1 Summary

Dimethyl ether (DME) is an essential industrial product and serves as a viable replacement for petroleum diesel and liquefied petroleum gas (LPG). It produces fewer pollutants such as CO2, NOx and particulates. Different raw materials, which include natural gas, coal, oil and biomass, can be used to create DME. Out of this list, natural gas is viewed as the most suitable raw material to be initially transformed into synthesis gas (syngas) and then into DME. This conversion process takes place in two ways: the indirect route and the direct conversion route. The indirect route entails two reactors handling each reaction stage independently. In the first stage of the process, syngas is converted into methanol. The second step involves catalytically dehydrating the methanol to produce DME. The direct method, which uses a single reactor with bi-functional catalysts, is generally considered more feasible than other methods. However, designing efficient reactors and catalysts for the direct method can be challenging due to the need to handle multiple reactions in a single-stage system. This article examines the various methods for producing DME, the catalysts used in these processes, and the challenges involved.

2 Introduction

The onset of the 19th century initiated the development of a society largely dependent on petroleum products due to their abundant reserves. However, with the advent of the present century, global communities realized the limitations of petroleum reserves and their detrimental impact on the earth’s natural habitat. In the current scenario, the energy demand is ever-increasing owing to the growing population and technological advancements. There is a need to explore alternative fuels that can politically, economically, and environmentally save future generations. In recent years, dimethyl ether has attracted attention as a potential alternative to replace or balance natural fuel consumption [1, 2].

DME is the simplest form of ether. Ethers are those chemical compounds that constitute the functional linkage of C-O-C. DME is a synthetic gaseous fuel, with two carbon, one oxygen, and six hydrogen atoms like ethanol, therefore it is considered an isomer of ethanol [2]. DME burns with a bluish flame without forming non-peroxides. It is a volatile substance that transforms into liquid at pressures exceeding 0.5 MPa. DME possess physical properties similar to that of liquefied petroleum gases (propane or butane). Therefore, DME is mostly used as a liquified petroleum gas (LPG) for industry, transport, and home cooking. Likewise, methanol and ethanol, DME considered clean fuels due to the presence of C-H and C-O bonds and higher hydrogen-to-carbon ratio than typical natural gas, diesel and gasoline [3, 4].

A higher cetane number of DME makes it the best alternative fuel for transportation due to limited emissions of particulates and toxins like NOx [5]. Its vapor pressure is also similar to other LPG gasses; therefore, it could be employed in LPG-based transportation and can be stored without any alteration of the existing LPG infrastructures [6]. Hence, DME has promising potential to replace conventional naturally depleting fuels, especially diesel. DME is mostly synthesized from syngas, which is prepared using natural gas, coal, oil, and waste products. Among these available sources, natural gas is highly preferred due to its availability and lesser cost fluctuations like the crude oil.

3 Synthesis Methods

There are mainly two approaches to produce DME, as shown in Figure 1, indirect and direct. In the indirect approach, firstly, methanol is produced, which is then dehydrated and converted to DME. While in the second approach, which is more efficient, the synthesis of methanol and dehydration is done in a single stage using bifunctional catalysts. The methanol is produced using syngas (a mixture of H2 and CO). This syngas can be produced through gasification or steam reforming of hydrocarbons [7, 8].

Figure 1. Illustration of sources and methods of DME production [1]

3.1 Indirect Synthesis

Figure 2 shows the schematic illustration of the production of DME through the conventional two-step indirect approach. Initially, syngas is used to produce methanol, which is then converted to the required product after purification. Eq. (1) shows the chemical reaction involved in the commercial production of DME from methanol dehydration [1].

The dehydration of methanol using a solid-acid catalyst follows the mechanism defined by Langmuir-Hinshelwood or Eley-Rideal kinetic models.

Figure 2. Schematic illustration of the indirect method of DME production [9]

The rate equation determined for Eq (1) at a temperature above 250°C is given below [10]:

Where Pmethanol is the methanol partial pressure, K0 (1.21×106) is the methanol pressure measured in KPa. Kmol/m3 h KPa, Ea= 80.48 KJ/mol. Low temperature is favorable for methanol production as its formation is exothermic, while the by-products ethylene, CO, H2, and coke are produced at prevailing temperatures.

3.2 Direct Synthesis

The direct synthesis method for DME is a more recent method in which the synthesis of methanol from syngas and its eventual dehydration is done from syngas in a single stage, unlike the indirect approach (Figure 3).

The direct DME synthesis from syngas involves the following overall chemical reactions:

Eq. (3) shows DME production with water-gas shift reaction (WGS), while Eq. (4) displays its synthesis without WGS. The production of DME involves both methanol synthesis and the Water-Gas Shift (WGS) reaction. Methanol synthesis is a key step in the process and generates CO2 as a by-product. This CO2 can be used to produce syngas through a reaction with natural gas in the reforming unit, while maintaining a H2/CO molar ratio of 1.

The reactions involved in the production of DME from syngas are highly exothermic and temperature control measures are required to avoid any run-way situation during synthesis. DME is obtained after purification processes, in which the remaining H2, CH4, CO2, and N2 are removed through absorption, flash, and distillation processes. The feasible separation and purification process of DME is depicted in Figure 3. Methanol synthesis is a thermodynamic constraint step, therefore, methanol consumption shifts the reaction equilibrium towards higher DME production. In the presence of methanol, it is quite difficult to separate CO2 from DME. To tackle this issue, methanol is absorbed in water as a result of condensation, and then DME is obtained through the distillation of the liquid stream [11, 12].

Figure 3. Schematic illustration of the direct synthesis process of DME [13]

3.3 Direct vs Indirect Approach-A Comparison

The direct method permits higher CO conversion in a simple reactor, which makes the process cost-effective as compared to the indirect method.  However, the separation process of DME from unreacted syngas and CO2 is much more complicated than the indirect synthesis. Moreover, DME production is influenced by the WGS reaction that consumes much of the CO, limiting the conversion of DME. Therefore, the direct conversion of syngas to DME is not much suitable for commercialization. The production of syngas is an energy-consuming process and is not environmentally friendly. Nevertheless, there is a need to develop a more energy-efficient and environmentally friendly process. Much effort has been made to utilize the maximum amount of CO2 to produce various chemicals and products to neutralize the emissions of CO2 into the atmosphere. The conventional method is challenging to commercialize due to low selectivity and CO2 conversion. The thermodynamic limitation of DME and low yield of the conventional process can be overcome by the selective removal of H2O from a limited equilibrium reaction.

A two-step indirect approach is considered much more feasible to produce DME, in which methyl bromide is prepared by combining oxygen with hydrogen halide over a catalyst Rh-SiO2. The next step involves the conversion of methyl halide into DME by passing it over the surface of silica-based metal chloride. Here, methanol is produced as the by-product and a serious drawback of the process is the corrosion of plants due to the hydrolysis of corrosive halides.

4 Catalyst Preparation

Various types of catalyst synthesis schemes have been reported for improving DME selectivity and catalyst stability. It was found that the catalyst preparation method has a crucial effect on achieving desirable DME synthesis. At present, hybrid-type catalysts are employed for direct DME synthesis that are prepared using different methods, such as physical mixing, co-precipitation, impregnation, sol-gel, and ultrasound assistance sol-gel [1, 3, 14].

According to a few reports, the physically mixed catalysts exhibit higher activity than the co-precipitation and impregnation-based catalysts. The activity of sol-gel and co-precipitation derived γ-Al2O3 was compared in a few pieces of research, and it was concluded that the catalyst prepared with sol-gel exhibits higher activity. In addition, solvents for sol-gel were also compared, non-aqueous solvent was found to be more effective in producing better catalyst activity [14, 15]. The sol-gel method offers many benefits over other methods besides the conversion performance like high purity, the ability to prepare samples at low temperatures, easy control of particle size, and pore size/volume distribution [16].

Previously, it was believed that the composition of Cu/ZnO/Al2O3 catalyst i.e., Cu/Zn ratio has the main effect on the performance. However, recent researchers have shown that it does not imply a drastic impact on the conversion of CO2 and selectivity.  [17, 18]. Many researchers have concluded that the dispersing oxide matrix has an important role in the Cu/ZnO/Al2O3 system towards the catalyst’s conversion and selectivity performance. The dispersion of Cu over the surface of the support depends upon the surface morphology, surface area, and porosity [19]. The morphology, surface area, and porosity depend upon the catalyst preparation techniques. The conventional methods are not very effective in producing a higher feasible surface profile to achieve higher Cu metal dispersion Cu/ZnO/Al2O3 system [20].

In this regard, a research group Dasireddy and Likozar [21] have presented a few catalyst preparation methods and investigated their impact on methanol conversion and selectivity performance. In the study, co-precipitation, ultrasonic-assisted co-precipitation, and solid-state mixing methods were used. In all these methods, the ratio of Cu/Zn/Al was kept fixed at 50:30:20. Typically, in the co-precipitation method, the pH of dispersion and temperature was fixed at -8 and 60°C. The combined concentration of all metal ions was no more than 1 M. The integrated metal ions solution and sodium carbonate solution with a concentration of 1 M were added simultaneously in the reacting vessel with a minor amount of DI water. The obtained suspension was aged for 1 h before drying (12 h at 90°C) and calcination (300°C for 4h).

In ultrasonic-assisted co-precipitation, primarily the solution preparation method is the same as mentioned for co-precipitation above before adding in the reacting vessel. In this method, the two metal ions and sodium carbonate solution were transferred to the ultrasonic generator bath for 2h. The precipitates were dried and washed before calcination in air at 300°C for 4h.  in the sol-gel combustion method, the same precursors were used for the previously mentioned methods. A homogenous solution was prepared using all precursors, then added into the oxalic acid solution at 70°C with the ratio 1:1 with the total metal ion. Finally, the solution was treated at 300°C for 4h in an oven. In the end, in the solid-state mixing method, all precursor salts were first blended, with the addition of citric acid. The mixture was grounded in the mortar for 30 min. Later the mixture was dried and calcined at 300°C for 4h.

Table 1 shows the surface characteristics of Cu with the function of preparation methods, as mentioned above. It shows that the catalyst prepared with ultrasonication assistance possesses the highest surface area and the one equipped with the sol-gel combustion method the lowest. These results have shown that ultrasonication promoted the ZnO incorporation in the alumina matrix, which in turn exposed the total surface area by acting as a spacer. Moreover, the crystallite size of the prepared catalyst is more or less the same, indicating that ultrasonication assistance has reduced the degree of aggregation in particles. Therefore, after calcination, ultrasonicated catalyst particles were re-aggregated to a minor extent compared to other prepared catalysts [21].

Table 1. Surface characteristics of the catalyst prepared from different methods [21]

Moreover, according to another report, large surface area, defective Cu nanoparticles, and the number of reactive interfaces of Cu with ZnO are the main requirements for getting high activity. The successful achievement of the required microstructure of the catalyst depends upon the homogenous mixing of Cu and Zn species, which in turn depends upon the preparation method [22]. Like Dasireddy and Likozar [21], Kunkes, et al. [23] also suggested that homogenous distribution of cations from the initial catalyst preparation till the final implication is required to achieve maximum performance. That can be achieved best using the co-precipitation method. In the aforementioned work, it is evident that the impact of co-precipitation is further improved with the assistance of ultrasonication. The industrial-established co-precipitation method is given in Figure 11. It is composed of co-precipitation, ageing, mixing of Cu-Zn Al hydroxy carbonate precursors, thermal decomposition and finally activation. Many optimization studies for the composition of this catalyst have been reported.

Figure 4. Industrial preparation method of Cu/Zn/Al catalyst [24]

Figure 12 shows the productivity of Cu/Zn/Al catalyst at an industrial scale via the co-precipitation method. It shows the relationship between productivity, pH, and precipitation temperature. The highest productivity can be achieved at 6-7 pH and 70°C temperature. In a comparison of this work with Dasireddy and Likozar [21], it can be concluded that the addition of ultrasonication with the conventional established industrial co-precipitation method is quite useful. It has increased both activity and selectivity.

Figure 5. Catalyst productivity with the function of pH and precipitation temperature [24]

4.1 Deactivation of Catalyst

In common practices, the catalyst used for DME conversion deactivates due to sintering and coke deposition on the active sites. Its deactivation is also attributed to the poisoning and blocking of active sites through contaminants present in the syngas. There are two mechanisms defined for zeolite deactivation; active sites of the catalyst absorb coke particles and pores get blocked due to the deposition of carbonaceous compounds that limit the reactant’s access to reach the active sites of the catalyst. Moreover, coke deposition on the pores depends upon particle shapes and size. According to a previous study, large pores undergo coke deposition more readily as compared to medium-sized pores [25].

In the indirect DME synthesis, the deactivation of γ-Al2O3 was investigated by Raoof, et al. [4] in a fixed bed reactor. It was found that catalyst activity was lost by 12.5 times in the methanol-water system compared to pure methanol. The catalyst’s deactivation occurred due to the absorption of methanol over active sites assisted by water. The bifunctional catalyst CuO-ZnO-Al2O3/γ-Al2O3 were evaluated for deactivation in the presence of H2+CO2 and H2+CO feed. It was found that catalyst activity was lost more in the case of H2+CO feed. On the other hand, H2+CO promoted reverse WGS, which increased the concentration of water in the reactor leading to active site blockage. In the case of H2+CO2, the coke deposition was limited and less water absorption on the active sites was observed [26].

5 Latest Developments in Catalysis

Efforts have been made to explore such catalysts that have high selectivity towards DME and low affinity to give coke and hydrocarbons. In the indirect method of DME production, typically solid-acid type catalysts have been used. The bi-functional catalysts are used for the direct conversion of syngas to DME. It is composed of metal and solid-acid. The metallic characteristic is required to convert syngas to methanol, while solid-acid transforms the methanol to DME [26].

The thermal conduction of the bi-functional catalyst is low due to solid-acid particles. Therefore, the operating temperature is maintained in the range of 795°C to 946°C at a pressure of ~10 bar [27, 28]. Metal oxides such as CuO, Al2O3, ZnO, and Cr2O3 are  used to provide a metallic function [26, 29]. Whereas, solid-acids include γ-Al2O3, TiO2-ZrO2, silica-modified Al2O3, resins, and clays. Moreover, barium oxide, sulfates, silica, and phosphorous can be added to such catalysts to achieve suitable acidity that enables an increase in CO conversion and production of negligible by-products [14, 30].

Zeolites are being used extensively in industry as a catalyst, ion exchanger, and absorbent. They are composed of crystalline aluminosilicates. In DME production, they have been used to catalyze the methanol dehydration process as a solid-acid catalyst at 18 bar pressure and 250-400°C temperature [16]. The γ-Al2O3 has been used to catalyze the dehydration process in methanol for DME conversion due to low cost, high surface area, and higher selectivity. The catalyst’s higher activity was ascribed to the low contents of highly acidic sites that are favorable for DME formation. The issue with this catalyst was its affinity towards water, hence reducing its activity [31, 32].

Bonura, et al. [33] have studied the performance of the bi-functional catalyst Cu-ZnO-ZrO2 for CO2 hydrogenation reaction to DME and HZSM-5 zeolite for methanol synthesis. It was found that metal and metal oxide interface acid sites are crucial for methanol conversion to DME. The strong acidic character of the Cu-ZnO-ZrO2 catalyst is not favorable for DME production; on the other hand, the medium-weak acidity of HZSM-5 zeolite sites increases DME formation.  In another report, Bonura, et al. [34] reported the influence of various promoters on the catalytic activity of CuZn ferrierite towards DME conversion. It was found that the promoters Al, Ga, and Zr behave in a similar pattern. However, Ce and La promote the activity of CuZn ferrierite. The increase in the activation of the CO2 path occurs due to the balance of acid-base active sites in the catalyst rather than the metallic character. On the other hand, weak-medium acidity promotes DME conversion. Rate of DME synthesis with the function of weak-medium acid sites is depicted in Figure 15.

Figure 6 Displaying the DME synthesis rate as a function of acidic sites (weak and medium) [34]

The CuO-ZnO-Al2O3 catalyst has a metallic copper cluster that is active for WGS reaction and methanol synthesis. The formation of product is governed by Cu metal surface area, while ZnO maintains the activity of copper during the chemical reaction by exposing a higher number of active sites. On the other hand, Hadipour and Sohrabi [35]  found out that the excessive amount of ZnO in such kind of catalysts had a negative influence on the performance. The purpose of adding a trivalent metallic compound is to enhance the catalyst’s surface area and dispersion; however, it is found that a trivalent ion also inhibits the sintering of Cu particles under processing conditions. Moreover, the same authors further demonstrated that in contrast to ZnO, the CuO, and Al2O3 in excessive amounts promote the surface area and thus active sites in the catalyst.

Yang, et al. [36] presented zeolite catalyst Cr/ZnO-S-Z synthesized through a dual-layer approach in which bimetallic Cr/ZnO remains at the core while neutral silicate at the shell and H-type h-ZSM-5 zeolite at the interface. This layered structure enabled direct formation of DME at a higher temperature. It happend by reducing the formation of by-products (alkane/alkenes). The core shell can be seen in the Figure 16.

Figure 7 Core-shell assembly of Cr/ZnO-S-Z catalyst [36]

The particle size and dispersion of Cu in the Cu/Zn catalyst affect the WGS reaction. It can be controlled by preparation conditions, for instance, the corresponding molar ratio, calcination temperature, and the precursor type. It was found that a low Cu/Zn molar ratio favors WGS reaction. However, a higher Cu/Zn ratio activates the methanol synthesis reaction. It might be due to a better interaction of the two oxides, i.e. CuO and ZnO [37].

In this regard, Cu–Fe–La and Cu–Fe–Ce combined with HZSM-5 bifunctional catalysts were reported for direct synthesis of DME. In these catalysts, La and Ce were used as promoters and it was found that Ce and La reduced the outer-shell electron density of Cu in the Cu-Fe catalyst, which in turn enhanced the reduction ability of the catalyst, leading to a dramatic improvement in the selectivity for DME. On the other hand, Ce promoted dispersion and increased the surface area of the catalyst; consequently, selectivity towards DME and CO2 conversion improved considerably [38].

In another study, Ren, et al. [39] reported Cu-ZnO-based catalysts prepared via the co-precipitation method for the direct synthesis of DME. Authors found that the catalyst’s microstructure and surface area is crucial for high activity and selectivity. These physical characteristics of catalysts were further dependent upon the preparation method and parameters. A low precursor amount gives better catalyst dispersion and high porosity, thus higher activity and selectivity. The highest yield of around 22% for DME and 30.5% CO2 conversion efficiency with DME selectivity of 72% was obtained.  Figure 17 shows the selectivity and conversion concerning the run time (TOS).

Figure 8 Performance of Cu-ZnO HZSM-5 bi-functional catalyst [39]

Similarly, Naik, et al. [40] evaluated the activity and durability of hybrid functional catalyst 6CuO–3ZnO–Al2O3 based on γ-Al2O3 and 6CuO 3ZnO 1Al2O3 based on HZSM-5 for direct conversion of CO2/H2 in fixed bed and slurry reactors. It was found that the first catalyst promoted reverse WGS reaction, thus lowering DME’s selectivity. Simultaneously, the second catalyst showed the relatively higher synthesis of DME from CO2 in both types of reactors.

6 Conclusions

DME (dimethyl ether) has gained attention as a potential replacement for traditional fuels like diesel and gasoline due to its lower emissions of pollutants. While it can be produced through both indirect and direct methods, the indirect method has a low yield and is not practical for commercial use. The direct method, on the other hand, faces challenges with separating DME and CO. Researchers have been exploring the use of bi-functional or hybrid catalysts made with nanotechnology to improve the efficiency of the direct conversion process. Further investigation is needed to understand how synthesis parameters impact the physical properties and conversion performance of these catalysts. If successful, the development of highly efficient catalysts could lead to the commercialization of DME in the future.