Activity, selectivity, and sulfur resistance of Pt/WOx-ZrO2 and Pt/Beta catalysts for the simultaneous hydroisomerization of n-heptane and hydrogenation of benzene

摘要

Bifunctional Pt/WOx-ZrO2 (Pt/WZr, 12.7 wt% W) and Pt/Beta (Si/Al = 12) catalysts have been studied for the simultaneous n-alkane hydroisomerization and aromatic hydrogenation using a n-heptane/benzene feed mixture (25 wt% benzene) at 3.0 MPa and temperatures in the 473-573 K range. The catalysts were characterized by X-ray diffraction, adsorption-desorption of N-2, laser Raman spectroscopy (for WZr), temperature-pragrammed desorption (TPD) of NH3, CO chemisorption, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). In the absence of sulfur, the Pt/WZr catalyst was more active than Pt/Beta, indicating that the former has some acid sites of a higher acid strength, as suggested also by NH3-TPD. At constant n-C-7 conversion, the selectivity to iso-C-7 was similar for both catalysts (ca. 87% for Pt/Beta and 90% for Pt/WZr at 75% conversion), but the concentration of high octane di + tribranched C-7 isomers was slightly higher for Pt/WZr. Under the reaction conditions used, benzene was totally hydrogenated on both catalysts, with a selectivity to cyclohexane (CH) + methylcyclopentane (MCP) above 90%. The MCP/CH ratio was higher for Pt/Beta. In the presence of 200 ppm sulfur, the Pt/Beta catalyst retained a much higher isomerization activity than Pt/WZr. Furthermore, the conversion of benzene was kept at 100% for the zeolite-based catalyst in the range of TOS studied, while it decreased from 100% to ca. 0 and 4% after ca. 300 min on stream for Pt/WZr samples containing 0.6 and 1% Pt, respectively. The lower sulfur resistance of Pt/WZr could be explained by a too strong interaction of part of the Pt with surface W6+ species in the WOx-ZrO2 support, as evidenced by TPR and XPS measurements, although the possibility of a different deactivation mechanism for both Pt/Beta and Pt/WZr catalysts and/or the formation of sulfur-resistant electron-deficient Pt species in the zeolite cavities cannot be discarded. Deactivation of the hydrogenation sites in the presence of sulfur was a reversible process for the two catalysts, while the loss of isomerization activity was seen to be almost reversible for Pt/Beta and irreversible for Pt/WZr. (C) 2000 academic. [References: 36]