Monsanto acetic acid process reaction steps

Fig Reaction steps involved in the catalytic Monsanto acetic acid process. 

Organometallic compounds are used widely as homogeneous catalysts in the chemical industry. For example, if the alkene insertion reaction continues with further alkene inserting into the M C bond, it can form the basis for catalytic alkene polymerisation. Other catalytic cycles may include oxidative addition and reductive elimination steps. Figure above shows the steps involved in the Monsanto acetic acid process, which performs the conversion... 

The Monsanto acetic acid process produces acetic acid from methanol and CO gas under fairly mild conditions (I80 C, 30-40 atm). The process utilizes a square planar Rh(l) catalyst. As shown in Figure 19.33, the first step in the catalytic mechanism is the OA of methyl iodide to form an 18-electron compound. In the second step, CO insertion (alkyl migration) occurs, resulting in a 16-electron species. Carbon monoxide adds to the vacant coordination site to r enerate a saturated compound, which then undergoes RE of CH3COI to regenerate the catalyst. The CH3COI product is further processed by reaction with water to make acetic acid and HI. The latter... 

The Wacker oxidation was discovered by Smidt and co-workers at Consortium fiir Electrochemie (a subsidiary of Wacker Chemie and Farbwerken Bayer). It is actually a combination of known reactions and thus not a catalytic reaction in the strictest sense (Scheme 1). The first and most basic reaction, the oxidation of ethene in aqueous solution was first discovered by Phillips in 9AP The precipitation of palladium metal from a palladium(II) chloride solution was used as a test for olefins. However, it was the discovery by Smidt and co-workers that the Pd(0) formed could be regenerated by cupric chloride that made the reaction a commercial success. The final step, the oxidation of CuCl to CuCl2 is one of the fastest reactions in inorganic chemistry, The three reactions add up to the simple air oxidation of ethene to ethanal. At one point over two billion pounds a year of ethanal was produced by the Wacker process. Presently, the Monsanto acetic acid process has largely replaced the Wacker procednre.t" ... 

Another important process involving oxidative addition of alkyl halide is found in the hrst step in Monsanto process, an important industrial process to convert methanol into acetic acid with carbon monoxide at 150-200°C under 30-60 bar of CO [24]. In the Monsanto process methyl iodide is formed first from methanol and hydrogen iodide, added as the promoter to the catalytic system. The subsequent reaction of methyl iodide with a rhodium(I) species forms a methylrhodium(III) iodide species hy oxidative addition (Eq. 1.2). 

The synthesis of acetic acid from methanol and CO is a process that has been used with commercial success by Monsanto. The mechanism of this process is complex a proposed outline is in Figure 14.20. The individual steps are the characteristic types of organometallic reactions described previously the intermediates are 18- or 16-electron species having the capability to lose or gain, respectively, 2 electrons. Solvent molecules may occupy empty coordination sites in the 4- and 5-coordinate 16-electron intermediates. The first step, oxidative addition of CH3I to [Rhl2(CO)2] , is rate determining. ... 

The kinetics and mechanism of the carbonylation of methanol to acetic acid using Monsanto s rhodium complex catalyst has been extensively studied. The reaction is first order in both rhodium and CH3I promoter but zero order in CO pressure. It is believed that oxidative addition of CH3I is the rate-controlling step in this process. This is a unique example of designing a catalyst system with commercial viability in which the substrate (methanol) is first converted to CH3I... 

Another example of successful SILP gas-phase reaction is the rhodium-catalyzed carbonylation of methanol [37]. The technical importance of this reaction is indicated by the Monsanto process, the dominant industrial process for the production of acetic acid (and methyl acetate), carried out on a large scale as a homogeneous liquid-phase reaction [38]. Using [Rh(CO)2l2] anions as the catalyticaUy active species, Riisager and coworkers have developed a new silica SILP Monsanto-type catalyst system [39] 21, in which the active rhodium catalyst complex is part of the IL itself. The SILP system was prepared by a one-step impregnation of the silica support using a methanoUc solution of the IL [BMIM]I and the dimeric precursor species [Rh(CO)2l]2, as depicted in Scheme 15.5. 

Rhodium-catalyzed carbonylation of methanol is known as the Monsanto process, which has been studied extensively. From the reaction mechanism aspect, the study of kinetics has proved that the oxidative addition of methyl iodide to the [Rh(CO)2l2] is the rate-determining step of the catalytic cycle. It was also observed that acetyl iodide readily adds to [Rh(CO)2l2], indicating that the acetyl iodide must be scavenged by hydrolysis in order to drive the overall catalytic reaction forward. An alternative to sequential reductive elimination and the hydrolysis of acetyl iodide is the nucleophilic attack of water on the Rh acetyl complex and the production of acetic acid. The relative importance of these two alternative pathways has not yet been fully determined, although the catalytic mechanism is normally depicted as proceeding via the reductive elimination of acetyl iodide from the rhodium center. The addition of iodide salts, especially lithium iodide, can realize the reaction run at lower water concentrations thus, byproduct formation via the water gas shift reaction is reduced, subsequently improving raw materials consumption and reducing downstream separation. In addition to the experimental studies of the catalytic mechanism, theoretical studies have also been carried out to understand the reaction mechanism [17-20]. 

Although this process shows similarities to the Monsanto process for the carbonylation of methanol to produce acetic acid (Sec. 5.1.1), there are some important differences. In addition to the difference in the catalysts and the corresponding mechanistic aspect of the reactions, the methyl acetate carbonylation reaction [Eq. (25)] has a much smaller Gibbs free energy change than the methanol carbonylation reaction [Eq. (1)]. Thus, to maintain a substantial net rate of reaction, the methyl acetate carbonylation process is operated at 175190°C up to a conversion between 50 and 70% and at over 5 MPa pressure (50 atm). Acetic anhydride is separated from the rest of the material in the effluent of the reactor by a series of distillation steps. Acetic acid is a by-product. Most of the other material in the reactor effluent is recycled back to the reactor. A small amount of tar is removed. In this process, acetic anhydride with purity up to 99.7% could be obtained. The main impurity is acetic acid. 

The attainment of optimum rate at relatively low [H2O] is a significant benefit for the iridium system, since it results in less costly product purification. A typical configuration for an iridium-catalyzed methanol carbonylation plant is shown in Figure 2. The feedstocks (MeOH and CO) are fed to the reactor vessel on a continuous basis. In the initial product separation step, the reaction mixture is passed from the reactor into a flash tank where the pressure is reduced to induce vaporization of most of the volatiles. The catalyst remains dissolved in the liquid phase and is recycled back to the reactor vessel. The vapor from the flash tank is directed into a distillation train, which removes methyl iodide, water, and heavier byproducts (e.g., propionic acid) from the acetic acid product. At the relatively high water levels used in the rhodium-catalyzed Monsanto process, three distillation columns are typically required. In the Cativa process, a lower water concentration means that the necessary product purification can be achieved with only two columns. 

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