Catalysis Today p. 235 - 240 (2018)
Update date:2022-08-11
Topics:
Fukutake, Tatsuhiro
Wada, Kenji
Liu, Gang Chuan
Hosokawa, Saburo
Feng, Qi
The crystalline structure of titania supports for iridium catalysts markedly affected their low-temperature activity for the dehydrogenative synthesis of N-containing heteroaromatics, namely benzimidazole and indole. While solid iridium catalysts supported on anatase showed moderate to poor activity for the synthesis of 2-phenylbenzimidazole (3) from o-phenylenediamine (1) and benzyl alcohol (2) at 100 °C, the reaction in the presence of rutile-supported catalysts proceeded smoothly to give 3 in high yields of up to 88%. Similar results were observed for the dehydrogenative conversion of 2-(2-aminophenyl)-ethanol (4) to indole (5). The reaction at 100 °C for 18 h in the presence of 1.0 mol% iridium on rutile gave 5 in a yield of 73%, while the use of anatase-supported catalysts resulted in significantly lower yields. TEM analysis showed the formation of small (ca. 2 nm in diameter), homogeneously-dispersed iridium nanoparticles on rutiles, while the inhomogeneous loading of iridium species was observed on anatase supports. CO pulse experiments revealed that there is a strong correlation between CO uptake by iridium nanoparticles and the activities at 100 °C. These results suggest that the predominant formation of small, well-reduced iridium nanoparticles is one major reason for the excellent activities of rutile-supported catalysts at low temperatures.
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