Mesoporous structure, surface acidity and catalytic properties of Pt/Zr–MCM-41 catalysts promoted with 12-tungstophosphoric acid

The influence of the Si/Zr molar ratio on the crystalline structure, textural properties, morphological features and surface acidity of Zr–MCM-41 mesoporous molecular sieves was investigated by X-ray diffraction (XRD), N2 adsorption–desorption isotherms, 29Si MAS NMR, TEM and FTIR spectroscopy of pyridine adsorption techniques. It is found that the structural ordering of Zr–MCM-41 materials varies with the Si/Zr ratio, highly ordered mesoporous molecular sieves could be obtained for a Si/Zr ratio larger than 4. Calcination may significantly improve the structural regularity. After impregnation with 25 wt% of H3PW12O40 (denoted as HWP hereafter), the surface Brönsted acidity of the Pt/Zr–MCM-41 samples was greatly enhanced by 4–8 times. In the hydroisomerization reaction of n-heptane, the n-heptane conversion over all the Pt/HPW/Zr–MCM-41 catalysts linearly increases with increasing the reaction temperature, while, the selectivity to isoheptanes gradually decreases. The molar ratio of multibranched to monobranched isoheptanes (R) in the isomerization products varies within a very narrow range, between 0.8 and 1.2, which is several times greater than those reported in the literature at similar reaction conditions using zeolite-supported catalysts. The high R value obtained in this work is assumed to be related to the mesoporous geometry of the novel catalysts. In the products distribution, 2-methylhexane is predominant in the monobranched isoheptanes and 2,3-dimethylpentane is the prevailing compound being more than 50% of the multibranched isoheptane products. In the cracking products, butane (isobutane and n-butane) and propane are exclusively formed.