to 96% [259]. Maghzoub et al. [260] conducted a study on 3D printed HZSM-5 catalysts, which showed that they can enhance the conversion of methanol to DME by controlling the density of acidity and porosity of the catalyst. This modification in acidity and porosity resulted in a significant increase in selectivity to DME, and they found that the DME selectivity was increased up to 96% with the use of 3D-printed HZSM-5. In another research investigation, they focused on assessing the performance of a 3D-printed catalyst composed of 4% Ga/ZSM-5 [261]. This catalyst demonstrated an impressive 85% MeOH conversion rate and a remarkable 74% selectivity for DME production. Notably, it exhibited no significant performance decline even after operating for 5 h, confirming its exceptional effectiveness as a catalyst for fuel synthesis. Bonura et al. [262] conducted a study to evaluate the efficiency of utilizing 3D-printed catalysts to produce hybrid catalysts for direct hydrogenation of CO₂ into DME. The study showed that the selectivity of DME was increased to 51.6% over the 3D-printed hybrid CuO-ZnO-ZrO₂/ZSM-5. Previous studies have not thoroughly examined this new topic, and there is a lack of literature on it, highlighting the need for further attention and consideration.

6.4. Catalysts Deactivation Issues

Despite extensive efforts to enhance catalyst stability, catalytic deactivation remains a challenge in CO₂ hydrogenation. This phenomenon involves a gradual decline in catalytic activity and selectivity, ultimately impairing process efficiency and product quality. Such challenges are paramount for researchers and industries engaged in CO₂ hydrogenation, as they can lead to escalated production costs and diminished product quality. The extent of catalyst deactivation varies based on catalyst type, synthesis method, and incorporation of promoters. Moreover, reactor configuration plays a crucial role in the deactivation mechanism. Thus, an overall comprehension of these factors is imperative for developing effective strategies to alleviate catalyst deactivation and ensure continuous stability and efficiency of CO₂ hydrogenation.

Deactivation mechanisms primarily include: (1) metal sintering, (2) acid sites, (3) water formation, and (4) coke deposition with prolonged operation. Notably, metal sintering, water formation, and acidity strength are pivotal among numerous factors influencing deactivation, as discussed below.

Metal sintering: Catalyst deactivation poses a common challenge in DME production using hybrid catalysts, notably copper-based ones, due to issues like sintering, Cu oxidation, and ion exchange. Strategies to stabilize Cu structures, such as adding hydrophobic promoters, are crucial for extending catalyst lifespan and enhancing performance. Lower reducing temperatures are suggested to prevent Cu sintering during hydrogenation [203]. Various techniques, including copper particle size reduction within mesoporous alumina and zeolite supports, have been proposed to improve catalyst stability in direct DME synthesis. Thermal sintering, a slow and irreversible process, requires careful control to maintain catalyst efficiency in industrial settings. Migration and sintering of copper in Cu-based catalysts with zeolite supports can reduce Brønsted acid sites, necessitating strategies like particle size reduction and zeolite structure modification [263]. Zeolite-induced coking adversely affects catalyst stability, underscoring the importance of controlling coking for sustained effectiveness. Zeolite morphology, particularly increased external surface area and surface Al concentration, influences catalyst deactivation, with strong acid sites converting DME components to heavy hydrocarbons and inducing coke formation [243]. Modification with sulphate species can mitigate coke formation by altering acidity strength and inducing weak Lewis acid sites [226].

Water formation: In CO₂ hydrogenation to MeOH and DME, water is generated via the RWGS reaction. However, the presence of water has both benefits and drawbacks. Among the advantages, water can act as a catalyst promoter, enhance active site dispersion, decrease coke deposition, increase stability, and optimize reaction conditions. While water can be absorbed by acidic sites, this is a minor effect and does not appear to impact coke formation. Conversely, the presence of water can occupy oxygen vacancies on active sites, potentially reducing methanol formation and DME production. Furthermore, excessive water content can negatively impact catalytic activity, while water adsorption may reduce acidity and catalytic activity.

Acidity strength: In CO₂ hydrogenation to DME, strong acidity in the catalyst support refers to the presence of strong acid sites, like Brønsted or Lewis acid sites, enhancing catalytic activity by activating reactants. However, excessive acidity can lead to catalyst deactivation over time, necessitating control of acid strength and reaction temperature, especially for Brønsted acid sites to prevent heavy hydrocarbon formation. Optimal catalysts must balance acidity and hydrophobicity to maximize methanol dehydration efficiency, as high acidity can produce inhibitory by-products. Zeolites such as H-ZSM-5 exhibit strong acid sites, while γ-Al₂O₃ supports offer moderate acidity, reducing by-product formation.

7. Reactor Configuration and Process Operation

The selection of reactor type for the direct synthesis of DME is based on various methods for controlling temperature and catalyst choice, which can influence the selectivity of DME production. Controlling the heat transfer during exothermic reactions is crucial for enhancing the selectivity of DME production. Inadequate heat transfer can result in localized high temperatures on the catalyst surface, which can lead to the formation of unwanted by-products. In addition, the RWGS reaction occurs more prominently at high temperatures, leading to increased steam production. This steam generation contributes to accelerated catalyst deactivation through sintering. Therefore, the reactor configuration is crucial to avoid catalyst deactivation and achieve a higher DME yield. Previous studies have investigated different reactor types for DME production [121,264,265,266,267]. Figure 20a,b illustrate the primary reactor types that are used in CO₂ hydrogenation to DME and the distribution of published papers for various configurations.

Table 4. Evaluation of conventional and advanced reactors in CO₂ hydrogenation process.

Conventional Reactor Types	Advantages	Disadvantages
Fixed-Bed Reactor	Simple and cheap design Facilitates the contact between the catalyst phase and the reactant phase Used on a laboratory scale or in pilot plants Remove any limitations in the diffusion process when gas and solid materials come into contact. Achieve an ideal temperature profile that is consistent from the inlet to the outlet of the reactor.	Poor efficiency of a single run in the process Costs associated with continuously feeding the reactants and operating the process Challenges associated with precise temperature regulation and cooling systems Deactivation of the metal catalyst through the deposition of coke and water; hot spot formation inside the reactor
2. Fluidized-Bed Reactor	High conversion of CO2 Low pressure drops Potential to utilize a significant quantity of catalyst Minor restriction on diffusion caused by the utilization of small particles Easily circulated for regeneration of catalyst Effective mixing to maintain constant temperature conditions and regulate it effectively Effective heat removal due to movement of the catalyst	Catalyst loss due to particle collision
3. Slurry Reactor	Lower costs associated with reactor materials and construction due to lower temperature and pressure Feasibility for large capacity without sacrificing product quality Commercial purposes due to lower energy requirements Higher process efficiency and reduced operating cost due to better heat transfer and mixing Precise control over the reaction due to online removal and addition of catalyst Higher efficiency due to slower transfer of bulk particles Proper temperature distribution and heat transfer due to the large heat capacity of the solvent leads to lower deactivation and lower capital expenditure Simple separation process and dosing catalyst	Require complicated equipment Mass transfer limitation between phases Reduction the bubble size due to increasing pressure Formation of aggregates, coking of catalyst, and abnormal distributions due to lack of wettability of the catalyst Changing the morphology of the catalyst under the influence of water Require more data for scaling up the reactor and optimizing the process
Advanced Reactor Types	Advantages	Disadvantages
Membrane Reactor	Improved process efficiency, higher selectivity, and better control of reactant concentrations Higher DME yield with improved process stability via removing water Better control of reaction conditions and preventing the build-up of reaction by-products Reduction in coke deposition and sintering due to higher catalyst wettability	Challenging to design and operate High-cost reactor due to membrane cost, energy consumption Difficulty in scaling up to larger production volumes Pore-blockage of membrane Membrane fouling and degradation due to high temperatures, harsh chemical environments, and mechanical stresses
2. Micro Reactor	Better heat and mass transfer due to the relatively high surface area to volume ratio, as well as the short distance to the reactor wall Precise control of reaction conditions Maintaining laminar flow Avoid problem of overheating with thermal instability Safe operation due to smaller amounts of reactants Reduced waste and a more sustainable process	Complex design with specialized equipment and expertise for fabrication and operation Limited production capacity Higher clogging or fouling due to small size of reactor Difficult in loading supports due to extensive agglomeration poses
3. Reactive Distillation (RD) or Catalytic Distillation (CD)	Higher selectivity of DME, higher conversion, and lower operation costs Reduction in capital and operating costs due to elimination separation unit Low design pressure leads to reduced energy consumption for compression and pumping, less stress on the equipment	Complexity of the design Restricted using in CO2 hydrogenation to DME due to operation in low temperature Fouling or poisoning by reaction products or impurities

illustrates the advantages and disadvantages of conventional and advanced reactor types used in CO₂ hydrogenation to DME. It includes the advantages and disadvantages associated with the reactor utilization in the process of CO₂ hydrogenation to DME, aligning with findings from prior studies. Despite extensive research in academia and the development of pilot-scale systems by some companies, the commercial implementation of these advanced technologies in large-scale industrial processes remains limited, necessitating further research and development to optimize their performance and scalability for industrial applications.

8. Conclusions

This review paper has comprehensively summarized the process of direct CO₂ hydrogenation to DME. This process has attracted significant attention from researchers investigating various aspects, such as thermodynamics, reaction mechanisms, catalyst development, catalyst synthesis techniques, catalyst deactivation, and reactor configurations. Clearly, CO₂ hydrogenation to DME typically involves catalysts comprising Cu/Zn/Zr/HZSM-5, but integrating them into 3D-printed catalysts has shown improvements in DME selectivity. A promising mechanistic pathway involves the formate intermediates to achieve higher yields. As observed in various studies, optimal operating conditions generally fall within the temperature range of 200 to 300 °C and pressure range from 1 to 3 MPa. Despite the significant progress made, challenges remain, including improving DME selectivity and developing catalysts that can minimize deactivation, making membrane reactors the preferred choice for water removal in this process. Additionally, the H₂ spillover mechanism represents promising paths for future research. Continued research in this area has the potential to advance the process of CO₂ hydrogenation to DME.