(329b) Sustainable Synthesis of C2-Building Blocks Via Ethanol Oxidative Dehydrogenation on Iron-Molybdenum Mixed Oxide Catalysts

Modern chemical industry is increasingly emphasizing highly efficient and sustainable processes. Rather than relying on crude oil, coal and natural gas as primary feedstocks, a rapidly growing number of waste reduction, reutilization, and circular strategies is being engineered. While not being exclusively motivated by the goals of defossilization, many sustainability approaches incorporate the usage of bio-based raw materials for the development of drop-in technologies. One chemical that is already available in large quantities and produced based on renewable resources is ethanol. Currently over 100 billion litres of bioethanol are produced annually by the fermentation of starch- and sugar-based feedstocks alone.¹ Hence, a key objective of current catalysis research is dedicated to the development of ethanol upgrading processes for the sustainable production of platform chemicals.² One possible route for the utilization of ethanol is its conversion to acetaldehyde via oxidative dehydrogenation (ODH), a route analogous to the well-established Formox process used in industry for the large-scale production of formaldehyde from methanol. This heterogeneously catalyzed process utilizes iron molybdate catalysts. Remarkably, only few studies are available regarding the applicability of these catalysts for the conversion of ethanol to acetaldehyde, even though this route provides a sustainable alternative to the currently applied Wacker-Hoechst process for acetaldehyde production via liquid phase ethylene oxidation.³ Therefore, this study evaluates the iron molybdenum mixed oxide system as a prospective catalyst for the ODH of ethanol to acetaldehyde.

Materials and Methods

Iron molybdenum mixed oxide catalysts of varying composition were prepared by coprecipitation of their respective metal salts. Performance and stability of these catalysts were investigated in a continuous flow fixed bed tubular reactor. Steady state as well as transient response methods were applied for kinetic investigations. Off-gas was analyzed by a quadrupole mass spectrometer and an online gas chromatograph. Structural characterization of catalysts was done by N₂-Physisorption, NH₃-TPD, ICP‑OES, Raman spectroscopy, XRD, SEM and XPS.

Results and Discussion

The evaluation of catalysts with varying compositions highlights an interesting finding: while pure iron oxide promotes the combustion reaction of ethanol, even the introduction of small molybdenum quantities provides catalysts with excellent acetaldehyde selectivity, allowing for single pass yields exceeding 90 %. Furthermore, these iron rich catalysts demonstrate superior activity compared to the Fe₂(MoO₄)₃ benchmark and enable almost full ethanol conversion at low temperatures while maintaining high selectivity (refer to Figure 1a). Alongside achieving equivalent yields at lower reaction temperatures, the use of iron-enriched catalysts has the additional benefit of extending the catalysts lifetime.

Our research demonstrates that the deactivation of molybdenum oxide-based catalysts in the ODH of ethanol proceeds via the depletion of active material by volatilization, analogous to the deactivation when employing methanol as the substrate.⁴ However, the deactivation is kinetically hindered at lower temperatures allowing the highly active iron rich catalysts to retain their activity (refer to Figure 1b), while the benchmark shows decreasing ethanol conversion and acetaldehyde selectivity over time. We will delve into the immense changes in both performance and stability among catalysts with different compositions, considering disparities in their structural and surface composition, as corroborated by a range of characterization techniques; i.e. we were able to correlate differences in product selectivities with differences in specific surface acidity, as evidenced by NH₃-TPD experiments (refer to Figure 1c).

Finally, steady state kinetics for the ethanol ODH, kinetics of bulk oxygen diffusion and desorption kinetics were studied for our highly active iron-rich catalyst. Through variation of feed composition and investigations into the consecutive oxidation of acetaldehyde and acetic acid kinetic parameters were obtained. Furthermore, we employed transient response methodologies to unveil novel mechanistic insights. Specifically, the involvement of bulk oxygen in the reaction was validated through pulse experiments (refer to Figure 1d). Lastly, a deeper understanding of surface reaction and desorption kinetics was obtained via temperature programmed surface reaction techniques (refer to Figure 1e and Figure 1f).

Significance

The improved mixed oxide catalysts developed in this work show outstanding performance in ethanol oxidation and provide an excellent example for the sustainable synthesis of C2-building blocks which are necessary for the defossilization of chemical industry in the near future.

Acknowledgements

The authors gratefully acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) ‑ CRC 1487 – Project number 443703006. The authors also thank Marcus Rose, Kathrin Hofmann, Felix Reinauer and Christian Hess for their insights and their help with this study.

References

1. R. Melendez, B. Mátyás, S. Hena, D. A. Lowy, A. El Salous, Renewable and Sustainable Energy Rev. 2022, 160, 112260.
2. A. Dagle, A. D. Winkelman, K. K. Ramasamy, V. Lebarbier Dagle, R. S. Weber, Ind. Eng. Chem. Res. 2020, 59, 4843.
3. Oefner, F. Heck, M. Dürl, L. Schuhmacher, H. K. Siddiqui, U. I. Kramm, C. Hess, A. Möller, B. Albert, B. J. M. Etzold, ChemCatChem 2022, 14.
4. V. Raun, L. F. Lundegaard, J. Chevallier, P. Beato, C. C. Appel, K. Nielsen, M. Thorhauge, A. D. Jensen, M. Høj, Catal. Sci. Technol. 2018, 8, 4626.

Figure 1: (a): Conversion of ethanol (EtOH) and selectivity to acetaldehyde (AcH), acetic acid (AcOH), diethyl ether (DEE), ethene and CO_x at different reaction temperatures for an iron-molybdenum mixed oxide catalyst. (b): Ethanol conversion and selectivities to the different reaction products during a stability test at 240 °C. (c): Comparison of the selectivities to diethyl ether and ethene with the specific surface acidity of different iron-molybdenum mixed oxide catalysts. (d): Transient response of a catalyst during a pulse experiment used to study bulk oxygen diffusion. (e): Acetaldehyde signals during a temperature programmed desorption experiments performed at various heating rates. (f): Redhead-Plot of the desorption experiments used for the determination of the activation energy for acetaldehyde desorption.