Unmodified Cobalt Catalysts

Since the desired product from propylene hydroformylation is -butyralde-hyde, considerable attention has been devoted to increasing the selectivity this focussed attention on the mechanism, especially the step where the propylene inserts into the Co-H bond, since this can be either Markovnikov or anti-Markovnikov. 

CoH(CO)4 which decomposes to cobalt metal at low pco- Thus, reasonable reaction rates in the range 110-180°C require rather high pco and total Ha/ CO pressures of 200-300 bar. Higher CO partial pressure decreases the hydroformylation reaction rate and also decreases the amount of alkene 

In addition to the hydroformylation reactions, side reactions of the product alcohols and aldehydes occur to form heavy ends, particularly at higher reaction temperatures, and usually account for 9% of the product distribution. Industrial reactors usually start using high boiling solvents, but after a while these heavy ends become the solvents. 

To recover the catalyst, BASF oxidizes CoH(CO)4 with oxygen to form water soluble Co salts that are extracted and reduced under syngas to reform CoH(CO)4, while Exxon deprotonates CoH(CO)4 with aqueous NaOH to make Na[Co(CO)4]. 

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High CO partial pressure promotes the formation of unmodified cobalt catalyst [211,212]. Using a 50% excess of syngas (Fig. 23B), a reduced activity was observed. The conversion decreased from 96% (run a ) to 12.2% (rim d ) after four runs. In addition, chemoselectivity decreased considerably. Both hydrogenation, particularly to octane, and isomerization increased. 

Scheme 3.3 Alkene hydroformylation mechanism for an unmodified cobalt catalyst.

Although the overall reaction mechanisms (catalytic cycles) written for hydroformylation reactions with an unmodified cobalt catalyst (Scheme 1) and the rhodium catalyst (Scheme 2) serve as working models for the reaction, the details of many of the steps are missing and there are many aspects of the reaction that are not well understood. 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

Scheme 3. Catalytic cycle of hydroformylation with unmodified cobalt catalysts.

The most detailed and generally accepted kinetic study on triphenylphosphine-modified rhodium catalysts was published in 1980 [109]. It was concluded from the coefficients obtained (Table 2) that the fast alkene insertion is followed by the rate-determining step involving CO or TPP [110]. The apparent activation energy for propene hydroformylation was found to be 84 kJ/mol, very similar to the value obtained for unmodified cobalt catalysts. 

The total pressure is considerably lowered in comparison with unmodified cobalt catalyst. 

Unmodified cobalt catalyst Phosphine modified cobalt catalyst Phosphine modified rhodium catalyst... 

The first commercial oxo processes were based on unmodified cobalt catalyst. Several companies, including Eastman, BASF, ICI, and Kuhl-mann (PCUK), developed their own versions of the oxo process and built plants during the 1940s and 1950s to compete in this market. 

The desired reaction in catalytic hydroformylation is the addition of carbon monoxide and dihydrogen to the olefin substrate usually to obtain aldehyde. To some extent, however, concurrent reactions of the olefin (substrate) such as hydrogenation, isomerization, and special carbonylations, and consecutive reactions of the aldehyde product such as hydrogenation to alcohol, aldol reaction, trimer-ization, and formate formation take place under the reaction conditions of hydroformylation, which affect both yield and selectivity of the aldehyde products. For an example of product composition obtained using an unmodified cobalt catalyst in the BASF process, see Table 3. 

For the extent of hydrogenation imder hydroformylation of various olefins using unmodified cobalt catalyst, see Ref. (75). 

Formation of isomerized olefin in the case of cobalt catalysts occurs with a higher rate than hydroformylation only at low partial pressures (<5 MPa) of carbon monoxide in the temperature range of 80-120°C. At higher partial pressures (>5 MPa) of carbon monoxide, the rate of olefin isomerization become slower than the rate of olefin hydroformylation. Tables 5 and 6 show relevant product composition in pentene hydroformylation at two different carbon monoxide partial pressures using unmodified cobalt catalyst. 

Depending on the reaction conditions, more or less of the aldehyde product is hydrogenated when the olefin hydroformylation is performed in the presence of unmodified cobalt catalyst see Table 3, for example. Trialkylphosphine modified cobalt catalysts are more active in this hydrogenation. By raising the temperature and the H2 CO ratio, the final product of hydroformylation is the alcohol instead of the aldehyde see the product of the Shell process in Table 1. 

Formate Ester Formation. In the case of unmodified cobalt catalyst, the formation of formate ester is observed in minor amovuits. With other catalysts, either trace amovuits or none of the formate esters are fovuid in the product of hydroformylation. 

Other Aldehyde Consuming Side Reactions. Beside consecutive hydrogenation and hydroformylation of the aldehyde product, other reactions as well can cause the lowering of the aldehyde yield in olefin hydroformylation. Especially in hydroformylation with unmodified cobalt catalyst, aldol formation from the aldehyde product is one of the sources of several high boiling components. 

Alternatively, unmodified cobalt catalysts have been used [98]. Jiao and coworkers [101] provided evidence in a theoretical study that also with cobalt the hydroformylation of the alkyne proceeds faster than hydrogenation to the olefin. 

The first trials to hydroformylate vinyl chloride were conducted with unmodified cobalt catalysts [9]. At 90-110°C and 200 bar syngas pressure, by applying a slight excess of CO (H2/CO = 1.35) up to 90% 2-chloropropionaldehyde was produced within 90 min. At higher temperatures. 

Acrylonitrile was reacted with an unmodified cobalt catalyst to produce fi-formyl-propionitrile, which in turn was selectively reduced to y-hydroxy-butyronitrile [64]. In contrast, the use of a Rh complex modified with P(OPh)3 afforded mainly a-formyl-butyronitrile, which is a starting material for the synthesis of the PMMA monomer methyl methacrylate [65]. Occasionally, trapping of the formed cyanopropionaldehyde as acetal proved advantageous [66]. 

It should be noted that some metal catalysts can initiate 1,2-sigmatropic rearrangement, which may lead to further modification of the olefin serving as a substrate of the hydroformylation. A frequently cited example is the isomerization-hydroformylation of a-pinene with an unmodified cobalt catalyst (Scheme 5.9) [65]. In strong contrast to rhodium, the cobalt catalyst produced 2-formylbornane. The 1,2-sigmatropic rearrangement was explained by the acidic nature of HCo(CO)4. 

In strong contrast to unmodified cobalt catalysts, rhodium congeners exhibit poor regiodiscriminating ability and usually produce equal amounts of linear and branched aldehydes when a terminal olefin is used as a substrate. This difference has been rationalized by the larger size of the metal center therefore, steric effects on the coordinated olefin are less pronounced [66]. 

Under the same conditions, ethylene oxide underwent primarily isomerization and polymerization, which could be prevented by use of a phosphine-modified Co catalyst (see below). By application of nonsymmetric epoxides, the regioisomeric ring-opening reaction has to be taken into consideration, which may lead to the terminal or branched aldehyde, or both. Especially, isomerization of the epoxide to the corresponding ketone or aldehyde is a serious side reaction, as illustrated in Scheme 6.104 by means of an unmodified cobalt catalyst [7]. In the absence of syngas, the cobalt-hydroxyalkyl complex can collapse under elimination of an enol, which rapidly undergoes tautomerization to give the stable ketone or aldehyde (with R = H). 

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