Acetic anhydride acetate carbonylation

The first anhydride plant in actual operation using methyl acetate carbonylation was at Kingsport, Tennessee (41). A general description has been given (42) indicating that about 900 tons of coal are processed daily in Texaco gasifiers. Carbon monoxide is used to make 227,000 t/yr of anhydride from 177,000 t/yr of methyl acetate 166,000 t/yr of methanol is generated. Infrared spectroscopy has been used to foUow the apparent reaction mechanism (43). 

Fig. 2. Flow sheet for methyl acetate carbonylation to anhydride. To convert kPa to psi multiply by 0.145.

A related but distinct rhodium-catalyzed methyl acetate carbonylation to acetic anhydride (134) was commercialized by Eastman in 1983. Anhydrous conditions necessary to the Eastman acetic anhydride process require important modifications (24) to the process, including introduction of hydrogen to maintain the active [Rhl2(CO)2] catalyst and addition of lithium cation to activate the alkyl methyl group of methyl acetate toward nucleophilic attack by iodide. 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

Figure 7 shows the results of methyl acetate carbonylation in the presence of water. Methanol and dimethyl ether were formed up to 250 C suggesting that hydrolysis of methyl acetate proceeded. With increasing reaction temperature, the yield of acetic acid increased remarkably, while those of methanol and dimethyl ether decreased gradually. Figure 8 shows the effects of partial pressures of methyl iodide, CO, and methyl acetate in the presence of water. The rate of acetic acid formation was 1.0 and 2.7 order with respect to methyl iodide and CO, respectively. Thus, the formation of acetic acid from methyl acetate is highly dependent on the partial pressure of CO. This suggests that acetic acid is formed by hydrolysis of acetic anhydride (Equation 6) which is formed from methyl acetate and CO rather than by direct hydrolysis of methyl acetate. 

At high temperatures with low catalyst concentration the formation of acetanilides is favored. Maleic anhydride and acetanilides may be formed directly from the mixed anhydride by an initial attack of the nitrogen on the acetate carbonyl, but this process would involve a seven membered ring transition state. Another possible route to the formation of maleic anhydride and the acetanilides is participation by neighboring carbonyl in loosening the amide carbon-nitrogen bond to the extent that the amine can be captured by acetic anhydride as shown in path D. 

Acetic Anhydride. A total of 1.9 billion lb of acetic anhydride was produced in the United States in 1999. Commercial production of acetic anhydride is currently accomplished through two routes, one involving ketene and the other methyl acetate carbonylation. A former route based on liquid phase oxidation of acetaldehyde is now obsolete. 

Figure 4.14 Simplified block diagram of the methyl acetate carbonylation process for acetic anhydride production.

The basic organometallic reaction cycle for the Rh/I catalyzed carbonylation of methyl acetate is the same as for methanol carbonylation. However some differences arise due to the absence of water in the anhydrous process. As described in Section 4.2.4, the Monsanto acetic acid process employs quite high water concentrations to maintain catalyst stability and activity, since at low water levels the catalyst tends to convert into an inactive Rh(III) form. An alternative strategy, employed in anhydrous methyl acetate carbonylation, is to use iodide salts as promoters/stabilizers. The Eastman process uses a substantial concentration of lithium iodide, whereas a quaternary ammonium iodide is used by BP in their combined acetic acid/anhydride process. The iodide salt is thought to aid catalysis by acting as an alternative source of iodide (in addition to HI) for activation of the methyl acetate substrate (Equation 17) ... 

Methyl Acetate Carbonylation. Anhydride can be made by carbonylation of methyl acetate [79-20-9] (28) in a manner analogous to methanol carbonylation to acetic acid. Methanol acetylation is an essential hist step in anhydride manufacture by carbonylation. See Figure 1. The reactions are... 

Methyl acetate carbonylation demands only 0.59 kg acetic acid per kg anhydride manufactured.)... 

Not until the concentration of water has decreased to zero is the consumption of methyl acetate to form acetic anhydride by carbonylation initiated at a constant acetic acid concentration (eq. (25)) ... 

Pyrones react with acetic anhydride at carbonyl oxygen to produce 4-acetoxy-pyryliums, in situ, allowing nucleophilic substitution at C-4 the reaction of 2,6-dimethylpyrone with methyl cyanoacetate is typical. " Phosphorus pentachloride likewise converts 4-pyrones into 4-chloropyryliums. ... 

The catalyst system for the modem methyl acetate carbonylation process involves rhodium chloride trihydrate [13569-65-8]y methyl iodide [74-88-4], chromium metal powder, and an alumina support or a nickel carbonyl complex with triphenylphosphine, methyl iodide, and chromium hexacarbonyl (34). The use of nitrogen-heterocyclic complexes and rhodium chloride is disclosed in one European patent (35). In another, the alumina catalyst support is treated with an organosilicon compound having either a terminal organophosphine or similar ligands and rhodium or a similar noble metal (36). Such a catalyst enabled methyl acetate carbonylation at 200°C under about 20 MPa (2900 psi) carbon monoxide, with a space-time yield of 140 g anhydride per g rhodium per hour. Conversion was 42.8% with 97.5% selectivity. A homogeneous catalyst system for methyl acetate carbonylation has also been disclosed (37). A description of another synthesis is given where anhydride conversion is about 30%, with 95% selectivity. The reaction occurs at 445 K under 11 MPa partial pressure of carbon monoxide (37). A process based on a montmorillonite support with nickel chloride coordinated with imidazole has been developed (38). Other related processes for carbonylation to yield anhydride are also available (39,40). 

Acetic anhydride capacity in the United States in 1995 was approximately 2960 million lbs./year. Forty five percent of this total is produced by Eastman Chemical by methyl acetate carbonylation in their coal to chemicals plant. The remainder is produced by others using the ketene cracking route [11]. 

Acetic Anhydride via Methyl Acetate Carbonylation. The second major development, pioneered by Tennessee Eastman (25) is the carbonylation of methyl acetate to acetic anhydride, which also comprises the first example of a totally integrated synthesis gas-based process for such chemistry, dependent entirely on coal as feedstock. 

Alternatively, if dimethyl ether is used as raw material (see the section Acetic Anhydride via Methyl Acetate Carbonylation) the overall reaction becomes... 

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. 

A. Plant configuration. (1) The hydrogen-to-carbon monoxide ratio of the syngas is well suited to combined production of methanol and acetic anhydride. Although the hydrogen is not needed for production of acetic anhydride via carbonylation, it is used to decrease the hydrogen deficiency in the feed gas and thus minimizes use of the water-gas... 

The methyl acetate carbonylation process was successfully started and operated in the early 1980s as part of a coal-to-syngas-to-acetic anhydride complex. This new process introduction resulted in a major improvement in acetic anhydride production economics. In this process, methyl acetate, itself the product of a one-step esterification of acetic acid and methanol, is reacted with carbon monoxide in the presence of a promoted rhodium-iodide catalyst. Figure 22.20 illustrates this process... 

Frg. 22.20. Acetic anhydride by carbonylation of methyl acetate, Halcon/Eastman process. Chem Systems PERP Report No. 88-7. Copyright Chem Systems, /nc. and used by permission of the copyright owner.)... 

---