Designed Catalysts from Pt Nanoparticles Supported on Macroporous Oxides for Selective Isomerization of <i>n</i>‑Hexane

Selective isomerization toward branched hydrocarbons is an important catalytic process in oil refining to obtain high-octane gasoline with minimal content of aromatic compounds. Colloidal Pt nanoparticles with controlled sizes of 1.7, 2.7, and 5.5 nm were deposited onto ordered macroporous oxides of SiO₂, Al₂O₃, TiO₂, Nb₂O₅, Ta₂O₅, and ZrO₂ to investigate Pt size- and support-dependent catalytic selectivity in <i>n</i>-hexane isomerization. Among the macroporous oxides, Nb₂O₅ and Ta₂O₅ exhibited the highest product selectivity, yielding predominantly branched C₆ isomers, including 2- or 3-methylpentane, as desired products of <i>n</i>-hexane isomerization (140 Torr <i>n</i>-hexane and 620 Torr H₂ at 360 °C). <i>In situ</i> characterizations including X-ray diffraction and ambient-pressure X-ray photoelectron spectroscopy showed that the crystal structures of the oxides in Pt/oxide catalysts were not changed during the reaction and oxidation states of Nb₂O₅ were maintained under both H₂ and O₂ conditions. Fourier transform infrared spectra of pyridine adsorbed on the oxides showed that Lewis sites were the dominant acidic site of the oxides. Macroporous Nb₂O₅ and Ta₂O₅ were identified to play key roles in the selective isomerization by charge transfer at Pt–oxide interfaces. The selectivity was revealed to be Pt size-dependent, with improved isomer production as Pt sizes increased from 1.7 to 5.5 nm. When 5.5 nm Pt nanoparticles were supported on Nb₂O₅ or Ta₂O₅, the selectivity toward branched C₆ isomers was further increased, reaching ca. 97% with a minimum content of benzene, due to the combined effects of the Pt size and the strong metal–support interaction