Rhodium catalysts carbonylation, effect

DMPO has been used in the synthesis of the first metalloporphyrin nitrone complex (443). On the basis of nitrone ligands (L) (Scheme 2.81) the synthesis of rhodium (I) carbonyl complexes of the type [Rh(CO)2ClL] was carried out. These complexes are used as effective catalysts of methanol carbonylation into acetic acid and its ester (444). 

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. 

As indicated above, hydrogen has a definite influence upon the catalyst. This is manifest in several ways, (a) Small amounts of hydrogen effectively keep this same rhodium catalyst active and in solution in the carbonylation of methyl acetate. (b) The rates of... 

The synthesis of acetic acid (AcOH) from methanol (MeOH) and carbon monoxide has been performed industrially in the liquid phase using a rhodium complex catalyst and an iodide promoter ( 4). The selectivity to acetic acid is more than 99% under mild conditions (175 C, 28 atm). The homogeneous rhodium catalyst is also effective for the synthesis of acetic anhydride (Ac O) by the carbonylation of dimethyl ether (DME) or methyl acetate (AcOMe) (5-13). However, rhodium is one of the most expensive metals, and its proved reserves are quite limited. It is highly desirable, therefore, to develop a new catalyst as a substitute for rhodium. 

Carbonylation of methanol to form acetic acid has been performed industrially using carbonyl complexes of cobalt ( ) or rhodium (2 ) and iodide promoter in the liquid phase. Recently, it has been claimed that nickel carbonyl or other nickel compounds are effective catalysts for the reaction at pressure as low as 30 atm (2/4), For the rhodium catalyst, the conditions are fairly mild (175 C and 28 atm) and the product selectivity is excellent (99% based on methanol). However, the process has the disadvantages that the proven reserves of rhodium are quite limited in both location and quantity and that the reaction medium is highly corrosive. It is highly desirable, therefore, to develop a vapor phase process, which is free from the corrosion problem, utilizing a base metal catalyst. The authors have already reported that nickel on activated carbon exhibits excellent catalytic activity for the carbonylation of... 

In this system higher CO pressures lead to lower linearities simply by shifting the complex equilibria to the species containing less phosphine. When isomerization plays a role under the reaction conditions applied, higher CO pressures will also give lower initial linearities by suppression of reaction (8i), the isomerization reaction as outlined above for the carbonyl or phosphite rhodium catalysts. These two effects can be clearly distinguished by monitoring the isomerization reaction when alkenes other than ethene and propene are used. 

Reactions of Phosphonium Ylides. - 2.3.1 Reactions with Carbonyl Compounds. This year we are able to report several variations of the traditional Wittig olefination which employ the addition of catalysts to effect the reaction. For example, Lebel et al. have reported a new salt-free process for the methyl-enation of aldehydes, in which the phosphorane is generated in situ from triphenylphosphine and a diazo precursor with either a rhodium- or rhenium-based catalyst (Scheme 6). It was found that the most effective combination of catalyst and diazo-compound were Wilkinson s catalyst [RhCl(PPh3)3] and... 

Prior to these investigations by HCC the promotional effect of iodide on the oxidative addition of Mel was investigated by others [9, 39, 40]. Foster demonstrated that the rate enhancement of this reaction in anhydrous medium was attributable to increased nucleophilicity of the rhodium catalyst with added iodide. The rationale for this observation was the generation of an anionic rhodium carbonyl complex, [Rh(CO)2l(L)]. Generation of this species was observed only with iodide added to certain neutral Rh species. No rate enhancement occurred with iodide added to the anionic complex, [Rh(CO)2l2] [39]. Similarly, in solvents with a high water concentration, iodide salts exhibited no rate enhancement in the presence of [Rh(CO)2l2] [11]. Maitlis and co-workers, in more recent investigations, reported a promotional effect of iodide in aprotic solvents on the oxidative addition of CH3I on [Rh(CO)2l2] [9a, 9c]. 

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]. 

The choice of catalyst can have a significant effect on these ratios. For reaction 26.5, a cobalt carbonyl catalyst (e.g. HCo(CO)4) gives a 80% C4-aldehyde, 10% C4-alcohol and 10% other products, and an h ratio 3 1. For the same reaction, various rhodium catalysts with phosphine cocatalysts can give an n i ratio of between 8 1 and 16 1, whereas ruthenium cluster catalysts show a high chemo-selectivity to aldehydes with the regioselectivity depending on the choice of cluster, e.g. for Ru3(CO)i2, a 2 1, and for [HRu3(CO)ii], 74 1. Where the hydroformylation... 

It is clear from the examples reported that carbon monoxide, when coordinated to a metal in a neutral complex, is not sufficiently activated to react with organic nitro compounds under mild conditions. More precisely, the first act of this reaction is the electron transfer from the metal to the nitro group to give a radical couple and this requires a very basic metal. This explains why basic ligands usually activate transition metal carbonyls in these catalytic reactions. Moreover, basic ligands such as Bipy favor the in-situ formation of the [Rh(CO)4] species from rhodium clusters. The effect of co-catalysts such as halide anions is more subtle, but even the action of these might, at least in part, be directed toward an increase of the electron density of the metal. 

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). 

Rhodium catalysts are known to be effective in the hydroformylation of unsaturated bonds and also in the decarbonylation of aldehydes. This efficacy arises from facile formation of rhodium carbonyl complexes from either carbon... 

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