Catalytic methanol carbonylation acetic acid

Low-water operation can be accomplished with modifications to the process which include significant changes in the catalyst system [23]. The main catalytic cycle for high-water methanol carbonylation is still operative in the low-water process (see Section 2.1.2.1.1), but at low water concentration two other catalytic cycles influence the carbonylation rate. The incorporation of an inorganic or organic iodide as a catalyst co-promoter and stabilizer allows operation at optimum methyl acetate and water concentrations in the reactor. Carbonylation rates comparable with those realized previously at high water concentration (ca. 10 molar) are demonstrated at low reaction water concentrations (less than ca. 4 molar) in laboratory, pilot plant, and commercial units, with beneficial catalyst stability and product selectivity [23]. With this proprietary AO technology, the methanol carbonylation unit capacity at the Celanese Clear Lake (TX) facility has increased from 270 X 10 metric tons per year since start-up in 1978 to 1200 X 10 metric tons acetic acid per year in 2001 with very low capital investment [33]. This unit capacity includes a methanol-carbonylation acetic acid expansion of 200 X 10 metric tons per year in 2000 [33]. 

Acetic Acid. Manufacture of acetic acid [64-19-7] by homogeneous catalytic methanol carbonylation has become the leading commercial route to acetic acid (eq. 8) (34,35). 

Industry uses a multitude of homogenous catalysts in all kinds of reactions to produce chemicals. The catalytic carbonylation of methanol to acetic acid... 

Iodide and acetate salts increase the rate of reaction of Li [1] with CH3I at 25 °C in acetic acid. The effects of water, LiBF4, and other additives are also reported. Iodide salts also promote catalytic methanol carbonylation at low water concentrations. In the case of Lil promoter, lithium acetate is produced. The promotional effects of iodide and acetate on both the model and catalytic systems are rationalized in terms of iodide or acetate coordination to (1) to yield five-coordinate RhI anions as reactive intermediates for rate-determining reactions with CH3I.11... 

The most common oxidation states and corresponding electronic configurations of rhodium are +1 (tf8), which is usually square planar although some five coordinate complexes are known, and +3 (T) which is usually octahedral. Dimeric rhodium carboxylates are +2 (oxidation states —1 (industrial applications include rhodium-catalyzed carbonylation of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to tf-butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

As mentioned in the previous section, the carbonylation of methanol to acetic acid is an important industrial process. Whereas the [Co2(CO)s]-catalyzed, iodide-promoted reaction developed by BASF requires pressures of the order of 50 MPa, the Monsanto rhodium-catalyzed synthesis, which is also iodide promoted and which was discovered by Roth and co-workers, can be operated even at normal pressure, though somewhat higher pressures are used in the production units.4,1-413 The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster.414 The anionic complex m-[RhI2(CO)2]- (95) initiates the catalytic cycle, which is shown in Scheme 26. 

Reaction (78) regenerates Mel from methanol and HI. Using a high-pressure IR cell at 0.6 MPa, complex (95) was found to be the main species present under catalytic conditions, and the oxidative addition of Mel was therefore assumed to be the rate determining step. The water-gas shift reaction (equation 70) also occurs during the process, causing a limited loss of carbon monoxide. A review of the cobalt-, rhodium- and iridium-catalyzed carbonylation of methanol to acetic acid is available.415... 

The Monsanto carbonylation of methanol to acetic acid catalyzed by Rh/H is a well-understood example of an organometallic catalytic cycle and can act as a good model with well defined steps (shown schematically in Chapter 4, Section 4.2.4). The starting material is the square planar Rh(I) complex, [Rh(CO)2l2] which is easily accessible by reaction of rhodium trichloride in solution with CO in the presence of iodide. This undergoes oxidative addition with Mel very readily to give the methyl-Rh(III) complex [Rh(Me)(CO)2l3] as an unstable... 

Like Rh/1 systems, the Ir4(CO),2/l2 system catalyzes the carbonylation of methanol to acetic acid [70]. In homogeneous hydrogenation of CO (Fischer Tropsch reaction), Ir4(CO)i2 shows a relatively high catalytic activity compared with other transition metal carbonyls (eq (62)) [71]. 

Methanol carbonylation has been the subject of numerous reviews [6-17], including the seminal mechanistic studies by Denis Forster at Monsanto. This chapter does not seek to repeat all the information included in those reviews, but instead is focused on the important recent advances in process development and mechanistic understanding along with recent research efforts to identify new ligand-modified, supported, or promoted catalysts. The increasing use of computational methods to model catalytic mechanisms and potential alternative catalytic routes to acetic acid are also summarized. 

A particularly broad potential for application in syngas reactions is shown by ruthenium carbonyl clusters. Iodide promoters seem to favor ethylene glycol (155,156) the formation of [HRu3(CO),i]" and [Ru(CO)3l3]" was observed under the catalytic conditions. These species possibly have a synergistic effect on the catalytic process. Imidazole promoters have been found to increase the catalytic activity for both methanol and ethylene glycol formation (158-160). Quaternary phosphonium salt melts have been used as solvents in these cases the anion [HRu3(CO)i,] was detected in the mixture (169). Cobalt iodide as cocatalyst in molten [PBu4]Br directs the catalytic synthesis toward acetic acid (163). With... 

Most of these processes currently make use of homogeneous catalysts. These are usually soluble complexes of transition metals, e.g., Co, Rh, and Ru. For example, conversion of methanol into acetic acid requires catalysis by either Co carbonyl or Rh carbonyl complexes and co-catalysis by iodine. Under reaction conditions iodine is most likely present as HI and CH3I, the latter probably being the agent by which the catalytically active ion (Rh or Co) is alkylated " before a methyl migration to the co-ordinated CO takes place. 

Scheme 1.8. Mechanism of catalytic carbonylation of methanol to acetic acid.

The carbonylation of methanol in acetic acid represents an important industrial process, which has been developed by Monsanto Corporation using a homogeneous rhodium complex. Extensive investigations on the rhodium catalytic system have been carried out and Liu et al. [77] have studied the use of PVP-stabilized Rh nanoparticles for this reaction (Scheme 11.10). The stable PVP-Rh colloid presents a lower activity than Monsanto s homogeneous catalyst under the same drastic conditions (140°C, 54bar). However, the colloidal metal catalyst could be reused several times with an increased activity (TON = 19700 cydes/atom Rh), which... 

In their pioneering work, Periana et al. [88] reported that Pd(II)/H2S04 catalytic system catalyzed direct conversion of methane to methanol and acetic acid with a combined selectivity of >90% at 455 K in liquid sulfuric acid. It was concluded that carbon atoms in acetic acid originate from methane and methanol, with the latter being primarily formed from methane. The reaction is initiated by the electrophilic CH activation with Pd(II) to yield Pd-CHs species. The activation is accelerated by sulfuric acid. The primarily formed Pd-CHs species is further transformed to methanol with simultaneous reduction of Pd(II) to Pd(0). The reduced Pd species are also formed upon methanol oxidation to some CO species. The latter participate in the carbonylation of the Pd-CHs species. To close the catalytic cycle Pd(0) is oxidized to Pd(II) by sulfuric acid. Since gas-phase O2 did not influence the reaction rate and the selectivity, free radicals were excluded as reaction intermediates. 

Two processes of direct methanol carbonylation are well established already The direct carbonylation of methanol yielding acetic acid, the Monsanto process and carbonylation of methyl acetate giving acetic anhydride, a technology commercialized by Tennessee Eastman Kodak. By adjusting the CO H2 ratio, catalytic systems for the reductive carbonylation of methyl acetate can be tuned to the production of acetic anhydride, ethylidene diacetate or acetaldehyde. 

The results of batch carbonylation of methanol to acetic acid and its ester in the presence of complexes 1, 2, fra 5 -[Rh(CO)Cl(PPh3)2] and [Rh(CO)2l2] as catalyst preciusors are shown in table 1. GC analyses of the products reveal that as the reaction time increases from 0.5 to 1.5 hoin, the total conversion as well as Tium Over Number (TON) for the different catalyst precin ors increases irrespective of the complexes. Maximum conversion of methanol with corresponding maximum TON were observed dining the entire catalytic reaction period for the complex 1 compared to others and the highest conversion (95 %) and TON (1563) were observed for 1.5 hour reaction period (table 1). Under the same... 

Detailed mechanistic and theoretical analysis of the key mechanistic steps of the Cativa process, for the Cativa process See Ref 257,257a that is, the iridium-based catalytic carbonylation of methanol to acetic acid, have allowed several groups, " particularly Haynes and co-workers, to unravel the mechanism of the catalytic process. Ir(l) complexes [Ir(CO)(L-L)I] (LL = dppms, dppe, dppmo) provided important mechanistic information about the influence of stereoelectronic ligand effects on the organometallic reactivity of modified metal centers with Mel. The carbonylation of methanol promoted by iridium and rhodium complexes which is at the basis of both Cativa and Monsanto processes for the synthesis of acetic acid will be described in detail in a different chapter of this volume. 

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. 

All the forward reactions are important steps in industrial homogeneous catalytic processes. Reaction 2,3.1.1 is the OA of CH3I to a square planar anionic Rh complex. It is the first step in the catalj c carbonyl-ation of methanol to acetic acid. Reaction 2.3.1.2, OA of hydrogen to a cationic Rh complex, is a step in the hydrogenation of an alkene with an acetamido functional group. 

Commercial production of acetic acid has been revolutionized in the decade 1978—1988. Butane—naphtha Hquid-phase catalytic oxidation has declined precipitously as methanol [67-56-1] or methyl acetate [79-20-9] carbonylation has become the technology of choice in the world market. By-product acetic acid recovery in other hydrocarbon oxidations, eg, in xylene oxidation to terephthaUc acid and propylene conversion to acryflc acid, has also grown. Production from synthesis gas is increasing and the development of alternative raw materials is under serious consideration following widespread dislocations in the cost of raw material (see Chemurgy). 

Acetic Acid and Anhydride. Synthesis of acetic acid by carbonylation of methanol is another important homogeneous catalytic reaction. The Monsanto acetic acid process developed in the late 1960s is the best known variant of the process. 

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