Intracrystalline concentration profiles

Abstract In this chapter the main macroscopic experimental methods for measuring diffusion in microporous solids are reviewed and the advantages and disadvantages of the various techniques are discussed. For several systems experimental measurements have been made by more than one technique, and in Part 3 the results of such comparative studies are reviewed. While in some cases the results show satisfactory consistency, there are also many systems for which the apparent intracrystaUine diffusivities derived from macroscopic measurements are substantially smaUer than the values from microscopic measurements such as PFG NMR. Recent measurements of the transient intracrystalline concentration profiles show that sirnface resistance and intracrystalline barriers are both... 

Fig. 36 Intracrystalline concentration profiles of isobutane in silicalite-1 along the z direction during adsorption a,b profiles measured by interference microscopy c,d simulated profiles, assuming that the internal interfaces serve only as transport barriers. For the simnlated profiles the time nnit is 10 elementary diffnsion steps. The eqnilibrinm valnes of C(y, z) after the end of adsorption are eqnal to 1...

Fig. 37 profile of methanol InEquilibrium intracrystalline concentration profile of methanol in a CrAPO-5 crystal. The color intensity is proportional to the integrals of local concentration in the y direction (a) and z direction (b). Darker regions correspond to larger concentration integrals. x,y, and z are the crystallographic directions (the channel direction is z)... 

Fig.40 Intracrystalline concentration profiles of water in CrAPO-5 (al-3) and SAPO-5 (bl-3) crystals integrated along the y direction under equilibrium with water vapor at 1 mbar (al, bl, a3, b3) and 20mbar (a2, b2). The profiles under the pressure of 1 mbar were recorded after the change of the water pressure from 0 to 1 mbar (al, bl) and from 20 to 1 mbar (a3, b3). The profiles are shown for only the crystal surface marked in c. The channels run along the z axis. Darker regions correspond to higher concentration integrals...

Fig. 53 Transient profiles of 2-methylpropane as a guest molecule in a nanoporous material (crystal of type silicalite-1). a Overview of the intracrystalline concentration profile to s after the onset of adsorption, b Evolution of the guest profiles along the x axis at 0 to xm during uptake, c Host crystal with indication of the cross section to which the transient concentration profiles in d refer, e Evolution of the guest profiles along the x axis during release over different cross sections as indicated by c...

Analyzing Intracrystalline Concentration Profiles via Boltzmann s Integration Method... 

In all the transient concentration profiles considered so far (Figs. 36, 42, 45, 46, 48, and 50), in no case did the boundary concentration immediately assume the equilibrium value. The consequences of this finding on the relation between the transport resistances exerted by the intracrystalline bulk phase and by the crystal surface on the overall uptake and release behavior will be... 

Fig. 2. Benzene concentration profiles in the intercrystalline space, C(Z,t), along the normalized bed length, Z = z/i, and in the intracrystalline pores, Q(Z, X, t), along the normalized crystallite radius, X = r/R, for crystallites located either at the bottom of the bed (Z = 0) or at the heigth of Z = 0.8. The three upper and lower diagrams correspond to the bed lengths, i = 5 mm and f = 15 mm respectively.

Xe NMR spectra are simulated from the adsorbate concentration profiles. The fit of experimental with calculated spectra (Fig. 3a) using the adjustable parameter, Tintra, allows the determination of the intracrystalline diffusion coefficient of benzene in HZSM-5 zeolite during its adsorption under constant (saturation) pressure. Dint = 7x10 m s" . This value agrees with that obtained with other techniques and reported in literature [4]. The equilibrium time, too, of 8 and 13 h, depends on the bed length, 5 and 15 mm, respectively. The calculation... 

Xe NMR spectroscopy of adsorbed xenon, largely used to investigate static properties of porous solids, appears very useful to study the diffusion of coadsorbed molecules when the local concentration of these molecules changes as for example during the adsorption process. Coefficient of intracrystalline molecular transport can be obtained from the simulation of the NMR spectra using the solutions (adsorbate concentration profiles) of the diffusion equations. 

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