Monsanto acetic acid production

In 1970, the first rhodium-based acetic acid production unit went on stream in Texas City, with an annual capacity of 150 000 tons. Since that time, the Monsanto process has formed the basis for most new capacities such that, in 1991, it was responsible for about 55% of the total acetic acid capacity worldwide. In 1986, B.P. Chemicals acquired the exclusive licensing rights to the Monsanto process, and 10 years later announced its own carbonylation iridium/ruthenium/iodide system [7, 8] (Cativa ). Details of this process, from the viewpoint of its reactivity and mechanism, are provided later in this chapter. A comparison will also be made between the iridium- and rhodium-based processes. Notably, as the iridium system is more stable than its rhodium counterpart, a lower water content can be adopted which, in turn, leads to higher reaction rates, a reduced formation of byproducts, and a better yield on CO. 

Flash distillation of the product where very high vacuums are applied at moderate temperatures so the solvents and products vaporize, which are collected and condensed in a condenser, leaving the catalyst behind in the vessel. In the Monsanto acetic acid process, the catalyst rhodium iodide is left behind in the reboiler once the products are flashed off (see Section 4.9). 

A block diagram of the Monsanto process for acetic acid production is shown in Fig. 4.13. The process flow sheet is simple since the reaction conditions are mild (180°C/30-40 bar) when compared to the BASF process (250°C/700 bar). More than 40% of world s acetic acid is made by the Monsanto process. One of the problems with this process is the continuous loss of iodine. A block diagram of the Eastman process for acetic anhydride production is shown in Fig. 4.14. The process generates minimum waste, and all process tars are destroyed to recover iodine and rhodium. 

Figure 4.13 Simplified block diagram of the Monsanto process for the acetic acid production.

AO Plus [Acid Optimisation Plus] A process for making acetic acid by carbonylating methanol. Based on the Monsanto Acetic Acid process, but an improved catalyst (rhodium with lithium iodide) permits operation at lower levels of water. Developed by Celanese in the 1980s and operated by that company in Clear Lake, TX. Residual iodide in the product is removed by the Silverguard process. 

Monsanto acetic acid A process for making acetic acid by carbonylation of methanol, catalyzed by rhodium iodide. Operated by BP. A variation of this process, the low water process, used added Group 1 metal iodides such as lithium iodide to enhance the productivity this was practiced by Celenese and by Daicel. 

As is the case of hydroformylation, the use of rhodium allows much milder conditions to be used. Such a process was started by Monsanto in 1966 it operates at 30-60 bar and 150-200°C and is now the world s largest process for acetic acid production (>5 million tons per year). In view of the corrosive nature of the reagents, Hastalloy or zirconium reactors have to be used. 

The hydrocarboxylation reactions discussed above have been proposed to involve direct addition of water to the metal center prior to elimination of the product, analogous to the oxidative addition of hydrogen to a metal center at the end of a hydroformylation catalytic cycle. Another class of hydrocarboxylation reactions is more analogous to the haUde-promoted Monsanto acetic acid process, where one has a reductive elimination of an acyl halide species that is rapidly hydrolyzed with free water to generate the carboxylic acid and HX. 

Acetic acid is a key commodity building block [1], Its most important derivative, vinyl acetate monomer, is the largest and fastest growing outlet for acetic acid. It accounts for an estimated 40 % of the total global acetic acid consumption. The majority of the remaining worldwide acetic acid production is used to manufacture other acetate esters (i.e., cellulose acetates from acetic anhydride and ethyl, propyl, and butyl esters) and monoehloroacetic acid. Acetic acid is also used as a solvent in the manufacture of terephthalic acid [2] (cf. Section 2.8.1.2). Since Monsanto commercially introduced the rhodium- catalyzed carbonylation process Monsanto process ) in 1970, over 90 % of all new acetic acid capacity worldwide is produced by this process [2], Currently, more than 50 % of the annual world acetic acid capacity of 7 million metric tons is derived from the methanol carbonylation process [2]. The low-pressure reaction conditions, the high catalyst activity, and exceptional product selectivity are key factors for the success of this process in the acetic acid industry [13]. 

Catalyst performance has of course been a permanent theme in industry. For example, the catalytic activity of oxo catalysts (in hydroformylation) has improved in the past 50 years by a factor of 10 000 change from diadic and triadic process technology to continuous plant operation, replacement of cobalt by rhodium, tailoring of the ligand sphere (phosphines), change of phase application (from mono- to two-phase processes). At the same time, an improvement of selectivity has been achieved, apart from the ease of product/catalyst separation [132]. A similar development seems to occur in the Monsanto acetic acid process [49]. 

This is the simplest of all carbonylations and is best illustrated by the conversion of iodomethane to acetyl iodide, a reaction crucial to the success of the low pressure Monsanto process for acetic acid production from methanol (equation 1). 

Acetic acid is used mostly in making cellulose acetate and vinyl acetate. The process developed by Monsanto but later exploited by BP Chemicals has provided new and robust demand for methanol as a raw material for acetic acid production. 

All known ACS enzymes are bifunctional in that they possess a C cluster with COdFI activity in addition to an A cluster (the ACS active site. Scheme 9). In the enzymes, a CO tunnel is described through which GO can pass directly from the C cluster, where it is generated from CO2, to the A cluster, where acetyl GoA synthesis takes place. Again, two mechanisms were proposed that differ in the order of binding events and redox states involved. In essence, however, GO binds to an Ni-GHs species, followed by insertion and generation of an Ni-acetyl species, which upon reaction with GoA liberates the acetyl GoA product. It is interesting to note that methylation of Ni occurs by reaction with methyl cobalamin (Scheme 7). In M. thermoacetica, the cobalamin is the cofactor for a rather unique protein called the corrinoid iron sulfur protein (GFeSP). The above process, even if mechanistic details still remain in question, resembles the industrial Monsanto acetic acid synthesis process (Scheme 9, bottom). In this case, however, the reaction is catalyzed by a low-valent Rh catalyst. 

The period from 1970 to 1985 saw radical changes in the production of acetic acid and acetic anhydride. By 1985, both products would be generated not from ethylene, but from synthesis gas which in turn could be generated fi om abundant resources such as coal, natural gas, and in the future, biomass. At the end of this period, acetaldehyde became a very small contributor to the total acetyl product stream since it was no longer required to make acetic acid or acetic anhydride and ethylene would only be required to produce vinyl acetate and to meet a much diminished acetaldehyde market. These advances were the result of two significant process breakthroughs - the Monsanto Acetic Acid Process and the Eastman Chemical Company Acetic Anhydride Process which will be discussed below. 

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... 

Reductive elimination is the product-forming step in some of the most important catalytic cycles, including hydrogenation, the Monsanto acetic acid process, and various types of cross-couplings. For this reason, detailed studies of this process have been conducted. Hrese studies have revealed examples of reductive eliminations to form H-H and C-H bonds, as well as reductive eliminations to form C-G and C-X bonds (in which X = halide, amide, alkoxide, thiolate, and phosphide). The mechanisms of these processes include the same pathways as have been deduced for oxidative addition (i.e., concerted, ionic, and radical), because reductive elimination is the same as oxidative addition, but in the reverse direction. 

Roth and co-workers [84] at the Monsanto company developed an acetic acid production process by the reaction of methyl alcohol with carbon monoxide in the presence of rhodium carbonyl as the major catalyst. 

The representative carbonylations with rhodium catalysts is the Monsanto acetic acid process which started in 1970 with a production amount of three million pounds per year [82-91]. In this carbonylation, the 0x0 process of the Reppe reaction is carried out at 250-270°C, 200-300atm with nickel catalysts, and the BASF process is carried out at 210°C, 530atm with a Co/I catalyst. However, the Monsanto acetic acid process shown in eq. (18.37) is carried out under mild reaction conditions in a high selectivity of acetic acid with rhodium catalyst. The catalyst is RhCl3 3H20 and the active species is considered to be [Rh(CO)2l2] ... 

In the BASF process, methanol and CO are converted in the liquid phase by a homogeneous Co-based catalyst. The reaction takes place in a high-pressure Hastelloy reactor. In recent decades the BASF process has been increasingly replaced by low-pressure alternatives mainly due to lower investment and operating costs. In the low-pressure Monsanto process methanol and CO react continuously in liquid phase in the presence of a Rhl2 catalyst. In 1996, BP developed a new attractive catalyst based on iridium (Cativa process) the oxidative addition of methyl iodide to iridium is 150-times faster than to rhodium. The search for acetic acid production processes with even lower raw material costs has led to attempts to produce acetic acid by ethane oxidation. In the near future ethane oxidation will most likely not compete with methanol carbonylation (even though ethane is a very cheap and attractive raw material) because of the low ethane conversions, product inhibition problems, and a large variety of by-products. 

For the immediate future, chemical synthesis of acetic acid as a commodity chemical from petrochemicals is the major method for production throughout the world. O Figure 1.5 diagrams the major chemical intermediates (precursors of the major industrial products) formed from acetic acid. Production of vinyl acetate, chloroacetic acid, acetic anhydride, and acetate esters requires glacial acetic acid, which is most inexpensively produced by the carbonylation of methanol, the Monsanto process, or by catalytic oxidation... 

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. 

This process may be competitive with butane oxidation (see Hydrocarbon oxidation) which produces a spectmm of products (138), but neither process is competitive with the process from synthesis gas practiced by Monsanto (139) and BASF (140) which have been used in 90% of the new acetic acid capacity added since 1975. 

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