Heterogeneously Catalyzed Hydroformylation Reactions

Research in hydroformylation has also been driven towards a catalyst that may be immobilized on a surface, without losing activity, but is as active as an homogeneous counterpart. Deloxan rhodium (HK I 2 % (Rh))-based catalysts may be used for these reactions. 

It has been shown that oct-l-ene can be hydroformylated in a continuous flow reactor using SCCO2 as a solvent. It is possible to obtain a total conversion of up to 

however, only an actual yield of 88.5 % aldehyde is achievable. Lower temperatures have been shown to increase the n iso ratio of the reaction, which can reach 2.2 1. There have been several reports of catalytic materials that possess promising properties, which have now been shown to work within a flow reactor system without loss of activity [80]. Perhaps this is one of the most impressive successes of SCF as solvent systems in a continuous reactor to date. The large-scale production of aldehydes by the petrochemical industry is currently based upon an aqueous extraction, which is dependent on solubility of organic compounds in water [81]. This limits the production mainly to aldehydes from olefins with chain lengths of less than five carbons. The discovery that higher alkanes may be hydro- 

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Aldridge and Jonassen (7) have attempted to distinguish between the similar poisoning effects of thiophene and heavy metals on metal carbonyls and fresh metal surfaces. They concluded that Eq. (64) was heterogeneously catalyzed by cobalt metal. This is a point which is difficult to prove unequivocally, however. Attempts to vindicate the early assumption that the hydroformylation reaction is heterogeneously catalyzed (5, 6) have received no further support in recent years. 

The hydroformylation of alkenes is commonly run using soluble metal carbonyl complexes as catalysts but there are some reports of heterogeneously catalyzed reactions of olefins with hydrogen and carbon monoxide. Almost all of these are vapor phase reactions of ethylene or propylene with hydrogen and carbon monoxide catalyzed by rhodium, " 20 ruthenium,nickel, 22,123 cobalt, 23,124 and cobalt-molybdenum 23 catalysts as well as various sulfided metal catalysts. 23,125,126... 

The hydroformylation or oxo reaction has been chosen for particular study for several reasons (a) The reaction was discovered by Roelen 2) in the course of an investigation of the mechanism of the Fischer-Tropsch reaction, and a study of the hydroformylation reaction could furnish information on the course of this heterogeneously catalyzed synthetic fuel process (6) hydroformylation involves the activation of hydrogen by a molecularly dispersed catalyst (c) there are few side reactions (d) the catalyst for the reaction, Co2(CO)s, is easily prepared, is relatively nontoxic, and is consequently readily available for study and (e) the reaction is of great industrial importance. 

Hydroformylation of ethylene was catalyzed by the cluster anion [Ru5G(GO)i6Me] , supported on a highly dehydrated silica (823 K). The catalysis was nearly 100% selective at 398 K. The retention of the cluster framework under the reaction conditions was shown by extended X-ray adsorption fine-structure curve fitting. The cluster anion does not catalyze this reaction in homogeneous solution, and a heterogeneous Ru-SiOz catalyst prepared by conventional methods gave ethane rather than propanal. "... 

The discovery of hydroformylation by Otto Roelen was made while investigating the influence of alkenes on the Fischer-Tropsch reaction using a heterogeneous cobalt oxide catalyst supported on silica. Later it was concluded that hydroformylation is actually a homogeneous process catalyzed by ECo(CO) formed in situ. Many metals catalyze hydroformylation, but the most active catalysts contain cobalt, rhodium, palladium, and platinum as the central metal. The discussion in this chapter centers on the most utilized catalysts ECo(CO), ECo(CO)3PR3, ERh(CO)3(PR3)j, and HRhfCOljfdiphosphine). 

Homogeneous catalysts play an important role in industry as well as in research laboratories. Established applications include, for example, polymerization processes with zirconocene and its derivatives, rhodium- or cobalt-catalyzed hydroformylation of olefins, and enantioselective isomerization catalysts for the preparation of menthol. In contrast to heterogeneous catalysts, more experimental studies of reaction mechanisms are available and the active species can be characterized experimentally in some cases. Most catalysts are based on transition metal compounds, for which electronic structures and properties are well studied theoretically. A substantial number of elementary reactions, such as reductive elimination, oxidative addition, alkene or carbonyl migratory insertion, etc., have been experimentally Studied in detail by means of isotopic, NMR, and IR studie.s, as well as theoretically. ... 

The hydroformylation of alkenes was accidentally discovered by Roelen [4] while he was studying the Fischer-Tropsch reaction (conversion of syn-gas to liquid fuels with a heterogeneous cobalt catalyst) in the late thirties. In what w s probably designed as a mechanistic experiment Roelen examined whether alkenes were intermediates in the Aufbau process for converting syn-gas (from coal, Germany 1938) to fuel. It took more than a decade before the reaction was taken further, but now it was the conversion of petrochemical hydrocarbons into oxygenates that was the driving force. It was discovered that the reaction was not catalyzed by the supported cobalt, but in fact by HCo(CO)4 which was formed in the liquid state. 

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