2025-04-03 マサチューセッツ工科大学（MIT）
＜関連情報＞
- https://news.mit.edu/2025/surprise-discovery-could-mean-improved-catalysts-industrial-reactions-0403
- https://www.science.org/doi/10.1126/science.ads7913
Pd触媒による酢酸ビニル合成における均一-不均一二官能性 Homogeneous-heterogeneous bifunctionality in Pd-catalyzed vinyl acetate synthesis
Deiaa M. Harraz, Kunal M. Lodaya, Bryan Y. Tang, and Yogesh Surendranath
Science Published:4 Apr 2025
DOI:https://doi.org/10.1126/science.ads7913
Editor’s summary
There are two broad classes of chemical catalysis, one in which the catalyst dissolves with the reagents in homogeneous solution and another in which it persists in a separate heterogeneous solid phase. Harraz et al. reported the surprising finding that catalytic palladium (Pd) appears to operate in both regimes in the commercial synthesis of vinyl acetate (see the Perspective by Tway and Filip). The mechanism of this process, which couples ethylene with acetic acid using oxygen, has remained uncertain despite more than 50 years of vigorous study. Through multiple electrochemical experiments, the authors found evidence that heterogeneous Pd reduces oxygen to release Pd(II) into solution for the coupling step and redeposition. —Jake S. Yeston
Structured Abstract
INTRODUCTION
Although it has long been appreciated that heterogeneous catalyst materials can serve as precatalysts for homogeneous active species, and vice versa, seldom are both phases invoked in a single catalytic cycle. The assessment of catalyst phase encodes subsequent approaches for scientific study and rational catalyst design. The question of catalyst phase is particularly pertinent for palladium (Pd)–catalyzed vinyl acetate synthesis, a large-scale industrial process. In this reaction, heterogeneous Pd metal catalysts are used, and thin acetic acid wetting layers form on the catalyst surface that enable the formation of soluble Pd(II) species. The mechanism of vinyl acetate synthesis remains poorly understood, owing to uncertainty regarding the roles of both heterogeneous Pd(0) and homogeneous Pd(II). In this work, we used electrochemical probes to study vinyl acetate synthesis, revealing that interconversion of heterogeneous Pd(0) and homogeneous Pd(II) is required for catalysis, with each species playing a complementary role in the catalytic cycle.
RATIONALE
We envisioned that electrochemistry could provide a distinctive lens for elucidating the role of Pd(II) in this reaction, as the formation of Pd(II) proceeds via corrosion of metallic Pd, intrinsically an electrochemical process. Thus, the electrochemical potential of the catalyst, and how reaction rates scale with potential, illuminate the role of corrosion in the catalytic cycle. We conducted potentiometric and voltammetric experiments in an acetic acid–potassium acetate electrolyte as a model liquid-phase reaction medium and further extended our studies to more industrially relevant vapor-phase conditions using a solid electrolyte potentiometry technique that we developed.
RESULTS
In our model acetic acid–potassium acetate reaction medium, we found that Pd/C is subject to corrosion to form Pd(II) acetate, driven by O2 or anodic electrolysis. In this same solution, Pd(II) acetate rapidly reacts with ethylene to selectively produce vinyl acetate with concomitant formation of Pd(0). Under thermochemical reaction conditions, variation in O2 partial pressure changes both the rate of vinyl acetate production as well as the electrochemical potential of the Pd/C catalyst. From these findings, we hypothesized that vinyl acetate synthesis can proceed via either O2 driven, or external polarization driven, corrosion of Pd(0) to Pd(II) followed by rapid Pd(II)-mediated ethylene acetoxylation. We compared the rate-potential scaling of thermochemical aerobic vinyl acetate synthesis (from measuring the open circuit potential and thermal reaction rate of Pd/C under ethylene and varying amounts of O2), electrolytic vinyl acetate synthesis (from external polarization of Pd/C under ethylene in the absence of O2), and electrolytic corrosion (from external polarization of Pd/C under inert atmosphere). All three of these processes display a common scaling of product formation rate versus catalyst potential, signifying that they share a common kinetically relevant charge transfer step, arising from the Pd corrosion half-reaction. These observations support a mechanistic model for thermochemical vinyl acetate synthesis in which heterogeneous nanoparticulate Pd(0) serves as an active oxygen reduction electrocatalyst to furnish the high potentials required for corrosion to form homogeneous Pd(II), which then mediates selective ethylene acetoxylation, and redeposits as heterogeneous Pd(0). To extend these mechanistic investigations to the vapor phase, we used solid electrolyte potentiometry with a sodium β″-alumina solid-state electrolyte to evaluate the catalyst potential in the absence of a bulk liquid phase. Under vapor-phase reaction conditions, we observed the same rate-potential scaling for vinyl acetate synthesis as was observed in the liquid-phase reaction, suggesting that a similar mechanism is operative under vapor-phase conditions. Additionally, inhibiting the corrosion of Pd(0) to Pd(II) by galvanic protection resulted in reversible poisoning of catalysis in both the liquid and vapor phases, highlighting the essential role of phase conversion in this catalytic cycle.
CONCLUSION
Our results point to a mechanism in which catalyst phase conversion is an on-pathway process, required to furnish a bifunctional catalyst system in which heterogeneous Pd(0) and homogeneous Pd(II) carry out complementary roles of oxygen reduction and olefin functionalization, respectively. This paradigm challenges the tacit assumption that catalysis proceeds via either homogeneous or heterogeneous modes and instead highlights how dynamic phase interconversion can serve to harness and couple complementary reactivity across molecular and material active sites.
Mechanistic model for vinyl acetate synthesis, highlighting the complementary roles of metallic Pd(0) and soluble Pd(II).
Efficient electrocatalysis of the oxygen reduction reaction by metallic Pd furnishes the high potential required for Pd(II) formation in an acetic acid–acetate medium. Pd(II) carries out the selective acetoxylation of ethylene to form vinyl acetate and also Pd(0), which redeposits onto the heterogeneous catalyst surface.
Abstract
Presently, mechanistic paradigms in catalysis generally posit that the active species remains either homogeneous or heterogeneous throughout the reaction. In this work, we show that a prominent industrial process, palladium (Pd)–catalyzed vinyl acetate synthesis, proceeds via interconversion of heterogeneous Pd(0) and homogeneous Pd(II) during catalysis, with each species playing a complementary role. Using electrochemical probes, we found that heterogeneous, nanoparticulate Pd(0) serves as an active oxygen reduction electrocatalyst to furnish the high potential required for corrosion to form homogeneous Pd(II), which then catalyzes selective ethylene acetoxylation with reformation of heterogeneous Pd(0). Inhibiting the corrosion of Pd(0) to Pd(II) by galvanic protection results in reversible poisoning of catalysis, evincing the essential role of phase conversion in this catalytic cycle. These results highlight how dynamic phase interconversion can harness and couple complementary reactivity across molecular and material active sites.
