Ruthenium carbonyl iodide catalysts promoters

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... 

With a ruthenium promoter (added as [Ru(CO)4l2]), r(CO) bands due to Ru iodo-carbonyls dominated the spectrum, precluding the easy observation of iridium species. Before injection of the Ir catalyst, absorptions due to [Ru(CO)2l2(sol)2], [Ru(CO)3l2(sol)] and [Ru(CO)3l3] are present. After injection of the iridium catalyst (Ru Ir = 2 1), [Ru(CO)3l3] becomes the dominant Ru species (Figure 3.11(b)). The observations indicate that the Ru(II) promoter has a high affinity for iodide and scavenges Hl(aq) as H30 [Ru(CO)3l3] . An indium promoter is believed to behave in a similar manner to form H30 [Inl4] . These promoter species also catalyse the reaction of Hlj q) with methyl acetate (Eq. (3)), which is an important organic step in the overall process. 

Monometallic ruthenium, bimetallic cobalt-ruthenium and rhodium-ruthenium catalysts coupled with iodide promoters have been recognized as the most active and selective systems for the hydrogenation steps of homologation processes (carbonylation + hydrogenation) of oxygenated substrates alcohols, ethers, esters and carboxylic acids (1,2). 

The behaviour of the ruthenium catalysts is quite different from that previously reported for cobalt carbonyl catalysts, which give a mixture of aldehydes and their acetals by formylation of the alkyl group of the orthoformate (19). The activity of rhodium catalysts, with and without iodide promoters,is limited to the first step of the hydrogenation to diethoxymethane and to a simple carbonylation or formylation of the ethyl groups to propionates and propionaldehyde derivatives (20). 

A range of compounds enhance the activity of an iridium catalyst. The promoters fall into two categories (i) carbonyl or halocarbonyl complexes of W, Re, Ru, Os and Pt and (ii) simple iodides of Zn, Cd, Hg, Ga and In. The preferred ruthenium promoter is effective over a range of water concentrations the maximum rate being attained at ca. 5% wt H2O, as in the absence of promoter. By contrast, ionic iodides such as Lil and BU4NI are strong catalyst poisons. 

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]. 

Alcohol Homologation Solvent and promoter effects on the cobalt carbonyl catalysed methanol homologation have been studied under synthesis gas pressure.The main product in a methanol/hydrocarbon two-phase system is 1,1-dimethoxyethane (ca. 70 selectivity).Using similar iodide promoted cobalt catalysts, R2C 0Me)2 and dimethylcarbonate are converted to acetaldehyde with up to 87 selectivity.Ruthenium in the presence of Co, 12 and dppe improves the ethanol selectivity in the homologation of dimethylether. Best results are achieved in inert solvents with high dielectric constants, e.g. sulfolane (e = 44), and with BF3 as activator. 

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