Oxo Process

Abstract

The oxo process, also known as hydroformylation, is the reaction of carbon monoxide (qv) and hydrogen (qv) with an olefinic substrate to form isomeric aldehydes (qv) as shown in equation 1. The ratio of isomeric aldehydes depends on the olefin, the catalyst, and the reaction conditions. RCH==CH 2 + CO + H 2 − −−−− → catalyst RCH 2 CH 2 CHO + R(CH 3)CHCHO (1) If a double-bond shift occurs, the number of aldehyde isomers is increased. Synthesis gas, a mixture of CO and H 2 , also known as syngas, is produced for the oxo process by partial oxidation (eq. 2) or steam reforming (eq. 3) of a carbonaceous feedstock, typically methane or naphtha. The ratio of CO to H 2 may be adjusted by cofeeding carbon dioxide (qv), CO 2 , as illustrated in equation 4, the water gas shift reaction. 2 CH 4 + O 2 −→ 2 CO + 4 H 2 (2) CH 4 + H 2 O −→ CO + 3 H 2 (3) CO 2 + H 2 − −− → ← −− − CO + H 2 O (4) 2 CH 4 + CO 2 + O 2 −→ 3 CO + 3 H 2 + H 2 O (5) The overall process for producing a 1:1 CO to H 2 ratio by partial methane oxidation and the water gas shift reaction is represented by equation 5. 1 2 OXO PROCESS 1. History The oxo reaction proceeds most frequently in the presence of a Group 8–10 (VIII) metal catalyst in the liquid phase, most particularly with members of Group 9, the Co–Rh–Ir triad. The earliest catalyst, hydrocobalt tetracarbonyl [16842-03-8], HCo(CO) 4 , was an outgrowth of Fischer-Tropsch investigations carried out prior to World War II on the effect of olefins on hydrocarbon synthesis (1). The hydroformylation reaction, as practiced in the early days using cobalt catalysis, presented formidable requirements of high pressure, containment of the hydrogen, containment of carbon monoxide, and handling of the toxic and unstable metal carbonyls. These conditions were challenging for the experimentalist as well as for the plant operator. However, because the oxo reaction provided a direct route for converting inexpensive olefins into valuable oxygenated building blocks, widespread industrial research and usage occurred throughout Europe, Japan, and the United States. The search for catalyst systems which could effect the oxo reaction under milder conditions and produce higher yields of the desired aldehyde resulted in processes utilizing rhodium. Oxo capacity built since the mid-1970s, both in the United States and elsewhere, has largely employed tertiary phosphine-modified rhodium catalysts. For example, over 50% of the world's butyraldehyde (qv) is produced by the LP Oxo process, technology licensed by Union Carbide Corporation and Davy Process Technology. Propylene (qv) [115-07-1] is the predominant oxo process olefin feedstock. Ethylene (qv) [74-85-1], as well as a wide variety of terminal, internal, and mixed olefin streams, are also hydroformylated commercially. Branched-chain olefins include octenes, nonenes, and dodecenes from fractionation of oligomers of C 3 –C 4 olefins as well as octenes from dimerization and codimerization of isobutylene and 1-and 2-butenes (see Butylenes). Linear terminal olefins are the most reactive in conventional cobalt hydroformylation. Linear internal olefins react at less than one-third that rate. A single methyl branch at the olefinic carbon of a terminal olefin reduces its reaction rate by a factor of 10 (2). For rhodium hydroformylation, linear α-olefins are again the most reactive. For example, 1-butene is about 20–40 times as reactive as the 2-butenes (3) and about 100 times as reactive as isobutylene. Oxo aldehyde products range from C 3 to C 15 , ie, detergent range, and are employed principally as inter-mediates to alcohols, acids, polyols, and esters formed by the appropriate reduction, oxidation, or condensation chemistry. The oxo reaction has been the subject of various reviews (4). The classic challenges in oxo technology are simultaneously to achieve high reaction rate, high selectivity to the desired aldehyde, and to utilize a highly stable catalyst. Since the early 1970s, considerable progress has been made using ligand-modified rhodium catalysts that address these problems. In addition, progress has been made in the development of high reactivity rhodium catalysts for the conversion of internal and mixed-olefin feed streams. These latter are considerably less reactive than simple unsubstituted α-olefins. Development of catalysts which give improved process selectivities to the straight-chain isomer, generally more valuable, and of more efficient ways to recover product from rhodium catalyst solutions, have occurred. Additionally, progress has been made in asymmetric hydroformylation by using chiral ligands as a potential route to chiral pharmaceuticals.