Embed Size (px)
Citation preview
Front the ACS meeting
Visible light partially oxidizes hydrocarbons Visible light and oxygen gas. That's all it takes to partially oxidize small al-kenes, alkanes, and alkyl benzenes with unprecedented selectivity, according to Lawrence Berkeley National Laboratory (LBNL) senior scientist Heinz Frei. The trick is to cage the reactants inside the pores of just the right kind of synthetic zeolite.
Frei described the simple route to partially oxygenated hydrocarbons— important building blocks for plastics, synthetic fibers, and intermediates in fine chemical syntheses—to a Division of Organic Chemistry symposium on supramolecular photochemistry. The research is supported by the Depart-
Zeolite stabilizes charge-transfer state
CH3
I H3C—C — H + 0 2
I CH3
Isobutane
CH3 I
H3C—C—OOH
CH3
terf-Butyl hydroperoxide
Blue light 1
Zeolite
CH3* I
H3C—C —H 02
CH3
Charge-transfer state
CH3 I
H3C — C · +
Radical pair
ment of Energy's Chemical Sciences Division.
"Our goal is to find useful chemistry that you can drive with abundant sunlight/' Frei said. "Photochemical oxidation within zeolites might replace processes that are now done with more expensive reagents or in environmentally harmful ways."
Many industrial-scale oxidations using oxygen are plagued by unwanted by-products. The desired epoxide, alcohol, or carbonyl products often are themselves further oxidized unless conversions are kept very low, he noted.
But by exploiting the unique envi
ronment inside certain zeolites, the LBNL chemists are able to use low-energy visible light to initiate oxidation of hydrocarbons, minimizing unwanted reactions. Frei and his coworkers— postdoctoral fellows Fritz Blatter (now with Ciba-Geigy in Switzerland) and Hai Sun—selectively produce, for example, benzaldehyde from toluene, acrolein from propylene, and acetone from propane.
The key is to use type Y zeolites, rich in aluminum oxide. The walls of such zeolites carry a formal negative charge that is balanced by cations such as sodium or barium. Unlike the more familiar acidic zeolite catalysts, these alkali or barium zeolites are chemically inert. But the large electrostatic field of the supercage of type Y zeolites profoundly influences the photochemistry of reactants trapped inside, the LBNL team has shown.
The electrostatic field stabilizes the excited charge-transfer state formed when a photon is absorbed by a collisional pair—a complex between a hydrocarbon molecule and an oxygen molecule held in close proximity within the zeolite pore. That stabilization means the charge-transfer state can be reached with visible light rather than the more energetic ultraviolet. The primary products are formed with minimal excess energy, so are not prone to fragment or to undergo unwanted coupling.
"The method is intrinsically stable against
overoxidation," Frei told the symposium. "We have never seen secondary oxidations."
The primary products are always the corresponding hydroperoxides, which usually dehydrate spontaneously at room temperature to aldehydes or ketones, Frei said. For example, terf-butyl hydroperoxide—itself a major industrial oxidant—is produced with 98% selectivity from isobutane. The team has shown that the hydroperoxide can be used in situ to stereospecifically epoxi-dize olefins, thus decreasing the risks of explosion.
Frei and coworkers are using time-
•OoH
resolved Fourier-transform infrared spectroscopy to probe the mechanism of these transformations. "Then," Frei said, "we will look for other types of nanoporous materials to carry out the unique chemistry of these collisional pairs in electrostatic fields."
Pamela Zurer
Judge ousts members of breast implant panel Clearly frustrated at the slow progress in settling Dow Coming's bankruptcy case, a federal judge has dismissed a nine-member committee set up to represent claims against the company from silicone breast implant recipients. The judge says eight of the nine panel members are lawyers who have conflicts of interest.
Instead, Judge Arthur J. Spector of the U.S. Bankruptcy Court for the Eastern District of Michigan, Northern Division, in Bay City, Mich., has ordered the U.S. bankruptcy trustee to appoint a new Tort Claimants Committee (TCC) composed of U.S. and foreign recipients of breast and perhaps other silicone implants. The trustee appointed the current members last May 31, just 15 days after Dow Corning filed for bankruptcy protection (C&EN, May 22,1995, page 6).
Spector asserts that the lawyers' conflicts of interest have hurt settlement negotiations: The committee and Dow Corning are "warring. They are not getting closer to a consensual plan." A Dow Corning spokeswoman adds that the company expects the judge's action "will help us to move efficiently through the Chapter 11 [bankruptcy reorganization] process."
The judge says the TCC should consist of people who claim implant-related illnesses. Only one of the trustee's appointees was an implant recipient. And, Spector notes, that appointee— implant recipient Sybil Niden Gold-rich—does not represent women whose cases may go to trial because she has already lost her own case against implant makers.
Targeting the lawyers on the TCC, Spector's opinion notes that "It is well known that in preparing their many clients' cases for trial, some of the members of the TCC have made large investments of their own money. What if some of an attorney-member's clients
APRIL 1,1996 C&EN 5
