Methanol carbonylation reaction pathways

Water is indigenous to the methanol carbonylation reaction. It is introduced to the reaction as a part of the solvent, reactants, or catalyst. It can also be formed in situ through steps of the main reaction pathway or via other competing reactions. 

The reaction conditions for the methanol carbonylation reaction are even milder than the commercial conditions indicate. Indeed, the complete catalytic cycle can be realized at room temperature and 1 atm of carbon monoxide This allowed Forster to specifically interpret the macroscopic kinetic observations in mechanistic terms (14) by demonstrating the component reactions under ambient conditions. Since then, a great deal of effort has been directed to determining the generality of this mechanism with other alcohols. In fact, a wide array of alcohols can be carbonylated by this catalyst system (e.g., Table I), though not necessarily by the same mechanistic pathways. 

The Ir-catalyzed methanol carbonylation reaction has been studied extensively by several groups 9f-h. The mechanism for the reaction is more complex than for the Rh reaction. The reaction involves a neutral and an anionic catalytic cycle. The extent of participation by each cycle depends on the reaction conditions. The anionic carbonylation pathway predominates in the Cativa process. The active Ir catalyst species is the iridium carbonyl iodide complex, [Ir(CO)2l2]. The carbonylation reaction proceeds through a series of reaction steps similar to the Rh catalyst process shown in Figure 1 however, the kinetics involve a different rate determining step. 

This represents one pathway to the formation of methane, a knovm by-product in iridium catalysed methanol carbonylation. The hydrogenolysis reaction was severely retarded by the presence of excess CO, indicating a mechanism involving initial dissociation of CO from [MeIr(CO)2l3] , prior to activation of H2. The mechanism therefore resembles that for hydrogenolysis of Rh acetyl complexes in hydroformylation. 

The catalysts or catalysts precursors employed in these studies were anionic group 6 carbonyl complexes ( ) or group 8 metal carbonyl clusters (37— 0) where reaction conditions were 500 psi (CO2/H2) and 125 C. For the group 6 metal catalysts, the turn-over numbers obtained for the methyl formate production were ca. 15 using methanol as solvent for a 24 hour period. The anionic metal carbonyls examined as catalysts precursors included HM2(CO)io , HC02M(C0)5, and CH3C02M(C0)s as their PPN salts (PPN = bis(triphenylphosphine)-iminium and M Cr or W). The proposed reaction pathway is depicted in Scheme 3. 

This reaction pathway is usually favoured when an aromatic moiety and an alkene bear electron-withdrawing and electron-donating substituents, respectively (or vice versa). This addition involves a charge transfer and the course of the reaction is sensitive to the solvent polarity. Such a mechanism may resemble that of [2 + 2] photocycloaddition of alkenes to aji-unsaturated carbonyl compounds (Section 6.3.2). Scheme 6.81 shows examples of two intermolecular processes and one intramolecular [2 + 2] photocycloaddition reaction (a) crotononitrile (196) is added to anisole (197) to yield several stereoisomers of 198 in 38% chemical yield and with high regioselectivity, which is linked to bond polarization in the exciplex 818 (b) hexafluorobenzene (199) reacts with 1-ethynylbenzene (200) to form the bicyclo[4.2.0]octa-2,4,7-triene 201 in 86% yield 819 and (c) irradiation of 202 in methanol leads to the single photoproduct 203. 820... 

A slurry phase concurrent synthesis of methanol using a potassium meth-oxide/copper chromite mixed catalyst has been developed. This process operates under relatively mild conditions such as temperatures of 100-180°C and pressures of 30-65 atm. The reaction pathway involves a homogeneous carbonylation of methanol to methyl formate followed by the heterogeneous hydrogenolysis of methyl formate to two molecules of methanol, the net result being the reaction of hydrogen with carbon monoxide to give methanol via methyl formate ... 

At least two reaction pathways may be postulated for the formation of INMF. The first (Scheme B, path a) involves initial reaction of CO with ammonia to give formamide, followed by methylation with Me-Ru generated via ruthenium-catalyzed CO hydrogenation. An alternative pathway would proceed via initial formation of methanol from CO/H2, to be followed by the production of methylamines and subsequent carbonylation (path b). 

The initial step of methanol conversion is composed of carbonylation to methyl acetate and dehydration to DME in parallel (see scheme below). DME is successively carbonylated to form methyl acetate. Methyl acetate is further carbonylated to acetic anhydride, which is rapidly hydrolyzed to acetic acid. Hydrolysis of methyl acetate to acetic acid and methanol proceeded also on the carbon support. Thus, carbonylation of methanol proceeds through a set of parallel and series reactions that produce methyl acetate and DME as primary products with acetic acid as the ultimate product, according to following reaction pathways (32,33) ... 

Although the term methanol carbonylation is usually associated with acetic acid manufacture, an alternative carbonylation pathway involves base-catalyzed addition of CO to alkoxide ions to provide a simple route to formate esters (see also the section Direct Synthesis of Methanol from CO/H2). In the case of methanol as the alkanol, the reaction is carried out industrially on a large scale to produce formic acid. The reaction proceeds at ca 30 bar and 80°C using sodium or potassium methoxide as the catalyst and involves nucleophilic attack of methoxide on CO ... 

Two different reaction pathways have been h5q>othesized for the Passerini reaction the first one is supposed to be ionic (Fig. 5.1) the second one, concerted (Fig. 5.2). In polar solvents such as methanol or water, the reaction proceeds hy protonation of the carbonyl followed by nucleophilic addition of the isocyanide to produce the nitrilium ion, 3 (below). Addition of a carboxylate gives the intermediate, 4. Acyl group transfer and amide tautomerization yield the desired ester, 5. 

Methyl acetate probably originates from the reaction of methanol with the intermediate cobalt-acyl complex. The reaction leading to the formation of acetaldehyde is not well understood. In Equation 8, is shown as the reducing agent however, metal carbonyl hydrides are known to react with metal acyl complexes (20-22). For example, Marko et al. has recently reported on the reaction of ri-butyryl- and isobutyrylcobalt tetracarbonyl complexes with HCo(CO) and ( ). They found that at 25 °C rate constants for the reactions with HCo(CO) are about 30 times larger than those with however, they observed that under hydroformylation conditions, reaction with H is the predominant pathway because of the greater concentration of H and the stronger temperature dependence of its rate constant. The same considerations apply in the case of reductive carbonylation. Additionally, we have found that CH C(0)Co(C0) L (L r PBu, ... 

By far the most important synthesis gas reaction is its conversion into methanol, using copper/zinc oxide catalysts under relatively mild conditions (50 bar, 100-250°C). Methanol is further carbonylated to acetic acid (see Section 22-7), so that CH3C02H, methyl acetate, and acetic anhydride can all be made from simple CO and H2 feedstocks. Possible pathways to oxygenates in cobalt catalyzed reactions are shown in Fig. 22-6. 

A third possibility of chemical modification is conversion into an acylsilane which reduces the oxidation potential of the corresponding ketone by approximately 1 V. A peak potential of 1.45 V (relative to Ag/AgCl) for the oxidation of undecanoyltrimethylsilane has been reported. Preparative electrochemical oxidations of acylsilanes proceed in methanol to give the corresponding methyl esters. A two-step oxidation process must be assumed because of the reaction stoichiometry —oxidation of the acylsilane results in the carbonyl radical cation which is meso-lytically cleaved to give the silyl cation and the acyl radical, which is subsequently oxidized to give the acyl cation as ultimate electrophile which reacts with the solvent. A variety of other nucleophiles have been used and a series of carboxylic acid derivatives are available via this pathway (Scheme 49) [198]. 

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