Ligands modifiers

Rhodium Ca.ta.lysts. Rhodium carbonyl catalysts for olefin hydroformylation are more active than cobalt carbonyls and can be appHed at lower temperatures and pressures (14). Rhodium hydrocarbonyl [75506-18-2] HRh(CO)4, results in lower -butyraldehyde [123-72-8] to isobutyraldehyde [78-84-2] ratios from propylene [115-07-17, C H, than does cobalt hydrocarbonyl, ie, 50/50 vs 80/20. Ligand-modified rhodium catalysts, HRh(CO)2L2 or HRh(CO)L2, afford /iso-ratios as high as 92/8 the ligand is generally a tertiary phosphine. The rhodium catalyst process was developed joindy by Union Carbide Chemicals, Johnson-Matthey, and Davy Powergas and has been Hcensed to several companies. It is particulady suited to propylene conversion to -butyraldehyde for 2-ethylhexanol production in that by-product isobutyraldehyde is minimized. 

The stringency of the conditions employed in the unmodified cobalt 0x0 process leads to formation of heavy trimer esters and acetals (2). Although largely supplanted by low pressure ligand-modified rhodium-catalyzed processes, the unmodified cobalt 0x0 process is stiU employed in some instances for propylene to give a low, eg, - 3.3-3.5 1 isomer ratio product mix, and for low reactivity mixed and/or branched-olefin feedstocks, eg, propylene trimers from the polygas reaction, to produce isodecanol plasticizer alcohol. 

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... 

Ligand-Modified Rhodium Process. The triphenylphosphine-modified rhodium oxo process, termed the LP Oxo process, is the industry standard for the hydroformylation of ethylene and propylene as of this writing (ca 1995). It employs a triphenylphosphine [603-35-0] (TPP) (1) modified rhodium catalyst. The process operates at low (0.7—3 MPa (100—450 psi)) pressures and low (80—120°C) temperatures. Suitable sources of rhodium are the alkanoate, 2,4-pentanedionate, or nitrate. A low (60—80 kPa (8.7—11.6 psi)) CO partial pressure and high (10—12%) TPP concentration are critical to obtaining a high (eg, 10 1) normal-to-branched aldehyde ratio. The process, first commercialized in 1976 by Union Carbide Corporation in Ponce, Puerto Rico, has been ficensed worldwide by Union Carbide Corporation and Davy Process Technology. 

FIG. 5 Hydroformylation of higher molecular weight olefins with a ligand-modified cobalt carbonyl catalyst [HCo(CO)3PR3] (Shell process). 

Clarke and Shannon also supported copper bis(oxazoline) complexes onto the surfaces of inorganic mesoporous materials, such as MCM-41 and MCM-48, through the covalent binding of the ligand, modified by alkoxysilane functionalities [59]. The immobilized catalysts allowed the cyclopropanation of styrene with ethyldiazoacetate to be performed as for the corresponding homogeneous case, and were reused once with almost no loss of activity or selectivity. 

Table 3. XPS data for the ligand-modified Pt nanoclusters and ligands.

In an ionic compound, the partial covalence of a bond formed between a transition metal ion and its ligand modifies the magnetic properties of the cation. It can be seen, for example, that if electrons were... 

A major advance in homogeneous catalysis was the introduction of a trialkyl-phosphine to supplement the role of carbon monoxide in catalyst stabilization. [3] A ligand modifier such as trialkylphosphine serves three principal roles in a homogeneous catalytic process. It stabilizes the metal, it influences the reaction rate, and it influences process selectivity. 

An alkene which will give a polar aldehyde product and syn gas are introduced into the reactor containing a non-polar ligand modified rhodium catalyst. Catalyst solution exiting the reactor enters a Flash stage where CO/H2 are purged. The catalyst solution then enters an extractor where it is contacted with a polar solvent. The product aldehyde is captured in the polar solvent in the extractor, then concentrated in the Solvent Removal Column. Polar Solvent is recycled to the Extractor. The Non-Polar catalyst solution is recycled to the reactor (see Figure 2.5). 

In hydroformylating with a polar ligand modified rhodium catalyst to give a relatively non-polar aldehyde product, after the flash column, the catalyst solution is extracted with a non-polar solvent. Polar catalyst recycles from the extractor to the reactor. The non-polar solvent is removed and recycled to the extractor (see Figure 2.6). 

Stability refers to the stability of the product, to the stability of the ligand and to the stability of the ligand-modified rhodium complex. 

Recently, various rhodium carbene complexes were investigated as catalysts for hydrosilation of olefins, acetylenes, and dienes to see whether carbene ligands modify catalytic activity. All reactions were... 

This chapter describes atropisomeric biaryl bisphosphine ligands modified DIOP-type ligands P-chiral bisphosphane ligands other bisphosphane ligands and their applications in the enantioselective hydrogenation of olefins. 

One approach to enantioselective reduction of prochiral carbonyl compounds is to utilize chiral ligand-modified metal hydride reagents. In these reagents, the number of reactive hydride species is minimized in order to get high chemo-selectivity. Enantiofacial differentiation is due to the introduced chiral ligand. 

A similar pattern has always been discussed for rhodium, with hydri-dotetracarbonylrhodium H-Rh(CO)4 as a real catalyst species. The equilibria between Rh4(CO)i2 and the extremely unstable Rh2(CO)s were measured by high pressure IR and compared to the respective equilibria of cobalt [15,16]. But it was only recently that the missing link in rhodium-catalyzed hydroformylation, the formation of the mononuclear hydridocomplex under high pressure conditions, has been proven. Even the equilibria with the precursor cluster Rh2(CO)8 could be determined quantitatively by special techniques [17]. Recent reviews on active cobalt and rhodium complexes, also ligand-modified, and on methods for the necessary spectroscopic in situ methods are given in [18,19]. 

In the late 1960s, Wilkinson postulated the reaction cycle of the ligand-modified rhodium catalyzed hydroformylation (Fig. 6). 

Fig. 6 Ligand-modified rhodium-catalyzed hydroformylation reaction cycle...

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 ... 

Going around the reaction system in Fig. 16, the first problem are poisons for rhodium such as traces of sulfur compounds in the raw materials. 3 valent P-compounds as ligands are highly prone to oxidation according to PR3 + [O] -> 0=PR3. In a continuous process, even traces of peroxides in the starting olefin and traces of oxygen in the synthesis gas accumulate over the time, so meticulous purification steps are a must if ligand-modified rhodium catalysts are used. 

Butanal by hydroformylation of propene is the most important oxo product in terms of volume. Six million metric tons per year of butanals were consumed in 2003, vis-a-vis a capacity of 7.6 million metric tons. Highly chemo- and regioselective processes based on ligand-modified rhodium catalysts have been developed and replaced the original cobalt high pressure technology. 

The linearity of the product of the Shell process is higher, 75-90% versus 60-70% for the non-ligand modified process. The reason for this is not entirely clear on steric grounds one might expect that the linear alkyl and acyl complexes are more stable leading to a higher linearity. Electronically the effects on rate and selectivity cannot be easily rationalised. 

The hydrogenation activity is an added advantage of this route, since most of the hydroformylation-products are converted to the alcohols anyway. Concurrent with the hydrogenation of aldehyde to alcohol, however, hydrogenation of alkene feedstock to alkane occurs, which may be as high as 15% under certain conditions (versus 2-3% for the non-ligand-modified... 

As a result, the second-generation processes used rhodium as the metal. The first rhodium-catalysed, ligand-modified process came on stream in 1974 (Celanese) and more were to follow in 1976 (Union Carbide Corporation) and in 1978 (Mitsubishi Chemical Corporation), all using triphenylphosphine (tpp). The UCC (now Dow) process has been licensed to many other users and it is... 

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