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), N-2 adsorption-desorption isotherms, Si-29 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 Bronsted 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-MCM41 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. (C) 2007 Elsevier Inc. All rights reserved.