Aromatization of n-hexane by platinum containing molecular sieves and distribution and motion of organic guest molecules in zeolites.

摘要

A vapor phase impregnation technique with Pt(acac){dollar}sb2{dollar} has been developed and used to load Pt into aluminosilicate (KL, BaKL, KBaKL, NaY, CsNaY, FAU, EMT, ZSM-12 and SSZ-24) and aluminophosphate (AlPO{dollar}sb4{dollar}-5 and VPI-5) molecular sieves. {dollar}sp{lcub}13{rcub}{dollar}C MAS NMR, TEM and H{dollar}sb2{dollar} chemisorption measurements reveal that Pt can be loaded into the micropores of molecular sieves with both charged and neutral frameworks. Pt containing molecular sieves were tested as catalysts for the aromatization of n-hexane at 460-510{dollar}spcirc{dollar}C and atmospheric total pressure in order to study the influence of Pt cluster size and support acidity/basicity, microstructure and chemical composition on activity and selectivity. High selectivity to benzene over most of the zeolite samples demonstrates that support acidity/basicity and microstructure do not contribute directly to the aromatization selectivity over Pt catalysts. A clear trend of increasing benzene selectivity with decreasing Pt cluster size is found. These observations suggest that the exceptional reactivity of Pt/KL for the aromatization of n-hexane results from the lack of any acidity in the support and the ability of zeolite L to stabilize the formation of extremely small Pt clusters. Pt/AlPO{dollar}sb4{dollar}-5 and Pt/VPI-5 show high selectivity to n-hexane with little formation of benzene while opposite is observed for Pt/SSZ-24. The differences in catalytic behavior are attributed to variations in the environment of Pt clusters which are situated in either an aluminophosphate or silicate micropore.; Proton multiple-quantum NMR spectroscopy is used to investigate the distribution of the Hexamethylbenzene (HMB), adamantane and naphthalene adsorbed within the cavities of NaY by a vapor phase impregnation method, and organic species occluded during the synthesis process into the intracrystalline voids of the cubic and hexagonal polytypes of faujasite and ZSM-18. The correct cluster size for single, isolated tri-quat cations rigidly held within the cages of ZSM-18 is observed. At bulk concentrations of 0.5, 1.0 and 2.0 HMB molecules per supercage, the HMB molecules undergo anisotropic motion inside the supercages of NaY. The intracrystalline distribution of HMB is two molecules per supercage at bulk concentrations of 0.5, 1.0 and 2.0 molecules per supercage. The other multiple-quantum NMR measurements appear to be complicated by the fast motion of the organic guest molecules trapped inside the intracrystalline voids of the zeolites. Results from emission spectroscopy suggest that the HMB pairs are uniformly dispersed among the intracrystalline supercages of NaY.