Methyl iodide promoters

The hydrogen iodide liberated then reacts with methanol, methyl acetate (or dimethyl ether) to regenerate methyl iodide promoter (eq. (3)) ... 

The reaction chemistry of the rhodium-catalyzed methanol carbonylation process under Monsanto conditions has been investigated extensively [6-8, 10, 12, 21, 26-29] (cf Section 2.1.2.1.1). The overall reaction kinetics are first order in both rhodium catalyst and methyl iodide promoter. The reaction is zero order in methanol and zero order in carbon monoxide partial pressure above 2 atm (eq. (6)) [27]. The kinetics agree well with the basic mechanism common to the three carbonylation reactions (see Section 2.1.2.1.1 and Tables 1 and 2). 

Hou et al. [34] studied the synthesis of dimethyl carbonate from methanol and CO2 as both solvent and reactant, with a methyl iodide promoter. They foimd that as the conversion increased, the mixture critical temperature and pressure increased. Thus, in order to maintain a single-phase mixture, the pressure needed to be increased or correctly chosen a priori. They also foimd that pressure affected the conversion differently, depending on whether the reaction was performed in a single phase or in a vapor-liquid regime. In a two-phase regime, an increase in pressure slightly increased the conversion, while the opposite was true in a single-phase mixture. 

The first commercialized homogeneous methanol carbonylation route to acetic acid was established at BASF in 1955, using a homogeneous Ni catalyst. In 1960 BASF developed an improved process it used an iodide-promoted CO catalyst and operated at an elevated temperature (230 °C) and pressure (600 bar) [2]. In 1970, Monsanto commercialized an improved homogeneous methanol carbonylation process using a methyl-iodide-promoted Rh catalyst [3-5]. This process operated at much milder conditions (180-220 °C, 30-40 bar) than the BASF process and performed much better [6]. Celanese and Daicel further improved the Monsanto... 

AAdiile the active catalyst species has not been fully defined, the results obtained with this system are similar to those found in liquid-phase studies (5,6). The catalyst (3% Rh/C) is effective at low temperatures (175-250°C) and low pressures (1-14 atm) with very high selectivity (>99%) in the presence of methyl iodide promoter (Table 7). Even at atmospheric pressure reasonable conversions of methanol are obtained. At higher pressures essentially total conversion of CH3OH is obtained at only 10-30% of CH3I level used at atmospheric pressure. Hydrogen iodide can be used in place of methyl iodide as the promoter. 

The production of another important chemical and polymer intermediate, acetic acid, was revolutionized by the Wacker process that was introduced in 1960. It was a simple, high yield process for converting ethylene to acetaldehyde, which replaced the older process based on ethanol and acetylene. In the Wacker reaction, the palladium catalyst is reduced and then reoxidized. Ethylene reacts with water and palladium chloride to produce acetaldehyde and palladium metal. The palladium metal is reoxidized by reaction with cupric chloride, which is regenerated by reaction with o gen and hydrochloric acid. In 1968, BASF commercialized an acetic acid process based on the reaction of carbon monoxide and methanol, using carbonyl cobalt promoted with an iodide ion (74). Two years later, however, Monsanto scored a major success with its rhodium salt catalyst with methyl iodide promoter. Developed by James F. Roth, this new catalyst allowed operation at much milder conditions (180°C, 30-40 atm) and demonstrated high selectivity for acetic acid (75). 

This reaction is rapidly replacing the former ethylene-based acetaldehyde oxidation route to acetic acid. The Monsanto process employs rhodium and methyl iodide, but soluble cobalt and iridium catalysts also have been found to be effective in the presence of iodide promoters. 

Earlier catalysts were based on cobalt, iron, and nickel. However, recent catalytic systems involve rhodium compounds promoted by methyl iodide and lithium iodide (48,49). Higher mol wt alkyl esters do not show any particular abiUty to undergo carbonylation to anhydrides. 

The optimized methylation conditions (Scheme 6.18) provided >99% conversion and 92% isolated yield of 35 after in situ crystallization, filtration and drying. Addition of at least 1 equiv of water was essential for complete conversion of the O-Me to N-Me product. Under these reaction conditions Mel is released at the reaction temperature, resulting in an initial 4 1 mixture of 35 49. In situ, iodide-promoted, demethylation of 49 followed by remethylation recycled the undesired O-methyl isomer 49 to 35 in a single-pot reaction. The reaction was generally complete in about 3-6 h at 100 °C. 

A modification of this method, related to the Beckmann rearrangement, entails treatment of a ketoxime with one equivalent of CDI, then four to five equivalents of a reactive halide such as allyl bromide or methyl iodide (R3X) under reflux in acetonitrile for 0.5-1.5 h. Quatemization of the imidazole ring effectively promotes the reaction by increasing the electron-withdrawing effect. The target amides then are obtained by hydrolysis. High yields, neutral conditions, and a very simple procedure make this modification of the synthesis of amides by azolides a very useful alternative. 1243... 

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 only dependencies noted in the kinetic studies were first-order dependencies on iodide promoter and rhodium concentrations. Thus there was no observed effect of varying methanol concentration, and the partial pressure of carbon monoxide had no effect on the reaction rate. Similarly, the concentration of the products, methyl acetate and acetic acid, has no effect on the reaction rate. Thus we have the unusual situation of a reaction, CH3OH + CO — CH3COzH, in which the concentrations of the reactants and product have no kinetic influence. 

Similar acetic acid conversions and higher acid yield distributions using ruthenium(IV) oxide in combination with methyl iodide, ethyl iodide and hydrogen iodide as the added iodide promoter under comparable conditions. This is consistent with these different starting materials ultimately forming the same catalytically active species. 

The intramolecular cyclization reaction of 4-thiazolidinones 290, promoted by iodine in the presence of potassium iodide (or by reaction with methyl iodide in methanol), gives thiazolo[4,3-A [l,3,4]oxadiazoles 110 and 341 (Equation 63) (Table 54) <2001IJC(B)440, 1996JCR(S)388>. 

One approach which enables lower water concentrations to be used for rhodium-catalysed methanol carbonylation is the addition of iodide salts, especially lithium iodide, as exemplified by the Hoechst-Celanese Acid Optimisation (AO) technology [30]. Iodide salt promoters allow carbonylation rates to be achieved at low (< 4 M) [H2O] that are comparable with those in the conventional Monsanto process (where [H20] > 10 M) while maintaining catalyst stability. In the absence of an iodide salt promoter, lowering the water concentration would result in a decrease in the proportion of Rh existing as [Rh(CO)2l2] . However, in the iodide-promoted process, a higher concentration of methyl acetate is also employed, which reacts with the other components as shown in Eqs. 3, 7 and 8 ... 

Promoters, 34 84 see also specific promoters HI or methyl iodide in carbonylation of methanol, 34 88... 

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 formation of the active catalyst can be retarded with high carbon monoxide partial pressure. High CO partial pressure leads to more CO in solution which competes with the ligand over the tricarbonyl species, Ni(C0)3, and forms the inactive nickel tetracarbonyl. The active complex stability was retained by increasing the promoter concentration. The complex formed between nickel and promoters is more stable than Ni(C0)4. In addition, promoters may impart higher electron density to the central atom and increase its nucleophilic character towards methyl iodide. 

Acetaldehyde is obtained from the reaction of synthesis gas with methanol, methyl ketals or methyl esters. The reactions are carried out with an iodide-promoted Co catalyst at 180-200 °C and 2000-5000 psig. In comparing the various feedstocks, the best overall process to make acetaldehyde involves the reductive carbonylation of methyl esters. In this case, acetaldehyde selec-tivities are > 95% ut acceptable rates and conversion. 

The reaction is catalyzed by a group VIII metal species, particularly that of rhodium or palladium. The initial metal species may be any variety of complexes (e.g., PdCl2 Pd acetate, etc.). A source of halide is necessary iodide is especially effective. The most convenient source is methyl iodide, since it is likely a reaction intermediate. In addition, an organic promoter must be included for catalytic activity. These promoters are generally tertiary phosphines or amines. Also, chromium complexes were found to have an important promotional effect. 

The catalyst is generally a palladium compound promoted with a trivalent amine or phosphine in the presence of methyl iodide as described earlier. Systems proven to bias acetaldehyde are utilized, of course (e.g. see Table I, run 12). A yield of 85% acetaldehyde from methyl acetate is typical by this method. It can be utilized in stoichiometric addition to easily prepared acetic anhydride resulting in EDA formation. When considering that the "boiling pot" reaction by-products are recyclable acetic acid, acetic anhydride and small amounts of EDA, the yield to vinyl acetate related products is 95%. 

It was found that a nickel-activated carbon catalyst was effective for vapor phase carbonylation of dimethyl ether and methyl acetate under pressurized conditions in the presence of an iodide promoter. Methyl acetate was formed from dimethyl ether with a yield of 34% and a selectivity of 80% at 250 C and 40 atm, while acetic anhydride was synthesized from methyl acetate with a yield of 12% and a selectivity of 64% at 250 C and 51 atm. In both reactions, high pressure and high CO partial pressure favored the formation of the desired product. In spite of the reaction occurring under water-free conditions, a fairly large amount of acetic acid was formed in the carbonylation of methyl acetate. The route of acetic acid formation is discussed. A molybdenum-activated carbon catalyst was found to catalyze the carbonylation of dimethyl ether and methyl acetate. 

The synthesis of acetic acid (AcOH) from methanol (MeOH) and carbon monoxide has been performed industrially in the liquid phase using a rhodium complex catalyst and an iodide promoter ( 4). The selectivity to acetic acid is more than 99% under mild conditions (175 C, 28 atm). The homogeneous rhodium catalyst is also effective for the synthesis of acetic anhydride (Ac O) by the carbonylation of dimethyl ether (DME) or methyl acetate (AcOMe) (5-13). However, rhodium is one of the most expensive metals, and its proved reserves are quite limited. It is highly desirable, therefore, to develop a new catalyst as a substitute for rhodium. 

Table I shows the effects of Mel/DME and CO/DME ratios in the feed gas on product yields. With increasing Mel/DME ratio both methyl acetate yield and selectivity increased. The yield of methyl acetate increased with an increase in the CO/DME ratio whereas its selectivity decreased. In the case of methanol carbonylation on Ni/A.C. catalyst, the product yield and selectivity were strongly affected by CO/MeOH ratio but not by Mel/MeOH ratio (14-16). The promoting effect of methyl iodide on the methanol carbonylation reached a maximum at a very low partial pressure, that is 0.1 atm or lower. However, both CO/DME and Mel/DME ratios were important for regulating the product yield and selectivity of the dimethyl ether carbonylation. This suggests that the two steps, namely, the dissociative adsorption of methyl iodide on nickel (Equation 4) and the insertion of CO (Equation 5) are slow in the case of dimethyl ether reaction.

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