1-Hexene isomerization over sulfated mesoporous Ta oxide: The effects of active site and confinement

Article abstract of DOI:10.1021/ja076584n

The isomerization of 1-hexene at 343 K was conducted over a series of sulfated mesoporous Ta oxides with pore sizes ranging from 12 to 30 A. The results were compared to the popular industrial catalysts HY-zeolite, H-ZSM5, and Amberlyst 15. The conversion of 1-hexene to trans/cis-2-isomers can reach 95.89% in 4 h, which is roughly 2 times faster than Amberlyst 15 and 20 times faster than HY-zeolite and H-ZSM5. GC analysis confirmed that trans- and cis-2-hexene isomers formed through a double bond shift process as the only two principal products. The molar ratio of trans/cis-2-hexene isomers varies with pore size, clearly showing confinement effects in this process. The selectivity of our best material toward the trans-isomer is at least 3 times greater than that of either zeolite or Amberlyst 15. The best catalytic results in this studywere achieved when using C₁₂ H₂SO₄ mesoporous Ta oxide, which possesses a pK_a of -8.2 and 19.8 mmol/g acid sites. The X-ray diffraction pattern showed complete retention of the mesostructure throughout catalysis as evidenced by a single broad reflection. Pyridine adsorption and infrared spectroscopy (IR) suggest that both Bronsted sites (1538 cm^-1) and Lewis sites (～1443 cm^-1) coexist on the surface of these materials. The large amount of Bronsted acid sites, high BET surface areas, high acid strength Ho, and superior diffusion properties of C₁₂ H₂SO₄ mesoporous Ta oxide are probably responsible for its extremely high catalytic activity and selectivity. Copyright