Steric Effects of Tetraalkylammonium Hydroxides on Nafion in Adiponitrile Production
Authors: Kim Mihyun , Akashige Toshihiro , Modestino Miguel A 
Published: August 2024

Over the last two decades, the chemical industry has witnessed a 43% increase in greenhouse gas emissions, with organic chemicals contributing to 75% of the total chemical production [1]. While organic electrosynthesis is a promising method for reducing the environmental impact of chemical manufacturing, advancing this technology faces significant challenges due to the limited viability of membranes used in divided electrochemical reactors. Despite the need for suitable membranes for electrosynthesis, only a few studies have explored the use of ion-conducting membranes for electro-organic reactions [2-7]. In this study, we aim to provide additional insights into the role of membranes in electrosynthesis and their behavior when exposed to electrolytes containing organic components.

We selected adiponitrile electrosynthesis, one of the largest industrial electro-organic processes, as a model reaction [2,3]. We used Nafion as a model membrane material. Electrolytes used in adiponitrile electrosynthesis contain tetraalkylammonium (TAA) ions to enhance the selectivity of electro-organic reactions. Here, we present a systematic study exploring how TAA ions impact Nafion's structure and transport properties. We exposed Nafion membranes to solutions with varying concentrations and sizes of TAA ions, including tetramethyl (TMA), tetraethyl (TEA), tetrapropyl (TPA), and tetrabutyl (TBA). We quantified TAA ion sorption in the membranes using ATR-FTIR. The results reveal that TAA ion sorption saturates when the solution concentration exceeds 0.02 M. Compared to the conductivity of Nafion equilibrated in pure water (9.8 x 10-2 S/cm), the membrane conductivity after exposure to TAA solutions exhibits a substantial decline, decreasing by up to 4 orders of magnitude. We performed small-angle X-ray scattering experiments to correlate the conductivity results with structural changes in Nafion. The measurements revealed that the spacing between Nafion's ion-conducting domains decreases as the TAA ion concentration increases. For example, when membranes are exposed to 1.0 M TBA solutions, the domain spacing drops from 4.9 nm to 3.76 nm compared to membranes equilibrated in pure water. Consequently, we infer those organic ions reduce the size of ion-conducting domains due to the diffusion of water from the membranes into the surrounding electrolyte, resulting in decreased conductivity. These results represent an important step in elucidating structure-property relationships in ion-conducting membranes used for organic electrosynthesis.

Reference

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[4] Baizer, M.; Lust, E.; Williamson, S.E.; Connor, R.F. "Electrochemical Synthesis of Adiponitrile." Industrial & Engineering Chemistry Process Design and Development, 1966, 5 (1), 56–62.

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[6] Blanco, D.; Prasad, P.; Dunningan, K.; Modestino, M. React. Chem. Eng., 2020, 5, 136

[7] Katzenberg, A.; Angulo, A.; Kusoglu, A.; Modestino, M. Macromolecules 2021, 54, 5187−5195

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