Rhodium Catalyzed Hydroformylation of Propene

9 RHODIUM CATALYZED HYDROFORMYLATION OF PROPENE Overall Reaction 

Propene from oil refinery crackers, and syngas from coal or natural gas. Product Use 

Butanal is reduced to 1-butanol, which is used as a solvent. Butanal is dimerized with base to give 2-ethylhexan-l-ol which is used as the phthalate ester in PVC as a plasticizer. 

The estimate is that 3 million tons of butanal are produced annually via the process described here. 

The low pressure oxo process was jointly developed by Union Carbide, Davy Powergas (Davy McKee), and Johnson Matthey. The latter two companies possess the rights of the Wilkinson patents. The catalyst is a rhodium complex, with a large excess of triphenylphosphine. Temperature and pressure have to be controlled very carefully because the linearity strongly depends on these parameters. Ligand and rhodium (300 ppm) are very sensitive to impurities and the feed must be very thoroughly purified. The rhodium stays in the reactor, apart from a small purification cycle, employed for the reactivation of inactive rhodium 

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Rhodium-catalyzed hydroformylation of propene is industrially realized in three process variations that differ in the way the products are separated from the Rh-catalyst after or during the homogeneous catalyzed reaction. 

These assembly ligands will be tested in suitable catalytic reactions that leave the assemblies intact. Salt-forming reactions are not attractive as the salts might interact with the assembly, nor is the use of catalytic metals that compete with the assembly metal for the salen type positions in the ditopic ligand ideally, all potential problems can be avoided if the same metal could be used. Rhodium-catalyzed hydroformylation of 1-octene is a suitable reaction, with the only disadvantage that high pressures are needed, but hydrogen or CO do not interfere with our assemblies. Metal salts do not interfere with the rhodium hydrides involved in the hydroformylation catalysis, as for instance the most effective industrial process today for propene hydroformylation... 

Catalyst decomposition is, overall, receiving little attention in academic work on homogeneous catalysis, and only in recent years has research on decomposition and stabilization of organometallic catalysts started to expand (116), with emphasis on reactions of significant commercial interest such as hydroformylation (117), metathesis 118), crosscoupling, and polymerization 119). Ligand decomposition seems to be a key issue for industrial application, because it affects the total number of turnovers, TON. Phosphine decomposition is an unavoidable side reaction in metal-phosphine complex-catalyzed reactions and the main barrier for commercial application of homogeneous catalysts. There are a few exceptions to this statement for example, the rhodium tppts-catalyzed hydroformylation of propene, a process developed by Ruhrchemie-Rhone Poulenc (now Celanese). 

High-pressure in situ ETIR and polymer matrix techniques were used to study the rhodium-catalyzed hydroformylation of 1-octene, 1-butene, propene, and ethene using Rh(acac)(CO)2 or Rh(acac)(CO)(PPh3) in a polyethylene matrix as the catalyst precursor. The acyl rhodium intermediates, RC(=0)Rh(C0)4 and RC(=0)Rh (CO)3(PPh3), were observed. It was found that the acyl rhodium tetracarbonyl intermediates easily react with ethene to form acyl rhodium tricarbonyl species RC(=0)Rh(C0)3(C2H4) [61]. Deuterioformylation of l-phenyl-l-(n-pyridyl)-ethenes in the presence of a phosphane-modified Rh4(CO)i2 as catalyst precursor was carried out at 100 bar of CO D2 = 1 1 and 80 °C at partial substrate conversion. On basis ofa direct NMR analysis of the crude reaction mixture, it was concluded that the branched alkyl rhodium intermediate is almost exclusively formed [62]. 

Biphasic catalysis in the presence of water-soluble catalysts has been the most significant development in recent years. After the report of Kuntz on the synthesis of sulfonated triarylphosphine TPPTS (Figure 14.1) and its successful industrial application in Rh-catalyzed hydroformylation of propene, great attention has been focused on the scientific study and industrial application of water-soluble catalysts, especially on water-soluble phosphines [6, 7], phosphites, and other phosphide compounds as well as their rhodium complexes [8]. Among them, TPPTS is the most widely studied and applied. Other important phosphine hgands will he introduced later. 

The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

Homogeneous rhodium-catalyzed hydroformylation (135,136) of propene to -butyraldehyde (qv) was commercialized in 1976. -Butyraldehyde is a key intermediate in the synthesis of 2-ethyIhexanol, an important plasticizer alcohol. Hydroformylation is carried out at <2 MPa (<290 psi) at 100°C. A large excess of triphenyl phosphine contributes to catalyst life and high selectivity for -butyraldehyde (>10 1) yielding few side products (137). Normally, product separation from the catalyst [Rh(P(C2H2)3)3(CO)H] [17185-29-4] is achieved by distillation. 

In 1975 Kuntz has described that the complexes formed from various rhodium-containing precursors and the sulfonated phosphines, TPPDS (2) or TPPTS (3) were active catalysts of hydroformylafion of propene and 1-hexene [15,33] in aqueous/organic biphasic systems with virtually complete retention of rhodium in the aqueous phase. The development of this fundamental discovery into a large scale industrial operation, known these days as the Ruhrchemie-Rhone Poulenc (RCH-RP) process for hydroformylation of propene, demanded intensive research efforts [21,28]. Tire final result of these is characterized by the data in Table 4.2 in comparison with cobalt- or rhodium-catalyzed processes taking place in homogeneous organic phases. 

The hydroformylation of alkenes to give linear aldehydes constitutes the most important homogeneously catalyzed process in industry today [51]. The hydroformylation of propene is especially important for the production of n-bu-tyraldehyde, which is used as a starting material for the manufacture of butanol and 2-ethylhexanol. Catalysts based on cobalt and rhodium have been the most intensively studied for the hydroformylation of alkenes, because they are industrially important catalysts. While ruthenium complexes have also been reported to be active catalysts, ruthenium offers few advantages over cobalt or... 

Rhodium compounds and complexes are also commercially important catalysts. The hydroformylation of propene to butanal (a precursor of hfr(2-ethyUiexyl) phthalate, the PVC plasticizer) is catalyzed by hydridocarbonylrhodium(I) complexes. Iodo(carbonyl)rhodium(I) species catalyze the production of acetic acid from methanol. In the flne chemical industry, rhodium complexes with chiral ligands catalyze the production of L-DOPA, used in the treatment of Parkinson s disease. Rhodium(II) carboxylates are increasingly important as catalysts in the synthesis of cyclopropyl compounds from diazo compounds. Many of the products are used as synthetic, pyrethroid insecticides. Hexacyanorhodate(III) salts are used to dope silver halides in photographic emulsions to reduce grain size and improve gradation. 

Example 5.2. Hydroformylation of propene [2]. Hydroformylation converts an olefin to an aldehyde of next higher carbon number by addition of carbon monoxide and hydrogen. The reaction is catalyzed by dissolved hydrocarbonyl complexes of transition-metal ions such as cobalt, rhodium, or rhenium. The carbon atom of the carbon monoxide can attach itself to the carbon atom on either side of the olefinic double bond, so that two aldehyde isomers are formed. If the catalyst also has hydrogenation activity, the aldehydes are converted to alcohols and paraffin is formed as by-product. For propene and such a catalyst the (simplified) network is ... 

More extensive use has been made of TPPTS than of TPPMS as a ligand for preparing water-soluble homogeneous catalysts. The major reason for this is that the presence of three sulfonate groups on TPPTS causes it to have a greater solubility in aqueous solution than does TPPMS. The principal application of TPPTS has been as a ligand for rhodium in catalyzed hydroformylation reactions. In the hydroformylation of propene with such catalyst systems, the reaction conditions use an equimolar mixture of CO and H2 at 40-bar pressure and 125°C in an aqueous solution of pH 6.0... 

Looking at the hydroformylation of propene, the technology of the cobalt- catalyzed processes has remained basically unchanged over the years, whereas the introduction of rhodium as catalyst has founded a new generation of hydroformylation processes. 

The diphosphines 1 and 2 were tested as ligands in the rhodium-catalyzed bipha-sic hydroformylation of propene. Both catalysts were found to exhibit higher activities and gave rise to higher l/b ratios [4] than TPPTS. Furthermore, it was shown that displacement of the biphenyl unit of 2 by a binaphthyl unit in 1 leads to an in-... 

The most important process with the solvent water is the hydroformylation of propene to butyraldehydes, known as the Ruhrchemie/Rhone-Poulenc process. This reaction is catalyzed by a rhodium complex containing the water-soluble ligand triphenylphosphane trisulfonate (TPPTS). The aldehydes are formed with an annual capacity of approx. 3000001. 

The thermal instability of rhodium-based hydroformylation catalysts has already been overcome commercially in the Ruhrchemie/Rhone-Poulenc process for propene hydroformylation in which the sodium salt of a sulfonated triphe-nylphosphine ligand (TPPTS, la) is used to solubilize the catalyst in the aqueous phase. In this process, the second phase is toluene and the reaction is carried out as a batch process with rapid stirring to intimately mix the two immiscible phases. After reaction, the system is allowed to separate and the organic phase is simply decanted from the aqueous catalyst phase. Both water-soluble polymers and PAMAM dendrimers have been reported as supports for rhodium-catalyzed hydroformylation under aqueous biphase conditions, but reactivities and regioselec-tivities were only comparable to or worse than those obtained with the reference TPPTS ligand. The aqueous biphase approach has found limited application for the hydroformylation of longer-chain alkenes, because of their very low solubility in water leading to prohibitively slow reaction rates, but there have been a variety of approaches directed at the solution of this problem. 

IR spectroscopy has proved that SILP catalysts have metalorganic complexes dissolved in the liquid layer, which then worked as a homogeneous catalyst Riisager et al. [12] have made spectroscopic measurements of a rhodirun-sulfoxantphos complex which was immobilized in an SILP system. This SILP catalyst was tested in the continuous-flow fixed-bed hydroformylation of propene. Spectroscopy of the SILP system was performed in situ under conditions closely related to the reaction conditions, that is, imder various gas atmospheres and at 100 °C. The result was that the Rh-sulfoxantphos complex of the SILP catalyst behaved similar to an analogous rhodium-xanthene catalyst dissolved in the homogeneous phase. Analysis of the CO stretching band showed that the catalyst was in equilibrium between a dimeric form and two monomeric forms (Scheme 8.3) and, consequently, that the hydroformylation reactions were indeed homogeneously catalyzed. 

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