Rhodium catalysts, for methanol carbonylation

Scurrell (201) recently briefly reviewed the literature on heterogenized homogeneous rhodium catalysts for methanol carbonylation up to 1976. 

Methanol Carbonylation. Some researchers have described the possibility of supporting a rhodium compound on an ionic resin such as a copolymer of styrene and 4-vinylpyridine alkylated with methyl iodide forming a methylpyridinium-functionalized polymer (16). They have concluded that their ionic polymer-supported rhodium catalyst for methanol carbonylation in liquid phase is approximately equal in catalytic activity to the dissolved complex and that leaching of the complex could be minimized by suitable choice of solvent and by selecting high resin-to-rhodium ratios. However, experiments carried out only at low temperature (120°C) and low pressure were reported. 

The industrial manufacture of acetic acid by methanol carbonylation (Equation (1)) has utilized catalysts based upon all three of the group 9 metals, since the initial development by BASF of a cobalt/iodide-based system. " The BASF process required harsh conditions of temperature and pressure, and suffered from relatively low selectivity. It was soon superceded by highly selective, low-pressure rhodium/iodide-based catalysts developed by Monsanto. The Monsanto process (and related variants operated by other manufacturers) quickly became dominant and remains one of the most successful examples of the commercial application of homogeneous catalysis.Rhodium catalysts for methanol carbonylation are discussed in Chapter 7.03. 

Hydroformylation and Related Carbonylation Reactions.—Reviews on the synthesis of commercially important organic intermediates via rhodium-catalysed hydroformylation reactions, carbonylations with triarylphosphine-palladium catalysts, diene carbonylations, and heterogenized rhodium catalysts for methanol carbonylation have been published. 

SUPPORTED RHODIUM CARBONYLATION CATALYSTS FOR METHANOL CARBONYLATION... 

The RhI2(CO)2 species considered to be the active catalyst for methanol carbonylation has also been observed as the only carbonyl-containing rhodium complex during the carbonylation of other alcohols (Fig. 1 See Sections II,C and D). 

The efficacy of an iridium/iodide catalyst for methanol carbonylation was discovered by Monsanto at the same time as their development of the process using the rhodium/iodide catalyst [5]. Mechanistic investigations by Forster employing in situ HPIR spectroscopy revealed additional complexity compared to the rhodium system [115]. In particular, the carbonylation rate and catalyst speciation were found to show a more complicated dependence on process variables, and three distinct regimes of catalyst behavior were identified. At relatively low concentrations of Mel, H20, and ionic iodide, a neutral iridium (I) complex [Ir(CO)sI] was found to dominate, and the catalytic reaction was inhibited by increasing the CO partial pressure. Addition of small amounts of a quaternary ammonium iodide salt caused the dominant iridium species to become an Ir(III) methyl complex, [Ir(CO)2l3Me]. Under these conditions, the rate... 

Besides the "immobilized" CF3SO3H, another homogeneous catalyst is anionic [Rh(00)212]. This was the first active rhodium catalyst for the carbonylation of methanol to acetic acid. Recently, Chiyoda and UOP introduced the Acetica process, a novel technology based on an "immobilized" [Rh(CO)2l2] on a polyvinyl pyridine resin. Compared with the existing homogeneous process, immobilization increases catalyst concentration in the reaction mixture. 

The high stability of the block copolymer-colloid approach was also illustrated by the use of poly(A-vinyl-2-pyrrohdone) protected rhodium colloid (Rh-PVP) that was used as a catalyst for methanol carbonylation under elevated temperature (140 °C) and high pressure (5.4 MPa). During the reaction, the catalyst was still in a colloidal state as verified by TEM observations, even after repeated uses and a total TON reaching 19 700 cycles per atom of rhodium. Toshima and Shiraishi also demonstrated the possibility to enhance the catalytic activity of silver colloids (Ag-PVP) in the oxidation of ethylene by the addition of alkali metal ions such as cesium. Bimetallic catalysts in colloidal dispersions composed of two distinct metals also appeared in the literature with often better activity... 

Comparison of Specific Activities for Methanol Carbonylation with Various Heterogeneous Rhodium Catalysts 1... 

The BASF cobalt/iodide catalyzed process for methanol carbonylation was quite quickly superseded by a rhodium/iodide catalyzed process discovered at Monsanto and first commercialized in 1970 at a plant in Texas City. The Monsanto process was a significant advance and became one of the few large tonnage processes to use a homogeneous transition metal catalyst. It was later... 

The Cativa process is based on a promoted iridium catalyst, and offers a considerable improvement over the rhodium-based system as a result of increased catalyst stability at lower water concentrations, decreased by-product formation, higher rates of carbonylation, high selectivity (>99% based upon methanol), and improved yields on carbon monoxide. This is a more cost-effective process for methanol carbonylation owing to lower energy consumption and fewer purification requirements. Implementation of this new process has now been achieved in four plants worldwide. 

Ditertiary phosphines such as (86), (92), and (98) (100) (Scheme 6) have found important uses as ligands for metal-catalyzed transformations, including e.g., palladium-catalyzed Grignard cross couplings,194,205 rhodium-catalyzed Michael additions,2 hydrocyanations,206 copolymerizations,20 and palladium-catalyzed animations.208 Rhodium complexes of (86) are catalysts for the carbonylation of methanol.188 More recently the ligand bite angle of ditertiary phosphines such as (100) has been shown to influence catalytic activity/selectivity in several important catalytic processes.209-213... 

A process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

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 commercial processes for methanol carbonylation discussed above all employ homogeneous rhodium complex or iridium complex catalysts that require an iodide cocatalyst. The highly corrosive nature of acidic iodide-containing solutions and the costly product separation steps mean that catalytic process that avoid these problems are potentially attractive,... 

The effect of iodide and acetate on the activity and stability of rhodium catalysts for the conversion of methanol into acetic acid have been studied. Iodide salts at low water concentrations (<2 M) promote the carbonylation of methanol and stabilize the catalyst. Alkali metal iodides react with methylacetate to give methyl iodide and metal acetate the acetate may coordinate to Rh and act as an activator by forming soluble rhodium complexes and by preventing the precipitation of Rhl3. A water-gas shift process may help to increase the steady-state concentration of Rh(I). The labile phosphine oxide complex (57) is in equilibrium with the very active methanol carbonylation catalyst (58) see equation (56). 

Early studies by Scurrell and coll, demonstrated the use of rhodium zeolites as catalysts for the carbonylation of methanol into methyl acetate in the presence of methyl iodide (65). It was hoped that due to their electrostatic field zeolites would effect the direct carbonylation of methanol without the help of the iodide promoter. In fact, as the CH3OH/CH3I ratio increased, increasing amounts of CH4 and CO2 were produced indicating that the reaction... 

---