Investigation of the catalytic activities of sulfated mesoporous Ti, Nb, and Ta oxides in 1-hexene isomerization

Abstract

In the present work, a series of porous transition metal (Ti, Nb, and Ta) oxides with different pore sizes (12–30 Å) were synthesized using a ligand-assisted amine templating approach. The as-synthesized samples were further treated with 1M sulfuric acid and were characterized by nitrogen adsorption/desorption, powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and elemental analysis (EA). The experimental results in the isomerization of 1-hexene were compared with the commercially available zeolites (HY-zeolite and H-ZSM5) and the ion exchange resin Amberlyst 15. The best results were achieved when using C 12H 2SO 4 mesoporous Ta oxide, which possesses a Ho of −8.2 and 19.8 mmol g −1 acid sites. The conversion of 1-hexene to trans/cis 2-isomers reaches 95.89% in 4 h and the ratio of trans/cis isomers reaches up to 3.7 after 6 h, which is very close to their theoretical ratio in thermodynamic equilibrium (3.37) under the same conditions. This conspicuously high selectivity and short reaction time were not detected with the Ta materials synthesized with C 18 or C 6 templates, nor with Ti and Nb materials of any pore size under the same reaction conditions. Temperature-programed desorption (TPD) of ammonia was used to investigate the much higher activity of the Ta materials. The results showed that the Ta materials had a much greater concentration of Brønsted sites in the active Ho range for this reaction than the Nb materials. The unique performance of the C 12H 2SO 4 Ta material was thus attributed to its high BET surface area, increased concentration of effective Brønsted acid sites and optimal pore size for this particular reaction.