Sustainable Electrochemical Synthesis of Dry Formaldehyde from Anhydrous Methanol
Authors: Schwarz Florian , Larenz Elizabeth , Mechler Anna K. 
Published: December 2024

Facing climate change, we switch our energy sources from fossil fuels to renewable electricity. This often requires the electrification of industrial processes that have so far been thermally driven. One of these large-scale processes is the methanol oxidation to formaldehyde with a yearly production of ca 45 MT worldwide. Formaldehyde is a versatile C1 building block that is used, for example, in the syntheses of polymers, resins and complex organic molecules. Conventionally, it is obtained by methanol oxidation with air to formaldehyde and water.[1] Attempts to electrify this process have been mainly studied in aqueous electrolytes, where a reasonable formaldehyde production was proven on the lab scale, but these reactions still struggle with the subsequent oxidation to CO or CO2 due to the presence of water.[2] Furthermore, for many products, formaldehyde is required in its dry form, such that energy intensive drying processes are needed subsequently.[3]

The first promising results for the electrochemical conversion of anhydrous methanol to formaldehyde on platinum electrodes were obtained in the 1960s. Faraday efficiencies of up to 75% were reported at low current densities (~3 mA cm-2) for batch systems under ice-bath cooling.[4]

In this work, we report the successful oxidation of anhydrous methanol to formaldehyde and hydrogen.[5] We not only verify the initial reports, now with modern reference electrodes and reliable measurement procedures, but also make the next step in the scaling of the process. Based on the reported setup, we changed to a non-aqueous reference electrode and worked at room temperature. Chronoamperometries over 2 hours yielded Faraday efficiencies of up to 78% at current densities of up to 10 mA cm-2, which already surpassed the reported results.

To scale the reaction up, we switched from an H-cell to a flow reactor with a surface area of 12 cm2 and applied current densities of up to 100 mA cm-2, i.e. a total of 1.2 A. This scaled-up system performed even better and reached Faraday efficiencies of up to 91% (Figure 1) and formaldehyde concentrations of up to 7.5 g L-1 in 30 minutes. Studying the impact of various reaction parameters, like current density, flow rate and electrolyte concentration, we furthermore reveal a fascinating potential dependent reactivity for the oxidation of anhydrous methanol. Our experiments indicate a vastly different behavior for the two mechanisms regarding mass transport and electrolyte viscosity and basicity.

Our results proof that the electrochemical synthesis of formaldehyde is selective and able to produce significant amounts of formaldehyde. The whole synthesis can be carried out anhydrously, which avoids energy intensive water removal for the industrial synthesis of resins, glues and polymers, and further recycles H2 from methanol as the cathode product. The robust behavior at elevated current densities is vital for further scale up and shows potential for industrial application.

References

[1] A. W. Franz, H. Kronemayer, D. Pfeiffer, R. D. Pilz, G. Reuss, W. Disteldorf, A. O. Gamer, A. Hilt, in Ullmann’s Encyclopedia of Industrial Chemistry, John Wiley & Sons, Ltd, 2016, pp. 1–34.

[2] J. Baessler, T. Oliveira, R. Keller, M. Wessling, ACS Sustainable Chem. Eng.
2023, 11, 6822–6828.

[3] M. Deitermann, Z. Huang, S. Lechler, M. Merko, M. Muhler, Chemie Ingenieur Technik
2022, 94, 1573–1590.

[4] K. Sasaki, S. Nagaura, BCSJ
1965, 38, 649–653.

[5] F. Schwarz, E. Larenz, A. K. Mechler, Green Chemistry
2024, DOI 10.1039/D3GC04978G.

Figure 1