Hydrogenation Without Metals

Fullerenes can catalyze hydrogenation of organic compounds as effectively as metals can, according to a study by researchers in China (J. Am. Chem. Soc., DOI: 10.1021/ja9061097). The discovery may lead to replacing precious-metal catalysts with carbon-based substitutes, which could reduce costs and the environmental effects of heavy-metal pollutants.

Transition-metal catalysts lie at the heart of global-scale hydrogenation processes, such as the ones used to refine crude oil and to synthesize the common fertilizer ammonia. Nonmetal hydrogenation catalysts could provide industry with substantial cost savings. But such catalysts typically require high temperatures and pressures or are ineffective at mediating hydrogenations with molecular hydrogen; they work instead with other hydrogen sources.

Now, Nanjing University chemists Baojun Li and Zheng Xu report that nitroaromatic compounds are readily and selectively hydrogenated to aromatic amines by molecular hydrogen under mild conditions in the presence of a fullerene catalyst. Specifically, the team finds that bubbling hydrogen at atmospheric pressure through a room-temperature solution of nitrobenzene containing a small quantity of C₆₀, while irradiating the reagents with ultraviolet light, yields aniline in nearly 100% yield. That level of catalytic performance is considered the hallmark of noble metals such as palladium and platinum.

The reaction runs well without UV radiation but requires higher temperature and pressure, the team notes. For example, a 2:1 mixture of neutral and anionic forms of C₆₀ also leads to nearly 100% yield of aniline but requires raising the temperature above 120 °C and the pressure to about 4 megapascals. The group reports that C₇₀ gives similar results.

“These results are quite interesting” and are likely to catch the attention of researchers in many fields, including green chemistry, says Yoshiaki Nishibayashi, a chemistry professor at the University of Tokyo. The exact role of the fullerenes remains unclear and needs to be explored for further development of new catalysts, he adds.

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