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A GREENER ROUTE TO PROPYLENE OXIDE
Lab-scale catalyst system integrates several steps to yield commodity chemical
STEVE RITTER
A tungsten catalyst system developed by a team of researchers in China holds promise for a more economical and environmentally friendly route to production of propylene oxide. The apparent front-runner among several possible processes that have emerged to replace the two industrial methods currently in use is the catalytic epoxidation of propylene with hydrogen peroxide. Researchers at the Chinese Academy of Sciences' Dalian Institute of Chemical Physics have now come up with a novel catalyst system for that process [Science, 292, 1139 (2001)].
Chemistry professor Xi Zuwei and coworkers Zhou Ning, Sun Yu, and Li Kunlan discovered a tungsten-containing catalyst, [C5H5NC16H33]3[PO4(WO3)4], that initially is insoluble in organic solvents. But under the action of in situ-generated H2O2 it forms a soluble active species that catalyzes propylene epoxidation. As H2O2 is used up, the catalyst becomes insoluble again, simplifying its recovery.
The only products of the reaction are propylene oxide and water. Typical runs carried out at 65 ºC for six hours give propylene oxide yields of 85%, the researchers report, with catalyst recovery of 90%. The performance of the catalyst remains consistent through at least several reaction cycles.
The new system is amenable to scale-up, Xi notes, particularly since the anthraquinone-based generation of H2O2 using O2 from air is already widely used in industry. He adds that the work has been carried out without a corporate partner and that the process has been patented. It is open for licensing worldwide, but he thinks it will likely take four or five years for commercialization.
"The authors have cleverly meshed together several previously known industrial reaction sequences into an integrated process," observes chemistry professor D. Wayne Goodman of Texas A&M University. The separation of the inactive catalyst component is particularly attractive, he says, and the producers of propylene oxide likely will want to scrutinize the new findings.
A potential drawback to the new system, according to Rick Fontenot, vice president of R&D for Lyondell Chemical and Equistar Chemicals, is that a commercial plant using the Dalian process would require three reaction units. Lyondell, one of the major producers of propylene oxide, has looked at similar technology involving multiple reaction units, he says, but is currently working on a "lower cost single-step process." The other major producer of propylene oxide, Dow Chemical, has recent patents describing the same overall peroxidation process as the Dalian group, but carried out in the vapor phase over gold-containing catalysts.
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