(153c) From Art to Science: Connecting Synthesis Parameters of Fine-Tuned Cu/ZnO/ZrO2 (CZZ) Catalysts with Activity in Methanol and Direct Dimethyl Ether Synthesis

The use of carbon dioxide (CO₂) to produce chemical energy carriers contributes to establish a closed carbon cycle. In this regard, sustainable process chains such as Power-to-X (PtX) technologies are intensively investigated. In particular, the conversion of atmospheric CO₂, synthesis gas (CO₂, CO, and H₂) from variable sustainable sources and renewable hydrogen to methanol (MeOH) and subsequently dimethyl ether (DME) is a promising pathway because these products are key intermediates in the chemical industry, for sustainable fuel production (e.g., gasoline and kerosene), and as a potential energy carrier for hydrogen storage [1,2]. State-of-the-art catalysts have been developed for the conversion of synthesis gas with low CO₂ content at industrial scale, based on copper/zinc and alumina as promotor. Increasing CO₂ content in the reaction feed system leads to additional challenges for catalyst and process development, especially with respect to thermodynamically limited conversion and to catalyst stability over long process periods [3].

Several studies showed that ZrO₂-promoted copper/zinc catalysts have promising activity regarding the hydrogenation of CO₂-rich syngas, due to its higher tolerance towards water in comparison to typical used Al₂O₃-promoted copper catalysts, however, the exact cause of the promotional effect is still under debate. [4,5,6]

In this contribution, highly stable copper-zinc-zirconia-based catalysts were prepared and optimized. A micromixer was used to precisely vary key synthesis parameters such as zirconium (Zr) content and pH. A parallelized and automated catalyst test setup is used to quickly provide information on the stability and activity of the catalysts under technically relevant conditions (e.g. high pressure and temperature) for the direct synthesis of DME.

Methods

The process optimization study to achieve higher efficiencies, such as yields and conversions, in the direct synthesis of DME uses methanol and dehydration catalysts in a physical mixture. A commercially available zeolite (H-FER-20) is selected as the dehydration component. This ferrierite (FER) type zeolite is used due to its high activity and high stability towards water [7]. As a methanol catalyst, an in-house produced Cu/ZnO/ZrO₂ (CZZ) catalyst is used.

The CZZ catalysts are prepared by continuous co-precipitation [8], which allows precise control of synthesis parameters, and consequently fine-tuning of the catalytic properties. In this study, a series of CZZ pre-catalysts were prepared with varying one synthesis parameter each (e.g. zirconium (Zr) content, pH) while keeping the Cu/Zn ratio constant. For comparison, different methanol catalysts are characterized by physical methods (e.g., N₂-Physisorption, N₂O‑Chemisorption) and tested in catalytic performance tests. The experimental screening of these different methanol catalysts for direct DME synthesis using CO₂-rich synthesis gas as feed was performed in an automated, parallelized test rig consisting of six fixed bed reactors. The productivities, yields of DME (P_DME, Y_DME) and cumulative MeOH (P_{MeOH, cum.,} Y_MeOH,cum.) as well as CO₂-conversion (X_CO2) are the key performance indicators for activity evaluation at 30 bars, using various temperatures (210 °C, 230 °C and 250 °C) and CO₂/CO_x ratios from 0.25 to 0.75. The catalytic reaction data of the parameter variation were measured at reaction times from 310 hours to 490 hours.

Results and Discussion

It is assumed that the use of zirconium significantly increases the activity, which was correlated with the analytical characterizations when looking at the specific surface area and copper surface area.

Figure 1 shows the cumulative methanol yields (including DME and methanol yields) at different CO₂/CO_X ratios and a reaction temperature of 250 °C, here, the screening catalytic systems contain different amounts of zirconium in the methanol catalyst at a pH of 7. First, it can be seen that the yield decreases approximately linearly with increasing CO₂ content in the syngas feed which agrees with previous studies. [7,9] From the figure it can be seen that a higher activity of the methanol catalysts is obtained by introducing zirconium. The use of 1 mol.-% Zr as catalyst component increases the activity between 70 % (for CO₂/CO_X = 0.25) and 46 % (for CO₂/CO_X = 0.75) in terms of cumulative methanol yield. It is also shown that the activity reaches a peak/maximum of 9.2 mol.-% Zr at the study conditions.

Compared to the catalyst system without zirconium, the addition of 9.2 mol.-% Zr increases the cumulative yield between 120.6 % and 80.5 %, depending on the CO₂-ratio in the feed. CZZ-catalysts with a zirconium content between 5.2 and 9.2 mol.-% show similar activities with a relative deviation of approx. 3 %. Therefore, no further increase in catalytic activity is expected with higher Zr content.

In addition of the study of CZZ catalysts at different Zr contents, CZZ catalysts were precipitated at different pH values. The same composition was used in the synthesis to obtain a Zr content of 9.2 mol.-%. Figure 2 shows that adjusting the pH has the potential to increase the activity to methanol. The catalyst systems with a pH of 6 and 7.4 are less active in comparison to a pH of 7 and 8, which show similar activities. In the catalytic tests, the catalyst system with the catalyst produced at pH 6.7 clearly shows the best results.

By comparing the metal compositions, the smallest deviations between the expected and actual measured values of the metal compositions were observed at a pH of 6.7 in the catalyst synthesis.

The dashed line in Figure 2 represents the yield obtained with the catalyst system with 9.2 mol.-% Zr and a pH of 7 (highest yields from Fig. 1). A comparison with the results of the CZZ catalyst at pH 6.7 shows that an increase in the cumulative methanol yield of up to 6.3 % is obtained by adjusting the pH from 7 to 6.7.

Conclusion

In conclusion, a precise control over catalyst synthesis parameters is shown to be necessary to get the highest catalytic activity for their application. In addition, trends and insights for catalytic testing can be derived from analytical methods after catalyst production, which are confirmed by the experimental evaluation such as that higher specific surface area and copper surface area lead to higher yields.

When comparing the analytical and the test results of the catalysts with systematically varied Zr content and pH, it is noticeable that the two catalysts with a zirconium content of 9.2 mol.-% and a pH of 6.7 give the best properties and highest cumulative activity yields. By optimizing the Zr content and adjusting the pH value compared to the initial starting catalyst (Zr content of 5.2 mol.-% Zr and pH of 7), the yield is increased by up to 11 %, depending on the CO₂ ratio in the feed.

References

[1] DME Handbook, Japan DME Forum, 2007

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[3] J.D. Grunwaldt et al., Journal of Catalysis, 194 (2000), 452-460

[4] Warmuth et al., ACS Appl. Mater. Interfaces 2024,16, 7, 8813−8821

[5] Arena F. et al., J. Catal. 2007, 249, 185-194

[6] Larmier K. et al., Angew. Chem. Int. Ed. 2017, 56, 2318 - 2323

[7] Wild et al., RSC Advances, 11 (2021), 2556-2564

[8] Polierer et al., Catalysts,10 (2020), 816

[9] S.Wild et al., React. Chem. Eng. 2022, 7, 943