Cobalt Process

A nucleophilic attack by 4.1 on CH3I produces 4.2 and iodide. Conversion of 4.2 to 4.3 is an example of a carbonyl insertion into a metal alkyl bond. Another CO group adds on to the 16-electron species 

3 to give 4.4, which in turn undergoes nucleophilic attackhy iodide to eliminate acetyl iodide. 

Apart from these basic reactions, there are a few other reactions that lead to product and by-product formations. The hydrido carbonyl HCo(CO) formed by reactions 4.2.1.3 and 4.2.1.4 catalyzes FT-type reactions (see Section 4.7.2). The attack on CH3I by 4.1 is a comparatively slow reaction. High temperatures are therefore required to achieve acceptable rates. This in turn necessitates high pressures of CO to stabilize 4.1 at high temperatures. 

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Unmodified Cobalt Process. Typical sources of the soluble cobalt catalyst include cobalt alkanoates, cobalt soaps, and cobalt hydroxide [1307-86 ] (see Cobalt compounds). These are converted in situ into the active catalyst, HCo(CO)4, which is in equihbrium with dicobalt octacarbonyl... 

Ligand-Modified Cobalt Process. The ligand-modified cobalt process, commercialized in the early 1960s by Shell, may employ a trialkylphosphine-substituted cobalt carbonyl catalyst, HCo(CO)2P( -C4H2)3 [20161 -43-7] to give a significantly improved selectivity to straight-chain... 

Mechanism ofLP Oxo Rea.ction. The LP Oxo reaction proceeds through a number of rhodium complex equilibria analogous to those ia the Heck-Breslow mechanism described for the ligand-free cobalt process (see Fig. 1). 

As normally practiced in a cobalt process, the aldehyde product contains about 10% alcohol, formed by subsequent hydrogenation. Marko (34) reported that the hydrogenation is more sensitive to carbon monoxide partial pressure than is the hydroformylation reaction and, in the region between 32 and 210 atm, is inversely proportional to the square of the partial pressure. The full kinetic expression for alcohol formation is expressed by Eq. (17). 

Considerable effort over the years has been devoted to a search for new oxo catalysts. This has been motivated by a desire to minimize the less valuable isobutyraldehyde/alcohol and also to lower oxo reaction temperatures and the high pressures (3-4000 psi) associated with the conventional cobalt process for reduced capital investment and increased energy savings. 

Even at temperatures of 180-200°C, space time yields remain lower than in the conventional cobalt process. At the elevated temperatures the cobalt ligand catalyst is a highly active hydrogenation catalyst that converts a significant portion (10%) of the propylene to propane and most of the butyraldehyde to butanol. 

The next breakthrough of importance for future 2-ethylhexa-nol plants occurred in the mid seventies. This was the development of the rhodium-catalyzed oxo process by Union Carbide, Davy Powergas and Johnson-Matthey (See Chapter 6). This process not only operates at lower temperatures and pressures than the conventional cobalt-catalyzed process but also gives a far lower yield of the less valuable isobutyraldehyde by-product. The net result is improved economics vs. the cobalt process for n-butyr-aldehyde - the intermediate for 2-ethylhexanol. Although outside the U.S. this new technology has already been licensed and plants are now operating(16), no new plants were constructed in the U.S. specifically for 2EH manufacture in the seventies. However,... 

Three commercial homogeneous catalytic processes for the hydroformyla-tion reaction deserve a comparative study. Two of these involve the use of cobalt complexes as catalysts. In the old process a cobalt salt was used. In the modihed current version, a cobalt salt plus a tertiary phosphine are used as the catalyst precursors. The third process uses a rhodium salt with a tertiary phosphine as the catalyst precursor. Ruhrchemie/Rhone-Poulenc, Mitsubishi-Kasei, Union Carbide, and Celanese use the rhodium-based hydroformylation process. The phosphine-modihed cobalt-based system was developed by Shell specih-cally for linear alcohol synthesis (see Section 7.4.1). The old unmodihed cobalt process is of interest mainly for comparison. Some of the process parameters are compared in Table 5.1. 

In 1968 Monsanto reported a chemically related process based on rhodium iodide complexes. Due to its high reaction rates, high selectivity and different kinetics, the process differs substantially from the cobalt process. Commercialization was achieved in 1970. Operating conditions are remarkably mild 30 bar, 180°C. 

The cobalt process is difficult to operate, partly because of the high pressures involved, and partly because of the need to recycle volatile cobalt carbonyls. Another disadvantage is the loss of 15% of the alkene due to hydrogenation condensation and ketone by-products are also formed. A Shell modification adding trialkylphos-phine to the cobalt catalyst increased selectivity for linear aldehydes and allowed lower reaction pressures, but gave lower activity and increased hydrogenation. 

The formation of hydrogenation by-products such as alcohols and hydrocarbons is favored at low p(CO). Extensive hydrogenation was often the aim of special cobalt process variants, in order to produce alcohols in one step - for instance, butanols. Especially for short-chain olefins, this technique has been replaced by two-step processes rhodium 0x0 synthesis along with a separate hydrogenation step. 

With all the characteristics of the cobalt process in mind, it can be seen that there was a sizable incentive to improve on this performance. Fimdamental work by... 

The industrial hydroformylation of short-chained olefins such as propene and butenes is nowadays almost exclusively performed by so-called LPO (low-pressure oxo) processes, which are rhodium-based. In other words, the former high-pressure technology based on cobalt has been replaced by the low-pressure processes, which cover nearly 80% of total C4 capacity due to their obvious advantages (cf. [8]). Nevertheless, some cobalt processes are still in operation for propene hydroformylation, for example as second stages in combination with a low-pressure process serving as the first stage [8, 9]. 

This reaction mechanism is supported by model studies. Paricularly advantageous are the mild reaction conditions (30-40 bar, 150-200°C) and the high selectivity with respect to methanol (99 %) and CO (> 90 %) compared to the older cobalt process. Methanol carbonylation is one of the few industrially important catalytic reactions whose kinetics are known in full [7]. 

Fisher, K.G. (2011) Cobalt processing developments . Proceedings of the 6th Southern African Base Metals Conference, 18-20 July, 2011, Phalaborwa, South Africa, p. 237-258... 

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