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Reaction & Synthesis: Selective Oxidation Gets a Boost

March 24, 2016
Catalyst promises to avoid byproducts and reduce energy consumption

Researchers at New Mexico State University (NMSU), Las Cruces, N.M., say they have developed more-efficient, less-expensive selective oxidation catalysts that use molecular oxygen as the oxidant, eliminate the need for a co-reductant, and don’t produce co-products or byproducts.

“Our new catalysts contain a transition metal, such as zirconium or hafnium, bound to several organic ligands and act as a bank for oxygen atoms and electrons,” notes Thomas Manz, chemical and materials engineering assistant professor at the university, “Substrate molecules, such as organic compounds containing double bonds, can extract oxygen atoms from this bank to produce oxidized products.”

Figure 1. Researchers Thomas Manz (left) and Bo Yang, have developed a new more-efficient selective oxidation catalyst. Source: Tiffany Acosta, NMSU.

The process relies on an eta-3 ozone intermediate in which two oxygen atoms from molecular oxygen can be easily separated from each other and passed on to two different substrate molecules, he explains. “This makes the reaction more efficient by eliminating the need for a reaction co-product…. This should enable new selective oxidation processes to be commercially developed that lower waste generation and energy consumption.”

A recent article in Theoretical Chemistry Accounts contains more detail on the process.

Manz believes the new catalyst can achieve selective oxidation of organic substrates in two different manners. “In a one-step process, it could be used to produce epoxides without allylic hydrogen atoms and aromatic N-oxides.

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In a two-step process, our catalyst could be combined with a Ruporphyrin-based catalyst to produce a wider variety of selective oxidation products including epoxides with allylic hydrogen atoms, ketones, or alcohols. We are interested in studying propylene oxide production, particularly because its commercial production currently consumes enormous amounts of energy… if … our two-step process cannot be competitive on propylene oxide production, there are more expensive and lower volume fine chemicals that would offer attractive opportunities,” he explains.

The researchers intend to focus on selective oxidation of 2,6-dimethylpyridine to 2,6-dimethylpyridine N-oxide. “The reason we want to focus on this reaction is that 2,6-dimethylpyridine N-oxide can be used as an oxidant over Ru-porphyrin-based catalysts to selectively oxidize challenging substrates. The 2,6-dimethylpyridine regenerated in the second reactor containing Ru-porphyrin-based catalyst could be recycled to the first reactor containing the Zr_benzol catalyst. We believe this will provide a new route for selectively oxidizing challenging substrates using molecular oxygen oxidant without co-reductant. This has the potential to dramatically reduce energy requirements and waste byproducts,” explains Manz.

The separation of catalyst and reaction products from the reaction mixture does pose a challenge, he admits. “Tethering the catalyst to a solid support material is one approach that could be tried. Using a fluorous biphasic system is another approach that could facilitate easy catalyst separation. Both of these approaches have been demonstrated for other kinds of organometallic catalysts.”

In addition, to make the catalyst commercially interesting, the researchers still must demonstrate it has high enough reactivity, selectivity and stability — as well as is inexpensive and easy to separate from the reaction mixture. The team plans to collaborate with other researchers to synthesize and test the catalysts.

“Designing a commercially viable catalyst is hard, because it must meet all of these criteria. We expect our catalyst to be available for commercial use within six years,” notes Manz.

Making the catalyst in industrial quantities likely won’t pose any issues, he adds. “The reagents for making the catalyst are not rare. We believe catalyst synthesis could be readily scaled up after it is demonstrated on the laboratory scale,” he explains.

“Our long-term goal is to work with industrial companies to commercialize this new technology,” Manz says. “We are interested in developing commercial applications for fine chemical production, commodity chemical production, or both.”

NMSU’s Arrowhead Center has filed a patent for the catalyst.

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