009ChemInform Abstract The results of 1H NMR self-diffusion experiments show that H3PO4 penetrates into the channel network of the ZSM-5 zeolite. At higher H3PO4 concentrations, an enrichment of phosphorus deposits near the external surface is observed. 27Al, 29Si, and 31P MAS NMR results show that dealumination and formation of polyphosphoric acids occurs during calcination of the H3PO4-modified zeolite. The decreased number of Broensted acid sites in the modified zeolite after heat treatment at 500 rc C is a direct result of the framework dealumination. A study of the influence of H3PO4 impregnation of H-ZSM-5 on the selective toluene disproportionation reveals a higher paraselectivity and lower catalytic activity of the modified zeolites.
Adsorption equilibrium and kinetic data were measured for the system p-ethyltoluene/Na,H-ZSM-5 zeolite at 343 K. The equilibrium data can be described by the superposition of two Langmuir isotherms. The kinetic data follow a complex mechanism of Fickian diffusion superimposed on the penetration of a surface barrier.The evaluation of the model parameters was carried out by simulation of sorption uptake curves including interactions of the system with both the apparatus and the intrinsic sorption kinetics. The dependence of fitted model parameters with respect to loading and pressure, respectively, is consistent with mechanistic understanding of the mobility within the crystal interface layer and in the crystal bulk.
Sorption uptake of hydrocarbons by molecular sieves with nonuniform micropore systems such as MFI-type zeolites may be governed by a complex of mechanisms instead of pure intracrystalline diffusion. In the particular case of sorption kinetics of n-hexane on silicalite-I, processes occur on the microcrystal level which comprise both Fickian diffusion and sorbate immobilization/mobilization. The rate processes connected with the immobilization of the sorbing species are due to both geometrical constraints and differences in the interaction potential topology between straight and sinusoidal channels within the zeolite crystals. A full quantitative description of this complex transport phenomenon has been derived. A strategy has been developed to reduce the three-parameter problem to that with one parameter only, which is the prerequisite of a practical parameter-fitting procedure. In this way, rate coefficients of the particular composite processes were calculated on the basis of experimental uptake data. The latter were fitted by use of a Volterra integral equation technique.The coefficient of intracrystalline diffusion of the system n-hexaneMFI structure at 323 K amounts to 5 x m2/s, which is a value independent of loading (as the product of the immobilization and mobilization rates is). It is impossible to interpret the measured uptake curves utilizing a model that encompasses intracrystalline diffusion only (Le., neglecting the presence of sorbate immobilization). Neglecting the strong deviation in uptake curve shape by utilizing equations for pure intracrystalline diffusion (e.g., the method of statistical moments), diffusivities were obtained that are lower by up to 3 orders of magnitude.
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