Carbonylation and Manufacture of Acetic Acid

The worldwide production of acetic acid is more than 10 million tons per year of which about 80% is based on methanol carbonylation technology. Methanol can be carbonylated to give acetic acid by using metal complexes of cobalt or rhodium or iridium as catalysts. All the three processes require the presence of some water and methyl iodide in the 

The discovoy of methanol carbonylation to acetic acid, with cobalt iodide as the catalyst, goes back to 1913. In 1960 BASF operated a small Co-based methanol carbonylation plant. The Co-catalyzed process requires high pressure and temperature ( 00 bar, 230°C) and is of moderate selectivity. The selectivity with respect to CO and methanol are -70% and -90%, respectively. Acetic acid production is accompanied by unwanted side products such as acetaldehyde, ethanol, and propionic acid. 

In the 1970s, the Rh-based carbonylation process was commercialized by Monsanto. The operating conditions for the Rh-based process are milder ( 30 bar, -180°C), and the selectivity (-90% and -99% with respect to CO and methanol) is higher than that of the Co-based process. Although the Rh-based process is highly selective and efficient, it must be operated within a stringent set of conditions to avoid precipitation and loss of rhodium as insoluble Rhij. 

In 1996, BP Chemicals introduced Ir-based methanol carbonylation technology called Cativa . The Ir-based process operates under conditions similar to that of the Monsanto process. However, it has a better tolerance to lower CO partial pressures and to low water content. This is advantageous because separation of a large amount of water from acetic acid by distillation adds to the cost of its manufacturing. In the Ir-based process, apromoter has to be used for increasing catalytic activity. It can be operated with water content as low as 5%. 

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It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

The Wacker process reached a maximum production capacity of 2.6 Mt/a worldwide in the mid 1970 s. The cause of the decline in the following years (1.8 Mt/a in 2003) was the increase in the manufacture of acetic acid (the most important product made from acetaldehyde) by the carbonylation of methanol. In future new processes for chemicals, such as acetic anhydride and alkylamines (which were also made from acetaldehyde) will probably further decrease its importance. With the growing use of syngas as feedstock, the one-step... 

The future of the commercial acetaldehyde processes mainly depends on the availability of cheap ethylene. Acetaldehyde has been replaced as a precursor for 2-ethylhexanol ( aldol route ) or acetic acid (via oxidation cf. Sections 2.1.2.1 and 2.4.4). New processes for the manufacture of acetic acid are the Monsanto process (carbonylation of methanol, cf. Section 2.1.2.1), the Showa Denko one-step gas-phase oxidation of ethylene with a Pd-heteropolyacid catalyst [75, 76], and Wacker butene oxidation [77]. Other outlets for acetaldehyde such as pentaerythritol and pyridines cannot fill the large world production capacities. Only the present low price of ethylene keeps the Wacker process still attractive. 

Propionic acid is produced commercially by several different processes. It is a by-product of the liquid phase oxidation of hydrocarbons for the manufacture of acetic acid. It is also made from carbon monoxide and ethylene by the 0x0 process through a propionaldehyde intermediate or by the carbonylation of ethylene with a nickel-based catalyst. BASF uses the one-step Reppe carbonylation process with a nickel propionate catalyst to produce 40,000 metric tons per year of propionic acid in Ludwigshafen, Germany. The hydrocarboxylation chemistry is shown in Eq. (29) ... 

Sunley, G.J. and Watson, D.J. (2000) High productivity methanol carbonylation catalysis using iridium. The Cativa process for the manufacture of acetic acid. Catal. Today, 58, 293-307. 

The industrial manufacture of acetic acid by methanol carbonylation (Equation (1)) has utilized catalysts based upon all three of the group 9 metals, since the initial development by BASF of a cobalt/iodide-based system. " The BASF process required harsh conditions of temperature and pressure, and suffered from relatively low selectivity. It was soon superceded by highly selective, low-pressure rhodium/iodide-based catalysts developed by Monsanto. The Monsanto process (and related variants operated by other manufacturers) quickly became dominant and remains one of the most successful examples of the commercial application of homogeneous catalysis.Rhodium catalysts for methanol carbonylation are discussed in Chapter 7.03. 

The filled arrows in Figure 1.2 are processes either based on homogeneous catalysts or having great relevance in homogeneous catalysis. Conversion of synthesis gas into methanol is achieved by a heterogeneous catalyst, while the manufacture of acetic acid is based on the homogeneous catalytic carbonylation of methanol. Similar carbonyla-tion of methyl acetate, the ester of methanol and acetic acid, yields acetic anhydride. These reactions are discussed in Chapter 4. 

Acetic Acid. Methanol carbonylation has become the process of choice for production of this staple of the organic chemical industry, which is used in the manufacture of acetate fibers, acetic anhydride [108-24-7] and terephthaUc acid, and for fermentation (see Acetic acid and derivatives). 

The third and now preferred method of acetic acid manufacture is the carbonylation of methanol (Monsanto process), involving reaction of methanol and carbon monoxide (both derived from methane). This is discussed in Chapter 12, Section 3. 

The recent dramatic increase in the price of petroleum feedstocks has made the search for high selectivities more urgent. Several new processes based on carbon monoxide sources are currently competing with older oxidation processes.103,104 The more straightforward synthesis of acetic acid from methanol carbonylation (Monsanto process) has made the Wacker process obsolete for the manufacture of acetaldehyde, which used to be one of the main acetic acid precursors. Several new methods for the synthesis of ethylene glycol have also recently emerged and will compete with the epoxidation of ethylene, which is not sufficiently selective. The direct synthesis of ethylene... 

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]. 

The processes for the manufacture of acetic anhydride have included, initially, the distillation of wood pulp, which was followed by the ketene route from acetic acid or acetone and finally the ethylene based oxidation of acetaldehyde. The carbonylation of CH3OAC to acetic anhydride has in part replaced anhydride capacity from the more expensive processes. 

On the basis of this development afforded by Eastman and Halcon, in 1983 the Eastman Chemical Company (Kingsport, TN) started the commercial process for the manufacture of acetic anhydride (Figure 5). Methyl acetate, the feedstock for the carbonylation reaction, was produced in a separate esterification step from acetic acid and methanol. The process was designed to produce 225 000 tons of acetic anhydride and 75 000 tons of acetic acid/year. The overall yield of acetic anhydride based on methanol is approximately 96 % [2, 47]. 

The concept of co-carbonylation of methanol/methyl acetate mixtures was first introduced by BASF in the early 1950s, but the reaction chemistry was not fully developed to commercial realization [75]. Not until the mid-1980s, after the development of carbonylation processes to produce acetic acid and acetic anhydride, were co-carbonylation processes patented using homogeneous rhodium/iodine catalyst systems (Table 2) [2, 56]. The basic process concept is to manufacture acetic acid and acetic anhydride from methanol and carbon monoxide as the only raw materials and to generate methyl acetate within the process. Similiarly, the suitability of dimethyl ether as a raw material for the generation of the anhydride equivalent in addition to or as a substitute for methyl acetate was revealed by Hoechst [76]. To produce a small fraction of acetic acid besides acetic anhydride as the main product, the carbonylation of methyl acetate could be conducted with small amounts of water or methanol. This variant, first demonstrated by Hoechst [56], is practiced by Eastman Kodak [2]. 

A simple example will indicate the scope of the operation and some of its characteristics. Acetic acid is produced by methanol carbonylation or oxidation of acetaldehyde, or as a by-product of cellulose acetate manufacture. In all three cases, a mixture of acetic acid and water must be separated to give glacial acetic acid (99.8 wt%, min). Separation by distillation is expensive because of the need to vaporize large amounts of water with its very high heat of vaporization. Accordingly, an alternative, less expensive liquid-liquid extraction process is often used. 

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