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DIRECT MECHANISMS CHALLENGED
Study fingers impurities as initiators in methanol-to-olefin chemistry
In a report that's sure to spark controversy in catalysis circles, researchers at the University of Southern California say methanol-to-olefin chemistry does not occur directly on solid-acid catalysts when all reagents are highly purified. Rather, they claim, the widely studied conversion process is a result of reactions initiated by low-level organic impurities [J. Am. Chem. Soc., 124, 3844 (2002)].
Discovered some 25 years ago by scientists at Mobil, zeolite-catalyzed reactions that convert methanol to olefins and other hydrocarbons have been the focus of many research programs because of their industrial importance. Having the technical know-how to convert abundant feed materials like methanol to valuable chemicals or fuels can reduce a country's dependence on imported oil, for example.
||CAGED Initially formed inside a zeolite cage from reagent impurities, methylbenzenes and other aromatic compounds make up a hydrocarbon pool that is essential for converting methanol to propene and other olefins, say USC chemists.
FIGURE COURTESY OF UNIVERSITY OF SOUTHERN CALIFORNIA
But figuring out precisely how simple compounds like methanol or dimethyl ether are converted to more complex hydrocarbons in the microscopic pores of zeolite-type catalysts has proven challenging. Proponents of so-called direct conversion routes have proposed 20-odd mechanisms for CC bond formation. These scientists postulate that the simple molecules, in the absence of other compounds, are converted to more complex molecules by way of a variety of exotic intermediates such as oxonium ylides, carbenes, carbocations, free radicals, or other molecules.
According to USC chemistry professor James F. Haw, however, the reactions cannot be ascribed to direct conversion processes. Haw, who led the study, asserts that the reactions occur indirectly and are initiated by impurities that form methylbenzenes and other species in a hydrocarbon pool. The species interact with catalyst surfaces, thereby forming active sites for conversion reactions. As olefins are formed, some of them are converted to methylbenzenes--eventually reaching steady-state conditions, at which point the impurities have little effect on the reaction rate, Haw says. "The oft-studied reaction, which has even prompted a number of theoretical investigations, is an artifact of impurities or other sources of contamination," he says.
To assess the role of impurities, Haw, postdoctoral associate Weiguo Song, and graduate students David M. Marcus, Hui Fu, and Justin O. Ehresmann purified methanol to rid the reactant of parts-per-million levels of ethanol, acetone, and other common impurities. The team members also modified their catalyst preparation procedures after discovering that a single high-temperature treatment left the catalyst with low levels of phenanthrene, naphthalene, and other aromatic residues. And the researchers took precautions to rid their already high-purity flow gases of residual contaminants.
Stripping the experimental setup of the ubiquitous impurities, Haw and coworkers found that methanol and dimethyl ether do not react on HZSM-5, an aluminosilicate, or on HSAPO-34, a silicoaluminophosphate compound--both of which have been used as conversion catalysts.
Northwestern University professor of chemistry Peter C. Stair notes that "the experiments seem unambiguous in terms of showing that, under the conditions that the USC group runs the reactions, impurities play a significant role during the initiation of the reaction. The question is--is the same thing true under conventional reaction conditions?"
Other catalysis experts, however, commenting off the record, strongly dispute the findings. Among the objections raised are that impurity-driven reactions would have far lower rates than those observed.
The work's merits or shortcomings will be established in due time. For now, the jury's out.
Chemical & Engineering News
Copyright © 2002 American Chemical Society