Catalytic methanol carbonylation process

The ionic attachment strategy for catalytic methanol carbonylation has recently seen a resurgence of interest from both industry [49-53] and academic groups [54-57]. Most significantly, in 1998 Chiyoda and UOP announced their Acetica process, which uses a polyvinylpyridine resin tolerant of elevated temperatures and pressures [8,58]. The process attains increased... 

In 1996, BP Chemicals announced a new methanol carbonylation process, Cativa , based upon a promoted iridium/iodide catalyst which now operates on a number of plants worldwide [61-69]. Promoters, which enhance the catalytic activity, are key to the success of the iridium-based process. The mechanistic aspects of iridium-catalysed carbonylation and the role of promoters are discussed in the following sections. 

Commercial methanol carbonylation processes have employed each of the group 9 metals, cobalt, rhodium and iridium as catalysts. In each case acid and an iodide co-catalyst are required to activate the methanol by converting it into iodomethane (CH3OH + HI CH3I + H2O) catalytic carbonylation of iodomethane into acetyl iodide is followed by hydrolysis to acetic acid. A problem common to all these processes arises because the mixture of HI and acetic acid is highly corrosive this necessitates special techniques for plant construction involving the use of expensive steels. We discuss each catalyst system in turn below. 

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]. 

A key property of catalytic processes is selectivity. Catalysis has revolutionized process chemistry by replacement of wasteful, unselective (i.e. multiple-product-forming) reactions with efficient, selective (i.e. one-product-dominating) ones. For example, selective catalytic methanol carbonylation (practiced by BP, BASF Monsanto, Eastman) has to a large extent substituted unselective non-catalytic n-butane oxidation (Celanese, and Union Carbide processes). 

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 proposed reductive elimination of acetyl iodide from the Rh(III) coordination sphere is an important step in the Monsanto methanol carbonylation process (2) [73]. In the proposed catalytic cycle (Scheme 30), the oxidative addition of iodo-methane, formed from HI and methanol, is followed by the carbonyl insertion into the Rh-Me bond. The reductive elimination of acetyl iodide followed by its rapid hydrolysis furnishes the acetic acid and regenerates free HI. 

CH3I to [Ir(CO)2l2]. The first step in the iridium system is several hundred times faster than in the Monsanto process the second step, involving alkyl migration, is much slower, and it is rate determining for the Cativa process. In addition to the catalytic cycle involving the anion [Ir(CO)2l2] , an alternative neutral cycle involving Ir(CO)3l or Ir(CO)2l has been reported. Two excellent reviews of developments in catalytic methanol carbonylation are available. ... 

A. Haynes (2010) Adv. Catal., vol. 53, p. 1 — Catalytic methanol carbonylation An up-to-date review of the Monsanto and Cativa processes including background information. 

Ca.ta.lysis, The readily accessible +1 and +3 oxidation states of rhodium make it a useful catalyst. There are several reviews of the catalytic properties of rhodium available (130—132). Rhodium-catalyzed methanol carbonylation (Monsanto process) accounted for 81% of worldwide acetic acid by 1988 (133). The Monsanto acetic acid process is carried out at 175°0 and 1.5 MPa (200 psi). Rhodium is introduced as RhCl3 but is likely reduced in a water... 

Of the three catalytic systems so far recognized as being capable of giving fast reaction rates for methanol carbonylation—namely, iodide-promoted cobalt, rhodium, and iridium—two are operated commercially on a large scale. The cobalt and rhodium processes manifest some marked differences in the reaction area (4) (see Table I). The lower reactivity of the cobalt system requires high reaction temperatures. Very high partial pressures of carbon monoxide are then required in the cobalt system to... 

It was discovered by Monsanto that methanol carbonylation could be promoted by an iridium/iodide catalyst [1]. However, Monsanto chose to commercialise the rhodium-based process due to its higher activity under the conditions used. Nevertheless, considerable mechanistic studies were conducted into the iridium-catalysed process, revealing a catalytic mechanism with similar key features but some important differences to the rhodium system [60]. 

The commercialisation of an iridium-based process is the most significant new development in methanol carbonylation catalysis in recent years. Originally discovered by Monsanto, iridium catalysts were considered uncompetitive relative to rhodium on the basis of lower activity, as often found for third row transition metals. The key breakthrough for achieving high catalytic rates for an iridium catalyst was the identification of effective promoters. Recent mechanistic studies have provided detailed insight into how the promoters influence the subtle balance between neutral and anionic iridium complexes in the catalytic cycle, thereby enhancing catalytic turnover. 

First of all, DMC is a nontoxic compound since the middle 1980s, in fact, it has not been produced from phosgene, but by catalytic oxidative carbonylation of methanol with oxygen through a process developed by Enichem (Italy) and UBE Industries (Japan) (Scheme 4.3) ... 

In SILP carbonylation we have introduced a new methanol carbonylation SILP Monsanto catalyst, which is different from present catalytic alcohol carbonylation technologies, by using an ionic liquid as reaction medium and by offering an efficient use of the dispersed ionic liquid-based rhodium-iodide complex catalyst phase. In perspective the introduced fixed-bed SILP carbonylation process design requires a smaller reactor size than existing technology in order to obtain the same productivity, which makes the SILP carbonylation concept potentially interesting for technical applications. 

There has been a recent resurgence of interest in iridium catalysed methanol carbonylation, arising from the commercialisation by BP Chemicals of the Cativa process. This uses a promoted iridium catalyst and has now superseded the rhodium catalyst on a number of plants. Its success relies on the discovery of promoters which increase catalytic activity, particularly at commercially desirable low water concentrations. HP IR spectroscopy has been used to investigate the behavior of... 

All the forward reactions are important steps in commercial homogeneous catalytic processes. Reaction 2.2 is a step in methanol carbonylation (see Chapter 4), while reaction 2.3 is a step in the hydrogenation of an alkene with an acetamido functional group. This reaction, as we will see in Chapter 9, is... 

Interest in iridium-catalyzed methanol carbonylation was rekindled in the 1990 s when BP Chemicals developed and commercialized the Cativa process, which utilizes an iridium/iodide catalyst and a ruthenium promoter. This process has the important advantage that the highest catalytic rates occur at significantly lower water concentration (ca. 5% wt) than for Monsanto s... 

Reactions of this type are termed alkyl migration (see Alkyl Migration). These reactions are very important in several catalytic reactions, such as hydroformylation, methanol carbonylation, and homogeneous CO reduction (see Carbonylation Processes by Homogeneous Catalysis). 

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]. 

The process chemistry of the methanol carbonylation reaction is summarized in Scheme 1. This catalytic reaction scheme depicts the balanced relationship between the methanol carbonylation, the WGSR and the iodide cycles under both regimes of water concentration. Within the scope of methanol carbonylation in an aqueous/acetic acid medium, the overall reaction rate depends not only on the nature of the rate-determining step(s), but also on reaction conditions influencing the steady-state concentration of the active Rh species, [Rh(CO)2l2]. ... 

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. 

The homogenously-catalyzed process involved with step c will be the focus of the following discussion. Note here that a Pd catalyst was used instead of one containing Rh or Ir, but the steps of the catalytic process are very similar to those for methanol carbonylation. Scheme 9.19 outlines the catalytic cycle. 

When palladium salts are used for methanol oxy-carbonylation to DMC, reaction conditions are milder than using copper only however, methanol and CO selectivities are lower owing to the formation of DM0 as a by-product and to the higher ratio between CO2 and DMC production rates. Despite the large amount of work on the catalytic systems, no process based on gas-phase direct methanol oxy-carbonylation to DMC has been established. 

Dimethyl carbonate (DMC) is a versatile compound that is an attractive alternative to phosgene [68, 69] and which could be synthesized in a eco-friendly process by catalytic oxidative carbonylation of methanol with oxygen (Enichem, Italy [70] and... 

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