Zeolites uptake curves

Fig. 2 displays a set of FTIR spectra obtained for the uptake of benzene into H-ZSM-5 at 415 K employing the experimental device and procedure as described in the Experimental Section. One recognizes the increase in absorbance of the typical benzene band at 1478 cm as a function of time (spectra 1 to 4). The maximum absorbance, A, of such bands can be used as a measure of the amount sorbed, M, at time t into the porous structure of the zeolite crystallites. Therefore, evaluation of the sequence of these spectral uptake curves can provide data which may be used in the appropriate solution (equ. 1) of Fick s second law, and this generates the desired diffusivities [22] ... 

The Co-salophen / zeolite catalyst, prepared by template synthesis method was active in the oxidation of hydroquinone to benzoquinone (Fig. 2) and produced similar oxygen uptake curves as the free complex. It was also possible to reuse the catalyst in a subsequent run with a similar activity as in the first run. 

The Co(salophen)/zeolite catalyst was used in the oxidation of benzyl alcohol. The activities of the zeolite-encapsulated catalysts were compared to the free complexes in the oxidation reaction. For this comparison, we were using the same amount of Co(salophen) in both cases, and the oxygen uptake curves are given in Figure 1. 

The intracrystalline diffusivities of the hydrocarbons were measured under the conditions of the temperature range of 373-773 K and the pressure range of 0-1.33 kPa by the constant volume method [9]. The apparatus and the procedure are the same as employed by Hashimoto et al. [4,5]. Change in the total pressure caused by adsorption was recorded by use of a piezometric sensor with a transducer, the response of which is first enough to measure accurately the pressure change. To eliminate the influence of several factors (such as mass conductivity between the sorbate and the pressure sensor) on the pressure change, the blank tests were conducted without zeolites. Comparing these data obtained with those with zeolites, an uptake curve of the amount adsorbed was obtained. 

The uptake curve of the amount adsorbed usually includes the amount adsorbed in the zeolite crystallite and that on the outer surface of the crystallite [10]. Hence, these two kinds of the amounts adsorbed were evaluated seperately in calculating the diffusivity from the uptake curve by a non-linear least square method. The magnitude of the amount adsorbed on the outer surface was about 5-30 % of that in the crystallite. Hence, in order to avoid several effects (such as the amount adsorbed on the outer surface of the crystallite and the temperature rise at the early period of the adsorption [11]), the diffusivity was calculated using all data of the uptake curve by use of a theoretical equation (Eq.[l]) [9]. 

Fig. 2 Experimental uptake curves for CO2 in 4A zeolite crystals showing near isothermal behavior in large (34 and 21.5 Jim) crystals (D 9 x 10 cm s at 371 K and 5.2 X 10 cm s at 323 K). The solid lines are the theoretical curves for isothermal diffusion from Eq. 2 with the appropriate value of Ddr. The uptake curves for the small (7.3 jim) crystals show considerable deviation from the isothermal curves but conform well to the theoretical nonisothermal curves with the values of Dc estimated from the data for the large crystals, the value of p calculated from the equilibrium data, and the value of a estimated using heat transfer parameters estimated from uptake rate measurements with a similar system under conditions of complete heat-transfer control. The limiting isothermal curve is also shown by a continuous line with no points. From Ruthven et al. [8]...

From sets of spectra such as those shown in Fig. 3 and uptake curves displayed by Fig. 8 not only isotherms and isosteres could be derived, using the respective plateaux for the temperatures and pressures indicated, but also from the ascending branches (measured via FTIR after an upward pressure jump) or the descending branches (determined after a downward pressure jump) the kinetics of adsorption and desorption into zeolitic pores could be derived. These processes were assumed to be diffusion controlled. Their evaluation required a fit of the appropriate solution of Tick s second law as provided by Crank [39] to the experimentally measured uptake (or removal) points, which are indicated in Fig. 6 by filled crosses for the case of ethylbenzene uptake. 

FIGURE 5.7. Experimental uptake curves for Nj in three different size fractions of 4A zeolite tals. Lines are theoretical curves calculated acc s . (From ref, 48, with permission.)... 

In the preceding analysis we considered the diffusivity as constant but if the uptake curve is measured over a large concentration step this may be a poor approximation. In many zeolitic systems the concentration dependence of the intracrystalline diffusivity is given approximately by Eq. (5.6), with Dq independent of concentration. If the adsorption equilibrium isotherm obeys the Langmuir equation this gives as the expression for the concentration dependence of the diffusivity... 

FIGURE 6.13 Experimental uptake curves for CO2 in 4A zeolite crystals showing near isother ma behavior in the large (34- and 21.5-pm) crystals (/)as9x 10" cm s at 371 K and... 

FIGURE 6.15. Experimental uptake curves for CO2 in 5A zeolite crystals at 273 K showing limiting case of heat transfer control [Eq, (6.70)]. Note that the rate of approach to equilibrium is faster in the thin bed as a result of the greater surface area-volume ratio. Curve (b) shows a case where the heat transfer limited uptake curve lies (fortuitously) close to the ideal curve for isothermal diffusion except in the initial region. (From ref. 20, with permission.)... 

FIGURE 6.19. Experimental uptake curves showing nonisothermal behavior, (a) Adsorption bf Nj at 195 K on 4A zeolite crystals, b) Adsorption of /j-heptane on 5A zeolite pellets. Measurements of the temperature rise show little difference betwieb the center and surface of the particle. [After refs. 23 and 25 (a) reprinted with permission from ref. 23. Copyright 1971 American Chemical Society, b) Reprinted with permission from CAem. Eng, Sci. 35, Ilavsky et al., Copyright 1980, Pergamon Press, Lid.] ... 

FIGURE 6.22. Experimental uptake curve for n heptane benzene mixture on 13X zeolite at 359 K ( bcn.=ne = 24.3 torr, = 50.6 torr) showing the... 

Figure 3.15. Uptake curves ofN2 (5.0) on dry zeolite Kostrolith SX6 (upper curve) ( )and zeolite SX6 pre-loaded with 1.88 mmol MiO/g SX6 (lower curve) (0) at 313.6 K after increase of gas pressure from 0.5 MPa to 0.7 MPa.

Figure 4.8 shows some experimental uptake curves for the adsorption of CO2 in a 5A zeolite which agree very well with equation (4.39) expressing rates of adsorption limited by heat transfer. 

Figure 4.8 Experimental uptake curves for CO2 on 5A zeolite demonstrating the limiting behaviour of heat transfer control. Adsorption temperature 273 K. Figures on curves represent various adsorbate pressures which relate to differing effective heat capacities. Curve 1,4.3 -3.6 torr curve 2, 20-17 torr curve 3,68 - 63 torr curve 4,234 - 204 torr.

Thus the shapes of the curves in Figure 6a are such as to suggest a low Xe-Xe interaction hump superposed upon a monotonically declining contribution to gst arising from Xe-lattice interaction. For 0 < v < 300 the largest heats for Kr were observed for La-enriched L, but for larger uptakes in this zeolite qBt for La-L declined rather sharply. The smallest heats were observed with the H- and K-enriched varieties. The sequence of heats varied with amount sorbed as exemplified below ... 

Figure 3 shows the uptake of n-pentane on 5A zeolite [7] at 523 K (13 mg sample) measured under a mean presure level of 24 torr. This system exhibits much faster uptake than the previous one. The data can again be quantitatively described by the model as shown by curve a in the figure. 

The experimental method used in TEOM for diffusion measurements in zeolites is similar to the uptake and chromatographic methods (i.e., a step change or a pulse injection in the feed is made and the response curve is recorded). It is recommended to operate with dilute systems and low zeolite loadings. For an isothermal system when the uptake rate is influenced by intracrystalline diffusion, with only a small concentration gradient in the adsorbed phase (constant diffusivity), solutions of the transient diffusion equation for various geometries have been given (ii). Adsorption and diffusion of o-xylene, / -xylene, and toluene in HZSM-5 were found to be described well by a one-dimensional model for diffusion in a slab geometry, represented by Eq. (7) (72) ... 

If the values of the effective self-diffusion coefficients, D rf [calculated from the complete xit) curves in TD NMR experiments, assuming diffusion-limited uptake (52)] are below the corresponding intracrystalline data, Dintra (measured directly by PFG NMR), the existence of additional mass transfer resistances in a layer near or on the outer surface of the zeolite crystals is indicated. 

Several types of zeolites with different cations have been analyzed for their hydrogen uptake by Langmi et al. [47] They report the highest H2 uptake (1.8 wt%) for NaY at —196 °C and 15 atm at room temperature the measured storage capacities are less than 0.3 %. The authors suggest that the adsorption is due to physisorption, because no hysteresis was measured between adsorption and desorption isotherms and the curve they obtained is a type I isotherm. Otero Arean et al. were able to measure the stretching mode of the H2 molecule in the cages of Li-ZSM-5 zeolite... 

F g. 3. Mass increase due to monomer uptake, Qt vs yfi dots at several constant vapour fnessures of moimmers (indicated at top right of each curve) top left kinetic curve for a zeolite containing 6.16% water by kind permission from Academic Press Inc. 

In both Figures the uptake of CO2 (upper curves) and the gas temperature (lower curve), measured immediately below the microbalance connected sorbent sample are shown as functions of time, approaching equilibrium after ca. 5 hours. For the dry zeolite experiment an overshooting of the uptake of... 

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