Mass Transfer Requirements for Alternating Polarity, Alternating Voltage Synthesis

Abstract

Operation of electrosynthesis reactors with 60 Hz transmission power modulates the electrode potential as a sine wave with no dc offset voltage. Alternating polarity alternating voltage (APAV) describes the synthesis because an electrode changes polarity every half cycle. In APAV electrosynthesis, products from electrode reactions must be removed from the electrode to prevent back reaction and loss of net current efficiency before the next half-cycle. An experimental study was made of the bromine current efficiency in the indirect electrosynthesis of alkene oxides. Reduction of soluble electrogenerated bromine at polarity reversal decreased the current efficiency for the production of bromine. High mass transfer, rapid homogeneous reaction, and low current density conditions produced high bromine current efficiencies. Bromine current efficiency was correlated as a function of three dimensionless numbers incorporating the homogeneous reaction rate constant, mass transfer coefficient, frequency, average current density, and exchange current density. A mass transfer criterion predicted process conditions required to produce bromine at 95% current efficiency. The correlations were applied to determine the suitability of a candidate chemical pathway for APAV electrosynthesis. Electrogenerated species in direct and indirect syntheses can be produced at 95% current efficiency with km/(D~) ~ above 5.6. Electrogenerated species in indirect syntheses with rapid homogeneous reactions can be produced at 50 to 95% current efficiency with 0.

Comprehensive Comparisons between Directing and Alternating Current Electrolysis in Organic Synthesis

“…Moreover, short‐live time species can be stabilized dynamically near the electrode surface and undergoes next steps in a very short time (Figure 19). [12d,6a] …”

Applications of Alternating Current/Alternating Potential Electro­lysis in Organic Synthesis

“…Lisius and Hart demonstrated that the mass transfer is a significant contributory factor to the prevention of the back reaction to achieve current efficiencies of up to 95% via removing of the working electrode product (Br 2 ) in the electrosynthesis of propylene oxide under alternating current conditions. 17 In the first half cycle of the alternating current electrolysis, Br 2 can be produced by oxidation of bromide anions and when the polarity is reversed, the Br 2 diffuses away from the electrode surface to initiate the desired transformation from propene to propene oxide.…”

Physico-chemical principles of AC electrosynthesis

“…Electrosynthesis is enjoying a renaissance in sustainable chemistry as an economical alternative to heat-driven reactions [1][2][3][4][5][6]. While electrochemical transformations using direct current (DC) -including constant current and and constant potential -are well-developed [7][8][9][10], considerably less effort has been made with alternating current (AC), whereby electrons reverse their flow periodically [11][12][13][14]. The latter has the advantage of introducing additional controllable parameters such as oscillation frequency and waveform shape, creating more opportunities for reaction control [15][16][17][18][19][20][21].…”