Rhodium-based hydroformylation

A convenient catalyst precursor is RhH(CO)(PPh3)3. Under ambient conditions this will slowly convert 1-alkenes into the expected aldehydes, while internal alkenes hardly react. At higher temperatures pressures of 10 bar or more are required. Unless a large excess of ligand is present the catalyst will also have some isomerization activity for 1-alkenes. The internal alkenes thus formed, however, will not be hydroformylated. Accordingly, the 2-alkene concentration will increase while the 1-alkene concentration will decrease this will slow down the rate of hydroformylation. This makes the rhodium triphenylphosphine catalyst 

Propene hydroformylation can lead to products with a linearity ranging from 60 to 95%, depending on the phosphine concentration. At very high phosphine concentration the rate is low, but the linearity achieves its maximum value. Low ligand concentrations, with concomitant low linearities, will give turnover frequencies in the order of 10,000 mol mol-1 h 1 at 10 bar and 90°C. In the presence of carbon monoxide this rhodium catalyst has no activity for hydrogenation and the selectivity, based on starting material, is virtually 100%. The butanal produced contains no alcohol and can be converted both to butanol and to other products, as desired. 

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Ligand (136), an analog of PPh3 with amphiphilic character, was used for making [Rh(CO) (136)(acac)]. The rhodium-based hydroformylation of 1-hexene using catalysts formed in situ... 

Three commercial homogeneous catalytic processes for the hydroformyla-tion reaction deserve a comparative study. Two of these involve the use of cobalt complexes as catalysts. In the old process a cobalt salt was used. In the modihed current version, a cobalt salt plus a tertiary phosphine are used as the catalyst precursors. The third process uses a rhodium salt with a tertiary phosphine as the catalyst precursor. Ruhrchemie/Rhone-Poulenc, Mitsubishi-Kasei, Union Carbide, and Celanese use the rhodium-based hydroformylation process. The phosphine-modihed cobalt-based system was developed by Shell specih-cally for linear alcohol synthesis (see Section 7.4.1). The old unmodihed cobalt process is of interest mainly for comparison. Some of the process parameters are compared in Table 5.1. 

Rhodium is an expensive metal, and the commercial viability of the rhodium-based hydroformylation process depends crucially on the efficiency of the catalyst recovery process. In the past this has been achieved either by a complicated recycle process or more commonly by energy-requiring distillation. A major advancement in catalyst recovery in recent years has been the introduc-... 

For a more detailed outline of the various cobalt- and rhodium-based hydroformylation catalysts and for additional related references, see G. O. Spessard and G. L. Miessler, Organometallic Chemistry, Prentice Hall, Upper Saddle River, NJ, 1997, pp. 257-265. 

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. 

Most metal-containing complexes, particularly rhodium-based hydroformylation... 

During the course of studies [27, 30, 31] on a range of different phosphines and phosphites for solubilizing rhodium-based hydroformylation catalysts in SCCO2, it... 

Because of the large success of the technical application of rhodium-based hydroformylation, the associated industrial and academic research is also mainly focused on this metal. By a rough estimate of the publishing activities over the last decade, it can be concluded that more than 80% of all publications and patent activities summarized under the keyword hydroformylation are connected in any form with the use of rhodium. 

Currently, a wide range of methods are available to generate active rhodium hydroformylation catalysts from catalyst precursors based on rhodium in oxidation states of O-III. Because of the almost unmanageable amount of protocols concerning the rhodium-based hydroformylation in the literature, a clear conclusion about the efficiency and duration of catalyst formation processes prior to the hydroformylation is hard to draw. A deeper understanding of these processes occurring prior to the hydroformylation would be of interest in order to distinguish between different catalyst precursors. 

Because of the technical success of rhodium based hydroformylations, it is understandable that since the 1970s the vast majority of academic and industrial investigations in this area dealt with the development of new rhodium catalysts. However, the worldwide demand of rhodium for chemical and technical processes and its enormous price stimulate the search for alternative transition-metal catalysts up to now [1]. A particular focus was given to ruthenium [2]. 

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