Researchers discovered a new cationic cobalt bisphosphine hydroformylation catalyst system that is highly active and extremely robust. Catalysts help transform one chemical substance into another, while remaining unchanged themselves. Cobalt, a common mineral, does well in accepting atoms from other molecules and forming complex molecules.
Majority of industries–about 75 percent–choose to use rhodium-based catalysts because of the low-pressure technologies and cheaper-to-build facilities, but Stanley said not only can cobalt-based catalysts make more–and better versions–of certain aldehyde products, but the price of rhodium is excessive in comparison.
“A cationic cobalt bisphosphine catalyst is only about 20 times slower than the best rhodium catalysts,” he said, “despite being 10,000 times less expensive.” Today, the price of rhodium has reached closed to $9,800 an ounce, while cobalt has been steady around only 90 cents per ounce.
Louisiana, alone, has three large hydroformylation chemical plants: the ExxonMobil facility in Baton Rouge that uses the high-pressure cobalt catalyst technology; the Shell plant in Geismar that uses the medium-pressure phosphine-modified cobalt catalyst system; and the Dow chemical plant in Taft that uses low-pressure phosphine-modified rhodium catalysts.
“About 25 percent of products produced by hydroformylation require high-pressure cobalt or rhodium technologies,” he explained. “This new cationic cobalt bisphosphine technology offers a far more energy-efficient catalyst that can operate at medium pressures for these reactions.”
Hydroformylation, or oxo, is the catalytic reaction that converts alkenes, carbon monoxide, and hydrogen into more complex organic products, like plasticizers–a substance added to produce flexibility and to reduce brittleness–and cleaning detergents.
Although the group’s new cobalt catalyst has low selectivity to the generally desired linear aldehyde product for simple alkenes, Stanley said it has excellent activity and selectivity for internal branched alkenes that are difficult to hydroformylate.
For example, researchers are finding that washing detergents are less likely to dissolve in cold water because of their linearity–a trait found in rhodium catalysts. Cobalt catalysts can make detergent molecules with more “branches” that can react to grease and water in a more efficient way.
Charging up cobalt for hydroformylation
Hydroformylation reactions are applied at massive scale in the chemical industry to transform olefins into aldehydes. The original catalysts were neutral cobalt complexes. Hood et al. now report that positively charged cobalt complexes, stabilized by chelating phosphine ligands, show higher activities at lower pressures than their neutral counterparts, approaching the activities of precious rhodium catalysts. These charged catalysts are particularly adept at accelerating the reactions of internal olefins while avoiding decomposition. Spectroscopic studies implicate the involvement of 19-electron intermediates in the catalytic cycle.
The cobalt complexes HCo(CO)4 and HCo(CO)3(PR3) were the original industrial catalysts used for the hydroformylation of alkenes through reaction with hydrogen and carbon monoxide to produce aldehydes. More recent and expensive rhodium-phosphine catalysts are hundreds of times more active and operate under considerably lower pressures. Cationic cobalt(II) bisphosphine hydrido-carbonyl catalysts that are far more active than traditional neutral cobalt(I) catalysts and approach rhodium catalysts in activity are reported here. These catalysts have low linear-to-branched (L:B) regioselectivity for simple linear alkenes. However, owing to their high alkene isomerization activity and increased steric effects due to the bisphosphine ligand, they have high L:B selectivities for internal alkenes with alkyl branches. These catalysts exhibit long lifetimes and substantial resistance to degradation reactions.
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