Catalysis methanol carbonylation

Polymer-supported catalysts incorporating organometaUic complexes also behave in much the same way as their soluble analogues (28). Extensive research has been done in attempts to develop supported rhodium complex catalysts for olefin hydroformylation and methanol carbonylation, but the effort has not been commercially successful. The difficulty is that the polymer-supported catalysts are not sufftciendy stable the valuable metal is continuously leached into the product stream (28). Consequendy, the soHd catalysts fail to eliminate the problems of corrosion and catalyst recovery and recycle that are characteristic of solution catalysis. 

It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

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. 

Catalysis. 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 woddwide acetic acid by 1988 (133). The Monsanto acetic acid process is carried out at 175°C and 1.5 MPa (200 psi). Rhodium is introduced as RhCl3 but is likdy reduced in a water... 

The basic organometallic reaction cycle for the Rh/I catalyzed carbonylation of methyl acetate is the same as for methanol carbonylation. However some differences arise due to the absence of water in the anhydrous process. As described in Section 4.2.4, the Monsanto acetic acid process employs quite high water concentrations to maintain catalyst stability and activity, since at low water levels the catalyst tends to convert into an inactive Rh(III) form. An alternative strategy, employed in anhydrous methyl acetate carbonylation, is to use iodide salts as promoters/stabilizers. The Eastman process uses a substantial concentration of lithium iodide, whereas a quaternary ammonium iodide is used by BP in their combined acetic acid/anhydride process. The iodide salt is thought to aid catalysis by acting as an alternative source of iodide (in addition to HI) for activation of the methyl acetate substrate (Equation 17) ... 

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). 

Figure 2. Catalytic breakthrough of rhodium vs. cobalt in homogeneous catalysis the methanol carbonylation.

Figure 2.13 Examples of three concepts of catalysis in supported ionic liquids (a) hydroformylation catalysis [121] (b) methanol carbonylation [122] (c) supported ionic liquid phase catalysis combined with SCCO2 [127].

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). 

In a further variation, the PVP-supported rhodium catalyst was used for methanol carbonylation in supercritical carbon dioxide [100]. This reaction medium has complete miscibility with CO and dissolves high concentrations of methanol and methyl iodide, while being a poor solvent for ionic metal complexes. Catalytic reaction rates up to half of those obtained in conventional liquid-phase catalysis were achieved with minimal catalyst leaching. 

The metal complexes most often studied as polymer-bound catalysts have been Rh(I) complexes, such as analogues of Wilkinson s complex. The catalytic activity of a bound metal complex is nearly the same as that of the soluble analogue. Rhodium complexes are active for alkene hydrogenation, alkene hydroformylation, and, in the presence of CH3I cocatalyst, methanol carbonylation, etc. Polymer supports thus allow the chemistry of homogeneous catalysis to take place with the benefits of an insoluble, easily separated catalyst . ... 

Acetic acid is another commodity chemical with a large market requirement (ca. 2.9 MMM Ib/yr in the U.S.) that has been targeted as being potentially made directly from synthesis gas using melt catalysis (eq. 4) (9)9 rather than via conventional methanol carbonylation... 

The application of SIL catalysis for continuous methanol carbonylation was reported [33]. The authors developed a siHca-SIL rhodium iodide Monsanto-type catalyst system, [BMIM][Rh(C0)2l2]-[BMIM]I-Si02, which used less catalyst material and allowed a simple process design. Compared to conventional and IL-based carbonylation systems, the advantage of this process was without recirculation and pressure change of tlie catalytic system. Moreover, the SIL catalyst exhibited excellent activity and selectivity toward acetyl products in fixed-bed, continuous gas-phase methanol carbonylation at industrially relevant reaction conditions. 

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