Investigating the Effect of Alkali Metal Cations on Kolbe Electrolysis

Abstract

Carboxylic acids, a major fraction of pyrolysis oil can be transformed electrochemically in alkanes and alcohols by Kolbe and Hofer Moest reaction. The selectivity of electrochemical decarboxylation depends on various factors, i.e. electrode material and reaction conditions (supporting electrolyte, pH and current densities) which are well explained in the literature1–3. Alkali metal cations (Li+, Na+, K+, Cs+) are widely considered inert,4 nevertheless in recent studies, a significant influence on different electrochemical reactions such as methanol5,6, ethanol7, and formic acid8 oxidation is reported. It is assumed that oxygenated species present on the electrode surface form (strong/weak) non-covalent interaction with the alkali metal cations.In this work, we studied the electrochemical oxidation of acetic acid (Kolbe electrolysis) and explored the influence of monovalent alkali metal cations (Li+, Na+, K+ and Cs+). It is observed that the product distribution and overall electrochemical activity are influenced by the presence of cations with K+ being most suitable for Kolbe electrolysis (order of activity Li+<Na+<K+~Cs+). The trend was observed irrespectively of the electrolyte conditions throughout the entire pH range at low current densities (25 mA/cm2), i.e. at pH 3 where OER is dominating, pH 5 where OER transitioned to Kolbe electrolysis and pH 9 where Hofer Moest reaction occurs. With increasing current densities (>50 mA/cm2), the influence of cations on electrooxidation diminishes showing the limitations of the effect of non-covalent interactions.To confirm the surface coverage and transition of OER to Kolbe reaction with increasing potential, we probed reaction selectivity in the inflection zone potential regime by means of RRDE. The analysis confirmed that the OER is suppressed completely in the presence of K+ cations after reaching potentials of ~2.5VRHE (inflection zone), while for Li+, OER still proceeds as confirmed by the ORR reaction at the ring electrode. Because surface coverage reflects the high activity of Kolbe electrolysis, we confirmed that the smaller cations interfere more strongly with the electrode surface adsorbed oxygenates due to strong non-covalent interaction and thus affecting the reaction. References F. J. Holzhäuser, J. B. Mensah, and R. Palkovits, Green Chem., 22, 286–301 (2020).C. Stang and F. Harnisch, ChemSusChem, 9, 50–60 (2016).T. Ashraf, A. P. Rodriguez, B. Mei, and G. Mul, Faraday Discuss. (2023)H. J. Schäfer, in Comprehensive Organic Synthesis, B. M. Trost and I. Fleming, Editors, p. 633–658, Pergamon, Oxford (1991)K. Silambarasan, J. Joseph, and S. Mayavan, Applied Surface Science, 489, 149–153 (2019).D. Strmcnik et al., Nature Chem, 1, 466–472 (2009).C. Han et al., ACS Appl. Mater. Interfaces, 14, 5318–5327 (2022).B. A. F. Previdello, E. G. Machado, and H. Varela, RSC Advances, 4, 15271–15275 (2014). Figure 1