Comprehensive comparison of various methanol synthesis catalysts

Based on the changes in environmental pressures and energy structures, on the one hand, methanol can be synthesized from CO2, and on the other hand, methanol can be used as a raw material for synthesizing propylene. Therefore, the application of methanol is increasing year by year. Currently, over 80% of the world's total methanol production is synthesized using the ICl process and the Lugri process, both of which use copper-zinc-aluminum-based catalysts, which are key to methanol synthesis.

After years of development, although copper-zinc-aluminum catalysts have become increasingly mature, experts from both at home and abroad have actively conducted research on methanol synthesis catalysts to improve conversion rates and reduce methanol synthesis costs.

1. Methanol Synthesis Catalyst

Methanol can be synthesized from CO2 using a catalyst, which can be divided into copper-based catalysts (including the most mature and widely used gas-solid phase catalysts in industry and new gas-liquid phase methanol synthesis catalysts) and non-copper-based catalysts (mainly including zinc-chromium catalysts (first developed and successfully commercialized by BASF in Germany in 1923), precious metal active component catalysts, metal alloys catalysts, and palladium-based catalysts, which are gas-solid phase catalysts with high pressures of 25 to 35 MPa).

1.1 Copper-based Catalysts

There are mainly three types of copper-based catalysts: a ternary copper-zinc-aluminum system catalyst (working temperature of 227-257℃ and working pressure of 5-10 MPa), a copper-based non-zinc-aluminum-based multi-component catalyst (with copper as the base and added third and fourth components as the catalyst), and a new low-temperature gas-liquid phase copper-based catalyst.

1.1.1 Copper-Zinc-Aluminum Ternary Catalyst

The copper-zinc-aluminum ternary catalyst, also known as Cu-ZnO-Al2O3 catalyst system, is the most commonly used catalyst system for the reaction of CO2 to form methanol. Cu is the active center of the reaction, ZnO acts as an adjuvant to the catalyst, Al2O3 serves as a carrier for the catalyst and also enhances its activity. Various studies have been conducted by scholars at home and abroad on the optimal ratio of the three components in the catalyst: Cu, ZnO, and Al2O3. For example, Denise, Baiker, etc. have systematically studied the key role of Cu in CO2 catalytic hydrogenation activity, methanol selectivity, and temperature impact, and found that at 225℃, methanol selectivity can reach up to 98%. Baiker also studied the reaction activity of other IB group metals replacing Cu, and found that Cu is best suited for catalytic hydrogenation reactions. Dai Chengyong, Li Jitao, Xu Yong et al. conducted similar research using a Cu-ZnO-Al2O3 catalyst, and found that it is suitable for use in the reaction. In English:

CuZnAl ternary catalysts are mainly composed of copper, zinc, and aluminum elements and their oxides. These catalysts include copper zinc aluminum ternary catalysts (working temperature of 227-257℃), copper based non zinc aluminum based multi component catalysts (with copper as the base and added third and fourth components), and new low temperature gas liquid phase copper based catalysts. These catalysts are commonly used for the reaction of CO2 to form methanol. Cu is the active center of the reaction, ZnO acts as an adjuvant to the catalyst, Al2O3 serves as a carrier for the catalyst and enhances its activity. Various studies have been conducted by scholars at home and abroad on the optimal ratio of these three components in the catalyst. For example, Denise, Baiker et al. have studied the key role of Cu in CO2 catalytic hydrogenation activity, methanol selectivity, and temperature impact,and found that at 225℃, methanol selectivity can reach up to 98%. Baiker also studied other IB group metals replacing Cu in their reaction activity

The CO2 conversion rate can reach 10%-30% under different conditions, and the methanol selectivity reaches 40% or above; Hania Ahouari, Ahce'ne Soualah et al. prepared a series of Cu-ZnO-Al2O3 catalysts by coprecipitation method and tested their catalytic effect on CO2 hydrogenation to produce methanol in a fixed bed reactor. The results showed that the catalyst with a Cu mass fraction of 51% and a Zn mass fraction of 22% had the highest CO2 conversion rate and methanol yield.

1.1.2 Copper-based non-zinc-aluminum series multi-component catalyst

(1)ZrO2-based copper-based catalysts

ZrO2 has good chemical stability and possesses both acidic and basic properties, as well as oxidation and reduction abilities, making it a catalyst that has attracted considerable attention in the field of catalysis. Studies have shown that increasing the amount of ZrO2 leads to an increase in methanol production rate, while the specific surface area of the catalyst's CuO/ZrO2 aerogels is related to catalyst activity to some extent. With respect to copper load, when copper loading is low, methanol production rate with CuO-ZrO2 is higher than that with Cu-ZnO. Additionally, reaction temperature has a significant impact on catalyst activity and selectivity.

Researchers such as J. Toyira and R. Miloua suggest that adding ZrO2 to the Cu-ZnO base can improve the dispersion of Cu particles in the catalyst, thereby enhancing catalytic activity. Congming Li, Xingdong Yuan, and Kaoru Fujimoto have studied the improvement in catalytic performance of copper-zinc-aluminum-based catalyst systems with the addition of Zr. The catalyst exhibits good tolerance for water vapor, and the addition of Zr increases CO2 conversion, inhibits the influence of water vapor, and suppresses catalyst passivation. The reason is that Zr promotes in-situ reduction of CuO (formed by reaction with water) in the reaction, thereby enhancing catalyst activity; inclusion of Zr in the catalyst enhances its reducing ability, which inhibits crystallization growth of CuOx and thereby suppresses catalyst passivation.

(2)Copper-based multi-component catalysts

have been studied extensively by scholars both domestically and internationally, with the addition of precious metals, rare earth elements, and silica being attempted. Other components such as Ga2O3 and Cr2O3 have also been added to the Cu-based system to investigate their effects on catalytic activity, selectivity, and catalyst lifespan. For example, J. Toyira, R. Milouac et al. developed a catalyst based on Cu/ZnO with the addition of Ga2O3 and Cr2O3, and their research showed that the addition of these materials can increase the catalytic activity per unit Cu surface area, while the addition of SiO2 can inhibit the crystallization of ZnO, thereby improving the catalytic performance.

Pawel Mierczynski, Piotr Kaczorowski and others studied the effect of adding 5% Pd or 2% Au to the CuO-ZrO₂-Al₂O₃ catalyst at a reaction temperature of 260℃ and pressure of 4.8 Mpa on the catalyst activity. The results showed that the addition of Pd or Au both decreased the catalyst's specific surface area. The order of methanol yields for the three catalysts was 5% Pd/CuO-ZrOz-Al₂O₃ > CuO-ZrOz-Al₂O₃ > 2% Au/CuO-ZrO₂-Al₂O₃, and the addition of Pd or Au significantly improved the catalyst's methanol selectivity. The results showed that Pd could enhance catalyst activity and promote the reduction of the ternary oxide.

Lin Minggui and others studied the effects of manganese and lanthanum on the synthesis of methanol with Cu/ZrO2 catalyst, and used BET, XRD, TPR, Hz-TPD and CO-TPD methods to study the structure and adsorption properties of the catalyst. The results showed that both manganese and lanthanum can effectively improve the activity of the catalyst, and the simultaneous introduction of the two can further improve the activity of the catalyst, showing a strong synergistic effect. The Chengdu Institute of Organic Chemistry of the Chinese Academy of Sciences has also developed ultra-fine copper chromium oxide catalysts. Under conditions of 90-150℃ and 3.0-5.5MPa, the single-pass conversion rate of synthesis gas reaches 90%, and the total selectivity for methanol and methanol acetate exceeds 98%, with a methanol selectivity of 80% and a space-time yield of 80.4g/(L h).

1.1.3 Novel gas-liquid phase copper-based catalys

Novel low-temperature gas-liquid phase copper-based catalysts are composed of cuprous salt and alcohol salt, which have higher catalytic activity and selectivity compared to gas-solid phase copper-based catalysts. The catalytic reaction temperature and pressure are lower, but the catalyst preparation process is more complex and the conditions are more demanding. Chen et al. used ultrafine CuB catalyst to synthesize methanol under liquid phase at 140-180℃, and the total reaction can be represented by equation 1-2. The optimal activity of the reaction occurs at 150℃, and it requires the addition of ThO2 and Cr2O3 as additives.

CO+2H₂→ CH₃OH

Reaction, ultimately resulting in methanol; the reaction temperature is around 170℃, and the alcohol serves as a solvent and auxiliary catalyst

1.2The role of copper in catalysts

Pic 3 Schematic diagram of the morphological changes of Cu particles attached to ZnO

Copper is the active center in copper-based catalysts, and there are three main views: the Cu center model represented by Klier, the Cu⁰ center model represented by Chinchen, and the Cu and ZnO collaborative (hydrogen overflow) model represented by Burch. With the development and application of in-situ characterization techniques, scholars have studied the electrical properties, crystal structure, and morphological and morphological changes of copper during the reaction, and proposed the following theories and assumptions. Peter C.K. Vesborg, Ib Chorkendorff, etc. used time-resolved methods to test the methanol synthesis reaction of Cu/ZnO catalysts and found that when the synthesis gas is a mixture of CO and H₂, there will be a sudden peak in the production of methanol during the initial stage of the reaction. Researchers used ETEM methods to observe the changes in the attachment morphology of Cu particles on ZnO (as shown in Figure 3). The morphology of Cu particles changes during the methanol synthesis reaction, and particles with a relatively flat shape have a higher methanol yield. After a period of time, the morphology of Cu particles changes from flat to spherical, leading to a decrease in methanol production. Therefore, there is a sudden peak during the initial stage of the reaction. Evgeny Kleymenov, Jacinto Sa et al. used HERFD, XAS, and EXAFS methods to characterize the Cu-ZnO-Al₂O₃ catalyst for methanol synthesis. They found that Cu* is the precursor for catalytic reactions. After a period of time, the catalyst mainly contains Cu⁰. It is only after all accessible copper is reduced that methanol synthesis officially begins. The catalyst structure that has already been reduced does not change with temperature or pressure. In addition, Timur Kandemir, Igor Kasatkin, Frank Girgsdies et al. studied catalyst samples prepared with different aging times and catalyst samples without Al₂O₃ from Cu-ZnO-Al₂O₃, respectively, and analyzed the surface crystal structure of copper. They found that catalyst activity is not only related to smaller microcrystallite size but also to the concentrated distribution of lattice defects, especially stacking dislocations.

Table 2-1 Comprehensive comparison of various methanol synthesis catalysts

2.1 Catalyst Comparison

(1) The classical copper-based catalyst Cu-ZnO-Al2O3 is the most mature process, but due to its low single-pass conversion rate, high energy consumption, and high requirements for synthesis gas, various copper-based multi-component catalysts and non-copper based catalysts have emerged, each with their own characteristics.

(2) Elements such as Zr and Si added to copper-based catalysts can promote the dispersion of Cu in the catalyst or facilitate the reduction of Cu, thereby improving conversion rates. Elements added to non-copper based catalysts such as Pd, Ru, Pt, etc. can enhance methanol selectivity or give the catalyst anti-poisoning properties.

(3) New low-temperature gas-liquid phase catalysts can catalyze the methanol synthesis reaction under low temperature (90-150℃) and low pressure conditions, significantly reducing gas consumption compared to traditional gas-solid phase catalysts.

2.2 Catalyst Development Trends Outlook

In the future, catalysts will continue to evolve and develop in response to various challenges and opportunities. New types of catalysts with improved properties and enhanced performance will be developed to address various industrial processes and meet increasingly stringent environmental standards. In addition, catalyst research will focus on reducing costs and improving efficiency while maintaining high conversion and selectivity. Furthermore, sustainable and environmentally friendly catalysts that are less harmful to the environment will be developed to address concerns related to environmental sustainability.

Outlook on the development trend of CO₂-to-methanol catalysts

2.2.1 Improving Single-Cycle Conversion Rate

Traditional copper-zinc-aluminum catalysts have a maximum single-cycle conversion rate of about 10%, which leads to problems such as high energy consumption, excessive by-product production, and cycle ratio. some scholars have tried to add MnOx, which has been found to increase the single-cycle conversion rate of CO2, but with a decrease in selectivity and difficulties in product separation.

2.2.2 Improving Catalyst Lifespan

In the process of coal-based synthesis gas to methanol, the raw gas usually contains sulfur and halogen elements, which easily react with the active center of copper-based catalysts, causing the catalyst to become inactive and seriously affecting its lifespan. To extend the catalyst lifespan, the current industrial practice is to reduce the content of sulfur and halogen in coal-based synthesis gas, which results in an increase in the purification cost of synthesis gas, making it one of the development trends of methanol synthesis catalysts.

2.2.3 Increasing Catalytic Activity

Zhang Xitong and others used two-step precipitation method with surface active agent to prepare super-fine methanol synthesis catalysts with high surface copper concentration, which increased the activity of copper-based catalysts by 9.3% and 16.8% respectively. Increasing activity is one of the development trends of methanol synthesis catalysts.