Kinetic Analysis for Selective Semi-Hydrogenation of Diphenylacetylene on Pt1Pd99/C Catalyst by PEM Electrolysis

Abstract

Introduction For carbon neutrality, organic synthesis should utilize renewable electricity. Alkyne semi-hydrogenation has been researched for a long time to prevent a side reaction: over-hydrogenation to produce alkane. Although the Lindlar catalyst, composed of Pd/CaCO3 added with lead acetate and quinoline, has been a dominant catalyst for stereoselective Z-alkene synthesis, lead toxicity is a concern. In contrast, PEM electrolysis enables synthesis under mild conditions without lead and hydrogen gas as a reactant, unlike the Lindlar catalyst.Our previous research revealed that Pt1Pd99/C (at%) catalyst had high activity and selectivity towards the semi-hydrogenation of diphenylacetylene (DPA) to form cis-stilbene due to the high proton reduction activity of Pt even with a small amount and the selective hydrogenation of DPA at Pd sites.1,2) It also qualitatively figured that electrolysis conditions that reduce residence time in the catalyst layer (low catalyst loading, high flow rate, high temperature) enhance selectivity.3) This study conducted essential quantitative kinetic analyses to evaluate performance under specific conditions and develop practical conditions. Experimental DSE® (De Nora Permelec Ltd) was used for the oxygen evolution as the anode and Nafion® 117 (DuPont) as the PEM, respectively. Carbon paper (39BB, SGL Carbon), loaded with 0.14 mg-PtPd cm−2 of Pt1Pd99/C (ISHIFUKU Metal Industry Co., Ltd.) and cut into 3.3 cm × 3.5 cm, was employed as the cathode. The cathode was hot-pressed on the PEM at 120 ℃, 6 MPa, for 3 min to make a membrane cathode assembly. In the anode compartments, 1 M (= mol dm−3) sulfuric acid circulated, and 1 M cyclohexane solution of DPA in the cathode compartments either flowed once-through or circulated.As electrochemical measurements, chronoamperometry at 1.3―2.1 V for once-through experiments and chronopotentiometry at 18 mA cm−2 for batch electrolysis were conducted. The concentrations of cis-stilbene and dibenzyl in electrolytes were quantified with high-performance liquid chromatography (HPLC) after collection during the last 2 min out of 5 min of electrolysis at 80 ℃. The partial currents were calculated by eqns. 1, derived from Faraday’s laws of electrolysis. Results and Discussion Figure 1 shows Tafel plots of the total and each partial current density derived by eqns. 1 at a 1 M DPA once-through flow rate of 2 mL min−1 and 80 ℃. The kinetic parameters, i 0 and α, were determined by the fitting lines of mass transport corrected Tafel equation (eqns. 2), shown in Table 1. The solid lines in Figure 1 represent the calculated partial current densities from the parameters, describing their behavior well.Figure 2 shows concentration-time profiles of 12-h batch electrolysis of 1 M DPA 30 mL at 18 mA cm−2 and 80 ℃. Initially, the concentration of DPA steeply decreased as the semi-hydrogenation to form cis-stilbene proceeded mainly. As a result, cis-stilbene at 65% concentration was produced with approximately 80% selectivity. The reactivity significantly changed around 9 h of electrolysis since the concentration of DPA became deficient, and thus the semi-hydrogenation rate dropped and the over-hydrogenation rate increased.The solid lines in Figure 2 were simulated from the estimated once-through partial current densities of 1 M DPA at 18 mA cm−2 in total by Figure 1, using eqns. 3. The rate constants k 1 and k 2 were respectively obtained by dividing the estimated once-through partial current density i 1 and i 2 by 2Fc A and 2Fc B . The computed kinetic parameters are shown in Table 2. The parameters derived by once-through electrolysis data described the concentration profiles in Figure 2 until 7.5 h of the batch electrolysis, when sufficient DPA existed. The high selectivity was quantitively explained by the high k 1/k 2 value at 18 mA cm−2. In addition, this simple kinetic model would predict products under a variety of operating conditions. Conclusion The kinetic analysis quantitively described the once-through partial current densities and the batch electrolysis concentration profiles. The high k 1/k 2 enabled cis-stilbene formation at 65% concentration with approximately 80% selectivity with the batch electrolysis. The suggested concise kinetic model would be beneficial to estimate the compositions in a batch electrolysis with once-through data. Acknowledgment This work was financially supported by CREST (JST Grant 18070940). The authors are grateful to ISHIFUKU Metal Industry Co., Ltd. for providing the cathode catalysts. References 1) S, Nogami, K. Nagasawa, A. Fukazawa, K. Tanaka, S. Mitsushima, M. Atobe, J. Electrochem. Soc., 167, 155506 (2020).2) S. Nogami, N. Shida, S. Iguchi, K. Nagasawa, H. Inoue, I. Yamanaka, S. Mitsushima, M. Atobe, ACS Catal., 12, 5430 (2022).3) H. Nakamura, J. Harada, N. Shida, K. Nagasawa, Y. Kuroda, M. Atobe, S. Mitsushima, ECS Meet. Abstr., MA2023-01, 2703 (2023). Figure 1