Catalytic methanol carbonylation reaction mechanism

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 chapter has focused particularly on the mechanistic aspects of catalytic methanol carbonylation and how the underlying organometallic chemistry has an impact on process considerations. Although there is often an element of serendipity in catalyst discovery, a thorough fundamental understanding of reaction mechanisms will play a crucial role in developing the next generation of catalysts. 

The overall response to the reaction variables is very similar in the carbonylation and reductive carbonylation reactions. This may indicate similar catalysts and reaction mechanisms. In the carbonylation reaction Co(CO) " was identified by its characteristic CO stretching frequency ( v(CO) r 1890 cm" as the dominant species present in high pressure infrared experiments carried out at 170 °C and 5000 psig. Similar results were obtained in the reductive carbonylation of methanol. It is known that Co(CO) " rapidly reacts with CH I to yield CH C(0)Co(C0) (J9) however, in the carbonylation and reductive carbonylation reactions acyl-cobalt complexes are not observed by infrared under catalytic conditions. This indicates that once formed, the acyl complex rapidly reacts as outlined by Equations 7 and 8. 

Synopsis of Kinnunen and Laasonen (2001), Reaction Mechanism of the Reductive Elimination in the Catalytic Carbonylation of Methanol. A Density Functional Study . 

The acetic acid-forming part of the catalytic cycle for methanol carbonylation consists of reactions between acetyl iodide and water to give acetic acid and HI (Fig. 4.2, bottom left). The hydroiodic acid reacts with methanol to regenerate CH3I and water. A similar mechanism operates for the carbonylation of methyl acetate. Acetic acid and acetyl iodide react to give acetic anhydride and HI. The latter reacts with methyl acetate to regenerate acetic acid and methyl iodide. These reactions are shown in Fig. 4.9 by the large, left-hand-side loop. 

The proposed mechanism for this carbonylation reaction, which occurs in the absence of water, involves basic catalytic steps similiar to the rhodium-catalysed methanol carbonylation process (see Section 2.1.2.1.1). The mechanism leads to the formation of acetyl iodide, which reacts with methyl formate to produce the mixed anhydride [133]. 

This reaction mechanism is supported by model studies. Paricularly advantageous are the mild reaction conditions (30-40 bar, 150-200°C) and the high selectivity with respect to methanol (99 %) and CO (> 90 %) compared to the older cobalt process. Methanol carbonylation is one of the few industrially important catalytic reactions whose kinetics are known in full [7]. 

This chapter will briefly review the fundamental organometallic reactions that play a key role in almost all metal-catalyzed processes. It will then apply these reaction steps to explain currently accepted mechanisms for some major catalytic cycles hydrogenation, hydroformylation, methanol carbonylation, Pd-catalyzed coupling reactions, and alkene polymerization and metathesis. Each of these catalytic reactions is covered in considerably more detail in later chapters, so the discussion here will be limited to relating and using the various fundamental reactions to build up and describe multistep catalytic cycles. 

Rhodium-catalyzed carbonylation of methanol is known as the Monsanto process, which has been studied extensively. From the reaction mechanism aspect, the study of kinetics has proved that the oxidative addition of methyl iodide to the [Rh(CO)2l2] is the rate-determining step of the catalytic cycle. It was also observed that acetyl iodide readily adds to [Rh(CO)2l2], indicating that the acetyl iodide must be scavenged by hydrolysis in order to drive the overall catalytic reaction forward. An alternative to sequential reductive elimination and the hydrolysis of acetyl iodide is the nucleophilic attack of water on the Rh acetyl complex and the production of acetic acid. The relative importance of these two alternative pathways has not yet been fully determined, although the catalytic mechanism is normally depicted as proceeding via the reductive elimination of acetyl iodide from the rhodium center. The addition of iodide salts, especially lithium iodide, can realize the reaction run at lower water concentrations thus, byproduct formation via the water gas shift reaction is reduced, subsequently improving raw materials consumption and reducing downstream separation. In addition to the experimental studies of the catalytic mechanism, theoretical studies have also been carried out to understand the reaction mechanism [17-20]. 

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. 

Pseudo-first-order rate constants for carbonylation of [MeIr(CO)2l3]" were obtained from the exponential decay of its high frequency y(CO) band. In PhCl, the reaction rate was found to be independent of CO pressure above a threshold of ca. 3.5 bar. Variable temperature kinetic data (80-122 °C) gave activation parameters AH 152 (+6) kj mol and AS 82 (+17) J mol K The acceleration on addition of methanol is dramatic (e. g. by an estimated factor of 10 at 33 °C for 1% MeOH) and the activation parameters (AH 33 ( 2) kJ mol" and AS -197 (+8) J mol" K at 25% MeOH) are very different. Added iodide salts cause substantial inhibition and the results are interpreted in terms of the mechanism shown in Scheme 3.6 where the alcohol aids dissociation of iodide from [MeIr(CO)2l3] . This enables coordination of CO to give the tricarbonyl, [MeIr(CO)3l2] which undergoes more facile methyl migration (see below). The behavior of the model reaction closely resembles the kinetics of the catalytic carbonylation system. Similar promotion by methanol has also been observed by HP IR for carbonylation of [MeIr(CO)2Cl3] [99]. In the same study it was reported that [MeIr(CO)2Cl3]" reductively eliminates MeCl ca. 30 times slower than elimination of Mel from [MeIr(CO)2l3] (at 93-132 °C in PhCl). 

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. 

Several C-labelled tetraphenyl arsonium salts of Rh(III) containing complex anions have been synthesized to investigate, by the N.M.R. method, the mechanism of the rhodium/iodine-catalysed industrial carbonylation of methanol used for acetic acid manufacture. A revised catalytic cycle for the reaction has been proposed (equation... 

---