Catalytic methanol carbonylation rhodium-complex

An alternative strategy for catalyst immobilisation uses ion-pair interactions between ionic catalyst complexes and polymeric ion exchange resins. Since all the rhodium complexes in the catalytic methanol carbonylation cycle are anionic, this is an attractive candidate for ionic attachment. In 1981, Drago et al. described the effective immobilisation of the rhodium catalyst on polymeric supports based on methylated polyvinylpyridines [48]. The activity was reported to be equal to the homogeneous system at 120 °C with minimal leaching of the supported catalyst. The ionically bound complex [Rh(CO)2l2] was identified by infrared spectroscopic analysis of the impregnated resin. 

Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes... 

The rate of the methanol carbonylation reaction in the presence of iridium catalysts is very similar to that observed in the presence of rhodium catalysts under comparable conditions (29). This is perhaps initially surprising in view of the well-recognized greater nucleophilicity of iridium(I) complexes as compared to their rhodium(I) analogues. It can be seen from the above studies that the difference in the chemistry of the metals at the trivalent stage of the catalytic cycle serves to produce faster rates of alkyl migration with the rhodium system thus, overall the two metal catalysts give comparable rates. 

A mechanistic study by Haynes et al. demonstrated that the same basic reaction cycle operates for rhodium-catalysed methanol carbonylation in both homogeneous and supported systems [59]. The catalytically active complex [Rh(CO)2l2] was supported on an ion exchange resin based on poly(4-vinylpyridine-co-styrene-co-divinylbenzene) in which the pendant pyridyl groups had been quaternised by reaction with Mel. Heterogenisation of the Rh(I) complex was achieved by reaction of the quaternised polymer with the dimer, [Rh(CO)2l]2 (Scheme 11). Infrared spectroscopy revealed i (CO) bands for the supported [Rh(CO)2l2] anions at frequencies very similar to those observed in solution spectra. The structure of the supported complex was confirmed by EXAFS measurements, which revealed a square planar geometry comparable to that found in solution and the solid state. The first X-ray crystal structures of salts of [Rh(CO)2l2]" were also reported in this study. 

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. 

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. 

Mechanistic Pathways for Ligand Substitution Processes in Metal Carbonyls, 21, 113 Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes, 17, 255... 

Carbonylation of organic substrates was investigated using these well defined complexes. These carbonyl compounds exhibited catalytic properties in the carbonylation of organic substrates. In particular methanol carbonylation to methyl acetate in the gas phase was successfully attempted. Mechanistic and kinetic studies of this reaction over rhodium and iridium zeolites showed the similarities between the homogeneous and the zeolite mediated reactions. Aromatic ni-tro compounds were also converted to aromatic isocyanates using similar catalytic systems. The mechanistic aspect of this reaction will be also examined. 

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... 

SCHEME 1 Catalytic cycle for the rhodium-complex-catalyzed methanol carbonylation. 

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 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... 

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