Elucidating the Role of Rh/C on the Pathways and Kinetics of Ketone‐to‐Secondary Amines Reaction

• Published in: ChemCatChem 2022-01, Vol.14 (1), p.n/a
• Main Authors: Arteaga‐Pérez, Luis E., Manrique, Raydel, Ortega, Maray, Castillo‐Puchi, Francisca, Fraile, Juan E., Jiménez, Romel
• Format: Article
• Language: English
• Subjects: amination; Amines; Aniline; Cyclohexanone; Dehydrogenation; Disproportionation; Hydrogenation; ketone; Ketones; Kinetics; Palladium; reaction pathway; Rh/C; Rhodium; Selectivity
• ISSN: 1867-3880; 1867-3899
• DOI: 10.1002/cctc.202101270

Biomass‐based amines have received a lot of attention due to their sustainability and carbon economy. Herein, the roles of the metal sites (Rh0 or Pd0) and operating conditions on the kinetics and reaction pathways of the heterogeneous catalytic amination of cyclohexanone with aniline have been discussed. Rh/C provides secondary amines with remarkable selectivity toward the hydrogenation product (ΣSi=72 %). The hydrogenation rate over Rh0 is 1.5‐fold higher than that observed over Pd0, while its dehydrogenation capacity diminished (rD_Rh ¡=0.74 h−1 and rD_Pd=1.32 h−1). This difference in the hydrogenation/dehydrogenation performance allows control over the selectivity via disproportionation of an imine intermediate. The kinetic observations can be represented using the Langmuir‐Hinshelwood model, indicating that the formation of the aminal intermediate is the rate‐limiting step. The apparent activation energy for this step on Rh/C (55 kJ/mol) was higher than that previously reported for Pd/C (48 kJ/mol). Catalytic amination of cyclohexanone with aniline is a sustainable method for producing secondary amines. In this paper, the full characterization of the catalyst (Rh/C) is reported. Moreover, a reaction pathway describing the catalytic cycle has been included and validated with experimental observations integrated into DelPlot analysis. Finally, a kinetic model based on Langmuir‐Hinshelwood formalism was elucidated and validated.