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Monsanto process

The Monsanto process is an industrial method for the large-scale production of acetic acid via the catalytic of using as the carbonyl source. The reaction, CH₃OH + → CH₃COOH, proceeds under conditions with high selectivity (>99%) and employs a rhodium-iodide catalyst system, typically involving species such as [Rh(CO)₂I₂]⁻ and methyl iodide as a promoter. Developed by the Company in the 1960s and first commercialized in 1970 at a plant in City, the process operates at moderate temperatures (150–200°C) and pressures (30–60 bar) in the presence of water and , marking a significant advancement over earlier methods like the process. The involves of methyl iodide to the rhodium center, followed by CO insertion to form an acyl , to acetyl iodide, and subsequent to acetic acid, regenerating the catalyst and iodide components. By the 1970s and 1980s, it became the dominant technology for acetic acid manufacture, accounting for over half of global production and enabling efficient synthesis of a key chemical used in , , and other derivatives. Although later supplemented by the more economical iridium-based Cativa process due to rhodium's high cost and sensitivity to low water levels, the Monsanto process remains a cornerstone example of organometallic in industry.

Overview

Chemical Reaction

The Monsanto process synthesizes acetic acid through the catalytic of using as the carbonylating agent. The core is given by the equation \ce{CH3OH + CO -> CH3COOH} This represents a stoichiometric 1:1 molar ratio between and , yielding acetic acid as the primary product. The reaction is exothermic, with a standard enthalpy change (ΔH) of approximately -138 kJ/, indicating significant heat release during the process. Thermodynamically, the strongly favors the formation of acetic acid due to the reduction in the number of gaseous moles (from one gaseous reactant, CO, and liquid to liquid acetic acid), which is promoted by high-pressure conditions typical of the industrial setup. Elevated temperatures are employed primarily to enhance kinetic rates, as the remains favorable across the operating range. Methanol feedstock for the Monsanto process is commonly derived from , a mixture of and produced via of or of coal. The input must be of high purity, exceeding 99 wt%, to minimize and ensure efficient reaction selectivity.

Catalysts Involved

The Monsanto process utilizes a homogeneous rhodium-iodide system to facilitate the of to acetic . The primary consists of species, most commonly rhodium triiodide (RhI₃) or the square-planar complex [Rh(CO)₂I₂]⁻, which serves as the active form under reaction conditions. Typical rhodium concentrations range from 1000 to 3000 ppm by weight of the reaction mixture, balancing catalytic activity with economic considerations given rhodium's high cost. Methyl iodide (CH₃I) functions as a critical promoter or co-catalyst, present at concentrations of 1-5 wt% in the reaction medium, where it accelerates the overall rate by promoting the step to the center. Additional sources, such as () or iodides (e.g., or ), are incorporated to maintain the necessary ionic equilibrium and stabilize the anionic species, often at levels up to 20 wt% for the latter. Catalyst stability poses a challenge in the process, particularly the risk of rhodium precipitation as insoluble Rh₂(CO)₄I₂ or related species when the partial pressure of drops below approximately 5 during downstream purification stages, necessitating careful control of operating conditions to prevent losses of the expensive metal.

History

Early Research

The research into methanol for acetic acid production originated in the 1940s at , where chemists developed a high-pressure process operating at 500–700 bar and 250–300°C using catalysts. This approach, pioneered by Walter Reppe, marked a significant advancement over earlier thermal methods by enabling catalytic under elevated pressures, though it suffered from low selectivity and high energy demands due to byproduct formation. Reppe's contributions extended to -catalyzed low-pressure variants for related substrates, laying foundational principles for transition metal-mediated insertions that influenced subsequent industrial processes. In the , researchers at Monsanto Company shifted focus to more efficient catalysts, leading to the discovery of a -based system that dramatically improved reaction kinetics and selectivity. Initial laboratory experiments identified promoters as essential for enhancing activity, with methyl emerging as a key species formed from and . This promoter facilitates to the center, accelerating the under milder conditions. The breakthrough culminated in a 1968 patent filing (issued as US 3,769,329 in 1973) by team members Frank E. Paulik, Arnold Hershman, Walter R. Knox, and James F. Roth, detailing the rhodium-iodide catalyst system for . Compared to the system, the rhodium-iodide approach enabled operation at 30–60 bar and 150–200°C while achieving over 99% selectivity to acetic acid. These lab-scale studies validated the system's potential, setting the stage for further optimization prior to commercialization.

Commercialization

The Monsanto process transitioned to commercial production in 1970 with the startup of its first industrial plant in , boasting an initial capacity of 150,000 tonnes per year of acetic acid. This facility marked the culmination of Monsanto's research efforts and demonstrated the viability of rhodium-iodide for large-scale . The plant's successful operation validated the process's high selectivity and efficiency, paving the way for broader adoption. Following the Texas City launch, the process underwent rapid expansion through Monsanto's licensing strategy, which facilitated and generated substantial royalties. Numerous plants worldwide were operating under the Monsanto process by the 1980s, including facilities licensed to major chemical firms such as Hoechst-Celanese and Eastman. This proliferation was driven by the process's economic advantages over older methods like oxidation, enabling licensees to meet growing demand for acetic acid in and production. By the mid-1970s, the Monsanto process had achieved market dominance, accounting for more than 50% of global acetic acid production and solidifying its role as the preferred industrial route. Monsanto's licensing model not only accelerated global capacity growth but also established the company as a leader in technology. In 1986, BP Chemicals acquired the exclusive licensing rights to the Monsanto process.

Mechanism

Catalytic Cycle

The catalytic cycle of the Monsanto process for acetic acid production via methanol carbonylation centers on a rhodium(I)-iodide system, where the key active species is the square-planar anion [\ce{Rh(CO)2I2}]^-, which acts as the predominant resting state under operating conditions. The cycle initiates from rhodium precursors, such as \ce{RhCl3} or \ce{Rh2O3}, which are reduced in situ under carbon monoxide pressure to form neutral rhodium carbonyl species like \ce{Rh(CO)4}, ultimately converting to the active [\ce{Rh(CO)2I2}]^- complex in the presence of iodide. Once activated, the cycle proceeds through a sequence of organometallic transformations: oxidative addition of methyl iodide (\ce{CH3I}) to [\ce{Rh(CO)2I2}]^- generates a methyl-rhodium(III) octahedral complex [\ce{(CH3)Rh(CO)2I3}]^-; migratory insertion of carbon monoxide into the Rh-CH₃ bond forms an acetyl-rhodium(III) species [\ce{(CH3CO)Rh(CO)I3}]^-; a second CO ligand coordinates to yield [\ce{(CH3CO)Rh(CO)2I3}]^-; and reductive elimination expels acetyl iodide (\ce{CH3COI}), regenerating [\ce{Rh(CO)2I2}]^-. Outside the rhodium cycle, \ce{CH3COI} undergoes rapid hydrolysis with water and iodide to produce acetic acid (\ce{CH3COOH}) and regenerate \ce{CH3I}, completing the overall transformation of methanol and CO to acetic acid. The rate-determining step in the catalytic cycle is the oxidative addition of \ce{CH3I} to the Rh(I) species, which follows second-order kinetics: \text{rate} = k [\ce{Rh(CO)2I2}^-] [\ce{CH3I}], reflecting the sensitivity of the process to methyl iodide concentration and underscoring the role of iodide promoters in enhancing this step. This step's activation barrier dominates the kinetics, with subsequent insertion and elimination steps occurring more rapidly under typical reaction conditions. Water is essential for maintaining the cycle's efficiency, with concentrations of 5-15 wt% required to solubilize the ionic species and prevent of inactive rhodium iodides, while also driving the of \ce{CH3COI} to acetic acid and \ce{HI}. Below this threshold, catalyst instability leads to reduced activity and potential deactivation.

Key Reaction Steps

The Monsanto process catalytic cycle consists of four primary elementary steps that facilitate the carbonylation of to acetic acid using a - system. These steps include , migratory insertion, , and , which collectively regenerate the active and close the promoter cycle involving methyl . The first step is the of methyl to the rhodium(I) complex, forming the anionic alkyl-rhodium(III) intermediate [Rh(CH₃)(CO)₂I₃]⁻. This rate-determining step proceeds via an SN₂-like mechanism where the methyl group from CH₃I adds to the metal center, accompanied by coordination: [\ce{Rh(CO)2I2}-] + \ce{CH3I} \rightarrow [\ce{Rh(CH3)(CO)2I3}-] This exothermic reaction generates the octahedral complex, which is crucial for subsequent carbonylation. In the second step, carbon monoxide undergoes migratory insertion into the rhodium-methyl bond of the intermediate. The methyl group migrates to a coordinated CO ligand, forming a five-coordinate acyl complex [Rh(CH₃CO)(CO)I₃]⁻ and creating a coordination vacancy: \ce{[Rh(CH3)(CO)2I3]- \rightarrow [Rh(CH3CO)(CO)I3]-} This step is relatively low-barrier and exothermic for , enabling efficient acyl formation essential to the pathway. The third step involves coordination of a second ligand to the acyl complex, followed by , expelling acetyl iodide and regenerating the rhodium(I) species: \ce{[Rh(CH3CO)(CO)I3]- + CO \rightarrow [Rh(CH3CO)(CO)2I3]- \rightarrow CH3COI + [Rh(CO)2I2]-} This process reverses the in a thermodynamic sense, with a modest activation barrier, and returns the catalyst to its active form. The final step encompasses the of acetyl iodide to acetic acid, coupled with regeneration of methyl iodide from the promoter: \ce{CH3COI} + \ce{H2O} \rightarrow \ce{CH3COOH} + \ce{HI}; \quad \ce{HI} + \ce{CH3OH} \rightarrow \ce{CH3I} + \ce{H2O} These non-metallic reactions complete the promoter cycle, recycling CH₃I for the oxidative addition while producing the desired acetic acid product. A notable side reaction in the system is the water-gas shift, where CO reacts with water to form CO₂ and H₂: \ce{CO} + \ce{H2O} \rightarrow \ce{CO2} + \ce{H2} This competes with the main carbonylation by consuming CO and reducing selectivity to approximately 85% based on CO, but is managed under standard operating conditions with 5-15 wt% water, iodide promoters, and temperature control.

Process Details

Operating Conditions

The Monsanto process for acetic acid production via methanol carbonylation operates in a homogeneous liquid-phase reactor under carefully controlled conditions to ensure high reaction rates, catalyst stability, and selectivity exceeding 99%. The temperature is typically maintained between 150 and 200°C, with an optimal value around 180°C that maximizes the carbonylation rate while minimizing side reactions and catalyst deactivation. Higher temperatures accelerate the kinetics but can promote unwanted water-gas shift activity, whereas lower temperatures reduce overall productivity. Pressure is set at 30–60 bar, predominantly driven by the partial pressure of at 15–30 bar, which is essential for suppressing precipitation and maintaining the active Rh(I) species in solution. The reaction medium consists of acetic acid as the primary solvent, incorporating 10–15 wt% to solubilize the promoter and prevent catalyst instability, alongside 5–10 wt% , which participates in the by generating methyl iodide . These compositions ensure efficient of the promoter and support the homogeneous environment required for the rhodium-iodide system. The process utilizes a configuration, leveraging the rapid kinetics of the —with a turnover frequency of approximately 1000 h⁻¹—to achieve short residence times of 20–60 seconds and high throughput. The gaseous feed is primarily , which may contain small amounts of . These parameters collectively enable the process to operate efficiently while depending on the underlying for sustained performance.

Reactor Design

The Monsanto process utilizes a (CSTR) to facilitate the of with , ensuring thorough mixing and efficient gas-liquid contact in a back-mixed configuration. This design accommodates the homogeneous rhodium-iodide catalyst system and handles the while maintaining steady-state conditions. Reactor vessels are constructed from corrosion-resistant alloys, such as Hastelloy B-2 or titanium linings, to mitigate degradation caused by the highly corrosive mixture of hydrogen iodide, acetic acid, and methyl iodide present in the reaction medium. These materials provide durability under the acidic and iodide-rich environment, with typical designs incorporating a corrosion allowance of several millimeters. Following the reactor, the effluent is directed to a flash separation unit where pressure is reduced, vaporizing light components including methyl iodide, carbon monoxide, and methyl acetate for recycling to the reactor inlet. The resulting crude liquid stream undergoes multi-stage distillation in light ends, drying, and heavy ends columns to yield high-purity acetic acid exceeding 99.8%, while iodide promoters and unreacted species are recycled to minimize losses. The homogeneous catalyst remains dissolved in the heavy phase and is recovered via this liquid recycle stream, typically without the need for filtration. Industrial implementations feature single-train configurations capable of producing up to 800,000 metric tons of acetic acid annually, optimizing capital efficiency for large-scale operations. Safety provisions include detectors to monitor toxic gas levels and pressure relief systems to manage the high-pressure environment, preventing overpressurization risks.

Related Processes

Tennessee Eastman Process

The Tennessee Eastman process is an adaptation of the Monsanto acetic acid carbonylation technology, specifically tailored for the production of acetic anhydride. Developed by (a division of ) during the late 1970s and early 1980s in response to volatile oil prices and the need for alternative feedstocks, the process leverages coal-derived synthesis gas to enable large-scale, sustainable production. The first commercial plant commenced operations on October 6, 1983, in , marking a pioneering use of for ; this facility initially had a capacity of approximately 227,000 metric tons per year of acetic anhydride, later expanded in 1991 to over 450,000 metric tons per year. The process achieves the net transformation of two molecules of into and , $2 \ce{CH3COOH -> (CH3CO)2O + H2O}, but employs a route for efficiency: first reacts with to form , \ce{CH3COOH + CH3OH -> CH3COOCH3 + H2O}, followed by of with , \ce{CH3COOCH3 + CO -> (CH3CO)2O}. This pathway integrates seamlessly with upstream production from , avoiding the energy-intensive direct of . The combined effectively mirrors the but shifts the focus to anhydride formation under controlled conditions to minimize side reactions. The catalytic system builds on the rhodium-iodide foundation of the Monsanto process but incorporates lithium iodide as a key promoter to enhance activity and stability, with typical concentrations of 250–1,300 ppm rhodium, 1,500–3,700 ppm lithium iodide, and 7–35 wt% methyl iodide. Operations occur at 160–220°C and 300–1,200 psig, with drier conditions limiting water content to less than 5% (ideally anhydrous in the methyl acetate feed) to prevent anhydride hydrolysis and maintain high selectivity. A small amount of hydrogen (2–7 vol% in the CO feed) suppresses tar formation and boosts catalyst performance by favoring the active \ce{[Rh(CO)2I2]-} species. These modifications address the less favorable thermodynamics of methyl acetate carbonylation compared to methanol, enabling selectivities exceeding 95%. This process offers distinct advantages through its integration with on-site cellulose acetate manufacturing, where serves as the primary acetylating agent for producing fibers, films, and plastics, while co-produced acetic acid recycles back into the system. Compared to the conventional process—which involves high-temperature of acetic acid to followed by reaction with additional acetic acid—the Eastman route provides higher overall yields (approaching 99% selectivity) and reduced energy consumption by avoiding the endothermic generation step. These benefits, combined with feedstock utilization, have sustained the process's commercial viability for over four decades.

Cativa Process

The Cativa process represents an advanced -catalyzed evolution of the methanol carbonylation technology originally pioneered in the , focusing on enhanced efficiency and stability for acetic acid production. Developed by Chemicals, it was first commercialized in a plant in Texas City, USA, in , with formal announcement in 1996. The catalyst system employs iridium precursors such as IrI₃ or [Ir(CO)₂I₂]⁻, promoted by compounds like Ru(CO)₁₂ or RuI₃ to improve stability and activity under low-water conditions.00263-7) Key advancements in the Cativa process enable operation at water concentrations below 5 wt%—compared to about 15 wt% required in the rhodium-based —while maintaining high reaction rates and catalyst stability down to 0.5 wt% water. This low-water regime achieves selectivity exceeding 99% for acetic acid, with byproducts like reduced to one-third of Monsanto levels (typically 400–600 ppm versus 1200–2000 ppm). Additionally, the process improves carbon monoxide utilization from approximately 85% to over 94%, and reduces overall energy demands by about 30% through lower and cooling usage in , as less water needs separation from the product stream.00263-7) Mechanistically, the Cativa process shares the same overall as the Monsanto process, involving of methyl iodide, CO insertion, and , but iridium's higher reactivity makes the step approximately 150 times faster than with , rendering it tolerant to low CO and HI concentrations without precipitation issues. Consequently, the rate-determining step shifts to the methyl migration (CO migratory insertion), rather than the seen in the rhodium system.00428-0) By the early 2000s, the Cativa process had become the standard for new acetic acid facilities worldwide, with the first major plant starting up in in 2000 at 500,000 tonnes per year capacity; retrofits of existing plants were also widespread, often increasing throughput by up to 75%, as demonstrated in the 1997 Samsung-BP in .

Industrial Applications

Production Scale

The process achieved peak global adoption during the to , when it accounted for the majority of carbonylation-based acetic acid production, with methanol-derived methods capturing about 50% of total output. As of 2025, legacy plants contribute a smaller share, estimated at around 20%, to global , while over 70% derives from the Cativa or hybrid variants, amid a total world capacity of approximately 20 million tons per year dominated by (over 80% of output). Major producers including and maintain operations using technology in older facilities, supporting ongoing but diminished reliance on the original rhodium-catalyzed system. The process relies on methanol feedstock primarily sourced from natural gas or coal, paired with carbon monoxide generated from syngas.

Economic and Environmental Impact

The Monsanto process has significantly lowered production costs for acetic acid due to the relatively inexpensive feedstocks involved, with methanol priced at approximately $802 per metric ton in as of 2025. This affordability, combined with efficient carbon monoxide utilization, contributes to high profit margins in the industry, as the process converts low-cost syngas-derived inputs into a high-value chemical essential for downstream products like and (PET). The subsequent Cativa process, developed by Chemicals, further enhanced economic viability by replacing rhodium with the cheaper and more abundant catalyst, effectively eliminating rhodium usage and reducing catalyst-related costs by up to 90% compared to the original technology. This shift allows for smaller reactor sizes and lower capital expenditures, with overall operating costs estimated at around $560 per ton of acetic acid produced. Additionally, the Cativa process operates under milder conditions (30-40 bar pressure), cutting energy requirements and contributing to savings in utilities and maintenance. Compliance with updated environmental regulations, such as EPA guidelines on toxic CO emissions and management, adds to operational considerations for legacy plants. Environmentally, the Monsanto process offers advantages over the older oxidation route by requiring fewer raw materials and generating lower volumes, with a (GWP) of about 1.31 kg CO₂ equivalent per kg of acetic acid and demand (PED) of 50.5 MJ/kg. However, challenges include handling toxic , which poses emission risks if not managed, and managing iodide-based corrosion that leads to streams. The Cativa process mitigates some issues by reducing formation and energy use, though exact savings vary; alternative assessments indicate up to 30% lower energy intensity in optimized setups. In terms of , the process supports key industrial chains for monomer (used in adhesives and coatings) and (for ), with global production exceeding 17 million tons annually enabling efficient material flows. As of 2025, efforts to integrate carbon capture in production—such as using biogenic CO₂ from for synthesis—aim to achieve net-zero goals, potentially reducing the GWP by 47% in hybrid bio-routes compared to traditional methods. Transitioning to bio-methanol, derived from or , further lowers fossil dependency, though scalability remains limited. Key challenges include rhodium scarcity in the original process, with prices fluctuating between $6,000 and $8,700 per ounce in 2025 due to supply constraints from South African , driving up catalyst expenses and incentivizing the shift to iridium-based alternatives. Iodide and CO handling also necessitate robust to minimize ecological risks.