Ruthenium carbonyl iodide catalysts

It is not surprising that homogeneous WGS catalysts are in two categories—those which operate in acidic and those in basic media. Of the acid-based systems, the most active are the rhodium carbonyl iodide combinations ", the PtCl4 /SnCl3 preparation, and the system based on the ruthenium carbonyl cluster catalyst precursors Ru3(CO)i2 and H4Ru4(CO),2 . The Rh carbonyl iodide system under more vigorous conditions (185°C, 23 atm ) shows a catalytic rate of 400 tumovers/h. 

Carbonylation of acetic acid to higher carboxylic acids can occur in presence of ruthenium/iodide catalysts. The reaction involves reduction and several carbonylation steps. The overall reaction may be written as follows ... 

Acetic acid has been generated directly from synthesis gas (CO/H2) in up to 95 wt % selectivity and 97% carbon efficiency using a Ru-Co-I/Bu4PBr "melt" catalyst combination. The critical roles of each of the ruthenium, cobalt and iodide catalyst components in achieving maximum selectivity to HOAc have been identified. Ci Oxygenate formation is observed only in the presence of ruthenium carbonyls [Ru(C0)3l3] is here the dominant species. Controlled quantities of iodide ensure that initially formed MeOH is rapidly converted to the more reactive methyl iodide. Subsequent cobalt-catalyzed carbonylation to acetic acid may be preparatively attractive (>80% selectivity) relative to competing syntheses where the [00(00)4] concentration is optimized that is, where the Co/Ru ratio is >1, the syngas feedstock is rich in 00 and the initial iodide/cobalt ratios are close to unity. 

In a more detailed examination of the ruthenium-cobalt-iodide "melt" catalyst system, we have followed the generation of acetic acid and its acetate esters as a function of catalyst composition and certain operating parameters, and examined the spectral properties of these reaction products, particularly with regard to the presence of identifiable metal carbonyl species. 

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... 

Homogeneous hydrogenation of carbon dioxide to methanol is catalyzed by ruthenium cluster anions in the presence of halide anions. The catalyst system was Ru3(CO)i2 and alkyl iodides in A -methylpyrrolidone (NMP) solution at 513 K. Some methane was also formed. FT-IR spectra of the reactions allowed identification of several ruthenium carbonyl anions. 

Data in Table V illustrate the production of acetic acid from 1/1 syngas. A variety of ruthenium-containing precursors - coupled with cobalt halide, carbonate and carbonyl compounds - at different initial Co/Ru atomic ratios, have been found to yield the desired carboxylic acid when dispersed in tetrabutylphosphonium bromide. In a more detailed examination of the ruthenium-cobalt-iodide melt catalyst system, we have followed the generation of acetic acid and its acetate esters as a function of catalyst composition and certain operating parameters, and examined the spectral properties of these reaction products, particularly with regard to the presence of identifiable metal carbonyl species. 

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. 

Carbon monoxide partial pressures exceed ca. 70 bar (5). Spectroscopic studies of typical Ru3(C0)i2 Co2(C0)8-l2/ Bu4PBr catalyst solutions have served to aid considerably in unraveling the differing roles of the ruthenium, cobalt and iodide-containing catalyst components in these syntheses. A typical solution spectrum in the metal-carbonyl region (5) shows the presence of significant concentration s of both [Ru(CO)3I3] (2108 and 2036... 

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). 

Relatively high temperatures and pressures are required for these carbonylations. When complex (88) is used as catalyst precursor in presence of an iodide source, two ruthenium(II) complexes have been shown to be formed. They are n s-[RuI2(CO)4] (89) and /nc-[RuI3(CO)3] (90). 

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. 

Suitable catalysts for this type of process must be capable of hydrogenating both carboxylic acids and their esters to alcohols, but also of carbonylating these compounds to their homologous acids. The best catalytic systems known contain either Rh or Ru in the presence of iodide. Ruthenium iodide systems are the most active ones in the hydrogenation reaction, but suffer from low activity in the carbonylation step, whereas rhodium iodide systems are very active when carbonylating alcohols to their acids (cf. Section 2.1.2.1). 

Water-soluble ruthenium complexes RuHCl(tppts)3, RuCl2(tppts)3, RUH2 (tppts)3, or the rhodium complex RhCl(PTA)3, are also effective catalysts for the hydrogenation of the carbonyl function of aldehydes [16], carbohydrates [17], and keto acids [13], provided that the iodide salt Nal is added for ruthenium complexes. 

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]. 

Carbohalogenation of various terminal or internal alkynes, via addition of perfluoroalkyl iodides or bomides, is catalyzed by carbonyl complexes of iron, cobalt or ruthenium. In this case, dichlorotris(triphenylphosphane)ruthenium(II) is not active as a catalyst. rram-Addition products are usually obtained in good yield under mild reaction conditions36. 

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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