Towards Electrochemical Process Design for the Production of Valeric Acid

Abstract

In order to achieve considerable CO2 reductions there is an urgent need to develop alternative sustainable processes based on bio-based feedstock and renewable energy. The energy sector is currently in a transition towards cost competitive energy generation from renewable sources. Where the availability of cheap electricity presents an opportunity to electrify the chemical industry, as it can benefit from the progressive decarbonisation of the energy sector.1 Furthermore, the shift towards bio-based feedstock represents the largest abatement potential for CO2 emissions.2 Valeric acid (pentanoic acid) is currently produced via hydroformylation (oxo-process) of 1-butene followed by oxidation of valeraldehyde to the valeric acid. Levulinic acid, or 4-oxopentanoic acid, is one of the renewable platform chemicals and can be derived from lignocellulosic biomass via acid catalysed hydrolysis. Typically, levulinic acid is reduced in one step to valeric acid utilizing lead electrodes, with γ-valerolactone (gVL) being a minor by-product.3 Recently, we have demonstrated the viability of other cathode materials (indium, cadmium and zinc). Where indium exhibited superior selectivity, as no formation of the side product gVL has been detected.4 Although the electrochemical ketone reduction to the methylene functionality of levulinic acid and similar compounds is known for more than 100 years, no study on the influence of reaction conditions is reported to the best of our knowledge. Therefore, to come towards an optimal electrochemical process design, it is vital to understand the influence of reaction parameters. We therefore focused in our study on the influence of reaction conditions next to the applicability of other cathode materials. Typical high overpotential electrodes (In, Pb, Cd and Hg) exhibited the highest selectivity and activity towards VA, whereas Pt/C showed the highest selectivity and activity towards gVL. The influence of acidity and temperature on conversion and selectivity was studied for In, Pb and Cd together with the influence of anode material on the design of the electrochemical reactor. Interestingly we found that the selectivity is highly dependent of the acidity (pH) and the nature of the cathode material (see Figure). The increasing selectivity towards VA at decreasing pH is remarkable considering that lactonization of 4-hydroxy-pentanoic acid to gVL is catalyzed under acidic conditions. Another interesting aspect is the increasing conversion of LA at decreasing pH and thus increasing suppression of the electrochemical evolution of hydrogen.We tentatively explain these observed phenomena by an increased coverage of the electrode surface by protonated levulinic acid which in turn influences the activity and selectivity of the process.[1] DECHEMA, Low carbon energy and feedstock for the European chemical industry (2017).[2] Stork M., de Beer J., Lintmeijer N., den Ouden B., Chemistry for Climate: Acting on the need for speed Roadmap for the Dutch Chemical Industry towards 2050, February 2018.[3] Tafel J., Emmert B., Zeitschrift für Elektrochemie 17 (1911), 569-572; Chum H. L., Electrochemistry Applied to Biomass, October 1980 – September 1981, SERI, 1982; Nilges P., dos Santos T.R., Harnisch F., Schröder U, Energy Environ. Sci. 5 (2012), 5231-5235; Xin L., Zhang Z., Qi J., Chadderdon D.J., Qiu Y., Warsko K.M. Li W., ChemSusChem 6 (2013), 674- 686; Dos Santos T.R., Nilges P., Sauter W., Harnisch F., Schröder U., RSC Adv. 5 (2015), 26634-26643.[4] Bisselink R.J.M., Crockatt M., Zijlstra M., Bakker I.J., Goetheer E., Slaghek T.M., van Es D.S., ChemElectroChem 6 (2019), 3285-3280. Figure 1