Carbonylation methyl acetate

Currently, almost all acetic acid produced commercially comes from acetaldehyde oxidation, methanol or methyl acetate carbonylation, or light hydrocarbon Hquid-phase oxidation. Comparatively small amounts are generated by butane Hquid-phase oxidation, direct ethanol oxidation, and synthesis gas. Large amounts of acetic acid are recycled industrially in the production of cellulose acetate, poly(vinyl alcohol), and aspirin and in a broad array of other... 

The reaction mechanism and rates of methyl acetate carbonylation are not fully understood. In the nickel-cataly2ed reaction, rate constants for formation of methyl acetate from methanol, formation of dimethyl ether, and carbonylation of dimethyl ether have been reported, as well as their sensitivity to partial pressure of the reactants (32). For the rhodium chloride [10049-07-7] cataly2ed reaction, methyl acetate carbonylation is considered to go through formation of ethyUdene diacetate (33) ... 

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.

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

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. 

Reaction rates have first-order dependence on both metal and iodide concentrations. The rates increase linearly with increased iodide concentrations up to approximately an I/Pd ratio of 6 where they slope off. The reaction rate is also fractionally dependent on CO and hydrogen partial pressures. The oxidative addition of the alkyl iodide to the reduced metal complex is still likely to be the rate determining step (equation 8). Oxidative addition was also indicated as rate determining by studies of the similar reactions, methyl acetate carbonylation (13) and methanol carbonylation (14). The greater ease of oxidative addition for iodides contributes to the preference of their use rather than other halides. Also, a ratio of phosphorous promoter to palladium of 10 1 was found to provide maximal rates. No doubt, a complex equilibrium occurs with formation of the appropriate catalytic complex with possible coordination of phosphine, CO, iodide, and hydrogen. Such a pre-equilibrium would explain fractional rate dependencies. 

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. 

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. 

The methyl acetate carbonylation process was successfully started and operated in the early 1980s. In this process, methyl acetate, itself the product of a one-step esterification... 

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

In methyl acetate carbonylation the plot of rate against the concentration of lithium iodide has a small positive intercept. Do you expect it to be the same for two different rhodium ion concentrations ... 

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... 

SCHEME 3 Catalytic cycle for Lil-promoted methyl acetate carbonylation. 

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). 

---