Rhodium acetic anhydride process

The acetic anhydride process employs a homogeneous rhodium catalyst system for reaction of carbon monoxide with methyl acetate (36). The plant has capacity to coproduce approximately 545,000 t/yr of acetic anhydride, and 150,000 t/yr of acetic acid. One of the many challenges faced in operation of this plant is recovery of the expensive rhodium metal catalyst. Without a high recovery of the catalyst metal, the process would be uneconomical to operate. 

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. 

Other companies (e.g. Hoechst, now Celanese) have developed a slightly different process in which the water content is low in order to save CO feedstock [1], In the absence of water it turned out that the catalyst precipitates. Also, the regeneration of ihodium(III) is much slower. The formation of the trivalent rhodium species is also slower because the HI content is much lower when the water concentration is low. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilisation of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts (Li, ammonium, phosphonium, etc). Later, we will see that this is also important in the acetic anhydride process. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives [1]. 

The heavy end products of acetic anhydride processes are separated from catalyst streams and distillation residues. The high affinity of the heavy ends for rhodium components affords specified procedures to separate rhodium and recycle it to the reaction stage of the process. Different methods for rhodium recovery were tested during the process development, including extraction methods [65], precipitation [66], complexing, and electrochemical methods [67]. 

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. 

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). 

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. 

Even with added iodide salt formation of the inactive [Rh(CO)2l4] can be a problem, since under anhydrous conditions this Rh(III) species cannot be reduced to the active [Rh(CO)2l2] by reaction with water. In the Eastman process, this problem is addressed by addition to the CO gas feed of some H2 which can reduce [Rh(CO)2l4] by the reverse of Equation 8. However, the added H2 does lead to some undesired by-products, particularly ethylidene diacetate (1,1-diacetoxyethane) which probably arises from the reaction of acetic anhydride with acetaldehyde (Equation 19 from hydrogenolysis of a rhodium acetyl) ... 

A very closely related process is the Tennessee Eastman (Kodak) carbonylation of methyl acetate to produce acetic anhydride. The rhodium-catalyzed portion of the mechanism is the same as shown in Scheme 19. Differences occur in the iodide-promoted pre- and post-rhodium reactions shown in Scheme 20. 

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 carbonylation process incorporates a rhodium salt, lithium iodide, and methyl iodide as primary catalyst components [80]. The active catalyst form is maintained by the lithium iodide promoter and hydrogen in the carbon monoxide feed to the reaction system. Preferred reaction conditions are a temperature of nearly 190 °C and a pressure of 5 MPa H2/CO. The conversion of methyl acetate to acetic anhydride per passage of reactor is between 50 and 75 %. 

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 process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

Acetic anhydride is used in the manufacture of cellulose acetate-based film, cigarette filters, and plastics. Eastman Chemical developed a process that is based on gasification of coal in a Texaco gasifier to make synthesis gas which then is converted to methanol. The methanol is converted to methyl acetate by esterification with acetic acid and then carbonylated. The carbonylation process uses rhodium salt catalysts with ligands and an iodine promoter [30]. 

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). 

However, Halcon have now developed a process, catalysed by rhodium (or nickel) with iodine and other promoters, for the carbonylation of methyl acetate (or dimethyl ether) to acetic anhydride. Like the ketene route, this technology fits in well with acetylation processes. 

Eastman Chemical s carbonylation of methyl acetate to produce acetic anhydride is closely related to the rhodium-catalyzed carbonylation of methanol to form acetic acid. Eastman s carbonylation process was commercialized in 1983 and produces over... 

The minor by-products of the process include ethylidene diacetate (1,1-diacetoxyethane), acetone, carbon dioxide, methane, and tar. The rhodium trapped or bound to the nonvolatile tar must be recovered for process economic reasons. The quantities of the organic products are very small. Carbon dioxide is formed by degradation of acetic anhydride. 

The presence of a lithium salt (Lil) as cocatalyst and hydrogen is very important for efficient production of acetic anhydride. The proposed reaction mechanism is shown in Figure 5 [42,43,47]. In this mechanism, there are two catalytic cycles for the formation of methyl acetate a rhodium-catalyzed cycle and a lithium-catalyzed cycle. The rhodium-catafyzed cycle is similar to the Monsanto process of methanol carbonylation (Fig. 1). The participation of the second cycle was discovered when it was found that the reaction rate was much enhanced when hydrogen and a lithium salt were added [43,44]. The role of hydrogen is to reduce the catalytically inactive Rh(CO)2l4 to the active Rh(CO)2l2. In the anhydrous medium used in the reaction, the formation of hydrogen by the reaction of carbon monoxide with water as in the water-gas shift reaction is not possible. Thus hydrogen must be added. 

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... 

One of the main uses of acetic anhydride is in the manufacture of cellulose acetate. Cellulose acetate is used in the production of plastics, coating chemicals, etc. Eastman Chemical Company has operated a highly successful rhodium-based acetic anhydride manufacturing process, based on the carbonylation of methyl acetate, for over 25 years. 

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