Additive-free N -methylation of amines with methanol over supported iridium catalyst

• Published in: Catalysis science & technology 2021-01, Vol.11 (10), p.3364-3375
• Main Authors: Wang, Jing, Qiang, Wenwen, Ye, Sen, Zhu, Longfei, Liu, Xiang, Loh, Teck-Peng
• Format: Article
• Language: English
• Subjects: Amines; Catalysts; Continuous flow; Flow velocity; Iridium; Mesitylene; Methanol; Methylation; Nanoparticles; Selectivity; Zinc oxide; Zinc oxides
• ISSN: 2044-4753; 2044-4761
• DOI: 10.1039/D0CY02442B

An efficient and versatile zinc oxide-supported iridium (Ir/ZnO) catalyst was developed to catalyze the additive-free N -methylation of amines with methanol. Mechanistic studies suggested that the high catalytic reactivity is rooted in the small sizes (1.4 nm) of Ir nanoparticles and the high ratio (93%) of oxidized iridium species (IrO x , Ir 3+ and Ir 4+ ) on the catalyst. Moreover, the delicate cooperation between the IrO x and ZnO support also promoted its high reactivity. The selectivity of this catalytic N -methylation was controllable between dimethylation and monomethylation by carefully tuning the catalyst loading and reaction solvent. Specifically, neat methanol with high catalyst loading (2 mol% Ir) favored the formation of N , N -dimethylated amine, while the mesitylene/methanol mixture with low catalyst loading (0.5 mol% Ir) was prone to producing mono- N -methylated amines. An environmentally benign continuous flow system with a recycled mode was also developed for the efficient production of N -methylated amines. With optimal flow rates and amine concentrations, a variety of N -methylamines were produced with good to excellent yields in this Ir/ZnO-based flow system, providing a starting point for the clean and efficient production of N -methylamines with this cost-effective chemical process.