A Review of DME Manufacturing: Process and Catalyst Studies

. Consumption of fossil-based energy is increasing every year which has an impact on air, water and soil pollution. Therefore, alternative energy is needed to replace fossil fuels. Dimethyl Ether (DME) is considered suitable to replace LPG because of its better physical and chemical properties than LPG. This review article discusses the differences between direct and indirect DME synthesis methods and studies their reaction mechanisms. In addition, the types of promoter addition and their effects on the characteristics and performance of the catalyst are also studied in this article. The final part of this article discusses the effect of operating conditions (temperature, pressure, time on stream (TOS), room velocity, and H 2 /CO ratio) on catalyst performance, which is sourced from several literatures. It is hoped that this article can obtain an effective DME manufacturing method both in terms of process and catalytic


Introduction
Energy is the main human need.Fossil energy is energy that is often used in several countries.The three types of fossil energy that are often used are Crude Oil, Natural Gas, and Coal.Consumption of fossil energy in the form of petroleum is the highest in the world in 2020 compared to coal and natural gas [1].The use of this fossil energy has an impact on air pollution in the form of CO2 emissions, the cause of global warming, climate change, and environmental damage [2,3].Therefore, an environmentally friendly alternative energy is needed to replace fossil energy.One of the best substitutes for fossil energy is dimethyl ether (DME), which is the simplest ether, without C−C bonds.Recently, DME has been extensively investigated for its ability to replace LPG.LPG was replaced by DME because of its similar physiochemical properties, biodegradability, non-toxicity, environmental friendliness and good combustion performance [4].DME can also be used as a substitute for diesel fuel because it has a high cetane number [5] and low soot gas emissions due to the absence of C−C bonds [6].In addition, DME is also an intermediate for the manufacture of methyl acetate, aromatics, acetic acid, light olefins, gasoline, high ethers, oxygenates, formaldehyde, ethanol [7].
Corresponding author: budiman@staff.uns.ac.idOne of the best substitutes for fossil energy is dimethyl ether (DME), which is the simplest ether, without C−C bonds.The interest in DME production lies in its use as a fuel and source of raw materials used in the DME production process, as well as the capacity of the process to multiply syngas from renewable sources (biomass and CO2) [8].

DME synthesis 2.1 Indirect Method (Conventional Method)
The indirect method is the synthesis of DME by a two-step catalytic process [9,10].The first stage is the catalytic process of forming methanol from syngas and CO2 in the first reactor through a hydrogenation reaction (Equation 1 and 2).The second stage is the catalytic process of forming DME in the second reactor through the dehydration reaction of methanol (Equation 3).The scheme of the DME indirect synthesis process is shown in Figure 1.where ∆H °is the standard enthalpy.
Fig. 1.Indirect Process Scheme [11] The first reaction (hydrogenation) requires a metal catalyst.Meanwhile, the second reaction (dehydration) requires a solid acid catalyst [9].The mechanism of CO2 hydrogenation with Cu as a catalyst is carried out in two ways: The first pathway produces CO intermediates, which are produced from RWGS reaction: the reaction CO2 +H2 → CO + H2O via carboxyl species (*HOCO ) and then hydrogenated to methanol; The second pathway is related to the intermediate formate (*HCOO) which is formed by the hydrogenation of CO2 which finally produces methanol through the breaking of the C-O bond and the intermediary *H2CO [12].

RWGS Reaction Pathway
Formate Pathway Fig. 2. Hydrogenation Mechanism [12] The differences in the hydrogenation mechanisms of CO2 and CO are shown in Figure 3. CO tends to form methanol via formyl.Meanwhile, CO2 tends to form methanol through formate.CO and CO2 hydrogenation processes are capable of forming methanol via the carboxyl pathway.

Fig. 3. Hydrogenation mechanism of CO2 and CO [14]
The mechanism of dehydration using a solid acid catalyst is shown in Figure 4.The reaction for the formation of DME involves two active sites on acidic solids, namely the acidic side (H + ) and the basic side (O2 -).Methanol is protonated on the acid side to [CH .OH ] + which is converted to CH 3+ and H2O (Equation 4).Meanwhile, the base site converts methanol to CH3O − and OH -(Equation 5).DME is formed due to the combination of the active species CH3O − and CH 3+ (Equation 6) [15][11].Some literature shows the suitability of the mechanism for the formation of DME following the Langmuir-Hinshelwood or Eley-Rideal kinetic models [17,18] with DME and water as reaction inhibitors [19,20,21].2.2 Direct method DME synthesis using the direct method involves 3 reactions, namely hydration reaction, methanol dehydration and the water-gas shift (WGS) reaction (Equation 7-10).The CO will be hydrogenated to produce methanol.Then, methanol is dehydrated to DME and H2O.The WGS reaction occurs between CO or CO2 and H2O produced by the dehydration reaction.
The product of the WGS reaction is H2 which will react again with CO according to equation 7. The total reaction is shown by equation 10.All of these reactions take place in one reactor with the help of a bifunctional catalyst.This catalyst is a combination of metals and solid acids which play a role in hydrogenation and dehydration reactions respectively [10].Several methods of preparing bifunctional catalysts have been studied extensively such as precipitation [22,23], adsorption [24], impregnation [25,26] and ion exchange [27].The advantages of direct synthesis are numerous, such as high CO2 conversion, simpler reactor design because it only uses one reactor, and low production costs.However, the drawbacks of this method are the large number of by-products produced and the low selectivity of DME [11,28].Therefore, purification and separation are important components in this method to produce the maximum amount of DME.
The DME purification process and DME separation lead to the loss of residual reactants and by-products, such as H2, N2, CH4, CO2, and H2O.The methanol will be recycled to the inlet stream to obtain high purity DME.This purification and separation cause a shift in the equilibrium towards higher methanol conversion [29].One of the purification processes developed by Yuanyuan et al. [30], the resulting methanol and water are condensed and then absorbed by the water; then the water vapor containing DME is distilled to get the final product of high purity DME.The schematic of this process is shown in Figure 5.Meanwhile, a study conducted by Kiss et al. [31], also showed that the use of divide-wall column (DWC) technology in DME synthesis resulted in higher DME purity compared to conventional processes (without distillation) resulting in a 28% reduction in energy requirements and a 20% reduction in equipment costs [31].3 Catalyst Many studies have been carried out to produce catalysts with maximum performance.The performance of the catalyst is seen based on the selectivity of DME and its tendency to form coke and other hydrocarbons.There are two types of catalysts for the indirect process, namely metals for the hydrogenation reaction to form methanol and solid acids for the methanol dehydration reaction to form DME [9].Meanwhile, the direct process uses one type of catalyst, a combination of metal and solid acid, which is called a bifunctional catalyst [32].Some metals that are often used are Cu, Zn, Mg, Zr, Al2O3 , and others [22,33,34,35,36,37,38].While the acid solids that are widely used are ZSM5, HY, Mordenite, SAPO, Ferrite, γ-Al2O3 , and others [39,40,41,19,42,43,44].

Catalyst Modification
The drawbacks of using the direct method of DME synthesis can be reduced by optimizing the performance of the catalyst used by adding a metal promoter that carries special properties to improve the catalyst.The addition of a promoter will increase the distribution of active hydrogenation sites, affect the acidity of the carrier, increase or decrease the surface area of the catalyst, etc. depending on the type of metal promoter, catalyst, and synthesis method used.

Mg promoter
ZnO/γ-Al2O3 increased the surface area of the γ-Al2O3 from 79.2 m 2 /g to 95.9 m 2 /g.The addition of Mg to the Cu/ZnO/γ-Al2O3 catalyst increased Cu dispersion as evidenced by a shift in the Malasite XRD peak due to the assimilation of Mg 2+ into the malachite structure [45].The addition of MgO metal to the Cu/γ-Al2O3 (CA) catalyst shows that Cu dispersion and surface area is greater than the addition of ZnO and ZrO2 [46].The addition of Mg into the Cu/Mordenite catalyst was carried out by Din et al (2020) [22].A slight increase in the surface area of the catalyst occurred by 6.4% after the addition of 0.3% by weight of Mg [22].Mao and Guo added alkaline earth metals (MgO, BaO, and CaO) to ZSM5.MgO provides the largest surface area of the ZSM5 (274.1 m 2 /g).MgO /ZSM5 has the largest acid sites (8.80x10 -4 mol g -1 ) compared to BaO /ZSM5 (7.52x10 -4 mol g -1 ) and CaO /ZSM5 (8.66x10 -4 mol g -1 ).MgO has the weakest degree of basicity compared to CaO and BaO.
MgO only reacts at strong acid sites, whereas BaO and CaO react at weak and strong acid sites.Therefore, the base site of MgO /ZSM5 is higher than BaO /ZSM5 and CaO /ZSM5 [34].The addition of MgO to the Cu/ZnO/γ-Al2O3 catalyst reduces the total acidity of the catalyst due to the strong basicity of MgO.MgO will react with strong acid sites so that it will reduce or modify them to become moderate or weak acid sites [45].Mao et al. [47] used MgO as a neutralizer for strong acids to weak acids.Strong acid sites lead to further degradation of DME to other hydrocarbons with shorter C chains [47,48].
The addition of Mg into a bifunctional catalyst also affects the catalytic activity.The addition of 20% Mg to Cu/ZnO /γ-Al2O3 catalyst showed the best activity results compared to the addition of 0% Mg, 5% Mg, 10% Mg, and 30% Mg.The increase in DME selectivity from 50% to 83% occurred with an increase in Mg concentration from 0% to 20%.The best performance of the catalyst with the addition of 20% Mg is related to the high acid sites in the medium formed with the addition of 20% Mg [45].This media acid site plays a major role in the dehydration reaction of methanol to DME and prevents the formation of byproducts in the form of coke and other hydrocarbons [49].The addition of 0.3% Mg to Cu/Mordenite catalyst can increased the selectivity of methanol from 40% to 50% [22].The addition of Mg to the Cu-ZnO-ZrO2/Al2O3 catalyst increased the selectivity of methanol from 22.44% to 35.98% and CO2 conversion from 10.87% to 12.12 % [46].MgO is able to reduce the acidity of Al2O3 so that no methanation reaction (formation of CH4) occurs [46,48].

Zr promoter
One of the promoters that is often used is Zr.The addition of 7,5% Zr to activated carbon caused a decrease in the specific surface area from 1.280 m 2 /g to 1.132 m 2 /g and the pore volume from 0.95 to 0.86 cm 3 /g.This proves that the addition of Zr covers the micropores [49].The addition of the Zr catalyst to Cu/γ-Al2O3 (CA) also caused a decrease in the specific surface area from 234.3 m 2 /g to 224.8 m 2 /g and the pore volume from 0.45 to 0.45 cm 3 /g [46].The addition of 1% Zr into the Cu-Fe/ZSM5 catalyst caused an increase in the specific surface area from 94.24 m 2 /g to 107.22 m 2 /g.However, increasing the Zr concentration up to 3% decreased the specific surface area to 92.62 m 2 /g [50] .
The addition of 5.25% Zr to H3PO4 impregnated activated carbon increased the weak acid sites from 125 to 175 mol/g due to the formation of Zr-O-P bonds..The increase in Zr concentration from 0.25% to 7.5% is proportional to the increase in methanol conversion due to the formation of more weak acid sites which are active sites for the formation of DME [51][52][53].The addition of Zr to the CuO/ZnO/Al2O3 (CZA) catalyst increases the stability of the catalyst due to Zr's hydrophobic nature which repels water (a by-product of methanol synthesis) from its active site from the catalyst.Therefore, the catalyst is not easily deactivated [36].
The addition of ZrO2 into the Cu-ZnO catalyst showed better catalytic activity than ZnO .Cu-ZnO-ZrO2 catalyst showed relatively high CO2 conversion (23%) and high methanol productivity (331 g CH3OH kgcat -1.h -1 ) under mild reaction conditions (280 °C, 50 bar, and GHSV 10,000 h -1 ) [54].The increase in Cu-ZnO-ZrO2 activity is related to the hydrophobic nature of zirconium oxide which inhibits strong water adsorption and increases surface alkalinity, which facilitates CO2 adsorption and methanol productivity.ZrO2 is also thought to have the ability to activate adsorbed CO2 , favoring the formation of CO2* species.These CO2* species can then react with H2* species to form intermediate species (i.e.formate/dioxoethylene/methoxy) during methanol synthesis [54].In addition, ZrO2 supports the formation of oxygen vacancies during reduction which facilitates Cu dispersion and enhances Cu-ZnO contact.The increased stability of Cu δ+ sites through interaction with zirconia explains the increased Cu dispersion and increased Cu-ZnO contact [55] .

Other promoters
The addition of several metals from groups B and F (Ga, La, Y, and, Zr ) was carried out on a Cu/Zn/γ-Al2O3 catalyst.The addition of Y showed the greatest increase in specific surface area and Cu metal, namely 114 and 11.7 m 2 /g.Ytrium (Y) is an element with a high affinity for oxygen and tends to inhibit the growth of Cu particles so that agglomeration can be avoided Cu/Zn/Y/γ-Al2O3 activity is better than other catalysts.CO conversion and DME yields were up to 60 % and 40 % at 250 °C and 600 psig pressure, respectively.Good Cu/Zn/Y/γ-Al2O3 activity is associated with higher specific surface area and Cu metal compared to other catalysts [71].Modification of Cu/Zn/Zr catalysts with rare earth metals (La, Ce, Nd and Pr) was carried out by coprecipitation method.The addition of rare earth metals decreases the specific surface area of the catalyst.The addition of Ce increased the performance of the catalyst for the synthesis of methanol from CO2. CO2 conversion increased from 19.6% to 22.8%.The biggest increase in methanol selectivity also occurred in the addition of Ce from 44.4% to 53%.CeO2 is reported to have a very good oxygen storage capacity.Therefore, the Cu/Zn/Zr/Ce catalyst has the ability to absorb CO2 better than other catalysts so that more methanol is obtained [72] [50].The addition of Ce catalyst to Cu-Mn-Zn/Y Zeolite showed the best activity compared to other rare earth metals such as La, Pr, Nd, Sm and Eu.CO conversion with Cu-Mn-Zn/Ce-Zeolite Y catalyst reached 77.1% at 245 °C, 2.0 MPa, H2 /CO = 3/2 and 1500 h -1 .In addition, this catalyst has the highest DME selectivity of 66.7% and coke formation of at least 21.6 mg/g-paint.The activity of Cu-Mn-Zn/Ce-Zeolite Y is the best because the formation of weak and medium acid sites is the highest due to the addition of Ce metal, namely 1.76 and 0.76 mmol.g -1 [74].

Preparation of Bifunctional Catalysts
Bifunctional is made by physically and mechanically mixing methanol synthesis catalyst (metal) and methanol dehydration catalyst (acidic solid).The mixing of the two ingredients must be homogeneous and uniform.Several chemical methods that are often used in mixing metals and acidic solids are coprecipitation (sol-gel), sequential precipitation, chemical metal precipitation, impregnation, sonochemical assisted impregnation, and physical sputtering.An important factor in the synthesis of bifunctional catalysts is the dispersion of the active metal and the acid phase and the interactions between the metal and acid sites.Therefore, the selection of the metal and solid acid mixing method is very important to produce good dispersion and suitable interaction.

Co-precipitation
This method is carried out by depositing metal precursors on an acid/solid carrier.Precipitation is carried out by changing conditions such as pH, temperature, or evaporation.The coprecipitation method scheme is shown in Figure 6.Zincian malcite has previously been reported as a desirable precursor for the formation of active methanol catalysts [75].The crystalline phase is formed during the aging process after the initial deposition, which generally forms an amorphous product due to the fast kinetics of the deposition process.The malcite produced by zincian is 5-15 nm wide, and forms the general meso structure of the catalyst.With Zn incorporation, the close interaction of Cu−Zn allows the formation of small Cu particles during calcination and subsequent reduction [75].The precipitate formed at pH 7 at 65 °C and was sprayed immediately to allow separation of the precipitation and aging stages.The bifunctional catalyst was prepared from CuO/ZrO2 with montmorillonite K10 carrier using the coprecipitation method [79] .There are three methods for producing CuO/ZrO2 bimetal, namely: 1. Citric acid method, 2. Co-precipitation with the addition of NaOH, 3. Coprecipitation with the addition of Na2SO3.Among all the synthesized catalysts, the best results in the CO2 hydrogenation reaction were obtained with the CuO/ZrO2 catalyst synthesized using the citric acid method, because this preparation resulted in a better interaction between the metal sites and the acid involved in the process.The coprecipitation method can be combined with other methods.Khosbin et al.
[80] compared impregnation, physical coprecipitation and combined coprecipitation-ultrasound methods to prepare CuO-ZnO-Al2O3 /HZSM-5 catalysts.The combination of ultrasound with coprecipitation has the highest catalyst reduction ability compared to other methods.This is because the use of ultrasonic energy increases the nanocatalyst dispersion and catalyst activity [81].

Precipitation
Similar to co-precipitation, precipitations are related to the precipitation of precursor solutions through changes in pH, temperature, or evaporation, resulting in the formation of metal compounds with low solubility (turning into metal hydroxides).Precipitation is carried out by gradually increasing the concentration of the supporting compound to prevent the formation of a bulk phase in the solution.Precipitation on the support is achieved because inclusion of the support in solution causes a reduction in the free energy of the thin surface nuclei or stabilizes the precipitate, lowering the energy barrier for nucleation.Therefore, there are conditions where nucleation can only occur on the support and not in bulk solution, so that the surface of the support functions as a feed for nucleation [82].Precipitation of metal species is generally carried out by changing the pH so that compounds with low solubility are formed.When precipitation is carried out by injecting a precipitating agent (e.g.alkaline solution), great care must be taken to prevent concentrations exceeding supersaturation conditions, which will cause bulk precipitation.Therefore, a homogeneous precipitation precipitation method is often preferred, in which precipitation is induced homogeneously throughout the reaction vessel.This can be achieved by adding urea (at room temperature), which when heated to 90 °C decomposes slowly, resulting in the formation of -OH, thereby slowly increasing the pH upon decomposition [81].The addition of CO to TiO2 was carried out with variations ranging from 4% to 24%.Although some small 2 nm particles were found using urea and cobalt nitrate, large particles were observed, resulting in an average particle size of 20 nm (4 wt% Co) and 50 nm (24 wt%).Precipitation of cobalt carbonate on titania by evaporation of ammonia resulted in a homogeneous 9 nm particle distribution with the addition of 4 wt% Co, only increasing to 15 nm for samples prepared at 24 wt% Co.These results indicate an important precursor and support interaction for precipitation precipitation methods; when the interaction is too strong, such as cobalt nitrate and silica, a mixed phase between metal and support is formed, whereas when the interaction is too weak, such as cobalt nitrate and CNF or TiO2 , large particles are generated.In all reported cases, the precipitation of cobalt carbonate via ammonia evaporation resulted in a more optimal interaction [82].

Physical Method
The most frequently used method for combining the catalytic functions of metals and acids in catalysts for the direct synthesis of DME is physical mixing.The hybrid catalyst made by physically mixing CuZnZr (CZZ) with HZSM-5 showed better activity results compared to mixing with the grinding method.This method causes destruction of the catalyst as well as uneven distribution of metal on the carrier after mortar treatment [83].The results presented by Jiang et al. [83], explained the differences in the properties and strengths of metal-acid interactions in hybrid catalysts prepared by coprecipitation and physical mixing methods [84].The hybrid catalyst was formed by mixing Cu/ZnO-ZrO2 with amorphous mesoporous aluminosilicate substrate as an acid solid for methanol dehydration.Physical mixing of Cu/ZnO-ZrO2 with aluminosilicate showed a higher yield of DME (41 gcat -1 h -1 at 260 °C and 20 bar) than mixing with coprecipitation.The low activity generated by the coprecipitation method is due to the close contact between the Cu and Brønsted acid sites which allows the stabilization of Cu nanoparticles and their incorporation into the silica structure leading to loss of the Cu-ZnO active sites [84].The weakness of the physical method is that the resulting metal is not perfectly distributed on the surface of the carrier.The physical method combined with coprecipitation shows lower activity than the combination of ultrasound and coprecipitation because physical mixing cannot increase the catalyst dispersion compared to ultrasound [81].Therefore, catalysts formed by a combination of physical and coprecipitation methods show lower stability [81].

Nanostructuring (Core-Shell Capsule Catalyst)
Conventional methods (precipitation, physical methods) have weaknesses, for example controlling the size and distribution of metal solids and acids is difficult and the resulting catalyst has an open structure which causes low product selectivity due to easy catalyst access E3S Web of Conferences 481, 01002 (2024) ICSChem 2023 https://doi.org/10.1051/e3sconf/202448101002for all substances [74].This problem can be overcome by making metal solids and acids into nanostructures in the form of capsule catalysts with a core-shell structure (Figure 7).This limited access to catalysts leads to a higher selectivity of DME when synthesized from CO2 or CO [85].Based on Figure 7, the hydrogenation reaction occurs at the core to form methanol.Meanwhile, the methanol dehydration reaction occurs on the surface.Synthesis of core-shell catalysts can be carried out by multiple layer methods, hydrothermal synthesis, single crystallization, or physically adhesive methods [74].Synthesis of core-shell catalysts from zeolite requires high temperature and pH [86].Cu/Zn/Al2O3 metal was used to synthesize the core-shell catalyst with HZSM5 using the dual layer method as shown in Figure 6.The use of silicalite-1 zeolite as an intermediate layer aims to prevent core damage during the hydrothermal process.The resulting core-shell catalyst has a higher DME selectivity (79%) than the bare catalyst.In addition, no side products were found in the form of alkenes with the formation of core-shell catalysts [86].

Effect of operating conditions 4.1 Temperature
Temperature is an important part of the catalyst test.Several studies have explained the effect of temperature on catalyst performance.Increasing temperature tends to increase the selectivity of methanol less than 275 °C by CuO/ZnO/ZrO2/HZSM5 fiber catalyst.An increase in temperature of more than 275 °C increases the formation of CO2 by the water gas shift reaction so that the selectivity and yield of methanol decrease with increasing temperature.Therefore, the formation of DME at 300 °C is very low due to the reduction of the methanol formed [87].The selectivity of DME produced by the ACP2800ZR5.25 catalyst decreases in proportion to the increase in temperature above 425 °C due to the increased formation of side products in the form of CH 4 and CO2 [49].Mn/SZ Sulfate Catalyst results in increased DME selectivity with increasing temperature.Above 180 °C, DME selectivity decreases with increasing temperature.This is because at temperatures above 180 °C, by-products of long carbon chains are formed.This is evidenced by the drastic increase in the formation of light olefins and aromatic carbon at temperatures above 180 °C [88].Thermodynamically, olefins/aromatic carbons are easily formed at high temperatures [89].Cu-Zr/SAPO-18 produced maximum DME (35%) at 275 °C.However, DME yield decreased when used at 300 °C.The decrease in DME yield at higher temperatures is caused by the formation of reaction byproducts in the form of olefins [69].Cu-ZnO-MgO(20)-γ-Al2O3 is used in the syngas reaction to DME (STD).An increase in temperature from 240 to 280 °C causes a greater conversion of CO.DME selectivity increases with increasing temperature.However, at temperatures above 260 °C, DME selectivity decreases [45].The decrease in DME selectivity is caused by the formation of by-products at high temperatures such as CH4, olefins and aromatics [89].

Pressure
Increasing the pressure from 1 bar to 20 bar causes an 85% increase in the conversion of methanol to DME using an HSiW /TiO2 catalyst [90].Increasing the pressure from 20 bar to 80 bar at 250 °C had a positive effect on increasing the DME yield from 17% to 25% [91].The same results were also obtained for the Cu-Zn-Zr/SAPO-18 catalyst, increasing the pressure from 20 bar to 40 bar, and consequently increasing the DME yield from 36% to 42% [68].Synthesis of DME from CO2-rich syngas using Cu/ZnO/Al2O3 /$ -Al2O3 catalyst (50:50) showed a CO conversion of 32% at 50 bar pressure, CO conversion decreased to 11% at reduced pressure to 25 [92].The increase in pressure causes the gaseous reactants to move faster so that more intensive collisions will occur.Therefore, the higher the pressure, the higher the product obtained.

Stream time (TOS)
Time on stream is the rate of reactants used per unit time.TOS measurement is related to the stability of the catalyst.The more stable the catalyst, the more constant the selectivity value of DME and methanol for each TOS value.Increasing the stability of the catalyst is usually done by adding certain metals which are useful for minimizing coke formation.The addition of Zr to activated carbon H3PO4 increases the stability of the catalyst.Methanol conversion was stable at 69% with TOS for 350 minutes and DME selectivity was stable at 97% with TOS for 72 hours [49].The same thing happened to the addition of Zr to the Cu/ZnO/Al2O3/HZSM5 catalyst which resulted in a stable DME selectivity of 58.7% for 100 hours.Meanwhile, without the addition of Zr, DME selectivity decreased when it reached 20 hours [36].The increased stability with the addition of Zr is due to Zr's hydrophobic nature, thereby removing water from the catalyst as a by-product of DME formation from syngas [36].Water triggers the deactivation of the catalyst because it triggers sintering on the active side so that the catalyst will form a lot of coke [69,36].The addition of 3 wt% CeO2 into the CuO-Fe2O3/HZSM-5 catalyst increased the stability of the catalyst.CO2 conversion and DME selectivity stabilized at 19.9% and 58.9% for 15 hours.Without the addition of the CeO2 catalyst, CO conversion decreased from 13% to 10% and DME selectivity from 19% to 5% for 6 hours [93].Cerium metal is hydrophobic which is thought to have the same role as Zr in maintaining catalyst stability [94].

H2 / CO
Increasing the H2/CO ratio from 0.5 to 2 which was fed to the CuO-ZnO-Al2O3/γ-Al2O3 catalyst increased the DME yield from 13% to 67% [94].CuO / ZnO /Al2O3/H-ZSM-5 catalyst activity test by changing the H2 /CO ratio from 1 to 1.5 at SV 500 mL/(gcat.h) showed an increase in DME yield from 40% to 55% and DME selectivity from 58% to 63% [93].FCZZ25(N)-10Z catalyst activity test at 275 ℃, SV 4800 mLn /(g-cat.h ) by changing the H2/CO ratio from 1 to 2 led to an increase in DME productivity from 1.83 to 2.63 mol DME g-cat -1 s -1 .In addition, CO conversion also increased from 80.8% to 91.6% [87].The increase in DME yield in proportion to the increase in H2/CO indicates a dominant role of H2 over CO in synthesizing DME from syngas [87].

Conclusion
The direct method has advantages over the indirect method, namely high CO2 conversion, simpler reactor design because it only uses one reactor, and low production costs.Optimization of this method is carried out by increasing the performance of the catalyst so as to increase the yield and selectivity of DME.The addition of a promoter is one way to improve the performance of the catalyst.Promoters have useful properties for correcting weak catalysts.Zr, Mg, Y, and Ce promoters have been proven effective in increasing catalyst performance based on several literature studies.The increase in catalyst performance is caused by an increase in the distribution of active sites, a decrease in the acidity of the catalyst to be weak or moderate, and an increase in the specific surface area after adding the promoter.The nanostructuring method is the best method for making bifunctional catalysts compared to conventional methods (co-precipitation, precipitation-precipitation, physical methods, etc.) because it can limit the access of other compounds (other than reactants for DME synthesis) to interact with the catalyst.Therefore, this method is able to increase the selectivity of DME.The better the catalyst performance, the higher the DME yield, DME selectivity, and the resulting higher CO2 conversion.These three things are strongly influenced by operating conditions.The increase in temperature is proportional to the increase in catalyst performance because it provides more energy for the reaction to take place.The same thing happens when the pressure is increased; catalyst performance is getting better.Increasing the pressure causes the motion of the reactants to become more active thereby increasing the contact time between the reactants and the catalyst.Time on stream (TOS) measures the stability of the catalyst for a certain time.Several literatures show that the addition of Zr and CeO2 promoter increases the stability of the catalyst.The promoter is hydrophobic so that it can reject H2O from the catalyst causing deactivation of the active site.Increasing the space velocity (SV) decreases catalyst performance due to reduced contact time between catalyst and reactants.The higher the ratio of H2/CO can improve the performance of the catalyst because more H2 is needed compared to CO.

Fig. 6 .
Fig.6.Synthesis of CuO-ZnO-Al2O3 /zeolite using coprecipitation method[74]    The combination of methanol synthesis and methanol dehydration catalyst can be carried out by coprecipitation of oxalate gel from Cu-Zn-Zr precursors in a solution containing zeolite[76].In addition to the catalyst preparation method, the chemical composition of the catalyst is very important to ensure the appropriate texture and surface properties of the Cu-Zn-Zr catalyst [68].High CO2 conversion and methanol selectivity were observed in the catalyst prepared by co=precipitation (gel-oxalate) of Cu, Zn, and Zr precursors (Cu concentration 57 wt%).By combining different zeolites with a copper-based catalyst through the coprecipitation method, Bonura et al. [77] and Frusteri et al. [78] demonstrated the superiority of FER zeolite over classic MFI, MOR, and Y zeolite in the conversion of CO2 to DME.Superior DME production was achieved through a Cu-Zn-Zr/FER hybrid system prepared by coprecipitation (gel-oxalate) method [78].The bifunctional catalyst was prepared from CuO/ZrO2 with montmorillonite K10 carrier using the coprecipitation method [79] .There are three methods for producing CuO/ZrO2 bimetal, namely: 1. Citric acid method, 2. Co-precipitation with the addition of NaOH, 3. Coprecipitation with the addition of Na2SO3.Among all the synthesized catalysts, the best results in the CO2 hydrogenation reaction were obtained with the CuO/ZrO2 catalyst synthesized using the citric acid method, because this preparation resulted in a better interaction between the metal sites and the acid involved in the process.The coprecipitation method can be combined with other methods.Khosbin et al. [80] compared impregnation, physical coprecipitation and combined coprecipitation-ultrasound methods to prepare CuO-ZnO-Al2O3 /HZSM-5 catalysts.The combination of ultrasound with coprecipitation has the highest catalyst reduction ability compared to other methods.This is because the use of ultrasonic energy increases the nanocatalyst dispersion and catalyst activity [81].

Table 1 .
Several studies related to the use of the Mg promoter in the synthesis of methanol and DME catalysts

Table 2 .
Several studies related to the use of the Zr promoter in the synthesis of methanol and DME catalysts