Catalysts and process advancements for the one-step oxidation of ethane to acetic acid

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Preface: For decades, the acetic acid production process has relied on a carbonylation process using methanol as the raw material. Although catalysts have been continuously improved, no fundamental breakthroughs have been achieved in this process. Therefore, the author came up with the idea of exploring research on the one-step synthesis of acetic acid from ethane. It was found that significant progress has been made in the foundational research, and industrial implementation has also been well proven; it is believed that in the near future, the technology for converting ethane into acetic acid will be able to be put into practical use. No corrections are indicated in the text; please bear with us and point out any errors. I. Traditional acetic acid process: As one of the most fundamental organic acids, acetic acid is produced industrially through the carboxylation of methanol. In 1960, the German company BASF was the first to develop a high-temperature and high-pressure methanol carbonylation process using cobalt as a catalyst. In 1970, the American company Monsanto made a significant breakthrough by developing a low-pressure methanol carbonylation process using rhodium/iodide as a catalyst; this technology employed RhI3 as the catalyst, achieving a selectivity of over 99% for acetic acid based on methanol. Subsequently, Celanese developed the Acid-Optimized Low Water Content process (AO Plus), while BP introduced the Cativa process in 1996. This process uses a new catalyst system based on iridium, along with various additives such as rhenium, ruthenium, and osmium; it offers higher activity than rhodium catalysts, produces fewer by-products, and can operate at water concentrations of less than 5%, thereby reducing production costs by up to 30%. Currently, over 90% of new acetic acid production facilities worldwide use the methanol carbonylation process, and acetic acid produced by this process accounts for more than 60% of the world’s total output. II. Economic advantages of one-step oxidation of ethane to acetic acid. The main advantage of converting ethane into acetic acid through one-step oxidation is that, compared with the traditional methanol carbonylation method, this process eliminates the complex steps of producing methanol from syngas and then carrying out methanol carbonylation, thereby reducing raw material costs. Secondly, it does not use precious metal complexes or iodide promoters, thereby completely eliminating the impact of equipment corrosion and trace iodine residues on product quality. The product separation and purification process is significantly shortened, resulting in higher safety and environmental sustainability. In recent years, significant advances have been made in high-performance catalysts, leading to simultaneous improvements in ethane conversion rate and acetic acid selectivity. The excellent yields of carbon-based products along with high atom economy make this route a highly competitive alternative for the green chemical production of acetic acid. III. Reaction mechanism for the one-step oxidation of ethane to acetic acid: Taking the common ethane oxidation catalyst, the Mo-V-Nb composite oxide catalyst, as an example, the oxidation reaction follows the Mars–van Krevelen (MvK) redox mechanism – lattice oxygen is involved in the activation of C-H bonds and the formation of products, after which the catalyst is re-oxidized by gaseous O₂. Dynamical and isotope studies indicate that the activation of the C-H bond is the only kinetically relevant step in the reaction. There are two pathways for the reaction: one is the generation of ethylene through the decomposition of ethoxy-hydroxy species, and the other is the conversion of hydroxyalkoxy intermediates into acetaldehyde, which is then rapidly oxidized to acetic acid. Water, as a byproduct or additive, can promote the desorption of adsorbed acetate species, thereby enhancing acetate selectivity. IV. Current Research Status at Home and Abroad Researchers at home and abroad have conducted extensive fundamental research on the one-step conversion of ethane into acetic acid, with four main technical approaches: heterogeneous catalysis, homogeneous catalysis, photocatalysis, and electrocatalysis. (1) Multiphase catalytic Mo-V-Nb oxides are currently the most prominent catalytic systems in the field of ethane partial oxidation, capable of producing both ethylene and acetic acid. The addition of trace amounts of Pd (0.0025–0.01 wt%) can significantly improve acetic acid selectivity—Pd, in the form of nanometal particles, is dispersed on the catalyst surface, facilitating the deeper oxidation of the ethylene intermediate to acetaldehyde, which is then rapidly converted into acetic acid. The main challenge with MoVNbOx catalysts is that ethylene, produced by the oxidation of ethane as a reaction intermediate, consumes the surface lattice oxygen, thereby inhibiting the C-H bond activation step and limiting the increase in ethane conversion rate. Using ethane and water as raw materials, the reaction takes place at around 300°C and at atmospheric pressure, thanks to a palladium-containing catalyst and a localized aqueous environment. At present, this pathway is mainly in the mechanism model stage, with ethylene and acetic acid as the predicted co-products ； By referring to the classic Mo-V-Nb polymetal oxide thermal catalysis system, an ethane conversion rate of about 16% can be achieved, with a total selectivity for acetic acid and acetaldehyde of approximately 90%. (2) Homogeneous catalysis: Ethane, oxygen, carbon monoxide, and water are used as raw materials; the reaction takes place under mild liquid-phase conditions, and sulfuric acid must be added as a catalyst. The product was mainly acetic acid, achieving an ethane conversion rate of 15.7% and an acetic acid selectivity of 92.1%, with a space-time yield of acetic acid reaching a record value of 372.7 mol·molPd⁻¹·h⁻¹. (3) Photocatalysis: Using ethane, oxygen, and water as raw materials, the reaction takes place at normal temperature and pressure under light stimulation (even at 0°C freezing conditions), with acetic acid as the main product. In typical studies, the selectivity for acetic acid in the liquid-phase product is close to 100%, the yield can reach 5.65 mmol·g⁻¹·h⁻¹, and stable operation is possible for 102 hours ； Another photocatalytic system achieved 98.7% acetic acid selectivity and a conversion frequency of 278 h⁻¹. (4) Electrocatalysis: Using ethane and oxygen as raw materials, the reaction is carried out at room temperature and atmospheric pressure with commercial copper electrodes and by periodically reversing the current. The product is a co-production mixture of ethylene and acetic acid, with an acetic acid formation rate of 6.2 µmol·cm⁻²Cu·h⁻¹, a total selectivity of over 96%, and by-product carbon dioxide at less than 3%. V. Progress in the development of engineering processes Regarding the one-step oxidation of ethane to acetic acid, the main industrial developments to date focus on the early route developed by Saudi Basic Industries Corporation (SABIC), as well as the second-generation technology route jointly developed by Linde and Clariant. Essentially, both technologies are processes for the oxidation of ethane to produce ethylene and acetic acid. The differences lie in the performance of the catalysts and the structure of the reactors; ultimately, this results in different selectivities for the two products. This route was initially developed by SABIC as a one-step gas-solid phase catalytic oxidation process. Its world’s first industrial plant with an annual capacity of 30,000 tons came online in 2005, and the acetic acid produced there is mainly used in the production of PTA downstream. SABIC once claimed that this technology was 40%–50% cheaper than the methanol carbonylation method, but it seems to have not granted further large-scale licenses thereafter. In 2021, Linde and Clariant introduced catalysts specific for the EDHOX™ process. The ratio of acetic acid to ethylene produced in the ethane oxidation process can be adjusted flexibly within the range of 2.5–4.5, with a total selectivity for both compounds exceeding 93%. This project has completed its commercial validation and is advancing multiple potential projects. The reaction takes place at temperatures below 400°C, saving 70% energy compared to traditional steam cracking; it also reduces direct carbon emissions by at least 60%, while CAPEX is reduced by 5%–20%. In China, in July 2025, Liaoyang Petrochemical’s \"world’s first industrial test unit for ethane oxidation dehydrogenation with a capacity of 3,000 tons per year\" was successfully commissioned. This process can also produce acetic acid as a by-product while manufacturing ethylene. VI. Future trends in technology: The key to ethane oxidation for producing ethylene lies in catalysts. Future catalyst development will focus on precise control of the crystal phase (such as directed synthesis of the M1 phase), further enhancement of the synergistic effects among multiple metals, and meticulous design of carriers/dopants, in order to improve activity at low temperatures and the selectivity for target products ； At the same time, it evolves in the direction of non-precious metal alternatives, long-term stability of catalysts, as well as resistance to carbon deposition/sintering, laying the foundation for large-scale application. Based on innovative oxidation catalysts, process development will evolve from optimizing single-pass conversion rates to overall process integration, with a focus on aspects such as heat recovery and recycling from reactions, the use of new reactors to achieve high space velocities with low levels of oxidation by-products, and rapid catalyst regeneration processes. References: 1. “Peroxidation Activation of Ethane with Hydrogen and Oxygen by Au-Based MWW Zeolite Acidic Catalyst”, ACS. 2. “Palladium-catalyzed selective oxidation of ethane to acetate acid”, National Science Review. 3. “Efficient and Selective Hydroxyl-Mediated Photocatalytic Ethane Oxidation to Acetic Acid”, Journal of the American Chemical Society. 4. “Electrochemically Promoted Activation of Light Alkanes at Ambient Conditions”, Angewandte Chemie International Edition.