Xenon adsorption

Tablel Nature, relative xenon adsorption capacities and relative crystailinities of the intermediate phases obtained at different stages of the SAPO-37 crystallization.

The distribution of cations in Na-Y, La,Na—Y and Cs,Na—Y zeolites were studied using xenon adsorption isotherms and Xenon- 129 NMR spectroscopy. 

A known amount of zeolite was loaded into a 10 mm NMR tube with an attached vacuum valve. The sample was evacuated to about 2xl0 < torr for 3 days at room temperature, then it was heated to 350 C with a heating rate of 0.2 C/min, the sample was allowed to maintain at this temperature for about 30 hours (2x10 torr). After cooling to room temperature, a known amount of xenon gas was introduced into the sample tube and was sealed by the vacuum valve. All the xenon adsorption isotherms were measured by volumetric method at room temperature. 

The pretreatment temperature of the sample submitted to xenon adsorption and NMR experiments was low enough (393 K) to prevent any hydrolysis. Evacuation to KT4 ton ( 1 ton = 133.322 Pa) for 15 h at this temperature ensures the elimination of most of the water of hydration. Moreover, Gdddon et al. (79) showed that when NaY is less than 15% hydrated the remaining water molecules are in the sodalite cages. Such a small amount of water cannot affect xenon adsorption, which occurs only in the supercages. 

Xenon Adsorption Experiments. Gaseous xenon was co-adsorbed onto the samples on a vacuum manifold the xenon equilibrium pressure was measured by an absolute-pressure transducer (MRS Baratron) capable of measuring pressure with accuracy 0.1 torr. The adsorption isotherms of the co-adsorbed xenon in the samples were measured volumetrically at 22 °C. 

Xenon Adsorption Isotherms and 129Xe NMR Measurements. Figure 1 displays the room temperature (22 °C) xenon adsorption isotherms of the coadsorbed xenon for the three different zeolite samples loaded with various amounts of benzene. A consistent decrease of adsorption with increasing 6 was found for each benzene/zeolite system. By comparing the slope at low xenon pressures, i.e. in the Henry s Law region, we obtained for the adsorption strength NaX(1.23) > NaY(2.49) > NaY(2.70). Moreover, the saturation benzene concentration in faujasite-type zeolites with different Si/Al ratios follows the relation NaX(1.23) < NaY(2.49) < NaY(2.70). 

Figure 1. (a) Xenon adsorption isotherms (at 297 K) of the size-selectively modified Na,H-ZSM-5 zeolites having different coke contents --uncoked A--1 wt % coke B--12 wt % coke, (b) Xenon adsorption isotherms (at 297 K) of fully protonated H-ZSM-5 zeolites having different coke contents -uncoked A—1 wt%coke 12wt%coke. (Reproduced with permission from ref. 16. Copyright 1991 Academic Press Inc. 

Amphlett, C. B., Greenfield, B. F., Krypton and Xenon Adsorption Isotherms on Charcoal Irradiated with 1 Mev Electrons, At. Energy Establishment, Ukaea Research Group, AERE C/R 2632 (July 1958). 

Figure 1. Isotherm of xenon adsorption on LiNaX-1 zeolite...

Figure 1 shows the representation of the experimental isotherm (B. G. Aristov, V. Bosacek, A. V. Kiselev, Trans. Faraday Soc. 1967 63, 2057) of xenon adsorption on partly decationized zeolite LiX-1 (the composition of this zeolite is given on p. 185) with the aid of the virial equation in the exponential form with a different number of coefficients in the series i = 1 (Henry constant), i = 2 (second virial coefficient of adsorbate in the adsorbent molecular field), i = 3, and i = 4 (coefficients determined at fixed values of the first and the second coefficients which are found by the method indicated for the adsorption of ethane, see Figure 4 on p. 41). In this case, the isotherm has an inflection point. The figure shows the role of each of these four constants in the description of this isotherm (as was also shown on Figure 3a, p. 41, for the adsorption of ethane on the same zeolite sample). The first two of these constants—Henry constant (the first virial constant) and second virial coefficient of adsorbate-adsorbate interaction in the field of the adsorbent —have definite physical meanings. 

Table I. Henry Constant, Ki, for Xenon Adsorption on Zeolite LiNaX-1 at —45°C (Mmole/G. Torr)...

Let us consider first the case of Na - faujasite denoted NaYx where x represents the Si/Al ratio (1.28 < x < 54.2). The signal shift increases linearly with the adsorbed xenon concentration [Xe] but is practically independent of the value of X, therefore of the number of Na+ cations [1]. This result proves that in the Y supercages the time-average electric field <5e> due to these cations is negligible at 25°C. The results relative to HYx are similar to the previous ones. At very low [Xe] the motion of each atom is disturbed only by cage walls. Consequently the chemical shift 5s (58 2 ppm) obtained by extrapolation of the line 5 = [Xe] to [Xe] = 0 can be considered as characteristic of the zeolite with respect to xenon adsorption. The increase of 5 with [Xe] results from mutual interactions between Xe atoms. 

Figure 3 shows the adsorption isotherms of xenon in NaX and in AgX following various pretreatments. In comparison with NaX, the adsorption of xenon in dehydrated AgX as well as in the material treated in oxygen is strongly enhanced, especially at low pressures. After reduction with hydrogen at 100 and 300°C xenon adsorption decreases, yielding adsorption isotherms slightly above and distinctly below that of NaX, respectively. Compared to NaX with the linear 5 vs. N dependence of monovalent - ion - exchanged X and Y type zeolites, the shifts in dehydrated and oxidized AgX are distinctly lower over the range of concentration studied (Fig. 4). Most remarkably, the shifts decrease with concentration down to negative values in the range -40 to -50 ppm at low xenon concentration.

Cd located in the supercages is higher than that of the cations in ZnX. This conclusion is also confirmed by the small slope of the part of the 5 vs. N curve before the minimum and by the fact that the xenon adsorption capacity for CdX is greater than that for ZnX. 

At 1 mbar thiophene pressure at room temperature the amount of adsorbed thiophene decreased with increasing metal (Ni or Co) loading of the zeolites (Figure 2). Differences in adsorption capacity may reflect differences in zeolite crystallinity, empty pore volume and density of adsorbate thiophene. Differences in crystallinity were not found by XRD. The pore volume of supercages of oxidic Ni(X)NaY zeolites are identical according to xenon adsorption measurements [7]. The low adsorption... 

Figure 1. Xenon adsorption isotherms at 25°C on (a) dealuminated samples, (b) fresh and used catalysts...

Xenon adsorption isotherms are given in Figure 2. At low equilibrium pressure there is an increase in the amount of adsorbed xenon due to stronger interaction with the coke as compared to that with the zeolite surface. This effect is all the more important when the coke content is high. Conversely, at high equilibrium pressures, there is a decrease in the amount of adsorbed xenon due to a decrease in the pore volume by coke deposit and partial pore blocking. 

Figure 2. Xenon adsorption isotherms of HY pellets coked at various rates (x) 0%, ( ) 2.5%,...

Simonyan, V.V., Johnson, J.K., Kuznetsova, A., and Yates, J.T. Jr. (2001). Molecular simulation of xenon adsorption on single-walled carbon nanotubes. J. Chem. Phys., 114, 4180-5. 

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