Decomposition reaction, high pressure process

This equation shows that the reaction rate is neither first-order nor second-order with respect to species A. However, there are two limiting cases. At high pressures where [A] is lar e, the bimolecular deactivation process is much more rapid than the unimolecular decomposition (i.e., /c2[A][A ] /c3[A ]). Under these conditions the second term in the denominator of equation 4.3.20 may be neglected to yield a first-order rate expression. 

The hypothesis that the cobalt carbonyl radicals are the carriers of catalytic activity was disproved by a high pressure photochemistry experiment /32/, in which the Co(CO), radical was prepared under hydroformylation conditions by photolysis of dicobalt octacarbonyl in hydrocarbon solvents. The catalytic reaction was not enhanced by the irradiation, as would be expected if the radicals were the active catalyst. On the contrary, the Co(C0)4 radicals were found to inhibit the hydroformylation. They initiate the decomposition of the real active catalyst, HCo(C0)4, in a radical chain process /32, 33/. 

As a result of the higher stability the process can be (and must be ) operated at lower pressure (25-100 bar versus 200-300 bar for HCo(CO)4). The higher stability can be explained by the electron donation of the phosphine to the electron deficient cobalt carbonyl thus strengthening the Co-CO bonds. The phosphine complex is less active than the tetracarbonyl complex and therefore the reaction is carried out at higher temperatures (170 °C versus 140 °C). The temperature is "dictated" by the rate required the high pressures in the tetracarbonyl system are needed to prevent decomposition of the carbonyls to metal and CO. 

It is interesting to examine Eq. 9.138 in the limits of very high and very low pressure. At very high pressure the rate of collision of the excited intermediate C with other species M is very high. Thus the stabilization process is expected to be much faster than the decomposition and reaction rates ... 

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