Methanol cobalt catalysis

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

The addition of increasing amounts of iodine promoters accelerates the hydrocarbonylation of methanol, but at the same time detioriates the hydrogenation ability of the cobalt catalysis. To obtain a high ethanol selectivity under these conditions, catalysts active for hydrogenation in the presence of iodine have to be added. Ruthenium compounds have been proved to be most suitable, as was mentioned earlier. Althou no detailed studies on the ruthenium intermediates involved are available, it is well known that aliphatic aldehydes... 

The original catalysts for this process were iodide-promoted cobalt catalysts, but high temperatures and high pressures (493 K and 48 MPa) were required to achieve yields of up to 60% (34,35). In contrast, the iodide-promoted, homogeneous rhodium catalyst operates at 448—468 K and pressures of 3 MPa. These conditions dramatically lower the specifications for pressure vessels. Yields of 99% acetic acid based on methanol are readily attained (see Acetic acid Catalysis). 

It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

Several differences between the cobalt- and rhodium-catalyzed processes are noteworthy with regard to mechanism. Although there is a strong dependence in the cobalt system of the ethylene glycol/methanol ratio on temperature, CO partial pressure, and H2 partial pressure, these dependences are much lower for the rhodium catalyst. Details of the product-forming steps are therefore perhaps quite different in the two systems. It is postulated for the cobalt system that the same catalyst produces all of the primary products, but there seems to be no indication of such behavior for the rhodium system. Indeed, the multiplicity of rhodium species possibly present during catalysis and the complex dependence on promoters make it... 

An important modern example of homogeneous catalysis is provided by the Monsanto process in which the rhodium compound 1.4 catalyses a reaction, resulting in the addition of carbon monoxide to methanol to form ethanoic acid (acetic acid). Another well-known process is hydro-formylation, in which the reaction of carbon monoxide and hydrogen with an alkene, RCH=CH2, forms an aldehyde, RCH2CH2CHO. Certain cobalt or rhodium compounds are effective catalysts for this reaction. In addition to catalytic applications, non-catalytic stoichiometric reactions of transition elements now play a major role in the production of fine organic chemicals and pharmaceuticals. 

Figure 2. Catalytic breakthrough of rhodium vs. cobalt in homogeneous catalysis the methanol carbonylation.

Matsui and coworkers reported the use of cobalt ion MIPs for chromatography based recognition studies on imprinted compounds. The authors chose to utilize an imprinting system described previously for the catalysis of aldol condensations (vide supra). This system was shown to be amenable to the study of MIP-metal ion mediated recognition. Preliminary studies were conducted to provide evidence for the complex formation between cobalt, polymerizable ligands, and dibenzoyl-methane, 28. Compleximetric titration of 28 in a model prepolymerization reaction mixture containing cobalt (II) acetate and pyridine in chloroform/methanol (5 1) showed formation of a complex with 1 1 stoichiometry between 28 and Co(II) (Fig. 19). 

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. 

Decomposition of 4-hydroperoxyhexahydrocolupulone (207, Figs. 87 and 112) is enhanced tenfold by catalysis with various metal ions (95-97). The conditions are standardized at 75°C with a solution of the hydroperoxide (2%) in cyclohexane, carbon tetrachloride, benzene, 1,4-dioxane or ethyl acetate. To 1 ml is added a solution of a metal salt in methanol (0.1 M 90 ml). Cobalt, copper, lead and manganese salts have been investigated (98). The reaction mixtures are analyzed by LC on a styrene -divinylbenzene anion exchange resin with UV detection at 254 nm. Two main products are formed, namely 4-hydroxyhexahydrocolupulone (208, Fig. 87) and tetrahydrocohulupone (206, Fig. 87). The ratio 206 208 depends on the nature of the metal ion and varies from 0.05 for lead(IV) acetate in cyclohexane to 3.5 for cobalt(ll) acetate in 1,4-dioxane. In the last case the yield of 206 is 45%, while the peroxide is completely decomposed after 4 h. In the absence of metal ions the decomposition of the peroxide takes 55 h. 

---