Pd(II) efficiently catalyzes the selective oxidation of ethane to acetate acid by in-situ formed H2O2 and •OH
image: The Gibbs free energy profiles and reaction pathways of H2O2 generation (a), ethanol formation (b), acetaldehyde formation (c), and acetate acid formation (d).
Credit: ©Science China Press
This study is led by Prof. Landong Li and co-led by Prof. Xiufang Xu and Dr. Xin Deng. Building on their previous studies on methane oxidation, Li’s group has successfully designed a simple Pd-catalyzed reaction system of C2H6-O2-CO-H2O. In this system, ethane can be selectively transformed into valuable acetate acid with high efficiency. This manuscript provides a promising new route for the efficient utilization of shale and natural gas resources under relatively mild conditions.
Even under the initial reaction conditions, the catalytic system exhibited exceptional performance with ethane conversion of 10.7% and acetate acid selectivity of 80.7%. Notably, with the addition of sulfuric acid (H2SO4), high ethane conversion of 15.7% and high acetate acid selectivity of 92.1% could be achieved at the same time, offering a state-of-the-art acetate acid space-time-yield of 372.7 molCH3COOH molPd−1 h−1, showing great potential for practical use.
Given the exceptional ethane oxidation performance, the reaction pathway and mechanism are explored through kinetic studies, spectroscopic techniques, and theoretical calculations. Kinetic studies show that the coexistence of O2 and CO is crucial. Spectroscopy reveals the reaction begins with the water-gas shift reaction (WGSR). In this step, the generated hydrogen (H2) reacts with O2 on Pd centers to produce hydrogen peroxide (H2O2) and hydroxyl radicals (·OH). These highly active oxygen species are key to breaking the C–H bonds, which drives the efficient and selective oxidation that follows.
Clear isotopic labeling experiments (using 13C and 18O) identified the origin of all atoms in the products. Ethane is confirmed as the sole carbon source for acetate acid and other liquid oxygenates. CO does not incorporate into the product carbon chain while it generates active oxygen species though WGSR. Oxygen source analysis further clarifies that the oxygen in ethanol originates from O2, while the oxygen in acetate acid and acetaldehyde comes from H2O.
Density functional theory (DFT) calculations are performed to comprehend the reaction pathway of ethane oxidation to acetate acid as well as to identify the active species. The reaction cycle initiates with [PdCl4]2− and involves the in situ generation of H2O2 and ·OH species, the stepwise conversion of ethane via ethanol and acetaldehyde intermediates, and finally to acetate acid. The formation of ethanol is identified as the rate-determining step. The calculation results also demonstrate that H2SO4 acts as a promoter by providing protons (H+), which facilitate the key proton-transfer steps and lower the overall energy barriers. The good agreement between theoretical and experimental results supports the proposed mechanism thereof.
This study not only develops a high-performance catalytic system for ethane oxidation, but also provides clear insight into mechanism investigation. It establishes a fundamental theoretical basis and reveals significant potential for the selective activation and functionalization of alkanes under relatively mild conditions.
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See the article:
Palladium-catalyzed selective oxidation of ethane to acetate acid
https://doi.org/10.1093/nsr/nwaf355