By carbonylation of methyl acetate

Methyl Acetate Garbonylation. 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 first step in anhydride manufacture by carbonylation. See Figure 1. The reactions are... 

FIGURE 1 Acetic anhydride manufacture by carbonylation of methyl acetate... 

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.)... 

The Eastman acetic anhydride [108-24-7] process provides an extension of carbonylation chemistry to carboxyUc acid esters. The process is based on technology developed independendy in the 1970s by Eastman and Halcon SD. The Eastman acetic anhydride process involves carbonylation of methyl acetate [79-20-9] produced from coal-derived methanol and acetic acid [64-19-7]. 

This process is one of the three commercially practiced processes for the production of acetic anhydride. The other two are the oxidation of acetaldehyde [75-07-0] and the carbonylation of methyl acetate [79-20-9] in the presence of a rhodium catalyst (coal gasification technology, Halcon process) (77). The latter process was put into operation by Tennessee Eastman in 1983. In the United States the total acetic anhydride production has been reported to be in the order of 1000 metric tons. 

Eastman-Halcon A process for making acetic anhydride from syngas. The basic process is the carbonylation of methyl acetate. Methanol is made directly from the carbon monoxide and hydrogen of syngas. Acetic acid is a byproduct of the cellulose acetate manufacture for which the acetic anhydride is needed. The carbonylation is catalyzed by rhodium chloride and chromium hexacarbonyl. 

Evidence has been presented that iodide salts can promote the oxidative addition of Mel to [Rh(CO)2l2]"> the rate-determining step in the Rh cycle [12]. The precise mechanism of this promotion remains unclear formation of a highly nucleophilic dianion, [Rh(CO)2l3]2 , has been suggested, although there is no direct spectroscopic evidence for its detection. Possible participation of this dianion has been considered in a theoretical study [23]. An alternative nucleophilic dianion, [Rh(CO)2l2(OAc)]2 , has also been proposed [31,32] on the basis that acetate salts (either added or generated in situ via Eq. 7) can promote carbonylation. Iodide salts have also been found to be effective promoters for the anhydrous carbonylation of methyl acetate to acetic anhydride [33]. In the absence of water, the catalyst cannot be maintained in its active form ([Rh(CO)2l2]") by addition of Lil alone, and some H2 is added to the gas feed to reduce the inactive [Rh(CO)2l4]. ... 

The direct carbonylation of methanol yielding acetic acid, the Monsanto process, represents the best route for acetic acid. Carbonylation of methyl acetate, obtained from methanol and acetic acid, gives acetic anhydride, a technology commercialized by Tennessee Eastman (22). It is noteworthy that this process is based on coal derived synthesis gas to give as the final product cellulose acetate. A combination of Monsanto and Tennessee Eastman technology opens the door for the combined synthesis of acetic acid and acetic anhydride. 

By adjusting the C0 H2 ratio, catalytic systems for the reductive carbonylation of methyl acetate can be tuned to the production of acetic anhydride, ethylidene diacetate or acetaldehyde. 

While the direct carbonylation is well accepted by industry, the reductive and oxidative carbonylations are still in the research and development stage. Using Texaco technology (j, 7/ ) the combined synthesis of ethene and ethanol is feasible via homologation of acids according to Figure 3. Ethene can also be obtained from the reductive carbonylation of methyl acetate to ethyl acetate followed by pyrolysis (2 ). Both routes, so far, lack selectivity. 

In anhydrous mixtures, the rhodium catalyzed carbonylation is enhanced by the presence of hydrogen. Introduction of hydrogen to a rhodium catalyzed carbonylation of methyl acetate increases the reaction rate and maintains catalyst stability (26) when the hydrogen partial pressure is rather low. It leads to reduced products formation, e.g. acetaldehyde and ethylidene diacetat with higher hydrogen partial pressure, in excess of 50 psi (27, 28). This is a clear indication that hydrogen is added to the coordination sphere of the rhodium catalyst. However, in the case of methanol carbonylation, the presence of hydrogen does not enhance the reaction rate or lead... 

Concurrent with acetic anhydride formation is the reduction of the metal-acyl species selectively to acetaldehyde. Unlike many other soluble metal catalysts (e.g. Co, Ru), no further reduction of the aldehyde to ethanol occurs. The mechanism of acetaldehyde formation in this process is likely identical to the conversion of alkyl halides to aldehydes with one additional carbon catalyzed by palladium (equation 14) (18). This reaction occurs with CO/H2 utilizing Pd(PPh )2Cl2 as a catalyst precursor. The suggested catalytic species is (PPh3)2 Pd(CO) (18). This reaction is likely occurring in the reductive carbonylation of methyl acetate, with methyl iodide (i.e. RX) being continuously generated. 

An alternative scheme to simultaneous formation of acetaldehyde and acetic anhydride could entail the carbonylation of methyl acetate to acetic anhydride which is subsequently reduced to acetaldehyde and acetic acid. The reaction of acetaldehyde with excess anhydride would form EDA. In fact, Fenton has described production of EDA by the reduction of acetic anhydride using both rhodium and palladium salts as catalysts when modified with triphenylphosphine (26). Two possible mechanisms for the reduction are postulated in equation 16. 

Acetic anhydride is also produced by the Rh-catalyzed carbonylation of methyl acetate. The method is called the Eastman process (Scheme 3.11). The Rh-catalysed production of acetic anhydride from methyl acetate can be understood by the formation of Mel and acetic acid by the reaction of methyl acetate with HI. Finally, attack of AcOH on the acetylrhodium affords the anhydride and HI, or acetyl iodide reacts with AcOH to give acetic anhydride and HI. 

Hydrogenative carbonylation of methyl acetate to 1,1-diacetoxyethane followed by cleavage to vinyl acetate and acetic acid. Only syngas is involved as raw materials. 

Acetic anhydride was then obtained by the catalytic carbonylation of methyl acetate with carbon monoxide.16... 

The recovery of heavy metals from solid waste poses more challenges. The Eastman Chemical Company process for the manufacture of acetic anhydride by the carbonylation of methyl acetate involves a proprietary process for the continuous recovery of rhodium and lithium from the process tar (see Section 4.6). 

In this chapter we discuss the mechanistic and other details of a few industrial carbonylation processes. These are carbonylation of methanol to acetic acid, methyl acetate to acetic anhydride, propyne to methyl methacrylate, and benzyl chloride to phenyl acetic acid. Both Monsanto and BASF manufacture acetic acid by methanol carbonylation, Reaction 4.1. The BASF process is older than the Monsanto process. The catalysts and the reaction conditions for the two processes are also different and are compared in the next section. Carbonylation of methyl acetate to acetic anhydride, according to reaction 4.2, is a successful industrial process that has been developed by Eastman Kodak. The carbonylation of propyne (methyl acetylene) in methanol to give methyl methacrylate has recently been commercialized by Shell. The Montedison carbonylation process for the manufacture of phenyl acetic acid from benzyl chloride is noteworthy for the clever combination of phase-transfer and organometallic catalyses. Hoechst has recently reported a novel carbonylation process for the drug ibuprofen. 

The acetic acid-forming part of the catalytic cycle for methanol carbonylation consists of reactions between acetyl iodide and water to give acetic acid and HI (Fig. 4.2, bottom left). The hydroiodic acid reacts with methanol to regenerate CH3I and water. A similar mechanism operates for the carbonylation of methyl acetate. Acetic acid and acetyl iodide react to give acetic anhydride and HI. The latter reacts with methyl acetate to regenerate acetic acid and methyl iodide. These reactions are shown in Fig. 4.9 by the large, left-hand-side loop. 

Thermodynamically, the carbonylation of methyl acetate (AG298 -10 kJ/mol) is considerably less favourable than that of methanol (AG298 -74 kJ/mol). This means that the reaction does not reach completion but attains an equilibrium which is dependent on the temperature and the CO pressure. Two variants are currently practised commercially that developed by Tennessee Eastman, based on a Halcon process, and a BP process in which acetic acid and the anhydride are co-produced in proportions which can be varied according to demand. Syngas for the Eastman process is made from coal which is mined close to the plant in Tennessee and the acetic anhydride produced is used to make cellulose acetate for film production. The BP process uses syngas generated from North Sea gas which is piped directly to the BP plant in EIull. [Acetic anhydride manufacture M. J. Eloward, M. D. Jones, M. S. Roberts, S. A. Taylor, Catalysis Today, 1993, 18, 325]. 

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) ... 

On the basis of the carbonylation of methyl acetate using Co, Ni or Fe catalysts by BASF [48] in the 1950s and of the initial results from the Rh catalyzed carbonylation of methanol by Monsanto [21, 49] in the early 1970s, Halcon [49, 50], Eastman [41b, 51], Ajinamoto [52], Showa Denko [53], BP [2, 20, 54, 55], and Hoechst [56] worked on substantial developments for the Group VIII metal-catalyzed manufacture of acetic anhydride. Promising catalyst metals are Rh, Pd, Ni, and Co among these, Rh has an essential position due to its exceptional carbonylation activity [20]. 

The mechanism of the Li/Rh-catalyzed carbonylation of methyl acetate was formally described by a catalyst cycle by Zoeller et al. based on kinetic measurements under high pressure [41b]. The organic reaction cycle is combined with the rhodium catalyst cycle in the Li/Rh catalyst system (Figure 3). 

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]. 

Carbonylation of methyl acetate, developed in particular by Hakoru This method is based on the use of synthesis gas to produce successively methanol, methyl acetate, and acetic anhydride jointly with ethyiidene diacetate, which then decomposes to vinyl acetate and acetic add, which is recycled to methyl acetate synthesis. The overall reaction is the following ... 

The same homogenous anionic rhodium catalyst is also active for the carbonylation of methyl acetate to acetic anhydride. The "immobilized" form on Reillex , a polyvinyl pyridine resin, has been reported for the carbonylation of methyl acetate to acetic anhydride. Literature procedures readily allow for the preparation of 2 to 5% by weight of rhodium on Reillex " (7). These catalysts... 

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