Phosphine Modified Catalysts

By the 1970s, the chemistry of the process was being investigated in an effort to improve performance. The main objectives were as follows  

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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). 

Phosphine-modified catalysts have been prepared from phosphine ad-... 

Reaction of 3 with 1 equivalent of a phosphine results in formation of "phosphine-modified catalysts (4). The complex formed from 7r-allyl-nickel chloride, tricyclohexylphosphine, and methylaluminum dichloride (4a) has been isolated and its structure determined crystallographically (see Fig. 1) (57) The phosphine is bonded to the nickel atom, and interaction with the Lewis acid takes place via a chlorine bridge. The bridging chlorine atom is almost symmetrically bound to both the nickel... 

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. 

The use of phosphine-modified catalysts prepared in situ may be highly risky. Indeed, recent studies have shown that, depending on the chelating diphosphine, the preparation of the catalyst precursor in situ may give much lower productivities as compared to reactions where a preformed Pd" complex is used. For example,... 

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-... 

Of prime importance for utilizing the new catalyst was the observation that the products of propylene dimerization with phosphine-modified catalyst system, XVI, are strongly influenced by the nature of the phosphine PR3 (24, 25). To understand the phosphine effect, it is necessary to examine the dimerization of ethylene and of propylene in some detail. The dimerization of ethylene formally involves the addition of the C-H bond of one olefin molecule across the double bond of a second one ... 

The influence of the phosphine PRh in the phosphine-modified catalyst, XVI, on the distribution of the isomeric C(j olefins is shown in Table III. 

The dimerization reaction has been carried out under two different conditions. In laboratory experiments, the reaction is conveniently carried out under 1 or less than 1 atmosphere and at a temperature of —20° to — 10°C. These relatively low temperatures are necessary to obtain a sufficient concentration of ethylene or propylene in the catalyst solution. The dimerization catalyst for laboratory experiments is usually prepared by mixing, for example, chlorobenzene solutions of a 7r-allylnickel halide and an aluminum halide (or alkylhalide) in molar ratio of at least 1 1. The phosphine-modified catalyst is obtained by simply adding 1 mole of a phosphine per mole of nickel to the solution of the catalyst. When ethylene or propylene is introduced into the catalyst solution, reaction starts immediately, as evidenced by a sudden rise in temperature. Dimerization is exothermic to the extent of about 28 kcal./mole propylene dimer. Hence, the mixture must be stirred and cooled intensively during the reaction. Under these conditions (Table V), reaction rates of about 6 kg. 

Heteroditopic hgands have been also proposed for the copolymerization reaction. The catalytic activity of systems containing one phosphorus ligating moiety is lower fhan fhat of fhe diphosphines. For fhe imine-phosphine-modified catalyst precursor 30 (Scheme 8.8), a low turnover frequency of 6 mol (mol h) [59] has been reported. Complex 30 was also used to investigate olefin and carbon monoxide insertions [60]. 

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. 

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. 

The Shell process is a variant of the cobalt-catalyzed process in which phosphine-modified catalysts of the type [HCo(CO)j(PR3)] are used. Such catalysts, which are stable at low pressures, favor the hydrogenation of the initially formed aldehydes, so that the main products are oxo alcohols. However, a disadvantage is the lower catalyst activity and increased extent of side reactions, especially the hydrogenation of the olefin starting material. The superiority of the low-pressure rhodium process can be seen from the process data listed in Table 3-3. 

Both factors together with the reduced fixed costs and the usage of an own technology (no license fees in case of Celanese plants) makes the RCH/RP process ca. 10 % cheaper in manufacturing costs (costs for ligand synthesis already included) compared to classical rhodium process applying a homogeneous phosphine-modified catalyst. 

However, with phosphine-modified catalyst systems (see chapter 1/3.6), oxygen and water should be completely excluded since they would react with the phosphines to produce inactive phosphine oxides. 

Three different arguments have been put forward to explain the increased stereoselectivity of the phosphine-modified catalysts ... 

In case of low boiling aldehydes (propionaldehyde, butyraldehyde, etc) and carbonyls of low volatility (Rh-carbonyls, phosphine modified cobalt or rhodium catalysts), the aldehydes may be separated from the reaction product by flash distillation [150]. In this operation the metal carbonyls remain in the bottoms. Depending on temperature, length of treatment and stability of the carbonyls, varying amounts of them are decomposed. The corresponding metals formed also remain in the bottoms. This method is limited to laboratory operations (with the exception of phosphine modified catalyst — see chapter on modified catalysts). It cannot be recommended for cobalt carbonyls since they are too volatile. 

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