CARBONYLATION OF METHANOL AND METHYL ACETATE

Table 1. Catalyst systems for carbonylations of methanol and methyl acetate.

The reaction course of the co-carbonylation of methanol and methyl acetate (Figure 6) can be interpreted in three phases. 

Industrial applications are toluene and xylene oxidation to acids, oxidation of ethene to aldehyde, carbonylation of methanol and methyl acetate, polymerization over metallocenes (Figure 2.2), hydroformylation of alkenes, etc. 

Mixed oxides, typically containing bismuth and molybdenum, are used as catalysts and these have been improved over the years so that the conversion of propylene to acrylonitrile is now over 80% per pass through the reactor. HCN is produced in a side reaction and is used mainly to make acetonitrile or methyl methacrylate [23]. Acetic acid and acetic anhydride, which are made in high yields by the carbonylation of methanol and methyl acetate, respectively, using an iodine promoted rhodium catalyst, can now be made in a variable ratio, to match market demand, using the same plant [24]. 

Carbon monoxide in the synthesis gas can be separated from hydrogen. Carbonylation of methanol and methyl acetate to give acetic acid and acetic anhydride, respectively, are examples of CO-based Cj... 

The single step conversion of methyl acetate to ethylidene diacetate is catalyzed by either a palladium or rhodium compound, a source of iodide, and a promoter. The mechanism is described as involving the concurrent generation of acetaldehyde and acetic anhydride which subsequently react to form ethylidene diacetate. An alternative to this scheme involves independent generation of acetaldehyde by reductive carbonylation of methanol or methyl acetate, or by acetic anhydride reduction. The acetaldehyde is then reacted with anhydride in a separate step. 

Early studies by Scurrell and coll, demonstrated the use of rhodium zeolites as catalysts for the carbonylation of methanol into methyl acetate in the presence of methyl iodide (65). It was hoped that due to their electrostatic field zeolites would effect the direct carbonylation of methanol without the help of the iodide promoter. In fact, as the CH3OH/CH3I ratio increased, increasing amounts of CH4 and CO2 were produced indicating that the reaction... 

In 1992, about 6.5 billion lb acetic acid was produced worldwide, of which about 3.6 billion lb was produced in the United States [1]. The current commercial processes for its production include oxidation of ethanol (acetaldehyde), oxidation of butane-butene mixture or naphtha, and carbonylation of methanol or methyl acetate. These are catalytic processes. The last, liquid-phase carbonylation of methanol using a rhodium and iodide catalyst, has become the dominant process since its introduction in the late 1960s, and accounted for about half the production of acetic acid in the United States [2]. That represents a conversion of 1.5 x 106 ton per year of methanol into 2.8 x 106 ton per year of acetic acid. In the United States, 80% of actual plant operation capacity is based on this technology [3]. The reaction is thermodynamically favorable [4], and the theoretical conversion is practicalty 100% at 389 K ... 

Reppe reaction involves carbonylation of methanol to acetic acid and methyl acetate and subsequent carbonylation of the product methyl acetate to acetic anhydride. The reaction is carried out at 600 atm and 230°C in the presence of iodide-promoted cobalt catalyst to form acetic acid at over 90% yield. In the presence of rhodium catalyst the reaction occurs at milder conditions at 30 to 60 atm and 150-200°C. Carbon monoxide can combine with higher alcohols, however, at a much slower reaction rate. 

The carbonylation of methanol to give acetic acid, according to Eq.(l), based on the catalyst [Rh(CO)2I2], is a major industrial process (Monsanto acetic acid process). However, ruthenium clusters as catalysts seem to favor the insertion of carbon monoxide into the O-H and not into the C-O bond, according to Eq.(2). Ru3(CO)12 in basic solution converts methanol to methyl formate with 90% selectivity (400-450 bar CO,... 

TABLE 1 Principal Features of Commercial Processes for Carbonylation of Methanol and of Methyl Acetate. 

The effect of iodide and acetate on the activity and stability of rhodium catalysts for the conversion of methanol into acetic acid have been studied. Iodide salts at low water concentrations (<2 M) promote the carbonylation of methanol and stabilize the catalyst. Alkali metal iodides react with methylacetate to give methyl iodide and metal acetate the acetate may coordinate to Rh and act as an activator by forming soluble rhodium complexes and by preventing the precipitation of Rhl3. A water-gas shift process may help to increase the steady-state concentration of Rh(I). The labile phosphine oxide complex (57) is in equilibrium with the very active methanol carbonylation catalyst (58) see equation (56). 

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... 

Formic acid is produced mainly by carbonylation of methanol to methyl formate followed by hydrolysis of this ester to formic acid and methanol [route (d) in Topic 5.3.3]. The applied reaction sequence represents formally the hydrolysis of carbon monoxide to formic acid. Owing to the growing worldwide interest in converting CO2 into useful chemicals, the catalytic hydrogenation of CO2 to formic acid has been investigated intensively but no commercial processes has been realized yet. Formic acid is also obtained as one of the side products in the catalytic oxidation of butane and light naphtha to acetic acid (see Section 6.15 for details). 

While the base-catalyzed carbonylation of methanol yields methyl formate, a versatile intermediate for formic acid and formamide synthesis, the transition metal-catalyzed carbonylation involves C—C coupling, giving acetic acid derivatives as C2 oxygenates. 

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. 

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 unit has virtually the same flow sheet (see Fig. 2) as that of methanol carbonylation to acetic acid (qv). Any water present in the methyl acetate feed is destroyed by recycle anhydride. Water impairs the catalyst. Carbonylation occurs in a sparged reactor, fitted with baffles to diminish entrainment of the catalyst-rich Hquid. Carbon monoxide is introduced at about 15—18 MPa from centrifugal, multistage compressors. Gaseous dimethyl ether from the reactor is recycled with the CO and occasional injections of methyl iodide and methyl acetate may be introduced. Near the end of the life of a catalyst charge, additional rhodium chloride, with or without a ligand, can be put into the system to increase anhydride production based on net noble metal introduced. The reaction is exothermic, thus no heat need be added and surplus heat can be recovered as low pressure steam. 

An analogue of the transesterification process has also been demonstrated, in which the diacetate of BPA is transesterified with dimethyl carbonate, producing polycarbonate and methyl acetate (33). Removal of the methyl acetate from the equihbrium drives the reaction to completion. Methanol carbonylation, transesterification using phenol to diphenyl carbonate, and polymerization using BPA is commercially viable. The GE plant is the first to produce polycarbonate via a solventiess and phosgene-free process. 

The final step in the process involves reacting purified carbon monoxide from the gas separation plant with methyl acetate to form acetic anhydride, using a proprietary catalyst system and process. Part of the acetic anhydride is reacted with methanol to produce acetic acid and methyl acetate, and the latter is recirculated to the carbonylation step. 

Carbonylation of methanol to acetic acid is fully discussed in Chapter 9. Another carbonylation process using a phosphine ligand to control the course of the reaction is a highly atom efficient route to the widely used monomer methyl methacrylate (Scheme 4.19). In this process the catalyst is based on palladium acetate and the phosphine ligand, bisphenyl(6-methyl-2-pyridyl) phosphine. This catalyst is remarkably (>99.5%) selective for the 2-carbonylation of propyne under the relatively mild conditions of <100 °C and 60 bar pressure. 

Historically, the rhodium catalyzed carbonylation of methanol to acetic acid required large quantities of methyl iodide co-catalyst (1) and the related hydrocarboxylation of olefins required the presence of an alkyl iodide or hydrogen iodide (2). Unfortunately, the alkyl halides pose several significant difficulties since they are highly toxic, lead to iodine contamination of the final product, are highly corrosive, and are expensive to purchase and handle. Attempts to eliminate alkyl halides or their precursors have proven futile to date (1). 

Recently, Eastman Chemical Company reported that ionic liquids can be successfully employed in a vapor take-off process for the carbonylation of methanol to acetic acid in the presence of rhodium and methyl iodide (3). While attempting to extend this earlier work to the carbonylation of ethylene to propionic acid, we discovered that, when using ionic liquids as a solvent, acceptable carbonylation rates could be attained in the absence of any added alkyl iodide or hydrogen iodide (4). We subsequently demonstrated that the carbonylation of methanol to acetic acid could also be operated in the absence of methyl iodide when using ionic liquids (5). 

General Procedure for Batch Carbonylation of Methanol in the Absence of Methyl Iodide. A complete set of procedures appears in ref. 5 bnt the following procedure is representative of a methanol carbonylation. To a 300 mL Hastelloy C-276 autoclave was added 0.396 g (1.5 mmol) of RhCl3 3H20, 112.0 g (0.507 mol) of N-methyl pyridinium iodide, 30.0 g (0.5 mol) of acetic acid, and 64.0 g (2.0 mol) of methanol. The mixture was heated to 190°C under 250 psi (1.72 MPa) of 5% hydrogen in carbon monoxide. Upon reaching temperatnre the gas feed was switched... 

Other companies (e.g., Hoechst) have developed a slightly different process in which the water content is low in order to save CO feedstock. In the absence of water it turned out that the catalyst precipitates. Clearly, at low water concentrations the reduction of rhodium(III) back to rhodium(I) is much slower, but the formation of the trivalent rhodium species is reduced in the first place, because the HI content decreases with the water concentration. 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. Stabilization of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives.8 The kinetics of the title reaction with respect to [MeOH] change if H20 is used as a solvent instead of AcOH.9 Kinetic data for the Rh-catalyzed carbonylation of methanol have been critically analyzed. The discrepancy between the reaction rate constants is due to ignoring the effect of vapor-liquid equilibrium of the iodide promoter.10... 

The reaction of alcohols with CO was catalyzed by Pd compounds, iodides and/or bromides, and amides (or thioamides). Thus, MeOH was carbonylated in the presence of Pd acetate, NiCl2, tV-methylpyrrolidone, Mel, and Lil to give HOAc. AcOH is prepared by the reaction of MeOH with CO in the presence of a catalyst system comprising a Pd compound, an ionic Br or I compound other than HBr or HI, a sulfone or sulfoxide, and, in some cases, a Ni compound and a phosphine oxide or a phosphinic acid.60 Palladium(II) salts catalyze the carbonylation of methyl iodide in methanol to methyl acetate in the presence of an excess of iodide, even without amine or phosphine co-ligands platinum(II) salts are less effective.61 A novel Pd11 complex (13) is a highly efficient catalyst for the carbonylation of organic alcohols and alkenes to carboxylic acids/esters.62... 

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