Science Concentrates PROCESS CHEMISTRY
A new low for palladium catalysis Palladium is the undisputed champion when it comes to catalyst metals, enabling chemists to carry out a versatile array of cross-coupling reactions. Bruce H. Lipshutz and his group at the University of California, Santa Barbara, have a goal to take this versatility further by seeing how green and efficient they can make palladium catalysis. In their latest endeavor, the Lipshutz group and its collaborators have devised a new structurally optimized phosphine ligand that helps facilitate palladium-catalyzed cross-couplings with only a partsper-million level of palladium, which is one to two orders of magnitude lower than that typically required (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201510570). Chemists have concerns about the cost and availability of palladium and about the amount of residual palladium left after synthesis of a product requiring FDA approval. Lipshutz has been chipping away at these concerns in recent years by developing an aqueous nanomicelle reaction system that minimizes the amount of palladium needed. The lipophilic “nanoreactors” allow common organic reactions to be run in water at room temperature with repeated in-flask recycling of the nanomicelles and catalyst.
O
CH3O
P OCH3
HandaPhos The new ligand, which is called HandaPhos after its creator, postdoc Sachin Handa, gives the researchers the ability to carry out Suzuki-Miyaura reactions in the nanomicelles using complex aryl coupling partners with less palladium than ever before, below 0.1 mol % compared with previous lows of about 1 mol %. “Lipshutz is a magician who uses his micelles as a wand to make the impossible possible,” says Thomas J. Colacot of Johnson Matthey Catalysis & Chiral Technologies. “The group’s work is having a great impact on our understanding of how Nature uses enzymes and trace amounts of metals to make biaryls.”—STEVE RITTER
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C&EN | CEN.ACS.ORG | MARCH 7, 2016
Improved route from syngas to light olefins Catalytic process has advantages over methanoland Fischer-Tropsch-based technologies A catalytic technique has promise as a new way of industrially producing olefins such as ethylene and propylene. Manufacturers use low-molecular-weight olefins to make plastics, solvents, paints, medicines, and other products. The largest-volume organic chemicals produced worldwide, these “light” olefins have traditionally been made, and are still primarily made, by Syngas
Ketene
(CO + H2) CH2
CO2
CO
its, which can degrade catalyst activity. MTO’s selectivity for light olefins is similar to OX-ZEO’s. But its zeolite catalyst deactivates quickly, and its two separate reaction steps make it potentially less efficient. FTO has lower selectivity, producing higher proportions of methane and other low-molecular-weight alkanes in addition to light olefins, and it is sometimes plagued by carbon.
CH2C0
CH2C0
+ CO
H+ H+ H+
CH2
Metal oxide (ZnCrOx)
Light olefins
Diffusion
In the OX-ZEO technique, activation of syngas on a metaloxide surface forms CH2 groups that help create ketene, which then diffuses to a zeolite with hydrogen active sites, giving rise to light olefins.
catalytic cracking of crude oil. Because of high oil prices and petroleum conservation efforts in past years, researchers developed two technologies as alternatives to catalytic cracking: the MTO (methanol to olefins) process and the FTO (Fischer-Tropsch to olefins) process. These methods use zeolite and metal catalysts, respectively, to convert syngas, a mixture of hydrogen and carbon monoxide, to olefins. The OX-ZEO (oxide-zeolite) technique, developed by Xiulian Pan and Xinhe Bao of Dalian Institute of Chemical Physics and coworkers, provides a third alternative (Science 2016, DOI: 10.1126/science.aaf1835). The researchers optimized a catalyst system—the partially reduced metal-oxide surface catalyst ZnCrOx and a zeolite called MSAPO—to convert syngas to ketene (CH2CO) and then into light olefins. OX-ZEO is highly selective for making light olefins over other products; it favors propene, which has been in short supply; its catalysts last a long time; its one-pot nature makes it streamlined; and it does not generate carbon depos-
Zeolite (MSAPO)
Eric van Steen of the University of Cape Town, a specialist in solid catalysts, comments that OX-ZEO “may find application in the direct conversion of syngas to predominantly propene, a fast-growing market.” Krijn P. de Jong of Utrecht University, an expert on oil and syngas conversion and catalysis, estimates that worldwide production of light olefins is more than 200 million metric tons per year and that a few percent of that, perhaps 10 million metric tons, is currently made from syngas by MTO and FTO, with most of the rest made from crude oil. A new technique competitive with MTO and FTO is thus not earthshaking in the short term. But de Jong notes that it could be important in the long term because the percentage of light olefins made by MTO and FTO has been growing significantly in the past few years as a hedge against high oil prices. He adds, however, that recent precipitous drops in the price of oil “spoil the party” and make future production trends much trickier to predict.—STU BORMAN
P. HUEY/ADAPTED FROM SCIENCE
GREEN CHEMISTRY
