A Platinum Nanocatalyst Confined within Carbon Nanotubes

Key words

carbon nanotubes - platinum - cinchonidine - asymmetric hydrogenation

Significance

A Pt nanocatalyst confined within nanochannels of carbon nanotubes (CNTs) was described. Treatment of multi-wall CNTs with 68 wt% HNO₃ (140 ˚C, 14 h) gave the CNTs with open ends. After ultrasonication of the CNTs and H₂PtCl₆ in H₂O for 3 h, the mixture was stirred at r.t. for 48 h, and then heated to 110 ˚C with a heat range of 1 ˚C/min and held at 110 ˚C for 24 h. ­Reduction of the catalyst precursor with sodium formate led to Pt nanoparticles within the channel of CNTs, which were modified with cinchonidine in AcOH. Asymmetric hydrogenation of α-keto esters was performed under H₂ using the chirally modified catalyst affording the corresponding α-hydroxy esters with 86-96% ee.

Comment

Pt nanoparticles encapsulated within the nanochannel of CNTs [Pt/CNTs(in)] were characterized by TEM, HRTEM (high-resolution transmission electron microscopy), N₂ chemisorption, and CO chemisorption. Pt nanoparticles located on the outer surface of CNTs [Pt/CNTs(out)], Pt/AC (active carbon), and Pt/Al₂O₃ showed lower catalytic activity under similar conditions. Pt/CNTs(in) were recovered by centrifugation (or filtration), and re-modified by cinchonidine in AcOH. The Pt/CNTs(in) catalyst was reused nine times without significant loss of catalytic activity (1^st: 99.9% conversion, 96.4% ee; 10^th: 99.9% conversion, 95.7% ee).