History and Technical Importance

Rhodium, besides cobalt, is the only metal that is used in technical-scale hydroformylation. Because of the classification of industrial hydroformylation processes made by Cornils [1], with rhodium, the third generation, after two generations of Co-based hydroformylation, process was ushered. The first plants went on stream in the 1970s (1974 Ruhrehemie (nowadays Celanese) 1976 Union Carbide Corporation (nowadays Dow) 1978 Mitsubishi Chemical Corporation). These units operate with P-ligand-modified Rh catalysts at low 

An aqueous two-phase hydroformylation went on stream at Ruhrchemie AG in 1984 (fourth generation) at their site in Oberhausen/Germany with an annual capacity of 100 kt/a [1]. The current capacity is 500kt/a. The Rh catalyst is immobilized in the aqueous phase. A sulfonated phosphine ligand (TPPTS, trisodium salt of 3,3, 3 Lphosphinidynetris(benzenesulfonic acid) confers the metal catalyst with high solubility in water. The catalyst is removed into the aqueous phase before distillation of the product, which avoids thermal stress. The loss of rhodium is in the range of parts per billion. 

Homogeneous unmodified or ligand-modified rhodium catalysts are predominantly utilized for the transformation of olefins with a chain length Cjo- Such Rh catalysts can be up to 1000 times more active than Co catalysts. The major advantages of rhodium catalysis are the reduced syngas pressure and lower reaction temperatures. These features have also been recognized by the chemical industry. Thus, in 1980 less than 10% of hydroformylation was conducted with rhodium, and by 1995 this had been increased to about 80% [3]. In some cases, a combination of Co and Rh can be advantageous [4]. 

The main problem of rhodium has been its high and very volatile price over the years. The price on the world market is dictated by the automotive industry, which consumes approximately 80% of the metal in catalytic converters for vehicles. 

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. 

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Information about the history and technical importance of the electrochemistry of chlorine is available from earlier reviews [62] and data collections [14]. 

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