Encapsulated metal catalyst for selective hydrogenation of quinoline under atmospheric conditions

Abstract

Simultaneous high activity and selectivity are highly desired in heterogeneous catalysis of various important organic intermediate compounds. In the present investigation, we realized ultra-high selective hydrogenation of quinoline to 1,2,3,4-tetrahydroquinoline (py-THQ) under atmospheric conditions by encapsulating 7 ± 2 nm Pd nanoparticles (PdNPs) inside the 10 ± 2 nm nanochannels of carbon nanotubes (CNTs). For the prepared Pd@CNTs catalysts, the encapsulated PdNPs was found to selectively absorb the nitrogen heterocyclic ring of quinoline, which led to the increase of average length of CC bonds in the heterocyclic ring, further resulting in selective activation of the heterocyclic ring and ~100% selectivity to py-THQ. The DFT calculations confirmed the selective hydrogenation feature of Pd@CNTs. In contrast, the Pd-CNTs catalyst with PdNPs supported on the outer surface of CNTs is incapable of selectively activate the heterocyclic ring of quinoline, showing poor selectivity and catalytic activity. The Pd@CNTs catalysts also exhibited universal ultra-high selectivity to other quinoline derivatives (7-Methylquinoline, quinaldine and quinoxaline) and oxygen heterocyclic compounds (2,3-benzofuran). Due to the confined effect, the Pd@CNTs catalysts presented superior cycling stability to the Pd-CNTs and activated carbon (AC) impregnated with Pd (Pd-AC) catalysts. The Pd@CNTs catalyst was also found to highly stable in air storage for months, without loss of activity.