A study of n-hexane hydroisomerization catalyzed with the Pt/H₃PW₁₂O₄₀/Zr-MCM-41 catalysts

A series of Zr-modified MCM-41 mesoporous materials with a variety of Si/Zr molar ratios were synthesized by the surfactant-templated method. Textural properties, crystalline structure and surface acidity were characterized by X-ray diffraction (XRD), low-temperature N₂ physisorption isotherms, solid-state nuclear magnetic resonance (MAS-NMR), UV-visible spectroscopy, and Fourier-transform infrared (FTIR) spectroscopy of pyridine adsorption techniques. Both the as-made and the calcined materials show the typical MCM-41 structure with hexagonal mesochannels. The structural regularity varies with zirconium content in the materials. A proper amount of zirconium ions incorporated into the Si framework may noticeably improve the structural order, but high zirconium content leads to the reduction of the structural regularity. The mean pore diameter of the materials increases from 2.8 to 3.0, 3.8 and 4.5 nm as the Si/Zr molar ratio decreases from 25 to 15, 8 and 4, respectively. Meanwhile, the surface area and pore volume diminish with the increase of zirconium content. As H₃PW₁₂O₄₀ (referred as HPW) is dispersed on the Zr-MCM-41 materials, the Brönsted acidity is greatly enhanced: from four to eight times. Three forms of heteropolyanions: (i) bulk-like HPW particles, (ii) highly dispersed HPW clusters with deformed Keggin structure and (iii) HPW species with partially fragmented Keggin structure, are adopted by the HPW deposited on the Zr-MCM-41 solids, due to the different interactions between the heteropolyacid species and the support. The 1 wt.%Pt/25 wt.%HPW/Zr-MCM-41 catalysts show high activity for the n-hexane hydroisomerization reaction, with 100% selectivity to isohexanes at temperatures below 260 °C. Increasing the zirconium content generally leads to a higher n-hexane conversion but it lowers the selectivity to hexane isomers that might be related to the different pore diameter distribution and surface density of the Brönsted acid sites.