Xenon adsorption isotherms

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. 

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. 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%,...

Figure 3. Xenon adsorption isothermes at 26°C Figure 4. Chemical shift...

Fig. 1. Phase diagram showing the evolution of xenon adsorption isotherms between 97 and 117 K on a homogeneous graphite surface (from Ref 6). G two-dimensional gas L twodimensional liquid S twodrmensional solid.

Anh et al. [233] prepared Pt-Cu clusters either by simultaneous co-exchange with platinum and copper ammino cations followed by 300°C calcination and reduction, or by reduction of a zeolite obtained by ion exchange of a Pt/NaY zeolite containing Pt-clusters with Cu + ions. These samples were characterized by Xe NMR, xenon adsorption isotherms, TEM and EXAFS. It was concluded that both samples contained Pt-Cu clusters with copper segregated on the surface of platinum. 

The adsorption isotherms of xenon were measured at 34°C using a classical volumetric apparatus. The 29xe-NMR measurements were performed at the same temperature on a Bruker CXP-200 spectrometer operating at 55.3 MHz. The n-hexane adsorptions were conducted at 90°C on a Stanton Redcroft STA-780 thermoanalyzer. The samples were submitted to a preliminary calcination under dry air up to 650°C with a heating rate of 10°C/min. 

The i29Xe chemical shift and the adsorption isotherm of xenon adsorbed on Y zeolites are dependent on the size, location and nature of cations in the zeolite intraframvork space. The variation of cation location in a partially cation-exchanged Na—Y can also be monitored by Na NMR. 

Measurements of xenon gas adsorption isotherms were performed at temperatures between 273 and 299.5 K (this latter temperature being that of the 129Xe NMR probe). 

Figure 3. Adsorption isotherm curves for xenon on NaY zeolite and NaY zeolite-V205 at 273 and 299.5 K.

Following the pioneering works of Ito and Fraissard (57) and Ripme-ester (58), 129Xe NMR of xenon adsorbed on zeolite has proven sensitive probe of its local environment due to its chemical inertness and excellent sensitivity (59). In this work, we used 11 and 13C NMR measurements of the adsorbed benzene in conjunction with 129Xe NMR and adsorption isotherm measurements of the co-adsorbed xenon to study the homogeneous adsorption behavior of benzene on faujasite-type zeolites with various Si/Al ratios. Detailed macroscopic and microscopic adsorption states of the benzene in various NaX and NaY zeolites are discussed in terms of NMR linewidths and chemical shifts and are compared with results obtained from other studies. 

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. 

Figure 1. Room temperature (22 °C) adsorption isotherms of xenon coadsorbed with various amount of benzene in various zeolites (a) NaX, Si/Al = 1.23, (b) NaY, Si/Al = 2.49, (c) NaY, Si/Al = 2.70.

Through the analysis of adsorption isotherms and 129Xe NMR results of the co-adsorbed xenon, we have shown that the dispersal of benzene molecules depends on not only the cation distribution but also the amount of benzene adsorbate within the supercage of zeolite adsorbents. We have also demonstrated for the first time that this well known indirect technique has the capability not only to probe the macroscopic distribution of adsorbate molecule in zeolite cavities but also to provide dynamic information about the adsorbate at the microscopic level. Conventional H and 13C NMR which directly detect the adsorbate species, although providing complimentary results, are relatively less sensitive. 

Figures 1A and 1B show the adsorption isotherms of xenon on the Na, H-ZSM-5 and H-ZSM-5 zeolites, respectively. From the comparison, one sees that xenon uptake decreases slightly (about 10%) with coke content in the Na, H-ZSM-5 with a low (1%) coke content, on zeolite H-ZSM-5, and decreases only slightly more with heavy coking (12%).

Microporous and, particularly, ultramicropous membranes are more difficult to characterize. Different procedures based on the low-pressure part of the N2 adsorption isotherm have been proposed [36], but they often require knowledge of the shape of the pores and of gas-surface interaction parameters which are not always available. Small angle X-ray scattering (SAXS) is another technique which is well suited to micro-porous powders, but difficult to execute in the case of composite layers, as in microporous membranes. Xenon-129 NMR has recently been proposed [37] for the characterization of amorphous silica used in the preparation of microporous membranes, but the method requires further improvement. Methods based on permeability measurements appear to be limited by the lack of understanding of the mass transport mechanisms in (ultra)microporous systems. 

Figure 4.2. Adsorption isotherms of xenon on FeCl2 (courtesy of Larher, 1992).

Figure 1.39. Adsorption isotherms of Xenon on Vycor glass. Temperatures relative to the bulk critical temperatures of xenon (289.7 K). Above T = 0.94 hysteresis is no longer observed. (Redrawn from S. Nuttall, Ph.D. Thesis. Univ. of Bristol (1974). See also C.G.V. Burgess, D.H. Everett and S. Nuttall, Pure AppL Chem. 61 (1989) 1845 (Courtesy of D.H. Everett).)...

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. 

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.

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