Hydroformylations modified catalysts

The substitution of trialkylphosphine for carbon monoxide also makes the metal-hydrogen bond more hydridic in character and results in increased reduction of the initially formed aldehyde to alcohol. Slaugh and Mullineaux (52) compared Co2(CO)g and [Co2(CO)8 + 2PBu3], each at reaction conditions of 150°C, 500 psi, H2/CO I.0, for the hydroformylation of 1-pentene. The products consisted of hexyl aldehydes and hexyl alcohols in the ratios of 95 5 and 30 70, respectively. In a negative aspect of the reaction, they observed 23% hydrogenation of alkene to alkane at a reaction temperature of 195°C with the phosphine-modified catalyst. Tucci (54) reported less alkane formation (4-5%) under more favorable reaction conditions (I60°C, H2/CO 1.2, 1 hour reaction time). 

Fig. 11. Growth patterns for cobalt and modified metal commercial hydroformylations (118). (A) Total production volume, 106 tons/year. (B) Production by modified catalysts, 10a tons/year.

The investigation of the kinetic aspects of hydroformylation is still an underdeveloped field. The reason is the complexity of the reaction, especially with ligand-modified catalysts. The reaction rate r will certainly depend on temperature T and on the following concentrations ... 

They constitute the first rhodium phosphine modified catalysts for such a selective linear hydroformylation of internal alkenes. The extraordinary high activity of 32 even places it among the most active diphosphines known. Since large steric differences in the catalyst complexes of these two ligands are not anticipated, the higher activity of 32 compared to 31 might be ascribed to very subtle bite angle effects or electronic characteristics of the phosphorus heterocycles. 

The ejfect of water on the conversion and selectivity of cohalt-catalyzed hydroformylations has long been noticed in industry [7,85,86], A systematic study [87] of this effect in hydroformylation of 1-octene with [Co2(CO)s] with and without P Bu3 revealed that addition of water, and especially when it formed a separate aqueous phase, significantly inaeased the hydrogenation activity of the phosphine-modified catalyst Under the same reaction conditions (190 °C, 56 bar CO H2 1 1, P Co 3 1), approximately 40 % nonanols were formed instead of 5 % observed with water-free solutions. No clear explanation could be given for this phenomenon, although the possible participation of water itself in the hydroformylation reaction through the water gas shift was mentioned. It was also established, that the [Co2(CO)g]-catalyzed hydroformylation was severly retarded in the presence of water. Under the conditions above, 95 % conversion was observed in 15 hour with no added water, while only 10 % conversion to aldehydes (no alcohols) was found in an aqueous/organic biphasic reaction. 

This becomes especially apparent in hydroformylation reactions of internal alkenes, since not only does (E)/(Z)-isomerization take place, but -aldehydes are obtained. Thus, in the hydroformylation of ( )-4-octene by Co2(CO)g, n-nonanal (78%), 2-methyloctanal (10%), 2-ethylheptanal (6%) and 2-pro-pylhexanal (6%) are obtained. This isomerization is supressed with the phosphine-modified catalysts, in the presence of excess phosphine and at high CO pressures. Both carbon monoxide and phosphine can react with a 16-electron complex to provide an 18-electron complex (e.g. 4 — 5 Scheme 2), the reverse (3-hydride elimination is prevented, a requirement for this elimination being the presence of a vacant co-... 

With respect to conversion, selectivity, and operation, the oxo synthesis is influenced by a plethora of parameters. By fine-tuning of the operation conditions, a broad band of product compositions is achievable. In accordance with the mechanistic discussion and the kinetics of the hydroformylation reaction, these issues will be treated separately for unmodified and modified catalysts. For operating processes and their reaction parameters, see Section 2.1.1.4. 

Yildiz-Unveren, H.H. and Schomacker, R. (2005) Hydroformylation with rhodium phosphine-modified catalyst in a microemulsion comparison of organic and aqueous systems for styrene, cyclohexene and l,4-diacetoxy-2-butene. Catal. Lett., 102, 83. 

Hydroformylation of various heterofunctionalized olefins can be carried out with a number of chirally modified catalysts. Asymmetric induction is usually higher than with unfunctionalized hydrocarbons, presumably, due to additional binding of the substrate to the catalysts65,75. Thus, this method is applicable to the synthesis of hydroxy and amino carboxylic acids and other conversion products of primary functionalized aldehydes. These results are compiled in Table 7. 

Such tert-phosphine-modified catalysts are used industrially in the Shell hydroformylation process. This is one of many examples of the influence of auxiliary ligands (cocatalysts) on homogeneous catalysis. 

A palladium complex with cyclodextrin modified with propionitrile and benzoylnitrile groups 73-74 was active in Wacker oxidation of higher 1-alkenes (Experiment 11-4, Section 11.7), and its activity was much higher than the activity of a catalj ic system prepared as a mixture of cyclodextrin and the palladium complex owing to the cooperative substrate binding and to the increase in the stability constant of the catalyst-substrate complex. As in hydroformylation, the catalyst was more active in the reaction with an aromatic substrate, styrene, than with linear alkenes [59,210-211], The catalyst activity depended on the 1-alkene chain length and was maximum for 1-heptene. 

It has been suggested that the acyl complex is [RCO-Co(CO)4], or [RCO Co(CO)3PBu3l for modified catalysts, but a site of unsaturation cis to the acyl ligand may be required (c.f.. Reference 60). A more probable formulation is therefore [RCO Co(CO)3], or [RCO Co(CO)2PBu3]. A binuclear, free-radical mechanism for the cobalt-catalyzed hydroformyla-tion of styrene or other conjugated substrates has also been proposed. These studies are far-reaching, especially because similar binuclear elimination steps have not received much consideration in studies of rhodium hydroformylation catalysts. ... 

Replacing some of the CO ligands with other substituent such as phosphine ligands gives the so-called modified catalysts. For example, the Wilkinson s hydroformylation catalyst (25) ... 

The homogeneous hydroformylation could take place both in conventional and environmentally benign (green) reaction media. The solubility of the catalyst is primarily determined by the applied ligand due to its polarity and/or functionality. The unmodified catalysts can be dissolved in the hydrocarbons such as alkanes and toluene, or in industrial scale the reaction is performed in the crude olefin mixtures (downstream) containing different unsaturated hydrocarbons. The solubility of the modified catalyst in the alternative reaction media is easily affected by the ligand variation of the metal complex for example... 

Rhodium-catalyzed hydroformylation using catalysts modified with alkylphosphines and arylphosphines was reported by Wilkinson s group [12]. Phosphine ligand variation hardly affected the rate and selectivity under the circumstances used (70 °C and 100 bar). Pruett (Union Carbide Corporation) found that phosphites can also be used, and the type of phosphite had a profound effect on rates and selectivities [13]. 

BASF patented the hydroformylation of 2,6-dimethylhept-l-en-6-ol, obtainable from the corresponding ketone via Grignard reaction (Scheme 6.41) [133]. The reaction with syngas was performed in the 1 kg scale with an unmodified Rh catalyst to afford 3,7-dimethyloctan-l-al-7-ol (hydroxycitronellal) in 90% yield. The use of a PPhg-modified catalyst did not improve this result. Usually, hydroxycitronellal is extracted from ethereal oils or can be alternatively produced by the hydration of citronellal bisulfite in an acidic medium [134]. The annual production is about 11001 [135]. The oil has a sweet floral scent, which is reminiscent of lilac, lily, lily of the valley, or lime. By the same protocol, also related hydroxy aldehydes with other interesting olfactory properties become accessible. 

Despite the fact that the Shell process operates at lower pressure and higher temperatures than the conventional processes, still higher n/iso ratios of the products formed are observed. Thus, in the hydroformylation of propylene an 88/12 ratio of n- over iso-product is obtained, whereas for comparison the distribution in the Ruhrchemie process is 80/20. This type of modified catalyst is not only a hydroformylation but also a hydrogenation catalyst. Thus, in the SheU process about 10-15 % of the olefin fed is lost through hydrogenation to the paraffin whereas the figures for the conventional 0X0 processes are only about 2-3 %. 

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