Adsorbents faujasite

The separation of fmctose from glucose illustrates the interaction between the framework stmcture and the cation (Fig. 5) (50). Ca " is known to form complexes with sugar molecules such as fmctose. Thus, Ca—Y shows a high selectivity for fmctose over glucose. However, Ca—X does not exhibit high selectivity. On the other hand, K—X shows selectivity for glucose over fmctose. This polar nature of faujasites and their unique shape-selective properties, more than the molecular-sieving properties, make them most useful as practical adsorbents. 

Fig. 5. Fructose—glucose separation on faujasite adsorbents, (a) Ca—Y adsorbent (b) Ca—X adsorbent (c) K—X adsorbent.

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

Faujasites are highly hydrophilic materials. In the transformation of apolar compounds such as hydrocarbons, all other molecules have higher polarity, so the rate of transformation is seriously decreased because the more polar products adsorb preferentially on the zeolite. This was a serious problem in the oxidation... 

Then, contrary to our previous hypothesis, the reaction proceeds via a Bai2 displacement of aniline on DMC. The product, mono-A -methyl aniline (PhNHMe), plausibly adsorbs into the zeohte in a different way with respect to anihne, because different H-bonds (N H — O-zeolite) take place with the solid. As recently reported by Su et al., A-methyl amines also may interact with NaY by H-bonding between the protons of the methyl group and the oxygen atoms of the zeolite this probably forces the molecule a bit far from the catalytic surface in a fashion less apt to meet DMC and react with it. This behavior can account for the mono-A-methyl selectivity observed, which is specific to the use of DMC in the presence of alkali metal exchanged faujasites in fact, the bis-A-methylation of primary aromatic amines occurs easily with conventional methylating agents (i.e., dimethyl sulfate). ... 

Zeolite/Desorbent Combination The desorbent used in the UOP Parex unit is p-diethylbenzene (PDEB) [28]. It has been found to have approximately the same affinity for the faujasite zeoHte as does p-xylene, balancing the amount of desorbent required for p-xylene desorption while not excluding the p-xylene from adsorbing in the adsorption zone. 

The faujasite zeolite in the UOP Parex process has some finite affinity for aU the aromatic species in the mixed xylene feed, indicated by the fact that selectivities between the components are typically less than five. Because the adsorbent has the tendency to adsorb all aromatic species in the feed to some extent, the fundamental variable dictating the adsorption zone operation is the ratio of zeolitic selective pore volume circulated past the feedpoint by the stepping action of the rotary valve per the volume of aromatics conveyed to the adsorption chambers. Typically this ratio is set to obtain a certain target recovery of p-xylene. 

The adsorbent is an alkali exchanged faujasite bound into a bead. Because m-xylene is by far the most basic of the four mixed xylene isomers, the selectivity of the zeolite is tailored for this chemical trait. 

In this study, we used the 29xe-NMR technique to examine the behavior of gaseous xenon adsorbed at different pressures on a series of intermediate phases isolated during the crystallization of a Faujasite-type silicoaluminophosphate, SAPO-37. Such a method has already proved successful in defining the different steps that successively occur during the crystaiiization process of zeoiites NaY, ZSM-5 and ZSM-20 [10] gel restructuration, increase of the crystallinity of the... 

Bull et al. (97) reported a systematic 2H-NMR and MD study of siliceous faujasite. MD calculations were performed for 1 molecule of benzene adsorbed in a single unit cell of faujasite. Full framework flexibility was incorporated, using potential parameters from MSI s cff91 force field (5). Simulations were performed for diffusion at 298, 350,400, and 450 K, using a time step of 1 fs for a 25-ps calculation run (following 5 ps of equilibration). 

In Faujasites. Bezus et al. (49) reported in 1978 statistical calculations on the low-coverage adsorption thermodynamics of methane in NaX zeolite (Si/Al = 1.48). As for single-atom adsorbates described earlier, the agreement between their calculated values and a range of experimental values was excellent. Allowing for different orientations of the molecule, they calculated a value of 17.9 kJ/mol for the isosteric heat of adsorption at 323 K. Experimental values available for comparison at that time (134-136) ranged from 17.6 to 18.8 kJ/mol. Treating the methane molecule as a hard-sphere particle, with a radius of 2 A, resulted in a far lower heat of adsorption (12.6 kJ/mol). Further calculations (99) yielded heats of adsorption of 19.8 and 18.1 kJ/mol for methane in NaX and NaY zeolites, respectively. 

Egerton and Stone (29), taking into account that synthetic sodalite zeolites did not adsorb CO molecules, concluded that CO does not enter the sodalite cages of the Y zeolites. However, the strong electric fields present in zeolites could also produce changes in the adsorptive properties of the solids thus the energies associated with the cationic sites in crystalline zeolites must be considered. From our IR results, we concluded that CO molecules were located in the volume of the sodalite cages. Thus, the steric effect alone cannot explain the different adsorptive properties exhibited by sodalite and faujasite. 

Olefin Separation. U.O.P. s Olex Process. U.O.P. s other hydrocarbon separation process developed recently—i.e., the Olex process—is used to separate olefins from a feedstock containing olefins and paraffins. The zeolite adsorbent used, according to patent literature 29, 30), is a synthetic faujasite with 1-40 wt % of at least one cation selected from groups I A, IIA, IB, and IIB. The Olex process is also believed to use the same simulated moving-bed operation in liquid phase as U.O.P. s other hydrocarbon separation processes—i.e., the Molex and Parex processes. 

The influence of the nonuniform character of the interior of zeolites on the photophysics of adsorbed guest molecules has been observed. Pyrene molecules included in zeolite faujasites show both monomer and excimer emission [232,233]. As in the case of silica surfaces, the excitation spectra of the emission corresponding to the monomer and the excimer differ (Figure 36), suggesting that there are at least two independent sites, each responsible for monomer emission and excimer emission. Time-resolved emission studies of pyrene included in Na + -X and Na + -Y (<0.1 molecule per cage) indicate... 

Relaxation times Tt and T2 have been determined as a function of temperature and surface coverage in various zeolites, particularly of the faujasite type. The early experiments have been troubled by the very strong dependence of relaxation rates on the concentration of paramagnetic impurities. In order for the relaxation values to be meaningful, such impurities expressed as Fe content must be below ca. 6 ppm. Figure 38 shows the variation of Tt and T2 for water adsorbed in a particularly pure sample of zeolite Na-X (248). The authors (248) account for the experimental results using a model of the intracrystalline fluid, which is about 30 times as viscous as bulk water at room temperature. It shows a broad distribution of molecular mobilities (the ratio T,/T2 at the minimum in Tt is much larger... 

Ripmeester (346) used MAS to study xenon adsorbed on zeolites Na-X and H-mordenite. In the case of faujasite containing excess sorbate, separate lines from liquid, solid, gaseous, and sorbed xenon could be distinguished (see Fig. 67). The presence of a line from adsorbed xenon at 160 K shows that sorbed xenon does not freeze at the bulk xenon melting point. The line from liquid xenon measured at 170 K shifts to high field (Fig. 67b), suggesting that sorbed xenon is more dense than bulk liquid. 

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

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